Abstract

The La Coipa ore deposit is part of an epithermal precious metal system located in the Tertiary volcanic belt of northern Chile. Two main mineralized zones are present at La Coipa, Ladera-Farellon and Coipa Norte, both hosted by Triassic black shales and sandstones and by late Oligocene to early Miocene tuffs and tuffaceous breccias. Latite dikes and sills and a dacite dome complex are spatially and temporally related to the volcanism that generated the host pyroclastic sequence. K-Ar ages demonstrate that the mineralization process took place during the volcanic event. Mineralization is structurally controlled, mainly by vertical to subvertical, north-south- to northeast-southwest-striking faults and associated fractures. However, the configuration of ore suggests that there was also an important lithologic control. The orebodies have semitabular to mushroom-shaped forms. Farellon and the eastern portion of Coipa Norte possess high gold grades associated mainly with advanced argillic alteration (alunite-kaolinite + or - dickite-quartz) and semitabular forms and are hosted mainly by the Triassic sedimentary rocks. Ladera and western Coipa Norte have high silver/gold ratios and mushroom-shaped forms and are hosted preferentially by the Tertiary pyroclastic unit. Ladera is largely supergene altered (jarosite-goethite + or - gypsum + or - barite) and western Coipa Norte is silicified and supergene altered in its upper parts and advanced argillic altered at depth. The most common precious metal-bearing minerals are cerargyrite, several other silver halides, argentitc, native silver, electrum, and native gold. Sulfides, such as pyrite, chalcopyrite, covellite, tennantite-tetrahedrite, enargite, galena, and sphalerite also have been observed in the unoxidized zone and in isolated patches in the oxidized zone.

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