Late-Hercynian gold mineralisation (Brués, Galicia, NW Spain): P-T-X conditions from fluid inclusion studies

The Brués Au-As deposit is located in the Boborás granite cupola, intrusive in the Schistose Domain of Galicia Trás-os-Montes (NW Spain). Gold is hosted in quartz veins at the granite exocontact. A sequence of quartz generation and mineral deposition was established: i) early Q1 (tension gashes and shear veins); ii) Q2a quartz accompanying arsenopyrite deposition in tension joints crosscut Q1; iii) Q2b in arsenopyrite fracture filling; iv) Q3 as clear quartz filling cavity (with phengite and bismuthinite) in arsenopyrite; v) native gold deposition, together with native Bi and tetradymite, a late Q4 quartz is likely coeval. Fluid inclusion trapped within the four quartz generation yield to the reconstruction of the P-T-X-t fluid evolution at Brués. Three main stages of fluid circulations are recognised. Two stages of aquo-carbonic fluid circulation: i) CO2-H2O-(CH4-N2) (c-w inclusions) are observed in early quartz generations (Q1 and Q2a quartz), trapped under lithostatic pressure (100-250 MPa) and in temperature range 400-450C; ii) CO2-CH4-H2O-(N2) are mainly observed in Q2a and Q3 quartz, trapped under hydrostatic conditions (50 to 100 MPa, 400-380C). Late stage of aqueous fluid circulation (2 to 12 wt% eq. NaCl, and Th 150 to 310C) is recorded in Q1 to Q3 as secondary inclusion planes and as primary in Q4. The P-T-t path appears as the succession of: (i) a near isothermal (ca. 400C) drastic pressure decrease (250 to 50 MPa), followed by (ii) an isobaric temperature decrease down to ca. 150C. These P-T-X evolution and correlated mineral deposition are broadly similar to other West European Variscan gold deposit (e.g. Boiron et al., 2003). In particular it is likely that at Brués as in other deposits, gold was deposited at the end of the evolution, at low P and T, and from aqueous fluids. On the other hand, no clear evidence of a locally derived magmatic brine was found until now in the Brués hydrothermal system, although the origin of chlorine in mildly saline aqueous fluids need to be further investigated. Boiron, M.-C., Cathelineau, M., Banks, D. A., Fourcade, S. and Vallance, J. (2003). Chemical Geology, 194, 119-141.