%0 Journal Article
%A Gómez Fernández, Fernando
%A Vindel, Elena
%A Martín Crespo, Tomás
%A Sánchez Pérez-Cejuela, Virginia
%A González Clavijo, Emilio
%A Matías Rodríguez, Roberto
%T The Llamas de Cabrera gold district, a new discovery in the Variscan basement ofnorthwest Spain: A fluid inclusion and stable isotope study
%D 2012
%@ 0169-1368
%U https://hdl.handle.net/20.500.14352/42307
%X The northwest of the Iberian Massif is well known to host numerous gold mineralizations which wereexploited during Roman times. This study presents a multidisciplinary approach leading to the mineralogical,fluid inclusions, and geochemical characterization of the recently rediscovered Roman gold district of Llamasde Cabrera. Gold occurs in extensional quartz veins hosted in the upper layers of the Lower to MiddleOrdovician Armorican quartzite and occasionally in the overlaying Middle Ordovician slates from LuarcaFormation. The veins are characterized by multistage ore deposition developed along three hydrothermalstages: As–Fe (I), As–Fe (II), and Au–Zn–Cu–Pb, followed by late supergene alteration processes. The fluid inclusionstudy undertaken on mineralized quartz revealed the presence of three types of fluids: (1) CO2–(CH4)fluid inclusions, (2) aqueous-carbonic fluid inclusions dominated by CO2 and subordinate CH4 related toquartz–arsenopyrite–pyrite deposition, and (3) aqueous fluid inclusions related to the Au-sulfide deposition.In conjunction with arsenopyrite geothermometry and fluid-inclusion data an attempt was made to determinethe P–T conditions of ore formation. Changes occurred in the P–T conditions fromarsenopyrite and pyrite depositionin quartz veins from aqueous-carbonic fluids at 300–390 °C and 200–220 MPa towards 180–310 °C andb200 MPa at the stage of gold from aqueous fluids.Mixing of two aqueous fluids of relatively contrasted salinitywere favorable factors for decreasing gold solubility and could allow gold deposition. The δ34S values are similarfor the two As–Fe stages, +8.0‰ to +16.3‰ and +9.0‰ to +19.5‰ respectively, and for the post-S1 pyritesfrom siliciclastic Luarca Formation suggesting a comparable sulfur source. Combining calculated δ18O values offluids of+6.7 to+9.4‰with fluid inclusion data indicates that fluids of unknown origin are largely equilibratedwithmetamorphic lithologies at medium–high temperatures. The ascent of some late Variscan magmatic bodies,not exposed at the present day erosion level, provides the heat source for convective water circulation. Goldquartz veins from Llamas de Cabrera display similar features and processes to related deposits of similar typeelsewhere in the Variscan belt of western Europe (northwest and central/western Iberia, French Massif Central,BohemianMassif). However Llamas de Cabrera shows certain differences, such as the non-appearance of igneousrocks in the area and the absence of Sb-rich minerals compared to some Variscan Sb–Au deposits, includingnorthern, central, and western Iberia. This last could probably be due to a greater depth of formation for theAs–Au veins in comparison to Sb–Au veins.
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