The magmatic-hydrothermal transition within intrusion-related gold and tin-tungsten deposits: Examples of late-hercynian Boborás and Beariz granites (Galicia, NW Spain)

The magmatic-hydrothermal transition is subject to debate in numerous cases where mineral deposits are spatially related to granite intrusions. In most of cases, intrusion only appears as a rheological trap for mineralizations (e.g. Pb-Zn deposits). Some rare examples suggest a magmatic origin for mesothermal gold deposit emphasis by magmatic to hydrothermal transition textures (Mustard, 2003). We studied two neighbouring deposits (Brués As-Au deposit and Mina Soriana Sn-W deposit), both emplaced on top of two different granite cupola (respectively Boborás granite and Beariz granite). For both granites, during emplacement, granitic melts injected hosting micaschists forming regular sills and dykes network. In both cases: i) Granitic dykes, sills and feldspar-quartz veins developed hydrothermal alteration within host micaschists (mainly tourmalinitisation at Mina Soriana and greisenification at Brués). Internal textures of the quartz-feldspar veins show gradual evolution from quartz-feldspar (border) to early single quartz (center). ii) this early quartz-feldspar veins are reopened and filled by a second quartz generation, followed by arsenopyrite deposition. iii) A third generation of quartz filled arsenopyrite cavities. Further fracturation of these composite veins allow trap of polymetallic sulphides, followed by bismuthinite stage. Differences are: Sn-W deposition during early quartz stage in Mina Soriana and Au-Bi-Te deposition with associated phengite during ultimate stage at Brués. Exceptional exposures and similarities of textural/mineralogical evolutions between these two deposits allow us to trace a continuous transition from magmatic to hydrothermal processes during ore deposits genesis. In spite of aquo-carbonaceous and low salinity fluid inclusions in Brués gold deposit, like other intrusion-related gold deposits (Lang and Baker, 2001), this continuous transition suggests the persistence of a magmatic input into fluid responsible for the mineralising stages. Lang J.R. and Baker T. (2001). Mineralium Deposita, 36:477-489. Mustard R. (2003).Proceedings of the 7th SGA Meeting, Athens, Greece, 24-28 Aug. 2003, 1:355-358.