Mineralogy, paragenesis and geothermometry of the tin and tungsten deposits of the eastern Andes, Bolivia

Abstract

Numerous geothermometers were applied to ores of the Eastern Andes of Bolivia in order to evaluate temperature as one factor in zoning and paragenesis of the deposits, and also to test the thermometers themselves where results could be checked against available field evidence. Some 225 specimens were analyzed representing 53 deposits and several ore types, chiefly the tin and tungsten ores.The stratigraphic record suggests that ores of the subvolcanic setting of central and southern Bolivia formed at depths on the order of 350 to 2,000 m and at total pressures of approximately 30 to 500 bars. Comparable estimates for the somewhat deeper deposits of plutonic association in the Cordillera Real are depths of 2,000 to 4,000 m and pressures of about 190 to 1,000 bars.A similar paragenesis is indicated for the tin and tungsten deposits and consists of (1) an early vein stage in which most tin and tungsten was deposited, (2) a base-metal sulfide stage, (3) hypogene alteration of pyrrhotite to pyrite, marcasite, and siderite and (4) late crusts and veinlets of siderite, fluorite, and a variety of hydrous phosphate minerals some of which are supergene.The Bolivian ores contain most of the sulfide minerals proposed, at one time or another, as geothermometers, but present attempts to use them have been largely unsuccessful. Pyrrhotite compositions were modified during replacement by pyrite, marcasite and siderite and most do not reflect initial depositional temperatures. Arsenopyrite gave only rough indications of temperature and was essentially useless as a geobarometer. Sphalerite displays complex polytypism in the Bolivian ores, but this could not be correlated with variables of geologic interest. The present iron content of sphalerite reflects initial equilibration with pyrrhotite or pyrite, but its significance in terms of temperature is debatable. Miscellaneous thermometers such as sulfide invariant points, the high-low quartz inversion, melting of native bismuth and sulfide exsolution temperatures are considered and some yield useful temperature limits. Fluid inclusion studies reveal highly systematic trends of salinity and depositional temperature during formation of the tin and tungsten ores. Tin-bearing fluids of the early vein stage were complex, NaCl-rich brines of low CO 2 content. During precipitation of cassiterite and early quartz, salinities reached values as high as 46 weight percent, but the later ore fluids were more dilute and gradually approached fresh water in closing stages of mineralization. Depositional temperatures first increased from about 300 degrees to 530 degrees C during the early vein stage and then declined steadily to less than 70 degrees C in the later stages. Minerals of the main sulfide stage formed as temperatures declined from about 400 degrees C to 260 degrees C. Fluid inclusions in siderite indicate temperatures of 260 degrees C to 200 degrees C for the hydrothermal alteration of pyrrhotite. Late fluorite of the Colquiri lode was precipitated at temperatures between 200 degrees C and 133 degrees C. The late hydrous phosphate minerals formed below 70 degrees C. Many events of the hypogene sequence could be attributed chiefly to cooling of the vein fluids and are so interpreted, but little is known of the wall rock alteration and its influence on paragenesis. Active boiling of the early vein fluids is indicated by the fluid inclusions in quartz and cassiterite. This boiling upgraded salt contents of the early vein fluids while driving CO 2 into the associated vapor phase. This vapor transported some mineral matter, as evidenced by daughter minerals in gas-rich inclusions, but the identity and quantitative significance of this material are unknown. The early boiling favored precipitation of quartz and cassiterite and may explain the restricted vertical distribution of very high-grade tin ore in several deposits of shallow origin in central and southern Bolivia. Evidence of boiling in several major deposits of the Cordillera Real suggests that these ores, though formed in the plutonic setting, were still deposited at depths of less than 3,000 m. This limit is compatible with that based on stratigraphic reconstructions of rock cover at the time of mineralization. All data gathered in this study are consistent with a single, prolonged event of mineralization for the individual tin and tungsten deposits throughout Bolivia. The deposits could differ in age, but there is no convincing evidence that they are hybrid products of hydrothermal regenerative processes. A magmatic source is preferred for the mineralizing NaCl brines and their contained metals, but the progressive cooling and dilution of these brines may reflect gradual influx of meteoric water to the magmatic hydrothermal system.