The composite gold-antimony vein deposit at Kharma (Bolivia)

      The Kharma antimony deposit in the Cordillera Oriental, Bolivia, is representative of vein-type antimony deposits that have stibnite as the only mineral of economic interest and a low gold content. In this deposit, recovery of gold is difficult because it is not known exactly where the gold is located within the ore shoots or the manner in which it is present in the main minerals of the deposit. Vein mineralization of this type is widespread, for example, in Spain, Germany, France, and Turkey. As with similar antimony deposits in Europe, this Bolivian Sb mineralization is bound to a fault zone that cuts dark, fine- grained clastic rocks of early Paleozoic age. Field evidence suggests that the mineralization is younger than Cretaceous. The country rocks have been subjected to regional dynamic metamorphism of greenschist facies, during which minute streaks of pyrite and other base metal sulfides formed parallel to the S planes of the siliceous pelites. Gold and antimony do not form discrete minerals in this wall-rock mineralization. Our studies of the fault-hosted mineralization reveal different genetic trends of gold concentration and depletion. During an early episode of vein formation, gold was added as "invisible gold" (up to 0.2 wt % Au) in arsenopyrite of substage Ia at temperatures somewhat higher than 400 degrees C, and as native gold (gold 1) containing little silver (up to 99.9 wt % Au) in substage Ib at temperatures somewhat higher than 300 degrees C. Arsenopyrite is considered the primary host for Au. It was converted by shear stress in the fault zone into As-bearing pyrite with a maximum of 268.8 ppm Au. Neither stibnite nor antimony sulfosalts, both of which are major stage II minerals in addition to quartz, is host to appreciable gold concentrations (max 0.8 ppm). Fluid inclusion studies on stage II quartz indicate a temperature of formation of 130 degrees to 234 degrees C. Stage III is characterized by an influx of Ag-bearing solutions at temperatures below 360 degrees C, which led to the conversion of primary gold (gold I) into argentiferous gold (gold II) and reaction of gold and stibnite to form antimony and aurostibite. Native gold was included by aurostibite during stage III and by Au-Sb oxides during stage IV. This inclusion of gold was accompanied by the precipitation of kaolinite, alunite, and schafarzikite (FeSb 2 O 4 ). The temperature of formation of this late-stage redeposition of the gold can be inferred only from present-day brines that emerge near the antimony vein at temperatures below 72 degrees C. Stage V reflects the adjustment of primary Sb mineralization to near-surface conditions by the formation of stibiconite and goethite...

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