The Au-bearing vein system of the Limarinho deposit (northern Portugal): Genetic constraints from Bi-chalcogenides and Bi-Pb-Ag sulfosalts, fluid inclusions and stable isotopes

- We characterize three stages of mineralization in a gold-bearing quartz vein system.
- We establish composition and origin of the hydrothermal fluids involved.
- We identify two gold parageneses separated in time: Bi-Te ± S and (± Cu)-Ag-Pb-Bi-S ± Te.
- Au scavenged by Bi-melt and later remobilization together with new Au deposition
- Bi-Pb-Ag sulfosalts as tools to constrain gold deposition temperature

The Limarinho gold deposit (northern Portugal), located in the Variscan Iberian Massif, is a system of gold-bearing quartz veins that crosscut a peraluminous synkinematic (syn-D3: 315-310 Ma) Variscan granite. The mineralized structures are subvertical and trend N30-40E parallel to the Penacova-Régua-Verín fault. The quartz veins record two main stages of mineral deposition (I and II), indicating successive periods of vein re-opening, and are crosscut by Stage-III irregular veinlets. Stage-I is characterized by milky quartz (mQ) and arsenopyrite (Apy-1), and produces a greisen-like alteration. Stage-II comprises clear quartz (cQ), K-feldspar, chlorite, rutile and metallic minerals, mainly arsenopyrite (Apy-2 ± Apy-3) and pyrite, with galena, chalcopyrite and sphalerite in lesser amounts. The Stage-III is characterized by chlorite and pyrite. The hydrothermal fluids in stages I and II are low salinity (< 2.4 wt.% NaCleq.) aqueous-carbonic, increasing the CH4 + N2 amount toward the Stage-II that indicates mixing with metamorphic fluids. Temperatures of 460 ± 40 °C and pressures from 230 to 350 MPa are inferred for the As-Fe mineralization of Stage-I, the highest P values indicate lithostatic conditions at a depth of around 13 km. The inferred P-T conditions for the Stage-II range from 340 to 425 °C and from 60 to 225 MPa. A decompression of more than 250 MPa through stages I to II, without a substantial cooling of around 150 °C, is observed in accordance with similar Variscan belt gold deposits of Western Europe. Estimated temperatures for Stage-III are from 300 °C to lower than 200 °C, the pressures being around 60 MPa. Late Stage-II and Stage-III are related to aqueous fluids with variable salinity as a result of a mixing between two end-members, a hotter and more saline fluid (6-7 wt.% NaCleq.) and a cooler and dilute one (< 2 wt.% NaCleq.) probably of meteoric origin. Stable isotopic study results also indicate fluid mixing processes. Although the calculated δ18O values for aqueous carbonic fluids (10.5 to 7.6‰) do not permit to discriminate between magmatic and metamorphic, the δ18O and δD values obtained for chlorite (4.0 to 5.0‰ and − 24.2 to − 23.1‰, respectively) are more indicative of a meteoric signature. The δ34S values of the fluid in equilibrium with arsenopyrite range from − 10.0 to − 7.8‰. However, the values obtained for pyrite (around − 2.6‰) can be only explained by the incorporation of an isotopically heavier sulfur from a different source, probably sedimentary. A detailed study of the ore has shown two different gold parageneses. The first (Bi-Te-(± S)) comprises native-Au, or maldonite, along with native-Bi, hedleyite and joséite-B. The second ((± Cu)Ag-Pb-Bi-S ± Te) comprises electrum, (± Cu)Ag-Pb-Bi sulfosalts such as lillianite-gustavite s.s., heyrovskyite, vikingite, berryite and matildite, Pb-Bi sulfotellurides and hessite. Lamellar exsolution textures observed in the lillianite-gustavite s.s. indicate initial temperatures above 400 °C. The paragenesis becomes Ag-enricher as temperature goes down. Matildite {111} exsolutions indicate a low limit of mineralization slightly above 200 °C. We propose different mechanisms of gold precipitation: 1) a gold scavenged by Bi-melts in the first case; 2) remobilization of this early gold together with new gold deposition due to sulfur extraction and/or fluid mixing from the hydrothermal fluid during the stage of prevailing sulfide precipitation. We conclude that the existence of at least two gold-bearing hydrothermal events may have enhanced the gold grade in this type of gold deposits.