The Salave Mine, a Variscan intrusion-related gold deposit (IRGD) in the NW of Spain: Geological context, hydrothermal alterations and ore features

Abstract

The Salave Gold deposit is located at the northern end of the Oscos Gold Belt, which forms part of the Asturian-Leonese Zone of the Variscan Massif. The Gold Belt and the Salave deposit are genetically related to the emplacement of post-kinematic granitoids, which were largely controlled by Variscan fracture systems. In the Salave area, three main igneous complexes, Porcia, Salave and Represas, outcrop in an approximately E-W direction. Gold deposits are only found in the intense hydrothermal alterations affecting the western part of the Salave outcrop, just where the Mondoñedo thrust reaches the Cantabrian Sea. The geometry of the mineralized body is approximately tabular and is roughly parallel to the Mondoñedo thrust. A network of fractures controlled the gold ore. It consisted of sub-horizontal and sub-vertical fractures, the latter acting as feeders. Along the fracture network an intense albitization replaced the granodiorite and partially filled the open fractures. Farther from the fracture system the granodiorite rock is affected by sericitic-propylitic alteration halo. The most external alteration halo is the chloritic alteration. The transition from one alteration to another is gradual and irregular. The albitized areas and some areas slightly beyond them are then intensely sericitized (advanced sericitic alteration), sulfurized and carbonatized along the network of fractures, and becomes the richest areas in gold. The sulfurized areas are rich in arsenopyrite, pyrite and stibnite, always over 10% and reaching as much as 40% in volume. All the altered rocks mentioned have associated late carbonates as veins or micro-veins, disseminated or filling open spaces.The isocon diagrams (using Zr and Nb as immobile elements) and representative analytical data show that in chloritic alteration there is a small gain of mass, in propylitic alteration the gain is negligible, in albitic there is no gain or loss, in less intense advanced sericitic there is a small loss of mass and in the intense advanced sericitic the loss of mass is significant. As far as the metallic elements, sulfur, arsenic and antimony, are concerned, significant amounts of S, Au, As, Sb and Bi were consistently added to the original rock, mainly during advanced sericitic alterations. Ag is added during albitization to advanced sericitic alteration. Pb, Cu, W, Mo and U are more erratic but appear to have been added from the beginning, together with some Bi, S and As, and to have remained quite constant during all the alterations as if they had been introduced previously to gold mineralization.Four mineral associations have been described at Salave, Cu-Fe-Zn-S, Mo-Bi-S-(W), S-As-Fe-(Au) and S-Sb-Au-(Pb-Bi-Ag-Cu-Zn). The two gold related ones, are post granodiorite consolidation. Visible gold (or electrum) is scarce. Invisible gold at Salave was detected using microprobe analyses. It accounts for most of the Salave gold. The invisible gold in arsenopyrite is in solid solution, while in pyrite a small amount appears in solid solution, but most is probably in the form of microinclusions. Invisible gold is higher in arsenopyrite than in pyrite. The Salave gold deposit is a fracture-controlled deposit whose characteristics, such as stable isotope data, fluid inclusions and geochemistry, indicate a strong igneous influence, albeit partially influenced by meteoric waters and host-rocks. Thus, the deposit should be considered as a reduced intrusion-related gold deposit.