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

• Tipo: Artigo em Revista Científica Internacional
• Data: 2016

Autores

Fuertes Fuente, M (Autor) Outra; Cepedal, A (Autor) Outra; Alexandre Lima (Autor) FCUP; Dória, A (Autor) Outra; Ribeiro, MA (Autor) FCUP; Guedes, A (Autor) FCUP

Revista

Título: Ore Geology Reviews

Vol. 72

Páginas: 213-231

ISSN: 0169-1368

Editora: Elsevier

Indexação

ISI Web of Knowledge - 22 Citações

Scopus - 23 Citações

Outras Informações

ID Authenticus: P-00G-XBK

DOI: 10.1016/j.oregeorev.2015.07.009

Abstract (EN): The Limarinho gold deposit (northern Portugal), located in the Variscan Iberian Massif, is a system of gold-bearing quartz veins that crosscut a peraluminous synldnematic (syn-D3: 315-310 Ma) Variscan granite. The mineralized structures are subvertical and trend N30-40E parallel to the Penacova-Regua-VerIn fault. The quartz veins record two main stages of mineral deposition (land 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 land II are low salinity (<2.4 wt.% NaCleq.) aqueous-carbonic, increasing the CH4 + N-2 amount toward the Stage-II that indicates mixing with metamorphic fluids. Temperatures of 460 +/- 40 degrees 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 degrees 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 degrees C, is observed in accordance with similar Variscan belt gold deposits of Western Europe. Estimated temperatures for Stage-III are from 300 degrees C to lower than 200 degrees 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 8180 values for aqueous carbonic fluids (10.5 to 7.6 parts per thousand) do not permit to discriminate between magmatic and metamorphic, the delta O-18 and delta D values obtained for chlorite (4.0 to 5.0 parts per thousand and -24.2 to -23.1 parts per thousand, respectively) are more indicative of a meteoric signature. The delta S-34 values of the fluid in equilibrium with arsenopyrite range from 10.0 to -7.8 parts per thousand. However, the values obtained for pyrite (around -2.6 parts per thousand.) 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 joseite-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 degrees C. The paragenesis becomes Ag-enricher as temperature goes down. Matildite {111} exsolutions indicate a low limit of mineralization slightly above 200 degrees 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.

Idioma: Inglês

Tipo (Avaliação Docente): Científica

Nº de páginas: 19