DYNAMICS OF SHALLOW LATE-VARISCAN GOLD MINERALIZATION - THE LE CHATELET AU-ARSENOPYRITE QUARTZ VEINS, MASSIF-CENTRAL, FRANCE

The Le Chatelet gold deposit is unique among the mined deposits of the French Variscan basement in that the gold is not free but caught up i n the arsenopyrite crystal lattice. Microstructural analysis shows tha t the Au-arsenopyrite quartz veins are hosted by north-trending dextra l strike-slip faults resulting from NNE-SSW compression during the ear ly Stephanian (around 300 Ma). Textural study indicates that vein empl acement was due to sequential dynamics related to spasmodic strike-sli p movements along these brittle structures (seismic regime). The class ic sequence begins with the slow crystallization of sulphide-free comb quartz followed by a co-deposition of microcrystalline quartz and nee dles of Au-arsenopyrite; the rapid crystallization of the Au-arsenopyr ite-bearing microcrystalline quartz was due to a sudden drop in fluid pressure. The veins correspond to dilational jogs formed by a suction pump mechanism during a late phase of the Variscan orogeny; they were trapped in migmatite about 1 km below surface.