Conditions of gold-bearing arsenopyrite crystallization in the Villeranges Basin, Marche-Combrailles shear zone, France; a mineralogical and fluid inclusion study

Abstract

The Au-As-Sb deposits from the Marche-Combrailles (northwestern part of the French Massif Central district) are spatially associated to the Marche-Combrailles shear zone. Host rocks are either Visean volcano-sedimentary tuffs or formations of the Paleozoic basement presenting a greenschist facies assemblage which resulted from extended hydrothermal metamorphism. The strong macro- and microfracturing that affected the Villeranges basin as well as the surrounding formations is related to late tectonic reactivation of the Marche-Combrailles shear zone around 300-315 Ma which favored intense circulation of hydrothermal fluids. Solubilization, transport, and deposition of Au, As, and Sb occurred together with specific rock alteration along faulted zones on distances ranging from several centimeters to a few decimeters. Wall rocks are affected by complete bleaching resulting from chlorite replacement by phengite (T = 240 degrees + or - 30 degrees C) followed by complete replacement of the previous minerals by an illite-quartz-ankerite-pyrite assemblage. Gold is carried in a combined state with arsenopyrite, occurring either within a diffuse network of quartz + ankerite veinlets or as isolated crystals in the altered tuff. Arsenopyrite crystals are characterized by important enrichments either at their periphery or within microcrack networks affecting their whole mass. Mass balance calculations on fresh and altered rocks show that during the ore stage, hydrothermal fluids contributed H 2 O, CO 2 , and S (Au, Sb) to authigenic minerals and leached almost completely Na, Ba, Sr, Si, Fe, Ca, Mg, Al, Ti, and K exhibit similar contents in fresh and altered rocks and are redistributed among the authigenic minerals.Ore fluids are characterized by an aqueous composition, a rather low salinity, and temperatures ranging from 160 degrees to 200 degrees C. Ore deposition occurred at a low pH, indicated by the complete alteration of the host rocks into a quartz-illite association, and at a low f (sub O 2 ) intermediate between the f (sub O 2 ) fixed by the hematite-magnetite and Ni-NiO oxygen buffers. Gold deposition probably resulted from a destabilization of Au bisulfide or thioarsenide complexes during the sulfide crystallization.Thus, the Au-As-Sb ores in the Villeranges area result from a single hydrothermal stage and occur during a late stage of brittle deformation, significantly posterior to the continental basin formation and the acid volcanism. Ore fluid circulation is linked mostly to the fluid convection produced by the abnormal heat flow characterizing the Marche-Combrailles shear zone.