Remobilisation of base metals and gold by Variscan metamorphic fluids in the south Iberian pyrite belt: evidence from the Tharsis VMS deposit

The Tharsis massive sulphide deposit, one of the major VMS-type deposits in the Iberian pyrite belt (IPB) was severely deformed by the Variscan tectono-thermal events. The question of whether or not these events affected the metal distribution in the deposit has been addressed by simultaneously studying the mineral parageneses (Tharsis stockwork) and the fluid circulation (at local and regional scales). The results are: (1) The early paragenesis in the stockwork (Q1 quartz-pyrite-chlorite-phengite +/- cobaltite +/- ankerite) was strongly overprinted by a late post-kinematic mineral deposition, including new quartz veins (Q3 quartz) and base metal sulphides (chalcopyrite, sphalerite, Bi and Te minerals, pyrite and galena) and gold. (2) At a regional scale, fluids accompanying the peak metamorphism conditions (ca. 300 MPa, ca. 300 degreesC) were of C-O-H-N-NaCl type, CO2-dominated with CH4 and N-2, and are considered to be "metamorphic" on the basis of microthermometry and geochemistry. The late- to post-kinematic evolution ("retrograde" stage) was characterised by a pressure drop, down to 40 MPa (lithostatic to hydrostatic transition), and a heat input leading to temperatures greater than or equal to430 degreesC, then decreasing to temperature around 170 degreesC. Fluids of the "retrograde" type exhibit both dilution of the C-O-H-N-NaCl fluid by a low salinity "meteoric" water and progressive loss of volatile components. (3) Fluids of the retrograde type pervasively percolated through the Tharsis stockwork and were responsible for the strong mineral overprint on the early (deformed) paragenesis. All the measurable fluid inclusions (f.i.) record these late fluids. There are primary fluid inclusions in Q1, but they are systematically imploded due to the external overpressure generated by the Variscan tectonic events. Although base metal distribution in the stockwork is basically the result of the "retrograde" fluid circulation, it remains unclear whether these metals were newly introduced into the stockwork from elsewhere or simply redistributed from the existing primary assemblages in the massive sulphide bodies. (C) 2002 Elsevier Science B.V. All rights reserved.