P-T-X conditions of late Hercynian fluid penetration and the origin of granite-hosted gold quartz veins in northwestern Iberia: A multidisciplinary study of fluid inclusions and their chemistry

Abstract

Quartz veins hosted by late Hercynian granites and their host rocks occur across the Variscides of the northern Iberian peninsula, and locally display Au-As mineralization. Four separate occurrences at Corcoesto, Tomino, Penedono, and Pino have been investigated to determine P-T-X conditions of formation and likely fluid sources. Special attention has been paid to fluid chemistry using a multidisciplinary investigation of fluid inclusion gases by Raman spectroscopy (individual fluid inclusion analysis) and mass spectrometry (bulk analysis), and ion chemistry using microthermometry and bulk leachate chemical analysis. Two major changes in the chemical and physical environment have been identified: (1) a progressive change in the bulk chemical composition from early CO 2-rich, C-H-O-(N) fluids, equilibrated with graphitic metamorphic host rocks, to late H 2O-dominated fluids, inferred from their halogen signature to result from an influx of meteoric or upper crustal fluids affecting the basement at the end of Variscan orogenesis, and (2) changes in the P-T conditions from early stage sulphide deposition in quartz veins, at ca. 450 °C and 150-300 MPa, towards epithermal conditions, ca. 260–310 °C and <75 MPa, at the stage of gold mineralization. Several chemical trends are shown by the fluid inclusions: (1) dilution of the early volatile-rich fluids, (2) a break of graphite buffering activity demonstrated by the CH 4 content increase in the volatile fraction of the latest As mineralizing fluids, and (3) increasing contribution of a relatively oxidizing fluid enriched in sulphate and bromide during the latest stages of fluid percolation (Au stage). These latest fluid stages are interpreted as indicative of extended fluid penetration downward in the crust enhanced by late brittle deformation and decompression, and played an important role in mass transfer at the end of the Hercynian orogeny, especially in transport of metals. Gold ores have formed mostly in granites because main fluid pathways developed in the main structurally active zones which favoured the emplacement of the granites. However, there is no evidence of genetic link between gold ores and their enclosing granites.