Abstract

The Chicote tungsten deposit is situated along a major north-northwest to south-southeast structural trend in lower Paleozoic sediments of the eastern cordillera of Bolivia. Kami and Amutara are ore deposits similar to Chicote; they occur along the same structural trend over a distance of 35 km. No igneous rocks are exposed in the vicinity of these mines. The tungsten-bearing veins at Chicote are restricted to a zone of pervasive hydrothermal alteration extending over an area of 3,250 by 1,000 m with a vertical range of over 1,600 m. Whole-rock K-Ar dating of the sericite-rich wall rock indicates a maximum age of 26.1 m.y. for the mineralization. The mineral paragenesis follows an oxide-sulfide-carbonate sequence. Peripheral to the tungsten mineralization is a zone of tin-bearing veins.Fluid inclusion studies demonstrate that the ore fluids are complex brines with salinities of 5 to 48 equiv wt percent NaCl and trapping temperatures of 195 degrees to 530 degrees C. The salinities and temperatures of the fluids display a progressive decrease with time. Stable isotope analyses for delta 18 O and delta D on vein quartz and siderite suggest that the mineralizing fluids were magmatic in origin.Despite strong similarities in mineralogy between Chicote and other tungsten deposits, such as Panasqueira in Portugal and Pasto Bueno and San Cristobal in Peru, there are important differences. The ore fluids were far more saline than the typical worldwide trend and were dominantly magmatic. The distal setting of the Chicote mineralization from the parent felsic intrusion is explained by migration of ore fluids along the major Amutara-Chicote-Kami structural feature. The hypersaline Chicote ore fluids provided a suitable medium for the transport of tungsten and precipitation was probably caused by a decrease in temperature and pressure.

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