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Distal alteration in the carbonate-hosted replacement and skarn systems at Yauricocha, central Peru Carrasco, Julio Jurado

Abstract

Despite numerous reviews of carbonate-replacement (CRD) and skarn deposits, there have been few exhaustive studies into the details of distal alteration features lateral and above to the mineralized centres at these deposits. These features record the passage of exhausted hydrothermal fluids and provide important guides for mineral exploration. Yauricocha, central Peru, is a Zn-Pb-Cu-Ag magmatic-hydrothermal system zoned around a late Miocene quartz monzonite-quartz diorite stock, which intruded limestone and marl of the Cretaceous Jumasha and Santonian Celendin formations. Northwesterly-trending folds controlled the emplacement of the stock, sulfide carbonate-replacement deposits at Mina Central, skarn deposits at Cachi Cachi, and ’Carlin-style’ Au mineralization at Purisima Concepcicin. Low-grade porphyry-Cu, Au, and Ag-Cu-base metal veins lie within the stock. CRD and skarn are fringed by a visible zonal arrangement of bleached and grey marble, dolomite, calsilicates-, orange brown- to white carbonates-, and Mn-oxides minerals, quartz and sulfide veins, and local jasperoid. Thermal aureole surrounding the intrusion contains an inner zone of bleached marble and an outer zone of grey marble. Cryptic geochemical halos are defined by trace element abundance and oxygen and carbon isotope composition. In Mina Central and Cachi Cachi, visible and cryptic alteration sequences extend up to 800m laterally from intrusive contact into Jumasha and Celendin formations, and over a similar vertical distance in proximity to the ores zones. At Mina Central, visible alteration is mainly structural controlled. Garnet skarns occur locally associated with quartz diorite sills. Bleached versus gray marble units extend up to 430m from intrusive contact. Mn-carbonate veinlets, decreasing in abundance with distance, are present in marbles and limestone, and are spatially associated with sulfide orebodies. At Cachi Cachi, alteration consists of andradite-grossular skarn, surrounded by bleached and grey marble. Pervasive bleaching characterizes rocks within 150m of the intrusive contact, while narrow bedding controlled zones of bleaching extend up to 450m away. Calcsilicate and carbonate vein networks are developed adjacent to ores, and decrease in abundance with distance away from intrusion. Visible and cryptic alteration halos can be used as a model to explore carbonate-hosted replacement and skarn orebodies in other areas with geological features similar to Yauricocha.

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