Geology and genesis of the Shalipayco evaporite-related Mississippi Valley-type Zn-Pb deposit, Central Peru: 3D geological modeling and C-O-S-Sr isotope constraints

The Shalipayco Zn-Pb deposit, in central Peru, is composed of several stratabound orebodies, the largest of which are the Resurgidora and Intermedios, contained in carbonate rocks of the Upper Triassic Chambara Formation, Pucara group. Petrography suggests that a single ore-forming episode formed sphalerite and galena within vugs, open spaces, and fractures. Three-dimensional (3D) geological modeling has allowed division of the Chambara Formation into four members (Chambara I, II, III, and IV) that better define lithological controls on sulfide formation. Diagenetic replacement of evaporite minerals with the organic matter (OM) presence likely generated secondary porosity and H2S accumulation by bacterial sulfate reduction (BSR), providing ground preparation for the later Zn-Pb mineralizing event. The least-altered host rocks have C-O isotope compositions of 1.8 +/- 0.1 parts per thousand (VPDB) and 29.9 +/- 2.1 parts per thousand (VSMOW), respectively, within the Triassic marine carbonate ranges. Early dolomite contains lighter C-O composition (1.1 +/- 0.9 and 23.8 +/- 2.9 parts per thousand, respectively) consistent with OM decomposition during burial diagenesis. Post-mineralization calcite has still lighter C-O composition (- 5.1 and 13.3 parts per thousand, respectively), suggesting meteoric water that had migrated through organic-rich strata. The strontium isotopes of Mitu group basalts (0.709654-0.719669) indicate it as a possible, but not the unique source of strontium and probably of other metals. Highly negative sulfide sulfur isotope values (- 23.3 to - 6.2 parts per thousand (VCDT)) indicate a major component of the ore sulfur derived ultimately from BSR. However, multiple lines of evidence suggest that preexisting H2S underwent thermochemical redox cycling prior to ore formation. The influx of hot metalliferous brines to dolomitized zones containing trapped H2S is the preferred model for ore deposition at Shalipayco.

Automated summary

The Shalipayco Zn-Pb deposit in central Peru consists of stratabound orebodies, mainly sphalerite and galena, formed in the Upper Triassic Chambara Formation. Geological modeling has divided the Chambara Formation into four members, enhancing understanding of lithological controls on sulfide formation. The replacement of evaporite minerals by organic matter likely generated H2S accumulation, preparing the ground for the Zn-Pb mineralizing event.