Geochemistry of Tertiary epithemml Ag-Pb-Zn veins

in Taxco, Guerrero, Mexico

Sheila E. Hynes Department of Earth Sciences

Lamentian University Sudbury, Ontario

Thesis submitted to the Faculty of Graduate Studies in partial fiiifillment of the requirements for

the degree of Master of Science

O Copyright by Sheila E. Hynes, 1999

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Abstract

The El Cobre, Esperanza and Hueyapa veins are northwest trending epithermal

Ag-Pb-Zn fissure-filling veins mined by Industrial Minera Mexico S.A., in Taxco,

Guerrero. Mexico. The veios are hosted by a Mesozoic sequence of rocks including the

Mexcala S hale, the Taxco Schist and the Morelos Limestone.

It is proposed that, rather than invoking a magmatic source for the hydrothermal

fluids and the metals, the veins were deposited fiom heated meteoric water that leached

met* and minerals from the Taxco Schist and the Mexcala Shale. The heat source was

probably buried felsic magma that produced Tertiary rhyolite flows to the north of the

area. Tectonic and hydraulic fiacturing provided conduits for the hydrothermal fluids.

Using 6"0 and g3c data, temperatures denved nom fluid inclusions and REE

abundances in carbonate minerais, it is suggested that the El Cobre vein represents the

early stage of ore formation that is dominated by the dissolution of carbonates in the

Mexcala Shale and the Taxco Schist. The Esperanza vein formed during intermediate

stages of fluid evolution when carbonate and silicate minerais reacted with the

hydro thermal fluids. Circulating fluids, leac hing predominantly silicate minerals fio m

the host rocks, deposited the Hueyapa vein during late, waning stages of the system.

This mode1 is supported by fluid inclusion data, Fe content in sphalerite and Ag

content in galena which show that the lower levels of the El Cobre vein have the highest

temperature (289°C). highest Ag content in galena and highest Fe content in sphderite.

This suggests that the earliest ore-forming fluids originated at depth in the El Cobre vein.

The temperature decreases upwards so that the upper levels of the vein have temperatures

similar to those of the Esperanza and Hueyapa veins.

Table of Contents

Page

Appendix 1

Appendix 2

Appendix 3

Appendix 4

Appendix 5

Appendix 6

Appendix 7

Appendix 8

Appendix 9

Appendix 10

Page

Figure 26

Figure 27

Figure 28

Figure 29

Figure 30

Figure 3 1

Figure 32

Figure 33

Figure 34

Figure 35

Figure 36

Figure 37

Figure 38

Figure 39

Ho mogenization temperatures of fluid inclusions in quartz samples fiom the El Cobre vein on levels O, 1, 2.5.7 and 9 - - - - - 73

~ " c ~ D ~ ~ - ~ ~ ~ o ~ ~ ~ ~ plots for cartmnate kom tbe ~ a x c o ~chist, Morelos Limestone, Mexcah Shale and ore samples__ _----_-__---- 76

613~pDBl-618~nMoW> values of the El Cobre. Esperanza and Hueyapa fluids calculated at 230°C fkom homogenization temperatures ---- 78

Log for trivalent REE ions substituthg into calcite as a function of ionic radius in 6-fold CO-ordination ----_--------~-------------------- 90

Stability constants of sulphate, fluoride, chloride and hydroxide complexes for lia)+ and LU^+ at 2 S T and 300°C 92

Schematic depiction of the evolution of the hydrothermal fluid with respect to REE distributions in the El Cobre v e i ~ _------------.----- 95

REE distributions in plagioclase as a function of atornic number------ 97

vui

List of Plates

Plate 1

Plate 2

Plate 3

Plate 4

Plate 5

Plate 6

Plate 7

Plate 8

Plate 9

Plate 10

Plate 1 1

Plate 12

Plate 13

Page

Sphalerite comding pyrite. Note chalcopyrite inclusions in sphalerite (reflected light, diagonal of photo = 1.49 rnm) ---_----- 17

AcicuIar bournonite forming dong galena grain boundaries (reflected light, diagonal of photo = 0.70 mm)--,,_---- 20

Irregular galena grains with quartz resulting from later quartz vehhg (refleçted light, diagonal of photo = 0.7 1 mm) ----------------- 25

Boumonite and argentian tetrahedrite replacing chalcopyrite and galena (?) (reflected iight. diagond of photo = 1.49 IKUD) ------~-_.- 28

Plate 14

Plate 15

ut

Page

List of Tables

Page

Table 1 Metal contents of &ale fiom the Taxço Mining District, Guanajuato, Valenciana mine, Leon and Zacatecas ------------------ 37

Table 3

Table 4

Isotopic enrichment factors for carbon species in hydro thermai fluids 81

Acknowledgements . . - . . . - - -- - - - - . - - . ----

1 wouId like to acknowiedge the management of Industrial Minera Mexico S.A.

de C.V. for giving permission to access the properties and coilect material nom their

operations in Taxco, Mexico. In particular, special thanks are extended to hg. Sergio

Ramùez P., Director of Mining Operatioas and Exploration, and hg. Rene Orozco G.,

Manager at Unidad Taxco.

1 am indebted to hg. Sergio Gana B., Exploration Geobgist, hg. Miguel Angel

Vazquez G., Chief Mine Geologist, and hgs. Luciana Ayala E. and Alejandro Garcia G.,

Mine Geologists, for their assistance in collecting specimens, guiding us through the

three mines and providing geological information.

1 am sincerely gratefûl to Dr. Robert Whitehead and Dr. J i . Davies for givïng me

the opponunity to participate in this research. The academic and life experiences 1

gained fiom working with them on this project have been invaluable to me. 1 also thank

hem for their bbopen-door policy", for critically reading my thesis on numerous

occasions, and providing helphil suggestions and comments. Their insight into various

topics and their unending encouragement were crucial to the successful completion of

this thesis and the maintenance of my sanity.

Valuable technical support and cornpu ter facilities were pro vided b y Lorraine

Dupuis and Dr. Harold Gibson.

1.0 Introduction

The town of Taxco de Alarcon, Mexico is known for its world class, low

sulphidation epithermal silver-lead-zinc deposits and has au extensive minhg history

dâting back to the 1500's. shortly after the Spanish conquest. In 1942, the mines in the

Taxco area were owned and operated by the American Smeiting and Renning Company

(ASARCO Inc.) but by 1974 the property was nationalized and came under the control of

Industrial Minera Mexico. S A de C.V. (IMMSA). The current operational mes on the

property are the Guerrero, the Remedios and the San Antonio. Proved and probable

reserves for the Taxco Mining District in 1982 were 15,146,440 tomes grading 98 g/t Ag,

1.10 % Pb and 3.40 % Zn (IMMSA, 1982 in, Osterman, 1984). Reserves and total ore

mined fiom 1950 to 1982 stood at 25,650,260 tonnes grading 0.30 g/t Au, 142.8 g/t Ag,

2.06 % Pb, 4.63 % Zn and 0.06 % Cu (IMMSA, 1982 in, Osterman, 1984).

The most economic deposits in Taxco are fissure-fjlling veins. Other styles of

mineralization include replacement veins, mantos. breccia chimneys and stockworks.

Mesozoic schist, shale and limestone of fiysch origin host the deposits. The mineraIi7iition

is of epithermal origin and fonned at temperatures of 200°C to 300°C which aïIowed

remobiiïzation of met& in the Taxco Schist and Phyiiite (Salas, 1991).

1.1 Location and A m s s

The Taxco Minhg District is situated between 189 1 'N and 1835'N and 9!1°32' W

and 9g039'W, covering an area of 200 W. just south of the town of Taxco in northern

Guerrero, Mexico. Taxco is accessible via Federal Highway 95 and located 163 km

south of Mexico City and 258 km north of Acapulco (Fig. 1). The area lies within the

Sierra del Sur Province in the transition zone between the Central Mexican Plateau to the

north and the Rio Balsas Valley to the south. The altitude in the Taxco MinUig District

ranges between 1 350 m and 2 410 m above sea kveL

1.2 Objectives of Thesis

The objectives of this thesis are as foilows: - to determine the variations in rare earth ekment values and patterns as well as the 613c-6% values in calcite fkom Merent veins

to deveiop a mode1 of the mineralization process which might be usehl as an exploration tool by geologists at IMMSA de C.V., Unidad Taxco.

In order to achieve these objectives, samples of the host rocks were collected in

Febmary, 1997 (Fig. 1) dong with ore minerals and calcite fiom the El Cobre, Esperanza

and Hueyapa veins, Hueyapa manto and veins intersected in drill holes 2)- Thse

samples were analysed for rare earth elements, carbon and oxygen isotopes and trace

elemenu. In addition. a limited number of fluid inclusions in carbonate and quartz gangue

yielded data on homogenization temperatures. Mineraiogic variations in sphalerite and

galena within and between the three veins were also studied.

1.3 Previous Work

Fowler et al (1950) and Osborne (1956) in, Ostennan (1984) and Fries (1960)

provided some of the earliest descriptions of the geology and the vein and manto deposits

in the Taxco area. Numerous unpublished Company reports dealt with metal ratios, ore

reserves, petrography and geochemistry. The age and genesis of the Taxco ScWt and

Fig. 1 Location map and geological map of the Taxco area showing the El Cobre, Esperanza and Hueyapa veins and mine shafts (after. IMMSA). Whole rock sample locations are indicated by cucies. The occurrence of Taxco Schist and PhyUite in the southeast part of the map area has been referred to as Taxco Roca Verde by others.

Hueyapr Vein N 45 degrecs W Iooking N 45 degrces E

Esperanza Vein N 12 degrees W looking N 78 d m E

El Cobre Vein N 62 degrees W looking N 28 degrees E

Fig. 2 Sample locations of the Hueyapa, Esperanui and El Cobre veins (after IMMSA, Unidad Taxco).

Phyllite were briefiy discussed in severai papers (DeCserna and Fries, 1974 and Campa et

ai, 1975 in, Osterman, 1984). The tectonic history of Mexico and its relationsùip to

mineral deposits and arc-related magmatism of the Sierra Madre Oççidental were

investigated by Campa and Coney (1983). Clark et al (1982). Damon et al (1983) and

McDoweli and Clabaugh (1979).

Recent work in the Taxco area is very limited. Osterman (1984) hypothesized that

the Guadalupe carbonate replacement deposit had a skarn-like origin although no definite

origin was known. He speculated that the source of metals in the Guadalupe silver deposit

was a metai-rich shaley component of the Taxco Schist and Phyllite. The metals were

remobilized and deposited in faults within the iimestone as a result of either crustal

weakness near the boundary between the Mixteca terrane and the Guerrero terrane or heat

kom plate fiction. Clark (1986) and the Department of Geology, IMMSA de C.V.,

Unidad Taxco (1990) provided summaries of the geology and the ore deposits in the

Taxco Mining District for a field trip spoasored by the Society of Economic Geoiogists.

1.4 Regional Setting

Campa and Coney (1983) concluded that 80% of the North American Cordilfera in

Mexico CO nsists of d o c htonous temanes bounded by major discontinuities in stratigrap hy

that may be faults. The western portion of Mexico is made up of several terranes that were

added to North America in late Jurassic to early Cretaceous tirne during the eastward

migration of a continental margin vokanic-plutonic regime within Mexico and the

southwestern United States (Damon et ai, 1983). The magmatic arc coincides with a

northwest trending belt of Ag-Pb-Zn deposits that extends about 800 km dong the

western margin of Mexico. The Taxco Miniag District is situated in the southem portion

of this belt which also encompasses the Pachuca, Guanajuato. Zacatecas, FresniNo and

Sombrerete muiing districts. These districts conskt predotninantly of northwest trending

epithermal veins. Clark et al (1982) suggested that Ag-Pb-Zn vein deposits in western

Mexico were formed during the late progression and the early regression of the migrating

magmatic arc during Eocene-Oligocene t h e (49 to 26 m.y. B.P.) rather than Mesozoic

age (76 m.y. B.P.) as suggested by Osterman (1984).

The regional geology in the area consists of Jurassic basement schist overiain by

Cretaceous hestone and marine sediments. A thick sequence of Eocene continental

conglornerates overlies the marine section and is, in tum, capped by younger volcanic

flo ws and ash flow tuffs of rhyolitic composition. Ag-Pb-Zn mineralization occurs as

northwest trendhg veins, mantos, stockworks and breccia chimneys within the Mesozoic

sequence of rocks and does not extend Uito the overlying volcanic pile.

1.5 Local Stratigraphy of the Taxco Mining District

The local stratigraphy of the Taxco Mining District consists of basement schist

(Taxco Schist and Phyiiite) overlain by a sequence of Cretaceous limeStone (Morelos

Formation), marine shale and sandstone (Mexcla Formation) as weii as volcanic and

p yroclastic flo ws (Balsas Group and T î p o tla Rhyolite) of Tertiary age (Fig. 3). The

stratigrap hy is intruded by plugs, dikes and sills of mafic, intermediate and felsic

CO m positio n. Ag-Pb-Zn v e h extend through the basement schist and Mesozoic sequence

of rocks but are no t found in the overlying Tertiary volcanic and pyroclastic flows.

:, T i p t h Rhyotite

Hornblende-Diabase Intrusion

Mercala Formation

Cretaceous Rhyotitic intrusion

Momlos Formation

Mdïc Intrusion

Taxco Schist and Phyllite

Fig. 3 Schematic depiction of the local stratigraphy of the Taxco Mining District ( IMMSA, 1990). Dashed line represents a thnist fault and wavy iines represent unconformities.

Taxco Schist and Phvllite

The basement rocks are metarnorphic rocks of greeaschist facies named the Taxco

Schist by Fries (1960). The Taxco Schist is made up of micaceous, talcose and chloritic

facies with phyilitic horizons and abundant quartz ienses. The parent rocks of the phyiiite

and schist are believed to be mudstone and rhyolitk tuff. respectively. These rocks make

up the Taxco-Zitacuaro massif which is believed to represent a metarnorphosed

sedimentary-volcanic arc sequence. The age of the massifis a matter of debate. It has

been dated late Precambrian using kad-alpha dates and ako as middie Jurassic due to the

occurrence of Jurassic ammonoids in an equivalent formation in central Guerrero

(DeCsema and Fries, 1974 and Campa et aï, 1975 in, Osterrnan, 1984). This unit is at

least 600 rn thick but its total thickness is unknown Typical exposures occur near the

town of Taxco (Fig. 1).

The rocks are strongly foliated in a northeasterly direction with dips less than 30".

Quartz lenses are found parailel to the foliation. The schist contains disseminated

sphalerite, galena and chaicopyrite within pyrite and specularite gangue (Osterman, 1984).

The Taxco 'Xoca Verde". or greenstone. was mapped by Fries (1960) as a

sequence of interstratified tuffs, breccias and lavas that are andesitic in composition. The

oniy exposure of this unit occurs southeast of the town of Taxco where it may

disconforrnably overlie Taxco Schist (Fig. 1). There is some confusion over the exact

stratigraphie relationship of the two occurrences and in the present study the basement

rocks are referred to coliectively as Taxco Schist and Phylïite.

Morelos Formation

The Morelos Formation was deposited dong the flanks of the Taxco-Zitacuaro

massif as a series of light to dark grey Lùnestones and dolostones. Osterman (1984)

concluded that the lower portion of the limestone consists of thick-bedded, coarse-grained

biosparite containing 10% to 20% aiiochems w& the upper section is thin-bedded, bladc

to dark grey micrite which may be fossiliferous.

The formation is early to mid-Cretaceous in age and is O m to 900 m thick. A

schematiç structural cross section across the Taxco Mining District (Fig. 4) shows the

attenuation of the Morelos Formation which was interpreted by Osterman (1984) to be a

tectonic feature, although it has also been attributed to basin edge deposition. The

Morelos Formation is separated f?om the undeclying Taxco Schist and Phyiïïte by a thnist

fault marked by a 1 m to 2 m thick zone of mylonite and fault gouge.

Mexcala Formation

The Mexcala Formation consists of upper Cretaceous interbedded shale and

sandstone with thin interstratified beds of limestone (Osterman, 1984). The base of the

formation is calcareous, with thin beds of Limestone, while the upper portion tends to be

clay-rïch with thick beds of Iimestone. Regionaliy* the formation may be as much as 1 200

m thick but localiy it is between 50 m and 400 m thick. The formation is strongly

deforrned and regionaüy folded (Fig. 4). The contact between the Mexcala Formation and

the underlying Morelos Formation is unconformable and shows signs of slippage due to

folding.

Fig. 4 Schematic structural cross section dong AB of Figure 1. The legend is the same as that of Figure 1.

Balsas Group

The east trending sedimentary and vokanic rocks of the Balsas Group are Eocene

to Oligocene in age and were deposited on top of the Mexcala Formation dong an

erosional unconformity (Osterman, 1984). The Balsas Group. which is 50 m to 400 m

thick, consists of a basal continental conglomerate containing clasts of limestone, shale

and volcanic fragments within a sandy matrix and an upper sequence of andesitic and

rhyolitic flows and tuffs.

Tilza~otla Rhvolite

The youngest rocks in the Taxco area are the Tilzapotla Rhyolite. This unit is

made up iargely of poorly differentiated rhyolite flows, tuffaceous brecçias, ignimbrites

and vitrophyres. Rhyolite displays fluidal, spherulitic and porphyritic textures. The

Tilzapotla Rhyolite is Oligocene in age and has been correhted with the Upper Vokanic

Series in the Sierra Madre Occidental by CIark et al (1982) and Clark (1986). In the

Taxco area, the sequence is 150 m thick and does not contain Ag-Pb-Zn veins.

Intrusive Rocks

The stratigraphy is intruded by three different types of plugs. dikes and siils that

form paraiiel to Ag-Pb-Zn veins. The oldest intrusive rocks are highly sericitized dikes and

sills with intermediate to mafic compositions and are restricted to the basement Taxco

Schist and Phyiüte. Strongly kaoiinized dikes of rhyoiitic to trachytic composition cut

through the entire Mesozoic sequence. Homblende-diabase plugs and dikes are younger

than the mineralization. These &es occur throughout the entire stratigraphic sequence

and may either replace or offset veins.

1.6 Structural GeoIogy of the Taxco Mining District

The Taxco district has undergone various stages of folding and faulting prior to

and foliowing the mineralizing event. Deformation accompanied greenschist facies

metamorphism of the Taxco Schist and Phyllite produchg a strong foliation and quartz

segregations in these basement rocks. Schistosity in these rocks strikes N4S0E to N90°E

and dips less than 30° indicating that the parent rock was compressed into recumbent folds

by stresses oriented southeast to northwest (IMMSA, 1990). The Morelos Formation and

the Mexcala Formation are folded into the San Antonio Anticline, the Tecalpulco Syncline

and the Juliantla Syncline (Fig. 1). The plunge and trend of the San Antonio Anticline are

70NW/N60°W and the Juliantla Syncline plunges gently northwest and trends N30°W.

Two stages of faulting ako occurred in the Taxco area producing faults pnor to

and foliowing rnîneralization, Stresses that produced the folding are aiso associated with

the first stage of faulting. Re-mineralization normal faults developed in the early Eocene

as a result of block faultïng after compression ceased. These faults strike either due north

or N60°W and offset the stratigraphic sequence fiom the Taxco Schist and Phyllite to the

Balsas Group. Normal faults have displacements of 10 m to 50 m (Osterman. 1984). Ag-

Pb-Zn mure-nlling veins n11 these paraNe1 normal faults but reverse faults are typicaily

unmineralized. The second system of faults fomed after the xnineraiizjng event and the

deposition of the Tilzapotla Rhyolite. The Cerro del Mueno Fault developed at this t h e

in the northern section of the area (Fig. 1). Its attitude is N70°W 60°NE with a dextrd

offset of 400 m.

1.7 Geology of the Guerrero, Remedios ancl San Antonio Mines

Guerrem Mine

The El Solar shaft of the Guenero mine is c o k e d in the Mexcala Formation at an

elevation of 1 730 m above sea levei. The major ore-producing vein in this mine is the El

Cobre vein. The El Cobre vein extends vertically 400 m, fiom level O to level9, and at

least 2 km dong strike. The width of the vein decreases with depth, so that it is 30 m at

level O and 1.5 m at level9. The vein bifurcates, splays, pinches and swelts and has

numerous barren sections. Generaiiy, the El Cobre vein is narrower in the Mexcala

Formation than in the Morelos Formation.

The mineralogy of the El Cobre, Esperanza and Hueyapa veins is essentially the

same excluding some ciifferences in carbonate and sulfosalt mineraiogy. Metaiüc minerals

in the El Co bre vein are pyrite, sphalerite, galena, chalcopyrite, marcasite, arseno pyrite,

hematite, rare p yrrho tite and sulfosalts such as boulangerite, bournonite and falkmanite.

Pyrargyrite replaces galena and constitutes up to 80 96 of the Ag in the El Cobre vein

(Sanchez-Torres, 1991). Occurrences of polybasite, stibnite and jamesonite are also

reported (Salas, 199 1). Generaily, iron and lead contents of the vein increase with depth

whereas zinc and silver content decrease with depth. Manto deposits typicaiiy have higher

zinc and lower lead and silver contents than the veins. Non-metaüic miner& in the El

Cobre vein include quartz, calcite, siderite, dolomite and fluonte. Bands of bluish, grey

and purplish amethystine varieties of quartz occur in phces. The carbonate gangue in the

mineralized veins becomes manganese-rich near the lirnestone-shale contact. There

appears to be several stages of vein formation that are recorded by brecciation and

crustiforrn textures.

Remedios Mine

The shaft of the Remedios mine is located in the Mexcala Formation and is

collared at an elevation of 1 610 m above sea leveL The two major veins in the mine are

the Esperanza and the San Pedro - San Pablo veins. For the purpose of this thesis, the

samples were taken fiom the Esperanza vein mg. 2) since access to the San Pedro - San

Pablo vein was very lunited and not reanily available.

The Esperanza vein has a vertical extent of 150 rn and is accessible on levels 1 ,2

and 3 of the Remedios mine. The vein extends along strike approximately 700 m and is

between 0.15 m and 2 m thick. It typically dips steeply to the north and northwest. In

parts of the mine, Taxco Schist and Phyiiite is overlain by Mexcala Formation shce the

Morelos Formation is absent here. The Esperanza vein is strongly sheared and contains

brecciated fragments of shale in these areas.

The Esperanza vein contains pyrite, sphalerite, galena, chalcopyrite, marcasite,

minor arsenopyrite, hematite and pyrargyrite with gangue minerals quartz, calcite, siderite,

dolomite, ankente and fluorite. Sphalerite varies in colour fYom honey brown near surface

to black at depth. Silver content decreases with depth in the mine and manto deposits

usually have higher silver contents than the veins.

San Antonio Mine

The Hueyapa shaft of the San Antonio mine is collared within an anticline in the

Mexcala Formation at an elevation of 1668 m above sea level. Only the fourth level of the

mine is accessible. The Hueyapa vein extends approximately 1 700 m along strike and is

between 0.01 m to 5 m thick. In some locations, the Morelos Formation is absent so

Taxco Schist and Phyliite is directly overlain by Mexcak Formation. The Hueyapa manto

is fo und at the eastern extent of the mine in a ramp immediately above b v e 1 4 (Fig. 2).

The metallic miner& in the Hueyapa vein are similar to those of the Esperanza vein;

however, chalcopyrite and galena are repiaced by süver-bearing tetrahedrite, freibergite

and bournonite. It is marked by the presence of honey coloured sphakrite rather than the

black variety. The Hueyapa manto is characterized by high grade silver values and honey

coloured sphalerite.

-- - - -

The mineralog y of the El Cobre, Esperanza and Hueyapa veins was determined

f?om the examination of hand specimens and the petrography of polished thin sections.

Ninety-nine analyses of sphalerite were conducted on the scanning electron microscope,

SEM, and 1 19 galena plus 29 sulfosalt samples were analysed on the electron micro probe

at the Ontario Geological Survey Laboratories (Appendixes 1 to 3). Whole rock anaiyses

for metals were performed on 19 samples fiom the El Cobre, Esperanza and Hueyapa

veins and veins intersected in driU hoks and 8 samples of host rocks by instrumental

neutron activation analysis, INAA, and total digestion ICP at Activation Laboratories

Ltd., Ancaster, Ontario (Appendix 4).

2.1 Petrogmphy and Paragenetic Sequenœ

2.1.1 The El Cobre Vein

Pyrite occurs as disseminated euhedral to subhedral cubes, coarse-grained

aggregates and inclusions within sphalerite. Galena and sphalerite commonly enclose

and corrode pyrite and fU fractures (Plate 1). Inclusions of galena and sphalerite are

common in pyrite but pinkish pyrrhotite inclusions are rare (Plate 2). Mutual boundary

texture is observed between pyrite and sphalerite.

Sphalerite commonly occurs as coarse aggregates and anhedral grains that display

zoning. Generally, sphalerite with higher iron content is darker in colour but ibis

correlation becomes unreliable where iron content is below 5% (Craig and Vaughn,

198 1). Sphalerite varies fiom a bhck variety at depth to a honey-brown variety near

surface. The Fe-rich variety contains chakopyrite as randomly distributed and

Plate 1. Sphalente corroding pyrite. Note chaicopyrite inclusions in sphalerite (reflected light, diagonal of photo = 1.49 mm)

Plate 2. Pyrite with pyrrhotite inclusion. Marcasite is intimately intergrom with pyrite (reflected iight, diagonal of photo = 0.71 mm).

crystallographically oriented rows of inclusions but pyrite and pyrrhotite inclusions are

l e s common (Plate 1). In places, sphalerite encloses and fills fkactures in galena but

galena &O appears to form at the expense of sphalente. Rismatic quartz crystals may

contain srnall inclusions of sphalerite.

Galena commonly occurs as subhedral cubic grains. It partly encloses and

corrodes pyrite and fills fkactures in pyrite and sphalente. In some places, mutual

boundary texture is displayed by smooth, regular cwved contacts between galena and

sphalerite. Galena rarely occurs as highly irregular and jagged grains with quartz,

Galena is also found as inclusions in prismatic quartz crystals and vice versa.

Chalcopyrite occurs primarily as small inclusions ui sphalerite although it also

f U fractures in sphalente and marcasite. Pyrite and rnarcasite rnay be enclosed in part

by chalcopyrite and corroded by it. Copper-bearing sulfosalts, boumonite and argentian

tetrahedrite, form at the expense of chalcop yrite.

Marcasite is intimately intergrown with pyrite (Plate 2). It is present as highly

fractured anhedral grains, feathery Iaths or bladed crysials. Minor inclusions of

pyrrhotite are observed within marcasite and fractures in marcasite are f i e d with

chalcop yrite.

Arsenopyrite occurs as subhedral to euhedral rhombs closely associated with

pyrite. It increases with depth in the El Cobre vein.

Hematite occurs as anhedral grains, radiating aggregates and needle-like crystals

dispersed throughout the vein.

The El Co bre vein is characterized by the presence of sulfosalts such as

boumonite, PbCuSbS. boulangerite. PbsSbrSii and fallunanite, PbSb&. Bournonite is

intimately associated with galena. It forms fine-grained, greyish-white needles dong

jagged galena boundaries or as fine-graùred yeilowish-grey blebs m g voids (Plate 3).

Boulangerite and falkmanite are observed within calcite as fme-graiaed,

yeliowish-grey needles under crossed-nicois in reflected iight plate 4) and as metaiiic

grey needles in vugs in hand specimen. According to Sanchez-Tomes (199 1) pyrarg yrite,

Ag3SbS3, replaces galena in the El Cobre vein and 80% of Ag in the El Cobre vein is in

p yrargyrïte.

Non-metallic minerals of the El Cobre vein consist of quartz. calcite, siderite and

dolomite plus localized patches of fluorite. Quartz may be present as euhedral prismatic

and hexagonal crystals and as anhedral grains or aggregates. Bluish. grey and purplish

amethystine varie ties of quartz occur and in places are banded (Plate 5). Carbonate

occurs as euhedral rhombs or as thin veialets. Fluorite is coarse-grained and is associated

with brecciated areas and strongly rnineralized zones. Lithic kagments are angular and

consist of mudstone and schist with moderate to strong chlorite and talc alteration.

2.1.2 The Esperanza Vein

Pyrite occurs as disseminated euhedral to subhedral cubes, coarse-grained

aggregates and inclusions within sp halerite. Galena and sphalerite commonly €iU

fractures in pyrite (Plate 6). Inclusions of galena and sphalerite are common in pyrite and

in places appear to be forming at its expense. Mutual boundary texture is observed

between pyrite and sphalerite. V t e rarely appears to have grown around calcite grains

(Plate 7).

Sphalerite commonly occurs as coarse aggregates and anhedrat grains that contain

randomiy distributed and crystallographicaily oriented rows of chalcopyrïte inclusions. B

Plate 3. Acicdar bournonite forrning dong galena grain boudaries (reflected light, diagonal of photo = 0.70 mm).

Plate 4. Blades and needles of boulangerite within calcite gangue (reflected light, diagonal of photo = 1.48 mm)

Plate 5. Hand speciinen with banding of purplish amethystine and white quartz

Plate 6. Galena replacing pyrite and Wing hctures (reflected Light, diagonal of photo = 1.49 mm)

Plate 7. Pyrite grown amund calcite grains (reflected light, diagonal of photo = 1.49 mm)

Plate 8. Zoned sphalerite displaying altemate light and dark bands @lane polarued light, diagonal of photo = 2.96 mm).

commonly dispiays zoning consisting of altemating light and dark bands (Plate 8).

Sphalerite varies fiom a black variety at depth to a honey-brown variety near surface. in

places, galena encloses and fills fractures in sphalerite but sphalerite also appears to form

at the expense of galena. Prismatic quartz crystals may contain srnail inclusions of

sphalerite-

Gaiena commonly occurs as subhedral cubic grains. It partiy encloses and

corrodes pyrite and filIs fractures in pyrite and sphalerite (Plate 6). In some places,

mutual boundary texture is displayed by smooth, regular curved contacts between galena

and sphalerite (Plate 9). Gakna is a h found as inclusions in prismatic quartz crystals

and vice versa. in brecciated samples, calcite, quartz and hematite rim galena producing

crustifed texture (Plate 10). Gdena rarely occurs as highiy irregular and jagged grains

with quartz, apparently resulting from the formation of later, crosscutting quartz veinlets

(Plate 11).

Chaicop yrite occurs primarily as smaii inclusions in sphalerite although it also

f W fractures in sphalerite, galena, pyrite and marcasite. Pyrite and m a s i t e may be

enclosed by chaicopyrite.

Arsenopyrite occurs in minor arnounts as subhedral rhombs that are intimately

associated with pyrite. It inçreases with depth in the Esperanza vein.

Hematite is found in the El Cobre, Esperanza and Hueyapa veins but it is

dominant in the Esperanza vein, especially in brecciated samples. It occurs as anhedral

grains, radiating aggregates and needles dispersed throug hout the vein (Plate 1 2).

P yrarg yrite is the only sulfosait found within the Esperanza vein. It occurs as

inclusions within galena

Plate 9. Smooth and curved contacts between galena and sphalerite exhibitiig muniol boundary textwe (reflected iight, diagonal of photo 4 - 4 9 mm)

Plate 10. Crustified texture exhibited by anhedral galena rimmed by quartz, calcite and hematite (plane polarized Iight, diagonal of photo = 1.48 mm)

Non-metallic minerais in the Esperanza vein are quartz, calcite, siderite, dolomite

and fluonte. Fluorite occurs as subhedral, coarse grains especiaily found in

brecciated areas. Quartz is observed as euhedral prismatic and hexagonal crystais and as

anhedral grains or aggregates. Euhedral rhombs of carbonate ffl open spaces and veins.

Carbonate also occurs as coarse aggregates and thin veinlets. Lithic fragments are

angular and consist of sandstone, mudstone and schist Schistose Bagments may have

chlorite alteration.

2.1.3 The Hueyapa Vein

The mineralogy of the Hueyapa vein is not fully known due to the insufficient

number of sarnples for thui section description. The sulphide and sulfosait mineralogy of

the Hueyapa vein consists of sphalerite, galena, pyrite, marcmite, boumonite, argentian

teuahedrite and freibergite. Quartz, calcite, dolomite. ankerite and fluorïte constitute the

gangue mineralogy. Quartz and carbonate fom coarse, anhedral grains and occur as

euhedral prismatic crystals and rhombs, respectively.

Pyrite occurs as subhedral to euhedral cubes. It is found as fine disseminations

and coarse aggregates. Lnclusions of galena and sphalerite occur in pyrite. Marcasite is

associated with pyrite and forms radiating crystals. Galena occurs as subhedral cubes and

anhedrai blebs. In places. galena commonly occurs replacing voids in pyrite. Sphaierite,

which is honey brown in colour, is present in minor amounts in the Hueyapa vein relative

to that of the El Cobre and Esperanza veins. It contauis little to no fine-grained

c halco p yrite inclusions. Arsenop yrite is no t found in the Hueyapa vein. Chalcop yrite

f l voids and fractures within pyrite. Copper-bearing sulfosalts commonly fonn at the

expense of chalcopyrite. The Hueyapa vein contains bournonite. argentian tetrahedrite

and fieibergite, ( A ~ , C U ) ~ ~ ( Z ~ , F ~ ~ S ~ ~ S ~ ~ - Argentian tetrahedite and fieibergite occur as

fuie bodies that are intimately associated with chalcopyrite that fills voids in pyrite and

galena whereas fine-grained bournonite replaces galena (Plates 13 and 14).

2.1.4 Pzuagenetic Sequence

The El Cobre, Esperanza and Hueyapa veins have variable textures, such as

vugg y, comb, drusy, brecciation, crustifkation, collo form bandhg and crosscutting,

which reflect several stages of vein formation anâ hypogene vein minerdimion. The

vehs are characterized by complex paragenesis in which various stages of deposition are

fo 110 wed by cracking and seaïhg events characteristic of low sulp hidation epithemial

deposits. For this reason, the paragenetic sequence is often difficult or impossible to

determine. Samples taken fiom the veins were not necessarily representative of the

mineralogy of the entire vein; therefore, the following paragenetic sequence for the El

Cobre and Esperanza veins is not f d y estabiished and is meant only as a general guide.

It is similar to the paragenesis of the El Cobre vein outlined by Sanchez-Torres (199 1).

Stage 1, the earliest stage in the paragenetic sequence, consists of quartz,

carbonate, pyrite, arsenopyrite and marcmite. Arsenopyrite and pyrite are

contemporaneous and intimately intergrown. Rare sphalerite may be deposited at this

tirne and fom mutual boundary textures with pyrite. Minor amounts of pyrrho tite form

as inclusions in pyrite and sphalerite.

Stage 2, the main mineralizing stage, is marked by the deposition of sphalerite,

gaiena, chalcopyrite, bournonite, boulangerite, falluaanite, argentiari tetrahedrite and

keibergite dong with quartz, carbonate. fluorite and hematite. Stage 2 Û divided into

two sub-stages. Sub-stage 1 consists of sphalerite, galena, quartz and carbonate

Plate 13. Boumonite and argentian tetrahedrite replachg chalcopyrite and galena(?) (reflected fight, diagonal of photo = 1.49 mm)

Plate 14. Chalcopyrite and galena replaced by boumonite and argentian tetrahedrite(?) (reflected light, diagonal of photo = 0.70 mm)

deposited as coarse-grained sulfides. Anhedral gaiena occurs as inclusions in pyrite and

prirnary sphalerite, forming at the expense of these mineals. Secondary sphalerite a h

forms at the expense of pyrite produchg reaction rims. Chalcopyrite and sulfosaits are

deposited in sub-stage 2. Chakopyrite EUS fractures in galena, pyrite and sphalerite.

Late copper-bearing fluids react with Fe-bearing sphalerite forming chalcopyrite

inclusions in sphalerite- The latest event of hypogene ore deposition involves Pb-, Sb-,

Cu- and Ag- bearing sulfosaIts. Cu- bearing bournonite, argentian tetrahedrite and

keibergite form in ctialcopyrite-rich areas of the veins whereas Pb- and Sb-bearing

boulangerite and falkmanite form in galena-rich parts of the veins. Pyrargyrite occurs as

incIusions within galena.

Stage 3 is comprised of massive, coarse-grahed &y white calcite and quartz

veins. These veins are typicaiiy barren but may contain minor disseminated pyrite as

weii as wallrock fragments. Cakite rnay occur as euhedral rhombs in vugs and open

spaces. Open spaces are filled with quartz as euhedral crystals or bands that may be

planar or deformed.

2.2 Electron Microprobe and SEM Analyses of Gaiena and Sphalerite

Figures 5 to 7 depict the relationships between Pb, Ag and Sb in galena Galena

fkom the El Cobre vein contains the highest Sb content whereas that of the Esperanza

vein has the lowest Sb values (Fig. 5). The Sb content of galena ftom the Hueyapa vein

is only slightly higher than that of the Esperanza vein. Gaiena fkorn the Hueyapa manto

has higher Sb content than that of the Hueyapa vein. Ag levels in galena are

1 I 1 I I I

0.00 0.40 0.80 I

1.20 Sb (wt ?4)

Fig. 5 Pb (wt %) plotted against Sb (wt %) in galena anaiysed by the electron microprobe. Pink syrnbols represent values with less than 3a in analytical background error.

Fig. 6 Pb (wt %) plo tted against Ag (wt %) in galena analysed by the electron microprobe. Pink symbols represent values with les than 30 in analytical background error.

Fig. 7 Ag (moles) plotted against Sb (moles) in galena analysed by the electron microprobe. Pink symbols represent values with less than 30 in analytical background error.

similar to Sb levels in that galena fkorn the El Cobre vein has the highest Ag contents.

The majority of the Esperanza vein samples have low Ag levels that are iess than 3a in

background error (Fig. 6). Gaiena from the Hueyapa manto has higher Ag contents than

galena from the Hueyapa vein. The El Cobre vein has the highest Ag and Sb contents in

gdena although the data is highly variable (Figs. 5 and 6). This variability is a reflection

of vertical zoning in the El Cobre vein where the Ag and the Sb contents in galena

decrease upward such that level9 of the El Cobre vein has galena with the highest Ag

and Sb contents. The Ag and Sb are incorporated into the galena structure by coupled

substitution of sb3+ and ~ g " for 2pb2' (Fig. 7) and the amount of substitution is

temperature-dependent.

In sphaierite, Fe, and to a minor extent Cd and Mn, isomorphousiy substitute for

Zn. A weli-defined correlation exists between Fe and Zn in sphalerite (Fig. 8). The

rnajority of the sphderite samples plot along a Zn:Fe line with a slope of 1. The addition

of Mn and Cd shift samples off the line along a dilution trend. The El Cobre and

Esperanza vein samples have the lowest Zn and the highest Fe contents (Fig. 8). The

Hueyapa manto samples have the highest Zn and lowest Fe content; however, there is a

wide spread in the data that can be attributed to gradation between the manto and the

Hueyapa vein. The outlier lÎom the Hueyapa vein has unusually low Zn and high Fe

contents. This sample contains 5.5 wt % Cu which can be explained by the presence of a

chalcopyrite inclusion.

Sphalerite contains chakopyrite as blebs and rods randomly distributed or

crystailographically oriented in rows. This texture has been interpreted as an exsolution

feature of cooling ores. However, studies have s h o w that the solubility of CuS in

Fe (aaomc proportions)

Fig. 8 Zn plotted against Fe (atomic proportions) for sphalerite analysed by the SEM, The dashed iine has a slope of 1.

sphalerite is too Iow and chalcopyrite does not dissolve in sphalerite in sufficient

quantities at temperatures below 5000C. According to Barton and Bethke (1987). the

chalcopyrite in sphalente forms by either replacement of Fe-bearing sphalerite by later

CO p per-bearing fiuids or epitaxial g o wth during sphalerite formation.

2.3 Whole Rock Metal Distributions

WhoIe rock metal analyses were performed on samples nom the EI Cobre,

Esperanza and Hueyapa veins, Hueyapa manto, veins intersected in drill holes, Taxco

Schist, Taxco Roca Verde and Mexcala Shale. Figures 9a to 9d depict the average metai

content as a fiinction of elevation above sea level in meters, normalized against total Pb +

Cu + Fe + Zn. Generaily, the Hueyapa vein has the highest contents of Ag, Au and Sb.

The El Cobre vein has the highest As values. In the El Cobre samples, Ag and Sb

decrease with depth whereas Au increases with depth. This variation is a reflection of

vertical zoning in the El Cobre vein as a whole, where Zn and Ag values decrease with

depth and Fe and Pb content hcrease with depth.

The whole rock metal contents of the Taxco Schist, the Mexcala Shale and the

Taxco Roca Verde are compared to metaliferous shdes in other parts of Mexico, viz.,

Guanajuato, Leon, Valenciana mine and Zacatecas in Table 1 (Unpublished data,

Laurentian University, Whitehead and Davies, 1998). The T a o Schist, the Taxco Roca

Verde and the MexcaIa Shale have about the same natural abundances of Pb and Zn.

These values are similar to those of shale fiom Guanajuato, Leon and Zacatecas although

the Taxco Schist and the Taxco Roca Verde are somewhat lower in Zn than the other

samples.

Fig. 9 Average whole rock metal content normalized against total Pb + Cu + Fe + Zn as a function of elevation for: (a) Ag; (b) Au; (c) Sb; and (d) As. Nurnbers in parentheses indicate the number of simples included in the average value.

Table 1 Average metal content in shale ftom Guanajuato, Valenciana mine, b o n and Zacatecas compared to Mexcala S u e , Taxco Schist and Taxco Roca Verde. Number in parentheses represents the number of samples included in the average metal calculations.

Me Werous Shales Guanajuato (38)

- - 1 1 1

AU samples above (140) 1 631 24 164

CU (ppm) 89

117 23 1 162

Valenciana mine (20) Leon (36) Zacatecas (46)

Ore samples from the veins have higher Pb and Iower Zn proportions than the

Taxco Schist and the Mexcala Shale; no whole rock copper analyses are avaiiable for

these samples but they tend to contain smalï amounts of copper (Fig. 10).

Multi-element plots of the Mexcala Shale, the Taxco Schist and the Taxco Roca

Verde are similar except for enrichment of Ca in the Mexcala Shale, possibly due to a

- Mexcala Shale (5) Taxco Schist and Taxco Roca Verde (6)

limey component in the sample (Fig. 11).

Pb (ppppm)- 23

33 68 51

(ppm) 145

17 22 30

46 6

17 17

136 82

Fig. 10 Pb-Zn-Cu ternary plot showing the average Pb, Zn and Cu values for the Taxco Schist, the Mexcaia Shaie and ore samples fkom the El Cobre, Esperanza and Hueyapa veins. Note that no Cu values are reported for the ore samples.

Fig. 11 Multi-element plots of the Mexcala Shale, Taxco Schist and Taxco Roca Verde.

3.0 Stable Isotopes and Rare Earth Ekments

Twenty-six samples of carbonate gangue fkom mineralized and non-mineraiized

portions of the El Cobre, Esperanza and Hueyapa veins and v e h intersected in drill

holes were analysed for 6 ' ' ~ ~ ~ and 6L80nMow, at G.G. Hatch [sotope Laboratory,

University of Ottawa (Appendix 5). Carbonate dissolved nom fifteen specimens of

Taxco Schist, Mexcala Shale and Morelos Limestone were analysed for 8l3ceDB) and

G ' ~ o ( ~ ~ ~ ~ at the same faciiity as weU as the organic carbon content of four samples of

Mexcala Shale and one sample of Taxco Schist (Appendix 5).

Thirty-one vein samples of carbonate gangue fiom mineralized and non-

mineraiized parts of the El Cobre, Esperanza and Hueyapa veins and veins intersected in

drïü holes were analysed for rare earth elements, REE, by laser ablation inductively

coupled plasma mass spectrometry (LAM-ICP-MS) at Memorial University,

Newfoundland (Appendix 6). Two vein samples and two samples of Morelos Limestone

were also analysed at Mernoriai University by whole rock ICP-MS (Appendix 6).

Whole rock buik analyses of eight samples of the Taxco Schist, the Mexcala Shaie and

the Taxco Roca Verde were conducted at Activation Laboratories Ltd in Ancaster,

On tario b y instrumental neutron activation analysis (INAA) (Appendix 4).

3.1 Theory of Carbon and Oxygen Isotopes

Isotopes are atoms of an element whose nuclei contain the sanie number of

protons but a different number of neutrons. Carbon has two stable isotopes and oxygen

has three stable isotopes with the following abundances:

''c = 98.89 %

Stable isotope ratios are given in parts per mil. 960. and are expressed in dei, 6. notation.

The del value of the ratio of "u'~c is calcuhted as foiIows:

18 O and 160 are emplo yed in the oxygen isotope ratio due to their high abundances and

the maximum difference between their atomic masses. The isotopic composition of

oxygen is calcuIated as a del value as folows:

Carbon isotopes are rneasured relative to the international reference standard

Belemnitella americuna Erom the Cretaceous Peedee formation. South Caroha. PDB.

Oxygen isotopes may be rneasured relative to either PDB or standard mean ocean water,

SMOW. The actual PDB standard has ken exhausted so the current standards k i n g

used are NBS-18 (carbonatite). NBS-19 (marble), NBS-20 (limestone) and NBS-21

(graphite).

Isotopes of an element can be fractionated through physical and chernical

processes as a result of mass differences between the isotopes. Fractionation of isotopes

is directly proportionai to mass differences between the isotopes and inversely

proportionai to temperature. Isotope ftactionation occurs in nature as isotope exchange

reactions between two substances, kinetic proçesses and physiochemical processes.

Meteoric water and magmatic water can be important ore-forrning fluids. The

OL3cPDB) values of magmatic water are between 4 and -û 8. and S1806MoW> values

range be tween 5 and 10 pig. 12; R O U ~ I W ~ , 1993). Meteoric water has a 8 1 3 ~ p D B )

value similar to atmospheric C a -7 Bo (Ohmoto and Rye. 1979). The 6 1 8 ~ e ~ ~ w value

of meteoric water in Taxco, Mexico is estimated to be -7.5 %O using the following

equation by Craig (1961) in, Field and Fifarek (1985):

6D z 86180 + 10 (%O)

where 6D = -50 in Taxco, Mexico.

Marine carbonate has 8 1 3 ~ p D B ) values between -1 and 2 %O and 81s~nMOw) values

between 25 and 30 %o(Fig. 12). In contrast, organic carbon and graphite are isotopicaiIy

iight, that is, they are depleied in 13c and enriched in 12c. Graphite and organic carbon in

sediments. coal and petrokum typicaliy have 6 I 3 c P ~ ~ ) values between -10 and -35 960

with an average of -25 (Ohmoto and Rye, 1979). Methane of organic ongin has

613c values as Iow as -80 %O (Rollinson, 1993).

3.2 Theory of Rare Earth Eïernents

Rare earth elements are comprised of La to Lu with atomic numbers 57 to 7 1. The

light rare earth elements, LREE, are made up of eiements with the lowest atomic numbers

and masses (La to Sm) and the heavy rare earth elements, HREE, consist of elements

with higher atomic masses and numbers (Gd to Lu) (Table 2). The REE comrnonly have

3+ oxidation States with exceptions. A small decrease in ionic radius occurs with

increasing atomic number. These ciifferences in ionic size aUow the REE to fiactionate

relative to one another. REE substitute for elements with similar ionic rad5 and

estinmted nideoric water in Tasce W c o

Fig. 12 8'3~por»-6'80~sMoW> plot showing the ranges of 6'% and 6'*0 for magmatic and meteoric water, marine carbonate and organic carbon. Data are taken fiom Ohmoto and Rye (1979), Rollinson (1993) and Craig (1961) in, Field and Fifarek (1985).

coordination polyhedra in minerais. LREE such as La (ionic radius 1-03 A0 in 6-fold CO-

ordination) substitute more readily for ca2+ (ionic radius 1.00 A" in 6-fold CO-ordination)

than do the W E such as Lu (ionic radius 0.86 A0 in 6-fold CO-ordination)-

Table 2 Atomic weights of REE and ionic radü of REE ions and ca2+ in 6-fold co- ordination

In reducing environments. EU" reduces to EU^* and will substitute more readily

for ca2+ than its neighbours. ~ d ~ + and Sn3+. producing a Eu anomaly. A Eu anomaly is

Atoraic Number

57

58

59

60

61

62

63

64

65

66

67

defmed as EuEu* where Eu* = d(sm x Gd) (Roiiinson. 1993).

Element

La

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

DY Ho

Atomic Weight

138.91

140.12

140-9 1

144.24

145.00

150.40

151.96

157.25

158.93

162.50

164.93

Ion

~ a *

Ce*

~ d ~ +

Pm*

SmN

EU^' ~ d -

Tb3'

Df+ HO"

Ionic radius in 6-fold CO-ordination

1.032

1 .O 10

0.990

0.983

0.958

O. 947

0,938

0.923

0.912

0.90 1

3.3 Carbon and Oxygen botope Data

Carbon and oxygen isotope data are measured according to the PDB and SMOW

scales, respectively. Three distinct populations are evident, correspondhg to the

Esperanza, Hueyapa and El Cobre veins (Fig. 13).

Carbonate gangue nom the El Cobre vein is eMched in 13c and "O relative to

samples fkom the Esperanza and Hueyapa veuis. The El Cobre vein has 613c values of

-9.766 to -1.298 %O and 6"0 values of 1 1.942 tu 19.395 %O. The mean values of 613c

and 6180 are -3.909 and 16.443%. respectively. The Hueyapa vein has a narrow range

of isotopic values with 6 " ~ ranging between -8.796 to -10.558 960 (mean = -9.502 %O)

while the 6"0 values range between 1 1.687 to 12.992 %O (mean = 12.464 %O).

Carbonate kom the Esperanza vein has 6% values ranging between -6.632 to - 10.087%0

(mean = -8.288 960). whereas the 6180 values are between 6.839 and 12.549 (mean =

9.895 Carbon and oxygen isotope values of the drill holes (613c= -9.013 to -9.818

%O. 6 % = 9.905 to 10.05 %O) are similar to the isotopic values of the Esperanza vein.

A specimen of altered Morelos Limestone from drill hole TA216 C3204.20 has

very different iso topic values (Fig. 13) than the other drill holes since it is enriched in

both 13c and 1 8 0 (613c = 0.246 960,6~~0x 13.031 BO).

There appears to be a few inconsistencies within the three populations. Fistly.

Hueyapa sample SH97-50a plots near the El Cobre vein population. Secondly, El Cobre

sample SH97- 17 and Esperanza sample SH97-44 fali within the Hueyapa vein population

(Fig. 13). Thirdly, El Cobre samples SH97-11 and -12, coarse carbonate in vuggy veins,

have 13c values shilar to the main El Cobre vein population but are depleied in "O in

cornparison to the other El Cobre samples (Fig. 13). F ï y , Hueyapa sample

Fig. 13 Plot of G'3~posl versus 6 1 8 ~ c s ~ o w for carbonate gangue fkom the El Co bre, Esperanza and Hueyapa veins and veins intersected in drill holes. Srnall inset shows sample numbers from the Esperanza and Hueyapa veins and veins intersected in drill hoIes.

SH97-57, taken from the Hueyapa manto. is enriched in ''0 relative to the other Hueyapa

samples.

The Morelos Limestone is significantly more enriched in 13c and "O than either

the Taxco Schist or the Mexcah Shale (Fig. 14). and ''0 data for carbonate miner&

dissolved from the Taxco Schist plot within the Hueyapa and Esperanza vein populations.

Carbonate in the Mexcala Shale has isotopic values that plot near the El Cobre vein

popuiation. Shale samples TS-18 and TS-33. taken proximal to the EL Cobre vein. have

values that are simiiar to carbonate in the vein whereas samples TS-8 to TS-12,

collected at distances of up to 8 km nom the vein system. have more positive 'b values

(Fig. 14). This suggests that carbonate in the host rocks is of hydrothemal origin and

indicates a regional carbonate alteration associated with a hydrotherrnai system. It is

unlikely that the divergence between the schist, shale and Limestone is the result of

diagenetic conversion of organic carbon to C@. Mexcaia Shale and Taxco Schist

contain organic carbon with an average 6I3c value of -24.04%. (Fig. 14).

3.4 Rare Earth Elernent Data

REE data were obtained for 3 1 samples of carbonate gangue Erom the El Cobre,

Esperanza and Hueyapa veins and veins intersected in drill holes. REE data were also

determined fkom whole rock analyses of the Mexcala Shale, the Taxco Schist. the Taxco

Roca Verde and the Morelos Limestone. The REE data are standardized according to the

North Ameriçan Shale Composite, NASC (Haskin et ai, 1968). which approximates the

REE concentrations of the average sedimentary rock found in the continental platform.

The El Cobre, Esperanza and Hueyapa veins and veins intersected in drill holes

a I El Cobm vein

+.

Fig. 14 6 1 3 ~ p ~ ~ ~ - G ' 8 ~ n ~ ~ w , plots for; (a) carbonate dissolved from Taxco Schist, Morelos Limestone and Mexcala Shale; and (b) carbonate gangue in ore samples.

proximal to the Esperanza vein have different NASC-normalized REE values and

patterns. The El Cobre samples have La/Lu that are genetally <l (Fig. 15). NASC-

normalized REE values generaiiy lie between 1 and 0.01 with a high number of samples

near 0.1. Many samples have low to moderate positive Eu anomalies. The LaLu of the

Esperanza samples are considerably el to >1 (Fig. 16). Eu anomalies range fiom

moderately positive to none to negative. MC-norrnalized REE values are generally

between 1 and 0.1. In the Hueyapa samples, La/Lu is significantly >1 with one sample

slightly tl (Fig. 17). NASC-normalized REE values are between 1 and 0.01. AU

Hueyapa samples have positive Eu anomalies. In general, there are no major differences

in the abundance of REE between the three veins.

The REE patterns of most of the El Cobre sarnples are characterized by depletion

in bo th LREE and HREE and positive Eu anomalies relative to NASC (Fig. 15). The

NASC-normalized REE cwves have positive slopes and Iess commonly flat slopes, that

is the samples are depleted in LREE relative to the HREE. There is a marked vertical

zonation in the absolute abundance of REE in the El Cobre vein such that the average

NASC-normalized REE content increases with depth in the vein (Fig. 18). Saniples

SH97-10 and SH97-11 are marked by unusuaiiy strong, positive Eu anomalies. These

samples are omitted from the calculation of average REE concentrations in the vein

because they have atypical Eu anomalies.

NASC-nomalized REE curves of carbonate fiom the Esperanza vein have slopes

that are either positive or negative (Fig. 16). The Esperanza samples are depleted in

LREE and HREE relative to the NASC standard. Al1 samples except those with high

hematite contents, specificaiiy samples SH97-29a and SH97-32, have positive Eu

100.000 1 1 1 1 1 1 1 1 1 l I l l 1941@@ I I l l l l l l l l l l l (a) SE97-OS (b) SH97-09

100.000 I l l l l l l l l l l l l (t) SH97-12

1 1 1 1 1 1 1 1 1 1 1 1 1 (d) SE97-ll

Fig. 15 NASC-normalized REE concentrations in carbonate gangue f?om El Cobre vein sarnples: (a) SH97-08; (b) SH97-09; (c) SH97- 10; (d) SH97- 1 1 ; (e) SH97- 12; (f) SH97-15; (g) SH97-16b; (h) SH97-Mc; (i) SH97-17; 03 SH97-18; and (k) SH97-24.

llO.OB0 I I I I I I I I I I I I I (b) SH97-16~ ,

t0.100 3

100.000 1 1 1 1 1 1 1 1 1 1 1 1 1 (J) SH97-18

Fig. 15 continued

lO...@. I I I I I I I I I I I I I (b) 589'1-298 -

10.0oo -j

- - - 0.001 I l I I I I I l I I I I I

L i Ce Cr I I P r Sm B i C1 Tb Dy H o T m Er Yb L i

100.000 l l I l l I 1 1 1 1 1 1 1 (d) SE9740

Fig. 16 NASC-normalized REE concentrations in carbonate gangue €tom Esperanza vein samples: (a) SH97-26; (b) SH97-29a; (c) SH97-32; (d) SH97-40; (e) SH97-42; (f) SH97-43; (g) SH97-46; and (h) SH97-47.

100.000 l I l I l I l I I I l I l - (t) S B 9 7 4 2 - 10.000 3

I0@.000 100.000 l l l l i l l 1 1 1 1 1 1 I I I I 1 I I I I I I I I (g) SE91-16 (a) SH97-41

Fig. 16 continued

100.000 I l l l l l l t l l l l l 1oo.eoe 1 1 1 1 1 1 1 1 1 1 1 1 1 (8) SE97-49 (b) SH97-508

100.000 1 1 1 1 1 1 1 1 1 1 1 1 1 (d) SH97-54.

Fig. 17 NASC-nomalued REE concentrations in carbonate gangue fkom Hueyapa vein samples: (a) SH9749; (b) SH97-50a; (c) SH97-53a; and (d) SH97-54a.

Fig. 18 Average NASC-normaked REE concentrations in carbonate gangue fiom the El Cobre vein on levels 1,5,7 and 9 of the Guerrero mine. Samples SH97- IO and SH97- 1 1 are omitted.

anomalies. AU of the Esperanza samples are depleted in REE relative to the standard.

The average NASC-nonnalized REE patterns for levels 1 and 2 of the Remedios mine

have positive Eu anomalies and are either tlat or have slight enrichment in LREE relative

to the HREE (Fig. 19). In contrast, the average NASC-normalized REE pattern of level3

has pronounced depletion in LREE relative to the HREE with no Eu anomaly. There is a

crude vertical zonation of REE content in the Esperanza vein such that average REE

abundances increase with depth; however, kvel3 does not fit this trend mg. 19).

The REE patterns of carbonates kom the Hueyapa vein are similar to those fiom

the Esperanza vein. WC-normalized REE patterns dispiay either enrichment or

depletion in LREE relative to HREE (Fig. 17). AU REE curves have positive Eu

anomalies. A plot of the average NASC-normalized REE abundances in the Hueyapa

vein reveals LREE enrichment relative to the HREE and a significant positive Eu

anomaly (Figure 20). AU samples are depleted in REE relative to the NASC standard

except sample SH97-53a which is enriched in REE relative to NASC. It is excluded fiom

the calculation of average NASC-normalized REE concentration since it has

unc haracteristicaliy high REE values.

The REE patterns of carbonate from veins intersected in drill holes are highly

irregular but are siinilar to the Esperanza samples since they are in close proximity to the

Esperanza vein (Fig. 2 1).

NASC-norrnalized REE curves for whole rock analyses of the Taxco Schist and

the Mexcala Shale are typified by depletion in LREE and minor enrichment or depletion

in HREE relative to NASC (Fig. 22). In contrast, the Taxco Roca Verde has REE values

almost identical to those of NASC. These NASC-norrnaiized REE distributions suggest a

Fig. 19 Average NASC-nomalized REE concentrations in carbonate gangue from the Esperanza vein on levels 1.2 and 3 of the Remedios mine.

Fig. 20 Average NASC-normalued REE concentrations in carbonate gangue fiom the Hueyapa vein on level4 of the San Antonio mine. Sarnple SH97-53a is omitted.

100.000 l I l l l l l l l T l l 1 100.000 I I I I I I I I I I I I I (c) TA 2 1 6 8 204.20 (d) TA216@ 77.15

Fig. 2 1 NASC-normalized REE concentrations in carbonate gangue from veins intersected in drill hole samples: (a) TA205 @ 165.85; (b) TA153 @ 177.85; (c) TA216 (9204.20; and (d) TA216 CB77.15.

+ Wre106 Limestone

+ hascdaSbaie

+ Taxmschist

Fig. 22 NASC-nomalized REE concentrations for whole rock analyses of the Taxco Schist, the Taxco Roca Verde, the Mexcala Shale and the Morelos Limestone.

sedimentary rather than an igneous origin for the Taxco Schist and the Taxco Roca

Verde. The NASC-normalized REE pattern for whole rock anaiyses of the Morelos

Limestone is flat to slightly positive with a srnail positive Eu anomaly. The NASC-

normalized REE values of the Morelos Lirnestone are generaliy < 0.05, which are

considerably less than values of the Taxco Schist, Mexcah Shale and Taxco Roca Verde.

3.4.1 Average REE Values as a Function or Depth

The NASC-normalized average values of La, Eu and Lu in carbonate gangue

îkom the El Cobre, Esperanza and Hueyapa veins and veins intersected in drill holes

proximal to the Esperanza vein exhibit vertical variati~ns in the individual veins as well

as spatial variations between the veins and the drill holes (Fig. 23). The El Cobre vein

shows the most prominent trends where average La, Lu and Eu values increase with

decreasing elevation.

In general, the average La concentration is highest in the Hueyapa vein excluding

anomalous average La values in level2 of the Esperanza vein. In order of decreasing La

abundance it is followed by the Esperanza vein, the El Cobre vein and veins intersected

in drill holes. The El Cobre vein is characterized by La contents that increase with depth.

In contrast, the Esperanza vein is marked by decreasing average La concentration with

depth although level2 of the vein has the highest values.

The distribution of average Eu with elevation is similar to that of average La. The

Hueyapa vein has the highest average Eu values and veins intersected in drill holes have

the lowest average Eu values, with the exception of drill hole TA205 @ 165.85 that has

the highest Eu values. The El Cobre vein exhibits vertical variation in average Eu

Fig. 23 Average NASC-nomalized La. Eu and Lu values plotted as a hinction of elevation above mean sea level (m): (a) average La; (b) average Eu; and (c) average Lu.

abundances such that the Eu content increases slightly with depth. The opposite trend is

observed in the Esperanza vein dthough level2 is marked by much higher Eu values.

The highest average Lu concentrations are found in the El Cobre vein. Veins

intersected in cirili holes have the lowest average Lu abundances. The Hueyapa vein has

higher average Lu concentrations than the Esperanza vein. Average Lu values increase

with decreasing elevation in the El Cobre and Esperanza veins. k v e l 2 of the Remedios

mine is again typified by reiativeIy high values.

4.0 Fluid Inclusions

Ruid inclusions are samples of fluid trapped within a mineral at the the it

crystallized or during a later deformation. Most fluid inclusions conskt of a liquid phase

and a vapour bubble but they rnay also contain soluble salts and ore elements. The

systematic examination and measurement of fluid inclusion populations provide

information about the composition, temperature and pressure of the ore-forming fluids.

Homogenization temperatures were determined for 138 fluid inclusions in quartz and

carbonate fiorn the Esperanza and Hueyapa veins in order to constrain the hydrothermal

fluid conditions during the fonnation of these veins and to find evidence of boiling.

4.1 Theory of muid Inclusion Analysis

The most abundant type of fluid inclusion contains liquid water and a vapour

bubble. Na. K, Ca, Mg. Ci and SQ" are the major constituents of the liquid but CR,

H2S, C&, CO and N2 may be present in minor amounts- The vapour bubble may consist

of either H20 or highly compressed gas such as C@. In the epitherrnai environment,

fluid inclusions typically consist of vapour bubbles and low-salinity HB-rich liquid, < 2

wt % NaCl equivalent, (Bodnar et ai, 1985) aithough solutes rnay range in value kom O

to 50 wt % NaCl equivalent (Roedder. 1984). Daughter minerals of halite and sylvite are

usuaily absent in fluid inclusions of epithermal deposits (Bodnar et al, 1985).

The data obtained fiom fluid inclusion analyses are only useful when the origins

and the temporal relationships of the fluid inclusions are known. Roedder (1984)

describes in detail the formation and the identification of fluid inclusions. Primary fluid

inclusions are those inclusions that are trapped dong crystal faces during crystal growth.

Secondary fluid inclusions form within planes that outline healed fractures afier the

crystailization of the host minerai is complete.

Vapow bubbles within fluid inclusions are the result of differential shruikage of

the iiquid and the host mineral during cooling nom the temperature of trapping, Tt. to

arnbient temperature (Sorby. 1858 in. Roedder. 1984). Tt is estimated by heating the

fluid inclusion to some temperature at which the vapour bubble disappears. This

temperature is referred to as the temperature of homogenization, Th. The assumptions of

the homogenization method are as follows: ( 1) the original fluid is a single homogeneous

phase; (2) the cavity enclosing the fluid does not change in volume after sealing; (3) the

inclusion does no t change its volume after sealhg ; (4) the effec ts of pressure are

insignificant; (5) the origin of the fluid inclusion is known; and (6) T h are preck and

accurate (Roedder. 1984). Except where the pressure is very low or where boiling has

occurred, Th is no t the same as Tt and it is necessary to have some estimate of the

pressure at which the fluid was trapped in order to calculate Tt fkom Th.

Liquid-rich fluid inclusions homogenize into the liquid phase by expansion of the

H20-rich Liquid and disappearance of the vapour bubble upon heating. Vapour-rich fluid

inclusions hornogenize to the vapour phase if a homogeneous fluid was trapped. If the

fluid was heterogeneous when trapped. that is. it contained some liquid as weil as vapour.

the fluid inclusions behave like Liquid inclusions by the contraction and disappearance of

the vapour bubbles at anomalously high temperatures.

Sample Selection and Instrumentation -

Samples of quartz and carbonate gangue fiom the Esperanuc and Hueyapa veins

were examined petrographicaily for nuid inclusion shapes. sizes, origin and classification

(Appendix 7). Primary and secondary inclusions were classified according to the criteria

discussed by Roedder (1984). Doubly polished quartz and carbonate chips, 100 microns

thick, were emplo yed in micro therrno metric anaiyses. Homogenhtion temperatures

were measured using the Linkam THMSG 600 heating-cooling stage (Appendix 7).

Calibration of the stage was conducted using synthetic fluid inclusion standards of pure

H20 and HD + Ca. Homogenization temperatures were measured for 105 fluid

inclusions in quartz and carbonate from the Esperanza vein and 33 fluid inclusions in

quartz and carbonate from the Hueyapa vein (Appendix 7).

4.3 Fiuid Inclusion Descriptions

The E s D ~ ~ I z ~ Vein

The Esperanza vein contains two-phase fiuid inclusions at room temperature. The

fluid inclusions are liquid-rich and consist of HzO-rich iiquid with &O vapour bubbles

typicaily constituting 5% to 10% vapour voIume (Plate 15a).

Primary fluid inclusions occw as single or clustered inclusions that are rounded or

irregularly shaped and range in size from 5pm to < 1 0 p . Negative crystal forms in

quartz and carbonate are unconmon. Few primary inclusions have vapour volumes

greater than 15%. Secondary fluid inclusions occur as fracture-controlied planar groups

and are thin, elongated and irregular in shape. The inclusions are < 5pm to Spm in size.

The Huevana Vein

At room temperature, the Hueyapa vein contains iiquid-rich and vapow-rich fluid

inclusions. The fluid inclusions typicaily contain H20-rich liquid with H20 vapour

Plate 15 Fluid inclusions in the Esperanza and Hueyapa vetos: (a) liquid-rkh inclusion in quartz gangue kom SH97-29; (b) vapour-rich inclusion in carbonate fiom SII97-32; and (c) rhombic-shaped fluid inclusion in carbonate fiom SH97-51.

bubbles. Liquid-rich inclusions have vapow bubbles constituting 596-78 of the inclusion

volume whereas the vapour-rich fluid inclusions have 3 5 - 0 8 vapour volume (Plate

15 b). Vapour-nc h inclusions are su brounded or irreguiarly shaped and occur as clusters

containhg one to three inclusions. These inclusions are larger in size, 5p.m to 12~un,

than the liquid-rich inclusions, <

Primary fluid inclusions are irregular or rounded and randomiy distributed in

carbonate and quartz. Rhombo hedrd-shaped inc1~ions in carbonate are no t uncommon

(Plate 1%). Secondary fluid inclusions form as planar groups of elongated inclusions.

The presence of fluid inclusions with variable liquid to vapour volumetric phase

ratios is the most common evidence of entrapment from boiling fluids (Bodnar et ai,

1985). The vapour-rich inclusions are believed to result korn boiiing fluids rather than

necking down processes due to their large size relative to the liquid-rich inclusions, the

absence of one-phase liquid inclusions and the presence of muid-rich inclusions with

constant 1iquid:vapow phase ratios and consistent Th. The occurrence of vapour-nch and

liquid-rich inclusions suggests the presence of two immiscible phases due to boiling at

the time that the fluid inclusions were trapped. Textures indicative of boiling are present

in the Hueyapa vein, but are not as common as those cited in other epithemal systems.

The El Cobre Vein

Gonzalez-Partida (1996) reported the coexistence of inclusions with liquid and

vapour and inclusions of pure vapour in the El Cobre vein. Sanchez-Torres (199 1)

reported similar observations in El Cobre samples. The presence of fluid inclusions with

variable Liquid to vapow volumetnc phase ratios suggests that these fluid inclusions were

trapped fiom boiling fluids.

4.4 Freezing and Eeating Measurements

Salinities of fluid inclusions are determined by measUrhg the freezing

temperatures of the fluid inclusions. According to Potter et al (1978). the fkzing point

of a fluid inclusion is the temperature at which the 1 s t ice crystal melts. The equation

used to determine the salinity of a fluid inclusion is based on the H20-NaCI system after

Potter et al (1978):

W, = 1.769580 - 4.2384 x 1û2e2 + 5.2778 x 10%~ (+/- 0.028 wt W NaCI)

where, w, = the weight percent NaCl in solution and

8 = fieezing point depression in OC

Freezing temperatures were measured prior to homogenization temperatures to

avoid leakage or decrepitation of the fluid inclusions (Roedder, 1984). Accurate

rneasurement of fieezing temperatures was Lunited to three Liquid-rkh inclusions in the

Esperanza vein due to their small sizes. The salinities of these fluid inclusions range

from 3.05 to 3.37 wt % NaCl equivalent, Gonzalez-Partida (1996) reported salinity

values of 0.8 to 18.6 wt % NaCI equivaient in the El Cobre vein (Appendix 8).

Homogenization temperatures were determined for primary and secondary

inclusions with constant iiquid:vapour volumetrïc phase ratios to avoid inaccurate

measurements resulting fiom necking d o m processes, leakage, stretching and

decrepitation (Bodnar et ai, 1985). The Th measurements were made at rates of

3"Uminute to S°C/minute. Several fluid inclusions were heated three times and the error

was determined to be +/- 1°C. No pressure correction was applied to the Th since it

approximates the Tt for HzO-NaCl fluid inclusions trapped from boiling fluids (Roedder

and Bodnar, 1980 in, Bodnar and Vityk, 1998).

4.5 Homogenization Temperatme Data

Homogenization temperatures of the fluid inclusions in the Esperanza vein range

lÏom 122OC to 278T (Fig. 24a). Three distinct fluid inclusion populations are

established based on probability analysis of the Th data using Systat 7.0 (Fig. 25a). The

three groups have Th of 122°C to 157"C, 16 1°C to 258°C and 265T to 278°C-

The Hueyapa vein has fluid inclusions that homogenize at temperatures ranghg

fiom 180°C to 360°C (Flg. 24b). Probabiiity analysis of the Th data (Fig. 2Sb) yields

three fluid inclusion populations with Tb intervals of 180°C to 247°C. 275°C to 3 15°C and

352°C to 360°C-

Gonzalez-Partida (1996) conducted a fluid inclusion study of quartz fÏom the El

Cobre vein. He found that the fluid inclusions homogenize at temperatures ranging from

170°C to 289°C (Fig. 24c; Appendk 8). There B an increase in the T h of the fluid

inclusions with increasing depth in the vein (Fig. 26).

The El Co bre, Esperanza and Hueyapa veins have similar fluid inclusion

homogenization temperatures. The primary fluid inclusions in the Esperanza and

Hueyapa veins yielded homogenization temperatures with a modal value of 230°C

although values range nom approximately 210°C to 250°C (Appendk 7). The primary

and secondary fluid uiclusions of the Esperanza vein have the lowest Th values cornpared

to those of the El Cobre and Hueyapa veins. The Hueyapa vein is characterized by some

vapour-rich fluid inclusions that homogenize at hig her temperatures of 352°C to 360°C.

These anomalously hig h temperatures are pro bably the result of a vapour-rich fluid that

trapped some liquid during boiüng. The middle population of inclusions, with

homogenization temperatures between 275°C and 315"C, may also be a result of

Fig. 24 Homogenization temperatures of fluid inclusions fiom: (a) quartz and carbonate samples kom the Esperanza vein; (b) quartz and carbonate samples fkom the Hueyapa vein; and (c) quartz samples fkom the El Cobre vein. The El Cobre data are taken fkom Gonzalez-Partida (1996). The n refers to the number of detennuiations in the Esperanza and Hueyapa samples; in the El Cobre vein, n refers to the number of samples.

3 j 1 . ; (a) Esperanza vein 2C

I !

; (c) Ei Cobre vein j 2-

Fig . 25 Pro babilit y plots of homogenization temperatures for the: (a) Esperanza vein; (b) Hueyapa vein; and (c) El Cobre vein. Populations are divided by iine segments.

Fig. 26 Hornogenization temperatures of fluid inclusions in quartz samples Erom the El Co bre vein at: (a) Level O; (b) Level 1 ; (c) Level2; (d) IRvel5; (e) k v e l 7 ; and (f) Leve19. Based on data fiom Godez-Partida (1996). The n refers to the number of samples analysed and not the number of fluid inclusions. :

heterogeneous trapped some Iiquid during boüing. The middle population of inclusions.

with homogenization temperatures between 27S°C and 3 1%. may also be a result of

heterogeneous entrapment.

5.0 Discussion

The isotopic composition of the hydrothennai fluid responsible for the deposition

of the El Cobre, Esperanza and Hueyapa veins may be detennined nom the carbonate

gangue in these veins. The evolution of the fluid may be explained by the mixing of

different fluids, such as meteork and magmatic water, that interacted with the

surrounding host rocks as part of a hydrothermal system.

The fluids that deposited the carbonates in the El Cobre, Esperanza and Hueyapa

veins appear to have distinct carbon and oxygen isotopic compositions that are reflected

in the 813c and S1'O values of the carbonates in the three veins. The El Cobre vein has

the highest 613c and 6180 values of the three veins (Fig. 27a). The Esperanza and

Hueyapa veins have sirnilar 613c values; however, the Hueyapa vein has slightly higher

6180 values than the Esperanza vein (Fig. 27a). The 6 " ~ and 6180 values of carbonates

in the Mexcala ShaIe are similar to those in carbonates of the El Cobre vein (Fig. 27b).

6180 values for Mexcala Shale samples taken closest to the El Cobre vein (TS-18 and TS-

33) are Iower than those for shale samples taken several kilometers away from the vein

(TS-8 to TS-12) (Fig. 27b). Sarnples TS-18 and TS-33 represent shale that has k e n

strongIy altered by the hydrothermal system and samples TS-8 to TS- 12 are l e s dtered

and closer to the 6I3c and 6180 values expected firom marine shale modifed by

diagenesis. Carbon and oxygen isotope values for carbonate in the Taxco Schist (Fig.

27b) approximate those in carbonates fiom the Esperanza and Hueyapa veins. It appears

that the carbonate in the Mexcala Shale and the Taxco Schist are of hydrothermal origin

and are located within a pervasive regional carbonate alteration zone associated with a

hydro thermal system.

Fig. 27 6L3~pow-6'8~GMoW, plots for: (a) carbonate gangue in the El Cobre, Esperanza and Hueyapa veins; and (b) carbonate in the host rocks.

The main factors that contribute to the evolution of the hydrothermal fluid are

changes in fluid sources and changes in source rock composition. Temperature is of

minor importance since the range of average temperature estimated fiom fluid inclusions

is 2 1 O°C to 250°C.

5-1 Isotopic Composition of the Hydrothennal Fid& Calculzited from Carbonate Gangue in the Vejns

The iso topic compositions of the hydrothermal fluids responsibIe for the

deposition of each of the three veins were calculated using homogenization temperatures

determined kom fluid inclusion studies. By using the follawing temperature-dependent

fractionation equations provided by Friedman and O'Neil(1977):

calcite-Ca 1000 ln a = 2.988(10~/r2)-7.666(10~/T) +2.461 T = 0-700°C

6 2 calcite-HzO 1000 in a = 2.78(10 f l )-2.89 T = O-500°C

and the relationship between 1000 ln aCw and the fractionation of and ''O between

carbonate and water as foliows:

18 i 000 in a', = - 6 O-,=

13 1000 in a', = 613~cuboaiie - 8 Car

the isotopic compositions of the fluids in equilibrium with each vein system were

calculated (Fig. 28; Appendix 9).

The variation in the isotopic composition of the three veins and the ore-forrning

fluids may be explained by the interaction of three possible sources of 1 8 0 and 13c.

These are meteoric water, magmatic water and the host rocks surroundhg the veins

through which the hydrothemal solutions passed. The rnixing of magrnatic Ca with

some combination of the Mexcala Shale and the Morelos Limestone can account for the

Fig. 28 Calculated 6 ' 3 ~ ( P D B ) and :&es of the El Cobre. Esperanvl and Hueyapa fluids at 230°C. The 813~eDB) and 6 onMow, vaiues of magmatic water are taken fiom Rolluison (1993). nie 613~(pDB) value of meteoric water is taken nom Ohmoto and Rye (1979) while the 61'~(sMow value of meteoric water at Taxco. Mexico is estimated using the equation by Craig (196 1) in, Field and Fifarek (1985).

613c and 6'*0 values of the El Cobre vein; however, this combination cannot account for

the 6 ' ) ~ and 6180 values found in the Esperanza and Hueyapa veins (Fig. 28). The

hydro thermal fluid responsible for the deposition of the El Cobre, Esperanza and

Hueyapa veins may be derived from either meteoric water that has reacted with the host

rocks or a combination of meteoric water and early, minor magmatic Ca that has reacted

with these rocks.

The differences in 6180 values between the three vehs cm be explained by

variation in the ratio of hydrothermal fluid to host rock with time. The 613c values of the

El Cobre vein can be related to the leaching of original caicite from the host rocks by the

hydrothermal fluid; however. the Esperanza and Hueyapa veins have 613c values that are

more negative than those of meteoric water and magmatic C a (Fig. 28). In order to get

S13c values that are isotopically Lighter than the meteonc water and the calcite in the

source rocks, the fluid must have hcorporated isotopicaiiy light 12c fiom another carbon

reservoir. The 6% values of organic carbon &om the Taxco Schist and the Mexcala

Shale (average 6I3c = -24.04%.) are considerably depleted in "C and enriched in 12c

relative to the PDB standard (Fig. 27b). The mixing of "C fkom the original carbonate,

organic carbon and methane in the host rocks can explain the range of 613c values in the

Esperanza and Hueyapa veins.

There is no direct evidence supporthg the contribution of magmatic water to the

hydro thermal fluid. The source of metals in the veins may be attributed to the Taxco

Schist and the Mexcala Shale whereas the heat source driving the hydrothermal system is

probably related to magmatism which produced the Tertiary rhyoiite flows and

ignimbrites in the area. Gonzalez-Partida (1996) reported saiinity values in the El Cobre

vein ranging fkom 0.8 to 18.6 wt 8 NaCI equivalent which may indicate a magmatic

component or simply high salinity values related to dissolution of carbonate by the fluid

and not mixing of meteoric water with saline, magrnatic water, Since there is no strong

evidence of a signifiant magmatic cornponent to the hydrothermal fluid, the proposed

mode1 w u describe the deposition of the El Cobre, Esperanza and Hueyapa veins &om

heated meteoric water that reacted with the wail rocks.

5.2 Isotopic Composition of the Hydrothertd Fluids Calculated From Host Rocks Interacting with Meteoric Water

5.2.1 Calculation of Organic Carbon in the Ruids

Two main sources of 13c in the surroundhg host rocks are marine carbonate and

organic carbon converted to C a by diagenetic processes. The diagenetic conversion of

organic carbon to C a and C h is shown by the following simplifred equations:

2CH20 + 2H20 + 2 C a + 8W

2C + 2H20 + C a + C&

CH4 + 2H20 + 8H+ + C a

Aqueous carbon species such as C@(aq), HzCO3, HCOi. CO*; and CIt(aq) are

important in hydrothermal fluids at temperatures below 600°C. The isotopic composition

of carbon species in solution is measured with reference to 813~~m3(.p) as foilows:

8l3ci = G 1 3 ~ ~ m 3 < w 1 + di

where Ai is the relative isotopic enrichment factor between the carbon species i and

Hs03(ap) and H2C03(ap) approximates C a (Ohmoto, 1972). Isotopic enrichment

factors are reported by Ohmoto (1972) for Ct4 (g. aq) and C(gcaphite) at 300°C and

400°C (Table 3).

Table 3 Isotopic e ~ c h m e n t factors (%O) for carbon specks (after Ohmoto, 1972)

Applying the enrichment factors (Table 3) to the average 613c value for organic carbon in

the Mexcala SMe and the Taxco Schist (-24.04 %O), if can be seen that a temperature of

300°C will induce an isotopic fiactionation of C(graphite) to produce H2C03(w with a

6 ' ) ~ value of-10.84%o. near the average value of the Esperanza and Hueyapa vein

carbonates, whereas a temperature of 400°C wiU produce a value of -12.44%0. This

suggests that 300°C is a slightly more reasonable temperature for these hydrothemal

fluids.

In order to mode1 the behaviour of I3c in the hydrothermal solution as it passes

through the host rocks, the initial meteoric fluids are assumed to have essentially no CO2

because the H a 3 kvels in these fluids are generally very low. Aiso, the dissolution of

calcite and dolomite found in the host rocks wiii overwhelm any smaU arnounts of HKO3

in the initial meteoric water. The 8 ' ' ~ value of the carbonates in the source rock are set

at -2 %O to O %O which is Iighter than the Morelos Limestone (-3.20 %O) but somewhat

heavier than the Mexcala Shale (-2.65 960). This is based on the diagenetic conversion

of organic carbon to CG.

The progressive deplet ion of carbonate, organic carbon and methane in the source

rocks and the associated reduction of "C in the fluids is achieved by incrementally

dissolving carbonate minerals f?om the host rocks and at the same t h e incrementdy

Carbon Species

C h C(graphite)

10 in a 300°C 4Oo0C

-25.20 - 19.60 -13.20 -1 1.60

GL3~H2C03(ap)

300T 400°C

1.16 4.44

-10.84 -12.44

converthg C(graphite) to C a and CH, to C a (Appendix 10). The key to the mode1 is

that the carbonate miner& will be completely dissolved before al i the C(graphite) is

converted to Ca and Ca, At this point, the C(graphite) accounts for ail the H2C@ in

the hydrothemal solution and accordingly, the 613c reaches a minimum value of -10.84

near the average value of the Esperanza and Hueyapa vein carbonates. This is the

minimum 6I3c value the fluid reaches based on the average 6I3c value of the Mexcaia

Shale and the Taxco Schist and the isotopic enrichment factor of C(graphite) at 300°C.

5.2.2 Caldation of Watec Rock Ratios

The oxygen isotope exchange between the source rocks and the hydrothemal

fluids is outlined by the following equation (Ohmoto and Rye ,1974; Appendix 10):

where 6'8dw = final isotopic composition of the water after equiübration with the rock

6"d, = initial isotopic composition of the rock

S1'O', = initiai isotopic composition of the water

A,, = temperature-dependent fractionation factor between the rock and the water

w/r = ratio of exchanged oxygen atoms (wt %) in the water to those in the rock

(ie) wt 8 oxygen in the water = 88.8 wt % = 1.8R wt % oxygen in the rock = 50.0 wt %

R = waterrock ratio, that is, the proportions of water and rock that have

isotopically equilibrated

If early reactions involve mainly the dissolution of carbonate whereas later reactions

involve silicates, the 1000 ln a wiii Vary from that for clay-H20 and calcite-Hz0 in the

early stages to quartz-&O in the hter stages (Table 4).

Table 4 Fractionation factors for mineral pairs at 300°C taken fÏom Friedman and O'Nei1(1977), O'Neil and Taylor (1967) and Anderson and Arthur (1983).

a From Friedman and O'Neil(1977) b From Friedman and 0TNeil(1977) c From O'Neil and Taylor (1967) d From Anderson and Arthur (1983) e From Anderson and Arthur (1983)

Mineral Pair

quartz-H20P

calcite-&Ob

plagioclase-H2OC

i i i i t e - ~ , ~ ~

chiorite-H20e

5.3 Stable Isotopes and Fîuid Evolution

1000 ln a (%O)

I

6.86

5.57

4.87 1

258

0.05

Differences between the 6 ' 8 ~ and 613c compositions of the ore-fonning fluids

cm be explained by oxygen isotope exchange involving waterxock ratios and the

incorporation of increasing amounts of organic carbon and methane as the migrating

hydrothermal fluid leached the source rocks. The 6180 values of the hydrothermal fluids

were determined at 300°C using weighted kactionation factors for illite-H20, chlorite-

H20, plagioclase-H20 and quartz-Hfl plus varying w/r values and waterxock mass ratios

(Figure 29, Appendix 10). The 6I3c values are arbitrariiy matched with #*O values of

the hydrothermal fluid to show the iacreasing importance of organic carbon in the fluid

with t h e . This curve is comparable to that generated by Taylor and Bucher-Nurminen

(1986) (Fig. 30).

Isdopic evdutïoo of tbe hyardbamrl fliid

. . . . . .

dissdutioo of silicates

O A

O

. . . . . . : :::.; A

. . . . . . . . dissdutioo of silicates A.::.::* . . - / &dm"

Fig. 29 613~pDB~-618~,sMOw pi0 t depicting the iso topic ~ V O ~ U tion of the El Cobre, Esperanza and Hueyapa fluids as the fiuids leached carbonates predominantly in the early stage and silicate miner& in the late stage of the system. The El Cobre, Esperanza and Hueyapa fluids are at 230°C.

WALL ROCK CALCrrE

WIN CALCITE

I ' Central Zone

Fig. 30 Variation in 6l3cPDB, and 6180(sMoW, of calcite as a result of mixing metasomatic fluid (613c = -5460; 6180 = 10960) with dol~mitic marble (613c = 0.5460; 6180 = %%O) for: (a) open system at 500°C and X(C@)& = 0.1; (b) closed system at 500°C and X(C&)@ = 0. I; (c) closed system at 400°C and X(COt)@ = 0.1; and (d) open system at 400°C and X(C&)G = 0.25. Data are taken fkom Taylor and Bucher- Nurminen (1986).

Increases in the water:rock ratios reflect the cumulative effect of inçreasing

amounts of meteoric water that leached the shale and the schist over t he . The ratios of

weight percent oxygen available for reaction in the meteonc water to that in the rock and

the watetlrock ratios used in the calculations are arbitrary; however, it is the increase in

the waterrock ratios, regardless of the exact value. that explains the differences in 'b

composition between the three veins. Various combinations of w/r and water:rock ratios

wïil produce similar models. The results of this modehg are Uustrated by pbtting the

@'O values of the three fluids responsible for depositing carbonates in the three veins, as

calculated fkom the carbonate gangue, and the paths of the fluids, as calculated fiom

meteoric water interacting with the source rocks (Fig. 3 1). The value of 6'*0 is inversely

proportional to the waterrock ratio. Of the three fluids, the El Cobre fluid has the highest

6180 values and lowest watecrock ratios representing the earliest fluid which was

dominated by dissolution of carbonate minerals in the shale and the schist(Fig. 31). The

613c values of the El Cobre fluid rnay be attnbuted to the mixing of significant quantities

of isotopicaliy heavy 13c of the carbonate and lesser amounts of organic carbon and

methane generated C a (Fig. 29). The low 6180 values of the Esperanza fluid are

explained by the interaction of larger amounts of water with carbonate minerals and

increasing reaction between water and silicate miner& in the source rocks (Fig. 3 1). The

progressive decrease in the 613c values of the Esperanza fluid is a function of increased

amounts of carbon derived from organic carbon and methane, relative to original

carbonate, incorporated into the hydrothermal fluid. As the source rocks became

depleted in carbonate, the conversion of o r g e carbon and methane to CQ became

more signifïcant and the 613c value of the fluid reached a minimum of -10.84 460 (Fig.

Fig. 3 1 618~(sMow plotted againsi water:rock ratios showing the effects of increasing meteoric water content on the 6'8~(sMow value of the El Cobre, Esperanza and Hueyapa fluids with time. Dashed curve represents early stage fluids influenced by carbonate dissolution. Solid c u v e represents late stage fluids Uustrating the greater influence of silicate reactions with the fluid.

29). The more positive 6"0 values of the Hueyapa fluid may be attributed to prolonged

reaction between silicate minerals in the source rocks and a reduced proportion of

meteoric water to rock (Fig. 31). The I3c content of the fluid was bufKered by organic

carbon and methane conversion to Ca. This implies that the Hueyapa fluid probably

represents the waning stages in the temporal evolution of the hydrothermal system while

the El Cobre fluid evolved in the earliest stages of ore formation. The Esperanza fluid

formed intermediate to the El Cobre and Hueyapa fluids, and portions may have formed

at the same t h e as the El Co bre vein.

5.4 REE Distributions

NASC-normaLized REE distributions differ between the El Cobre. Esperanza and

Hueyapa veins. The El Cobre samples have LaLu that are generally <l (Fig. 32a and b).

There is a marked vertical zonation in the absolute abundance of REE in the El Cobre

vein such that the average REE content and the La/Lu increase with depth in the mine.

The LaLu ratios of the Esperanza samples are cl to considerably >1 (Fig. 32c and d). In

the Hueyapa samples, WLu ratios are significantly >1 to slightly < 1 (Fig. 32e and f).

There is no major difference in the abundance of REE between the three veins studied.

The REE are relatively insoluble; consequently they rernain relatively immobile

during 10 w-grade metamorphism, wea the~g and alteration. Hydro thermal act ivity is no t

expected to signifiçantly affect the REE content of rocks unIess waterxock ratios are very

high. Distribution coefficients, log Kd. for the substitution of trivalent REE into calcite

plo tted against ionic radius in 6-fold coordination show that larger REE, such as La,

partition more readily into calcite (Fig. 33). The decrease in apparent mineral-water

100.000 10o.ee0

10.000 10.0mo El Cobrc vcln V

El Cobrc vcln rn U < Fn

5 1.000 < i.oo0 * - C)

R -

1 0.100 rn rn 5 o.iee

0.0 10 0.01e

Fig. 32 NASC-normalized REE distributions from El Cobre, Esperanza and Hueyapa vein samples: (a) SH97- 12; (b) SH97- 18; (c) SH97-26; (d) SH97-42; (e) SH97-49; and (f) SH97-53a.

Fig. 33 Partition coefficients, log ka, for trivalent REE substituting into calcite as a function of ionk radius in 6-fold CO-ordination. Data are taken from Zhong and Mucci (1995) and Shannon and Prewitt (1969) in, Rimstidt et al (1998).

partition coefficient with decreasing ionic radius results fiom the formation of REE

complexes whkh reduces the amount of fiee REE avaihble in solution for exchange with

the mineral phase. Wood (1990) showed that aqueous REE complexes are more stable at

higher temperatures and that Lu complexed with fluoride, hydroxide and sulphate is more

stable at higher temperatures than the La complexes (Fig. 34). He predicts that cd; and

HCO-3 also form stronger complexes with HREE than with LREE in near neutral to basic

pH sohtions where ca2- and HC03- predominate. Lah and LU% speciation in a

theoretical hydrothermal fluid at 300°C and pressures corresponding to liquid-vapour

saturation of H20, Pa, indicate that REE complexing increases considerably as pH

increases and that HREE cornplex more readily than LREE (Haas et al, 1995). The study

by Haas et al (1995) ako shows that REE are strongly complexed by Cl, F and OH under

acidic, neutral and basic pH conditions, respectively (Fig. 35).

Fluid inclusion studies of the El Cobre vein indicate that temperature decreases

upward (Fig. 36a) by about 90°C over a vertical distance of about 425 m (Gonzalez-

Partida, 1996). It should be noted that Wood (1990) concluded that complexing only

doubles between 2S°C and 300°C. therefore, temperature is not a controiiing factor in

REE complexation within the El Cobre vein. Boiling of the fluid increases the pH as a

result of the separation of an acid vapour phase which, in tum, increases REE

complexing, in particular the HREE. Consequently, the abundance of REE in carbonate

should decrease and LaLu should increase as a hnction of boiling and increasing pH.

A schematic explanation of the evolution of the hydrothermal fluid at the

Guerrero mine is presented in Figure 37. The hydrothermal fluid initially leached

original marine carbonate fkom the Mexcala Shale and Taxco Schist which have NASC-

Fig. 34 Stability constants of sulphate. fluoride, chloride and hydroxide complexes for: (a) LA-"; and (b) LU^' at 2S°C and 300°C. Data are talcen fiom Wood (1990).

Fig. 35 REE speciation as a hinction of pH in a simulated geothemal fluid at 300°C: and Pm. and LU^' are complexed with hydroxide ion, fluoride ion and chloride ion (after Haas et ai, 1990).

Fig. 36 (a) Average temperature (OC); (b) average NASC-normaiized La; and (c) average NASC-normalized Lu as a fiinction of elevation above rnean sea level (m) in the EI Cobre vein.

REE coaœntmtioa a d WLu ratio in tb crilcite decmases with time

I Conipositioa af d a t e

initiai f l d conposition upon dissdution of &ginai carbooate in host rocks

/ - Intermediate composition of

Z /

C

the residud fiuid /

4

C C

REE 4mXenhtioa and ImILAI ratio in tbe muid k m m e s witb îiuœ

Fig. 37 Schematic explanation of the REE distributions in the ElCobre vein.

norrnalized L a L u ratios slightly cl. The composition of the initial fluid reflects the REE

content of these rocks (Fig. 37). The calcite precipitated tkom the initial fluid has LaLu

ratios slightly more positive than the initial nuid as a result of the preferential partitioning

of La into the calcite (Fig. 33). The residual fluid became more depleted in REE, in

particular the LREE, with LdLu considerably 4. Precipitation of REE in calcite

continued with lower concentrations in subsequent calcite and fluids. The average REE

values in carbonate decrease upward in the El Cobre vein with La decreasing upward

much more rapidly (fkom 0.3 ppm on level9 to 0.02 ppm on level O) than Lu (fiom 0.6

ppm on level9 to O. 1 ppm on level O) (Fig. 36b and c). The WLu ratio in the El Cobre

vein ranges fkom 0.5 on level9 to 0.2 on level O of the Guerrero mine. It would appear

that the preferential partitioning of La over Lu into the calcite structure during the early

stages of calcite precipitation results in a fluid, that during the late stages of precipitation

is considerably enriched in Lu over La; therefore, in spite of the effect of boiling and

alkaline pH, which favour greater complexing of HREE, the LaLu ratio decreases.

The NASC-normalized REE distributions of the Hueyapa vein differ fiom those

of the El Cobre and Esperanza veins in that the REE patterns of the Hueyapa vein show

enrichment in LREE relative to HREE ( WLu BI) mg. 32e and 0. The shape of REE

patterns is controiied primarily by waterrock ratios and mineralogy and indirectly by

fluid pH and temperature (Hopf, 1993). REE in plagioclase plotted against atomic

number display REE patterns wifh LaLu ratios > 1 and positive Eu anomalies (Fig. 38).

The late stage reaction of feldspars in the schist and the shale with the fluid increased the

LREE content of the fluid relative to the HREE. Consequently, reaction of plagioclase,

facilitated by increasing water:rock ratios with the, increased the LREE content in the

Fig. 38 REE distributions in plagioclase as a fbnction of atomic number. Data are taken &O m Ro llinson (1 993).

fluid and produced carbonate with La/Lu>l.

NASC-normalized REE patterns of carbonates in the Esperanza vein are

transitional between the fluids responsible for carbonate deposition at the El Cobre and

Hueyapa veins. The REE patterns are dominated initially by carbonate in the Mexcala

Shaie and the Taxco Schist (La/Lu < 1) and later by silicate minerah in the hydrothermal

solution (LaLu > 1). Michard and Albarede (1986) studied REE concentration in

hydrotherrnal solutions Born geothemal fields in Tibet and Bulgaria and found that the

NASC-nonnalized REE patterns are highly variable and have LaLu ratios < 1 to > 1.

The average Eu anomalies of carbonate gangue differ between the three veins

such that the El Cobre vein has the lowest average Eu anomalies where the Hueyapa vein

has the highest average Eu anomalies (Fig. 39). The Esperanza vein has moderate

average Eu anomalies compared to the other two veins. Average Eu anomaly increases

upwards in the Esperanza vein. The high average Eu anomalies in the Hueyapa vein may

suggest that the carbonate which precipitated fiom hydrotherrnal solutions was probably

derived fiom plagioclase (Fig. 38). Samples SH97-10 and SH97-11 have anomalously

high average Eu anomalies (Fig. 39) that suggest these samples are late carbonate veins

that formed around the time that the Hueyapa vein precipitated.

5.5 Mineraiogy of the El Cobre, Esperanza and Hueyapa Veins

The mineral assemblages of the El Cobre, Esperanza and Hueyapa veins are very

similar except for differences in the Fe content of sphalerite, the Sb and Ag content of

galena and carbonate and sulfosalt mineralogy.

Sphalerite fiom the El Cobre and Esperanza veins have the highest Fe contents

Fig. 39 Average Eu anomaiy ploned against elevation above mean sea level (m) for the El Cobre, Esperanza and Hueyapa veins and theMorelos Limestone. Dashed lùie segments represent the range o f values for average Eu anomalies. Samples SH97-10 and SH97- 1 1 are omitted fkom the calculation of average Eu a n o d y .

whereas the Hueyapa vein has the lowest Fe content in sphalerite. Fluid inclusion

analyses of the El Cobre, Esperanza and Hueyapa veins yielded modal homogeaization

temperatures of approxhately 230°C; however, the temperatures range for the El Cobre

vein is 289T at level9 to 173°C at kvel O (Appendix 8). This indicates an evolving

hydrothermal fluid fiom high temperature at depth to lower temperature at shallower

levels. Only to a minor extent does temperature control the isomorphous substitution of

Fe, and to minor extents Cd and Mn, for Zn in sphaiente. The Fe content in sphderite

appears to be a function of the amount of Fe in the muieralizing fluid at the time of

sphalerite precipitation.

The high Fe content in sphalerite from the El Cobre and the Esperanza veins is

attributed to large amounts of Fe in the ore-fonning fluid and suggests that the earliest

and the highest temperature fluids, originated at the El Cobre and the Esperanza veins.

Furthermore, it appears that portions of these veins formed contemporaneously based on

similar Fe content in sphalerite. The Hueyapa vein bas sphalerite with low Fe content

that reflects decreased quantities of Fe in the fluid and suggests that the Hueyapa vein

formed later than the El Cobre and Esperanza veins.

GaIena fkom level9 of the El Cobre vein contains the highest Sb and Ag contents

whereas that of the Esperanza vein has the Iowest Sb and Ag values. Ag and Sb are

incorporated into the galena structure by coupled substitution of sb3' and ~ g " for 2pb2'

and the arnount of substitution is temperature-dependent. Galena fiom the lower levels of

the El Co bre vein has the hig hest Ag and Sb concentrat ions re flec ting the hig hest

temperature of ail the veins and suggesting that these ore-forming fluids were the earliest.

Galena with low Sb and Ag contents was deposited Erom the ore-forming fluid at a later

tirne when the temperature of the fluid decreased and only limited substitution of Ag and

Sb for Pb within the galena structure was accommodated.

5.6 Condusions

1. The sources of met& in the El Cobre, Esperanza and Hueyapa veins are probably the

Taxco Schist and the Mexcala Shale.

2. The El Co bre veh was deposited tiom early hydrohermal fluids where 6% values

were predomùiantly controiïed by "C derived fiom original carbonates, rather than

13c denved from organic carbon, in the Mexcala Shale and the Taxco Schist. 6180

vaiues were dominated by the carbonate minerals rather than the sikate minerals. At

this stage, water:rock mass ratios were reiatively low (< 1.5). These conditions are

reflected in the NASC-normalized REE patterns of the carbonate gangue. These REE

distributions are similar to the REE pattern of the Morelos Limestone which is

assurned to be the same as the original marine carbonate in the shale and the schist.

Both patterns exhibit low to non-existent Eu anomalies.

3. The Esperanza vein was deposited fiom hydrothermal fluids that represent

intermediate stages of carbonate and silicate dissolution where 13c derived fiom

organic carbon in the Mexcala Shale and the Taxco Schist began to dorninate the fluid

until the 6'" value of the fluid approached a minimum value of -10.84 Pio.. The "O

composition of silicate minerals significantly influenced the composition of the fluid.

The increase in water:rock mass ratios (3 to 5) suggests an intemediate stage in the

fluid evolution. The NASC-normalized REE patterns of carbonate in the vein reflect

the change in the composition of the fluid since the REE distributions alternate

between patterns that are similar to both carbonate and silicate minerais (La/Lu <1 to

> 1 ) The Eu anomaîies are variable and the higher values represent reaction of the

hydrothermai fluid with plagioclase in the shale and the schist-

4. The Hueyapa vein was deposited fÏom late fluids that leached silicate minerals from

the shale and the schist. 8180 values of the silicate niinerals caused the 6180 value of

the fluid to increase by approximately 5 %O. The 613c value of the hydrothermal fluid

achieved a minimum of -1O.84 %O and was comprised of ')c derived from organic

carbon in the rocks. In the case of the Hueyapa vein. Eu anomalies are considerably

higher than anomalies in the El Cobre vein, reflecting the increased reactions between

plagioclase and the hydrothermal fluid. This is also obsewed in the increase of LaLu

tO >l.

5. Sphalente fkom the El Cobre and the Esperanza veins has higher Fe contents than that

of the Hueyapa vein. This suggests the ore-forming fluids were initially enriched in

Fe and the highest temperature fluids, -289°C. originated ai the El Cobre and the

Esperanza veins. It appears that portions of the El Cobre and Esperanza veins formed

contemporaneously. The low Fe content in sphalerite fiom the Hueyapa vein

suggests that the Hueyapa vein formed later than these veins. Galena fkom level9 of

the El Cobre vein has the highest Ag and Sb contents refiecting the highest

temperature of aU the veins and suggesting the earliest fluids originated at depth in

the El Co bre vein.

6. This is a schematic depiction of the events during the deposition of Ag-Pb-Zn veins.

Volcanic-Hydrothecmal System

y-- co Mining Dii-ct

Volcanic-Hydrothecmal System (a)

Geothermal System 2o0°C - 3wC co Mining Dii-ct

ow sulphidation Au, Ag deposits

Stage Early \

-

i Stage 3

Fig. 40 Aschernatic iiiustrationof the (a) low and highsulphidationepithemal environrnents; and (b) events durhg the deposition of the El Cobre, Esperanza and Hueyapa veins.

5.7 Further Work

(a) MineraIization in the Taxco Mining District occurs within areas of the Mexcala Shaie

and the Taxco Schist that are characterized by varying degrees of hydrothermal

carbonate alteration, Luw 613c values are found in altered host rocks associated with

the mineralization. The 613c values of carbonate ftom the Mexcala Shale are similar

to those of carbonate in the El Cobre veh; simüarly, 613c values of carbonate î?om

the Taxco Schist approximate those of carbonate in the Esperanza and Hueyapa

veins. Low 6 ' ) ~ and 6180 values of dtered host rocks are proximal to mineralized

zones and further fiom the mineralized areas the 6I3c and 6180 values of the host

rocks increase. I3c and 1 8 0 appear to be good toois for exploration on a broad scale.

Regional scaie mapping and sampling of the Taxco Schist and th Mexcala Shaie are

required to determine the extent of the hydrothermal carbonate alteration and the

relationship between 613cC. 8180 and mineralization. This work is similar to the 1 8 0

work perforrned by F, Paquette-Mihalasky and H. Gibson in siiicified andesite of the

Amulet Upper Member of the Noranda camp, Quebec.

(b) In the Guerrero mine. the western end of the El Cobre vein is tmncated and o f k t by

a northwest trending normal fault. The displacement direction of the mineraikation

can be established by analysing drill samples of ore, interseçted on the far side of the

fault, for Fe content in sphalerite and fluid inclusion homogenization temperatures-

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Campa, M.F., and Cone y, P.J., 1983, Tectono-stratigraphie terranes and minerai resource distributions in Mexico: Canadian Journal of Earth Sciences, v. 20, p. 1040- 1051.

Clark, K.F., Foster C.T. and Damon, P.E., 1982, Cenozoic mineral deposits and subduction-reiated magrnatic arcs in Mexico: Geological Society of America Builetin, v. 93, p. 533-544.

Clark, K.F., 1986, Summary of the geobgy and ore deposits in the Taxco, Guanajuato Panchuca-Real del Monte region, Mexico: Society of Economic Geologists Guidebook, p. 13 1- 143.

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Darnon, P.E., Shafiqullah, M. and Clark, KF., 1983, Geochronology of the porphyry copper deposits and related mineralization of Mexico: Canadian Journal of Earth Sciences, v. 20, p. 1052-1070.

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Hopf, S., 1993, Behaviour of rare earth elements in geotherrnai systems of New Zealand: Journal of Geochemical Exploration, v. 47, p. 333-357.

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McDowell, F.W. and Clabaugh, S.E., 1979, Ignimbrites of the Sierra Madre Occidental and their relation to the tectonic history of Mexico, in, Emerson (ed.) Ash Flow Tuffs: Geobgical Society of America Special Paper 180, p. 1 13 - 125.

Michard, k and Atbarede, F., 1986, The REE content of some hydrothermal fluids: Chernical Geology, v. 55, p. 5 1-60.

Ohmoto, H., 1972, Systematics of sulfur and carbon isotopes in hydrothermal ore deposits: Economic Geology, v. 67, p. 551-578.

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O'Neil, J.R. and Taylor, H.P., Jr., 1967, The oxygen isotope and cation exchange chemistry of feldspars: American Minerdogist, v. 52, p. 1414-1437.

Osterman, C., 1984, Geology and genesis of the Guadalupe silver deposit, Taxco mining district, Guerrero, Mexico: Unpublished M.Sc. thesis, University of Arizona, 77 p.

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Rimstidt, J.D., Balog, A. and Webb, 1.. 1998, Distribution of trace elements between carbonate mineriils and aqueous solutions: Geochimica et Cosmochimica Acta, V. 62, p. 185 1- 1863.

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SEM data for sphalerite from the El Cobre, Esperanza and Hueyapa veins and veins intemted in drill holes

Sample Numher SH97-O 1 SH97-0 1 SH97-01 SH97-01 SH97-03 SH97-03 SH97-03 SH97-03 SH97-05 SH97-05 SH97-05 SH97-05 SH97-09 SH97-09 SH97-09 SH97-09 SH97-16b SH97-16b SH97-16b SH97- 16b SH97-17 SH97- 17 SH97- 17 SH97- 17 SH97- 19 SH97-19 SH97- 19 SH97- 19 SHW-2 1 SH97-2 1 SH97-2 1 SH97-21

Sum

(M w 99.35 10 99.9 l'?O w.5800 99.910 98,9440 99.1800 100.9910 99,9850 99,1570 100,1040 99.2330 98,9300 99.3340 99.4880 99,8530 99.6300 99,3290 99.2800 100,2690 100,0570 98,9540 99,8570 99.6720 98.5090 99,9760 99.1 110 100.6020 W.4130 100.3500 9.9850 99.2040 w.4460

Sample Numixr SH97-26 SH97-26 SH97-26 SH97-26 SH97-26 SH97-29a SH97-29a SH97-29a SH97-32 SH97-32 SH97-32 SH97-32 SH97-32 SH97-38 SH97-38 SH97-38 SH97-45 SH97-45 SH97-45 SH97-53a SH97-53a SH97-53a SH97-531i SH97-53a SH97-533 SH97-531 SH97-56 SH97-56 SH97 -56 SH97-56 SH97-56 SH97-56

Cu (wt %)

O. 1390 0,7980

O, 1240 3.2530 0.0200 0,0740

0.43 10 0.0460

0.09 10

0.4020

0,0780 0,0890 0.4340 2,3230 5.4660 0,0580 0.1 130

0.0 150

0.0200

Fe (wt %) 7.1320 8.0620 10.2820 8.3530 10.2360 6.1460 3.4300 7,7970 10.6500 11.4140 9,5520 9.89 10 1 1.4840 6.7270 9.1600 8.3160 12.1330 11,6440 12.5330 ?.<Ml0 8.8030 7,0170 5,3580 5.9330 7.9040 9,<)050 4,6140 5.0800 3,3730 1.8480 1.8690 4.3780

Sum (W w 99.2450 100.0530 lOo.27 10 99.8900 100,7460 100.3630 99,1400 99.5040 100,6850 99,4880 100,2940 98.6750 100.1430 100.6660 98,5760 99,8260 99,3780 99,8380 100,8560 99,0120 98,7770 98,9340 99.0330 99.7460 99,4030 98,8260 99.13W 99,5860 99,2010 100,0060 99.5180 99.0530

Sample Numher SH97-56 SH97-56 SH97-56 SH97-56 SH97-56 SH97-56 SH97-57 SH97-57 SH97-57 SH97-57 SH97-57 SH97-57 SH97-57 SH97-57 TA1 53@ 164.05 TA1538144.05 TA153@144,05 TA153816405 TA1538 177.85 TA1538 177.85 TAI538 177.85 TA1538177.85 TA2058 154.15 TA2058 154.15 TM058 154.15 TA2058 154.15 TA2O5@ 154.15 TA205 8 165.85 TA2058 165 $85 TA205 @ 165.85 TA2 16876.25 TA2 16876.25

Sum (wt %) 99.8590 98,9190 98,7700 98.6860 99.0991 98,6860 100.1OOO 99,0370 99.9950 99.0930 100.5320 10.9 100 100.4460 99.5 100 98,8200 99.7540 98.8750 99.91 30 99.2940 99,7210 99,7750 99.2160 99.0470 99.5300 99.6170 99.6680 99,7780 99.4410 99,8840 99.4510 98.9730 98.9330

Sample Number TA2 16@76.25 TA216877.15 TA2 l6@77.15

Mn Cd S Sum (wt %) (wt %) (wt %) (wt 96)

0.6950 32.9130 99.4610 0.63 10 32.6840 99.3600 0.7270 32.8550 99.3770

Electron microprobe data for gdena from the El Cobrc, Esperanza and Hueyapa veins and veins intersected in ml holes

Sample Number SH97-29 SH97-29 SH97-29 SH97-29 SH97-29 SH97-35 SH97-35 SH97-35 SH97-35 SH97-35 SH97-38 SH97-38 SH97-38 SH97-38 SH97-38 SH97-38 SH97-38 SH97-38 SH97-38 SH97-38 SH97-38 SH97-45 SH97-45 SH97-45 SH97-45 SH97-45 SH97-45 SH97-45 SH97-47 SH97-47 SH97-47 SH97-47

I'b (wt %) 86.2880 86.9 130 86,3760 86,3260 86,4540 86.1660 86,4250 87,404O 85,104O 87.4830 86.1495 86.8943 87.0553 87.3054 86.3422 86,6399 85,8121 86,7728 86.09 15 86,1737 86.2603 85.1776 86,167 1 85.6456 85.2521 85.59% 85,8537 86. I768 87.4289 86.801 1 86,6044 85.3759

Ag (wt %) 0.0880

0,0720 0.0480

0.0690 0.0570 0.0780

0.0507 0.0590 0.0479

0,0656

0,3640 0.3282 0.2305 0.5868 0,6701 0,2301 0,3040

0,2087 0,1652

Sum

twt 95) 99,8480 101.0920 99.8030 99.6280 lOO.l25O 99.7270 100,0170 101.0940 98,7520 101.1910 99.8 104 100.3250 100,6876 t 00.8492 99,7693 99,9935 99,1014 100,3800 99.4449 99.6597 99.5832 99,1455 100.1095 99.2836 99,5826 100,9227 99.6187 99.9480 100.8212 100.0377 100,3615 99,0600

Sum (wt %) 99.8073 100.9944 99,9333 99,8369 100,4250 10 1.0520 100.9900 100.1602 100,5023 100.3 1 15 lOI.l378 100.5376 99,4854 100.7456 10 1 , I437 lOl.1559 100.6942 lOl.3361 100,8606 100.5698 100.6456 99.5745 10,3745

99.7790 98.7253 100.1890 99.92 10 100,3344 100,4757 99,1723 100.8167 100.0561

Sample Number TA153@177.85 TA1538 177.85 TAI 53@ 177.85 TA153@177,85 TA153@177.85 TA2058 lM,l5 TUOS@ lM.15 TA2058 lM,U TA2058 l54,U TM058 154.15 TM058 154.15 TA2058 1 54.1 5 TA205@154,15 TA205Q lM.15 TA2058154.15 TA2 16877.15 TAU6877,15 TA216877.15 TA2l6877,lS TA2 l6@77.15 TA216877.15 TA2 16877.15 TA216877.15

Sum

(wt 'w 99,7923 99,4638 99,474 1 99.1378 100,6105 99 AS8 1 lûO.1380 !KM87 1 99.1849 101.1377 99.1445 w.9009 100.2058 99.77 17 99.8014 100,0509 100.5673 100.0268 100,2733 100,6722 99,7853 99.3224 101.0999

Electron microprobe data for s u l l ' t s from the El Cobre, lkpernats and ffueyapa veins and veins inte~~eeted in drill holes

Sample Numbrr SH97-07 SH97-07 SH97- 18 SH97- 18 SH97-18 SH97-47 SH97-53a SH97-53a SH97-53a SH97-53a SH97-53a SH97-53s SH97-53a SH97-53a Sf.197-53a SH97-533 SH97-53a SH97-53a SH97-53a SH97-53a SH97-S3a Sli97-53a SHO7-53a SH97-53a SH97-53a SH97-53a SH97-5% SH97-53a SH97-53a

I'h (wt %) 4 1.582 4 1.755 54.924 60.1 17 72.106

O, 146 41.610 42.092

4 1,787 41.999 41.875 41 .W3

42.628 42.505 41.570 42.05 1

4 1,747 4 1,958 41.835 4 1.953

42,587 42.464

Sample Number

SH97-07 SH97-07 SH97- 18 SH97-18 SH97-18 SH97-47 SH97-53a SH97-53a SH97-53a SH97-53a SH97-531 SH97-53a SH97-531 SH97-53a SH97-53a SH97-53a SH97-53s SH97-53a SH97-53a SH97-53a SH97-53a SH97-53s SH97-53a SH97-53~ Sli97-53a SH97-53a SH97-539 SH97-531 SH97-53a

1% atamic

proportion 0.20 1 0.202 0.265 0.290 0,348

0,001 0.201 0.203

0.202 0.203 0.202 0,203

0.206 0.205 0,201 0,203

0.20 1 0.203 0,202 0.202

0,206 0.205

S AI: Sb atomic ritomic atomk

proportion proportion prcyiottion 0.605 0.00 1 0.207 0.608 0,210 0,579 0.21 1 0.555 0,001 O. 177 0.492 0.003 0,096 0.564 0,533 0.193 0,777 0.008 0.24 1 0.607 0.205 0.613 0.203 0.7 19 O. 146 0.226 0.705 0.1 76 0,217 0.615 0,204 0,616 0.001 0.208 0.610 0,204 0.615 O, 204 0.725 0.151 0.224 0.6 15 0,204 0,605 0.00 1 0.206 0.607 0,205 0.613 0,203 0.7 19 O, 146 0,226 0,615 0.204 0,616 0,001 0.208 0.610 0,204 0.6 15 O. 204 0.725 0.151 0.224 0,615 0.204 0,605 0.001 0,206 0.740 0.045 0,233

Fe atomk

proportion

0.00 1 0.001 0,003

0,02 1 0,002 0.002 0.096 0.092 0,016 0.008 0.00 1 0.007 O. 103

0,002 0,002 0.m 0.016 0.008 0,001 0.007 O, IO3

Cu Zn atomk atomk

proportion proporîion 0.202 0.200

hournonite bournoniie

boulangerite falkmanite

pyl'artlytite tetrahebiîe bounronite bownonite

wgentian teuahedrite argentian tetrahednte

bownorrite bournonite bounionite boummite

argeniian (euahedriic baurnoriite bounwnrite boumonite boumonite

argcntian kItahedritc bournonite bounioni te bournonite bounionite

argcntian ielrahakite boumonite b o u m i t e freibergite

Appendix 4

Whole rock data analyzeà by INAA and total digestion ICP for spmpks from the El Cobre, Esperanza and Hueyapa veins, veins intersecteci in drill hdes, Taxco Schist,

Taxco Rocs Verde and Mexcaia Shale

Ssmple Number I N M 1's-8 TS- 1 1 TS- 19 TS-20 TS-2 1 TS- 1 TS-3 TS-4 SH97-0 1 SH97-03 SH97-09 SH97- 13 SH97- 17 SH97- 18 SH97-2 1 SH97-26 SH97-32 SH97-35 SH97-37 SH97-38 SH97-45 SH97-53a SH97-54a SH97-57 TAI 5 3 8 177.85 65 TA205 @ 165.85 1 TA2 l6@77.lS 219

Sample Numbw lNAA TS-8 TS- 1 1 TS- 19 TS-20 TS-2 1 TS- 1 TS-3 TS-4 SH97-01 SH97-03 SH97-09 SH97- 13 SH97- 17 SH97- 18 SH97-2 1 SH97 -26 SH97-32 SH97-35 SH97-37 SH97-38 SH97-45 SH97-53a SH97-Ma SH97-57 TA153@177,85 TA2058 165.85 TMl6@77.15

Sample Number 'Ibtal Digestion ICP SH97-01 SH97-03 SH97-O9 SH97- 13 SH97- 17 SH97- 18 SH97-2 1 SH97-26 SH97-32 SH97-35 SH97-37 SH97-38 SH97-45 SH97-53a SH97-54a SH97-57 TAI538 l77,85 TA2058 165.85 TA216877.15

TS-8 TS- 1 1 TS- 19 TS-20 TS-2 1 TS- 1 TS-3 TS-4

Sample Number Total Digestian ICI' SH97-O 1 SH97-03 SH97-09 SH97- 13 SH97- 17 SH97- 18 SH97-21 SH97-26 SH97-32 SH97-35 SH97-37 SH97-38 SH97-45 SH97-53a S H97-54a SH97-57 TAI 5 3 8 177.85 TA205 @ 165.85 TA216@77.15

TS-8 TS- 1 1 TS- 1s) TS-20 TS-2 1 TS- 1 TS-3 TS-4

Whole Rock Analvsîs

Whole rock data were obtained for the El Cobre, Esperanza and Hueyapa veins,

veins intersected in driU holes, Taxco Schist, Taxco Roça Verde and Mexcala Shaie. The

sarnples were analyzed at Activation Laboratones Ltd. in Ancaster, Ontario using

instrumental neutron activation analysis, INA& and to ta1 digestion ICP.

INAA is capable of determinhg up to 35 elements simultaneously. It measures

gamma rays emitted by radioactive isotopes in samples that are placed into the neutron

tlux of a neutron reactor. It requires approximately 2 to 32 gram of powdered rock

sample that is placed, with standards in a neutron reactor and are irradiated. Gamma-ray

spectrometry is performed at set intervals after irradiation.

Total digestion KP involves placing the sample into solution via 4 acid

technique. The solution is placed into radio Erequency excited pIasma at a temperature of

8000 K and the intensity of spectral lines of elements are measured.

~ " C ~ D B ~ and 61s~css~ow data for carbonate fmm the El Cobre, Esperanza and Hueyapa veins, veins intemected In drSU hdes, Taxa Schist, Mexcala Shaïe and

Morelos Limestone and

613~woa values of organic carbon àata h m the Taxa Sdiist and Mexcala Shale

Sample Number SH97-09 SH97- 1 1 SH97- 12 SH97- 15 SH97- 16b SH97-16c SH97- 17 SH97-18 SH97-24 SH97-26 SH97-32 SH97-33 SH97-40 SH97-42 SH9743 SH97-44 Sn9747 SH.9749 SH.97-H)a SH97-53a SH97-54a S-7-56 SH97-57 TA153@ 166.65 TA216@77.15 TA2 l6@ 20Q.20 TS-5 TS-6 TS-7 TS-8 TS-8 TS- 10 TS-Il TS- 12 TS- 18 TS-18 TS-33 TS-19 TS-20 TS-2 1 TS-2 1

Organic carbon data Sampïe dl'~~.om Number (%O) TS-8 -24.96 TS-10 -22.63 TS-18 -2356 TS-21 -24.06 TS-33 -24.98

Carbon and oxygen isotopic compositions of carbonate gangue from the El Cobre.

Esperanza and Hueyapa veins and veins intersected in drill holes were determiwd by

converting the carbonate gangue into C a gas and measuring the mass differences

between the isotopes in a triple coilector VG SIRA 12 mass spectrometer. The C a gas

was iiberated fiom the sample by total dissolution of the carbonate with an acid leach at

Io w temperature, circa 25°C. Routine precision on pure carbonate analysis was 0.10 %a

Analysis of carbonate in the host rock samples involved removing the carbonate

fiom the sample using low temperature weak acid leach. The carbonate was converted to

CO2 gas and analyzed in the VG SIRA 12 mass spectrometer.

Samples of Taxco Schist and Mexcala Shale were run through an off-line Ert to

ascertain the percentage of organic carbon in eac h sample. Organic carbon was then

converted to COz through combustion with copper oxide at 800°C. The C a was

analyzed on a Finnigan Delta Plus mass spectrometer c o ~ e c t e d to a Car1 Erba eiementai

analyzer and a Conflo interface. CO2 was carried through the system by Helium gas and

measured with respect to a pulse of reference gas. A linear correction was applied to the

samples using standards.

REE data for whole rock ICP-MS anlyses of the Morelos Limestone and LAM ICP- MS analyses of carbonate gangue h m the El Cobre, Esperanza and Hueyapa veim

and vein intesecteci in drill holes

Samplc Number LAM ICP- US SH97-08 SH97-OS SH97-O9 SH97-09 SH97-09 SH97-O!l SH97-09 SH97-09 SHY7-09 SH97-IO SH97-IO SH97-11 SH97- 1 1 SH97- 12 SH97- 12 SH97.12 SH97- 15 SH97- 15 SH97- 15 SH97- 1 6b SH97- 16b SH97-16~ SH97-16c SH97- 17 SH97- 17 SH97-17 SH87-18 SH87- 18 SH97- 18 SH97- 18 SH97-18

Sample Numbcr SH97-18 SH97-18 SH97-24 SH97-24 SH97-24 SH97-24 SH97-24 SH97-24 SH97-26 SH97-26 SH97-26 SH97-29a SH97-29a SH97-29a SH97-32 SH97-32 SH97-32 SH97-33 SH97-40 SH97-42 SH97-42 SH97-42 SH97-43 SH97-43 SH97-43 SH97-44 SH97-44 SH97-44 SH97-46 SH97-46 51497-47 SH97-47

VL -.-szgz i.

3 E o o o o o 9 O * E

V

- 2 S C g Z œ

0 E o o o o o 9 O = Y 2

Sample 1,ri Ce IDr Nd Sm u Cd 'I'b Dy Ho r Tm Yb Lu Number (ppm) (ppm) ( P P ~ ) ( P P ~ ) ( P P ~ ) ( P P ~ ) ( P P ~ ( P P ~ ) ( P P ~ ) ( P P ~ ) ( P P ~ ) ( P P ~ ( P P ~ ( P P ~ ) Whok rock lCPIMS SH97-03 1,18 2.24 0.27 1.06 0,18 0.02 0.14 0.02 0.10 0.02 0.04 0.00 0.05 0.00 SH97-17 !)9.91 197.60 26.92 103.03 18.47 3.13 14.24 1.80 9.94 1.98 5.67 0.82 5.01 0.76 SH97-17 114.94 226.66 31.10 120.75 2197 3.69 16.71 2.03 11.07 2.15 5,96 084 5.27 0.82 TS -5 3.401 5.766 0.765 2.767 0.547 0.196 0.599 0.092 0.557 0.108 0.304 0043 0,276 0.042 TS-6 0.332 0.319 0,076 0.310 0,031 0.017 0.076 0.008 0.070 0.020 0.049 0.012 0.023 0,002

REE Analvsis

REE data were obtained from carbonate gangue fiom the veins by laser ablation

microprobe inductively coupled plasma emission mas spectrometry, LAM ICP-MS at

Mernorial University, Newfoundland, LAM ICP-MS used a focused laser beam to

ablate a srnail amount of carbonate contained in a closed ceil. The ablated material was

transported as an aerosol in a continuous flow of argon plasma to an ICP-MS for isotopic

detection. In the ICP-MS, ions were extracted from a single solution piasma through a

pinhole-sized orifice into a vacuum system and focused with an ion lem ïnto a mass

spectrometer. Carbonate gangue ftom each veh sample was run through the LAM-ICP-

MS for a minimum of 2 to 8 analyses.

REE data were also obtained Erom two El Cobre vein samples and two samples of

Morelos Lirnestone using whole rock inductively coupled plasma (ICP-MS) emission

mass spectrometry at Mernorial University. The procedure was similar to that described

above.

Homogenization temperatures of fluid inddom in quartz and carbonate gangue from the Esperanza and Hueyapa veins

Sarnple Num ber

SH97-32 SH97-32 SH97-32 SM7-32 SH97-32 SH97-32 SB7-32 SH97-32 SH97-32 SH97-32 SH97-32 SH97-32 SH97-32 SH97-32 SH97-32 SH97-32 SH97-32 SH97-32 SH97-32 SU97-32 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 SH97-44 S H97-44 SH97-44 SH97-46 SH9746 SH97-46 SH97-46 SH97-46 S m 7 4 SH97-46 SH97-46

Type

liquid-rich liquid-rich liquid-ridi liquid-rich liqui&rich liquid-rich liquid-rich liquid-rich Iiquid-rich liquid-rich vapour-ach liquid-rich liquid-rich liquid-rich liquid-rich liquid-rich liquid-rich vapour-rich vapour-rich iiquid-rich liquid-rich tiquid-ricb liquid-rich liquid-rich tiquid-rich tiquid-rich liquid-rich liquid-rich liquid-rich liquid-rich liquid-rich liquid-rich liquid-rich liquid-ricb liquid-rich liquid-ricb liquid-rich liquid-rich liquid-rich iiquid-ri& Iiquid-rich liquid-rich Iiquid-ri& liquid-rich liquid-ri& liquid-rich liquid-rich liquid-rich liquid-rich

G=gue Mineral q- q- q- calcite q- calcite calcite calcite calcite calcite Calcite calcite q- calcite calcite calcite calcite calcite calcite calcite q- q- q- q- q- '4- q- '4- q u a '4- cl- q- q- q- q- q- q- q- q- q- q- Qu- q- q- q- q- q- q- q-

Shape

irreg ular rounded rounded rounded

pnSmatic roun&d roua&d rounded rounded rounded rounded

rounded

Size

(microns) d to 5 d m 5 d m 5 <5 to 5 4 to 5 d to 5 4 to 5 4 to 5 4 t o 5 <5 to 5 <5 to 5 4 105 4 to 5 4 to 5 4 to 5 c5 to 5 d to 5 <5 to 5 <5 to 5 4 5 to 5 d to 5 4 to 5 <5 to 5 4 to 5 d m 5 d to 5 <5 to 5 <5 to 5 4 5 to 5 <5 to 5 c5 to 5 c5 to 5 d to 5 4 to 5 4 to 5 4 to 5 <5 to 5 <5 to 5 d to 5 <5 to 5 4 to 5 <5 to 5 c5 to 5 d to 5 4 to 5 <5 to 5 4 to 5 <5 to 5 4 to 5

Type

liquid-rich tiquid-rich iiquid-rich liquid-rich liquid-rich tiquid-rich liquid-rich liquid-rich liquid-rich liquid-rich 1iquid-rich iiquid-rich liquid-rich liquid-rich liquid-rich liquid-rich Liquid-rich iiquid-rich liquid-rich iiquid-rich liquid-rich liquid-rich liquid-rich liquid-rich tiquid-rich liquid-rich liquid-rich liquid-rich liquid-rich liquid-rich liquid-rich liquid-rich liquid-rich Iiquid-rich liquid-rich liquid-rich liquid-rich liquid-ricb iiquid-rich liquid-rich Iiquid-rich liquid-rich liquid-rich Iiquid-rich liquid-rich liquïd-rich tiquid-rich tiquid-rich liquid-rich

G-gue Minerai

'4- q- q- q- q- q- q- q- q- q- q- Q- q- q- q- q- q- q- '3- q- q- q- q- q- q- q- q- q- q-

'3- q- q- q- q- q- q- q- q- q- q- q- q- q- q- q- q- q- q-

Sample

(microns) d to 5 <Sm5 <5 to 5 4 to 5 <5 to 5 <5 to 5

Gangue Shape

Sample Nwn ber

SW7-49 SH97-49 SH97-49 SH9749 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 SH97-5 1 sm7-5 1 SH97-5 1

liquid-rich Liquid-rich liquid-fich liquid-rich liquid-rich iiquid-rich Liquid-rich Liquid-rich tiquid-rich liquid-rich liqmd-ria liquid-rich liquid-rich Liquid-rich liquid-rich liquid-nch iiquid-rich Iiquid-ricb tiquid-rich Liquid-rich Liquid-rich liquid-rich vapour-fich tiquid-rich vapour-rich vapour-rich vapour-rich vapour-rich vapour-rich vapour-rich vapour-rich vapour-rich Liauid-rich

Gluigue Mineral q-

q- 'l- calaie cakite calcite calcite calcite calcite calate calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calcite calci te calcite

Shape

? ? ? ?

irreguiat munded rounded eloagate imgular irregular ilregalar irregular irregular irregular irregular

tear-shaped irregular (-1 c r y s d irregular irreguku irregular

irreguhr irregular irreg ular

tear-shaped irregular irregular elliptical üregular inegular irregular irrgeular eloneated

The Linkam THMSG 600 Stage

The Linkam THMSG 600 Stage is made by Linkam Scientific Instruments L t d in

Surrey, England. A doubly polished sample sits between the Iower sapphire window and

an overlying glas slide within a stainless steel crucible carrier. The crucible carrier is

loaded through the side door of the stage so that it sits on the thermal Ag block without

any air gaps. Nitrogen gas is passed between double g l a s windows beneath the chamber

containhg the thermal Ag block.

The Linkam THMSG 600 stage is used in conjunction with the TP 93

Programmer and the LNP93/2 Cooling h m p . The LNP9312 Cooling S ystem is able to

lower the temperature of a sample below -194OC. A 2L dewar flask with a fitted pipe is

attached to the THMSG 600 Stage via tubing. Liquid nitrogen is drawn from the dewar

flask via tubing with a filter where it is vapowized and then the N2 gas is passed through

a valve which allows manual control of the flow rate. The N2 gas flows between double

glass windows beneath the chamber containing the thermal Ag block.

The TP 93 Programmer controls the temperature control of the Linkha.cn THMSG

600 Stage. The stage uses a platinum resistor to sense the temperature. The plathum

resistor is mounted near the top of the Ag block. Output from the platinum resistor to the

stage is converted to temperature in the TP 93 to better than O. 1°C. Heating and c o o h g

rates range between 0.1 and 90°Umin. and can be increased and decreased incrementally

in steps of 0.1, 1 and 10 degree in te~als . Rate changes are implemented by a circuit that

amplifies and digitaüy linearizes the signal fiom the platinum resistant thermometer.

Homogenization temperatures anà saiinîty data for quartz from the El Cobre vdn taken from GonzalezlParada (1996)

Sample Number C2-06 C2-08 C2- 10 C2-12 C2- 14 Cl-12 Cl41 Cl43 Cl45 C 1-06 Cl47 Cl* CI-1 1 Cl-13 Cl-14 CI-15 ce01 Ce02 Ce03 ce05 c m ceos ce IO CO-1 1

Appendix 9

Derivation of isotope fractionation factors and calculation of the Isotopic composition of the hydrothennal fluid at 220°C, 230°C and 24û°C using nuid

inclusion data

The Derivation of Isotope Fractionation Factors

Isotopes of an element can be fractionated through physico-chernical processes,

kinetic processes and isotope exchange reactions as a result of m a s differences between

the isotopes. The fraçtionation of an isotope between two phases is defmed by a

fractionation factor, ac An isotope ffac tionation factor between two substances, A and B,

is the quotient of heavy to Iight isotope ratios in the two substances and is expressed as

follo ws:

eq. 1

We also know that,

and therefore,

so substituting equation 2 into equation 1 and subtracting 1 fiom each side yields:

1f aAB is 5 1.010, then & + 1000 n 1000 and,

and by approximation,

Experimental studies of isotope exchange reactions between minerah and fluids

have shown that a values are directly proportional to mass ciifferences between the

isotopes and inversely proportional to temperature such that:

1000 h ~ ~ - ~ ~ ~ = ~ ( 1 0 ~ / T~ ) + B

w here T is temperature in kelvin and A and B are experimentaily derived constants for

mineraI-mineral and mineral-fluid pairs. The bctionation factors for isotopes between

two or more phases at various temperatures have been experimentally and

mathematicaiiy derived for compounds of carbon, oxygen, hydrogen and suIfuf.

Snm ple Number

Sample Numhvr

Sample Number

Appendix 10

Calcula tion of water: rock ratios and the evolution of the hydrothermal nuid

Calculation of Water:Rock Ratios

The oxygen-iso topic exc hange between the source rocks and the hydrothermal

fluids is outlined by the foilowing equation (Ohmoto and Rye ,1974):

18 f where 6 0 , = final isotopic composition of water after equilibration with rock

6ISO', = initial isotopic composition of the rock

18 i 6 0, = initial isotopic composition of the water

= temperature-de pendent frac tionation factor between rock and water

w/r = the ratio of wt % oxygen available for dissolution in the

water and in the rock

R = "water:rock ratio", volume of water rehtive to volume of rock that

have isotopicaily equiiibrated

Weigh ted fkac tionation factors of calcite-&O, plagioclase- H20, illite-H20, chlorite-H20

and quartz- Hz0 are used to calculate A,, at 300°C and following equations are resolved:

18 f 6 0, = 20 W, - A~-, %, + (5.92R) (0 %O) eq. 1 1 + 5.92R

6"0\ = 22 %. - A,, %, + (5.92R) (O %O) eq. 2 1 + 5.92R

where 6180ir = 20 and 22 6180 values similar to the Mexcala shale

18 i 6 0, = O %,, the 6180 value of meteoric water which has equilibrated with the Mexcala shale

Ac-, = weighted fractionation factor at 300°C for calcite and silicate pairs

wlr = 5.92R, ratio of exchanged oxygen atoms in water relative to rock (ie) wt % oxygen in water = 88.8 = 5.92R

wt % oxygen in rock = 15

A

Calcite

(w %) 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15

0

Cwkite Factor

(W %) 0.95 0.85 0.75 0.65 0.55 0.45 0,35 0.25 0.15 0.05

0 0 0 0 0

D

lllite Factor

(wt w 0.020 0,059 0.W8 0,137 0.176 0.2 15 0,254 0,293 0.332 0.37 1 0.389 0.384 0.379 0,369 0,359

E

Chlorite

(* %) 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

F

Chlorite Factor

(W %) 0.020 0.059 O,W8 0.137 0,176 0.215 0,254 0.293 0,323 0,353 0,358 0,353 0.348 0,338 0,328

0

I'lagioc lase

(wt %) 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20

H

Plagloclase Factor

(wt 'ib) 0.005 0,025 0,045 0.065 0.085 0,105 0,125 0.145 0.165 0.185 0.205 0.2 17 0.228 0.240 0.252

J

Quartz Factor

(* %) 0,000 0.002 0.004 0.006 0.008 0.010 0,o 12 0,014 0,022 0,030 0,038 0,046 0,054 0,062 0,070

K

1000 In x weighted average

0,oo 0.42 0.73 1 .O3 1.34 1.64 1.93 2.23 2.59 2.94 3.18 3.27 3.37 3.47 3.56

L

R value leît curve

0.20 0.50 1.10 1,70 2.30 2.w 3.50 4.10 4.70 5.30 5.90 6SO 7,lO 7.70 8.30

M

R vaiue Right curve

o. 10 0.30 o. 50 0.70 O.m 1.10 1.30 1 S O 1.70 1.90 2,lO 2.30 2.50 2.70 290

N

w/r Leil curve

5 -92 5.62 5,32 5.02 4.72 4.42 4.12 3.82 3.52 3,22 2,02 2,62 2,32 2.02 l,72

O

wlr Right curve

5,92 557 522 4.87 4.52 4.17 3.82 3.47 3.12 2,77 2,42 2.07 1-72 1.37 1 .O2

P

d"O,SMOW,

Lell cuvre 9.16 5.14 2,81 1 ,w 1,57 1.33 1,17 1 .O7 0.99 0.94 0,92 0.93 0,95 1 .O 1 .O8

Q

d'80(mow, Rlght curve

13,82 8.08 5,89 4,76 4.08 3,64 3.36 3,19 3 .O8 3 ,O4 3.09 3.25 3.52 3.94 4,66

s CO2 (organlc)

1/2 converted O, 1 0 0.090 0.08 1 0.073 0.066 0.059 0,053 0.048 0.043 0,039 0.035 0.03 1 0.028 0.025 0.023

T CH4 (organic) Cu, = CO2

0,020 0,018 0.016 0.015 0.01 3 0.012 0.01 1 0,010 0.w 0.008 0.007 0.006 0.006 0,005 0.005

U

CO2 (carbonate)

(wtW 15.00 7.50 3.75 1.88 0.94 0.47 0.23 O, 12 0,06 0.03 0.0 1 0.0 1 0.00 0.00 0.00

v CO2 (carbonate)

ü2 removd 7.50 3.75 1.88 0.94 0.47 0.23 O, 12 0,06 0,03 0.0 1 0.0 1 0.00 0.00 0.00 0.00

w d'3~4m~1

Lem curve -O,l4 -0.25 -0.44 -0.75 - 1.27 -2.05 -3.12 4.37 -5.64 -6,72 -7 S2 -8 ,O6 -8,39 -8-58 -8.70

It

d"c(mi, Rigbt curve

-2.1 1 -2.19 -2.34 -2.58 -2.98 -3.59 -4.4 1 -5.38 4.36 -7.20 -7.82 -8.23 -8,49 -864 -8.73

Columns A, C, E, G and 1

Weight percent calcite, fite, chlorite, plagiocIase and quartz found in an arbitrary

sedimentary rock

Columns B, D, F, H and J

Weig hting factors of calcite, iiiite, chlorite, plagioclase and quartz in the hydrothermal

fluid. The calcite weighting factor decreases due to dissolution of calcite by the

hydrothermal fluid. The other weighting factors increase due to increased interaction

between the rocks and the hydrotherrnai fluid.

Column K

Weighted average isotope ftactionation factor between the source rock and the

hydrothermal fluid, It is calculated at 300°C as follows:

where 1 000 ln a for calcite-H20 = O %O, assurning total dissolution of carbonate by the

hydrothermal fluid and Little to no fiactionation of ''0

1 000 ln a for iilite-H20 = 2.58 960

1 000 In a for chlorite-H20 = 0.05 %O

1 O00 ln a for plagioclase-H20 = 4.87 %O

1 000 in a for quartz-H20 = 6.86 %O

Columns L and M

Water:rock ratios used to produce left and right curves in Figures 29 and 31.

Colwnns N and O

Weight percent oxygen avaïlable in the water and rock for isotope exchange used to

produce left and right curves in Figure 29.

Column P

n ie s's~(sMow, value of the hydrothermal fluid calculated ffom the following equation:

where 20 %O = 6L8~(sMoW, value of the source rock

A = weighted average isotope fiactionation factor

wfr = mas ratio for the left curve

R = waterrock ratios for the left c w e

O = 618~~sMoW> value for meteoric water that has equilibrated with the source

rock (Mexcala Formation)

Column Q

The 6 1 8 0 ~ s M ~ w value of the hydrothermal fluid calculated fkom the foilowing equation:

where 22 %O = 6180(sMoW> value of the source rock

A = weighted average isotope ikactionation factor

w/r = mass ratio for the left curve

R = waterrock ratios for the left curve

O %O = 6180(sMoW, value for meteonc water that has equilibrated with the rock

Column R

Weight percent organic carbon in the source rock.

Column S

Weight percent organic carbon converted to Ca. We estimate that 10 % of the initiai

organic carbon content is converted to C02and that the carbon hctionates between C a

and methane by the following equation:

For example, COz (organic) = 2.00 wt % x 10% = 0.200 wt %

and according to the above equation, Ca (organic) = 1/2 x 0.200 wt % =0.100 w t %

Column T

Weight percent organic carbon converted to methane. We estimate that 20 % of the C h

is converted to CO2- The remaining 80% partitions into vapour phase.

For example, C& (organic) = 0.100 wt % x 20% = 0.020 wt %

Column U

Weight percent carbonate in the source rock.

Column V

Weight percent carbonate rernoved from the source rock by leaching of the hydrothermal

fluid.

Column W

The s ~ ~ c ~ ~ ~ , value of the hydrothermal tiuid based on the isotopic enrichment factors by

Ohmoto (1972) for organic carbon, methane and carbonate in a hydrothermal fluid at

300°C. It is calcuhted as foiiows:

where the source rock is assumed to have a 6 ' 3 ~ ( p D B ) value of0 %a

Column X

The 6 L 3 ~ p D B ) value of the hydrothennal fluid based on the isotopic enrichment factors by

Ohmoto (1972) for organic carbon, methane and carbonate in a hydrothermal fluid at

300°C. It is calculated as foilows:

where the source rock is assumed to have a ~ ~ ~ ~ ~ o e ~ value of -2 %a