Geological Society of America Bulletin - Rice Earth...

Geological Society of America Bulletin

doi: 10.1130/0016-7606(1953)64[945:GAHAAB]2.0.CO;2 1953;64;945-992Geological Society of America Bulletin

BRONSON STRINGHAM GRANITIZATION AND HYDROTHERMAL ALTERATION AT BINGHAM, UTAH

Email alerting services to receive free e-mail alerts when new articles cite this articlewww.gsapubs.org/cgi/alertsclick

Subscribe to subscribe to Geological Society of America Bulletinwww.gsapubs.org/subscriptions/click

Permission request to contact GSAhttp://www.geosociety.org/pubs/copyrt.htm#gsaclick

presented in this publication do not reflect official positions of the Society.science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and

Notes

Copyright © 1953, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment.

on January 29, 2010gsabulletin.gsapubs.orgDownloaded from

http://gsabulletin.gsapubs.org//cgi/alerts

http://gsabulletin.gsapubs.org//subscriptions/index.ac.dtl

http://www.geosociety.org/pubs/copyrt.htm#gsa

http://gsabulletin.gsapubs.org/

BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICAVOL. 64. PP. 945-992. 9 FIGS.. 5 PLS. AUGUST 1953

GRANITIZATION AND HYDROTHERMAL ALTERATIONAT BINGHAM, UTAH

BY BRONSON STRINGHAM

ABSTRACT

The rocks exposed in the disseminated copper deposit at Bingham, Utah, consist of folded Pennsylvanianquartzites, limestones, and dolomites within which have been formed granite, actinolite syenite, graniteporphyry with associated aplites, and biotite-quartz latite porphyry. Field and petrographic evidence showsthat the granite porphyry and biotite-quartz latite porphyry are magmatic. That the granite and actinolitesyenite may have been formed by granitization is strongly suggested by: (1) the presence of a micrograniticrock in the granite, recognized as being granitized quartzite, and (2) the replacement of quartzite by feldspar,observable only with the aid of the microscope, that occurs along the contact of the granite. Other chemical,textural, compositional, and other evidences support this hypothesis.

The hydrothermal activity attending the ore deposition took place in seven stages following the formationof the igneous-appearing rocks. Except for the first two, which can be separated on good microscopic evi-dence, all stages can be differentiated by structural field relations. Stage I saw the formation of kaolinite andillite. During Stage II hydrothermal biotite and sericite were widely developed. In Stage III, chlorite andhydrothermal biotite were formed, but in restricted areas. In Stage IV, quartz and sericite formed over wideareas, while during Stare V quartz was deposited in open fissures without accompanying alteration. StageVI is the sulfide stage, and Stage VII saw the development of sericite and allophane in open fissures.

TEXT

PART I. GRANITIZATION .

Introduction 946Acknowledgments 947Limitations of the study 947Field work 948Chemical analysis 948General geologic setting 949Sedimentary rocks 949

General statement 949Structure 950Bingham quartzite within the mine 950

Pure quartzite 950Impure quartzite 950Impure limestone 951Dolomitic limestone 951Weathering 952

Granite 952General statement 952Megascopic features 952Microscopic features 953Chemical analysis 955Contacts and classification 955

Actinolite syenite 959General statement 959Megascopic features 959Microscopic features 959Special features 960

CONTENTS

Chemical analysis 960Page Contacts and classification 960946 Granite porphyry 960

General statement 960Megascopic features 961Microscopic features 962Chemical analysis 963Contacts 963Aplites 963Classification 964

Biotite-quartz latite porphyry 964General characteristics 964Megascopic features 964Microscopic features 964Chemical analysis 964Contacts and classification 964

Last Chance quartz monzonite 965Feldspar network 965

Microscopic features 965Classification and distribution 968Chemical analysis 968Origin 968

Granitized quartzite 968General statement 968Megascopic features 969Microscopic features 969Contacts 970Chemical analysis 971Origin and classification 971

945



946 BRONSON STRINGHAM—GRANITIZATION AND ALTERATION, UTAH

PageOrigin of the igneous-appearing rocks 971

General statement 971Origin by igneous action 971

Validity of evidence 971Last Chance quartz monzonite 971Actinolite syenite 972Granite 972Granite porphyry 973Biotite-quartz latite porphyry 973

Origin by granitization 973Actinolite syenite and granite 973Summary 975Granitization history 976

PART II. HYDROTHERMAL ALTERATION 976

Introduction 976Chemical analysis 977Hydrothermal Stage I—Kaolinite-illite 977

General statement 977Microscopic appearance 979Distribution 979Character of the altering solutions 979Paragenesis 980

Hydrothermal Stage II—Hydrothermal bio-tite-sericite 980

General statement 980Granite 981Actinolite syenite 982Granite porphyry 982Biotite-quartz latite porphyry 983Granitized quartzite 983Feldspar network 983Sediments 983Relations between hydrothermal biotite

and sericite 983Character of altering solutions 983Paragenesis 984

Hydrothermal Stage III—Chlorite-hydro-thermal biotite 984

General statement 984Microscopic features 984Chlorite orbicules 984Character of altering solutions 985Paragenesis 985

Hydrothermal Stage IV—Quartz-sericite 985General statement 985Field features 985Microscopic features 985Distribution 986Character of altering solutions 986

Paragenesis 987Hydrothermal Stage V—Quartz 987Hydrothermal Stage VI—Sulfides 987

General statement 987Sulfide minerals 987Distribution 988Alteration 988Nature of depositing solutions 988

Hydrothermal Stage VII—Sericite-allophane. . 989Summary of the hydrothermal alteration and

mineralization stages 989Origin of solutions 990General conclusions 990References cited 991

ILLUSTRATIONSFigure Page1. Index map showing the location of Bingham. 9472. Arkosic quartzite from outside the mine

area northeast of Bingham 9503. Granite showing sutured texture with

"granular" quartz 9554. Classification chart 9585. Actinolite syenite 9596. Classification chart 9637. Last Chance quartz monzonite 9658. Pseudomorphs of hydrothermal biotite 9809. Sericite in feldspar 983

FacingPlate page1. Geologic map and hydrothermal alteration,

Bingham Copper Mine, Utah 945Following

2. Photomicrographs3. Photomicrographs.

page

4. Photomicrographs: Granitized quartzite andaltered granite

5. Altered granite: granite and granitizedquartzite contact

966

TABLESTable Page1. Chemical analyses of relatively unaltered

Bingham rocks and minerals 9562. Chemical analyses of altered rocks from

Bingham 9773. Hydrothermal stages at Bingham 978

PART I.—GRANITIZATION

INTRODUCTION

The results of the most recent mineralogicalstudy of the copper mine at Bingham, Utah,were published in 1920, and the works of Bout-well (1905, p. 73-390), Keith (1905, p. 29-70),Beeson (1916, p. 2191-2236), and Butler (1920,p. 335-362) were the most extensive. Thewriter undertook a new studv in the summer of

1947; the objective was to investigate the hy-drothermal history of the disseminated copperore deposits. During the course of the work,the possibility that some of the rocks at Bing-ham were formed by granitization became evi-dent, and after further investigation it was de-cided to present the full results of the Binghamstudy in two parts. Part I deals specifically withdetailed descriptions of rock units as well as



INTRODUCTION 947

the granitization and related processes. Part IIcovers the hydrothermal history related to oredeposition.

OSALT LAKE CITY

BINSHAM

U T A H

FIGURE 1.—INDEX MAP SHOWING THELOCATION or BINGHAM

Early workers on the Bingham copper de-posit could observe considerably more area thanis now exposed since recent dumping of wastehas covered several critical areas. On the otherhand, since the ore body now has been cut muchdeeper, less weathered rock and ore are nowobservable.

The present work has been greatly facilitatedby a very favorable geographic coincidence, inthat mapping could be done in the field con-currently with laboratory investigations con-ducted at the University of Utah. This hascontributed immeasurably to the quick under-standing of the nature of the rocks and hasadded to the confidence and speed of the map-ping.

The Utah Copper open pit mine is situatedjust south of the town of Bingham, Utah, inthe Oquirrh Mountains, a typical basin andrange mountain about 20 miles southwest ofSalt Lake City (Fig. 1).

The Kennecott Copper Corporation minesthe disseminated copper deposit as an open-pitoperation employing the level and bank method.

The levels carry railroad, power-line, and elec-tric shovel facilities and are separated by ap-proximately 50-foot high banks from which therock is blasted. The ore contains values incopper, gold, molybdenum, and a little silver.

ACKNOWLEDGMENTS

Expressions of thanks and appreciation areoffered to the management of the KennecottCopper Corporation for permission to make thestudy and to the management and geologicstaff for their whole-hearted co-operation andencouragement during the work; to The Geo-logical Society of America and to the Univer-sity of Utah for their financial aid without whichthe study could not have been made; to T. S.Lovering for his helpful suggestions and guid-ance and to G. E. Goodspeed for a very stimu-lating discussion of the granitization problem;to Dan McLachlan, Jr., for invaluable adviceon crystallographic problems; to James F. Syl-vester and Allen O. Taylor for their very ableassistance in the field and laboratory; to EdwinW. Roedder who read the manuscript and of-fered valuable criticisms.

LIMITATIONS OF THE STUDY

No geologic map or other such informationregarding the mine was received from theoperating company, and the data and interpre-tations presented here were arrived at withoutany influence other than that gained from theliterature. The conclusions are strictly thewriter's and do not necessarily represent theviews of the operating company. For variousreasons, no direct intensive observations weremade on the ore body or in areas beyond theopen-pit boundaries, and hence no correlationof the disseminated copper deposits with thereplacement lead-sliver deposits of the districthas been attempted. Since the original plan wasto study the hydrothermal alteration and laterthe granitization, little attention was given tothe larger sedimentary or structural features.This present work should be considered as acontribution to only one phase of the generalproblem. So many new questions constantlyarise regarding the geology and mineralogy ofthe copper deposit proper that it would seemthat the present work could still be carried on




for a considerable length of time with profit,and much more work is necessary to completea full-scale geologic study of the district.

FIELD WORK

Intensive study began in the copper mine inthe summer of 1947, and mapping and samplingcontinued through the field seasons of 1948and 1949. During the summer months of 1950and 1951 occasional trips were made to themine for final checking.

Triangulation points were established on dif-ferent levels around the periphery of the openpit, and, from these, triangulations were madewith the plane table and alidade when accuratelocations within the pit were necessary. Mostof the work was done, however, with a steeltape and Brunton compass traverse along eachlevel tying into surveyed points at either endof each level. This established fairly accurately,regardless of high-voltage power lines, the posi-tions and distances on each level. The solidrock banks were examined foot by foot, andspecimens were collected wherever laboratoryexamination was thought necessary. In thismanner some levels were covered as many asfour times, while all levels were traversed atleast twice. There was much confusion in iden-tifying rock types during the early stages ofthe mapping, but considerable confidence wasgained after laboratory examination had es-tablished the true nature of the rocks. Duringthe entire mapping of the pit no thought wasgiven to granitization, and it was not until theend of the 1949 field season, when a peculiarrock type was definitely recognized as granitizedquartzite, that granitization was considered apossible origin for many of the igneous-appear-ing rocks in the deposit.

Contacts were found only on the banks be-tween the levels in the mine, and extension ofthese contacts was interpolated along the levelsbetween banks. Solid lines on the map (PL 1)indicate contacts observed, and dashed linesrepresent interpolated positions. Some inter-polated contacts were checked when later exca-vations exposed a new area, and these positionswere also placed on the map as solid lines. Asis readily apparent, different interpolations ofcontacts between banks can in many instancesproduce different map patterns. Some contacts

were drawn between banks with a fair degree ofcertainty, while others are drawn in positionsthought to be most reasonable. No majorcontacts were reinterpolated after the granitiza-tion was recognized, and the map stands es-sentially as it was when first completed. Con-tinued excavations make obsolete most mappedcontacts, and in some very active areas con-tacts are obsolete within a week. Few contactson cut banks are in exactly the same positionas when mapped. The map (PI. 1) representsgeologic conditions during 1947 to 1949 only.Checking in 1950 and 1951 revealed that mostof the rock units and contacts are at least inthe immediate vicinity of the previous survey.Constant careful mapping of an area beingmined at such a rapid rate as this would showsome interesting and worthwhile information onthe three-dimensional aspect of the deposit.

Almost all rocks collected were thin sectioned,and 803 sections have been studied. Seven largelantern-sized (3J4 by 4 inches) slides were pre-pared of critical contacts. Many x-ray studieson minerals were made, and in two instancesspecimens were sent to experts for precise de-termination. Differential-thermal analysis, uni-versal-stage, and other standard optical equip-ment was freely used.

The co-ordinates on the map (PI. 1) areentirely artificial and are placed there only asan aid to the description of the area. Eachsquare is numbered from 1 to 15. These sectionnumbers are used for conveniently locating aspecific area.

CHEMICAL ANALYSIS

Ninteen complete and six partial chemicalanalyses were made on the rocks and mineralsat Bingham. Some of these pertain to the hy-drothermal problem and are not presented inPart I. However, 13 complete analyses and 2partial analyses regarded as pertinent to theproblem of the origin of the rocks are included.

The quality of the samples available foranalysis was poor. Since so much alterationhas affected the Bingham rocks probably noentirely fresh specimen was obtained. The dif-ficulties of obtaining a representative sampleof Bingham rocks are manifold. The rockbreaks into very small fragments due to in-numerable post-mineral and pre-mineral frac-



CHEMICAL ANALYSIS 949

tures, and most of the latter are faced withquartz and/or pyrite which must be scaled off.In some instances, the analyst was suppliedwith only a very small fragment, and in twoinstances (Analyses 4 and 5, Table 1) not morethan 20 grams could be made available. Thelarge bulk samples which are so highly desir-able for analytical purposes were simply notobtainable. The interpretative value of theanalyses, therefore, is not so reliable as is usu-ally encountered.

The attempt to calculate a norm from severalof the analyses did not work out at all reason-ably, and the complications arising because ofthe extreme abundance of biotite with itsvariable composition made almost impossibleany calculation which would correspond withthe actual mineral content of the rock. Meas-ured modes were made in the thin section ofeach rock analyzed except four which wereestimated, and these are all given directly be-low each analysis. Except for analysis 12—feld-spar network, 11—Last Chance stock, and 9—•actinolite syenite, all the rocks show a low SiOzcontent and a high MgO, FezOj, and K2Ocontent for granitic type rocks. It is believedthat the extreme abundance of biotite, beinglow in SiO2 and high in MgO, iron, and K2O,accounts for these anomalies.

GENERAL GEOLOGIC SETTING

The oldest rocks within the area covered inthis study are Pennsylvanian quartzites, im-pure quartzites, limestones, and dolomites whichhave been folded and locally faulted and withinthe mine area dip moderately to the northeastapparently without serious displacement. Ayounger irregular body of quartz monzonitecalled the Last Chance stock occurs beyond theborders of the mine to the south and west,while within the mine area itself granite, actino-lite syenite, and an unusual rock, thought to begranitized quartzite, was formed. Intensive sili-cate mineral development is found around theperiphery of this complex mass in the limestonesand impure quartzites, as might be expectedalong a contact of a granite. The quartzitesnear the granite have been feldpathized to amoderate degree, visible only with the micro-scope. All these rocks were intruded first by alarge mass of granite porphyry with offshoot

dikes, then by a biotite-quartz latite porphyrywhich appears only in very small dikes andirregular pipes. Extensive hydrothermal activityaltered the rocks and culminated in the deposi-tion of the main metalliferous ores.

SEDIMENTARY ROCKS

General Statement

All the sedimentary rocks exposed in the pitbelong to one formation described and named"Bingham quartzite" by Keith (1905, p. 35),who estimated its total thickness to be 10,000feet or more. G. H. Girty (Keith, 1905, p. 36)determined its Pennsylvanian age on the basisof two collections of fossils. Gilluly (1932, p. 34),in an area south of Bingham but in the samemountain range, distinguished the upper partof this formation as the Oquirrh Series.

The sedimentary rocks within the pit properwere found to be altered. Since very little workwas done during this study on rocks beyond thepit, it is perhaps proper to quote excerpts fromthe excellent description by Keith (1905, p.33-35) for the unaltered formation.

"The Bingham 'quartzite' is composed in themain of quartzites and of sandstones that are moreor less silicified. The strata are for the most partwhite and frequently streaked. . . . In the upperpart of the formation are many layers of argil-laceous sandstone containing feldspars and micain fine grains. . . . These sandstones are also slightlycalcareous and locally developed into thin lime-stone beds . . . which are of the same lenticularnature as the limestones that are mapped, and theyare so small that it is not practicable to map themor trace them for any considerable distance. . . .Almost any section where the rocks are continu-ously exposed for 100 yards will show one or morelayers of limestone or calcerous sandstone. . . . Itis not possible, however, to connect the individuallayers of one series with another. Usually the con-tacts of the limestones and quartzites are sharpand distinct and the transition from one rock typeto another takes place within a few inches. In othercases. . . the quartzite passes downward into lime-stone through several feet of calcareous sandstoneand sandy limestone. In still other cases, there isan alteration of limestone and sandstone layers fora few feet. The limestone lenses thin out along thestrike and disappear in two ways. Either the closingquartzites approach each other and coalesce ... orelse the quartzites in the strike of the limestonesare calcareous. ... In a few places . . . thin layersof quartz conglomerate appear, but these are finegrained for that class of rock. . . . The uniformityof the grain makes the bedding of the quartzitesvery difficult to ascertain except where argillaceous,calcareous, or banded beds are present."




If the quartzite contains lime, even as littleas 3-4 per cent, local usage classifies it as lime-stone.

Structure

The general attitude of the sedimentaryformations in the pit is quite uniform. Manymeasurements on quartzite in regions consider-ably removed from the igneous-looking rocksshowed that an approximate strike of N.70°-85° E. and dip of 25MO° N. is predomi-nant over the entire area mapped. Near theigneous-looking rocks, alteration and cementa-tion obscure the bedding. Only one sedimentaryrock area close to the granite, near the lowercenter of Section 3, is questionable in regard toa change from the conventional regional dip.The rocks here are extensively altered andprobably somewhat faulted, but this faultingis believed to have preceded the formation ofthe granite. Measurements of the attitude ofthe beds here were highly questionable, andhence none were placed on the map. A fewfaults were noted in other areas; their recogni-tion was incidental to the mapping of the lime-stones. (See Section 5.) However, lack of timeprevented giving too much attention to thisfeature since it would not contribute greatlyto the major problem. Quartzites and impurequartzites could not be correlated from onelevel to another. Such a job would require verydetailed sampling with accompanying thin sec-tions as well as extremely close surveying con-trol. However, in the mapping of some readilyrecognizable limestones, it was necessary tocall upon faulting to explain their displacementfrom one level to another (Section 5). Somefaults are known to extend into the granite,as extensive slickensided breaks and crush zonesare found within it, but most of the majorfaulting is considered pre-granite. Joints inmany attitudes are noted, but no over-allpattern was evident, and an extensive studyinto this feature was not deemed advisable.The granite rocks and closely adjacent sedi-mentary rocks at Bingham break up readilyunder the hammer into very small fragmentsdue to the presence of minute fractures whichintricatelv traverse the rock.

Bingham Quartzite Within the Mine

Pure quartzite.—The grains in the quartziterange from very fine (J4 mm. in diameter) to asomewhat coarse grit (3-5 mm. in diameter).

FIGURE 2. ARKOSIC QUARTZITE FROM OUTSIDETHE MINE AREA, NORTHEAST OP BINGHAM

Cloudy grains are orthoclase. Note that ghostsof rounded quartz sand grains are still present.Opaque areas are pyrite. Tracing from photomicro-graph. X30.

No true conglomerate was discovered. Wherenot stained with limonite, the rock is usuallywhite, gray, or tan. On the upper levels inregions removed from the granite, bedding isplain, but near the granite the bedding isobscure, and the rock is massive and glassy dueto strong cementation by silica. Much pyriteis found in veinlets and as small disseminatedcrystals.

Under the microscope the pure quartzitesremote from the granite show some porosity,but the boundaries between grains are inter-locking, and ghosts of rounded quartz sandgrains are present. In most sections, a littlesericite is found interstitially. Rocks near thegranite show suturelike grain boundaries andno ghosts of sand grains. Occasional quartzitespecimens containing as much as 4 per centdetrital feldspar grains may be found withinthe pit. These feldspars, usually orthoclase,are scattered in more or less of a random dis-tribution (Fig. 2), and all exhibit smooth, sub-rounded boundaries.

Impure quartzite.—Many sedimentary rocksdiffer considerably in composition from the



SEDIMENTARY ROCKS 951

normal quartzite. Some beds are white andchalklike and much lighter in weight than nor-mal quartzite, suggesting considerable porosity.Some are white to tan, while others range fromtinges of green to an intense green. Individualspecimens from areas of greenish rocks showblotches, irregular patches, and randomly ori-ented stringers of deeper greenish material.In many exposures a green streak or veinlettraverses as much as 10 feet through an other-wise massive-looking dense tannish quartzite.

Thin sections of the white impure quartzitesshow fine interstitial crystals of sericite, tremo-lite, or, in a few instances, wollastonite. Thoughgenerally interstitial the tremolite and wollasto-nite commonly penetrate quartz grains. Originalimpurities in these types of rocks are dolomite,calcite, and clay. The pale-green and greenishrocks contain greenish biotite, actinolite, andchlorite. The chlorite is thought to be an un-usual variety since its index of refraction, near1.54, is lower than ordinary chlorites. Quiteevidently the greenish colors in the rock aredue to different concentrations of chlorite, greenbiotite, and actinolite. The green stringers thattraverse the quartzite may be explained on thebasis that the original interstitial impure sub-stances—lime, magnesia, alumina, and iron—were mobile enough during the developmentof the minerals to migrate through channelsand form veinlets or group themselves in clumpsand patches.

The impure quartzites may be found on allmine levels where sedimentary rocks occur.However, on the upper levels in regions moreremote from the granite, the greenish colorsare less prominent, indicating that here thedevelopment of these greenish minerals wasnot pronounced.

Impure limestone.—The impure limestones,which occur in thin beds, are found mainly onthe east side of the pit where eight limestonelenses were mapped (Sections 4, 5, 9, 10, 14,IS). One limestone lens was mapped on thewest side (Section 2), and two isolated limestonebodies mapped in Sections 2 and 7. Most ofthese rocks appear dense and greenish. Manyspecimens contain macroscopic quartz grains,but no carbonate was found. Occasionally someminerals, such as long blades of actinolite orpatches of epidote and chlorite, are readily

identifiable in hand specimen. The beds rangein width from a few feet to as much as 60 feetthick on some levels. The typical lenslike char-acter of the limestones seems to be quite pro-nounced in some of the beds, and the thicknessusually ranges rather markedly along the strike.Introduced pyrite is universally present andlocally constitutes as much as 50 per cent ofthe limestone bed.

Microscopic examination reveals actinolite,chlorite, and occasional epidote and tremoliteas the principal minerals of the impure lime-stone. Quartz grains appear scattered amongthese minerals. A greenish mineral, which other-wise looks like sericite, is common, as well asgarnet which is believed to be near grossularite(index 1.74) in composition. These garnets arerarely seen in hand specimen. One carefullytraced bed grades from a rock high in greenishsilicates near the granite to a rock lighter incolor and containing less silicate minerals, indi-cating that probably less silica and iron wereintroduced away from the granitic rocks.

Throughout the granite mass are large andsmall irregularly shaped inclusions of quartziteand impure quartzite with sharp megascopicborders. Many of the larger inclusions weremapped (Sections 12, 13, and 14 for example),but there are so many in parts of the area thatonly some could be represented on the map.Bedding on those observed is so obscure thattrue strike and dip could not be secured withconfidence. Petrofabric work on these inclusionsas well as the country rock showed no preferredorientation of the quartz grains, and hence nocorrelation of the inclusion attitude with thedipping country rock could be made usingthis method.

Dolomitic limestone.—One bed of what isbelieved to have been a more or less pure dolo-mitic limestone is found on the southern inter-mediate levels in Sections 12 and 13. Its miner-alogy differs from the other limestone beds.It was identified and mapped principally onthe presence of hydrothermal and skarn typeminerals, but the outlining of its boundariesseems to be fairly accurate. The bed is judgedto be between 30 and 80 feet thick and probablyrepresents a segment of either the Jordan orCommercial limestone member (Keith, 1905,p. 38-41) of the Bingham quartzite. The ap-




pearance of the rock is extremely varied, asdifferent minerals predominate in differentareas. The following minerals were found:andradite (index 1.873), grossularite (index1.74), very fine-grained massive-appearing diop-side, actinolite, chlorite, epidote, and muchcoarse glassy introduced quartz. These mineralsoccur not in streaks and lenses along bedding,as might be expected, but as irregular patchesup to 15 feet or more across. The boundariesbetween these different minerals are usuallyvery irregular, and veins of one mineral mayextend deeply into another. Veinlets of chal-cedony, some of which are yellowish, and a fewirregular small bodies of magnetite are intro-duced. In this area occurs the largest irregularveins and bodies of pyrite found in the pit.Fractures which ordinarily cause the rock tobreak into smaller fragments seem to be absentin this area, apparently because of the silicateminerals, and large blocks generally composethe talus on the bank of the level.

Weathering.—Weathering of the quartziteseems to be confined to the development oflimonite, hematite, and some nontronite. Onthe upper levels near the position of the originalground surface are extensive brown stains. Onthe east side are areas where limonite and hema-tite are concentrated enough to color the rockbrownish red. Also on the east side, apparentlydue to channeling of meteoric water, are largestreaks of limonite which extend down alongthe bedding into the white quartzite. On theupper levels, nontronite is extensively developedas a result of weathering of pyrite (Stringhamand Taylor, 1950, p. 1065).

GRANITE

General Statement

The term granite is here used in a broad senseto include several varieties of rock found in thepit. If classified according to Johannsen (1939,p. 141-159) they could be termed orthoclasegranite, granite, orthoclase syenite, quartz mon-zonite, monzonite, syenodiorite, quartz dio-rite, granodiorite, or diorite. In mapping, allthese types were grouped because no structuralboundaries separated them, and megascopicallythey could not be readily differentiated. Seem-ingly they were all formed by the same proc-

ess during approximately the same period. Allthe rocks are rich in biotite, and each rock nameprobably should be prefixed by the word bio-tite. The term granite is here used for all thegroup since normal granite constitutes at least75 per cent of the whole. The other types varywidely in amount and are found in irregularareas, large and small, randomly scattered overthe entire granite exposure.

Early workers classified this rock as a monzo-nite, quartz monzonite, and monzonite por-phyry; it was designated by Butler (1920,p. 351) as the "dark porphyry."

The granite constitutes the largest mass ofan igneous-appearing rock within the copperpit and covers about two-thirds of the area.The largest uninterrupted mass, on the lowereastern levels of the mine in Sections 4, 9, 13,and 14, extends high on the south side (Section12) where it presumably connects through tothe granite found in the first 1000 feet of theNiagara Tunnel of the U. S. Mine. A largemass with irregular borders also extends onthe upper levels toward the west in Sections6 and 7. A small isolated area is found on thenorth intermediate levels (Sections 2 and 3).

Megascopic Features

It is practically impossible to classify thegranite megascopically. The quartz, in all handspecimens observed, is in such small units that,except in rare cases, it could not be observedeven with a hand lens. Orthoclase and plagio-clase are generally both so altered that theyappear white to gray and are indistinguishablefrom each other. Biotite is always recognizablebut may be present in such fine flakes that itspresence is determined only by its dark color.A granitoid texture characterizes all specimensexamined, but four varieties were noted, and adescription of each follows.

A medium-grained granitoid texture is foundin places over the whole area mapped as granite,and it is by far the most abundant texturaltype (PI. 5, fig. 2). In the lower eastern partof the pit where the granite is exposed thistexture is very extensive and uniform. The grainsizes range from 1 to 3 mm. in diameter with anaverage of about 2 mm. The rock is gray. Dis-tinction between orthoclase and plagioclase is



GRANITE 953

practically impossible except when larger (5 to25 mm.) pinkish-lavender well-shaped ortho-clase crystal are present. In many places theseare as much as 20 or 30 feet apart, whereas insome areas they may be within 1-2 feet ofeach other. The biotite may be euhedral, or itmay be in ragged, irregular black or brownpatches and may vary in amount from a mini-mum of about 20 per cent, giving a spottedappearance to the rock, to about 50 per centwhen the rock becomes extremely dark oralmost black. Some of these dark granites onthe intermediate levels of the south side (Sec-tion 7) carry plates of biotite up to about halfan inch in diameter.

All minerals in rocks with fine granitoidtexture are so small that they are not readilydifferentiated, and the rock takes on a uniformmassive appearance. Close examination, how-ever, reveals the ever-present crystals and ir-regular patches of brownish biotite. The over-all color is usually a uniform dirty bluish gray,and the rock somewhat resembles a densespotted limestone. This fine-grained type iscommon near the edges of the granite and insmall apophyses, but small irregular areas ofit may also be found deep within the granitemass. The occasional large pinkish-lavenderorthoclase crystals are much rarer in this tex-tural type than in the medium-grained rock.

Coarse granitoid texture is simply a coarservariety of the medium-grained granitoid tex-ture. The individual crystals range from about2 to 7 mm. across with an average near 4 mm.and therefore offer a much more normal gran-itic appearance than any other rock in themine. Despite the larger grain size, biotite isthe only definitely identifiable mineral, exceptfor the frequent large pinkish-lavender ortho-clase crystals. The quartz, plagioclase, andorthoclase of the main part of the rock cannotbe differentiated from one another. This typeis found only on the upper western levels of themine in a fairly restricted area.

In many places the large well-shaped pinkish-lavender orthoclase crystals become so abun-dant that they make up 30 to 40 per cent ofthe rock. These are usually set in a groundmassof fine, medium, or coarse granitoid ground-mass, and the rock has a porphyritic (or por-phyroblastic) graniotid texture. Measurements

revealed no preferred orientation or alignmentof the crystals. This porphyritic type occursin scattered irregular small patches at irregularintervals. It is particularly abundant on thesouthwestern middle levels of the mine.

Near the old ground level the granite hasbeen turned somewhat whitish with brownstreaks due to surface weathering. In mostcases, however, where weathering is deep, thebiotite lightens in color, and a little brownishmontmorillonite develops on some of the al-tered feldspars giving the rock a dull tannish-gray appearance.

Microscopic Features

Except for the size of the crystals, there is onlya superficial correlation between the megascopicand microscopic appearance of these granite rocktypes. Of the 283 thin sections of granite examined,nearly each one is individualistic. However, themineral species in these varieties are generally quitesimilar. Thorough preliminary observations re-vealed six textures which seemed distinctive enoughto serve as standards to which all the texturescould be referred. However, all combinations ofthese types exist. The minerals, which are nearlythe same in all cases, occur in different proportions,and occasional absence of quartz, orthoclase, orplagioclase, or combinations of these, account forthe variety of classifications recognized. Charac-teristics of each mineral and each texture in thegranite type rock are here detailed.

MINERALS: Quartz is anhedral and interstitial.A texture similar to the sieve texture is very commonand could be described as a group of interstitialquartz anhedra from 0.1 to 1 mm. across whichextinguish simultaneously indicating their opticaland crystallographic continuity (PI. 2, fig. 1). Thistype is called "sieve" quartz. Quartz may also ap-pear as interstitial dimensional granules .05 to 1 mm.in diameter, but with very ragged outlines and ran-dom extinctions. This type is called "granular"quartz. These two types are not to be confusedwith the "introduced" quartz, which appears inveins, and "alteration" quartz, which is associatedwith fine hydrothermal biotite and sericite.

Two types of orthoclase occur. Type I crystalsare usually small and always euhedral against gran-ular quartz. Of particular note is its anhedral char-acter against itself with most boundaries irregularenough to resemble sutures. Most of this ortho-clase is perthitic with a plagioclase (Anos). However,all gradations of perthite or antiperthite have beenobserved from 90 per cent orthoclase and 10 per




cent plagioclase to essentially 90 per cent plagio-clase and 10 per cent orthoclase. The grain sizesare less than 1 mm. and average around 0.2 mmin greatest dimension.

Orthoclase of Type II forms the large pinkish-lavender crystals, seen in hand specimen, and arealways much larger than the other minerals in therock. The borders are usually ragged and verythoroughly scalloped, though the crystals as a wholeare roughly euhedral. The groundmass mineralsquartz, orthoclase, plagioclase, and biotite are inmany places incorporated within the large crystal,especially near the margins; the plagioclase inclu-sions usually are oriented with (010) parallel to theedge of the large phenocrysts. Islands of quartzwith scalloped borders are common within thecrystals; several islands may have unit extinction.A full chemical analysis and a partial one wasmade of two of these orthoclase crystals (Table 1,Nos. 13 and 14). The high Na2O (3.14 and 2.60)is probably largely due to included plagioclasecrystals, but some of it may be in solid solution inthe orthoclase.

The plagioclase composition ranges from Anos toAn2s; the more albitic plagioclase greatly pre-dominates. Almost all the crystals are euhedralto subhedral except where they constitute coreswithin orthoclase crystals (PI. 2, fig. 2). The sizeranges from 0.1 to about 2 mm. In most instancesalbite twinning, although usually present, is some-what obscured by alteration. Plagioclase cores withorthoclase rims (Type I) of various sizes (0.1-1.5mm. in diameter) are very common (PI. 2, fig. 2).The boundaries between the two are extremely ir-regular, and many are scalloped toward the plagio-clase as though the orthoclase were replacing plagio-clase. Several islands of plagioclase with unitextinction and optically continuous twinning mayremain within orthoclase. Relic or ghostlike altera-tion patches of sericite having an outline similarto a plagioclase cr>'stal in some places appearwholly within an orthoclase crystal suggesting thatthis was the position of a former plagioclase crystalwhich has now been replaced by orthoclase. Theproportion of core material to rim material may runfrom 90 per cent orthoclase to nearly 90 per centplagioclase.

Biotite, identified by x-ray methods (J. W.Gruner, personal communication), appears in allthe granites examined and in some specimens con-stitutes as much as 60 per cent of the whole. Micro-scopically much of the biotite forms well-shapedcrystals 0.05-1 mm. across. Some of the biotite isprominently scalloped and may poikilitically en-close orthoclase, plagioclase, and quartz. The biotitequite commonly is frayed and ragged due to ran-

domly oriented biotite "foils" around the edges,and some entire crystals, maintaining their originalshape, show a mat of fine-grained biotite withidentical optical properties; this in reality is apseudomorph of biotite after itself. In many cases,however, the biotite has altered to secondary biotiteaccompanied by quartz so that pseudomorphs mayrange in composition from pure biotite to as muchas 5 per cent biotite and 95 per cent quartz (Fig. 8).Fine flaky biotite, as well as sericite, also commonlyreplaces any of the principal minerals as veinletsor irregular patches. The whole problem of thissecondary biotite is more thoroughly described inPart II. A chemical analysis of the alkalies in somemechanically separated coarse biotite was made(Table 1, No. IS). The total alkali content, 10.8per cent, is near the theoretical figure for biotite,but in this specimen Na20 seems to be rather high,amounting to .75 per cent.

In all the slides of granite examined, no pyroxeneor amphibole was found. However, in 10 of theslides, the outline of the secondary biotite patchessimulates the outline of pyroxene and/or amphibole.

In every thin section apatite is an accessorymineral. In some cases it appears in fairly large(0.05 mm.) crystals, but in general crystals arevery small and occur interstitially. In almost allslides rutile is present as very small stubby crystalsoften grouped in clumps. Long needle-like crystalsin some slightly altered biotite are also thought tobe rutile.

MICROSCOPIC TEXTURES: A true evaluation of thedistribution of the different microscopic textures isalmost impossible since only the orthoclase pro-phyritic type can be recognized in the field, andthere was no attempt to delineate them in mapping.However, certain generalizations based on thelocations of each specimen can be offered. Thesutured texture with sieve quartz (2) is by far themost abundant and was found in all parts of thegranite area, while the sutured texture with granularquartz (3), next in abundance, was restricted moreto the lower eastern part of the pit (Sections 4, 9).Specimens showing the pure sutured texture (1)were encountered only occasionally and in randomlocations throughout the granite area. The por-phyritic and lamprophyric sutured textures ap-peared principally on the higher levels on the westside of the pit.

The six distinct types of microscopic texturesrecognized were:

(1) Sutured Texture. Where orthoclase pre-dominates, the extremely irregular boundaries be-tween the orthoclase grains become very striking(Fig. 3). Weinschenk-Johannsen (1916, p. 200)called this texture sutured, and the term fits very



GRANITE 955

well the texture of much of the Bingham granite.All the orthoclase grains are roughly equidimen-sional and average around 0.5 mm. across. Biotiteand plagioclase may be present between orthoclasegrains in this and all the other textures.

(2) Sutured Texture with Sieve Quartz. Ortho-clase is euhedral against the quartz but as in texture(1), anhedral and sutured against itself. The presenceof sieve quartz is the distinguishing characteristicof this texture.

(3) Sutured Texture with "Granular" Quartz.The quartz appears as small interstitial equidimen-sional granules between the extremely sutured ortho-clase crystals (Fig. 3). The individual quartz grainsextinguish independently, as contrasted with thesieve quartz. The boundaries between orthoclaseand quartz are irregular in most cases.

(4) Lamprophyric Sutured Texture. This texturemay be similar to any of the three textures above,except that most biotite is euhedral giving a texturesimilar to the type found in some lamprophyres.Biotite usually constitutes 40-50 per cent of therock.

(5) Porphyritic (or Porphyroblastic) SuturedTexture with Orthoclase of Type II. Large crystalsof orthoclase of Type (2) are set in a groundmassof the typical sutured type. In rocks containingabundant large orthoclase crystals, plagioclase ismore abundant and slightly basic in composition toAn2s.

(6) Porphyritic (or Porphyroblastic) SuturedTexture with Plagioclase. In a very few rocks,crystals of plagioclase of about An2s are found ina groundmass having a typical sutured texture.

Chemical Analysis

Chemical analyses numbers 1-4, Table 1,are all of granite type rock. Number 1 is theleast altered rock found, and this analysis givesthe best account of the original compositionof much of the granite. Most of the high con-tent of K2O in this rock is in perthitic ortho-clase. Analysis No. 2 was made on the porphy-ritic variety of granite since this type was al-ways high in plagioclase, and this accounts forthe high Na2O.

Number 3 is of a moderately altered rock ofgranitoid texture. It is characteristic of a largeportion of the granitic rock found particularlyin the southern and eastern portions of thegranite exposure. The slightly higher Fe2O3 andMgO reflect the abundance of the alterationmineral biotite, while the remainder of theanalysis approximates analysis Number 1. The

specimen for analysis Number 4 was securedabout 1 cm. from the granite-granitized quartz-ite contact (PI. 5, fig. 4). Here biotite andplagioclase are present in high concentrationas indicated by the lower K2O and high NajO

FIGURE 3.—GRANITE SHOWING SUTUREDTEXTURE WITH "GRANULAR" QUARTZ

Quartz is in small equidimensional clear grains(Q). Cloudy orthoclase (O) has sutured boundaries.Plagioclase (P) and biotite (B) are also present.Tracing from photomicrograph. X55.

and MgO. This rock is near quartz monzoniteor granodiorite in composition.

Contacts and Classification

Contacts between the granite and quartzite,limestone, or granite porphyry are very sharp,and thin sections across the contacts show thatit is fairly sharp even on the microscopic scale(PI. 5, fig. 6). On all contacts observed eitherin the field or under the microscope, no flowarrangement was found in the minerals of thegranite. A petrofabric analysis of the biotitein some of the thin sections across contacts,showed no preferred orientation. Howeverwhere the granite is next to quartzite, thequartzite exhibits the feldspar network feature,which will be discussed later.

The outline of the contact in places is ofparticular interest, as in Sections 9 and 10 onthe east side of the pit, where beds of limestoneand quartzite extend down into the granitemass for two or three levels without any ap-parent disturbance in the bedding. In Section



TABLE 1.—CHEMICAL ANALYSES OF RELATIVELY UNALTERED BINGHAM ROCKS AND MINERALSWalter J. Savournin, Analyst

University of Utah

Si02A1203Fe203MgOCaONa2OK2OTiO2H20

So.Gr

'

68.1615.231.292.30

.621.248.67

.661.78

99.952.61

'

61.7318.131.873.061 164.287.74

.751.59

100.312.54

66.7116.212.703.61

3082

6 83.67

1.75

99.60

4

65.4815.251.884.101 483 634 35

792 86

99.82

*

62.2816.522.723.481 214 294 87

.423.91

99.70

6

69.0415.771.261.961 164 553 91

441 68

99.772.57

7

67.5016.071.702.47

522 187 40

611 67

100.122.50

8

65.3914.913 903.27

981 717 00

541 84

99.542.63

9

54.6810.805.567.79

11 381 506 16

761 61

100.242.49

10

61.5814.526 544.661 412 025 06

693 77

100.25

ll

59.4215.106.523.934 564 044 03

761.01

99.372.69

12

89.682.781.951.10

.41

.572.23

.17

.20

99.09

13

65.20*20.46

.603.149.55

1.05

100.00

14

2.6010.25

is

759 05

W»iC/lO

3Oa

Modes Measured

QuartzOrtho incl PerthitePlagioBiotiteRutile and ApatiteSphene . . . . . .ActinoliteDiopsideAugiteHornblende

2562

571

125225101

13+664

152

20+2832182

19+303516

28+263412

23568

121

195110182

46121

21119

12342232

9233461

1215

751348

W

hHO

a5?K



Note: All results are calculated to a 105° C. moisture-free and sulfide-free basis. The compensation for the latter is based on the amount of sulfur and copperpresent calculated to the theoretical formulas of first chalcopyrite then pyrite. The Fe2O3 value represents total iron as Fe2O3, minus the iron used for pyrite andchalcopyrite correction. The presence of sulfide prevented analysis for FeO. Any P206 present is included in the figure for A12O3 but probably does not exceed0.25%. No calcite was found in any of the rocks chosen for analysis, and hence C02 was not determined. The H2O value was obtained by ignition at 1150°C.corrected for the weight change to be expected from the oxidation of the pyrite and chalcopyrite present.

* by difference.+ modes estimated.- less than 1%.

1. Granite, most unaltered rock available; fine granitoid texture2. Porphyritic granite, slightly altered; porphyritic granitoid texture3. Granite, moderately altered; granitoid texture4. Granite, moderately altered; granitoid; 1 cm. from contact with granitized quartzite; 2 cm. from No. 55. Granitized quartzite, slightly altered; 1 cm. from granite contact; 2 cm. from No. 4.6. Granitized quartzite, slightly altered7. Granite porphyry, moderately altered8. Granite porphyry, moderately altered O9. Actinolite syenite, slightly altered >

10. Biotite-quartz latite porphyry, moderately altered §11. Quartz monzonite, Last Chance quartz monzonite, very slightly altered £j12. Quartzite with feldspar network, slightly altered13. Orthoclase from porphyritic granite, mechanical separation14. Orthoclase from porphyritic granite, partial analysis; mechanical separation15. Biotite from granite, partial analysis; mechanical separation




10 the granite-sediment contacts suggest thatthe granite extends sill-like into the sedimentsfor a short distance, while in Sections 9 and14 mapping shows that an interconnecting

according to the Johannsen system, all of thesecould be classified as a variety of granite(Fig. 4). According to the estimated modessome rocks mapped as granite may have prac-

QUARTZ

OrthocloseSyenite

OrthoclaseGranite

QuartzDiorite

/ Monzonite ^ DJorj,e

/ \ V^ Diorite/ Monzonite I Syenodlorite_\jVx

ORTHOCLASE PLAGIOCLASE(An 05-2 5)

FIGURE 4.—CLASSIFICATION CHARTAfter Johannsen, showing the mineral composition of the granite. The nine circles represent measured

modes and are true granites. Other estimated modes of the granite type rock fall within the area enclosedby the solid heavy line.

dikelike network extends deeply into the quartz-ite in a rather intricate manner. Granite dikesare found near all the margins of the graniteand range in thickness from 2 inches to 6 and8 feet. Many extend very irregularly withunmatched borders, often pinching out or swell-ing abruptly. No flow structure was found onany of these small dikes, and owing to thehomogeneous character of the quartzite nosedimentary rock structures could be tracedacross dikes with or without offset; thus noevidence was discovered that fully and com-pletely supports either the injection or thereplacement origin of these dikes.

In attempting to classify the rock types com-posing the outcrop mapped as granite, esti-mates of the relative amounts of minerals weremade on all thin sections. Nine modes weredetermined with the integrating stage, and,

tically no quartz, some may contain very littleorthoclase, and others are essentially ortho-clase and biotite, giving a range which coversthe Johannsen classifications orthoclase sye-nite, syenite, orthoclase granite, granite, quartzmonzonite, granodiorite, quartz diorite, dio-rite, monzonite, and syenodiorite. No attempthas been made to estimate the areal extent orrelative amounts of each type, since they wereall determined on microscopic preparations.However, at least 75 per cent of the slides aretrue granites with perhaps quartz monzoniteand monzonite second in abundance. In theporphyritic types containing large amounts ofpinkish-lavender orthoclase crystals, the plagio-clase content usually increases while the quartzcontent generally decreases, placing this typegenerally in the monzonite group.



ACTINOLITE SYENITE 959

ACTINOLITE SYENITE

General Statement

The rocks classified as actinolite syenite aremineralogically and texturally similar to thoseclassified as granite but are high in actinolite,epidote, and diopside. This group was mappedas a separate field unit on the presence of thereadily recognized actinolite. The term actino-lite syenite is used because actinolite and ortho-clase are abundant in all these rocks, and quartzis usually absent or present in very smallamounts. However, some rocks contain differ-ent relative mineral proportions, and othernames such as actinolite-orthoclase syenite andactinolite granite are applicable.

The actinolite syenite is confined to areason the south and southwestern intermediatelevels of the pit (Sections 6, 7, 12, and 13) andforms irregular patches and dikelike masses inaltered limestone, quartzite, and granite. Sim-ilar rocks are also found in the Niagara Tunnelof the U. S. Mine to the south, associated withnormal biotite granite.

Megascopic Features

The overall texture of the rock is uniformand granitoid except where large patches ofbladed actinolite and massive to granular diop-side and epidote occur. The feldspar is graywith a very faint lavender tint and ranges insize from 0.5 to around 1.5 mm. The actinoliteis always dark green and is easily recognized;it may amount to as much as 30 per cent ofthe rock. Biotite can occasionally be seen, butno quartz is discernible. The actinolite, diop-side, and epidote, though disseminated through-out the rock in small crystals, may be presentalso in large (0.5-inch to 3-foot) concentratedpatches, streaks, and veinlets.


The over-all microscopic features of the sye-nite are very similar to the granite. Orthoclaseis the most prominent and persistent mineralwith actinolite, diopside, and epidote very com-mon. Plagioclase (An5) is rare. The feldsparsare roughly equidimensional, but the sizes varymarkedly, even within a single thin section.

Quartz, though more often absent than present,is found, as in the granite, interstitially between

orthoclase; a number of units extinguish togetherin the typical sieve texture.

Orthoclase of Type I is also similar to Type Ifound in the granite. It is roughly equidimensionaland often strongly perthitic and with boundaries so

FIGURE 5.—ACTINOLITE SYENITEThe turbid background of the whole illustration

is a single optical unit of orthoclase (O). Rods andirregular patches of clear quartz (Q) in left halfforms micropegmatite. Actinolite crystals are im-bedded poikilitically on right. Note the actinolitecrossing micropegmatite boundaries, left center.Traced from photomicrograph. XSO.

irregular they resemble sutures (PI. 2, fig. 3). Nearcontacts with quartzite, a structure akin to micro-pegmatite appears (Fig. 5; PI. 2, fig. 4), except thatthe proportions of orthoclase to quartz are usuallynot in the eutectic proportions; the compositionranges from 90 per cent quartz and 10 per centorthoclase to 10 per cent quartz and 90 per centorthoclase. The large pinkish-lavender orthoclasephenocrysts similar to those found in the granitehave been observed in only a very few instances.

Plagioclase is not very common, but where foundit is usually euhedral with a composition near Anos.Plagioclase cores with orthoclase rims, as describedunder the discussion of granite, are very common.

The biotite in the actinolite syenite is like thebiotite described in the granite but is less abundant.Crystals may be euhedral with ragged outline ormay poikilitically enclose quartz and orthoclase,and there may be, in rare instances, some fine-grained biotite pseudomorphs after biotite.

Actinolite is always abundant. It is very palegreen with X colorless, Y pale bluish green, Z palebluish green, a = 1.621, 0 = 1.632, y = 1.644,2V(-) = 72°, ZAC = 15°. Except for a few largecrystals, the sizes range from 0.2 to 2.0 mm. Thelarger crystals average 1-2 cm. in greatest dimensionand may be present as single crystals, unorientedclumps of crystals, or arranged in rosette pattern.The actinolite relationship to the other minerals




may be classified in four ways: (1) interstitial buteuhedral between orthoclase, plagioclase, biotite,and quartz; (2) poikilitically within crystals oforthoclase (Fig. 5); (3) subhedral crystals crossingboundaries of minerals and boundaries of the micro-pegmatitic structure (Fig. 5) without disturbingthe shape of the actinolite; and (4) irregular patchesand veinlets which cut all other minerals.

Epidote is not always present. Under the micro-scope it is yellowish, 2V(-) = 82°, XAC = 4°.Crystal sizes range from 0.1 to about 1 mm. Itoccurs in precisely the same manner as the actino-lite: (1) interstitial, (2) poikilitic within orthoclase(3) crossing crystal boundaries of orthoclase, quartz,etc., and (4) irregular patches and veins.

Colorless diopside (a 1.670, 0 1.674, y 1.694,2V(+) = 52°, ZAC = 38°) was found in severalslides. It is found principally interstitially but it isalways euhedral. Occasionally a large patch (3-6 cm.)of very fine-grained diopside was discovered, perhapsrepresenting an undigested dolomite inclusion.

In all slides examined, apatite was present asdisseminated crystals of various sizes, from ex-tremely small up to as much as 0.5 mm. in length.Small euhedral crystals of sphene were found inmost of the specimens examined, usually dissemi-nated but occasionally grouped in clumps.

Special Feature

Several small crosscutting dikes of actinolitesyenite were found in quartzite, having ex-tremely irregular edges, often pinching andswelling within 1-2 inches from a dike 3 incheswide to 0.5 inch wide. The microscope showedthat these dikes were rich in micropegmatite ofunusual proportions of quartz to orthoclase(PI. 2, fig. 4) and also rich in actinolite.

Chemical Analysis

Analysis Number 9, Table 1, was made onslightly altered actinolite syenite. SiOi andAljOs are comparatively low here, and Fe20s,MgO, and especially CaO are unusually highas compared to the granite, granite porphyry,and granitized quartzite. The K2O and Na2Ovalues are near that of the granite analyses 1and 3 and granite porphyry analyses 7 and 8.The high Fe2Os, MgO, and CaO are due to theabundance of diopside and actinolite. Sincethis rock contains very little modal plagioclase,the high CaO (11.38) content raises the ques-tion of why basic plagioclase is not present in

large amounts. This anomaly is discussed inthe section on the Origin of the ActinoliteSyenite through Granitization Processes.


All contacts of the actinolite syenite withlimestone and quartzite are sharp. Small dikesextend into the quartzite, but none was foundextending into the limestone. The trace of thecontacts themselves are quite irregular, andwall-rock prominences extending into the sye-nite are common.

The contact with the granite covers 5-10 cm.,and two large thin sections (3J4 by 4 inches)covering this contact show that there is noappreciable difference in the texture of therock, from one type to the other. The onlydistinction is that the amount of actinolite andepidote of the syenite gradually diminishes tozero, and biotite increases toward the granite.Near the center of the contact, actinolite andepidote are altered partially or completely toaggregates of biotite. This biotite is much moregreenish, in contrast to the brown color foundin the secondary biotite of the granite. Nowherealong any of the contacts was there any sem-blance of alignment of crystals.

Only estimates were made of the relativeamounts of the minerals in the actinolite sye-nite. The great majority of specimens containedbut a very small amount of quartz and only alittle plagioclase. Therefore, the term syeniteis applicable in its truest sense to most ofthese rocks. However, hi some slides quartzmade up more than 5 per cent of the rock, andthese could probably be called quartz syenite,nordmarkite, or perhaps even granite. A fewrocks lack plagioclase and quartz and couldproperly be called orthoclase syenite. Sincemost of the rocks come under the classificationof syenite, however, it seemed advisable to usethe term for the group, and, since actinolite is'also always present, it should be included inthe name.

GRANITE PORPHYRY

General Statement

The term granite porphyry includes all theporphyritic rocks found in the pit with the



GRANITE PORPHYRY 961

exception of the porphyritic granite and actin-olite syenite, which contain large pink ortho-clase crystals, and a biotite-quartz latite por-phyry. The relative amount of phenocrysts togroundmass varies rather widely. If a classifica-tion based upon whether the phenocrysts con-stituted more or less than 50 per cent of therock were employed, nearly one-third of therocks could be classified as rhyolite porphyry.Compositionally two classifications are possible.In a very large majority of the rocks, orthoclasegreatly predominates over plagioclase, result-ing in a granitic or rhyolitic type rock. In manyrocks, however, the plagioclase-orthoclase ratiois near 1, resulting in a quartz monzonite typerock.

Butler (1920, p. 352) distinguished this rocktype with a generalized term "light porphyry";this term is still in popular use.

About one-third of the total area of igneous-appearing rocks in the pit is granite porphyry.The largest uninterrupted mass is near thenorthwestern central part of the mine (Sections3, 8). A large arm extends the length of thepit toward and west where it disappears underdump material (Section 1). Some medium-sized dikes and stringers extending southwest-ward from the main mass are found in Sections6 and 7. One small dike here is isolated, andgranite surrounds it in outcrop. In Sections 8and 13 a medium-sized apophysis projects fromthe main mass toward the south. In the easternpart of the pit the main mass connects withfive large dikes and several small plugs (Sec-tions 4, 5,9,10,14,15). The two large southern-most dikes (Sections 9, 10, 14, 15) crosscutsedimentary beds at a small angle and extendto the southeast beyond the borders of the pit.The southernmost of these was not found tobe connected to the main mass in outcrop (Sec-tion 9). Near the borders of the large centralarea are innumerable small aplite dikes whichextend into all the different types of adjacentrock. Only a few of these could be shown on themap.

Megascopic Features

All specimens mapped as granite porphyryare distinctly porphyritic with orthoclase orplagioclase and occasionally biotite as pheno-crysts. In some cases pheoncrysts may make

up 80 per cent of the rock, and unless carefuloptical observation is made these types maybe confused with granite. In some instances,the phenocrysts are so small and so few thatthe rock appears uniformly dense and maysimulate some of the adjacent quartzites (PI. 2,fig. 5). As a result, thin sections were made todetermine the position of several of the con-tacts. In general, phenocrysts range from 0.5mm. to 8 mm. in greatest dimension, but in afew instances some are considerably larger. Theaverage, however, range between 1 and 5 mm.

Quartz phenocrysts are found in only a fewareas, and most of them are fairly large, glassyequidimensional crystals. Orthoclase is by farthe most abundant phenocryst mineral. It isusually white to pinkish, and in many placeshydrothermal alteration has made many ofthe crystals chalklike. In some the rims onlyare altered. Plagioclase phenocrysts are whitewhen fresh or only slightly altered, but mostare intensely altered and then take on an earthyclaylike appearance with colors near gray totan. Biotite phenocrysts are present as brownto black euhedral crystals or in ragged patchesof fine-grained hydrothermal material.

The groundmass is very dense and fine-grained, and hence the minerals are not re-solvable in hand specimen except where blackspecks, presumably biotite, are noticed in somerocks having a slightly coarser groundmass.Depending upon the stage of alteration, thegroundmass varies from very pale green towhite and, where intensely altered, to tan.

Weathering and hydrothermal alteration re-sulted in two broad general types of changesin the typical granite porphyry.

Type I. Hydrothermal alteration not con-fined to fractures or other visible channels hasproduced considerable kaolinite, illite, biotite,and sericite. These minerals weather to mont-morillonite which is yellowish brown; in thelarge central area these brownish-yellow pheno-crysts give the rock a distinctly spotty appear-ance. In hand specimen the montmorilloniteis pale green when wet and first collected on afresh bank, giving the impression that the rockhas a high copper content. When dry, however,the brownish yellow of the montmorillonite isseen. Where the rock has been intensely alteredand later weathered, the brownish-yellow color




appears in the groundmass, and the whole rockis light tan and chalklike.

Type II. As a result of intense silicificationand sericitization, which in this case followsvisible channels, hard, dense, quartzose-appear-ing material traverses the rock in streaks, andthe Type I alteration is found interstitiallybetween the streaks. Where channels are soclose together that the whole mass is altered,phenocrysts disappear, and the rock resemblesdense quartzite with a few small brown patchesof biotite.

Phenocrysts predominate over groundmassin the rocks of the large central mass. To thewestward, and in the dikes of Section 6 and 7,phenocrysts, larger than average, make up lessthan 30 per cent of the rock and the groundmassis often slightly greenish. On the eastern sideof the pit, in the northernmost dike and thetwo southernmost dikes, phenocrysts are ex-tremely large but rather scarce. In these dikeslarge quartz phenocrysts become quite promi-nent. The groundmass here is also pale green-ish. In the small plugs and stringers in Section5 the phenocrysts are small, less than 0.5 mm.in diameter, and the groundmass is very dense,white, and sometimes chalky due to the in-tense alteration to sericite.


Quartz and orthoclase, which make up at least95 per cent of the groundmass, are anhedral andequidimensional, presenting a somewhat granularappearance (PI. 2, figs. 5, 6). The two minerals aregenerally about the same size. When the pheno-crysts are few, the groundmass has an extremelyfine texture, but the groundmass grain size gradesupward to a maximum of 0.2 mm. in rocks wherephenocrysts constitute 80 per cent of the rock.Groundmass plagioclase crystals, which are ratherscarce, are usually slightly larger than the quartz-orthoclase aggregate and generally are subhedral.A very few small flakes of euhedral biotite about thesame size as the quartz and orthoclase also appearin the groundmass. Apatite, the only accessory, isfound in clumps and disseminated crystals.

Quartz phenocrysts are rather large (2-3 mm.)subhedral crystals with extremely irregular borderssuggesting resorbtion.

Orthoclase phenocrysts present in all sectionsexamined, make up 30-70 per cent of the pheno-crysts. Most of the crystals are subhedral to eu-

hedral but exhibit very ragged borders (PI. 4,fig. 1). Very commonly quartz and plagioclase in-clusions the size of similar grains in the groundmassoccur within and along the rims of the orthoclase(PI. 2, fig. 6).

Misch (1949, p. 384; Pis. 3, 5 of Pt. II) found asomewhat similar condition in the rocks of theSheku area, China. It is believed that such featuresof the phenocryst (inclusion of groundmass mineralsand irregular borders) may be best explained ineither of two ways: (1) The phenocryst could havecontinued to grow after the groundmass had largelybeen formed by feeding of late-stage liquid to thephenocryst. (2) The larger phenocryst has lesssurface energy than the smaller groundmass ortho-clase crystals, resulting in an activity gradientwhere ions making up orthoclase migrate from thesmaller crystals to the larger. This results in thegrowth of the larger crystals at the expense of thesmall groundmass orthoclase crystals. One effectof this would be ragged borders. Also, the ground-mass quartz, and plagioclase originally occupyinginterstitial positions between similar sized randomlyoriented groundmass orthoclase, would now findthemselves surrounded poikilitically by orthoclaseparticularly near the margin. The problem of whysome of the plagioclase inclusions become generallyoriented with (010) parallel to the phenocryst edge(PI. 2, fig. 6) has not been satisfactorily solved.

Plagioclase phenocrysts occur in all the porphy-ritic rocks and usually constitute 5-50 per cent ofthe phenocryst content. They are generally eu-hedral to subhedral with fairly straight borders,and their composition ranges between Anoo andAn2o. Most of the phenocrysts are so highly alteredthat the albite twinning is obscured. No plagioclasecores with orthoclase rims similar to those found ingranite and actinolite syenite are present in thegranite porphyry.

Biotite phenocrysts are not abundant but arealways present, composing 10-20 per cent of thephenocryst content. The outlines of the biotitecrystals are similar to the outlines of the biotite inthe granite and actinolite syenite. Some crystals areeuhedral or have a general euhedral outline butwith ragged borders, some contain inclusions ofgroundmass material, and some are changed tosecondary biotite (pseudomorphous after biotite).Much biotite is slightly bleached with small needlesof rutile arranged within it in the commonly ob-served trigonal manner.

In hand specimen an alignment of crystals in-dicating flow is very obscure, but thin sections of afew specimens from near some of the contactsshowed a strong flowage alignment parallel to thecontacts.



GRANITE PORPHYRY 963

Chemical Analysis

Analyses numbers 7 and 8, Table 1, weremade from the least altered granite porphyryspecimens obtainable. The SiOs and Al2Os con-

Aplites

Small dikes and stringers related to thegranite porphyry are profuse near its bordersand extend out into adjacent rock as much as

QUARTZ

OrthocloseSyenite

ORTHOCLASEGranite

QuartzDiorite

Diorite

ORTHOCLASE PLAGIOCLASE(An 05-2 5)

FIGURE 6.—CLASSIFICATION CHARTAfter Johamisen, showing the mineral composition of the granite porphyry. Of the 8 measured modes

indicated, 7 fall in the granite field and one in the quartz monzonite field. All other estimated modes ofgranite porphyry fall within the area enclosed by the solid line.

tents do not vary much from the granite analy-ses 1 and 3. The amounts of Fe2O3, MgO,CaO, and KjO are also very similar, and Na2Ois somewhat higher. From these four analyses(], 3, 7, 8)—two of granite and two of graniteporphyry—it seems evident that there is notmuch difference in the chemical composition ofthe two types.

Contacts

All observed contacts of the granite porphyrywith granite, limestone, or quartzite are "knifeedge" and quite regular when carefully traced,so that these contacts were more confidentlyinterpolated from one level to another than werethe contacts of the other rocks.

500 feet. These dikes range in width from 5 mm.to 2 feet and are found in all types of rock ofthe pit except the granite porphyry itself andbiotite-quartz latite porphyry. In hand speci-men, the aplites are slightly pinkish and ratherfine-grained. They are predominantly quartzand orthoclase with a little plagioclase and avery little biotite. All the minerals are aboutthe same size and are roughly equidimensionalwith fairly regular borders. Thin sections acrossvery small dikes in granite show extremelysharp borders with a graduation in grain sizefrom the center of the dike to the margin, thesmaller crystals being next to the margin (PI. 3,fig. 1). Many more dikes are present than areindicated on the map, and they are most abun-dant near the large central main mass of granite




porphyry. No offshoot aplite dikes from thedikes or plugs of granite porphyry on the eastside of the pit were observed.

Classification

Eight modal measurements were made withintegrating stage employing extreme caution tominimize errors due to alteration features. Inall but one case these fall within the granitefield; the exception is quartz monzonite. Anestimation of the modes of all the other speci-mens showed that in a number of instancesplagioclase increases to the extent that theterm quartz monzonite porphyry is applicable.These measurements are plotted on a Johannsendiagram, (Fig. 6), and a field is drawn whichencompasses all the estimated modal composi-tions.

BlOTTE-QuARTZ LATITE PORPHYRY

General C/taract eristics

This rock, composed of large white pheno-crysts and abundant biotite phenocrysts in apale-greenish groundmass, is distinctive in handspecimen. It occurs very sparingly in the pit.The rock forms dikes 2-30 feet wide that in-trude granite, granite porphyry, and quartzitcin the north-central part of the pit. Two dikeswere mapped in Sections 2, 3, and 8, but thelargest and most accurately outlined mass isin the northwest corner of Section 9 where theboundaries were fairly well located becausebanks in this part of the pit were moving rapidlyat the time of mapping and several return checkscould be made.

Megascopic Features

The porphyritic texture is immediately evi-dent in all these rocks. White chalklike feldsparphenocrysts averaging about 3 mm. across arealways present, together with very abundantblack to brown biotite phenocrysts. The latterare commonly in alignment parallel to thewalls of the dike. The groundmass is usuallygreenish to greenish black; its texture is veryfine but in some places is coarse enough so thatthe grains are resolvable with a lens. Largepink orthoclase crystals, as much as 5 cm. long,occur sparingly.


All the smaller euhedral feldspar phenocrystsof the biotite-quartz latite porphyry are so alteredthat their true nature has been almost obscured.Probably most of them were plagioclase since theorthoclase in the goundmass has not been ap-preciably altered. The large pink euhedral ortho-clase crystals are very slightly perthitic, and someof their borders contain inclusions of groundmass.Quartz phenocrysts with ragged edges are seenonly under the microscope and are rather small andcomparatively rare. They may be present as singleisolated crystals or in groups. Some of the groupsof quartz anhedra are elongated as though they hadbeen stretched out due to flowage. Biotite crystalsare either euhedral, subhedral, or have very raggedborders, and some are completely changed to biotitepseudomorphs. Most of the biotite shows alignmentdue to flowage (PI. 3, fig. 2), and many crystals ap-pear to have been bent.

The groundmass is a granular aggregate ofquartz and orthoclase with much biotite mixedwith it. Alteration to a greenish biotite and chloriteis always a prominent feature, and no doubt impartsthe green color. A little rutile was found as well asconsiderable apatite; both occur in small, dissemi-nated crystals.

Chemical Analysis

Analysis Number 10, Table 1, is of biotite-quartz latite porphyry. Except for a high con-tent of iron and magnesia accountable for inthe profuse biotite, there is not much differ-ence in composition from that of granite, grani-tized quartzite, and granite porphyry.


All the contacts observed are quite sharpand fairly regular, and alignment of crystalsshowing flow is quite evident (PI. 3, fig. 2).The direction of intrusive movement, however,was not determined in the field.

The abundance of euhedral biotite pheno-crysts at first suggests that the rock is closelyrelated to the lamprophyres. If the alteredfeldspar phenocrysts are all plagioclase, and theorthoclase of the groundmass is considered, therock represents a composition near monzoniteor quartz monzonite. Since groundmass alwayspredominates over phenocrysts, the term quartzlatite porphyry is probably more applicable.The profuse biotite in the groundmass as wellas in the phenocrysts justifies the prefix biotite.



LAST CHANCE QUARTZ MONZONITE 965

LAST CHANCE QUARTZ MONZONITE

The Last Chance intrusive stock is anigneous-appearing rock outcropping in a largearea west, southwest, and south of the coppermine. None of the Last Chance rocks are be-lieved to occur within the mine proper, butprevious workers have made important refer-ence to it. The Last Chance quartz monzoniteis mineralized only slightly, and no appreciabledisseminated copper mineralization has beenfound within it. Some of the early workersthought that all the igneous rocks in the B ing-ham District were of the same type and thesame age. On his map, Boutwell (1905, PI.XXXIII) used the same pattern for both theBingham rocks and the Last Chance rocks.During this study approximately 50 thin sec-tions of Last Chance specimens were observed.

Keith (1905, p. 52) classified the Last Chancerock as monzonite. The rock is medium finegranitoid with grain sizes averaging usually1-2 mm. in diameter. Average mineral composi-tion ranges as follows: quartz 5-15%, plagio-clase (An26_.,o) 30-40%, orthoclase 20-30%, bio-tite 5-10%, augite and hornblende 20-30%,and the accessories, magnetite, rutile, and apa-tite. In augite a = 1.685, ft = 1.690, 7 =1.704, 2V(+) = 60° ZAC = 47°, it is faintlypleochroic from colorless to very pale green.The hornblende showed X pale green, Y green,Z green, ZAC = 15°, 2V = 84°. The textureand mineral composition are much more uni-form in the Last Chance rock than in the Bing-ham granite; the only exception is in some speci-mens from small dikelike bodies and along theedges of the mass which are porphyritic. Plagio-clase is generally euhedral, and orthoclase andquartz interstitial. In several instances horn-blende rims augite (Fig. 7). Analysis No. 11,Table 1, shows higher CaO and lower SiO2 thanany of the Bingham rocks other than the actin-olite syenite. Evidently the CaO in the LastChance stock has gone into the making ofplagioclase as contrasted with the CaO in theactinolite syenite (Analysis 9), which wentmainly into actinolite, epidote, and diopside.

The question as to whether the Binghamrock and the Last Chance rock are the sameor not has been a matter of debate for a num-ber of years, but from the evidence the writer

believes that the Last Chance rock is distinctlydifferent from Bingham rock.

FELDSPAR NETWORK


Early in this study it was noted that in sec-tions across granite-quartzite contacts the

FIGURE 7.—LAST CHANCE QUARTZ MONZONITELarge crystal in upper left is augite with a horn-

blende rim. Twinned plagioclase, orthoclase, asmall amount of quartz (clear), and three crystalsof biotite are present. Opaque is magnetite. Tracingfrom photomicrograph. X40.

quartzite always contains orthoclase and plagio-clase (An06 to Anls) interstitially, replacing theedges of quartz grains. Not until it was notedthat thin sections of specimens taken as muchas 300 feet from the granite contact also con-tain such introduced feldspar was the wideextent and full significance of this feature re-alized. Because of a late start on this problemand the monumental task of outlining thequartzite areas containing the feldspar throughmultitudinous thin sectioning, only a beginningon the problem of its areal extent could beattempted.



966 BRONSON STRINGHAM—GRANIT1ZATION AND ALTERATION, UTAH

Although the hand specimen containing feld- to other patches, supporting the belief that allspar network looks no different from the nor- or most of the network may be interconnected,mal quartzite, the characteristics observed in Under crossed nicols most of the sections appearthin section are very unusual and striking. In to be normal quartzite; it is only when nicolsmany specimens the quartz grains interlock in are uncrossed, the light cut down, and thethe conventional manner, but between many microscope set slightly out of focus that thegrains are irregular stringers and patches of network shows up well. Where feldspar is pres-feldspar (PI. 4, fig. 2). Only occasionally does ent in large amounts, quartz anhedra may befeldspar cut quartz anhedra, and this is usually surrounded. Also, in some cases where ortho-in a pattern suggesting that the quartz was clase is abundant, quartz-feldspar boundariesonce fractured. The feldspar is otherwise inter- may become fairly straight (PL 4, fig. 2), butstitial, interweaving, and connecting one patch feldspar-feldspar boundaries are always sutured,with another. Some replacement patches of An unusual feature appears in many of theseorthoclase are up to 2 mm. across. The indi- isolated or nearly isolated quartz grains. Theirvidual feldspar anhedra are usually quite small shape is euhedral to subhedral with rhombicwith sutured borders, but occasionally a single shapes predominating and occasionally otheroptical unit of feldspar may surround several short edges appearing (PL 4, fig. 2). Bayleyquartz grains. Stringers may extend from these (1893, p. 85-96) found a similar condition inpatches through the interstices of the quartzite the mottled quartzites near the Pigeon Point

PLATE 2.—PHOTOMICROGRAPHS

FIGURE 1.—FINE-GRAINED GRANITE, X NICOLSLeast altered granite observed. Light areas mainly quartz, and darker minerals largely orthoclase. Note a

typical patch of sieve quartz (isolated anhedra with unit extinction) within outlined area.FIGURE 2.—PLAGIOCLASE WITH ORTHOCLASE RIM IN GRANITE, X NICOLS

FIGURE 3.—ACTINOLITE SYENITE, X NICOLSThe predominant mineral is perthilic orthoclase (darker gray) with suturelike boundaries. Very few

small grains of quartz (white) present.FIGURE 4.—ACTINOLITE SYENITE FROM NEAR QUARTZITE CONTACT, X NICOLS

Micropegmatitic structure well developed with quartz (light gray) greatly predominating over ortho-clase (dark gray)

FIGURE 5.—GRANITE PORPHYRY (RHYOLITE PORPHYRY) SHOWING VERY FINE TEXTURE, X NICOLSPhenocrysts are altered feldspar in a groundmass of quartz and orthoclase.

FIGURE 6.—TYPICAL GRANITE PORPHYRY, X NICOLSQuartz, orthoclase, plagioclase (Ani»_i5), and little biotite compose the groundmass. Phenocrysts are

orthoclase. Note the oriented inclusions of plagioclase and the small islands of quartz in the large orthoclasephenocryst (lower center).

PLATE 3.—PHOTOMICROGRAPHSFIGURE 1.—CONTACT OF APLITE DIKE (UPPER) AND GRANITE (LOWER), UNCROSSED NICOLS

Orthoclase is turbid with low relief. Note sharp contact and gradation in grain size of quartz (clear andhigher relief) toward the contact. Opaque is sulfide.

FIGURE 2.—BIOTITE-QUARTZ LATITE PORPHYRY, X NICOLSAligned biotite phenocrysts are in a groundmass of quartz, orthoclase, and plagioclase.

FIGURE 3.—PORTION OF FELDSPAR CRYSTAL ALTERED ENTIRELY TO KAOLINITE AND ILI.ITF.(ALTERATION STAGE I), X NICOLS

Dark-gray to black areas kaolinite. Lighter-gray to white flakes illite, roughly oriented in grid pattern,FIGURE 4.—PLAGIOCLASE CRYSTALS IN GRANITE PORPHYRY PARTIALLY ALTERED TO FLAKY

SERICITE (WHITE), X NICOLSNote grid arrangement of sericite flakes with fresh feldspar between.

FIGURE 5.—VEINLET OF HYDROTHERMAL BIOTITE (B) (ALTERATION STAGE II)Traversing a felted mass of kaolinite and illite,(alteration stage I), X nicols.

FIGURE 6.—LARGE ORTHOCLASE CRYSTAL (O), IN GRANITE (Gr)Cut by a Stage IV alteration band (IV) composed of quartz and sericite, X nicols.



BULL. GEOL. SOC. AM., VOL. 64 STRINGHAM, PL. 2

--------- -- -- --------- - --

PI IOTOM ICROGRAPHS



BULL. OEOL. SOC. AM., VOL. 64 STRINGHAM, PL. 3

--------- - -- --------- - --

PHOTOMICROGRAPHS




--------- - -- ----------------- - --

PlIOTOMICnOGRAPIIS: GHANITIZliD QUAHTZITE AND ALTERED GRANITE




Al.TliKKl) (1UANITIC: GRANITIC AND (JUANI'l 'IZlin QUAKT/.n'E CONTACT



FELDSPAR NETWORK 967

Sill, Minnesota. Here, however, some of the ticular crystallographic position; after a timeinterstitial material was micropegmatitic. This the fragment, if extracted from the solvent,crystal orientation based on outlines could exhibits crystal faces (Buckley, 1951, p. 304).nearly always be checked by noting the extinc- Where feldspar replaces along the irregulartion position in relation to the above faces, and edges of quartz grains of the quartzite, theoften the c axis of the quartz could be predicted crystal structure of the quartz grain controls tobefore confirming with the gypsum plate or some extent where replacement will take place,quartz wedge. Irregular fragments of some and irregularities and prominences are prefer-crystallized substances, when allowed to dis- entially replaced until a fairly flat plane parallelsolve in certain solvents, are dissolved in par- to the rhombohedron or some other crystal face

PLATE 4.—PHOTOMICROGRAPHS: GRANITIZED QUARTZITE AND ALTERED GRANITEFIGURE 1.—EDGE OF ORTHOCLASE PHENOCRYST (Or) IN GRANITE PORPHYRY, X NICOLS

Groundmass (upper part) composed of granular orthoclase and quartz. Note extremely irregular outlineof edge of phenocryst and quartz inclusions near rim entirely surrounded by orthoclase phenocryst.

FIGURE 2.—FELDSPAR NETWORK IN QUARTZITE, UNCROSSED NICOLSLight-gray high-relief mineral is quartz, and turbid darker-gray mineral is orthoclase. Lower part is

quartzite only slightly replaced, but grading upward orthoclase becomes more abundant until some quartzgrains are isolated. Orthoclase replaces principally along quartzite grain boundaries. Several replaced quartzboundaries show straight borders some of which prove to be parallel to the more common quartz crystalforms.

FIGURE 3.—PHOTOGRAPH OF POLISHED SPECIMEN OF GRANITIZED QUARTZITEShows the very fine texture with the light and dark patches. Veinlets of clear quartz.

FIGURE 4.—GRANITIZED QUARTZITE, UNCROSSED NICOLSSmall, white, high-relief minerals are quartz. Turbid (grayish) low-relief areas are fine aggregates of

feldspar (orthoclase + plagioclase (An5_i0). Dark minerals are biotite. Near center is a trapezoidal area offeldspar aggregate, probably a former quartzite breccia fragment.

FIGURE 5.—GRANITIZED QUARTZITE, UNCROSSED NICOLSQuartz grains (Q) imbedded in fine aggregate of feldspar (turbid). Area containing high concentration of

quartz is believed to represent an incompletely replaced fragment of quartzite.FIGURE 6.—GRANITE MODERATELY ALTERED BY HYDROTHERMAL STAGE II (BIOTITE-SERICITE)

To extreme right of specimen may be seen half of a Stage IV alteration band (quartz-sericite) with a darkstreak of hydrothermal biotite between it and the main mass of the rock.

PLATE 5.—ALTERED GRANITE: GRANITE AND GRANITIZED QUARTZITE CONTACTFIGURE 1.—CHLORITE ORBICULE

Showing onionlike structure, developed in granite during alteration Stage III (biotite-chlorite).FIGURE 2.—POLISHED SPECIMEN OF GRANITE WITH MEDIUM-GRAINED TEXTUREDarker spots are biotite, lighter areas are either orthoclase, quartz, or plagioclase.

FIGURE 3.—PHOTOMICROGRAPH OF CONTACT BETWEEN QUARTZITE (EXTREME UPPER) AND GRANITIZEDQUARTZITE (LOWER), X NICOLS

Lower left is oblong aggregate of relatively pure feldspar. Unreplaced quartzite fragments (Q) appear inthe granitized quartzite area.

FIGURE 4.—LARGE THIN SECTION ACROSS CONTACT OF FINE-GRAINED GRANITIZED QUARTZITE(LOWER) AND GRANITE (UPPER), X POLAROIDS

Note that outlined contact is sharp and very irregular.FIGURE 5.—ALTERED GRANITE

Main portion of rock (dark) is granite altered by Stage II (hydrothermal biotite-sericite). Lighter hori-zontal streak is a Stage IV (quartz-sericite) band with chalcopyrite and pyrite in the center fracture. Verti-cal veins are vuggy clear quartz of Stage V, with no adjacent alteration, cutting through the Stage IV band.Small sulphide crystals are present in the vugs as well as disseminated throughout the rock.

FIGURE 6.—PHOTOMICROGRAPH OF GRANITE-QUARTZITE CONTACT, X NICOLSContact extends diagonally from upper right to lower center. Quartzite with feldspar network (does not

show under X nicols) right. No flow alignment of minerals found in the granite.




has been developed. This face will then bereplaced uniformly and the crystal eventuallydisappears1.

Much of the feldspar of the network is maskedby alteration minerals. In almost all cases onecan see a few flakes of fine biotite or sericiteas alteration products in the feldspar. Quiteoften it may be almost completely changed to asolid mat of fine biotite or fine sericite, andvery careful observation is necessary to detectthe orthoclase that remains. Many quartziteslides, where feldspar network has not beenproved, exhibit large interstitial uninterruptedpatches of the alteration minerals, but fromtheir sizes and outline it is believed that feldsparnetwork was once present and its original out-line represents the present outline of these al-teration minerals.

Classification and Distribution

The appearance of the interstitial feldspar inthe quartzite gives the over-all impression of anetwork, and that term is used here.

The determination of the areal extent of therock containing feldspar network is based onrandom locations of the thin sections of quartz-ite examined and thus is of limited reliability.The feldspar network is always found in quartz-ite and impure quratzite near the granite con-tact, including all the quartzite and impurequartzite inclusions in the granite. It was alsonoted in several rocks up to 300 feet from thecontact but is more common in specimens lo-cated nearer the contact. A few sections ofquartzite adjacent to the granite porphyryshow no feldspar network, but sampling here isinadequate to conclude that it is absent. Be-cause it cannot be seen in the field, it is notknown whether this feature is confined to cross-cutting streaks, certain beds, or is distributeduniformly. The dashed line on the map repre-senting the outer margin implies the probableextent of the orthoclase network. Much furtherwork is necessary to establish the true natureof its distribution. It is surely present, however,and must be reckoned with in any hypothesisregarding the origin of the rocks at Bingham.

1 Since this paper was written a study of thesecrystal faces which have developed through re-placement has been undertaken, and the report,now in press, will be published in the AmericanMineralogist.

Chemical Analysis

Analysis number 12, Table 1, of the quartzitewith feldspar network, as expected, shows ex-tremely high SiC>2 content. The iron and mag-nesia is contained in a little biotite which ispresent as an alteration product. The high K2Oindicates that in this specimen orthoclase pre-dominates over the plagioclase.

Origin

The sedimentary nature of the originalquartzite and the evident replacement rela-tionship between the quartz and feldspar indi-cate that feldspar and small amounts of biotitehave been introduced into the quartzite. Thepossibility was considered that arkosic feldspargrains may be the source of the network feldsparand that it has simply been recrystallized andredistributed. All the true arkoses observed inthin section were definitely low in feldspar con-tent as compared with the feldspar content ofmuch of the feldspar network type rock. Ofcourse if arkosic orthoclase grains were present,they probably were recrystallized and redis-tributed when feldspar was introduced, sinceno clastic grains were found in this rock. Theamount of feldspar in much of the networkexceeds in amount the orthoclase grains foundin the most arkosic quartzites. Further, noplagioclase grains were seen in the arkosictypes, and plagioclase is abundant in the net-work. These two features strongly indicate thatmost of the feldspar of the network has beenintroduced.

GRANITIZED QUARTZITE

General Statement

Owing to a circumstance in mapping, the(rue nature of the peculiar rock classified hereas "granitized quartzite" was not fully realizeduntil late in the study. Its origin has now beenwell established, and this may offer a clue tothe origin of the granite and actinolite syeniteat Bingham. Earlier workers in the district didnot notice this granitized quartzite for appar-ently two reasons: the writer believes that eitherthere was only a very small outcrop of it atthese earlier dates and it was therefore easilymissed, or, more likely, it was not exposed at all



GRANITIZED QUARTZITE 969

but has been uncovered by recent excavation.To the writer's knowledge, no identical rockhas previously been described. Details of therock vary markedly in different specimens,and the mine was revisited many times to tryto secure every type.

The granitized quartzite was found near thelower intermediate levels on the southwestside of the pit, specifically in the southern partsof Sections 7 and 8. This area is directly northof some large quartzite bodies which extendinto the granite on higher levels (Sections 12and 13). Contacts mapped are quite accuratelylocated, but interpolation between levels isa matter of judgment, and the true shape ofthe exposure may not be exactly as drawn.Subsequent location checks, as much as 2 yearslater, reveal that contacts now (1952) are insomewhat different positions (due to excava-tion), but the granitized quartzite rock isabundant in this same general area.

Megascopic Features

All the rock in the granitized quartzite areahas been so fractured without recementationthat rarely can a solid specimen be secured largeenough to make a large (3H by 4 inches)thin section. To observe the true nature of therock in the field it must be painstakinglychipped along edges to eliminate the maskingeffect of innumerable quartz veinlets and ex-pose a fresh surface of the rock. The over-allcolor is a gray, with white chalklike spots andpatches, and brownish spots (PI. 4, fig. 3).Intensive hydrothermal alteration resulting inabundant biotite has produced a few ratherdark rocks. No minerals can be determinedmegascopically, but in places, particularly wherebrown spots (presumably biotite) are abundant,the grain size appears slightly coarser. The rockcan be best observed on a polished specimenwhere irregularities in grain size are readilyapparent. Streaks and patches of light finematerial are seen adjacent to streaks andpatches of darker material. These patches mayrange in size from 0.1 mm. to 1-2 cm., and theborders of most of them appear to be fairlyeven and straight, giving a superficial appear-ance of a replaced or strongly recementedbreccia ted rock. However, the impression isgained that the rock fragments were not fully

rotated and separated, but that the fragmenta-tion was more the result of intensive fracturingwithout much movement.

Unreplaced quartzite fragments or skialiths(Goodspeed, 1948a, p. 520) which range fromapproximately 1 mm. to several yards in di-ameter, are very common. Superficially theboundaries of these fragments appear quitesharp, but on the polished specimen, wherequartz takes a better polish than the otherminerals, it is seen that most of these contactsare gradational. Distributed through the gran-itized quartzite are many small late quartz vein-lets as well as some very small (2 mm.-2 cm.across) discontinuous more coarsely crystalline,irregularly shaped patches and veinlets whichhave distinct gradational borders with the mainrock. The latter are not to be confused withthe thicker aplitic dikes which have very sharpborders. Frequently irregular streaks and bands,2 mm.-3 cm. wide, having a chalcedonic ap-pearance, occur in most of the granitized quartz-ite rocks examined. Thin-section observationshowed that these are due to a late hydro-thermal sericitic alteration which spreads moreor less symmetrically on either side of smallfractures.


MINERALS: Quartz grains ranging from .01 to0.2 ram. in diameter were found in all slides ex-amined. They are euhedral to subhedral and inmany cases exhibit at least two straight and pre-sumably rhombohedral edges and often what ap-pears to be a prism edge. High-power magnificationrevealed that many of the smallest quartz grainshave some semblance of crystal outline.

Where orthoclase is adjacent to orthoclase, theborders are always very irregular, but, where itborders quartz, boundaries are fairly straight. Theorthoclase grains are slightly larger than the quartzgrains, ranging from approximately .02 to 0.4 mm.in greatest dimension. Most of the orthoclase ob-served was un twinned and only slightly perthitic.

Plagioclase (Ano6-Ani2), with the conventionalalbite twinning (010) and an index near orthoclaseor slightly higher, is distinguished from orthoclaseprincipally by the presence of repeated twinning.Most of the plagioclase crystals are slightly largerthan the orthoclase anhedra and are more or lesseuhedral against orthoclase but anhedral againstquartz.

There are two types of biotite. One type, be-lieved to be hydrothermal, consists of very fine




flakes scattered somewhat evenly or occasionallyconcentrated in small clumps within the feldsparareas. The most abundant type, however, is muchlarger (up to 0.1 mm.) and is believed to have beenformed by the same processes that formed the ortho-clase and plagioclase. This latter biotite is generallyquite ragged in outline. In some cases several raggedand individually isolated crystals have unit extinc-tion; in other cases many crystals may be groupedtogether with random orientation.

A very small amount of rutile is scattered throughthe rock, but the crystals are so fine they are de-tectable only with the highest magnifications. Somebiotite, which shows slight bleaching, may containconsiderable rutile. Apatite, found throughout thegranitized quartzite, forms crystals less than .02mm. in length. These crystals may at times beconcentrated in areas associated with biotite.

The very fine and varied texture usually foundin these rocks made it extremely difficult and im-practical to make quantitative mineral measure-ments Since it was estimated that there is a con-siderable range in relative amounts of minerals inall the slides, it was reasoned that perhaps a meas-ured mode would not give a true picture of themineral composition of the rock. Estimations indi-cate the following: quartz 10-20%, orthoclase 30-60%, plagioclase 20-35%, biotite 5-50%, andrutile and apatite combined comprise no more than1 per cent of the rock.

TEXTURES AND STRUCTURES: The grain sizes ofquartz and feldspar vary from .01 to 0.4 mm. Thetexture at first suggests an evenly distributed ag-gregate. Various aggregate sizes may be presentwithin 1.5 mm. of each other. Quartz grains areusually detached equidimensional crystals dis-tributed in various concentrations in the feldspargroundmass. Though most of the slides showgradual changes in grain size, many unusual patternsand odd shapes outlined by abrupt grain-sizechanges are discovered. For instance, quartz, oc-curring as isolated crystals, may be present in anunusually high concentration in areas covering asmall angular patch 0.5 mm. in diameter or perhapslarger (PI. 4, fig. 5). These areas have fairly straightor regularly curved edges and may be roughly tri-angular, oblong, or completely irregular. They areconsidered to represent quartzite fragments partial!}'replaced by feldspar. Quartz, still as detached crys-tals, is found in straight or regularly curved stringerswhich often widen into larger angular patches(PI. 4, fig. 4).

Orthoclase and plagioclase are always interstitialbetween quartz grains. In no area is plagioclase ororthoclase particularly dominant over the other.Very often an area composed of an aggregate ofonly feldspar will be bounded by fairly straight or

broadly curved sides whose outlines may also re-semble triangles, oblongs, or trapezoids (PI. 4,fig. 4). These are thought to be quartzite fragmentscompletely replaced by feldspar. Most of the hydro-thermal biotite occurs in these relatively purefeldspar areas. In addition to these breccialikefeatures, large areas are found in which quartzand felspar are uniformly distributed. In two slides,quartz and orthoclase were intergrown in a micro-pegmatitic structure, but the relative amounts ofquartz and feldspar are not in the eutectic pro-portions.

Recognizable quartzite fragments are common,irregularly distributed throughout the rock (PI. 5,fig. 3), and show the feldspar network. Some of theslides indicate that the quartzite apparently wasso crushed that fractures traverse individual quartzgrains and replacing feldspar has penetrated thesecracks.

Three types of "veinlets" traverse the granitizedquartzite, in addition to the aplites related to thegranite porphyry: (1) discontinuous patches andstringers of slightly coarser orthoclase, plagioclase,and quartz, having gradational borders; (2) mega-scopally visible veinlets composed of a chalcedonic-looking material which under the microscope is seento differ from the bulk of the rock only in havingslightly more hydrothermal sericite; (3) definitesharp-bordered very small veinlets of colorlessquartz.

Contacts

There is a very evident gradational contactbetween quartzite with feldspar network andgranitized quartzite. This gradation is seen inmany of the granitized quartzite slides con-taining recognizable quartzite fragments (PI. 5,fig. 3). Around the borders of these fragments,which always contain feldspar network, thequartz and feldspar proportions vary with thedistance from the contact. Near the quartzite,quartz predominates, while farther from itfeldspar becomes more prominent until theusual proportions of quartz to feldspar ingranitized quartzite is reached.

Except for the gradational contact withquartzite and the sharp contact with apliticdikes, granitized quartzite is found in contactonly with the granite. In the field this contactappears sharp but irregular, and thin sectionsacross it show that it is very sharp and veryirregular, the small grains of the granitizedquartzite abutting against the larger grains ofgranitized quartzite (PL 5, fig. 4). In all the



GKANITIZED QUARTZITE 971

slides studied, no essential mineralogical differ-ence was recognized between the granite andgranitized quartzite. Grain size, therefore, isthe principal difference between the two, withthe granite being the coarser.

Chemical Analysis

Two samples of granitized quartzite wereanalyzed (Table 1, Nos. 5, 6). Number 5 wastaken 1 cm. from the contact with the graniteshown in the large thin-section illustration(PI. 5, fig. 4). The granite specimen, analysis 4,was taken 1 cm. from the same contact. Hence,analyses 4 and 5 represent rock about 2 cm.apart, with a contact between. The range ofcomposition between the two is very slight.Analysis 6 is of a specimen taken some distancewithin the granitized quartzite area; exceptfor a slightly lower silica content there is not agreat deal of difference in the compositions ofnumbers 5 and 6. Thus the composition of thegranitized quartzite seemingly is near that ofthe high-plagioclase granite (analyses 2 and 4).

Origin and Classification

The original rock from which the granitizedquartzite was derived is believed to be eitherquartzite or impure quartzite. Presumably thisrock was intensely fractured, but little rotationof fragments or slipping along fractures occur-red. The original rock probably contained muchSiO2 and perhaps a little Fe203, A12O3, CaO,K2O, Na2O, and MgO. From the abundance ofthe minerals of the granitized quartzite theintroduced substances must have included muchA12O3, K2O, and Na2O with perhaps a littleFeO, MgO, and CaO, and a very little P2OB

and Ti02. Of the constituents now in the rock,quartz may be considered residual as well aspossibly a small amount of orthoclase of thearkosic quartzite and a very little clay, calcitedolomite, and iron. The new minerals are ortho-clase, plagioclase, biotite, rutile, and apatite.

The substances which formed these mineralsare believed to have been introduced along theinnumerable small fractures and grain bound-aries. The fragments were to a certain degreeselectively replaced; some were completelychanged to feldspar, while others were changedonly partially to feldspar and considerablequartz remained. The reason for this selectivity

is not known. Much of the rock, where originalfracturing was probably not intense, shows themore or less uniform texture of much of thegranitized quartzite.

During the later stages of the replacementprocess, solutions may have become less con-centrated in introduced substances causing min-erals in localized portions of granitized quartz-ite to grow larger, resulting in the patches andstringers of coarser minerals.

The term microgranite adequately describesthe mineral composition and fine texture ofthis rock but does not convey the replacementorigin or peculiar structure. Since the rock isbelieved to have been made by a replacementprocess the name granitized quartzite is con-sidered suitable.

ORIGIN OF THE IGNEOUS-APPEARING ROCKS

General Statement

If a conventional intrusive origin for all theigneous-appearing rocks in the Bingham pit—granite, actinolite syenite, granite porphyrywith related aplite, and biotite-quartz latiteporphyry—were the only hypothesis to be con-sidered, the origin of the feldspar network andgranitized quartzite could perhaps be explainedas magmatic "fringe effects." Many unusualfeatures of the granite and actinolite syenite,however, need further explanation. Hence, twoideas, origin by igneous activity and by gran-itization or replacement, are offered here withcriteria for each, and the reader may judge thevalidity of either hypothesis. The writer at-tempts to present all the arguments on bothsides. As the criteria as now set up by propo-nents of each process do not apply completely,for either case, to the rocks at Bingham, thismethod of analysis may have merit.

Origin by Igneous Action

Validity of evidence.—Evidences for the se-quence of intrusion of the igneous rocks atBingham, excluding the Last Chance stock, arequite clear except for the granite-actinolitesyenite relationship. If the Last Chance rocksare included, there have been either four or fiveperiods of igneous activity, depending upon theorigin thought most probable for the actinolitesyenite.

Last Chance quartz monzonite.—The position




of the Last Chance rock in the sequence ofintrusion has not been definitely established,and this rock does not crop out in the maparea. For the purposes of clarity and complete-ness, its features were discussed previously.Early authors have related it directly to theBingham rocks. The writer concludes that theLast Chance stock was intruded independently.It is further indicated that the Last Chancemagma, of intermediate composition, may havebeen implaced by sloping and assimilation.Many dikes and offshoots are observed aroundthe periphery of the main intrusive, and in anumber of slides taken from these smallermasses a definite alignment of crystals is seen,indicating flowage. The boundary relationshipsbetween the minerals in this quartz monzoniteindicate that the sequence of crystallizationfollows essentially the conventional Rosenbuschsequence in that hornblende and augite arenearly always euhedral as is also plagiodase,but the boundary of the latter yields to theboundaries of the dark minerals, and orthoclaseand quartz are always interstitial. The factthat hornblende often rims augite may beregarded by some students to mean that mag-matic processes have more than likely beenoperative. The over-all constant uniformity oftexture and composition of the quartz monzo-nite mass and the abundance of a plagioclaseof intermediate composition (An25_4o), account-ing for the higher Na20 and CaO in the analysis,further strongly suggests that this rock ismagmatic. The petrographic and chemical cri-teria, as interpreted according to the classicalpetrology, together with the structural relation-ships, are fairly convincing evidence that theLast Chance quartz monzonite is magmatic.

Actinolite syenite.—The position of the actino-lite syenite in the intrusive sequence could notbe positively established, but contacts on Plate1 suggest that dikes of actinolite syenite seemto be cut ofi by the granite. Structural evidenceon the relative ages of the Last Chance rock andthe actinolite syenite is not forthcoming sincethe two have not been found together. However,petrographic features relate the actinolite sye-nite more closely to the Bingham granite thanto the Last Chance quartz monzonite. In thisrespect, the high content of orthoclase withsutured boundaries, the low amount of albite-oligoclase, and the sieve texture of the quartz

are all similar to the Bingham granite and quitedissimilar to the Last Chance rock.

There are several possible modes of originfor the actinolite syenite all of which are basedupon a strict magmatic interpretation.

(1) The actinolite syenite may have beenintruded early and independently as plugs anddikes in areas within or adjacent to a largebed of dolomitic limestone, the dark mineralsactinolite, diopside, and epidote being originalorthotectic material.

(2) The actinolite syenite may originallyhave been a part of a large granite magma.Contamination as a result of assimilation of thedolomitic limestone carrying abundant MgOand CaO could result in a silica-poor granitic-type rock containing enough calcium and mag-nesium to make actinolite, diopside, and epi-dote. Serious objection to this idea, however,is that the plagioclase in the actinolite syeniteis near albite and not high in An, and, further,it is not at all abundant.

(3) The actinolite syenite may have beenoriginally independently emplaced as a basicgabbroic magma with a chemical compositionnear the present rock. A gabbroic-type rockmay originally have crystallized, but subsequentcontact-metamorphic action has recrystallizedthe rock to actinolite syenite. The writer con-siders this possibility quite remote.

Metamorphism of the near-by dolomitic lime-stone may have been accomplished either atthe time of the intrusion of the actinolitesyenite or at the time of the granite intrusion.

Granite.—The next magmatic stage was theintrusion of the large body of granite which wasemplaced in the main by stoping and assimila-tion of country rock. Since the sedimentarybeds are disturbed but very little along themargins of the granite, the intrusion was prob-ably not injected very forcefully. However, asexpected along the periphery of a large bodyof magma, many dikes and small bodies havebeen intruded into the country rock. Uponexamination of the representative dike patternson the map, one could well imagine that, sincemany of these dikes interconnect, the stopingprocess is well illustrated. Inclusions of allsedimentary rock types are common in thegranite, and these could be thought of as de-tached fragments which "floated" downwardinto the magma from the roof of the magma



ORIGIN OF IGNEOUS-APPEARING ROCKS 973

"chamber." The crystallization history of thegranite could be interpreted to be that of atypical magmatic rock. Biotite is the first andprincipal dark mineral to develop. The plagio-clase crystallized early and was later rimmed byorthoclase. Orthoclase was formed still laterwith mutual crystal boundaries which are ex-tremely sutured, and quartz crystallized last,occupying interstitial positions. Thus the rockfollows the ideal conventional sequence forcrystallization from a magma. The wide varia-tions in texture and composition found through-out the granite mass may be explained by localcontamination, segregation, or a lack of thor-ough mixing of the chemical constituents, whilethe magma was still liquid.

On magmatic reasoning, the feldspar net-work and granitized quartzite may be explainedas a type of fringe effect around a magmaticbody where magmatic material to form feldsparand biotite seeps into the country rock andreplaces it to varying degrees. The contactmetamorphism noted around the edges of thegranite could have been the result of the gran-itic intrusion. The megascopically very sharpborders of the granite with country rock sug-gest strongly that this was once a solid-liquidcontact. Most important of all, the absence ofcontinuous relic country rock structure—inthis case bedding—-through the magmatic rockfurther substantiates the intrusive idea. Theseare felt to be very strong evidences in favor ofthe magmatic theory.

Granite porphyry.—Following the emplace-ment of the granite, the large mass of graniteporphyry was intruded. This porphyry wasprobably emplaced chiefly by stoping and assim-ilating the surrounding granitic and sedimentaryrocks. Along the sharp contacts of the graniteporphyry are found flow structures which, to-gether with a crosscutting relationship of largegranite porphyry dikes, seem to prove that thegranite porphyry is distinctly the result ofmagmatic activity. There are more phenocrystsnear the center of the outcrop than toward theedges, which should agree with the ideal crystaldistribution of a cooling magma. Since smallaplite dikes are found in all rocks except graniteporphyry and the later biotite-quartz latiteporphyry, the aplite dikes are probably off-shoots from the granite porphyry.

Biotite-quartz latite porphyry.—Following the

emplacement of the granite porphyry there wasa recurrence of magmatic activity resulting inthe intrusion of small dikes and irregular pipesof slightly more basic material. Many of thephenocrysts in this porphyry are aligned, show-ing flow structure, and undoubtedly this rockis truly magmatic.

Origin by Granitization

Actinolite syenite and granite.—The followingdiscussion is more or less confined to featureswhich favor origin by granitization for the gran-ite and the actinolite syenite. The relationshipof the Last Chance rock to the granitizationprocess has not yet been fully established, andhence discussion of it is omitted.

It is generally considered that one of the bestcriteria for the granitization process is an ob-vious gradation between true igneous-appear-ing rock and the surrounding unchanged coun-try rock. If this concept is true it is then one ofthe weakest points favoring granitization of thegranite and actinolite syenite at Bingham sinceborders with sedimentary rocks here appear tobe sharp. The feldspar network in the quartzite,though having a sharp juncture with the granite,may be regarded as distinctly a gradationalfeature between ordinary quartzite and granite.Furthermore, it is a question whether a grada-tional contact is absolutely necessary for a re-placement boundary since many replacementore bodies exhibit very sharp borders. Also,along the granite contact no flow arrangementof crystals or inclusions has been recognized,either in the field or through a petrofabricanalysis of thin sections of granite from areasnear contacts.

Perhaps the most convincing criteria favor-ing granitization is the continuation of reliccountry rock structures such as bedding, schis-tosity, etc., from wall rock into or through theigneous-appearing rock. Any such conclusiveevidence is lacking in the Bingham granite, butthe following could satisfactorily explain itsabsence. Except for one fairly thick bed of dolo-mitic limestone, the country rock is largelyquartzite with only a few thin limestone lenses.Therefore, sedimentary structures which couldbe preserved as relics in the granite are practi-cally absent except for the quartzite bedding.If some strong shale beds were present, the




chances for their relic preservation would befairly good. Since only a few limestone beds arepresent in the quartzite, possibly this materialcould be either entirely moved out into thecountry rock as a sort of feeble basic frontwhere it would be so thoroughly distributedthat it would be difficult to detect, or it mayhave been incorporated and so thoroughlydiffused in the replacing material that its iden-tity was lost. Backlund (1946, p. Ill) listsrocks in order of their susceptibility to the gran-itization processes. This is based principallyon the chemical dissimilarity of the replacedrocks to the granitizing material. He lists arkoseas first and easiest to transform, followed byshale or hornfels, quartzite, limestone, and last,basic rocks. The northwest corner of Section 9illustrates this, in that quartzite beds have beenpreferentially changed to granite while alteredlimestone beds project into the granite for ap-proximately 100 to 200 feet. In some drillingoperations an altered limestone was occasion-ally encountered having granite both directlyabove and below and its position when pro-jected is consistent with known limestone bedson the surface. This suggests strongly thatBacklund's sequence of the susceptibility togranitization of different standard rock types isperhaps operating here and further that thealtered limestone extensions occur either asconnected unsupported thin beds or isolatedundisturbed remnants, and are still in place, notblocks floundering in a magma. Since quartzitebedding is very obscure near the granite, aneffort was made to correlate and compare petro-fabric information taken from undisturbedquartzite country rock and from isolated quartz-ite inclusions to see if a common orientationwas forthcoming. No convincing preferredquartz diagrams were obtained. The large bedof limestone and adjacent quartzite masses inSections 12 and 13 are surrounded by granite,and yet their position seems to be fairly con-formable to the regional attitude of the countryrock. This bed and accompanying group ofinclusions could, however, be interpreted asthe lower portion of an attached pendant ascould also the apparently undisturbed isolatedblocks in the lower part of Section 9.

Relationships of wall-rock quartzite to thesmaller dikes of granite showed no structural

proof of replacement or dilation origin (Good-speed, 1940, p. 175), but their extreme irregu-larity, rapid pinching and swelling, and theunmatched wall configuration strongly suggest areplacement origin.

The space problem could be solved betterfor the granite at Bingham by a replacementprocess rather than by magma tic intrusion. Thestructure of the sedimentary beds surroundingthe granite show a remarkably uniform con-tinuity except for one small area in the lowerright center of Section 3, the disturbance ofwhich is believed to be due to pre-granite fault-ing. No crushing or brecciation of country rockis evident near the granite, and there seems tohave been no major doming or pushing asideof country rock to make room for the magma.Sloping, melting, and other methods of assimi-lation then are the principal processes to ex-plain the magmatic emplacement.

The concept of a basic front, wherein partsof the material composing the country rock inthe area now occupied by the granitized masshave been moved to the adjacent country rock,was first applied (Reynolds, 1947, p. 205) wherethe removed constituents were somewhat basic.A true basic front is absent at Bingham, butthe replaced rocks are quartzite. A silicic frontwould then be expected to develop, and atBingham this type of front is thought to beexpressed in the silication of the limestonebeds and near the granite the introduction ofsilica into quartzite which has transformed theminto rather glassy massive rocks. The true outerlimits of this silicic front are difficult to detectin the quartzite, but silication of the limestoneproceeds with lesser intensity away from thegranite borders.

The extreme suture of the orthoclase bound-aries are not what might be expected in a rockwhich has crystallized from a melt by growthfrom nuclei, as theoretically in such a casestraight boundaries should result (Dan McLach-lan, Jr., Personal communication). The suturedtexture, then, would appear to be more crystal-loblastic than magmatic. Also the extreme vari-ation of mineral distribution even within onesection suggests further that the mineral con-stituents were not uniformly distributedthroughout the mass. Some may regard as goodgranitization evidence the fact that the varia-



ORIGIN OF IGNEOUS-APPEARING ROCKS 975

tion in grain sizes is in no way related toproximity to the borders of the granite. Theratios of K to Na, which vary markedly indifferent parts of the Bingham granite, furthersuggest a nonuniform distribution of the al-kalis.

A reasonable explanation of the features ofthe actinolite syenite is probably one of thebest arguments for a replacement origin. Thelarger outcrops of this rock are close to thelarge bed of dolomitic limestone in Sections 12and 13, while the smaller bodies in Section 7are within or adjacent to quartzite and granitein a region approximately where this limestoneprojects. All the various textures and mineralassemblages in the actinolite syenite, except theactinolite, epidote, and diopside, are especiallysimilar to the granite. Thus the actinolitesyenite could have formed as the result of re-placement by the same materials which formedthe granite, except that they were contaminatedwith the CaO and MgO of the dolomitic lime-stone they were replacing. The capture of SiOsoriginally in the granitizing materials or thereplaced rock to make the abundant actinolite,epidote, and diopside, all of which contain highMgO and CaO, might explain the low quartzcontent of the syenite. Actinolite, epidote, anddiopside, associated with orthoclase and a smallamount of acidic plagioclase, is a low-tempera-ture mineral assemblage and definitely a pecu-liar one for an igneous rock. If the rock ismagmatic, one wonders why a gabbro-typemineral assemblage did not crystallize, ratherthan a syenitic type, and why the high CaOwas not accounted for in basic plagioclase ratherthan in the dark minerals. TiO2 is found as rutilein practically every specimen of granite, yetthe Ti02 of the actinolite syenite is largelytied up in sphene. This could mean that thesolutions that formed the granite found asource of CaO in the limestone and formedsphene instead of rutile. Further, the actinolite,epidote, and diopside may be poikiloblasticwithin orthoclase as well as interstitial betweenthe feldspar, quartz, and biotite, as thoughthey were formed fairly early. Yet these samethree minerals are found as euhedral crystalscrossing boundaries of quartz, orthoclase, andplagioclase, and also in solid monomineralicpatches and veinlets as though they were formed

at a late stage as well. This unusual feature isvery difficult to explain from a magmatic view-point.

In small actinolite syenite dikelets in quartz-ite where there may have been an oversupplyof Si02, micropegmatitic structure of unusualorthoclase quartz proportions is abundant. Sincethis structure is regarded by Drescher-Kaden(1948, p. 205) as a reaction phenomenon, itspresence further contributes to the argumentfor replacement.

Summary.—The above evidences against andin favor of a granitization origin for the graniteand actinolite syenite are summarized as follows:

Against Granilizalion

(1) No prominent continuous relic countryrock structures in granite or actinolite syenite.

(2) Sharp megascopic boundaries betweengranite and wall rock.

In Favor of Granilization

(1) The feldspar network may be considereda transitional zone along the contact.

(2) Limestone extensions and skialiths ingranite, aligned with limestone beds of un-disturbed country rock. Obscure quartzite bed-ding would likely not be preserved as a relic inreplaced granite.

(3) Periphery dikes very irregular with prom-inent pinch and swell features. Wall-rockborders largely unmatched.

(4) No flow structures along contacts, evenof smallest dikes.

(5) Silicic front present near contacts (glassyquartzite with obliterated bedding, limestonesilica ted).

(6) Wide variation of textures and rock typesin granite.

(7) No miarolitic cavities.(8) Grain-size variations bear no relation

to contacts.(9) Crystalloblastic nature of microtextures.(10) Fairly wide variation in K-Na ratios in

different areas.(11) High CaO rock (actinolite syenite) only

near large limestone skialith. (a) CaO makesactinolite, diopside, and epidote rather than




plagioclase. (b) Actinolite, diopside, and epi-dote formed late as well as early.

(12) Strong micropegmatitic structures ofunusual orthoclase-quartz proportions suggestreplacement.

Grantization history.—If granitization is re-garded as a possible origin for the granite andactinolite syenite, the subsequent intrusion ofthe granite porphyry with associated aplite,and biotite-quartz latite porphyry must beexplained.

All the igneous-looking rocks of the area maybe explained by two granitization hypotheses:(1) pure granitization, and (2) granitization re-lated to magmatic action.

According to (1) granite-forming materialswere transported upward from a source atdepth, possibly by hot solutions or vapor richin water. The quartzite and limestone areasnow occupied by the granite were slowly re-placed, and the granite and actinolite syenitewere formed. Part of this process was the de-velopment of the feldspar network and silicicfront in the country rock ahead of the advancinggranitization front. In one place the replace-ment was perhaps only partial, and the gran-itized quartzite rock remaining represents anintermediate stage between the formation ofthe feldspar network and the normal Binghamgranite.

With continued heat, temperatures of therock in some areas were elevated until the gran-ite became mobile through partial or completemelting forming a neomagma (Goodspeed,1948b, p. 72). This gave rise to the large massof granite porphyry which, around its borders,moved into cracks and openings in the graniteand quartzite and formed dikes. Differentia-tion was taking place at depth in this moltenmass, and after a portion of the granite por-phyry had solidified the slightly more basic bio-tite-quartz latite porphyry was intruded. The

porphyritic nature of the granite porphyrycould have developed in two ways. If only aportion of the rock were melted and some rem-nants of the larger minerals were still solid,upon cooling they would form nuclei for re-crystallization causing them to grow to ex-traordinary sizes and the interstitial meltedmaterial would crystallize as groundmass. Thealternative hypothesis would be that the granitewas completely melted and the phenocrystsdeveloped in the conventional magmatic man-ner. In either case the question could not beconvincingly settled since, under equilibriumconditions, there would be no difference betweentwo magmas of duplicate composition, one ofwhich had been cooled from a liquid until therewas 10 per cent of the whole present as crystalsin the liquid, and the other which was formedby gradually heating and melting the solidrock until only 10 per cent remained as crystalsin the liquid. The textures, subsequent crystal-lization sequence, etc., would be identical. Themore uniform chemistry and mineralogy of thegranite porphyry could perhaps be explainedby the stirring action which would accompanymovement and the chance for more thoroughdiffusion of chemical constituents afforded bythe fusion.

Hyphothesis (2) involves granitization result-ing from magmatic action. From a magma atdepth would develop granite, actinolite syenite,granitized quartzite, and feldspar network asabove, but the whole action would be a widefringe effect around the upper periphery of themagma. Later, the parent magma could havestoped or assimilated its way into and beyondthe granitized rock forming the granite por-phyry and later the biotite-quartz latite por-phyry.

Evidence for either hypothesis is lacking, andthe reader must fall back on his beliefs for hisselection.

PART II.—HYDROTHERMAL ALTERATION

INTRODUCTION

Following the formation of the igneous-ap-pearing rocks in the Bingham deposit, second-ary processes in the form of hydrothermal

alteration and mineral introduction becameoperative. The alteration and mineralizationtook place in seven distinct stages, all of whichcan be clearly delineated. The separation andparagenetic sequence of all but the first two are



INTRODUCTION 977

established on recognizable structural cross-cutting field relationships, while the first twoare separated on the basis of crosscutting struc-ture seen under the microscope. During thefirst four stages the original minerals in therocks were altered, while the last three stagesrepresent mineralization through introductionessentially without accompanying alteration.In Stage I, rock in a restricted area near thenorth-central part of the pit was altered withkaolinite and illite forming principally at theexpense of the feldspars. Stage II saw the ex-tensive development of sericite and hydro-thermal biotite in essentially all the rocks withinthe mine. The alteration minerals of both thesestages are disseminated throughout the rocksand do not follow fractures or other recog-nizable channels. The amount or intensity ofalteration varies widely. These effects may bethought of as having been accomplished bysolutions which more or less soaked uniformlythrough the rock. During Stage III, profusechlorite and hydrothermal biotite were formedwithin rather restricted areas of granite and arefound in wide bands, the positions of which wereobviously controlled by fractures. In Stage IV,quartz and sericite were formed on either sideof fractures and appear as bands and streaksin all the rocks in the area. Quartz, the onlymineral formed in Stage V, is found in sharpbordered veins with no accompanying altera-tion. In Stage VI disseminated sulphides weredeveloped in all the rocks within the mine.No alteration in the strict sense accompaniedthis mineralization stage. Sericite and allo-phane of Stage VII were introduced into cracksand vugs in somewhat restricted areas.

CHEMICAL ANALYSIS

The chemical analyses of the altered rockpresented in Table 2 were made by preciselythe same analytical methods as were used withthe unaltered rock samples. These figures repre-sent results obtained from rocks observed to bethe most intensely altered specimens of theirtype. The same caution in evaluation of theanalytical figures is imposed here as for theunaltered rocks, in that large representativebulk samples were not obtainable. With manyanalyses of altered rocks, authors have beenable to present analytical data that showed

progressive alteration intensity (Lovering et al.,1949, p. 41-60). The essential absence of freshrock, the rather wide compositional variationof rock types, and the complexity and over-

TABLE 2.—CHEMICAL ANALYSES OF ALTERED ROCKSFROM BlNGHAM

Walter J. Savournin, AnalystUniversity of Utah

Si02A12OSFezOs .MgOCaONa2OK2O . . .TiO2H2O

Total

Sp.Gr

l

68.1914.67

1.933.26

.341.966.25

.632.06

99.29

2.54

2

63.0615.833.744.121.484.054.68

.721.76

99.44

2.61

3

61.6014.295.107.44

.741.135.69

.992.49

99.47

2.70

4

74.1914.41

.06

.77

.451.005.60

.582.17

99.23

2.46

s

73.6115.76

1.08.89.45.78

3.58.42

2.6599.22

1. Altered granite porphyry. Stage I, Illite-kaolin-ite

2. Altered granite. Stage II, Hydrothermal biotite-sericite

3. Altered granite. Stage III, Hydrothermal biotite-chlorite

4. Altered granite porphyry. Stage IV, Quartz-sericite

5. Altered granite. Stage IV, Quartz-sericite

lapping of the numerous alteration stages atBingham prevented use of this valuable tech-nique.

HYDROTHERMAL STAGE I—KAOLINITE-!LLITE

General Statement

The earliest hydrothermal activity followingthe formation of the igneous-appearing rocksresulted in the development of kaolinite and"a poorly organized mica possibly in the illite2

group" (W. F. Bradley, Personal communica-tion). This alteration effect is found only inthe granite, granite porphyry, and biotite-quartz latite porphyry. The very small sizes(.001-.003 mm.) of these minerals and thesmall patches (.05-2.0 mm.) in which they

1 The name illite will be used hereafter to desig-nate this mineral.



TABLE 3.—HYDROTHERMAL STAGES AT BINGHAM

Minerals developed

Paragenetic evidence

Introduced materials

Rocks affected

Alteration Stages

Disseminated

Stage I

Illite

Kaolinite

Stage II

Hydrothermalbiotite

Sericite

Mineralization Stages

Channels

Stage III

Hydrothermalbiotite

Chlorite

Stage IV

Quartz

Sericite

Stage V

Quartz

Disseminated

Stage VI

Chalcopyrite

Channels

Stage VII

Sericite

Bornite : AllophaneDigeniteEnargiteMolybdenitePyriteGoldSilver

Petrographic Field Field Field Field Field

GraniteGranite por-

phyryApliteBiotite-Quartz

Latite Por-phyry

Fe Mg

All rocks

Fe Mg (much) • Si Si Cu Mo Fe S

Granite All rocks (ex-cept sedi-ments)

All rocks

As (little)Au Ag

All rocks

K Al SiO2 H20

All rocks

Time •



HYDROTHERMAL STAGE I 979

occur make it impossible positively to distin-guish this type of alteration in the hand speci-men, except in a superficial way where a brownchalky appearance is present caused by abun-dant brownish montmorillonite which has de-veloped through weathering of the alterationminerals. In some granite porphyry rocks in-tense alteration is represented on a fresh-cutbank where altered phenocrysts may break outreadily from the altered groundmass. However,in all cases a thin section was necessary todistinguish this alteration type positively.

Microscopic Appearance

Under the microscope the kaolinite and illiteare intimately mixed in almost all instances.Only in a few cases and under high magnifi-cation, was one mineral observed without theother. Optically the kaolinite is readily identi-fied by a low birefringena (.007) (PI. 3,fig. 3). The individual crystals of some aggre-gates of kaolinite are so small that they oftenappear isotropic. In most of the kaolinite ag-gregates, the individual crystal units are roughlyequidimensional, but some typical wormlikestructures with transverse flakes are present(Ross and Kerr, 1931, PI. 41). The relief isslightly higher than the relief of quartz.

Illite is readily distinguished from kaoliniteunder crossed nicols, by its slightly higherbirerefringence, with colors in first-order yel-lows. It has a distinctly flaky habit with indi-vidual flakes length slow and a relief slightlyhigher than kaolinite, the lowest index beingabout 1.569. Almost all the flakes of illite areintimately mixed with kaolinite, and many arearranged or oriented in a somewhat crudepattern with the angle of the grid being about90° (PL 3, fig. 3). No large patches of pureillite were discovered, and hence it was notpossible to work out its precise optical proper-ties. Because of the intimate mixture of thetwo minerals, it was impossible to obtain apure enough sample of either one for chemicalanalysis. Positive identification of the two min-erals was made by W. F. Bradley of the Il-linois State Geological Survey who x-rayed somecompletely altered phenocrysts of the graniteporphyry. He states that, from his powderpattern, he would judge the material to be"about 50 per cent kaolinite and 50 per cent a

poorly organized mica, possibly in the illitegroup."

The two alteration minerals have formed atthe expense of all the primary minerals in therocks except apatite and rutile. Plagioclase isusually most nearly completely altered, butorthoclase is quite often altered, and quartz isaffected around its borders only in the mostintensely altered rocks. Biotite is altered onlyrarely and then also very lightly around itsborders.

Wherever a Stage I alteration patch is foundit is composed of practically 100 per cent al-teration minerals. In only rare cases are thesealteration minerals scattered with fresh un-altered mineral between. Where alteration hasbeen very slight, the kaolinite-illite patchesmay be quite irregular and occupy only a por-tion of a crystal. With more intensity, a wholefeldspar crystal may be affected, and the out-line of the alteration patch is that of the crystal.

Distribution

The first alteration stage is most intense nearthe north-central portion of the pit in Sections2, 3, 4, 7, 8, and 9 (PI. 1). The outer boundaryof this area was determined by observing thmsections from well-located samples. Within theboundary are rocks altered as much as 75 percent, but most of them have been altered muchless. An arbitrary figure of 15 per cent wasselected to determine the outer boundary of thealteration area which appears on Plate 1. Thus,if the rock showed less than 15 per cent kaoliniteand illite, it was not included in the alteredarea. The boundary thus drawn is therefore notabsolutely accurate, but its position shouldbe a fairly good approximation of the outerlimits of the alteration.

Character of Altering Solutions

Since kaolinite forms principally in an acidenvironment below 350°C., its presence as analteration product here would definitely placethe pH of the altering solutions of Stage I inthe acid category and at temperatures below350°C. The field of formation of illite is not sowell known, but the intimate association be-tween illite and kaolinite would suggest thatthey were formed at the same time.




Analyses No. 1, Table 2, was made of aspecimen of granite porphyry composed ofabout 60 per cent Stage I alteration minerals.Comparison of these figures with the analysesof moderately altered granite porphyry, Nos.7 and 8, Table 1, shows that there is very little

HYDROTHERMAL STAGE II—HYDROTHERMALBIOTITE-SERICITE

General Statement

The alteration produced during the secondstage of hydrothermal activity resulted in the

A B CFIGURE 8.—PSETJDOMORPHS OE HYDROTHERMAI. BIOTITE

(A) former biotite crystal completely changed to variously oriented small hydrothermal biotite flakes.(B) former biotite crystal altered to quartz and hydrothermal biotite. (C) biotite crystal altered to hydro-thermal biotite on lower end. Note the ragged and distended border of the hydrothermal biotite. Cameralucida drawing, X32.

significant chemical difference between unal-tered rock and altered rock of Stage I type.The K20 and CaO are slightly lower than theunaltered rock, while SiC>2 is slightly higher.Thus it may be stipulated that the alteringsolutions of the first stage only effected mineralchanges, and very little material was added ortaken away from the rock.

The alteration follows no obvious channelswhich can be noted in the field or thin section.The solutions evidently penetrated the rockalong mineral boundaries or through micro-scopic fractures. Since there is no pattern of thelatter, it is believed that the solutions result-ing in the first stage of alteration penetrated therock as a "soaking action."

Paragenesis

Stage I is definitely the earliest alterationstage and occurred after the primary mineralsin the rocks were formed. The alteration min-erals of Stage I are all cut by veinlets or re-placed by all other types of secondary minerali-zation in the pit.

profuse development of fine flaky biotite (herecalled hydrothermal biotite) and sericite. Thesealteration minerals are disseminated through-out the rock and are rarely found in concen-trated clumps, patches, or veinlets. Both min-erals generally occur in the same rock but arerarely intermixed. Sericite, however, is ab-sent in some specimens, and in one area on theeast side of the pit some of the small dikesand plugs of granite porphyry contain abundantsericite but no hydrothermal biotite. On thewhole, however, hydrothermal biotite is byfar the more widespread and abundant. In thefield the hydrothermal biotite-rich rock with-out sericite is difficult to distinguish fromsericite-rich rock without hydrothermal biotite;consequently, they were not mapped separately.All the igneous-appearing rocks and a few ofthe sedimentary rocks in the pit have been af-fected to varying degrees.

The fine flaky dark mica occurring profuselythroughout the Bingham mine as an alterationproduct is biotite, as determined by x-ray, but,since it is believed to have been formed underdifferent conditions from the coarser biotite,



HYDROTHERMAL STAGE II 981

the term hydrothermal biotite is used. Theonly difference between the two is the size andform of the crystals.

The boundaries of many patches or clustersof randomly oriented hydrothermal biotiteflakes have a shape resembling a biotite crystal(Fig. 8A). The edges of many of these pseudo-morph patches are quite regular, but some areragged (Fig. 8C). Occasionally, they are com-posed of biotite and quartz (Fig. 8B), and somepseudomorphs may contain as much as 80-90 per cent quartz.

The intensity of the development of sericitewas not mapped because sericite could not bedistinguished in hand specimen. However, theease of recognizing hydrothermal biotite inthe field made it possible to note two areas(not mapped) where it is profusely developed.In an area on the intermediate levels in Sec-tion 7, the rock was so rich in hydrothermalbiotite (50-60 per cent) as to appear black, and,on the upper levels in Section 12, it is developedto a slightly greater degree than in the rest ofthe pit. The various rock types show somevariations in the general effects that alterationStage II had upon them; hence, each type isdescribed.

Granite

Hydrothermal biotite is developed best inthe granite. The small patches of concentratedflakes of hydrothermal biotite and its char-acteristic dark color make it fairly easy to de-tect in the hand specimen. The sericite of thisstage, however, cannot be detected without themicroscope. The dark mica flakes are found inclumps and clusters occasionally with raggedborders, disseminated throughout the rock; thesize approximately equals the grain size of theoriginal rock and may vary widely as the grainsize of the granite varies. Most of the patchesappear rather massive, presumably due to thevery small size and random orientation of thesmall mica flakes. Though usually ragged inoutline some of these patches which have beendeveloped from pre-existing biotite may bequite regular. Where the patches are fairlylarge, they are blackish, but, where they aresmall, a brownish color is most prominent im-parting a peculiar speckled salt and pepperappearance (PL 5, fig. 2). In some cases where

the patches are fairly large, they seem to beinterconnected and exhibit irregular and raggedborders. Some fairly coarse-textured dark rockscontain as much as 50 per cent hydrothermalbiotite while most of the extremely fine-texturedgranite has a very small amount of it. Nostreaks, veinlets, or large areas of concentratedhydrothermal biotite are seen.

The sericite is confined principally to thefeldspars and, where present in large amounts,colors them earthy white or tan. The kaolinite-illite development of Stage I also usually re-sulted in an earthy white or light brown color,and no field test was devised with which todistinguish confidently the presence of Stage Iminerals from the sericite of Stage II.

The hydrothermal biotite is readily identi-fied under the microscope by its prominentpleochroism, its high birerefringence (intensefirst-order colors up to second-order red), andthe cedar grain or birds-eye structure character-istic of mica. The flakes, usually slightly largerthan those of the sericite, are near .01 mm.to 0.1 mm. across, and various sizes may befound within one patch of hydrothermal biotite.It is found on or within all the minerals ex-cept quartz, and this includes the alterationminerals of Stage I (PI. 3, fig. 5). In many in-stances the flakes cross mineral boundaries.Pseudomorphs of hydrothermal biotite afterbiotite are discovered with straight boundarieson all sides, but more often a portion of theboundaries are ragged (Fig. 8). Some are en-tirely ragged. Occasionally the brown micaflakes composing the rims of pseudomorphs areeither coarser for finer-grained than those ofthe core. Very often fine-grained quartz ac-companies the brown mica in the pseudomorphs,and in some instances as much as 90 per centof the area may be quartz (Fig. 8B).

Up to half the original biotite in a specimenmay be unaltered, and in many cases an un-altered biotite crystal is adjacent to thoroughlyaltered biotite. Most of the 'fine-grained darkmica areas are extremely irregular in outline;the individual flakes have no particular align-ment or arrangement. These patches may beeither isolated or interconnected, with indi-vidual flakes extending in irregularly shapedstringers from one area to another. The sizes ofhydrothermal biotite flakes vary throughout the




rock, and there seems to be no relationshipbetween intensity of alteration and the size ofthe mica flakes.

Sericite is found in practically all specimens,accompanying the hydrothermal biotite. It isdistinguished microscopically by its lack ofcolor, high birerefringence (intense first-ordercolors up to second-order red), flaky form, andits intermediate relief with the lowest indexaround 1 56. The sericite is found only in thefeldspars and not intimately mixed with hydro-thermal biotite. However, it is often presenton or in a kaolinite-illite alteration area ofStage I. In the feldspars the sericite flakes arecommonly spaced wide apart with unalteredfeldspar between (Fig. 9); several characteris-tic arrangements of the flakes are seen. For in-stance, a "train" of randomly oriented sericiteflakes crosses an otherwise unaltered feldsparcrystal (Fig. 9A) as though the solution ef-fecting the alteration had proceeded through amicrofracture in the feldspar and the alterationhas occurred only near the fracture. Several ofthese "trains" of sericite flakes may criss-crosswithin a single crystal. Many feldspars havealtered only in part to sericite, while the restof the feldspar crystal is clear and unaltered(Fig. 9B). The sericite flakes are frequently ar-ranged with the long dimension of the shredsor flakes oriented in two directions forming agrid pattern over a feldspar crystal (Fig. 9C;PI. 3, fig. 4). However, more commonly thesericite flakes have a completely random orfeltlike arrangement with an even distributionthroughout the crystal (Fig. 9D). With in-creasing intensity of alteration the amount ofsericite increases, and the unaltered feldsparbetween the flakes decreases. In all the graniteobserved there was never more than 20 percent of the rock changed to sericite, and exceptin a very few instances hydrothermal biotitealways accompanied it.

Actinolite Syenite

The actinolite syenite was altered to a minordegree by Stage II type. The hydrothermalbiotite and sericite are found in the same rela-tionships and proportions and affecting the sameminerals as in the granite but are not so abun-dant. Around the borders of the actinolite

syenite areas where it is in contact with granitesome actinolite and diopside] have been alteredto a fine flaky greenish mica rather than brownmica. The most intensely altered actinolitesyenite contained about 10 per cent of StageII alteration minerals.

Granite Porphyry

As in the granite, sericite is not detectablein hand specimen except for the alteration foundin the small dikes and plugs of Section 5 and theupper part of Section 10 where hydrothermalbiotite is essentially absent and a silky luster onintensely altered feldspar phenocrysts distin-guishes the sericite. In the rest of the graniteporphyry, hydrothermal biotite is abundant,but it generally amounts to about one third ofthat present in the granite and occurs prin-cipally as pseudomorphs after biotite. Onlyabout one third to one half of the total biotitephenocrysts in the granite porphyry (which arereadily observed in the hand specimen) arechanged to pseudomorphic patches which gen-erally have a regular outline but show a verydense structure.

Under the microscope about 90 per cent ofthe hydrothermal biotite in the granite por-phyry is found in large patches with regularor irregular outlines and is believed to havedeveloped largely from previously existing bio-tite. About 10 per cent of the hydrothermalbiotite occurs in the quartz-orthoclase ground-mass or in the feldspar phenocrysts distributedin small disconnected patches. Many of thepseudomorphs here contain considerable ac-companying alteration quartz similar to thatfound in the granite. Some biotite phenocrystshave been bleached with the accompanyingdevelopment of many oriented rutile needles.The sericite is found here in precisely the samemanner as in the granite, being derived entirelyfrom the feldspar and found in similar rela-tionships (Figs. 9A, B, C, D).

Both second stage minerals are distributedthroughout the granite porphyry. Where alater stage (Stage IV) has altered large areas inthe granite porphyry, the hydrothermal biotiteand sericite of Stage II have been obliterated,and identity of Stage II in these areas is there-for lost.



HYDROTHERMAL STAGE II 983

In aplite the alteration minerals of Stage IIhave been developed to about the same extentand intensity as within the granite porphyryproper.

Q 0

are 0.5 mm. across and are believed to have beenformerly occupied by feldspar. In some speci-mens the feldspar is partially altered only tosericite.

a

FIGURE 9.—SERICITE IN FELDSPAR(A)—aligned in "trains." (B)—present only in part of a crystal. (C)—flakes oriented in a grid pattern.

(D)—flakes in felt-like or random arrangement. Camera lucida drawing, X26.

Biotile-Quartz Latite Porphyry

In practically all the dike rocks classified asbiotite-quartz latite porphyry most of the feld-spar phenocrysts have been completely changedto sericite, whereas hydrothermal biotite oc-curs principally on the fringes of the biotite andto a slight extent in the groundmass.

Granilized Quartzite

The typical granitized quartzite containsbut a very little hydrothermal biotite, and nowell-developed dark mica pseudomorphs werefound. In only one or two specimens was hydro-thermal biotite present in sufficient quantityto give the rock a brownish appearance. Thesmaller biotite flakes are more or less randomlyoriented and disseminated. The sericite in thevery fine feldspar aggregates is usually in suchsmall flakes that very high magnification wasnecessary to resolve them. When observable,however, they are found to be present in anopen disconnected manner similar to that foundin the feldspar of the granite porphyry andgranite.

Feldspar Network

In practically all cases quartzite having thefeldspar network contains very fine-grainedunmixed hydrothermal biotite and sericite bothof which are present only in the feldspars. Theflakes are generally quite small and randomlyoriented, but occasionally some areas or patches

Sediments

Some hydrothermal biotite was found in afew unfeldspathized quartzites. It is recognizedin hand specimen by the presence of some verysmall (0.5 mm.) widely spaced, brownish-look-ing spots disseminated throughout the rock.

In the limestones hydrothermal biotite isgenerally restricted to the edges of some of thexenoliths, or skialiths. In some places enoughdark mica has been introduced to make up asmuch as 50 per cent of the rock, and occasionallythe individual flakes become so large thatcleavage units can be discerned megascopically.

Relations between Hydrothermal Biotileand Sericite

In many slides, hydrothermal biotite andsericite adjoin, and the boundary relations sug-gest that the dark mica is later than the sericite.However, there are no crosscutting veinlets orother visible structural features to show thatthe two minerals were developed at separatestages, and it is therefore believed that the twoare closely related in time and environment offormation.


It has been fairly well established (Gruner,1944, p. 587) that true sericite (fine muscovite)probably forms in acid solutions above 350°C.and in alkaline solutions below 350°C. Thefield of formation of biotite is not so well known.




However, one might extrapolate that, sincebiotite is so similar to sericite, both might haveapproximately the same general environmentof formation. There is no critical mineral ac-companying this stage of alteration which wouldplace the temperature of formation above orbelow 350°C. or the pH of the solution aboveor below 7. Therefore neither the pH nor thetemperature of the solution can be determined.

Chemical analysis Number 2, Table 2, is of anintensely altered (50 per cent alteration min-erals) granite containing hydrothermal biotiteand sericite. This analysis is not radically dif-ferent from an analysis of the comparativelyunaltered rocks, Numbers 2 and 4, Table 1.The greatest difference is in the Fe2O3 content,which jumps from 1.88 per cent to 3.74 percent in the altered rock. All other figures re-main within a reasonable and expectable rangeconsidering the wide variation in compositionof the original rock. The introduction of ironwas no doubt responsible for the profuse forma-tion of biotite. Analogous percentages for silica,alumina, magnesia, soda, and potash in theanalyses indicate that not much if any of thesematerials were introduced.

Paragenesis

When Stage I minerals, kaolinite and illite,were found with Stage II minerals, the hydro-thermal biotite and sericite frequently formedon the Stage I minerals, and occasionally vein-lets of Stage II minerals traversed Stage Iminerals (PI. 3, fig. 5). This evidence is ad-mittedly purely petrographic, but it is fairlyconclusive that Stage II followed the develop-ment of illite and kaolinite of Stage I.

HYDROTHERMAL STAGE III—CHLORITE-HYDROTHERMAL BIOTITE

General Statement

Alteration Stage III, resulting in the de-velopment of profuse chlorite and hydrothermalbiotite in granite, is readily recognizable inthe field. The dark mica is of the same type asthat found in Stage II though the individualflakes are generally much smaller. The termchlorite here is used to cover several mineralspecies of the chlorite group. At the beginning

of the study seven chlorites—penninite, anti-gorite, clinochlore, delessite, diabantite, jenkin-site, and prochlorite—were distinguished op-tically. However, there seemed to be no spatialor areal relationship of these chlorites to oneanother, and in the present report it seemedproper to use the general term chlorite ratherthan any of the species names.

The granite which has been intensely alteredto chlorite and hydrothermal biotite of StageIII appears very dense, nearly black, and whenwet resembles black limestone. Where the al-teration is less intense, the original granitoidtexture may be discerned, though the graniteis still quite dark. Alteration is best developednear the western margin of the pit (Sections1 and 6) where it is readily seen as dark streaks.These streaks range in width from a few milli-meters to slightly over 200 feet. Three irregularareas were mapped on the southern middleand upper levels (Sections 7 and 12), but thespecimens in this region show less intense al-teration than do the rocks to the west. Manyadditional small pods or patches of this altera-tion type are found everywhere within thegranite area, but because of their small sizethey were not mapped.


Under the microscope, the hydrothermal bio-tite and chlorite always occur in a fine flakyform often intimately mixed. In the most in-tensely altered rocks no feldspar or biotite hasbeen left unaltered, and the quartz units havebeen replaced principally on their edges. Inless intensely altered rock some of the feldsparand original biotite may be unaffected. The al-teration minerals occur either as very fine-grained felted irregular solid mats or, in lessintensely altered rocks, as fine flakes scatteredthrough feldspar and biotite.

Chlorite Orbicules

Of special note are some chlorite orbiculeswhich have formed in granite as a result ofStage III alteration. In these orbicules thechlorite has formed with spherically curvedcleavages, giving a distinct onionlike structureto the mass. The largest orbicule seen was 22



HYDROTHERMAL STAGE III 985

mm. in diameter (PI. 5, fig. 1), but the averagesize is around 10 mm. with some as small as2 mm. in diameter. The chlorite orbicules ap-pear to have grown by replacing the mineralswhich surrounded it. It is not yet known whythese orbicules have developed in such a uniquemanner.


The minerals of the chlorite group have notbeen investigated extensively, and very littleis known of the hydrothermal physico-chemi-cal conditions under which they form. However,it is thought (T. S. Levering and J. W. Gruner,Personal communication) that chlorite prob-ably forms in neutral to slightly alkaline solu-tions at temperatures that range from near100° to about 400° or 500°C. The intimate mix-ture of fine biotite and chlorite suggests theywere formed at the same time and hence fromthe same type solutions. If biotite forms undersimilar conditions to sericite (alkaline solutionsbelow 350°C.), then it may be argued that,since chlorite probably forms in alkaline orneutral solutions, the temperature at the timeof Stage III alteration was probably lower than350°C.

Chemical analysis Number 3, Table 2, wasmade of an altered rock of Stage III. The onlysignificant chemical difference between this rockand the more or less unaltered granite (Table 1,Analyses 1, 2, 3, 4) is the expected higher mag-nesia and iron content. The percentages jumpfrom a maximum of 2.7 for iron in unalteredrock to 5.1 and 4.1 for magnesia to 7.44 in thealtered rock.

Paragenesis

The position of Stage III in the hydrothermalparagenetic sequence is very clear from struc-tural evidence. In the field alteration streaksor bands of Stage III minerals cut through al-teration Stage II, thus definitely dating thechlorite-hydrothermal biotite alteration.

HYDROTHERMAL STAGE IV—QUARTZ-SERICITE

General Statement

The fourth alteration stage finds quartz andsericite extensively developed along variouslyoriented fractures in granite, actinolite syenite,

granite porphyry, and granitized quartzite. Thealteration has developed symmetrically oneither side of the fractures producing bandseasily recognized in the field. Where the bandsare so wide and numerous as to coalesce leavingno unaltered rock between, as is common inthe granite porphyry, the resultant rock some-what resembles quartzite.

Field Features

The quartz-sericite alteration type clearlyfollows fractures in all cases. It colors the rockgray and converts it to a very dense chalcedonic-appearing material. Where the dark graniteand actinolite syenite have been affected, thesealteration areas appear as light streaks andbands (PI. 5, fig. 5), but where they traversethe light-colored granite porphyry they appeardarker than the host. It is difficult to recognizethem in the granitized quartzite since the colorsand textures are similar.

The alteration is always symmetrically de-veloped on either side of a fracture, and theband or streak maintains a more or less con-stant width for a given fracture. The width ofthese bands ranges from 2 mm. to 10 cm. withmost falling between 5 mm. to 3.5 cm., and theouter margins always appear as sharp contactswith the host rock. Along some outer bordersin the granite a concentration of hydrothermalbiotite formed, producing a darker streak oneither side of the alteration area. Some frac-tures are open and empty, but most of them arefilled either with glassy quartz of Stage V orsulfides of Stage VI, and frequently with both.

There seems to be no particular relation be-tween the width of the alteration band and thewidth of the fracture. The fractures controllingthe alteration are not preferentially orientedand are distributed entirely at random. Theyare usually fairly straight but may criss-crossone another at various angles in an intricatenetwork. Because they are so easy to detect inthe dark granite, some of the wider alterationbands (2-3 inches) have been traced in thefield as far as two or three levels (200 or 300feet).


Under the microscope the most intenselyaltered rocks of this stage are found to be com-




posed entirely of quartz and sericite in aboutequal amounts. The sericite, which occurs inirregular patches, is fine-grained, and the flakesare usually arranged in a feltlike manner,forming solid mats. The quartz is medium-grained with interlocking boundaries. Some ofthe alteration bands show less intense altera-tion, but this is also found to have quartz andsericite. Sericite is much less abundant and isfound only in the position of former feldspars,biotite, and hydrothermal biotite. Some un-altered feldspar may remain. Where alterationhas been less intense, the porphyritic texture ofthe granite porphyry is still readily determin-able. Where an alteration band cuts large pinkfeldspar phenocrysts, the alteration seems tobe predominantly sericite with sharp outerboundaries and contains quartz only near thecenter of the alteration band (PI. 3, fig. 6).

There seems to be no particular gradationin the intensity of the alteration from the cen-ter of an alteration band to the outer margin;the whole band shows either intense alterationor moderate alteration. Although in most rocksthe outer border of a band appears very sharpin hand specimen, under the microscope it isseen to be fairly gradational over a distance of0.1 to 0.2 mm. Of special interest are bands ofa dark mineral which sometimes occur justbeyond the border of the alteration bands ingranite (PI. 4, fig. 6). Under the microscope thisdark border is observed to be composed of hy-drothermal biotite in greater concentration thanin the granite, and within the alteration banditself are many bleached biotite crystals andhydrothermal biotite aggregates. This suggeststhat the altering solutions removed the ironand magnesia from the dark mica within theband to the position just beyond its border andthere formed the dark mica. This conditionmay be somewhat similar to that commonlythought of as a "basic front."

Distribution

The alteration bands, though observed overthe entire open pit, seem to be more concen-trated in the western part of the mine. Theamount of the altered rock generally rangesbetween 2 and 20 per cent of the whole, but,in manv cases where alteration bands are wide

and frequent, these figures may grade up to100 per cent. The areas mapped as Stage IV onPlate 1 indicate the areas where alteration con-stitutes 25 per cent or more of the rock. Somefairly large areas of 100 per cent altered rockare found principally in the granite porphyryon the higher levels to the west in Sections 1,2, 6, and 7. Only a few small patches of 100per cent altered rock are found in the granite.The intensely altered specimens often closelyresemble quartzite, and a thin section was neces-sary in many instances to determine the truenature of the rock.


The solutions that accomplished the StageIV alteration evidently obtained access intothe rock along fractures and soaked symmetri-cally into the walls on either side of the fracture.The only significant chemical addition wasSiOj since the material to form sericite is presentin the feldspars and biotite. The solutions ex-tracted MgO and FeO and perhaps a little K2Ofrom the biotite and hydrothermal biotite andeither removed them or deposited them nearbyas disseminated material or as dark mica streaksalong the edges of the alteration band. Nodiagnostic hydrothermal mineral was developedduring this alteration stage by which to esti-mate the temperature or pH of the solutions.It is believed that quartz may form under alltypes of physico-chemical environments and,as stated elsewhere, seicite is formed either inacid solutions above 350°C. or in alkaline solu-tions below 350°C. There is no evidence to indi-cate which condition existed at the time of thealteration.

Analysis numbers 4 and 5, Table 2, are ofrock showing alteration Stage IV. Of signifi-cance is the fact that the SiO2 has jumped from60-68 per cent to approximately 74 per centand that Fe2O3 and MgO have declined rathermarkedly. Analysis number 3, Table 1, is of acomparatively unaltered granite and was takenfrom rock approximately 3 mm. from an alteredband. Analysis number 5, Table 2, is of thealtered band. Note that SiO2 rises from 66.71per cent in unaltered rock to 73.61 per cent inthe altered band and that the MgO and Fe2O3

and Na-'O significantly decline.



HYDROTHERMAL STAGE IV 987

Paragenesis

The sequence of formation of Stage IV is veryclear from field evidence. These alteration bandstraverse or cut through the hydrothermal bio-tite-sericite rock of Stage III, the hydrothermalbiotite-sericite rock of Stage II, and the kaolin-ite-illite rock of Stage I and are in turn cut bybarren quartz veins of Stage V.

HYDROTHERMAL STAGE V—QUARTZ

The quartz of Stage V occurs in sharp-bordered veins which obviously filled fractures,and no minerals are altered as a result of itsformation, hence stage V cannot be strictlycalled an alteration stage (PL 5, fig. 5). Thequartz is clear to milk white and glassy andoften contains vugs. The veins traverse all therock types of the pit with random orientationand range in width from 0.5 mm. up to 10 cm.with the average near 1 to 7 mm. They aregenerally so widely spaced that no mappingof them was attempted. Sulfide crystals arefound in many of the vugs.

As quartz is not diagnostic of temperatureand pH of depositing solutions, no evidence asto the environment of formation of the StageV quartz is possible. Study of the liquid in-clusions in the quartz (not attempted here) couldreveal information regarding the temperatureof the quartz formation. In the field theseveins cut through alteration bands of StagesIII and IV (PI. 5, fig. 5) and in turn are cut bysulfide veinlets, thus placing Stage V as defi-nitely pre-sulfide and post-Stage IV.

STAGE VI—SULFTDES

General Statement

Although the presence of sulfides in the cop-per deposit at Bingham is the most importanteconomic consideration, intensive study of thesulfide minerals was not attempted here forvarious reasons. This study was confined to therelationships of the stage of sulfide develop-ment to the overall hydrothermal history of thedeposit. Evidence is conclusive that all thesulfides were formed during one clear-cut periodin the hydrothermal sequence, but earlier workshows that the different sulfide minerals were

probably deposited at different times withinthis period, and it is recognized that thesefeatures deserve special study. Beeson's (1916)early work on the sulfide minerals at Binghamshows a picture of the sulfide problem at thattime, but perhaps a new study is now war-ranted because of new and deeper exposures.The large and extensive problem of oxidationand enrichment of the ore body, though alsoimportant, would not materially add to thepresent study and was therefore neglected.

Sulfide Minerals

Sulfide minerals believed to have been de-posited by hydrothermal solutions have beendetermined as digenite (x-ray determination),chalcopyrite, bornite, enargite, molybdenite,and pyrite.

The digenite occurs in very small (0.1 mm.diameter) individual units disseminatedthroughout the deposit. It usually is accom-panied by one or several other sulfides. Chal-copyrite is the most widespread ore mineraland occurs predominantly as small (1.0 mm.maximum diameter) units also disseminatedthroughout the rock, but occasionally smallveinlets of it are noticed. In one instance, aveinlet of chalcopyrite, about 6 inches long and2 cm. wide was found in granite. Chalcopyriteis also found in many instances as well-shapedcrystals in vugs within the quartz veins ofStage V.

Most of the bornite is present in the lowerlevels of the pit. It occurs in a manner similarto the chalcopyrite—that is, as small dissem-inated grains and rarely as small and discon-nected veinlets.

Enargite is rare in the mine but, where found,is present in the vugs of the Stage V quartz.

Molybdenite occurs in very small units dis-tributed throughout all the rocks in the mine;it is almost universally associated with thecopper minerals. The largest molybdenite crys-tals are obtained, however, where it has crystal-lized in vugs in the quartz of Stage V.

Pyrite is by far the most abundant sulfidemineral in the mine. It forms small crystalsdisseminated throughout the rock, small vein-lets, and very often well-developed crystals invugs in the quartz of Stage V (PI. 5, fig. 5).




Pyrite is also abundant in the sediments aroundthe periphery of the igneous-looking rocks andin some cases may replace fairly large masses,particularly in the limestone.

Distribution

Pyrite is found in small crystals or large re-placement bodies in all types of rock within thepit. Enargite occurs only in vugs of Stage Vquartz while chalcopyrite, pyrite, bornite, anddigenite are found in varying amounts in all theigneous-looking rocks and to a minor extent insome of the quartzites and limestones near thegranite and granite porphyry.

The copper, iron, and molybdenum sulfideminerals do not seem to favor in occurrence anyspecial alteration type or fundamental rocktype within the mine except that they are gen-erally more sparse in quartzite and limestone.

The sulfide minerals disseminated in theigneous-appearing rocks seem to favor positionswithin or adjacent to biotite (Schwartz, 1947,p. 326).

Alteration

The sulfide minerals in the pit were formedby (1) replacement in the solid rock, and (2)precipitation in the vugs of the quartz of StageV. No rock-alteration effects were found whichcould be related to the development of thesulfide minerals.

Nature of Depositing Solutions

Available data indicate that none of theminerals formed during the sulfide stage arediagnostic of the physico-chemical nature of thedepositing solutions. The question is raised asto just how the sulfide minerals were dissem-inated so uniformly throughout the rocks atBingham. Many instances are found wherequartz of Stage V containing large vugs mayhave only a few sulfide crystals deposited onthe quartz, whereas, in the solid rock adjacentto the quartz vein, disseminated units of sulfideminerals in the usual concentration are present(PI. 5, fig. 5). If the sulfide-bearing solutionstraveled through the rock in a soaking manner,the connected vugs in the quartz veins wouldprobably act as channels, accelerating the move-ment of the solutions. It might be expected

then that the vugs, having a more constantand fresh supply of sulfide-depositing solutions,ought to be filled with sulfides. This is true onlyrarely, and the vast majority of quartz veinswhich show vugs contain but a small amountof sulfides. When the overall spatial distribu-tion of the sulfides in the rock is observed, thesulfide minerals are about equally spacedwhether there is a vein with vugs in the areaor not.

From this it is suggested that perhaps a newexplanation of the mechanism of the sulfide in-troduction and deposition might be forthcom-ing. The sulfide-bearing solutions could havepenetrated the rocks at Bingham and more orless come to rest without any deposition. Thispenetration when completed would soak notonly through the pores of the rocks but alsothrough the vugs in the quartz. Then, a moreor less uniform concentration of sulfide mate-rial would be accomplished through diffusionthroughout the soaked-up rock. At some timewhen conditions were right, crystallization ofsulfides began with nucleation at more or lessequally spaced points in the rock whether in avug or not. The nuclei then grew, being fed bydiffusion from the solutions in the immediateneighborhood of the growing crystal. Hence itmight be thought that each crystal or unit repre-sents the deposition of all the sulfide materialfrom the solutions which were present just be-fore precipitation. This would necessarily im-ply a rather concentrated ore solution. For ex-ample, if the total porosity of the rock (andhence volume of the ore solution) is assumed tobe 10 per cent, this formed 1 per cent coppersulfide ore by this process, the rock density istaken as 2.6. The solution would have to contain0.26 gm each of sulfur and copper per cc—i.e., it would contain 26 weight per cent each ofCu and S if it had a density of 1.0. At this stage,local cracks may have formed resulting in thestreaming in of sulfide-bearing solutions form-ing occasional small solid veinlets of sulfides.This whole process could thus be thought ofnot as a dynamic one but rather a comparativelystagnant soaking action.

There is a possibility that the uniform dis-seminated distribution of sulfides may be theresult of an original uniform disseminated dis-tribution of magnetite in the original igneous



STAGE VI—SULFIDES 989

rock and that the magnetite crystals, when thesulfide-bearing solutions traversed the rock,simply formed nuclei for sulfide replacement(Boutwell, 1905, p. 168). The present positionof each crystal of sulfide then would largely bein the position of a former magnetite crystal.Magnetite was discovered in the limestones ofthe upper middle levels on the south side, but,in all the sections of the ingeous-appearingrock examined, not a single magnetite unit wasfound. If the above process is correct, somemagnetite crystals should have been left un-replaced.

HYDROTHERMAL STAGE VII—SERICITE-ALLOPHANE

Following the deposition of the sulfides, seri-cite and allophane were formed in cracks andfractures. The two minerals have not yet beenfound together. The sericite occurs in veins upto an inch wide, principally on the east side ofthe pit. It is dense, white, with a satiny appear-ance and a very soapy feel. The allophane isquite widespread. One area near the middlelevels on the south side contains so much al-lophane that the rocks appear white from adistance. The allophane is white, dense, andporcelainlike and forms in veins, the same asthe sericite. Both minerals have been depositedon top of sulfide minerals, thus establishing theirage as post-sulfide.

SUMMARY OF THE HYDROTHERMAL ALTERATIONAND MINERALIZATION STAGES

In the first alteration stage following theformation of the rocks, kaolinite and illite wereformed as an alteration product from feldsparsin an area more or less confined to the north-central part of the pit. The temperature of thedepositing solutions was probably below 3SO°C,and the solutions were acid. No appreciableamount of material was added to or extractedfrom the rock as a result of this stage.

Stage II finds sericite and hydrothermalbiotite extensively developed in all the igneous-appearing rocks in the pit. Petrographic evi-dence separates Stage I from Stage II; micro-scopic veinlets of sericite and hydrothermalbiotite are found in kaolinite and illite. Iron and

magnesia were introduced by the solutions, butnot enough is known regarding the field offormation of hydrothermal biotite or sericiteto state confidently the temperature and pHof the solutions. On the basis of the presence ofsericite, the solutions either were at tempera-tures above 350°C and acid or below 350°Cand alkaline.

Stage III is found principally in the graniteas dark streaks and bands consisting of anintensive development of chlorite and hydro-thermal biotite. These streaks cutting the gran-ite establish this alteration as post-Stage II.Chlorite is believed to form in solutions whichare neutral to slightly alkaline, and the hydro-thermal biotite, so closely associated with thechlorite, probably forms in neutral to alkalinesolutions which are below 350°C. Therefore,the alteration Stage III might have been ac-complished by slightly alkaline solutions below350°C.

The quartz and sericite bands of Stage IVcut Stage III alteration as well as Stages I andII, thus establishing its age. Sericite or quartzare not diagnostic of any particular environ-ment, and nothing can be said regarding thetemperature and pH of the solutions. Silicawas introduced in profuse amounts.

Stage V saw the development of quartz onlyin veins and is not an alteration stage. In thefield the veins cut the quartz sericite alterationof Stage IV as well as all earlier alterationstages, and it is therefore later. Quartz is be-lieved to form in all types of solutions at alltemperatures, hence no statement can be maderegarding the temperature and pH of the solu-tions. Silica was the only substance introducedat this stage.

Sulfide crystals of Stage VI are in vugs in thequartz of Stage V, and small sulfide veinlets cutthe quartz veins. Thus the sulfide stage is sepa-rated from the quartz mineralization of StageV. Since various sulfides may be formed in alltypes of solutions in all ranges of temperatures,no interpolation can be made regarding thetemperature and pH of the solutions. Copper,iron, sulfur, molybdenum, a little arsenic, silver,and gold were introduced by these solutions.

The minerals of the last hydrothermal stage,allophane and sericite, coat sulfide crystals,thus establishing that the allophane-sericite




stage is post-sulfide. Neither sericite nor al-lophane is diagnostic of any hydrothermal en-vironment of formation, and the temperatureand pH of these solutions remain unknown.Potash, alumina, silica, and H2O were intro-duced.

The solutions of the first four stages defi-nitely altered some of the minerals in the rocks,and the solutions of Stages I and II soakedthrough the rock more or less uniformly. Thesolutions of the third and fourth stages tra-versed the rock through fractures and alteredthe rock adjacent to them. The solutions of thelast three stages introduced material, and noalteration in the above sense was effected. Solu-tions of the fifth and seventh stages followedfractures, while, in the sixth or sulfide stage,solutions penetrated the rock uniformly, sug-gesting a soaking action.

ORIGIN OF SOLUTIONS

The problem rises as to the origin of the post-magmatic or post-granitization solutions andthe localization of these solutions within theBingham copper mine area. The solutions evi-dently came from depth, being derived as adifferentiate of the original magma or theneomagma, which ever view is taken. Thebroader geographic localization may be due tothe presence of a deep-seated channel in thevicinity of Bingham through which the solu-tions moved from a fairly deep source. The localposition of the alteration stages and introducedminerals in the rock is probably the result of:(1) the original higher porosity of the igneous-appearing rock plus the intense fracturing whichwas localized in these same rocks and occurredconcurrently with the entrance of at least thelater solutions; (2) the original composition ofthe rocks for some reason caused the depositionof minerals from the solutions to favor theigneous-appearing rocks. The specific distribu-tion of alteration types among the rock typesis thought to be more a matter of spatial coin-cidence than a result of specific chemical "af-finities".

It is not within the province of this study tospeculate on the separation of the solutionsfrom the magma or to offer reasons for theseveral distinct stages of mineralization; these

problems will be left to others. Perhaps whenthe entire Bingham or whole Oquirrh Mountainarea has been intensively studied, some of theproblems along these lines will be solved satis-factorily.

GENERAL CONCLUSIONS

The studies of the hydrothermal alterationat Bingham have resulted in the recognitionof seven distinct mineralization periods. Fromthis information interpretations have been maderegarding the temperature and pH of the al-tering solutions of Stages I and III. Perhaps areasonable and acceptable hypothesis could bemade regarding the pH and temperature ofStages II and IV, but similar information re-garding the last three stages would be purelyguesswork.

The geologic and mineralization features ofthe Bingham disseminated copper deposit couldbe compared with other similar deposits, butno new information would be forthcoming sincean adequate comparison has already been madeby Schwartz (1947, p. 323-346), Kerr (1951, p.478-480), Anderson (1950, p. 621-625), andothers. A review of the hydrothermal studies ofporphyry copper deposits by Kerr (1951) andKerr et al. (1950) at Silver Bell and Santa Rita;Anderson (1950) at Bagdad; Gilluly (1946) atAjo; Peterson et al. (1946) at Castle Dome,and Schwartz (1947) at San Manuel shows per-haps as many dissimilarities as similarities tothe hydrothermal features at Bingham.

One may take the similarities of the groupsuch as the argillic (illite and kaolinite atBingham), biotitic, sericitic, and silicic altera-tion and propose that these features will likelybe present to some extent in all "porphyrycopper" deposits and that they might be usedconfidently in further exploration. At Bingham,however, the ore and protore mineralizationextends practically to the full limit of the al-tered area making the alteration target thereessentially no larger than the ore target; there-fore the hydrothermal alteration is of littlepractical use as a guide to the ore finder atBingham. This condition may not be true for alldeposits, and the characteristic alteration abovementioned may be of great value to the orefinder in other areas.



GENERAL CONCLUSIONS 991

The granitization and the feldspar networkhalo associated with the Bingham ore bodypossibly offers a new or different line of thoughtto the student of porphyry copper deposits.

There is a possiblity that some of the dis-similarities of the various deposits mentionedabove may be more apparent than real, dueto the differences in background of the differentworkers and the time and intensity of thestudies.

REFERENCES CITED

Anderson, C. A., 1950, Alteration and metallizationin the Bagdad porphyry copper deposit, Ari-zona: Econ. Geology, vol. 45, p. 609-628.

Backlund, H. G., 1946, The granitization problem:Geol. Mag., vol. 83, p. 105^17.

Bayley, W. S., 1893, The eruptive and sedimentaryrocks on Pigeon Point, Minn.: U. S. Geol.Survey Bull. 109, p. 1-118.

Beeson, J. J., 1916, The disseminated copper oresat Bingham: Am. Inst. Min. Metall. Engineers,Trans., vol. 54, p. 2191-2236.

Boutwell, J. M., 1905, Economic geology of theBingham mining district, Utah: U. S. Geol.Survey Prof. Paper 38, p. 73-390.

Buckley, H. E., 1951, Crystal growth: New York,John Wiley, p. 1-571 (References on dissolu-tion phenomena at end of Chapter 9, p. 336).

Butler, B. S., 1920, Ore deposits of Utah: U. S.Geol. Survey Prof. Paper 111, p. 335-362.

Drescher-Kaden, F. K., 1948, Die Feldspat-QuartzReactiongefuge der Granite und Gneise: Berlin,Springer-Verlag. p. 1-259.

Gilluly, James, 1932, The Stockton and Fairfieldquadrangles, Utah: U. S. Geol. Survey Prof.Paper 173, p. 1-171.

1946, The Ajo mining district: U. S. Geol.Survey Prof. Paper 209, p. 1-112.

Goodspeed, G. E., 1940, Dilation and replacementdikes: Jour. Geology, vol. 48, p. 175-195.

1948a, Xenoliths and skialiths, Am. Jour.Science, vol. 246, p. 515-525.

1948b, Origin of granites, in Gilluly, James

(Chairman), Origin of granite: p. 55-78, Geol.Soc. America Mem. 28, p. 1-139.

Gruner, J. W., 1944, The hydrothermal alterationof feldspars in acid solutions between 300°and 400°C.: Econ. Geology, vol. 39, p. 578-589.

Johannsen, Albert, 1939, A descriptive petrographyof igneous rocks: Chicago, Chicago Univ. Press,vol. 1, p. 1-318.

Keith, Arthur, 1905, Bingham mining district,Utah. Part I, Areal geology: U. S. Geol. SurveyProf. Paper 38, p. 29-70.

Kerr, P. F., 1951, Alteration features at Silver Bell,Arizona: Geol. Soc. America Bull., vol. 62,p. 427-440.

Kerr, P. F., Kulp, J. L., Patterson, Meade, andWright, R. J., 1950, Hydrothermal alterationat Santa Rita, New Mexico: Geol. Soc. AmericaBull., vol. 61, p. 275-348.

Levering, T. S., et al., 1949, Rock alteration as aguide to ore—East Tin tic district, Utah: Econ.Geology Mon. 1. p. 1-64.

Misch, Peter, 1949, Metasomatic granitization ofbatholithic dimensions: Am. Jour. Science,vol. 247, Part I, p. 209-245; Part II, p. 372-406; Part III, p. 673-705.

Peterson, N. P., Gilbert, C. M., and Quick, G. L.,1946, Hydrothermal alteration in the CastleDome copper deposit, Arizona: Econ. Geology,vol. 41, p. 820-840.

Reynolds, D. L., 1947, The association of basicfronts with granitization: Sci. Progress, vol.35, p. 205-219.

Ross, C. S., and Kerr, P. F., 1930, The kaolinminerals: U. S. Geol. Survey Prof. Paper 165-E,p. 151-180.

Schwartz, G. M., 1947, Hydrothermal alterationin the porphyry copper deposits: Econ. Geology,vol. 42, p. 319-352.

Stringham, Bronson, and Taylor, Allen, 1950,Nontronite at Bingham, Utah: Am. Mineralo-gist, vol. 35, p. 1060-1066.

Weinschenk, Ernst, and Johannsen, Albert, 1916,Fundamental principles of petrology: NewYork, McGraw Hill, p. 1-214.

DEPARTMENT OP MINERALOGY, UNIVERSITY OFUTAH, SALT LAKE CITY, UTAH

MANUSCRIPT RECEIVEH BY THE SECRETARY OFTHE SOCIETY, AUGUST 13, 1952



Geological Society of America Bulletin - Rice Earth...

Documents

Transcript of Geological Society of America Bulletin - Rice Earth...