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

The Bongara area, northern Peru, contains Mississippi Valley-type (MVT) Zn-Pb mineralization hosted in Late Triassic-Jurassic carbonate rocks of the Pucara 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 δ34S values are consistent with sulfate reduction in an open system with regard to sulfate. The δ34S 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 (δ34S = −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 δ34S 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 (δ34S ≈ +18 to +21‰) and evaporitic sulfate minerals in the Middle-Late Triassic Chambara-Condorsinga Formations (δ34S ≈ +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 δ34S 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 δ34S 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 δ34S variations lend support to the conclusion that in situ thermochemical sulfate reduction can occur during mineralization in these environments.