Multistage deformation of Au-quartz veins (Laurieras, French Massif Central): evidence for late gold introduction from micro structural, isotopic andfluid inclusion studies

The relative chronology of fluid migration, quartz and Au-deposition in a s ilicified fault from the main Au-district (Laurieras, St Yrieix district) f rom northern French Massif Central has been determined from microstructural , fluid inclusion, isotopic and ore mineral evidences. Three main stages of fluid circulation, microfracturing and quartz crystallization, and ore dep osition were distinguished on the basis of textural relationships and the p ressure, temperature and composition of the palaeo-fluids: (1) a series Of early fluid events was responsible for the localized drainage of retrograde metamorphic fluids along the main fault and the subsequent sealing by milk y and microcrystalline quartz preceeded the main Au-ore stages. Early fluid s were aqueous-carbonic, trapped under lithostatic to sublithostatic pressu res at temperatures in the range 350-500 degreesC. Subsequently, several ty pes of micro structures were developed in the early quartz matrix. (2) NS m icrofractures filled by clear quartz, arsenopyrite and boulangerite (I) con tain significant refractory gold concentrations. Clear quartz formed from a queous-carbonic fluids of lower densities than those of the earlier fluids. Significant pressure drops, down to pressures around 55 MPa were responsib le for a local immiscibility of the aqueous-carbonic fluids at temperatures of 340 +/- 20 degreesC. (3) The main ore stage is characterized by the for mation of dense sets of sub-vertical (EW) microfractures, healed fluid incl usion planes in quartz, and filled by ore minerals (native gold, galena and boulangerite II) when they crosscut earlier sulfides. The fluids are aqueo us with low and decreasing salinity, and probable trapping temperatures aro und 230 degreesC. Isotopic data, obtained on microfissured quartz, indicate these dilute aqueous fluids may be considered as meteoric waters that deep ly infiltrated the crust. Late microfissuring of a mesothermal quartz vein, originally barren (only w ith pyrite and arsenopyrite), appears to be the main factor controlling gol d enrichment. It can be related to late Hercynian deformational stages, dis connected from the early fault formation and silicification. These late sta ges which affected the Hercynian basement during its uplift, are of critica l importance for the formation of Au-ores. We concluded that this type of A u-ore formed under rather shallow conditions, is distinct from those genera lly described in most mesothermal Au-veins. (C) 2001 Elsevier Science BN. A ll rights reserved.