The genesis of the arsenopyrite gold veins from the Vall de Ribes District, eastern Pyrenees, Spain

Abstract

In the district of Vall de Ribes, Cambro-Ordovician schists contain several disseminated gold-bearing arsenopyrite occurrences. Close to these occurrences quartz veins parallel to the Hercynian foliation also contain arsenopyrite gold mineralization. Most of the ore is, however, within brittle structures (fractures and thrusts) crosscutting the Hercynian foliation. They occur as veins enclosed in schists and dolomites, and as replacement bodies in dolomites, and are representative of a large number of gold-bearing sulfide deposits that are known to occur in the Hercynian basement of the Pyrenees.The postfoliation veins mainly consist of quartz, siderite, ankerite, arsenopyrite, pyrite, and several minor sulfides. Gold is found in close association with the sulfides. According to fluid inclusion and arsenopyrite geothermometry, the veins formed at a temperature around 350 degrees to 400 degrees C and 2 kbars pressure. Mineralizing fluids were CO 2 -free NaCl + CaCl 2 + (KCl) brines with salinities ranging from 10 to 26 wt percent NaCl equiv. The pH of the fluid was near neutral, the sulfur fugacity was constrained by the stability of pyrite and arsenopyrite, and the oxygen fugacity was close to or higher than pyrite-pyrrhotite-magnetite equilibrium. Under these conditions, and as suggested by available experimental data, gold was most likely transported as AuCl (super -) 2 . The high dependence of gold solubility on temperature suggests a cooling of the solution as the most efficient precipitating mechanism, which is in accordance with the close association of native gold to sulfides and quartz found in the veins.The enclosing and vein carbonates are strongly depleted in 13 C and 18 O (delta 13 C = -10ppm; delta 18 = + 13ppm in vein ankerite) with respect to the regional dolomite value (delta 13 C = -0.4ppm; delta 18 O = +24ppm) but show a positive correlation between delta 13 C and delta 18 O, interpreted as the result of an exchange process between regional dolomite and an external fluid. The delta 18 O values of quartz range from 9.8 in synfoliation quartz to 15.8 per mil in postfoliation vein quartz from the Tarragona mine. Calculated delta 18 O values of water in equilibrium with carbonates and quartz range from 6.7 to 10.8 per mil. The delta D values of chlorites associated with quartz vary between -45 to -61 per mil. The delta D values of these fluids gave values between -5 and -30 per mil. These delta 18 O and delta D data are compatible either with a fluid of metamorphic origin or seawater infiltrating through the metamorphic pile. The heavy sulfur isotope composition of disseminated arsenopyrite (delta 34 S = 11ppm) and vein arsenopyrite (delta 34 S from 13-17ppm) indicates a sulfur of crustal origin. The carbon isotope composition of vein carbonates (delta 13 C 2 O-CO 2 -NaCl fluid generated during metamorphism (at 500 degrees -550 degrees C and 5-6 kbars), contemporaneously to the main foliation, is favored. This fluid could contain metals in solution as evidenced by the deposition of native gold and sulfides in quartz veins parallel to the foliation. The subsequent rise of the thermal gradient from 25 degrees to 45 degrees -70 degrees C/km caused the thermal expansion of the fluids, their migration into faults and thrusts, and the evolution of their chemistry to a CO 2 -free brine by the unmixing of a CO 2 -rich phase. The transport of gold and other metals as chloride complexes was ensured throughout the process. Although a surficial fluid, i.e., seawater, infiltrating through the metasediments could acquire a similar isotopic composition, several geologic uncertainties make this hypothesis less reliable.