The Llamas de Cabrera gold district, a new discovery in the Variscan basement of northwest Spain: A fluid inclusion and stable isotope study

Abstract

The northwest of the Iberian Massif is well known to host numerous gold mineralizations which were exploited 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 Llamas de Cabrera. Gold occurs in extensional quartz veins hosted in the upper layers of the Lower to Middle Ordovician Armorican quartzite and occasionally in the overlaying Middle Ordovician slates from Luarca Formation. The veins are characterized by multistage ore deposition developed along three hydrothermal stages: As–Fe (I), As–Fe (II), and Au–Zn–Cu–Pb, followed by late supergene alteration processes. The fluid inclusion study 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 to quartz–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 determine the P–T conditions of ore formation. Changes occurred in the P–T conditions fromarsenopyrite and pyrite deposition in quartz veins from aqueous-carbonic fluids at 300–390 °C and 200–220 MPa towards 180–310 °C and b200 MPa at the stage of gold from aqueous fluids.Mixing of two aqueous fluids of relatively contrasted salinity were favorable factors for decreasing gold solubility and could allow gold deposition. The δ34S values are similar for the two As–Fe stages, +8.0‰ to +16.3‰ and +9.0‰ to +19.5‰ respectively, and for the post-S1 pyrites from siliciclastic Luarca Formation suggesting a comparable sulfur source. Combining calculated δ18O values of fluids of+6.7 to+9.4‰with fluid inclusion data indicates that fluids of unknown origin are largely equilibrated withmetamorphic 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. Gold quartz veins from Llamas de Cabrera display similar features and processes to related deposits of similar type elsewhere 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 igneous rocks in the area and the absence of Sb-rich minerals compared to some Variscan Sb–Au deposits, including northern, central, and western Iberia. This last could probably be due to a greater depth of formation for the As–Au veins in comparison to Sb–Au veins.