La paragénesis aurífera del yacimiento de Limarinho (N Portugal) y su implicación en las condiciones de transporte y precipitación del oro

Abstract

The Limarinho deposit (Portugal) comprises a set of Au-bearing quartz veins hosted by a variscan two-mica synkinematic granite. The study of the ore paragenesis allows us to establish two stages of gold deposition. The first stage consists of arsenopyrite (from 350 to 435ºC) and native gold together with Bi-Te-(±S) minerals such as native-Bi maldonite or hedleyite deposited as a melt under low fO2, fS2 and fTe2. A melt of liquid bismuth could scavenge ionic gold from the hydrothermal fluid. The second stage is characterized by sulfides like pyrite, galena and bismuthinite, sulfotellurides and Bi-PbAg sulfosalts [lillianite homologous (Gu32-Gu62), vikingite, heyrovskyite], electrum, native-Bi and hessite. This stage began at a temperature slightly higher than 400ºC under higher fS2 than that of the first stage, in the pyrite and bismuthinite stability field, and evolved by decreasing temperature and fS2 until to reach the native-Bi stability field around 200ºC. A fracturation of the quartz veins took place between both stages and, together with the high temperature of the second stage hydrothermal fluid, favoured the gold remobilization from the previous ore and the gold deposition as a result of removing sulphur from the hydrothermal fluid during the sulphide formation.