アルゼンチン, Rio Negro州の螢石鉱床

抄録

In the area of 250 (east to west) by 200km (north to south) in Rio Negro Province, Argentina, numerous fissure-filling fluorite veins of epithermal type can be found. The wallrocks of these deposits are various. They are rhyolite, rhyolitic tuff and pyroclastic flow, dacite, granite, mica-schist, etc. However, the feature of the wallrock alteration in these deposits are quite similar to each other.
Frequently, innumerable rock fragments are cemented with fluorite and quartz in fractured zones. Veins, having a width of 1-3m and an extension of 200-500m along tree strike, are not uncommon. In these veins quartz, barite, sericite, calcite, and gypsum are closely associated with fluorite. In some deposits, small amounts of galena, sphalerite, chalcopyrite, and pyrite are recognized, occasionally, nacrite, montmorillonite, and stilbite are also found.
Filling temperatures of liquid inclusions in fluorite specimens were studied with the heating stage attached to the microscope, and it was revealed that the measured values are in a range from 90°C to 210°C with 95% of probability. Any relations were not recognized among the filling temperature, the mineral assemblage, and the wallrocks in these deposits.
It is suggested that fluorite precipitated when the temperature of the ore-forming solution decreased to 90-210°C, following the crystallization of higher temperatureminerals, and that montmorillonite, calcite, and gypsum were formed in later stage with decreasing temperature of the solution, Furthermore, it is considered that the chemical nature of the hydrothermal solution which formed these fluorite deposits varied between weakly acidic and weakly alkaline.