The Au–As (Ag–Pb–Zn–Cu–Sb) vein-disseminated deposit of Arcos (Lugo, NW Spain): Mineral paragenesis, hydrothermal alteration and implications in invisible gold deposition

Abstract

The Arcos deposit is located in the NW of the Iberian Variscan Massif, and consists of a pollymetallic Au–As (Ag–Pb–Zn–Cu–Sb) vein-disseminated mineralization. It is temporally and spatially related to post-Variscan rhyolite dikes and structurally controlled by Late-Variscan and Variscan structures. The mineralization in Arcos is divided into two stages. The first (Au–As) consists of fine-grained Au-bearing pyrite and arsenopyrite, with Au occurring as invisible gold. Sulfides occur filling veins as well as disseminated in hydrothermally altered (silicification, dolomitization, phyllic alteration and sulfidation) rhyolites and impure limestones and calcareous slates, Lower Cambrian in age. Several types of pyrite and arsenopyrite are defined according to their chemical and textural characteristics. Among these, the Py-II and Apy-II types are Au-bearing. Py-II is As-rich (up to 5wt.%) and is the main Au-bearing mineral (up to 0.2wt.% of Au) in Arcos, while Au content in Apy-II is local and low (up to 0.03wt.% of Au). The Py-II occurs as fine-grained pyritohedra and overgrowth rims of previous pyrite crystals, showing oscillatory As-zoning. The highest Au-contents are in the As-richer zones. The Au/As ratios of Py-II and Apy-II indicate that Au was mainly deposited as solid solution, suggesting that they precipitated from fluids undersaturated in Au. Sulfidation of iron derived from the alteration of ferroan silicates (mainly biotite) was the most important depositional mechanism of invisible gold in Arcos. The second stage of mineralization shows an Ag–Pb–Zn–Cu–Sb metallic signature. It replaces the first stage and consists of sphalerite, Ag-bearing tetrahedrite, bournonite, chalcopyrite and galena with Bi–Pb–Sb sulfosalts, pyrrhotite, Ni–Co–Fe sulfides and electrum as accessory minerals. The gangue minerals of this stage include Ca–Mg–Fe carbonates, quartz with minor fluorite and chlorite. In the most superficial levels of the area, secondary native Au, covellite and Fe oxide and hydroxide developed due to oxidation of the ore. The vein-disseminated mineralization of Arcos represents a distal deposit of an intrusion-related gold system developed around an unexposed, post-Variscan, calc-alkaline granitoid in the NW Iberian Massif.