Sulfur isotopes in the porphyry copper deposit at El Salvador, Chile

Abstract

Sulfur isotope analyses have been performed on 64 monomineralic concentrates from 37 samples that are representative of mineralization in time and space at E1 Salvador.The hypogene sulfates (mean +10.75o; range +7.3 to +17.05o) are enriched in s4S relative to supergene sulfates (--0.75ø; --4.6 to +3.67oo) and to hypogene sulfides (--3.05ø; --10.1 to --0.37oo). Coexisting hypogene sulfides are increasingly depleted in a4S in the order molybdenite, pyrite, chalcopyrite, and bornite. The isotopic data suggest that sulfur in the supergene sulfates was largely derived from the oxidation of hypogene sulfides and that supergene "chalcocite" probably replaced hypogene chalcopyrite or bornite, but not pyrite. Isotopic temperature estimates from sulfate-sulfide fractionation pairs range from 400 ø to 570øC and are only in crude agreement with temperatures (<300 ø to >600øC) indicated by other geologic evidence. Those estimated from pyrite-chalcopyrite fractionation pairs (95 ø to 185øC) are nmch too low. Fractionation between 13 coexisting hypogene sulfate-sulfide assemblages (21 mineral pairs) defines a rather narrow band in /o•-pH-T space and suggests that [o• and pHacted as internally controlled variables throughout mineralization. Mass balance estimates of •s4Szs indicate a value of about +6 per mil for the sulfate zone and a value probably significantly heavier than 0 per mil for the entire deposit as presently exposed. The •s4S per mil values of coexisting hypogene sulfate and sulfide pairs approximate linear trends when plotted against their respective delta (/x) values. These trends suggest that Early anhydrite-chalcopyrite-bornite assemblages were formed from a sulfur reservoir having $a4Szs of approximately +1.6 per mil whereas Late anhydrite-pyritechalcopyrite assemblages formed from a reservoir +6.8 per mil $a%.s. Speculative interpretation suggests that Late sulfur was derived either from remobilization of Early assemblages below the deepest levels of exposure or from volcanic wall rocks surrounding the deposit, rather than from continued emanations from the underlying magma chamber that was the source of Early mineralization. However, at least one totally different interpretation of these data is possible. Recent experimental work by Ohmoto and Rye (1975, written and oral commun.) indicates that our $a4S per rail values for pyrite may require a correction factor, which would reduce both Early and Late sulfate-.sulfide assemblages to approximately single linear trends. This would imply that the underlying magma chamber continued to be the predominant source of sulfur ($s•Szs • +2%•) throughout the entire sequence of alteration-mineralization. The isotopic data do not show any consistent trends of s4S depletion with either paragenesis or zoning that would suggest a restricted reservoir of sulfur in the hydrothermal system. More questions than answers are provided by these data. Andesite; peripheral, 2,400 m; sericite-chlorite Andesite; peripheral, 2,400 m; sericite-chlorite "L" porphyry; central, 2,400 m; "D" v...

10th Applied Isotope Geochemistry Conference

“…S values is close to values characteristic for magmatic sulfur, 0‰ ± 5‰ d 34 S (Field and Gustafson 1976;Ohmoto and Rye 1979; Shelton and Rye 1982; Weihed and Fallick 1994). However we do not want to overlook fact that magmatic fluids assimilated one small fraction of sulfur from adjacent rocks (Ohmoto and Goldhaber, 1997).…”

Genesis of the Mamut Porphyry Copper Deposit, Sabah, East Malaysia

“…The Mamut deposit is characterized by the absence of gangue sulfates such as anhydrite, which are common in other porphyry type deposits elsewhere (e.g., El Salvador, Chile: Field and Gustafson, 1976;Panguna, Papua New Guinea: Eastoe, 1983). The results are listed in Table 11.…”

Sulfate in plagiogranites from the Troodos ophiolite, Cyprus.

“…The values of magmatic-hydrothermal sul fate formed in aqueous fluids are mostly in the range of +8 to +14%0 (Field and Gustafson, 1976;Beane and Titley, 1981). The &34S values of about +13% of sulfate in our plagiogranite samples are within the range of those magmatic or magmatic hydrothermal sulfates.…”