Fluid inclusion study of the bournac polymetallic (Sb-As-Pb-Zn-Fe-Cu...) vein deposit (montagne noire, France)

Abstract

Mineralogical studies demonstrate that the Hercynian polymetallic antimony-rich deposit of Bournac can be described by four stages of ore deposition and one of partial ore remobilization. Fluid inclusion data permit calculation of the composition and temperature of the fluids associated with each stage of hydrothermal mineralization and concomitant wall-rock alteration. Stages I and II (Fe-As and Zn) are represented by moderate-salinity H2O-CO2-(NaCl) inclusions which correlate closely with early carbonate deposition. Stage III fluids which are responsible for the deposition of Pb-Sb ores are characterized by low-salinity H2O-(NaCl) inclusions. During the final stage of mineralization (IV), corresponding to the main phase of stibnite deposition, abundant aqueous inclusions confirm the continued involvement of low-salinity fluids and the intense development of potassic clays and secondary silica in the wall rocks. Homogenization temperatures suggest that the whole cycle of mineralization took place during a gradual decrease in fluid temperature of 380°–140°C. Stibnite deposition is restricted to the interval of 230°–140°C thus confirming an essentially epithermal environment. Stage V (partial remobilization) is distinguished by the presence of high-salinity CaCl2-rich inclusions which are tentatively related to Triassic barite mineralization in the region and therefore postdate the Bournac antimony ores. Homogenization temperatures for this stage range 140°–60°C.