Abstract

W,As,(Au) and As,Sb vein mineralizations in the Haut Allier (French Massif Central) are associated with ortho- and parametamorphic rocks containing biotite + or - sillimanite, which have been locally intruded by small granitoid bodies. Hydrothermal alteration is well developed around these veins and has been studied here from a petrographic and a chemical point of view. The results are compared with those which were obtained by microthermometric study of fluid inclusions. In both mineralized types, three superimposed hydrothermal stages are distinguished, showing good agreement with the results obtained by other methods.In the early stage, fluids enriched with CO 2 + or - CH 4 developed a quartz-2M1 muscovite assemblage (with 8% paragonitic substitution) at temperatures of about 400 degrees C. Minor chlorite appears, replacing biotite, near the transition of altered to fresh rocks. This early stage is predominant in veins with W,As,(Au) mineralization. In the intermediate stage, low-salinity aqueous fluids dilute the earlier fluids and control a new alteration with quartz-2M1 phengite + or - carbonate assemblages at slightly lower temperatures. This alteration stage is particularly important in the environment of veins with As,Sb mineralization. In the late stage, the last hydrothermal event occurs at a low temperature (<200 degrees C) and forms a 1M illite-kaolinite + or - quartz assemblage in small alteration veinlets.Chemically, the different phyllosilicates that successively occur in W,As,(Au) and As,Sb veins are very similar. The differences between these mineralization types can be explained by the proximity of the granitoid bodies to W,As,(Au) veins and by a greater fluid/rock ratio involved in the alteration processes for As,Sb vein deposition.

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