The Yanque Prospect (Peru): From Polymetallic Zn-Pb Mineralization to a Nonsulfide Deposit

The Yanque nonsulfide Zn-Pb deposit (26,491 kilotonnes of indicated resources at 2.37% Zn and 2.18% Pb [1.67% ZnEq cutoff]; 8,701 kilotonnes of indicated resources at 4.05% Zn and 3.32% Pb [4.00% ZnEq cutoff]) is located in the Andahuaylas-Yauri metallogenic province, southern Peru. The deposit occurs in a base metal district, with the Dolores porphyry copper deposit at its center. Sedimentary and igneous rocks predominate in the Yanque-Dolores area. The sedimentary rocks belong to the Soraya, Mara, and Ferrobamba formations (Upper Jurassic-Upper Cretaceous). Igneous rocks are mainly intrusive products associated with the Andahuaylas-Yauri batholith (diorite/tonalite, tonalite porphyry, and quartz diorite/monzonite porphyry). The Yanque deposit is hosted by two different sedimentary facies consisting of a siliciclastic conglomerate and a sedimentary dolomite breccia at the transition between the Mara and Ferrobamba formations. Hypogene mineralization consisted of galena, pyrite, and sphalerite, associated with a strong muscovite/illite hydrothermal alteration of the host rock (similar to alteration observed in porphyry copper deposits) and minor kaolinite, dolomite, and quartz. Minor element geochemistry, characterized by Sb, As, Mn, Ag, and Cu, suggests a magmatic-hydrothermal source for primary mineralization at Yanque. The Pb isotope compositions of Yanque and Dolores sulfides (pyrite and galena) are similar: (206)pb/Pb-204 ratios are between 18.545 +/- 0.003 and 18.656 +/- 0.002, (207)pb/Pb-204 ratios between 15.632 +/- 0.003 and 15.653 +/- 0.006, and Pb-208/Pb-204 ratios between 38.531 +/- 0.009 and 38.649 +/- 0.007. This indicates that the Yanque primary Zn-Pb mineralization likely formed from the same hydrothermal system that produced the Dolores porphyry copper deposit. The Pb isotope values of sulfides in both deposits are typical of the Tertiary magmatically derived ores in this part of Peru. Economic mineralization at Yanque consists of oxidized minerals resulting from weathering and alteration of hypogene sulfide mineralization. Sauconite (Zn smectite clay) and cerussite (Pb carbonate) with minor hemimorphite (Zn hydrosilicate) and smithsonite (Zn carbonate) predominate. Sauconite formed as replacements of detrital feldspar and hydrothermal muscovite/illite and kaolinite. In contrast to carbonate-hosted nonsulfide Zn-Pb deposits, sauconite at Yanque occurs in comparable amounts as in the world-class Skorpion deposit (Namibia). Cerussite replaced galena and precipitated as cement in the host rock. Hemimorphite and smithsonite mainly crystallized in late-forming veins. Unlike most supergene Zn-Pb deposits, the Yanque orebody shows comparable ore grades for both Pb and Zn with an inverse supergene chemical zoning. Zinc is most abundant at the top of the deposit, whereas lead increases with depth. These two elements have different mobility in solution, which implies that the primary source of zinc in the Yanque nonsulfide mineralization was originally located far from the primary galena mineralization, the remnants of which are present in situ. Supergene zinc mineralization was derived from the dissolution of primary sphalerite hosted in the now-eroded rocks above the Yanque mine site. Zinc-rich solution migrated through the sedimentary host rocks and precipitated Zn minerals, mainly sauconite, in pore spaces and replaced detrital and hydrothermal alteration minerals.