An intrusion-related gold deposit (IRGD) in the NW of Spain, the Linares deposit: Igneous rocks, veins and related alterations, ore features and fluids involved
Résumé
The Linares deposit is an intrusion-hosted gold deposit located in the NW of the Iberian Variscan Belt. The igneous rocks hosting the ore are the Linares adamellite and the Arganzúa leucogranite. They are high-K, calc-alkaline and slightly peraluminous post-collisional igneous rocks, a mixture of I- and S-types, the Arganzúa leucogranite representing a late-stage fractionated phase. The intrusions have a reduced to intermediate oxidation state which spans the boundary between the ilmenite and magnetite series. The gold mineralization occurs disseminated or along a network of microfractures and veins of variable size, commonly in sheeted vein array. The alteration observed includes both potassic, with K-feldspar and secondary biotite developed mainly in the Arganzúa leucogranite, and sericitic and propylitic alterations. A sericite–chlorite–carbonate alteration, related to hairline fractures, is broadly distributed but rarely pervasive. Wolfram-bearing mineralization is followed by sulphide precipitation, mainly pyrrhotite, chalcopyrite, pyrite, arsenopyrite, molybdenite and galena, with minor löllingite and sphalerite. Gold is commonly associated with Bi–Te-bearing minerals with Bi/Te(S + Se) ≥ 1, consistent with a pyrrhotite-buffered environment. This is reflected in the strong correlation observed between Au and Bi. The fluids involved in the sulphide precipitation are aqueous-carbonic (CO2 and ± CH4), with low salinity (0.5–6.3 wt.% NaCl eq.) and variable amounts of other volatiles (N2 and H2S). The fluid inclusion study registered an adiabatic drop from a lithostatic pressure of 1.8–2.6 kbar to a hydrostatic pressure of 0.3–0.9 kbar, at temperatures of 300–400 °C. The fault-valve phenomena could explain the pressure drop producing a volatile release from an early CO2-rich fluid. A later trapped fluid with a wide-ranging CO2/CH4 ratio suggests an isothermal mixture with a CH4(± N2)-rich fluid external to the granitoid (metamorphic). The effervescence and the fluid-mixing could be the possible mechanisms of ore precipitation in this system. The Linares deposit presents a number of features in common with intrusion-related gold deposits elsewhere in Phanerozoic orogenic belts. The existence of intrusion-related gold systems would have potentially important implications for exploration in the NW of the Iberian Variscides.