THE COMPOSITE GOLD-ANTIMONY VEIN DEPOSIT AT KHARMA (BOLIVIA)

The Kharma antimony deposit in the Cordillera Oriental, Bolivia, is re presentative of vein-type antimony deposits that have stibnite as the only mineral of economic interest and a low gold content. In this depo sit, recovery of gold is difficult because it is not known exactly whe re 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 o f 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 elastic rocks of early Paleozoic age. Field evidence suggests that the mineralization is younger than Cretaceous. The country rocks have bee n subjected to regional dynamic metamorphism of greenschist facies, du ring which minute streaks of pyrite and other base metal sulfides form ed parallel to the S planes of the siliceous pelites. Gold and antimon y do not form discrete minerals in this wall-rock mineralization. Our studies of the fault-hosted mineralization reveal different genetic tr ends of gold concentration and depletion. During an early episode of v ein 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 I) containing little silver (u p 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 pyrit e with a maximum of 268.8 ppm Au. Neither stibnite nor antimony sulfos alts, both of which are major stage II minerals in addition to quartz, is host to appreciable gold concentrations (max 0.8 ppm). Fluid inclu sion 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 le d to the conversion of primary gold (gold I) into argentiferous gold ( gold II) and reaction of gold and stibnite to form antimony and aurost ibite. 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 (FeSb2O4 ). The temperature of formation of this late-stage redeposition of the gold call be inferred only from present-day brines that emerge near t he 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. We have modeled the Au- Sb Kharma vein mineralization-stages I and II-in terms of regional met amorphism. Considering the compressional structural regime that existe d during most of the early Tertiary, the transformation of invisible g old into native gold may be a result of metamorphic devolatilization, whereas the reverse trend of Au depletion and Ag concentration-stages III and IV-may be genetically related to the waning hydrothermal activ ity of nearby (sub)volcanic domes which formed during the late Tertiar y when the study area was uplifted and an extensional tectonic regime replaced the earlier compressional regime. This deposit may thus be te l med a composite Au-Sb vein mineralization featuring the characterist ics of the thrust-bound Sb-As-Au ore deposits or arsenian subtype of A u-sb deposits in deformed metamorphic belts (stages I-II) as well as s imple epithermal Sb vein-type deposits (stages III-IV).