Sulfur isotope studies in the zinc-lead mine San Vicente, Central Peru

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Book Chapter

Reference

Sulfur isotope studies in the zinc-lead mine San Vicente, Central Peru

GORZAWSKI, Hendrik, et al.

GORZAWSKI, Hendrik, et al . Sulfur isotope studies in the zinc-lead mine San Vicente, Central Peru. In: Fontboté, L., Amstutz, G.C., Cardozo, M., Cedillo, E. & Frutos, J. Stratabound ore deposits in the Andes . Berlin : Springer, 1990. p. 306-312

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Sulfur Isotope Studies in the Zinc-Lead Mine San Vicente, Central Peru

H. GORZAWSKI1•2, L. FONTBOTE2 , C. W. FIELD³and R. TEJADA ⁴

1 Introduction

The stratabound Zn-Pb deposit of San Vicente, presently the largest zinc producer of Peru, is located about 300 km east of Lima in the tropical rain forest of Central Peru (Schulz 1971; Levin and Amstutz 1973; Fontbote and Gorzawski 1990). The San Vicente deposit appears to be associated with certain dolomitic facies within a transgressive carbonate sequence (up to 2000 m thick) in the eastern part of the Upper Triassic-Liassic Pucara Group. The ore-bearing dolomites belong to a peritidal facies belt that forms a transition between detrital-evaporitic sediments at the margin of the Brazilian Shield to the east, and open marine carbonates of the central Pucara to the west. Along this facies belt, that is recognized over a dis- tance of at least 200 km from north to south, are a number of nonexploited ore deposits and occurrences. These collectively belong to a Mississippi Valley-type province in the eastern Pucara, whose economic significance at present is not well known (Fontbote this Vol.).

The ores of San Vicente occur as lenticular bodies that are conformable with bedding at the scale of the deposit. Paragenesis is very simple, with sphalerite and galena as the only commercial minerals, and the Zn: Pb ratio is 10: 1 on the aver- age. The ore lenses display a remarkable facies control whereby the metal-rich ho- rizons are mainly bound to (1) layers of dolomitized mudstones and pel- let-grainstones with abundant cryptalgallaminations and sulfate pseudomorphs deposited in lagoonal and tidal flat subenvironments, and (2) to barrier calcarenites in the vicinity of intercalations of the former facies. Barrier calcarenites are the predominant ore-bearing facies. Both reflectivity measure- ments and Rock Eval analyses of organic matter indicate a high stage of maturity largely beyond that of oil and gas generation. Exact burial depth is unknown because of tectonics and erosion, but it exceeds 2-3 km.

The ore-forming process appears to have started in late diagenetic stages under burial of about 2-3 km (Fontbote and Gorzawski in press). Petrographic, geochemical, and isotopic (Sr, C, 0, Pb) evidence (Gorzawski 1989; Gorzawski et al. 1989; Fontbote and Gorzawski 1990) indicate the introduction of a metal-

1 Max-Planck-Institut ftir Chemie, Postfach 3060, D-6500 Mainz, FRG

2 Mineralogisch-Petrographisches Institut der Universitat Heidelberg, INF 236, D-6900 Heidelberg, FRG

3 Department of Geology, Oregon State University, Corvallis, USA

4 San Ignacio de Morococha S.A., San Vicente, Peru

Stratabound Ore Deposits in the Andes L. Fontbote, G. C. Amstutz, M. Cardozo, E. Cedillo, J. Frutos (Eds.)

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306 H. Gorzawski et a!.

B

³⁴

S [

⁰/oo,

COT]

10.4

9.7

9.4

Smm ₁

Fig. I. Thin section photograph of sample FSV-007. The results of sulfur isotope analyses in three consecutive sphalerite generations (SL/, SL//a, SL//b) are indicated

rich basinal brine that circulated through sediments rich in detrital material derived from old crust (Brazilian Shield).

This chapter reports the results of sulfur isotope studies carried out on samples from San Vicente. The aim was to constrain the sulfur sources and the precipitation conditions at the ore deposit.

2 Analytical Methods and Results

Complex textures and structures present in the ores and host rocks are one of the most striking features of the San Vicente Mine. A significant part of the ore occurs as diagenetic crystallization rhythmites (DCRs, Fontbote and Amstutz 1983).

These DCRs have been preferentially investigated with respect to the sulfur isotope analyses, because successive generations of diagenetic crystallization are megascopically visible and capable of being isolated as paragenetically distinct samples.

Such characteristic textures are illustrated in Fig. 1 that shows a thin section of an ore-bearing OCR (sample FSV-007). Here, as in other samples, it is possible to distinguish three different sphalerite generations within a typical crystallization sequence, which is as follows:

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Sulfur Isotope Studies in the Zinc-Lead Mine San Vicente, Central Peru 307

Generation I consisting of fine-grained xenomorphic sphalerite I (inter- grown with dolomite I).

Generation Ila consisting of subhedral, bipolar, brownish sphalerite Ila.

Generation lib consisting of subhedral, bipolar, yellowish sphalerite lib.

Generation III consisting of sparry white dolomite.

Analyses reported in this study are based on those of 20 individual sulfide con- centrates that were extracted from five samples. Where feasible, up to three generations of sulfide were separated from a single sample, usually by means of a den- tal drill and with the aid of a binocular microscope. Although most sulfide fractions were pure, having been extracted from relatively massive and coarsely crystalline aggregates, those associated with Generation I were finely crystalline dis- seminations abundantly contaminated with carbonate. These latter impure sulfide fractions were additionally concentrated by means of differential settling in bromoform heavy liquid. Sulfide-sulfur in each concentrate was converted to

so2

gas for mass spectrometric analysis of the sulfur isotope ratio according to the method described by Ohmoto and Rye (1979). The S02 gas was formed under vacuum at 1025

oc

by oxidation of the sulfide in the presence of CuO. Water and other noncondensable gases were removed from the S02 prior to isotopic analysis by Global Geochemistry Corporation of Canoga Park, California. The sulfur isotope data are given in conventional <5³⁴S per mil values, where the posi- tive or negative sign represents deviations (<5) in parts per thousand (%o) of ³⁴S in the sample relative to the Canon Diablo meteorite standard. Analytical precision

Table 1. Sulfur isotope-results on sulfide samples from the San Vincente Mine

Field no. Analysis no. Manto Min Gen

o

³⁴S/CDT

FSV-007 HGA-075 3t sl 10.4

HGA-076 sl II a 9.7

HGA-077 sl lib 9.4

FSV-041 HGA-078 3p sl I 13.0

HGA-079 sl II 12.8

HGA-080 ga III 6.9

FSV-079 HGA-124 Ayala sl 10.6

HGA-152 sl II 9.9

FSV-039 HGA-092 2 sl 12.1

HGA-149 sl 12.2

HGA-153 ga 6.8

FSV-044 HGA-081 3t sl 10.1

HGA-082 sl 11.0

HGA-083 sl 11.0

HGA-084 sl 9.9

HGA-085 sl 11.5

HGA-086 sl 10.8

HGA-087 sl 10.0

HGA-088 sl 10.8

HGA-089 sl 10.2

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308 H. Gorzawski et al.

(>³⁴$ (%o]

+13 78

~19 ₁₄₉

I ~

+ 12 ^I_I ₉₁

I 68S

a

I

+ 11 124

AB6.88.

^{83 . 82} A sphalerite I

75 ~ ⁴ spha lerit e n a

~ ^I ~89

+ 10 ^I ⁸¹ ⁸⁷ ^£fl.sphalerit e n b

f 6

¹⁵²4 ⁸⁴ ^\I^galena^m

11 v ^l:l,sphalerite (subsequent

generations not specifil!!d) .6. results of NIELSEN

v V in SCHULZ 119711

..:; _\I ^v

80 153

+6 FSV-7 FSV-41 FSV -79 FSV-39 FSV-44 sample no.

Fig. 2. Results of sulfur isotope studies in sphalerite and galena samples from the San Vicente Mine.

Dashed lines connect the results of consecutive sulfide generations in one given hand specimen. The numbers indicated correspond to Table 1 (analysis no. HGA-). The results obtained by Nielsen (in Schulz 1971) are also indicated

is calculated to be better than ±0.2%o. The data are listed in Table 1 and are graphically displayed in Fig. 2 that also includes the earlier results of Nielsen (in Schulz 1971) for comparison.

Results indicate that sulfides from the San Vicente Mine are remarkably isotopically homogeneous, significantly enriched in ³⁴S, and exhibit a nearly consistent weak isotopic trend with paragenesis. Many of these features are characteristic of Mississippi Valley-type deposits elsewhere, as summarized by Heyl et al. (1974) and Ohmoto and Rye (1979). The

o

³⁴S values of 18 sphalerite concen- trates range narrowly from +9.4 to + 13.0%o, and those of two galena concen- trates from

+

6.8 to

+

6.9%o. All sulfides are enriched in ³⁴S relative to most of those of presumed magmatic or magmatic hydrothermal origin (O%o± 3%o), which implies an isotopically heavy source of sulfur derived from sulfate in oceanic and/or connate waters, or marine evaporites. Small enrichments of ³⁴S in sphalerite with respect to associated galena are consistent with fractionation effects imposed by isotope exchange equilibria. The isotopic uniformity of sulfur in sulfides from San Vicente was previously noted by Schulz (1971), who attributed this to a later homogenization process. However, such a process is considered unlikely, based on the results of the present investigation and in the absence of a metamorphic imprint on host rocks and ores at San Vicente.

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3 Discussion

One of the more interesting aspects of this investigation is the presence of isotopic trends related to the paragenetic position of sulfides. These are manifest as small differences in the isotopic compositions of paragenetically sequential sphalerites that exhibit a general trend of progressive, but slight, ³⁴S depletion in the deposi- tional order from I through Ila to lib generations. This trend towards lighter

<5³⁴S values during sequential sulfide formation may result from fractionation

and mass-balance effects that accompany changing physico-chemical parameters of the system, and/or from changing and/or compositionally different sources of sulfur in the evolving ore-forming process. Moreover, it is associated with analo- gous trends from early and heavy to late and light per mil values for ¹³C and ¹⁸0 in the dolomitic DCRs as described by Fontbote and Gorzawski (1988, 1990) and Gorzawski (1989). This heavy to light trend is explained in terms of a slightly but continuously changing ore fluid composition during ore formation, probably related to increasing temperatures with advancing diagenesis. The similar trend for

34S could be attributed to decreasing fractionation between ZnS and H2S with increasing temperature as the host carbonate sediments underwent progressive compaction and burial throughout the paragenetic deposition of the several sphalerite generations. However, the high temperature differences calculated (> 100°C) for subsequent sphalerite generations appear to be quite unrealistic.

Other mechanisms to produce the ³⁴S trend observed might include the addition of ³⁴S-depleted organic sulfur or the increase in oxygen fugacity over the paragenetic interval of ore formation, but there is little evidence at present to sup- port either speculation.

A major objective of this investigation was to determine or set constraints on the source of sulfur contained in sulfides of the San Vicente deposit. For Zn-Pb deposits hosted by unmetamorphosed sedimentary carbonate rocks there are es- sentially three geologically feasible means by which to generate H2S for the fixation of metals as sulfides. These are by (1) the biogenic reductions of marine sulfates by bacteria, (2) release of organically bound reduced sulfur by alteration- maturation of hydrocarbons, and (3) abiogenic reduction of marine sulfates by organic carbon or ferrous iron. Our data suggest that the latter mechanism best applies at San Vicente.

According to Claypool et al. (1980), the sulfur isotopic compositions of marine sulfate have varied throughout geologic time, and have ranged from about + 12 to + 15%o during Middle to Late Triassic and from about + 13 to + 18%o during Liassic time. In addition, Field et al. (1983) have reported <5³⁴S values of + 13.1 and + 13.8%o for evaporitic anhydrite within the western part of the Pucara Group from the nearby Morococha district. Because the sphalerites of San Vicente are isotopically heavy (mostly + 10 to+ 12%o) and compositionally similar to the marine sulfates described above, it may be inferred that the source for much or all of this sulfide-sulfur was a reduced form of seawater or evaporite sulfate of Trias- sic-Liassic age. This assumption is supported by the fact that at San Vicente there is abundant evidence of sulfate replacement by carbonates. As pointed out by An- derson and Garven (1987), this may be indicative of sulfate reduction, because sulfate minerals can only be replaced by carbonates if there is a sink for the hydro-

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gen ions generated in this process. In the absence of other reactions, such as, for example, silicate alteration, this sink should be the result of sulfate reduction to H2S or S.

The isotopically heavy and uniform

o

³⁴S values of the sphalerites are largely inconsistent with a biogenic mechanism for the reduction of sulfate to reduced forms of sulfur. Although it is theoretically possible to obtain similar isotopic characteristics from H₂S derived by sulfate-reducing bacteria, assuming the quantitative (1000J'o) reduction of a finite reservoir of sulfate sulfur, such results would be fortuitous, as the necessary assumptions are not likely to prevail in a system as large and geologically complex as the San Vicente deposit.

The thermal degradation of organic sulfur-bearing hydrocarbons at elevated temperatures might readily supply H2S without fractionation. According to Hunt (1979), H₂S is easily released during the diagenesis of organic-rich sediments at temperatures higher than 100°C, and this reduced sulfur may become an important component of natural gas. Large amounts of gas and organic matter would be required to form this large Zn-sulfide deposit. Although the ore-bearing dolomite units contain some organic material, mass balance considerations do not favor the application of this mechanism at San Vicente. In addition, data pre- sented and reviewed by Vredenburgh and Cheney (1971) indicate that compositions of sulfur in petroleum and organic matter range largely from between -7 and

+

7%o and generally show a ³⁴S-depleted trend parallel to that of marine sulfates over geologic time. Accordingly, these forms of organic sulfur do not seem to represent isotopically reasonable sources of H2S for the fixation of metals at San Vicente.

Abiogenic processes of sulfate reduction by reaction with organic carbon, ferrous iron, or H2 may generate H2S that is only slightly to moderately fractionat- ed relative to the source. Barton (1967) proposed that the precipitation of Missis- sippi Valley-type ores was caused by H2S formed with the chemical reduction of sulfate by methane or organic matter. According to Orr (1974, 1977), hydrocarbons in combination with H2S may reduce sulfates at temperatures above 75

oc.

Investigations in organic geochemistry by Macqueen and Powell (1983) and Pow- ell and Macqueen (1984) have suggested that the thermochemical reduction of sulfate by bitumen may have formed sulfide ores of the Pine Point deposit at temperatures of about 100

oc.

This reaction, according to these authors, would in- volve two steps: (1) the reduction of sulfate to elemental sulfur by hydrogen sulfide, and (2) production of hydrogen sulfide by reaction of elemental sulfur with organic matter. The latter reaction is rapid (Nielsen 1985) and therefore the rate- controlling step may be the sulfate release from evaporites. It should be noted that the abiogenic reduction of sulfate in laboratory experiments has only been clearly attained above 250°C and thus some authors question its significance as an ore- forming process (see discussion in Trudinger et al. 1985). However, other investi- gators such as Orr (1974) and Powell and Macqueen (1984) have suggested that extrapolation of the kinetic data indicate the feasibility of this process at temperatures as low as 80° to 120 °C in geologic systems. Moreover, the likelihood and speed of this abiogenic reduction may be enhanced by the presence of partly oxi- dized intermediate sulfur species (Spirakis 1986).

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4 Conclusion

Thus, for reasons previously mentioned, the sulfur isotope distributions observed in sulfides of San Vicente together with other geologic and geochemical data are interpreted to have originated from H2S produced by the abiogenic reduction of evaporitic sulfate during diagenesis of the Pucara host sediments. The c5³⁴S values of sphalerites are isotopically heavy, narrowly spread, and compositionally similar to evaporitic sulfates of Liassic age from Peru and elsewhere. Isotopic temperature estimates of 75° and 92

oc,

which assume equilibrium between the sulfides of generations II and Ill, have been calculated for two sphalerite-galena mineral pairs (samples FSV-041 and FSV-039; Table 1) using the fractionation equations given by Ohmoto and Rye (1979). These calculated temperatures partly overlap those at which the thermochemical reduction of sulfate is considered to be possible, and they are consistent with scarce fluid inclusion data in dolomites (J.I. C. A. 1976) indicating a temperature range from 70° to 140 °C. The ore-bearing units of the dolomitic Pucara host are clearly associated with certain evaporite-bearing facies, indicating the possibility that the sulfur has been reduced from anhydrite and gypsum present in and in the vicinity of the deposition site as evidenced by abundant molds of evaporite salts. Considering the evaporite- bearing facies at the basin scale and taking into account that sulfate pseudomorphs are also abundant in non-ore-bearing parts of the sequence, the available amount of sulfur should have been sufficient for ore formation. Dispersed organic matter occurs within the dolomitic rocks and in addition, bitumen aggregates up to 15 em in thickness and a few meters in lateral extent are considerably abundant. The unit Bituminous Silty Limestone is also characterized by a high content of organic carbon that consists mainly of altered bitumen. This organic matter, as previously mentioned, exhibits a high stage of evolution (RMoil = 4.607o ), which is interpreted as having been attained in post-ore stages.

A basinal brine rich in Zn and Pb migrating into the host carbonate rocks may have dissolved at least part of the evaporitic sulfate. Organic matter at the site of ore formation might have served to reduce the so~- at temperatures around 80°-120

oc.

The H2S necessary to initiate this reaction might have been produced either by bacterial activity or by cracking of organic matter (bitumen).

In summary, the existence of abiogenic reduction of sulfate by organic matter seems to constitute an attractive model which could account for the sulfur in sulfides at the San Vicente Mine. In particular, it is consistent with the homogeneous c5³⁴S values and with the striking similarity of the ratios of the main sulfide phase sphalerite with the sulfur isotopic range for Liassic marine sulfates. Al- though the geochemical significance of abiogenic reduction is still uncertain, such a process best explains the sulfur isotopic patterns observed and is perfectly consistent with the paleogeographic context of the San Vicente deposit.

Acknowledgments. This investigation has been supported by the European Communities (Contract No. MSM-010-D) and by the Compafiia Minera San Ignacio de Morococha, Lima.

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