Abstract

Zoned, polymetallic base and precious metal (Ag-Bi-Pb-Cu-W-Au) mineralization at the Julcani district, central Peru, is genetically related to a geologically brief pulse of late Miocene ( approximately 10 m.y.) calc-alkalic magmatic activity. The Julcani volcanic center appears to have been localized by an anticlinal flexure in Paleozoic and Mesozoic strata, and perhaps by longitudinal and/or transverse faults. Early eruption of voluminous pyroclastic material was succeeded by the emplacement of interpenetrating volcanic domes of dacitic to rhyodacitic composition. Hydrothermal alteration and mineralization took place concurrently with the intrusion of late-stage volcanic domes and dikes. Complex systems of fractures and faults that channeled hydrothermal solutions and localized the various late-stage dikes were produced by repeated pulses of magmatic doming, perhaps aided by renewed movement along regional faults.Mineralization is almost exclusively confined to fracture fillings, with only subordinate replacement of sheared country rocks within the vein structure. Ore solutions spread upward and laterally from several centers, producing clearly defined zoning patterns. Three of these hydrothermal systems (Herminia, Mimosa, and Estela) have now been studied mineralogically and analyzed by detailed metal ratio contouring. The central part of the district, characterized by the pyritic gold-tungsten mineralization of Tentadora, appears to be the innermost zone of the Herminia system. Solutions moving southeast from Tentadora split, with a portion moving north into the Lucrecia vein system, a portion continuing southeast along the Docenita vein (which fed the veins in the old Herminia and Santo Domingo-San Jose mines), and much of the remainder flowing south along Luz-82 vein and then east and west along the Ano-2NW vein system. Along these paths they deposited first enargite-pyrite-tennantite/tetrahedrite, then a zone of complex silver and bismuth-sulfosalts with bismuthinite, and at greater distances galena followed by lead sulfosalts, orpiment, and realgar. In the Mimosa vein system, tennantite/tetrahedrite predominates in the lower or inner zones and galena in the outer zones; the antimony and silver content of tennantite/tetrahedrite increases systematically upward along the solution paths. In Estela, early pyrite-wolframite mineralization is followed first by tennantite/tetrahedrite-chalcopyrite-arsenopyrite-bismuthinite and then by galena-sphalerite mineralization.The zone of pyritic gold-tungsten mineralization, which corresponds approximately with a zone of premineral dikes of intrusive breccia and tuffisite and with the inferred location of the vent for the early pyroclastic rocks, overlies the permeable rock through which a large portion of the mineralizing solutions ascended. Detailed metal-ratio zoning studies have helped considerably in guiding exploration and development of Julcani, allowing a marked increase in ore reserves, as well as the estimation of larger amounts of prospective ore than was heretofore possible.

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