Constraints on gold metallogeny in NW Iberia: Insights from LA-ICP-MS analysis of trace elements, fluid inclusions, and stable isotopes in the unexplored Portas del Sil gold deposit

The NW Iberian Variscides is a well-known gold province with a variety of hydrothermal gold occurrences. Among them, the Portas del Sil gold deposit serves as a case study for gaining valuable insights into Variscides gold metallogeny and guiding gold exploration. The Portas del Sil deposit is a gold-bearing quartz-vein system intersecting an Ordovician sequence, metamorphosed to greenschist facies, comprising quarzites, carbonaceous slates, and iron-rich levels. This sequence underwent three coaxial Variscan deformation phases related to E-W shortening (D1-D3). The deposit is located in the Sil Synform, a narrow NW-SE D3 structure that refolded D1 folds. The mineralization is spatially associated with quartz veins (±arsenopyrite and pyrite) that cut the main rocks fabric and produced sericitization, chloritization and sulfidation haloes. A carbonate 'spotting' alteration extends beyond the haloes. Dissemination of fine-grained arsenopyrite occurred mainly in carbonaceous slates, along with As-rich zoned pyrite porphyroblasts. In the ironstones, sulfidation of siderite produced intense pyritization. Arsenopyrite crystals appear overprinting the main rock fabric, showing pressure shadows sometimes affected by an S3 crenulation foliation. These crystals have a porous core and a clear rim of variable thickness. LA-ICP-MS analyses revealed that Sb, Cu, Co, Ni are the main trace elements, Ni and Co showing oscillatory behavior. Au was detected mainly in the porous cores, while the rims are richer in Sb, Co, Ni and Mo, the latter showing an antithetic behavior to Au. “Invisible gold” also occurred in the As-rich zones of pyrite porphyroblasts. However, disseminated pyrite in the iron-rich levels is poor in As and the other trace elements except Cu, and is accompanied by disseminated gold. Another stage of gold deposition is observed, as isolated grains or associated with pyrite, sphalerite, chalcopyrite and galena, infilling fractures in early arsenopyrite and pyrite, pressure shadows or sealed fractures in quartz veins.The fluid inclusion study on quartz points to an immiscible fluid system: an aqueous-dominated fluid with low salinity (<5 wt% NaCleq.), and a carbonic (CO2 ± CH4 ± N2)-dominated fluid. Estimated P-T trapping conditions are 385 ± 25 °C in the range of 10 to 140 MPa. Local extension during faulting may have induced fluid pressure fluctuations, facilitating the unmixing process, although the limited solubility of methane in aqueous saline fluids could also have played a role. A later CO2-rich aqueous carbonic fluid shows similar P-T trapping conditions, from 95 MPa to 155 MPa and from 350 °C to 400 °C. The δ18Ofluid values obtained range from 8.3 to 10 ‰, typical of orogenic gold deposits. The δ34SCDT values of sulfides (from 3.0 to 9.7 ‰) support the idea of a deep-source homogenized metamorphic fluid originated through a large-scale recycling of sulfur and metals during prograde metamorphism of the sedimentary record at depth. Two different mechanisms of gold deposition contributed to the deposit development: sulfidation reactions promoted by fluid-rock interactions, and fluid-unmixing as a result of the combination of local pressure loss and an increase in CH4 due to fluid interaction with the carbonaceous slates. Based on its characteristics, the Portas del Sil deposit could be classified within the Sediment-Hosted Orogenic Gold (SHOG).