Fluids in Hercynian Au veins from the French Variscan belt

Abstract

AbstractFluids, together with alteration and ore mineral assemblages, were studied in representative hydrothermal gold-bearing quartz veins from the western part of the Variscan belt in France (La Bellière, Montagne Noire district, Villeranges-Le Châtelet district, and Limousin province). Petrographic studies of the relationships between ores, fluid inclusions, microfracturing and quartz textures show that chronological and genetic relationships between gold deposition and fluid trapping may be very complex and difficult to establish for veins which show multi-stage fracturing and shearing. Systematic studies of secondary fluid inclusions in microcracks and recrystallized zones of the early quartz veins indicate two contrasting physical-chemical conditions: 1 relatively high temperature (250–400°C) and pressure (>1 kbar) event with CO2-CH4-H2S-N2 (±H2O-NaCl)-rich fluids related to the early sulphide deposition; 2 lower temperature (150–250°C) and pressure with aqueous fluids related to the late native-gold-sulphide (or sulphosalt) assemblage, which constitutes the economic ores in some deposits.In deposits where gold occurs predominantly in a combined state within arsenopyrite and pyrite (Châtelet and Villeranges), primary fluid inclusions in authigenic quartz combs cogenetic with arsenopyrite are almost purely aqueous (H2O-NaCl) and have a low salinity (1–4 wt. % NaCl). P-T conditions (150–250°C), nearly hydrostatic pressures) are similar to those of the second stage in the multi-stage quartz veins.Consideration of chemical equilibria in the C-O-H-N-S system using microthermometric and Raman spectrometric analysis for the fluids, together with data obtained from mineralogical studies, show that during gold deposition, fO2 was below hematite-magnetite buffer at Villeranges and around the Ni-NiO buffer at La Bellière and Montagne Noire. fS2 calculations based on H2S analyses are in good agreement with mineral assemblage estimates and close to that fixed by the pyrite-pyrrhotite boundary at high temperature. Ore fluid pH was significantly lower than in the host rocks as shown by the complete alteration of the host rocks into a quartz-K-mica assemblage. The data illustrate that during the late Hercynian, fluid circulation evolved from high P-T conditions, in some cases linked to late magma intrusions, towards conditions typical of later hydrothermal systems of the geothermal type.