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

摘要

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 approx 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 sigma~(34)S values are consistent with sulfate reduction in an open system with regard to sulfate. The sigma~(34)S values of sphalerite-1 from Florida Canyon (-13.7 to + 14.2 per thousand V-CDT; median = + 1.9 per thousand V-CDT) are lower and more variable than those for Florcita (+6.2 to +19.3 per thousand V-CDT; median = +15.9 per thousand V-CDT). Sphalerite-2 (sigma~(34)S = - 0.3 to +17.6 per thousand V-CDT; median = +2.4 per thousand V-CDT), postdating sphalerite-1, probably formed in a closed system with regard to sulfate. The geologic setting, fluid inclusion microthermometry, and (sigma~(34)S values indicate that thermochemical reduction of dissolved sulfate (TSR) was the principal H_2S production mechanism for metal precipitation. Possible sources of sulfate include Late Cretaceous seawater (sigma~(34)S = +18 to +21 per thousand) and evaporitic sulfate minerals in the Middle-Late Triassic Chambara-Condorsinga Formations (sigma~(34)S approx = +12 to +15 per thousand) and/or in the Late Jurassic Sarayaquillo Formation (approx +13 to +18 per thousand). Reduced sulfur from thermal cracking of organosulfur compounds, and probably also remnants of H_2S formed by bacterial sulfate reduction (BSR), might have been present during metal precipitation in Florida Canyon. Sample-scale sigma~(34)S variations observed in some samples from both areas reflect variable proportions of sulfate from different sources, temperature effects on isotopic frac-tionation from sulfate to sulfide, and the addition of reduced sulfur from organic compounds. Sample-scale sigma~(34)S 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 sigma~(34)S variations lend support to the conclusion that in situ thermochemical sulfate reduction can occur during mineralization in these environments.