Abstract

The San Vicente Zn-Pb ore deposit is situated 300 km east of Lima in central Peru, within the Upper Triassic-Lower Jurassic carbonate platform (Pucara Group) at the western margin of the Brazilian Shield. Production during the last 20 years and present reserves exceed 12 million tons of ore assaying about 12 percent Zn and 1 percent Pb. Sphalerite and galena, the only ore minerals, occur as lens-shaped bodies generally parallel to the bedding. Within the 1,400-m-thick Pucara sequence three ore-bearing dolomite units occur. The ore lenses are bound to dolomitized tidal flat and lagoon facies with cryptalgal lamination and evaporite molds and to adjacent oolithic grainstones of barrier facies.Strontium, carbon, oxygen, and sulfur isotope geochemistry was carried out on consecutive crystallization generations. The results obtained display systematic trends. The 87 Sr/ 86 Sr ratios range between 0.7077 and 0.7084. The first generations display values very similar to those of Upper Triassic-Lower Jurassic ocean water. Later generations are slightly enriched in radiogenic strontium. The degree of enrichment in radiogenic strontium is low compared to other Mississippi Valley-type deposits. The delta 18 O values range between -6 and -10 per mil PDB, and the delta 13 C values between +2 and -1 per mil PDB. The late crystallization generations are enriched in the light isotopes of oxygen and carbon compared to the first generations. The sulfur isotope ratios of sphalerite are relatively homogeneous, ranging between 9.9 and 13.0 per mil. A trend to lighter sulfur isotope ratios with advancing diagenetic stage can also be recognized.The San Vicente lead-zinc deposit formed during burial diagenesis. The temperatures indicated by sulfur isotope geothermometry (75 degrees -92 degrees C) would be consistent with temperatures reached at a burial depth of about 2 to 3 km. This might have been reached by the end of the Jurassic. A model based on abiogenic reduction of sulfates at or near the ore site, and the introduction of a zinc- and lead-bearing basinal brine characterized by strontium isotope ratios slightly higher than contemporaneous seawater and highly radiogenic lead, is favored. The association of the orebodies with peritidal facies with abundant sulfate molds is important evidence favoring the existence of two separate reservoirs for sulfur and metals. Leaching of detrital material derived from Precambrian upper crust from the Brazilian Shield would explain both strontium and lead isotope ratios.

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