Le Chaˆtelet gold-bearing arsenopyrite deposit, Massif Central, France: mineralogy and geochemistry applied to prospecting

Abstract

The abundant data available on the geochemistry and mineralogy of the Herynian gold deposit of Le Chaˆtelet (Massif Central, France) formed the basis for an analysis of the complex signature of a gold deposit and its hydrothermal alteration haloes. This study describes the deposit and its environment at various scales. The sulfide paragenesis of the Le Chaˆtelet deposit resulted from several stages, one of which was an event responsible for the deposition of Au-bearing arsenopyrite quartz veins; the Au is contained within the arsenopyrite lattice. Hydrothermal activity associated with these veins produced two superimposed alteration haloes: an outer halo of kaolinite + tosudite + siderite alteration 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 geochemical signatures such as Li, B, LREE, As, Sb, Au and W anomalies. Mineralogical studies and geochemical analyses of the hydrothermal facies show that: (1) the anomalous Li is trapped mainly in the tosudite lattice and to a lesser degree in the illite lattice; and (2) the B is probably located in tosudite and illite lattices. Arsenic, Sb and Au are contained in the arsenopyrite lattice. During weathering, the lithochemical 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 in the soill profiles indicate zones of hydrothermal alteration, whereas As and Au indicate zones of mineralization. At a regional scale, the association of Au, Li and B in stream sediments is characteristics of this type of mineralization. Therefore, in the hydrothermal setting described at Le Chaˆtelet, Au, As, Li and B appear, at all scales, to be dependable pathfinders for Au-bearing hydrothermal systems of possible economic interest.