Abstract

The Le Bourneix gold deposit in the French Massif Central occurs within a shear zone in gneisses of the Limousin Hercynian basement. Gold-bearing quartz lenses formed during extension phases, after cataclasis of initially barren quartz postdating mylonites (Bouchot et al., 1989). The economic mineralization (arsenopyrite, gold, pyrite, galena, and Pb-sulfosalts), hosted by microsaccharoidal quartz, formed after crystallization of arsenopyrite and pyrite, synchronous with white milky quartz. A later paragenesis associated with hyaline quartz comprises Ag-rich gold, sphalerite, galena, tetrahedrite, and sulfosalts. Arsenopyrites have a range of compositions, with a mean As content equivalent to an equilibrium temperature with pyrite, of about 400 degrees + or - 30 degrees C. Hydrothermal alterations synchronous with ore deposition resulted in a phengite + chlorite association, and then a propylitic chlorite + carbonate assemblage.From fluid inclusion studies, it appears that the general evolution of the ore fluids is (1) circulation under high pressure (2 kbars ?) at 400 degrees + or - 50 degrees C of a low-salinity solution with minor volatile contents, (2) mixing of this solution with a CO 2 -rich vapor induced by a drop in pressure down to 0.6 kbars producing a heterogeneous fluid at 320 degrees C, and (3) circulation of volatile-free aqueous solutions at decreasing temperature (300 degrees - 120 degrees C) and salinity.Most of the economic ore probably formed at the transition from (1) to (2). Lead isotope data suggest a local source for metals, with mixing of poorly radiogenic lead from the basement orthogneiss with radiogenic lead from a supracrustal source.

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