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Sulfur Isotope Evidence for Thermochemical Reduction of Dissolved Sulfate in Mississippi Valley-Type Zinc-Lead Mineralization, Bongara Area, Northern Peru

N. I. Basuki

Department of Geology, University of Toronto, 22 Russell St., Toronto, Ontario, Canada M5S 3B1

B. E. Taylor

Geological Survey of Canada, 601 Booth St., Ottawa, Ontario, Canada K1A 0E8

E.T.C. Spooner

Department of Geology, University of Toronto, 22 Russell St., Toronto, Ontario, Canada M5S 3B1

Corresponding author: email, basuki{at}geology.utoronto.ca

The Bongara area, northern Peru, contains Mississippi Valley-type (MVT) Zn-Pb mineralization hosted in Late Triassic-Jurassic carbonate rocks of the Pucará Group. Mineralization is interpreted to have formed during the Late Cretaceous Peruvian orogenic event. Florida Canyon and Florcita, located within ~30 km of each other in the Bongara area, were studied to determine the origin and possible sources of sulfur in the mineralization.

One hundred and two in situ sulfur isotope analyses were determined on sphalerite, galena, and pyrite. The narrow and symmetric distributions of the sphalerite-1 ³⁴S values are consistent with sulfate reduction in an open system with regard to sulfate. The ³⁴S values of sphalerite-1 from Florida Canyon (–13.7 to +14.2 V-CDT; median = +1.9 V-CDT) are lower and more variable than those for Florcita (+6.2 to +19.3 V-CDT; median = +15.9 V-CDT). Sphalerite-2 (³⁴S = –0.3 to +17.6 V-CDT; median = +2.4 V-CDT), postdating sphalerite-1, probably formed in a closed system with regard to sulfate.

The geologic setting, fluid inclusion microthermometry, and ³⁴S values indicate that thermochemical reduction of dissolved sulfate (TSR) was the principal H₂S production mechanism for metal precipitation. Possible sources of sulfate include Late Cretaceous seawater (³⁴S +18 to +21) and evaporitic sulfate minerals in the Middle-Late Triassic Chambará-Condorsinga Formations (³⁴S +12 to +15) and/or in the Late Jurassic Sarayaquillo Formation (~ +13 to +18). Reduced sulfur from thermal cracking of organosulfur compounds, and probably also remnants of H₂S formed by bacterial sulfate reduction (BSR), might have been present during metal precipitation in Florida Canyon. Sample-scale ³⁴S variations observed in some samples from both areas reflect variable proportions of sulfate from different sources, temperature effects on isotopic fractionation from sulfate to sulfide, and the addition of reduced sulfur from organic compounds. Sample-scale ³⁴S variations can be interpreted as evidence for in situ sulfate reduction during mineralization. This study places constraints on the mineralization mechanism, the nature of sulfate reduction, and the source of sulfur in MVT mineralization in northern Peru. The observed sample-scale ³⁴S variations lend support to the conclusion that in situ thermochemical sulfate reduction can occur during mineralization in these environments.