Sulphide deposits-their originand processing

Sulphide deposits-their originand processing

Editorial Committee: P.M.] . Gray, G.]. Bowyer,J.E Castle, D.J . Vaughan and N.A. Warner

~) In~~ The Institution of~U~UU Mining and Metallurgy

I

Published at the office of~ The Institution of Mining and Metallurgy44 Portland Place London W1 EnglandISBN-13: 978-94-010-6851-2 e-ISBN-13: 978-94-009-0809-3DOl: 10.1007/978-94-009-0809-3 The Institution of Mining and Metallurgy 1990

Front cover is a false colour backscatter electron image, obta ined from Cameca Camebax electron microprobe, showing colloform coppersulphides from Cattle Grid orebody, Ml. Gunson, South Australia. Photograph kindly supplied by A.R. Ramsden and D.H. French, CSIRODivision of Exploration Geoscience, Australia

ForewordThe sulphide deposits pose a challenge to every technology that the mineral exploitation industry uses. Their structure,composition and multi-element content could hardly be further from those of the end-products of highly refined single metalswhich must be won from them . The location of the deposits is rarely on the surface, close to end-product markets, and thedisposal of the by-products of processing in environmentally acceptable ways is an unavoidable part of processing. The miningof sulphide depos its is probably more expensive per tonne of product than for any other of the tonnage metals since low unitcost mining methods cannot be used in deep veins or lodes or, if the sulphides are in disseminated form, the deposits are oflow grade.

Research engineers and scientists are striving to relate, in usable scientific terms, the properties of these natural deposits tothe physical and mechanical means of transforming a deposit into an engineering material.

The quantitative characterization of mineral textures has advanced considerably in recent years with the advent of morepowerful equipment. It is now possible to ident ify all minerals present in any but the very finest of microstructures, measure theproportions of each present and obtain an accurate and full analysis of all the elements contained in them.

Even with these measurements we are still some way from being able to forecast the reaction of a natural combination ofsulphide and gangue minerals to, for example, the application of mechanical forces such as are used in ore grinding, the ratesof reaction in contact with an aqueous solution of well-defined and homogeneous characteristics or the behaviour of discreteparticles of that texture when subjected to gravity, magnetic or surface forces in mineral separation.

This gap in rational understanding is not too comfortable for the process engineer to live with and, even if he has grownaccustomed to it in dealing with mineral feed, it is, nevertheless, possibly still at its widest in dealing with the sulphides.Production engineers are accustomed to organizing their data, and thus their manufacturing technology, into rigid programswhich can take command of day-to-day operation almost totally. It is a salutary experience for a mineral processing engineer toobserve a modern motor-car or electronic component production line and realize how far he still is from understanding thebasic parameters that still give him some unwelcome surprises - not infrequently at five o'clock on a Sunday morning!

This volume records some of the progress that has been achieved recently in understanding the processes by which sulphideminerals have been assembled by nature and the way in which the properties of those assemblages influence the metallurgicalprocesses that have been derived by trial and error on a bulk scale to treat sulphides. There is still far to go.

Great advances in the technologies of medicine, electron ics, telecommunications and data processing have followed fromresearch into the fundamentals of the most basic molecular, atomic and electron units. Perhaps basic research into thefundamental forces that have formed the Earth's crust will, in time, provide metallurgistswith the right tools for transformingminerals to metals on a controlled production line basis.

Meanwhile, the 'try-it-and-see' methods on the production or pilot plant and in the laboratory used by metallurgists for thesulphides are becoming more sophisticated and efficient.

The future for the major non-ferrous metals and all their many associated elements from the sulphide deposits is bright. Thechallenge to match the product quality and price demanded by the user is no less than it ever was.

Philip GrayTechnical Editor

July, 1990

ContentsPage Page

Foreword v Methods of recovering platinum-groupmetals from Stillwater Complex ore

Geology, petrology and mineralogy E.G. Baglin 155

Compositional and textural variations of theChina's sulphide deposits - theiroccurrence and treatment

major iron and base-metal sulphide minerals Yu Xingyuan, Li Fenglou and Huang Kaiguo 165James R. Craig and David J. Vaughan

Is flotation the unavoidable way forRio Tinto deposits - geology and geological beneficiating metal sulphide ores?models for their exploration and ore-reserve J. De Cuyper and Ch. Lucion 177evaluationF. Garcia Palomero 17 Concentrate processing and tailings disposal

The massive sulphide deposit of Aznalc6l1ar, Improved model for design of industrialSpain, Iberian Pyrite Belt: review of geology column flotation circuits in sulphideand mineralogy applicationsloseflna Sierra 37 R.A. Alford 189

Precious- and base-metal mineralogy of theComparison of methods of gold and silverextraction from Hellyer pyrite and lead-zinc

Hellyer volcanogenic massive sulphide flotation middlingsdeposit, northwest Tasmania - a study by D.W. Bilston et al. 207electron microprobeA.R. Ramsden et al. 49 Variables in the shear flocculation of galena

T.V. Subrahmanyam et al. 223Mineralogy and petrology of the lead-zinc-

Role of chloride hydrometallurgy incopper sulphide ores of the ViburnumTrend, southeast Missouri, U.S.A., with processing of complex (massive) sulphidespecial emphasis on the mineralogy and oresextraction problems connected with cobalt D.N . Collins and D.S. Flett 233and nickel Evaluation of the CANMET Ferric ChlorideRichard D. Hagn i 73

lead (FCl) process for treatment of complexOre processing and mineralogy base-metal sulphide oresw.j .S. Craigen et al. 255

Principles and practice of sulphide mineral lead production from high-grade galenaflotation concentrates by ferric chloride leaching andR. Herrera-Urbina et al. 87 molten-salt electrolysis

l.E. Murphy and M.M. Wong 271Chelating reagents for flotation of sulphide

Mercury production from sulphidemineralsA. Marabini and M . Barbaro 103 concentrates by cupric chloride leaching

and aqueous electrolysisMineralogy of and potential beneficiation l.E. Murphy, H.G. Henry and l.A . Eisele 283process for the Molai complex sulphide Arsenic fixation and tailings disposal inorebody, Greece METBA's gold projectM . Grossou-Valta et al. 119 M . Stefanakis and A. Kontopoulos 289

Studies of mineral liberation performance in Acid mine drainage from sulphide oresulphide comminution circuits depositsD.M. Weedon, L]. Napier-Munn and CL, Evans 135 Fiona M . Doyle 301

Geology, petrology and mineralogy

Compositional and textural variations of the major iron andbase-metal sulphide mineralsJames R. Craig

D~p~,!ment of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg,Virginia, U.S.A.David J. VaughanDepartment of Geology, The University of Manchester, Manchester, England

ABSTRACf

The crystal structures, stoichiometries, electrical andmagnetic properties, stabilities and mineral textures found inthe metal sulphides are briefly reviewed. Eight of the majoriron and base metal sulphide minerals, chosen because oftheir widespread occurrence (pyrite, pyrrhotite), role as themajor ore mineral of a particular metal (chalcopyrite, spha-lerite, galena, pentlandite), or importance as a carrier of rareor precious metals (arsenopyrite, tetrahedrite) are discussedin greater detail. The crystal structures and physical proper-ties of these minerals are discussed, along with phase rela-tions in the relevant sulphide systems. Particular emphasisis placed on the presentation of data on major and minorelement compositional variations in these minerals and textu-ral features commonly observed in ores containing them,both of which are of crucial importance in their metallurgicalprocessing.

ore deposits. These minerals range from sulphides in whichthe ~rincipal metal extracted is a necessary, and usuallydO~lna.nt, c~nstituent (e.g ., galena or sphalerite) to sul-phides In which the most valued metal is a minor to trace~onstituent (e.g., gold in pyrite or arsenopyrite). SphaleriteIS an excellent example of a sulphide that serves in both ofthe ways noted above; it is mined for its zinc content but itis today virtua~ly the only source of cadmium, an eiementpresent as a minor to trace component. It also serves as asource of gallium, germanium and indium. It is important toremember that the ~ulphide minerals, especially pyrite, havealso ~erved as maJ~r sources of sulphur (and more rarelyselenium and tellunum) as well as metals. A general listing?f the major types of sulphide-rich ore deposits, as presentedin Table 2, documents the abundance of these minerals in the

ore~. In addition, .these major sulphide minerals, especiallypynte and pyrrhotite, occur as common accessory phases ina large variety of rock types .

These same minerals, so valuable as metal sources or ashosts to the minerals that contain the metals, are today alsorecognized as the potential sources of major environmentalhazards, such as acid mine drainage and acid rain. The min-eral of greatest concern in this regard is pyrite also knownas "fool's.gold," becaus~ of its very superficiisimilarity to

~old and ItSa~undant WIdespread occurrence. Indeed, pyriteISoverwhelmingly the most abundant metal sulphide in thecrust of the earth. Its content in sulphide ores may rangefrom only a few pe~cent of th~ sulphid~mass in Sudbury-type and some stratiform massive sulphide deposits, to vir-tually 100%of the sulphide minerals in coal beds.

. The compositional variations of the major sulphideminerals are reasonably well characterized, both as a resultof numerous analyses of natural samples from a wide variety

Table 1. The major iron and base-metal sulfide minerals.

INTRODUCflON

The naturally occurring metal-sulphur compounds, col-lectively referred to as the sulphide ore minerals, serve bothas actual metal sources and as the hosts for many of theworld's precious, base and strategic metals. Distinct,named, sulphide species now number in the hundreds andhave been variously classified on the basis of chemistry andcrystal structure. Despite this large group, most completelylisted in Fleischer (1987), the majority of sulphide-bearingdeposits are dominantly composed of one or more of thesmall group of major sulphide minerals listed in Table 1.This listing of only eight minerals is admittedly arbitrary, butthese minerals constitute more than 95% (and in many cases99%) of the sulphide mineral volume in most sulphide-type

NameIdeal

FormulaPrincipal Elements Derived

(* by-product)pyrite (marcasite)pyrrhotitechalcopyritesphaleritegalenaarsenopyritetetrahedritepentlandite

FeS2Fel _xSCuFeS2ZnSPbSFeAsS

CU12Sb4S13(Fe,Ni)9Sg

Co, Au, SNi, S*CuZn, Cd*, Ge* , Ga* , In*Pb, Ag*, Bi*, Sb*As,AuCu, Sb, Ag, AsNi, Co, Pd*

1

Table 2. Abbreviated listing of the major types of sulfide ore deposits. Thisclassification is modified and much simplified from that of Cox andSinger (1987) .

Type Major Minerals" Metals Extracted Examples

Ores related to mafic and ultramafic inttllsionsSudbury nickel-copper po, pn, py, cpy, violMerensky reef platinum po, pn, cpy

Ni, Cu, Co, PGMNi, Cu, PGM

Sudbury, OntarioMerensky Reef, S. Af.1M Reef, Montana

Pine Creek, California

Ban Ban, AustraliaCarr Fork, UtahBingham Canyon, UtahClimax , ColoradoCamsell River, NWT

Zn,PbCu,AuCU,Mo,Au

Sn, W

py,cpy,go,sph,ttd

Zinc-lead skarnsCopper skarnsPorphyry copperImolybdenumPolymetallic veins

Ores related to felsic inttllsive rocksTin and tunsten skarns py, cass, sph, cpy,

wolfpy, sph, gopy,cpypy, cpy, bn, mbd

Ores related to marine mafic eXttllsiverocksCyprus-type massive py, cpysulfidesBesshi-type massive py, cpy, sph, gosulfides

Cu

CU,Pb,Zn

Cyprus

Japan

Ores related to subaerial felsic to mafic extrusive rocks~e-type epithermal py, sph, gn, cpy, ttd, Cu, Pb, Zn, Ag, Au Creede, Coloradovems aspAlmaden mercury type py, cinn Hg Almaden, Spain

Ores related tomarine felsic tomafic extrusive rocks

Kuroko type py, cpy, gn, sph, asp, Cu, Pb, Zn, Ag, Au Japanttd

Ores in clastic sedimentary rocksQuartz pebble py, uran, Au Au, U Witwatersrand, S. Af.conglomerate gold-uraniumSandstone-hosted lead- py, sph, go Zn, Pb, Cd Laisvall, SwedenzincSedimentary exhalative py, sph, gn, cpy, asp, Cu, Pb, Zn, Au, Ag Sullivan, BClead-zinc (Sedex) ttd, po Tynagh, Ireland

Ores in Carbonate rocksMississippi Valley type Py, go, sph Zn, Pb, Cd, Ga, Ge SE Missouri

Cxystal structures

STRUcruRES AND PROPERTIES OF TIlE MAJORSULPHIDE MINERALS

Several of the common sulphide minerals were amongthe first materials to be studied by X-ray crystallography,and since that time the structures of nearly all mineralogicallysignificant sulphides have been determined. It is possible tocategorize the mineral sulphides into a series of groups based

Abbreviations used as follows: po = pyrrhotite, pn = pentlandite, py = pyrite. cpy = chalcopyrite. viol =vio\arite, cass = cassiterite. sph = sphalerite. wolf = wolframite, go = galena. bn = bornite, mbd = molybdenite,ttd = tetrahedrite, asp= arsenopyrite, cinn = cinnabar, uran = uraninite.

of deposits, and as a result of systematic laboratory investi- ion probe and the proton probe (PIXE), have yielded newgations of the phase equilibria. We can here only present a data that appear to give much moreaccurate measurements offew relevant phase diagrams; for additional information the the sulphide mineral compositions, especially for minor andreader is referred to Barton and Skinner (1979) and Vaughan trace elements. Table 3 presents a listing of the maximumand Craig (1978,1990). Table 3 contains a tabulation of the contents of many elements in the major sulphides consideredmaximum concentrations of numerous elements in the com- in this paper. Sources used were limited to those employingmon sulphide minerals. Analytical data for the common sul- modem analytical techniques that should have largelyphide minerals is abundant but widely scattered and largely avoided contamination by mineral inclusions.redundant in displaying minor amounts of a variety of ele-ments. The large number of analyses results from theseminerals being abundant and from the desire of investigatorsto ascertain the distribution of valued elements so that theycan be effectively extracted. There have been relatively fewextensive compilations of sulphide mineral compositionalranges. The largest (Fleischer, 1955) is now 35 years oldand contains data derived primarily using analytical methodsthat indiscriminately included elements from mineral inclu-sions as well as from the mineral being studied. The devel-opment of the electron microprobe, which allows analysis ofareas as small as a few micrometers, and more recently the

2

(3) Ordered omission, e.g., monoclinic pyrrhotite(Fe7S8) is derived from the NiAs structured FeS byremoval of Fe atoms leaving holes (vacancies) thatare ordered (Fig. 2C).

(4) Distortion, e.g., the troilite form of FeS is simply adistortion of the parent NiAs structure form (Fig.2C).

are also, commonly, other minerals that have structuresbased on these "parent" structures and that can be thought ofas being "derived" from them. The relationship between aderivative structure and the parent structure may involve:

(I) Ordered substitution, e.g., the structure of chal-copyrite (CuFeS2) is derived from sphalerite (ZnS)by alternate replacement of Zn atoms by Cu and Feresulting in an enlarged (tetragonal) unit cell (seeFig. 2A). As also shown in Figure 2A, stannite(Cu2FeSnS4) results from further ordered substitu-tion of half of the Fe atoms in CuFeS2 by Sn.

(2) A stuffed derivative, e.g., talnakhite (Cu9Fe8S 16) isderived from chalcopyrite by the occupation ofadditional, normalIy empty cavities in the structure(Fig.2B).

on major structure types, or having key structural features incOrJ?11lon, as shown in Table 4 (modified after Vaughan andCraig, 1978). In many cases, these are the classic structuresof crystalline solids such as the rocksalt structure of thegalena group (Fig. lA), the sphalerite and wurtzite forms ofZnS (Fig. IB,C), or the nickel arsenide structure (Fig. lC).The disulphides are characterized by the presence of dianion(S-S, S-As, As-As, etc.) units in the structure; as well as thepyrite structure in which FeS6 octahedral units share comersalong the c-axis direction, there is the marcasite form ofFeS2 in which octahedra share edges to form chains oflinked octahedra along the c-axis (Fig. 10). The structuresof FeAs2 (loellingite) and FeAsS (arsenopyrite) are variantsof the marcasite structure that have, respectively, shorter oralternate long and short metal-metal distances across theshared octahedral edge (see Fig. ID). A few sulphides suchas molybdenite or covellite (Fig. IF) have layer structures,and a small number exhibit structures best characterized ascontaining rings or chains of linked atoms (e.g ., realgar,AsS). A diverse group of sulphides, referred to by Vaughanand Craig (1978) as the metal-excess group, is composed ofan unusual and diverse set of structures well illustrated bythe important example of the mineral pentlanditeNi,Fe)9S8, see Fig. 10).

As can be seen from Table 4, in many of these groups anumber of minerals share the actual structure type, but there

Table 3. Maximum concentratio-ns (hippmunless otherwise indicated) of numerous elements in the eight major sulfide mineralsdiscussed in the text All data are from studies employing techniques such as electron microprobe or PIXE that are bothsensitive and capable of avoiding contamination by mineral inclusions. References for the data are given in parenthesesafter the data : Full references are given at the end of text.

Element pyrite pyrrhotite chalcopyrite sphalerite galena arsenopyrite pentlandite tetrahedrite SS

References(1) Basu(1984)(6) Cabri (1989)(11) Fralick(1989)(16) Kieft(1990)(21) Nikitin (1929)(26) Scheubel (1988)(31) Burke (1980)

11.69% (17) 158 (3) 5.7% (I)essential essential 27.6% (17) essential essential 13.6% (29)415 (3) 9.9% (33) 52 .6% (20) 4.2% (8)719 (3) 0.21% (24) 4.3% (33) essential 3.5% (8)

essential 1.3% (26) 2000 (26) essential2570 (4) essential 1.19% (26) 12.7% (8)

0.16% (14)0.14% (14) 1.3% (28)

essential 30 .1% (8)180 (3) 4383 (5) 396 (3) 3681 (3) 682 (4) 41.1 % (13)

-

7 (10) 0.8% (28)80 (4)

5200 (4) 86 (4)- 1.42% (4)

1685 (3) 1.62% (19) 308 (5) 3.1% (10) 14.77% (27) 55 .0% (22)2.84% (7) 899 (3) 11.9% (23)

1085 (5) 10.4% (31) -2.34% (15) 286 (3) 7 (10) 14% (8)

900 (24) 7900 (24) 37.1 % (8)200 (10) 26.4% (18)

(2) Boldryeva (1973)(7) Craig(1983)(12) Harris (1984)(17) Kissin (1986)(22) Paar (1978)(27) Scott(1973)(32) Kovalenker (1980)

VCrMnFeCoNiCuZnGaGeAsSe'hMoRuRhPdAgCdInSnSbTeWPtAuHgrtPbBi

32 (11)11 (11)

essentialmajor ssmajor ss

40 (11)3334 (5)

8% (9)644 (3)37(11)

0.12% (5)

0.41% (5)

110 (30)17(11)51 (11)

1.8 (30) 7.7 (30) 3.4 (30) 1.6% (25) 2.26% (32)24% (8)

400 (10) 2.6% (28)0.38% (26) essential 6.3% (24)0.58% (24) 6.2% (10) 19.7% (2)

(3) Brill (1989) (4) Cabri (1984) (5) Cabri (1985)(8) DoeIter (1926) (9) Fleet (1989) (10) Foord(1989)(13) Johan(1982) (14) JOOan (1988) (15) Kase(1987)(18) Kovalenker (1986) (19) Loucks(1988) (20) Misra(1973)(23) Pattrlck(1985) (24) Pearson (1988) (25) Picot (1987)(28) Spiridonov (1988) (29) Godorikov (1973) (30) Cook (1990)(33) Klemm(1965)

3

Table 4. Sulfide structural groups.

1) THE DISULFIDE GROUPPyrite Structure Marcasite Structure Arsenopyrite StructureFeS2 pyrite \FeS2 marcasite FeAsS arsenopyriteCOS2 cattierite! FeSbS gudmundite

derived by As/S ordered substitution(Co,Fe)AsS cobaltite(Ni,Co,Fe)AsS gersdorffite (I)

2) THE GALENA GROUPPbS galenaa-MnS alabandite

Loellin2ite StructureFeAs2 loellingiteCoAS2 saffloriteNiAs2 rammelsbergite

3) THE SPHALERITE GROUPSphalerite Structure .. derived by ordered substitution - stuffed derivativesB-ZnS sphalerite CuFeS2 chalcopyrite Cu9FegS16 talnakhiteCdS hawleyite CU2FeSnS4 stannite CU9Fe9S16 mooihoekiteHg(S,Se) metacinnabar CU2ZnSnS4 kesterite CU4FeSSg haycockite

4) THE WURTZITE GROUPWurtzite Strucmre .. composite structure derivatives - ?further derivativesa-ZnS wurtzite CUFe2S3 cubanite CU2Fe2SnS6 hexastanniteCdS greenockite ?AgFe2S3argentopyrite

derived by ordered substitutionCU3AsS4 enargite

5) THE NICKEL ARSENIDE GROUPNiAs Structure .. distorted derivativesNiAs niccolite FeS troiliteNiSb breithauptite CoAs modderite

6) THE THIOSPINEL GROUPC03S4linnaeiteFeNi2S4 violariteCuC02S4 carrollite

7) THE LAYER SULFIDES GROUPMolybdenite Structure Tetragonal PbO StructureMoS2 molybdenite (Fe,Co,Ni,Cr,Cu)l+xSWS2 tungstenite mackinawite

.. ordered ommission derivativesFe7Sgmonoclinic pyrrhotiteFe9SlO, Fell S12 hexagonalpyrrhotite, etc.?

Covellite StructureCUS covellite-Cu3FeS4 idaite

Digenite Structure ---...derived by ordered substitutionCU9SS digenite CU7S4 anilite

9) RING OR CHAIN STRUCTURE GROUPStibnite Structure Realgar StructureSb2S3 stibnite AS4S4 realgarBi2S3 bismuthinite

8) METAL EXCESS GROUPPentlandite Structure(Ni,Fe)9Sg pentlanditeC09Sgcobalt pentlandite

Argentite StructureAg2S argentite

Chalcocite StructureCU2S chalcocite"" ?derivative

CU1.96S djurleiteNickel Sulfide StructuresNiS milleriteNi3S2heazlewoodite

Cinnabar StructureHgS cinnabar

In some cases, the relationships involved are more com-plex, as, for example, in certain of the sulphosalt minerals"where the resulting structure is composite and made up ofslabs or units of a parent structure (or structures) arranged insome ordered fashion (an important example of a sulphosaltmineral, tetrahedrite, is further discussed below). It is alsouseful in certain cases (such as the stuffed derivatives or ex-

Defined as minerals with a general formula AmTnXp inwhich common elements are A:Ag.CuPb; T:As,Sb,Bi; X:S. Theycontain pyramidal TS3 groups in the structure.

4

amples of ordered omission mentioned above) to regard themineral sulphide structures as a relatively "rigid" sulphurlattice framework from which metal atoms can be removed,or to which metal atoms may be added.

Stoichiometty

Many metal sulphides show evidence that the elementsthat comprise them are not combined in a simple wholenumber ratios, i.e., they exhibit non-stoichiometty.

In certain cases, the extent of deviation from a simpleratio is considerable. For example, the pyrrhotites are

(Bl~m and Kroger, 1956). Galena is apparently stable overa widerange of values of aSz, and at high aSz it has leadvacancies, whereas at low aSz there are sulphur vacancies.It has also been suggested that certain "polymorphic pairs"of minerals exhibit this type of non-stoichiometry, and thesereports conflict with the rigid definition of polymorphism.For e~ample, Scott and Barnes (1972) have suggested thatwurtzrte, formerly regarded as the high-temperature(> 1020C) polymorph of ZnS, is actually sulphur-deficientrelative to sphalerite. Electrical measurements indicate thatzinc vacancies occur in sphalerite and sulphur vacancies inwurtzite with a total range in non-stoichiometry of about 1atomic %. Sphalerite-wurtzite equilibria would, therefore,be a function of aSZ as well as temperature and pressure.However, another pair of minerals to which such reasoningmight be applied are pyrite and marcasite, and here Tossell etal. (1981) have offered an interpretation that is based on thereaction mechanism by which marcasite is formed(commonly in acid solution) as a metastable species relativeto pyrite. An important consequence of non-stoichiometry isthe effect that it has on physical (electrical, optical, hardness)and chemical properties, although the latter have been lesswell studied. Preliminary studies (Vaughan et al., 1987)suggest that rates of surface alteration, that would in turnhave a marked influence on such extraction methods asflotation, are strongly influenced by the stoichiometry ofsulphides.

Electrical and magnetic properties

As well as exhibiting a richness and diversity in struc-tural chemistry, the metal sulphides also show a tremendousrange of electrical and magnetic behavior. As Table 5 indi-cates, whereas such non-transition metal sulphides as spha-lerite and galena are diamagnetic, diamagnetism is also ex-hibited by pyrite and also marcasite (the other FeSz poly-morph). Substitution of iron for zinc in sphalerite leads toparamagnetic behavior, and many transition metal sulphidesshow various forms of magnetic ordering at lower tempera-tures including antiferromagnetism (e.g., chalcopyrite),ferromagnetism (e.g., cattierite) and ferrimagnetism (e.g.,monoclinic pyrrhotite, although "hexagonal" pyrrhotites areantiferromagnetic) . Other transition metal sulphides , whichare metallic conductors, exhibit the weak temperature-inde-pendent paramagnetism that is known as Pauli paramag-netism. As well as the metallic conductivity occurring inmany sulphides of diverse magnetic character, numeroussulphides are semiconductors, and a smaller number (e.g.,pure sphalerite) are insulators. Among semiconducting sul-phides, both intrinsic and impurity (or extrinsic) conductionmechanisms occur, and conduction via electrons~ orvia holes~ is common. Such variations are often aconsequence of very minor impurities being present , or ofslight non-stoichiometry (e.g., galena, PbS, exhibiting atotal variation in composition of 0.1 atomic %, shows p-typeconductivity in lead-deficient samples and n-type conductiv-ity in sulphur-deficient samples).Sulphide mineral stability

Much work has been done in the last 50 years to estab-lish the stability relations of sulphide minerals in terms ofvariables that include temperature, pressure, compositionand the activities of various components. Much of the data,whether derived from synthesis experiments or thermochem-ical measu:ements, are reviewed by Vaughan and Craig(1978), Mills (1974), Kostov and Mincheeva-Stefanova(1~8q and sources therein . Although it is not the primaryobjective of the present paper to review these data indeedthat ~ould not be possible in a relatively short contributioncertain aspects are worthy of discussion. '

c

--

r-, .: r-,

"" ----"""y

I-- L.- .....

r~ L.......,... -;j ~--.....

Table 5. Magnetic and electric properties of some major sulfide minerals(after Vaughan and Craig, 1978).

Mineral Species Magnetic Properties Electrical Properties

Sphalerite (ZnS)"Iron sphalerite" (Zn,Fe)SGalena (PbS)Pyrite (FeS2)Cattierite (COS2>Chalcopyrite (CuFeS2)

Covellite (CUS)(Monoclinic) Pyrrhotite(Fe7Ss)Carrollite (CUC02S4)Pentlandite (Ni,Fe>9Ss

diamagneticparamagneticdiamagneticdiamagneticferromagnetic (Tc = 110 K)antiferromagnetic(TN = 823 K)diamagnetic(?)ferrimagnetic (Tc = 573 K)

Pauli paramagneticPauliparamagnetic

insulator (Eg-3.7 eV)semiconductor (Eg-O.5 eV for -12 at % Fe)semiconductor (n- and p-type, Eg-OAI eV)semiconductor (n- and p-type, Eg-O.9 eV)metallicsemiconductor (n-type, Eg-O.5 eV)

metallicmetallic

metallicmetallic

Eg = band or "energy"gap, Tc = Curie temperature; TN= N6:1temperature.

+ (Fe,Ni)9Sgpentlandite

Fel-xSpyrrhotite

=(Fe,Ni)Smonosulphidesolid solution

textures produced by exsolution or unmixing, as exemplifiedby the Ni-Fe-S system (Fig. 4). Such changes occur be-cause at elevated temperatures in many sulphide systemsthere are extensive fields of solid solution that shrink withfalling temperature. In the case of the Ni-Fe-S system, it isthe breakdown of the so-called monosulphide solid solutionand segregation of pentlandite according to the reaction

OSl OSJ e si r

that is central to understanding the assemblages and texturesin the sulphide nickel ores. Such processes may lead tocompletely separate grains of the two phases being formedon exsolution, or the two phases may be intergrown as laths,blebs, etc., often with a clearly-defined crystallographicrelationship. The crystal structure of the two phases willpartly dictate the kind of texture that forms, but other crucialfactors are related to the monosulphide solid solution bulk"starting" composition and the cooling history. The impor-tance of kinetic factors is clear from such studies as havebeen undertaken (e.g., Kelly and Vaughan, 1983), althoughrelatively little work has been done in this field. The major-ity of binary, ternary and many quaternary sulphide systemshave been studied as regards temperature-composition rela-tions, yielding evidence of the maximum stability tempera-tures of phases, solid solution limits, and coexistence ofphases under the equilibrium conditions pertaining in suchexperiments.

B....

c o~.

~~~TalAMl'IIite

.....

~o o c f.() s.... s as

a===}

b C",""",n..

Figure 2. Parent and derivative crystal structures in thesulphide minerals: (a) the sphalerite (ZnS) structure with thechalcopyrite (CuFeS2) and stannite CU2FeSnS4 structures;(b) the sphalerite and chalcopyrite unit cells with an octahe-dron of metals outlined within which may be an additionalmetal ion in the minerals talnakhite (Cu9FeSS16), mooi-hoekite (Cu9Fe9S 16) and haycockite (CU4FeSSS), thearrangeinent of additional occupied metal sites being asshown (also shown are the dimensions of the parent spha-lerite cell in A); (c) the niccolite unit cell of high temperatureFeS that has vacancies in place of Fe atoms in monoclinicpyrrhotite (Fe7Ss) that are ordered as shown in the diagramthat has vacancies represented by squares (and only Fe atomlayers shown); also shown in a projection onto the basalplane are the distortions that occur in the troilite modificationof FeS.

In terms both of the interpretation of assemblages andtextures in the context of understanding the genesis of theores and from the point of view of mineral processing,temperature-composition relations in the subsolidus regionare of the most value. This is because many sulphideminerals undergo changes in the solid state down to rela-tively low temperatures compared with silicates and oxides(Fig. 3). These changes give rise to a wealth of intergrowth

6

Figure 4. (A) The condensed phase relations in the Fe-Ni-Ssystem at 650C. Note the absence of pentlandite and themonosulphide s?lid solution (m.s:s) that spans the systemfrom Fel_xS to NI1_xS. (B) A pornon of the Fe-Ni-S systemshowing the compositional limits of the monosulphide solidsolution (mss) in (A) at 600, 500, 400, and 300"C.Pentlandite flames, as those shown in Fig . 17, exsolve from!he mss as the sulphur-poor boundary retreats during cool-mg.

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..

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Figure 5. Sulphidation curves for several sulphides as afunction of temperature.

The stable phase relations may also be defined in termsof the activities of components and, in regard to the sulphideminerals, it is the activity of sulphur (aSV that is of the mostimportance. Thus, a "petrogenetic grid" commonly used topresent sulphide mineral relationships is that in which aS2 isplotted against temperature (actually log aS2 versus Iffbecause sulphidation boundaries then plot as straight lines inmany cases) as shown in Figure 5. Such diagrams show theprogressive development of assemblages through sulphida-tion from metal to monosulphide to (where appropriate)disulphide.

Another important type of phase diagram is that in whichthe activity of sulphur is plotted against that of oxygen (atconstant temperature). Such aS2-a02 diagrams may bepre-sented for one or several cations. In Figure 6 a plot of thistype is shown for the iron sulphides and oxides at 2S"C. Inthis case, of course, the value of the diagram lies in the in-terpretation of relationships and textures that may arise in theoxidative alteration of sulphide minerals during weatheringor some analogous process.

Figure 6. A log aS2 versus log a02 plot of iron SUlphidesand oxides at 25C contoured in terms of log aS02.SULPHIDE ORETEXlURES: PRIMARY ANDSECONDARY DEVELOPMENT

The major sulphide ore minerals occur not only in a widevariety of deposits (fable 2) but also in an amazing array oftextures. The textures, in effect the shapes and spatial distri-butions of mineral grains, are the products of the deposi-

zo

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~::I Sp~aJerite,E .... pynte,~ 'arsenopyrite

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Figure 3. Relative rates of equilibration of various commonsulphide minerals as a function of temperature. (AfterBarton, 1970.)

Go'-

7

tional and post-depositional histories of the minerals and thedeposits in which they occur. The terminology customarilyused to describe textures is sununarized below:

Primary - textures reflecting the form and distribu-tion of the minerals as they were origi-nally deposited

Secondary - textures that have formed as the result ofany post-depositional process

Hypogene - referring to the original minerals formedin a deposit

Supergene - referring to secondary minerals andtextures that have formed as a result ofmeteoric weathering.

It is not possible to herein exhaustively present or dis-cuss the many varieties of textures exhibited by the majorsulphide minerals. We shall attempt instead to review someof the most common textures and refer the reader to Craigand Vaughan (1981) and Craig (1990) for additional discus-sion and illustrations.

Primary textures of the major sulphides range widely',depending upon the individual mineral phase or phasespresent (e.g., pyrite that typically forms crystals, Fig. 7A,versus pyrrhotite that is generally anhedral or interstitial toother phases), the environment of deposition (e.g., openspace filling, Fig. 7B, versus replacement of preexistingphases, Fig . 7C), and chemical reactions (of multipledepositing solutions and between solutions and preexistingminerals).

Secondary textures of these minerals modify or replaceprimary textures by processes such as post-depositionaldeformation (e.g., brecciation, Fig . 8A, or flow), recrystal-lization (e.g., to form crystals or polycrystalline aggregates,

Fig . 8B), exsolution (Fig. 9A), or supergene alteration(e.g., to form alteration rims, Fig . 9B, or to differentiallycorrode coexisting phases).

~nalysis of the textures of the major sulphide mineralsprovlde~ not only much insight into the origin and history ofa de~slt or occurrence, ~ut also into the exploitability andP:Otenn~ problems of mineral processing. Thus, the very

flOe-gr~un~ nature of an ore (e.g., McArthur River,Australia, FIg. lOA) may result in its being very difficult toecon?mically ~eparate into clean concentrates by standardflotation techmq.ues. In con~~t, ores that are coarse grained

~s a !esult .of pnmary deposition or metamorphic recrystal-l~zanon (FIg. lOB) are relatively readily separated by flota-non . The.tex~s are ~so critical in determining which ores

m~y readily yield precious metals during cyanide leaching(FIg. I ~A) a~d thos~ ~at ar~ refra~tory (e.g., contain goldas fine inclusions within pynte grams where it remains un-reacted with cyanide solutions, Fig. liB).

The differences in sulphide mineral structures (bothcrys~l and elecl!0nic structures) result in a broad range inphysical properties such as density, thermal stability, hard-ness, reflectance, electrical conductivity, etc. These varia-nons ~s~lt i~ signifi~ant diff~rences in the rates and degreesof ~u.lhbration dw:ng coohng from original depositional

conditl~ns, and dunng post-.depositional metamorphism orw~thenng. !bus, among rmxtures of the conunon sulphideminerals, pynte, arsenopyrite and sphalerite are the most re-

fracto~ a~~ the sulph~~es most likely to retain the evidenceof their original de~SltlOn conditions, whereas chalcopyrite,

gal~na, and ~yrrhotlte most readily adjust their compositionsdunng c~l~ng (Barton and Skinner, 1979). Similarly,under conditions of moderate dynamic metamorphism, pyrite

8

Figure 7. (A) Euhedral pyrite crystals in a matrix of inter-stitial anhedral pyrrhotite (field of view = 1.1 mm). (B)Euhedral 7 mm sphalerite crystal formed atop a mass ofsaddle dolomite crystals in an open vug in the centralTennessee ores. (C) Fractured pyrite crystal undergoingreplacement by chalcopyrite as a result of reaction of thepyrite with copper-rich fluids.

remains a rigid phase retaining primary structure and com-position, whe~s p~ases such as pyrrhotite, chalcopyrite,an~ galena~y YIeldand deform plastically. This differ-ential behavior under stress has been examined for several ofthe major sulphide minerals by Kelly and Clark (1975).

MAJOR IRON AND BASE METAL SULPHIDES

In the more detailed discussion that follows the mostimpo~t of the sulphides are considered in turn ~d aspectsof their s~ctures, chemistries and textural relationships in

o~s considered. A total of eight sulphides are considered inthis way, chosen for their widespread occurrence (pyrite,

BFigure 8. (A) Cataclastic texture of pyrite resulting frompost-depositional deformation of sulphides (field of view =0.6 mm). (B) Recrystallized pyrite exhibiting typical poly-granular texture with 120 interstitial angles (field of view =0.6 mm),

pyrrhotite), role as the major ore mineral of a particular metal(chalcopyrite, sphalerite, galena, pentlandite), or importanceas a carrier of rare or precious metals (arsenopyrite, tetra-hedrite).

Pyrite (ideal formula FeS2) is the most abundant of allsulphide minerals, occurring as a major phase in many sul-phide ore deposits and as an accessory mine~ in~y o.therores and rocks. Pyrite has rarely been nuned for Its noncontent but has served as an ore of nickel and cobalt, both ofwhich may substitute for iron in the structure. !he pyrite ofthe Central African Copperbelt serves as a major source ofthe world's cobalt supply. Before the development of theFrasch technique for sulphur extraction, pyrite se~ed as. themajor source of sulphur; today, pynte from lbenan mme.sremains an important sulphur source for Europe. Because itis nearly ubiquitous and serves as a host for many otherminerals containing valued metals, pyrite has been muchanalyzed . The maximum concentrations of many elementsheld within pyrite are given in Table 3.

The crystal structure of pyrite is illustrated in Figure 10and, as already noted, is distinguished by the presence of S-S dian ion units. Pyrite is a diamagnetic semiconductor(Table 4) that may exhibit p- or n-type conduction mecha-nisms. Marcasite (ideal formula also FeS2) has generallybeen regarded as the dimorph of pyrite in which ~-S dianionunits again occur, and Fe is octahedrally coordinated ~o ~.However, the mode of linkage of FeS6 octahedral umts ISdifferent to that found in pyrite (Fig. 10).

9

Figure 9. (A) Exsolution lamellae of chalcopyrite frombornite (field of view = 1.1 mm) . (B) Veins of covelliteformed on chalcopyrite as a result of supergene alteration(field of view = 1.1 mm).

Figure 10. (A) Fine-grain~ , unmetam~rphosed pyrite-sphalerite ore from McArthur River, A.uStra!Ia. (B) Coarselyrecrystallized, metamorphosed sphalerite-pyrrhotite ore fromlorna, Norway. Both samples have been photographed atthe same scale (field of view =0.6 mm).

Figure II. (A) Gold grain along pyrite boundary whereit would likely be dissolved by cyanide leach solutions. (B)A gold grain completely enclosed in pyrite where it wouldlikely be protected from reaction from leaching solutions(field of view = 0.6 mm).

Pyrite, isostructural with cattierite (COS2) and vaesite(NiS2) exhibits temperature-dependent extensive solid solu-tions toward both of these phases. Generally, however, thevery -low geochemical abundances of cobalt (25 ppm) andnickel (75 ppm) relative to that of iron (56,000 ppm) resultsin the formation of pyrites that contain no more than a fewparts per million of cobalt and nickel. Pyrite is an importantsink for cobalt and nickel and, occasionally, individualgrowth zones have concentrated the cobalt and nickel to formbravoites. Because pyrite is often the dominant sulphide inmany ores and must be processed to extract other intimatelyintergrown ore minerals, much effort has been directed todetermine what valuable elements the pyrite itself maycontain (see Table 3). Many pyrites have been thought tocontain gold within their lattices, but the highest documentedvalue is 110 ppm (Cook and Chryssoulis, 1990).

Pyrite occurs in a very broad range of textures dependingupon mode of origin, abundance relative to other mineralsand post-depositional history. Although it can form in poly-crystalline porous colloform bands at low temperatures,pyrite is best noted for its very common appearance as c~bes(Fig. 7A) or pyritohedra. Its great tendency to form intoeuhedral crystals and its very great hardness (greater than allother sulphides and many silicates and oxides) results in ittypically appearing as cubes, even in matrices that haveundergone extensive deformation. Under conditions ofthermal metamorphism, polycrystalline masses of pyriterecrystallize to typical annealed textures displaying 120'interstitial grain boundaries. Despite its hardness and refrac-tory nature, pyrite, like all sulphides, is unstable in the oxi-dizing conditions of the earth's surface and most surfacewaters. The oxidation of pyrite, and sometimes coexisting

Figure 12. Diagrammatic presentation of the structuresof Fe7Sg, FegSIO and FellSl2. These are the 4C, 5C and6C pyrrhotites.pyrrhotite or marcasite, leads to the ~eneration of ~tronglyacid waters that have been responsible for considerabledamage to stream and vegetation. The solid phase resultingfrom pyrite oxidation commonly leads to the ~evelopment ofgoethite (FeO-OH) pseudomorphous after pynte cubes.

Pyrrhotite

Pyrrhotite (often simplistically referred to by the formulaFel-xS) is a major component in many important base metaldeposits. It has been mined for its iron content. The sn:uc-ture of "pyrrhotite" is based upon t~e hexagona! NIAs(niccolite) structure (Fig. IE) , b~t this st:ru~ture I~ onlystable at higher temperatures (5'150 C). StOlc~lOmetrlC ~~Sundergoes distortion at lower temperatures to give ~e troilitesuperstructure modification (see Fig: ~C). A v~~ unportantfeature of the pyrrhotites is the ability to omit tron. ~tomsfrom the structure, leaving vacancies up to a composlnon of-Fe7Sg. At higher temperatures (-100-300 C), thesevacancies are disordered, but at lower temperatures theyundergo ordering so as to result in a series of superstructuresbased on the parent NiAs-type structure. The b.est knownand characterized of these superstructures IS that of"monoclinic pyrrhotite" (ideal composition Fe7Sg) illustratedin Figure 2C. Here, the vacancies are ordered onto alternateiron atom layers parallel to the basal plane and alt~mate ro~sin these layers. The resulting structure has a urut cell w!thfour times the c-axis dimension of the parent structure Withsome collapse around the vacancies causing a monoclinicdistortion. The situation in the "intermediate pyrrhotites" atlow temperatures is more uncertain, but i~ seems likely. thatvarious other vacancy-ordered superlattices are possible,centered on such compositions as FegS 10, Fe lOS11. andFej j S12, and with 5C, llC and 6C superstructures as Illus-trated in Figure 12 and further discussed by ~aughan andCraig (1990). An important.aspect of the ch~ffi1~try of theseintermediate pyrrhotites anses from the ~nencs o~ suchvacancy ordering processes. Because the differences .m freeenergies between various vacancy OI~ered structures ISverysmall, a variety of metastable alternative .structur~s and fine-scale intergrowths can be formed on c

Atomic percent

The sphalerite lattice and the similarity to the size of theZn2+ ion permits the ready substitution of a number of diva-lent transition metal ions for the zinc. Indeed, naturally-occurring sphalerites always contain metals (dominantly

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Figure 13. (A) Phase relations in the central portion ofthe CU~F~-S system; (a) schematic relations at 300C.Abbreviations: c~, chalcopyrite; dj, djurleite; di, digenite; al,anilite; cv, ~ovelhte; bn, bornite ; id, idaite; cp, chalcopyrite;~' talnakhite; mh, mooihoekite; he, haycockite; cb, cuban-ite; mp?, monocli~ic pyrrhotite; hpo, hexagonal pyrrhotite;tr, trOllu7; py, pynte; ISS, Intermediate solid solution. (B)Schematic low-temperature phase relations in the Cu-Fe-S~ys!em (after Craig and Scott, 1974). The shaded arrowIn~cates the genera;I trend of copper-iron-sulphide mineralsdunng the weathenng process. The removal of iron andsubsequently copper, results in the formation of comPosi-!I_ons_ncher In sulphur.Sphalerite

Sphalerite (ideal formula, ZnS) is the only ore mineral ofzinc, although it commonly contains other metals replacingZn (most notably Fe, but also Cd, Co, Ni, Cu, Ga, Ge).The sphalerite or "zinc blende" structure is one of the fun-damental structure-types found in inorganic solids (Fig. IB).However, the other major structure type found in ZnS, thehexagonal structure of wurtzite, is also of fundamental im-portance (Fig . l C). The structural relationship betweensphalerite and wurtzite is such that, although both contain Znand S in tetrahedral coordination, linking of the tetrahedra(via comers) in sphalerite results in a cubic unit cell, whereasin wurtzite the cell is hexagonal . It is also a relationship thatcan be viewed in terms of close-packing of the sulphuranions in layers parallel to the basal plane and the stacking ofthese layers to give cubic or hexagonal symmetry.Variations in stacking sequence can give rise to differentpolytypes and a submicroscopic interlayering of sphaleriteand wurtzite structure types. As already noted, someauthors have proposed that a compositional difference existsbetween sphalerite and wurtzite, although others wouldregard wurtzite formed at low temperatures as a metastablephase resulting from the mechanism of nucleation of ZnS insolution (Vaughan and Craig, 1990).

room te~perature. This property is of value in geophysicalprosJ>e

Figure 14. (A) Plot of the FeS content of sphaleritecoexisting with pyrite and hexagonal pyrrhotite at 0, 2.5, 5,7.5, and 10 kbar at temperatures from 300 to 700C. (AfterScott, 1976.) (B) "Chalcopyrite disease" consisting of blebs(actually rods) of chalcopyrite dispersed with sphalerite. Seetext; field of view =0.6 mm.lamellae of matildite in the host galena. Silver, commonlysought in galena-rich ores, is not taken into the galenastructure except as a coupled substitution where the silverand bismuth atom are coupled to substitute for two leadatoms .

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iron) in addition to zinc. Iron, virtually always present,ranges from trace quantities to greater than 27 wt % and isthe primary cause of color in sphalerite. Sphalerites fromMississippi Valley-type deposits nearly always contain less 700!han 2% iron (commonly

The crystal structure of CUl2Sb4S13 tetrahedrite isshown in Figure 16. In each half-unit cell, ten M1and twoMOl atoms occupy six 4-fold and six 3-fold coordinationsites, the Mill atoms occupy the equivalent of a tetrahedralsite in sphalerite but are bonded to only three sulphur atomsresulting in a void in the structure; 12 S atoms are 4-coordi-nate and the other single S atom is 6-coordinate. Numerousstudies have been undertaken of the nature and extent of thevarious substitutions that occur in the tetrahedrites. Forexample, using a variety of spectroscopic techniques,Charnock et al. (1989) were able to show that silver goesinto trigonal rather than tetrahedral sites, cadmium into tetra-hedral sites and iron mainly into tetrahedral sites, although innatural tetrahedrltes of low silver content, it may enter thetrigonal site.

The tetrahedrite -tennantite series exhibits a broad rangeof solid solutions (Table 3; Johnson et al., 1986). Mosttetrahedrites are mined for their silver contents, and numer-ous studies have demonstrated that the silver contents corre-late with antimony contents. It is clear that silver and arsenichave little tolerance for each other in the tetrahedritestructure . However, as antimony content rises, the capacityfor silver substitution also rises.

Figure 16. (A) A half-unit cell of tetrahedriteCU12Sb4S13. (B) Coordination polyhedra in tetrahedrite(Ref 21).

Pentlandite

Pentlandite Ni,Fe)9Ss) is the major ore mineral ofnickel. A complete solid solution occurs between pentlanditeand cobalt pentlandite (CQ9SS). The crystal structure ofpentlandite is unique and contains "cube clusters" of tetrahe-drally coordinated metals with very short metal -metaldistances along the cube edges; the additional (ninth) metalatom in pentlandite occurs in octahedral coordination. Thecrystal chemistry and mineral chemistry of pentlanditeexhibits a number of interesting features. One of these isthat it undergoes substantial volume contraction on cooling,evidenced in natural samples by the presence of numerousfractures. It is also important to note that pentlandite almostalways forms as a subsolidus phase in the Fe-Ni-S system(see Fig. 4).

Pentlandite formation occurs primarily as a result of ex-solution in the form of so-called "flames" in nickeliferouspyrrhotite. Exsolution occurs such that pentlandite (Ill),(110) and (112) planes are parallel to the (001), (110) and(100) planes ofthe pyrrhotite (Francis et al., 1976). The ex-solution occurs as a consequence of the shrinkage of the(Fe,Nih-xS monosulphide solid solution (mss) field duringcooling. The mss phase that spans the Fe-Ni-S system athigh temperatures (Fig. 4A) beings to thin (Fig. 4B) as tem-perature falls . Consequently, pentlandite exsolves as the

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Figure 17. (A) Pentlandite "flames" fonned as a resu1~ ofcrystallographically-oriented exsolution from msson cooling(field of view = 0.6 mm). (B) Chain-like masses of pent-landite between grains of pyrrhotite. The characteristicfractures in the pentlandite are believed to result from thehigh coefficient of thermal expansion of the pentlanditeduring cooling.sulphur-poor boundary of the mss retreats towards moresulphur-rich compositions.

Kelly and Vaughan (1983) have experimentally tracedthe development of the pentlandite textures that result fromexsolution. Initial exsolution at the highest temperatures re-sults in the formation of discrete blebs of pentlandite alongthe grain boundaries of the mss. During contin.uedexs?lu-tion the blebs grow and coalesce to form connnuous nmsbetween the mss grains. At lower temperatures, where ratesof diffusion and exsolution are far slower, rims no longerform but rather there are developed the fine blades andflam~s shown in Figure 17A. No doubt, some of the earliestformed lamellae coalesce into the rims, but because thepentlandite exsolution continues during cooling, new flamescontinue to be formed as prior ones disappear. The pe~tlandite chains are characteristically highly fractured (Fig.17B), apparently because pentlandite has a coefficient ofthermal expansion two to 10 times grea.ter th!'" tha! of.~esulphides (e.g., pyrrhotite and chalcopynte) With which It IScommonly associated (Rajamani and ~ewitt, 1975). Thegreater shrinkage suffered by pentlandite results 10 greaterstress and subsequent cracking.

Pentlandite has traditionally been considered an oremineral of nickel that yields by-product cobalt. In the past20 years, however, pentlandites have been found that ~S?carry significant quantities of silver (Scott and Gasparrini,1973) and ruthenium and rhodium (Cabri et al., 1984) asshown in Table 3.

14

ACKNOWLEDGMENTS

The authors acknowledge support from NERC GrantGR3/6823 for collaborative research on iron sulphides andthe help of a large group of mineralogists and ore petrolo-gists who have stimulated their curiosity toward, andeducated them about, sulphides. JRC also acknowledgesUSBM allotment grant G1194151 to Virginia Tech that helpssupport his sulphide work, and DJV acknowledges SERCGrant GR/F/62841 that supports work on copper andcopper-iron sulphides.

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Goble, R.J. (1980) Copper sulfides from Alberta:yanowite, CU9SS, and spionkopite, Cu3QS28. CanadianMineralogist, 18,511-518.

Goble, R.J. and Robinson, G. (1980) Geerite, CU1.60S, anew copper sulfide from Dekalb Township, New York.Canadian Mineralogist, 18,519-523.

Godorikov, A.A. and Il'yasheva, N.A. (1973) Chemicalcharacteristics of fablores. Int'l Geology Review, 15,1413-1422.

Harris, D.C., Cabri , L.J. and Nobiling, R. (1984) Silver-bearing chalcopyrite, a principal source of silver in thelzok Lake massive-sulfide deposit: confirmation byelectron- and proton-microprobe analyses. CanadianMineralogist, 22, 493-498.

Hutchison, M.N. and Scott, S.D. (1981) Sphalerite geo-barometry in the system Cu-Fe-Zn-S. EconomicGeology, 76, 143-153.

Johan, Z. (1988) Indium and germanium in the.s~cture .ofsphalerite: an example of coupled substitution Withcopper. Mineralogy and Petrology, 39, 211-229.

15

Johan, Z., Picot, P. and Ruhlmann, F. (1982) Parageneticevolution of the uranium mineralization rich in seleniumat Chameane (Puy-de-DOme), France: chameanite, gef-froyite and giraudite, three new selenides of Cu, Fe, Agand As. Tscher. Mineral. Petrogr. Mitt, 29,151-167.

Johnson, N.E., Craig, J.R . and Rimstidt, J.D. (1986)Compositional trends in tetrahedrite. CanadianMineralogist, 24, 385-397.

Kase, K. (1987) Tin-bearing chalcopyrite from the Izurnovein, Toyoha Mine, Hokkaido, Japan. CanadianMineralogist, 25, 9-13.

Kelly, D.P. and Vaughan, D.J. (1983) Pyrrhotite-pent-landite ore textures: a mechanistic approach.Mineralogical Magazine, 47, 453-463.

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Kieft, K. and Damman, A.H. (1990) Indium-bearing chal-copyrite and sphalerite from the GAsborn area, WestBergslagen, Central Sweden. Mineralogical Magazine,54, 109-112.

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Kostov, I. and Mincheeva-Stefanova, J. (1981) SulphideMinerals: Crystal Chemistry, Paragenesis andSystematics. Bulgarian Academy of Sciences.

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16

Rio Tinto deposits - geology and geological models fortheir exploration and ore-reserve evaluationF. Garda PalomeroRio Tinto Minera, S.A. Minas de Riotinto, Hue/va, Spain

SYNOPSISThis naper describes the geology of the

. ,Rio Tinto area, and the morphological ancgenetic relationships between differentmineralisation types (stockwork, massivesulphide and gossan) in each of thethree deposits (San Dionisio, SanAntonio and Cerro Colorado). Thegeological relationships and spatialdistribution of the ore mineralisation,and associated alteration minerals,define a mineralisation model relatedto shallow submarine volcanic activity.

~lineralisation begins with precipi-tation of sulphides in fissures in thevolcanic hostrocks at estimated depthsof 400 metres below the seafloor, andtemperatures of 4002C. Sulphideprecipitation from ascending hydro-thermal fluids increases towards theseafloor, and leads to the developmentof mineralised stockworks as thetemperatures decrease to 1002C at thetop of the volcanic pile. The remainingsulphur and metals are deposited on theseafloor in massive sulphide lenses.The distribution of minerals inhaloes to both types of mineralisationare related to cooling of the orefluidsas they move outwards from the sourcearea.

After the cessation of volcanicactivity in the area flysch sedimen-tation continued followed by tectonismassociated ,dth the Hercynian orogeny.Subsequent erosion and strongweathering of the deposits, during thelate Tertiary period, led to theformation of extensive gossan deposits.

The geological model of metaldistribution in the sulphide and gossandeposits has been successfully usedas a guide in exploration for new

17

reserves, and also as a control onsubsequent reserve evaluation usinggeostatistical methods.

INTRODUCTION

The Riotinto sulphide deposits arelocated in the Huelva province ofsouthern Spain, 90 km northwest ofSeville and ?O km northeast of Huelva(Figure 1).

~uning activity in the area datesfrom pre-Roman times, with a peak ofactivity between the third and firstcenturies BC (Rothenberg et aI, 198?)1,as illustrated by the large amount ofslags (6 million tonnes) derived fromtreatment of gold and. silver ore.

After a long interval during whichonly sporadic mining occurred,production of copper from the zonesof secondary sulphide enrichment, withgrades of 3-5% Cu, began in middleof the last century. Towards theend of the last century, as this materialwas worked out, activity switched tothe exploitation of the primary massivesulphides for sulphur production. Withthe drop in pyrite priCes, at the endof the 1960's, new mining operationswere started for the production ofcopper from the low grade sulphidestockworks (enriched by supergeneactivity near the surface), andproduction of gold and silver from t hesurfical oxide cap (Gossan) formedby the weathering of underlyingsulphides.

GENERAL GEOLOGY

On a regional scale, the Riotintodeposits lie in the Iberian PyriteBelt, an upper Palaeozoic unit

con~aining volcanics (andesites andrhyolites) at the base of the Carbonife-rous, with whd ch the abundant sulphidedeposits are associeted both spatiallyand genetically (Figure 1).

Riotinto anticline wh er e it displaysintense chloritic alteration associatedwith the mineralisation process. Itahowa a fairly uniform thickness of400-500 metres in the Riotinto area.

Fig.- 1 GEOLOGY AND MAIN DEPOS ITS OF THE IBER IAN P YRI TE BELT

~~ Post-Paleozoic sed i rnen ts ~~r-===-1c.:==.::J

Volcani c 12v21

Devon ian sla tes

~ LOW2r Pal~oZ\)ic~ Gran ite

Figure 2 shows the geology of theRiotinto area, whe r-e sulphide minerali-sation is associated with the volcanicrocks of .t h e Pdotinto anticline (GarciaPalomero, 1974)2, being overlain bybarren rocks of Cul~ facies. The mainlithostratigra?hic units are describedbriefly b eLow,

Devonian

Thi s unit outcrops to the south of theRiotinto area, in the core of a mainlyvolcanic anticlinal structure. Itconsists mainly of grey argillaceousslates with some quartzitic levels nearthe top. The first evidence for Devono-Carboniferous volcanic activity, datedas Tournaisian ( Schermerhorn, 1971)3,occurs near the top of this unit,

Basic Complex

This unit conformably overlies theDevonian, and consists of alternationsof basic pyroclastics, vesicular andesi-tes, slates, fine grained acid pyro-clastics and a variety of breccias.

It outcrops to the south of the area,as well as in the core of the mai n

18

Acid Complex

This unit consists of very homogeneousacid volcanics (dacitic to rhyoliticin composition), with predominantpyroclastics, and locally abundantbreccias, autobreccias and lavas.These rocks have a porphyritic texturewith phenocrysts of quartz and feldsparin a microcrystalline matrix of thesame composition, which shows a clearschistosity and frequent alteration(chlorite, haematite, and sericite).

The thickness of the acid volcanichorizon is generally fairly uniform andsimilar to that of the Basic Complex(400-500m), although in the area ofthe mineralisation it diminishesconsiderably thinning to only 30-50metres in the centre of Corta Atalaya(Figure 2).

Transition Series

This is typically a heterogeneouspyroclastic level with abundant lateraland vertical changes in both grainsize and composition of the constituent

Fig .- 2

30:","" -' __ -: _"1 .' as MVALLE

GEOLOGY OF THE RIOTINTO AREA-..-.,===,,,,,,'000 m I ~

~ Devcni c n slc te sI-+'++1 Basic co rn p l exE=,:=:,:'-J S2dim2ntary-Pyroclast ic hor izon

~ A cid cornp te xb,- ,..~- \I Tr ans it i 0 n SUi2S

r:::-: .I Massiv2 s ul phi d esc=J Culm

~'In situ' Gossan

~ Ir cns por ted GossanCOlcio Pa lom et a F. "7 '

lithologies (polygenic breccias, litho-crystalline tuffs, finely crystallinetuffs, chloritised-haematised-serici-tised tuffs, massive sulphides, etc.).These represent the lithologies formedat the end of a phase of volcanicactivity with the transition to theflysch sedimentation of the Culm. Thethickness of this unit is very variable(5- 50 metres).

The most notable feature of thisseries is that mineralised massivesulphide lenses, of probable syn-sedi-mentary origin, occur within it.

Culm

This unit consists of a monotonousseries of turbiditic sediments, althoughwithin it different units can sometimesbe distinguished on the basis offossiliferous horizons, and differencesin the relative proportions of gre~~ackeand shale.

Tertiary

This is represented by subhorizontaldeposits, 1-10 metres in thickness,of a limonite cemented mixture offragments of volcanic rocks, slates,and iron oxides. These are laterallyfairly restricted, and occur at specific

19

elevations (400 metres, 350 metres, etc)coinciding with possible terracesassociated with the evolution of theriver systems.

All the units described, exceptfor the Tertiary, are affected by theHercynian orogeny. This resulted invarious phases of folding which formedthe main ea at--we at; folds, and anassociated schistosity (dipping northat 75-85 degrees), as we .LL as faultinG"and thrusting. A weak metamorphism ofgreenschist facies is also developed.

Geological studies of the unitsdescribed above have identified somefeatures whic h indicate the palaeogeo-graphic conditions prevailing at thetime of mineral isation. Some of thesephenomena are specific to the Rio Tintoarea, an~ may be related to the pr e s en ceof mi ne r a l i s a t i on , including thefollowing:1. The predominance of pyr o c l a s t i crock s wi t h respect to l avas, in theBasic Complex of the Pd o Tinto area.The associated l ava s contain abundan tvesicles of ~uartz and sulphides.These features disappear a way fromthe Pdo Tinto area.2. The decrease in thickness ofthe Acid Volcanic unit close to themineralisation (from 500 to 100 metres,'.d t h a minimum of 30 metres). A parallel

HI NERALI 3ATI ON

decreaso in the t hickness of t ho.Jyroclastic e.Lemerrts in t he transitionseries i s a l s o noted.3. At the e a stern and "{estern limit .::of the area, coincident ' :i th strongaccwnulc.:tions of pyroclastic r-cc.co :i..nt he acid volcanic level, abundantfragments of oxidised lapilli tuffand coarser pyroclastics are seen,suggesting very aha.Ll.ow "later volcani sm.J!.. At the base of the Culm frequentslump features are seen, and the b Qsalunit of the Culm is absent close tothe mineralised zone.

These features seem to indicate avery shallow marine e nvi r onme ntcoinciding wi t h the present-dayanticline, with marginal shallo\'!volcanic centres. This difference inrelief wa s eliminated during depositionof the Culm.

l-lin e r a l i s a t i on in the Riotinto areaoccurs in three separate zones, situatedin different parts of the Riotintoanticline. Although locally inter-connected, each zone belongs to aseparate g en e t i c unit, sharing a comnlong e n e t i c model.

Th e three mineralised zones are:Sa n Dionisio, PlaneS/ San Antonio,Cerro Colorado/Filon Norte/Filon Sur.Each of these zones is made up ofthree different types of mineralisation,namely;

SUlphide stockNorks in the volcanicr-o c k s ,St rat i f o r m mass~ve sulphide lensessituated on top of the stoek"lorkmineralisation, ' a n d intercalatedtith other rocks of t h e transitionseries,Gossan (iron oxide cap), formedl:l y ";e a t h e r i ng of outcropping massivesulphide and stock":ork mineralisation,a nd r estric'cecl to "lithin 70 metresof the surface.

Th e total amount of sulphide minera-lisa tion in the ~otinto area suggestst hat t h e area wa s the locus of one oft he most intense mi n e r a l isi ng episodesdocum e nt e d to date.

Th e i mportance of the mineralisingevent i s indicated by the followingstuti stics:

Sa n Dioni s io

sulphides:1900-2000 HtS 6%, Cu 0.155';,Zn 0.15%, Ph 0.06%,Ag 7ppm, Au 0.07Ppm

stock'-lorkAmountGr a de s

Ma s siv e 3ulphides:Amount 500-600 MtGrades S 50% cu 1%,

Zn 2~~, Pb 1%,Ag 30ppm, Au 0.3ppm

Stockwork mineralisation in San Dionisioconsists of irregular veins containingpyrite, chalcopyrite, galena, sphalerite,magnetite, quartz, chlorite, calcite a ndbarite, cutting the volcanic host rock s.

The top of the stockwork marks theb a se of the ma s s i v e sulphides, and itextends for up to 400 metres stratigra-phically downwar-ds from thi s contact,down to the Basic Volcanic complex.The minera i.isation takes the form ofa n originally s emi v e r t i cal chimney witha s emi c i r c ul a r cross section of about600- 700 metres in diameter. Foldings u b s e qu e n t to f o r mat i o n of the deposith as t ilted this chimney through 90

degree s~ so that it now lies in as ub - h or i zon t a l p o s i t i on , elongatednorth- south.

Stockwork

This zone is located on t he south fl ankof the Riotinto anticline, associatedwi t h a secondary synclinal structurewhich plunges east a t 30-3 5 degrees(Figure 2).

Figure 3 shows a transverse N- Ssection through the centre of thisdeposit, illustrating the interrela-tionship bet~een the different types ofmineralisation, and lithologies present.The different types of mineralisationare described beLow,

Of the total sulphide mineralisationformed, at least 400 mi l l i on tonnes ofmassive sulphides and 400 million tonnesof stockwork mineralisation have beenaffected by weathering and erosion.Part of this remains in the form ofoxide caps (Gossan) totalling in excessof 100l-It ,dthin whi c h the concentrationof less soluble metals (Au, Ag, Pb, Da,30, etc.) has been enriched by the"'e a t h e r i n g process.

The three different mineralised zonesare described separately be.l.ow,

4 km24 00- 500 m.

J.ii n e ral i s e d areaVertical ext e n t

20

sN 100

N

Stockwork. a nd .M assiv~ Sulph id ~

Fi g. - 3

STRUC TURE OF THE SAN DION ISI O DE POSI T

Ga.rcia PoiomHO F. 19 &3

Py PYR ITE

CPy CH A LC OP YRITE

o O UA R T Z .

C hi C H L OR IT E

Mog MAGNET ITE

S ph S PHA LE R I TE

G GALENA

B o BAR ITE'

So SER IC IT E

CuR : - _

Pta- Zn

lO Omis .-- -==o

4 7.20l!o .

II

.-l -:;Stockw!>rk . i ~

U.;.~-r':":";~-t-----8 a si c I)b c k s

--.

S lat~s

S~co n da rySu I ph j d2

Se r i c i t i c Rocks --\Chi - O-"t-CPy_'Sp h _ G - _---"\-_

W~ l l def i ne d contactStockwork- Massi v2 O r e

Se r i c it i cAc id Rock s

Variations in vein density, thickness,texture, orientation, mineralogicalcomposition, wall rock alteration, andrelationship with the massive sulphides,allow the definition of a spatial zona-tion within the stockwork mineral~sation.This reflects the physio-chemicalconditions affecting the hydrothermalsolutions as they ascended towards theseafloor. The zonation is"made up ofthe following three units:- Pyrite Chimney (stockwork nucleus),

consisting of very intense pyritemineralisation, with local chalcopy-rite, in which original wal l r oc klithologies are almost indistin-guishable. No clear contact withthe massive sulphides can be deter-mined (Figure 3).Three Dimen sional St oc kwor k (typicalat.ockwor-k ) , which is widely developedspatially, and surrounds the pyritechimney. It is made up of a threedimensional network of sulphideveins which do not cut the overlyingmassive sulphides, with a clearcontact between the two units.

'.Vithin this type of stockwork avaried mineral paragenesis is seen,along with intense alteration (quartz,chlorite, and sericite) of the hostrocks.

The mineral paragenesis present isindicated in Figures 3 and 4.

Sericite Envelope, consisting ofweak mineralisation in the outerfringes of the stoc~work zone. Itis restricted to the acid volcanicrocks within which strong serici-tisation occurs contiguous with thetop of the three dimensional stockwork.

Hassive Sulphides

!'lassive sulphide mineralisation in SanDionisio forms a large lens of sulphidessitting directly on top of the stockwork.This has a more irregular shape andgreater lateral extent than theunderlying stockwork.

Figure 3 shows the folded natureof this lens, and its stratigraphicl oc a t i on between the acid volcanicrocks below, and the Culm above. Itsspatial relationship with respect tothe stockwork is also ahown ,

Within the massive sulphi de bo dy,the accumulation of sulphides isfairly monotonous with very few otherminerals, apart from some thinpyroclastic horizons near the footwalland hangd.ngwa.lL contacts, and a weakinterstitial siliceous matrix which

21

Pyroc las t ic s

Acid

Volcan ics

Pyro cl a s I i cte ve t

Basiccornp t e x

/'/

..

c lJ.o lJ. ..c

o

Se r i c i t eEnve lope

F ig .-' I NF ER RED PARAGE NETIC FRAMEWORK OF THE SAN DION ISIO DEPOS IT

BEFORE FOLDING

Gn 'cia Pa to rnu o F "eo

.. CRYS TALlI l lE PYR ITElJ. COLL () IOA ~ CHA LCOPY RITEo "'AG NETI TE CHLORI TEo SEO'C ITES QUARTZ

_ SPHA LER ITE.GALEHA

never exceeds 2p by weight. The sulphideminerals are totally recrystallised andahow some relict textures of a primarysedimentary character (colloform,framboidal, etc.) (Read, 1967)4,particularly ldthin the pyrite. Lessstable sulphide minerals, such .aschalco~yrite, galena, tetrahedrite,stannite, and bornite, are recrystalli-sed in intergranular spaces bet"'een thepyrite and sphalerite crystals, or inmicrofractures ,dthill them. ilespitethe monotono~s character of the sulphiG3body, t wo well defined units can bec i s t i ngui s h e d on the basis of degreeof recrystallation, and mineral parage-nesis. These are as follows:a) Ga sul unit. This shows occasional

sul~hicle banding (pyrite-sphalerite-;;;al e n a ) , is nredLum-cfLne grained, andcontains the bulk of the base metals inthe massive sulphides. ':li t h i n thiswlit, metal distribution appears to berelated to structure and/or the under-lying stock~ork types.

Variations in paragenesis areillustrated in Figure 3, wi t h theureferential occurrence of pyrite-

~halcopyrite Lmmedd.a t.e.Ly above thepyrite chimney, and pyrite-chalcopyrite-

22

sphalerite-galena above the threedimensional stock",'ork. Beyond thelimits of the stockwork mineralisationpyrite with sphalerite-galena occurs.b) Upper unit. This is made up ofvery fine grained pyrite, wi t h a lowbase metal content, and occasionaldisseminated silica especially tiovrardsthe top. Local irregular concentrationsof galena and chalcopyrite are relatedto remobilisation associated withHercynian folding, which also causedstrong remobilisation of quartz.

The massive sulphides are overlainby shales of the Culm series, a sindicated in Figure 3. At the laterallimits of the massive sulphide minera-lisation interfingering ,-:itil theTransition Series occurs. This seriesthins over the centre of the sulphidelens where it is represented by a 1-3metre thick pyroclastic level.

Gossan

As indicated in Figure 3, the nearsurface part of the stockwork andmassive sulphide mineralisation ahowsintense we a t.her-Lng , Oxidation of the

sulphide minerals and leaching of thevolcanic rocks has led to the formationof an Oxide Cap (Gossan), which extendsdown to a max:imm depth of 70 metres belowthe surface in the most permeable partsof the orebody (within the massivesulphide lens).

This deposit is located at the easternend of the Riotinto anticline, wherethe anticlinal axis closes and plungeseastwards at 30-35 degrees beneath theCulm (Figure 2).

The same types of mineralisation,as in San Dionisio, occur although somedifferences in overall size and mineralparagenesis patterns are seen.

Stockwork

The Planes stockwork, down to thelimits of evaluation by drilling(Figure 5), is of reduced size andsomewhat different characteristics tothat of San Dionisio. Its structurein terms of mineral paragenesis, a n drelationship with the massive sulphides,is shown in Figure 5.

w

As in San Dionisio, this stockwork isa gain made up of three different units,namely:- Pyrite Chimney, formed by semimassivesulphide mineralisation, generallywithout base me t a l s , and lacking a clearcontact with the overlying massivesulphides. In this transition zone intostratiform massive sulphides, at thetop of the pyrite chimney, a strongconcentration of copper occurs.- Three Dimensional stockwork ispoorly developed here, and althoughlead-zinc sulphides are dominant overcopper, overal~ base metal contentsare low. The volcanic host rocksshow less alteration than in SanDionisio.- Sericite Envelope, is fairly welldeveloped with several small minera-lised zones containing sphalerite andgalena.

~s in San Dionisio, both the threedimensiona l stockwork, and the sericiteenvelope, show a clear contact withthe overlying massive sulphides withno evidence for s t.ockv-or-k veins cuttingthe mas sd,ve sulphides.

E

fig. - 5 LONGITUDINAL SEC TION OF THE PLANES SAN AN TON IO

&~'-"':..~ Culm,

1 ~ / ~ : ." 1 Hor izon Tr ansi t i onc==J Mass ive Sulphi de 1::::::::1 Ba sic Comp lczx

~ A ci d Complczx ~ P y r it ic - pipe

23

~ Stockwork

GarC i a PQt,),... crc r 1983

ha ssive 3u1i:>hidea

The massive sulphiu~s in this depositare planar in form, wi t h averagethicknesses of 10-15 metres. Theycontain enhanceu b r ade s of co~per, aswell as several mi n o r metals such as ~~ ,Au , As , and 3n . They appear to form

t~.O separate lenses 1d th some LsoLa't.e dl i n k s between the b ,o (Fibure 5). Thel en s directl y overl ying- the stock work,known as the Planes deposit, has beenalmost tota.Li.y wo r-k e d out. The otherlens, known as the .:Ja n An t on i o deposit,occurs dov.ndd.p , further a wa y from theat.ockwor-k , It VIa s di s c o ve r e d in the1960 I S and remains une xpLo Lt.ed ,

The Planes lens, occurs directlyabove the pyrite chimney and shows noclear contact ,d t h it. It consistsmainly of pyrite ,:i t h a bu n dan t chalco-.:>yrite. Apa r-t, from the mass i v e sulphidesthe transition series is only representedby only 2-5 me t re s of tuffs.

The 3a n An t on i o len s occurs outsidethe limits of the s t o ckwo rk in theunderlying rocks, ,dth a g r a du a l develop-men t of a thick Transition 3eriesbet.we en it and t he Culm e a s c war-ds , Anincrease in t ypicaL s lwuping textures,new mineral para geneses, and additionallithologies wi t h i n the pyroclasticrocks, are a l s o seen going eastwards.

Al t hough the avera;;;;e lens thicknessi s Lo wer- than usual at 10-15 me t.r-e s ,b .o parallel e a a t.wesf zones showsulphide thicknesses of up to 5U metres.' ;i t hi n these zones a g r e a t variety ofsep a r a t e sulphide l en ses , as well asintercalated p yroclastic horizons, arer ecognised. These include compactpolymeta l lic massi v e sulphides, banded-brecciated len s es, pyritic lenses, andrhyolite-sulphide breccia horizons. Inthe s e t 1,'0 zones the l e n s e s and mineralp a r-ag e n e s e s s now different l a t e r al andvertical distriqutions, suggesting thatthey occup y t.wo troughs, or paralleldepressions, predating the sulphideformation. The s e depressions we r-efi l led wi t h sylphide an d pyroclasticmaterial, di s placed from higherelevations, during movemerrt s of thesea f loor c aused by seismic or volcanicact ivity.

The tlistribution of the paragenesesahown in Figure 5, together with thechanges in the nature of the sulphidesand pyroclastics go i ng e astwards,

24

suggest that the sulphides originatedfrom the pyrite chimney Loc a t.ecl , to tihewe st., upslope from the 3an Ant o n i o l e ns(Garcia Palomero F, 19 80)5.

Cerro Colorado/Filon No r t e / Fi l o n Su r

This deposit, located a t the centreof the Riotinto anticline, to the eastof the Zduardo fault (Figure 2), is themost extensive mineralised zone in thearea. It consists of s mall remnantsof ma s s i v e sulphide (Filon Norte andFilon Su r ) connected by a large oxidecap and extensive at.ockvor-k mineralisa-tion in the underlying volcanics. Figu-re 6 ahows a NE- 3,-' s e c t i o n through thedeposit, indicating the relationshipbetween mineralisation t ypes an d theirhostrocks.

The ma i n g e ologi cal f e atures of thi szone are described be.l ow, alt h o u g h its houl d be noted that t h e "!ide s prea deffects of weatherin~, a nd min i nga c t i v i ty . developed in t his a rea makeit di fficul t to s tudy s ome of the mo s t

import~nt Ge o l ogi c a l asy c c t s .

:.s ah o wn in Figure 6, the stocl.:1:or' :.,lineralisation is very extensi v e , e:d~ 3nding over lOOOm from north to s o u th,and 2000m from east to ~'e st . It i sde v e l o p e d over a vertical thickn e s s of400-500m, affecting the t09 of the Ba s i cvolcanics and all the Ac i. d Complex.

.~ g e ol og i c a l r econstruction of theuneroded remnant s define s a s imil a rs t r u c t u r e to the s t ock"orks a Lr-e a d ydescribed, ~ :ith the p re s en c e of a p y r i techimney, t hree climens iona l stoc!,,:or ~( inchloriti sed volcanics, a n d a s e r i c i t i cenvelop e. ;\ b e n e r a 1. de s c r i p t i on of thes'cock wor-k md.ne r-a Ld a a t.Lo n i s not repe atec:,but some characteri stics particul ar tothis zone a r e l ist e d be.Lov i

wi ele spatial extent,l ol'l intensity of mi neral i s a t i o n inthe three dimensional at.oclcv-or-k (4- 6 ~~),l o c al i s a t i on a t t h e ed:;;e of a possihl erhyolite dome , inferred f'r-o m thethinnins of the a c i d volcanics fromnorth to south,,Jidespread disseminated coppermineralisation,1'!i de s p r e a d remobili sed copper minera-lisation due to its structural positionat an anticlinal crest,