LE-CHATELET GOLD-BEARING ARSENOPYRITE DEPOSIT, MASSIF-CENTRAL, FRANCE- MINERALOGY AND GEOCHEMISTRY APPLIED TO PROSPECTING

The abundant data available on the geochemistry and mineralogy of the Hercynian gold deposit of Le Chatelet (Massif Central, France) formed the basis for an analysis of the complex signature of a gold deposit a nd its hydrothermal alteration haloes. This study describes the deposi t and its environment at various scales. The sulfide paragenesis of th e Le Chatelet deposit resulted from several stages, one of which was a n event responsible for the deposition of Au-bearing arsenopyrite quar tz veins; the Au is contained within the arsenopyrite lattice. Hydroth ermal activity associated with these veins produced two superimposed a lteration haloes: an outer halo of kaolinite + tosudite + siderite alt eration that developed at the expense of the plagioclase minerals, and an inner halo of illite +/- siderite alteration that affected all the minerals. The hydrothermal alteration was followed by silicification of the wall rocks and then deposition of the Au-bearing arsenopyrite. The hydrothermal alteration related to the event also generated geoche mical signatures such as Li, B, LREE, As, Sb, Au and W anomalies. Mine ralogical studies and geochemical analyses of the hydrothermal facies show that: (1) the anomalous Li is trapped mainly in the tosudite latt ice and to a lesser degree in the illite lattice; and (2) the B is pro bably located in tosudite and illite lattices. Arsenic, Sb and Au are contained in the arsenopyrite lattice. During weathering, the lithoche mical signatures (determined by the analytical techniques used in this study) lose some of their specific characteristics, depending on the original concentrations. At deposit scale (0.25-1.0 km 2) , B and Li i n the soil profiles indicate zones of hydrothermal alteration, whereas As and Au indicate zones of mineralization. At a regional scale, the association of Au, As, Li and B in stream sediments is characteristic of this type of mineralization. Therefore, in the hydrothermal setting described at Le Chatelet, Au, As, Li and B appear, at all scales, to be dependable pathfinders for Au-bearing hydrothermal systems of possi ble, economic interest.