ORE MINERALOGY, ALTERATIONS AND HYDROTHERMAL FLUID COMPOSITION OF A VARISCAN PORPHYRY-STYLE AU(±CU) MINERALIZATION AT LINARES DEPOSIT, NORTHWESTERN SPAIN

The Linares Au(±Cu)-deposit is located in the Navelgas Gold Belt, one of the gold belts identified in the NW of the Iberian Peninsula. The Au-mineralization is hosted by post-Variscan biotite granites that intrude folded and thrusted Precambrian and low-Cambrian series, and unconformable Stephanian rocks also thrusted during the later stage of the Variscan orogeny. The area is partially covered by Tertiary sediments. There are two different types of granite: the oldest (290±6 Ma) are porphyritic in texture and the youngest (276±6 Ma) are equigranular. Both are calc-alkaline type-I granites, slightly peraluminous with high K-content, although the youngest shows a flat REE-pattern, enriched in HREE and poor in LREE, and a high Eu-anomaly. The Au(±Cu) mineralization occurs disseminated or along a network of quartz veins. The alterations observed include feldspar (albitization, mainly in the youngest granitoid, and microclinization), filitic and propilitic alterations. An early oxide-bearing stage occurs in the youngest equigranular granite, consisting of quartz veins with Nb and Ta-bearing zoned wolframite partially replaced by scheelite. Gold is associated with a later sulphide-bearing stage. Sulphide minerals are mainly löllingite, pyrrhotite, chalcopyrite and arsenopyrite, with minor molybdenite, sphalerite and stannite. Gold occurs as native gold and electrum and is associated with Sb-rich native bismuth, bismuthinite, hedleyite, joseite, joseite-B, Se-rich galena among others. Xenotime, monacite-(Ce) and monacite-(Nd) occur in the altered rocks associated with quartz, carbonates, muscovite, rutile, chlorite and sulphides. Rutile, which is relatively abundant, is zoned with variable W and Nb contents. Ore-bearing quartz veins were sampled for a fluid inclusion study. At the present stage of petrographic and microthermometric studies two types of primary fluid inclusions have been identified: 1) an aqueous carbonic (pure CO₂) fluid with low salinity (bulk composition of 83-92% H₂O; 6-14% CO₂; 1.8-3.2% NaCl); 2) an aqueous carbonic fluid, also with low salinity, but bearing others volatile together with CO₂. The latter fluid inclusions occur in quartz with μm-scale trails of droplets of opaque minerals.