Abstract

The Hualgayoc district of northern Peru is typical of several Pb-Zn-Ag-Cu deposits in the Peruvian Cordillera in which veins are closely associated with stratiform (manto) orebodies. Steeply dipping veins containing mainly galena, sphalerite, pyrite, chalcopyrite, tetrahedrite, and rhodochrosite are hosted by Miocene andesitic plutons and by the gently folded and faulted Cretaceous miogeosynclinal sediments they intrude. The sediments also host concordant pyrite-rich orebodies that are crosscut by the veins.A comprehensive survey of the isotopic composition of lead in crustal rocks of the Hualgayoc region demonstrates that the Tertiary plutons, which partly host the veins, contain less radiogenic lead than either the Cretaceous sedimentary rocks that host the ores or the regional metamorphic basement rocks believed to underlie the Hualgayoc district. Sedimentary and metamorphic rocks from the Hualgayoc region contain no component of the highly nonradiogenic lead found in ancient high-grade rocks like those of the southern Peru coast or inferred to exist in eastern Colombia and also differ from basement rocks of the Guyana shield in Venezuela.Lead in vein and manto galenas from Hualgayoc is isotopically indistinguishable and coincides with the composition of the Tertiary plutons. The vein and the manto ores thus probably formed from the same solutions. Ion microprobe traverses across the growth zones of large, euhedral vein galena crystals reveal little isotopic variability of Pb during their growth. These data indicate that the ore-forming solutions had essentially a single source of lead, that this source was the Tertiary plutons in the district, and that local mixing of lead from several sources was not important. Although the plutons may have assimilated some lead from crustal rocks at depth, these inputs were homogenized by the time of ore formation.The manto ores are interpreted to be metasomatic bodies formed during the same event that produced the veins, rather than syngenetic massive sulfide ores overprinted by a later episode of vein mineralization as proposed by other workers. This interpretation is supported by the lead isotope evidence, by the overlap of paragenetic sequences in the vein and manto ores and the orderly progression of fluid inclusion homogenization temperatures and salinities from the manto to the vein minerals, by the absence of local igneous activity during the interval of sedimentation, and by the abundance of replacement textures in the manto ore sufides.The absence in the ores of lead leached from supracrustal rocks indicates (1) that processes within the magma determined the supply of metals to the ore-forming system, (2) that supracrustal host rocks enriched in ore metals are not necessary for the formation of this type of deposit, and (3) that meteorically derived waters, though probably comprising part of the ore solutions, did not scavenge important amounts of crustal lead.

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