Fluid-inclusion and stable-isotope studies of gold-tungsten bearing hydrothermal deposits, Saucelle-Barruecopardo Area, Spain
Antona, Juan F.; Fallick, Anthony E.; Sánchez, Antonio García
European Journal of Mineralogy Volume 6 Number 6 (1994), p. 819 - 836
published: Nov 30, 1994
manuscript accepted: Jun 17, 1994
manuscript received: Oct 29, 1993
ArtNo. ESP147050606002, Price: 29.00 €
Abstract The Saucelle-Barruecopardo mineralized (Au-W) area is located in the northwest of Salamanca province, Spain, within the Centroiberian zone (the most internal part of Hercynian Cordillera System). The quartz veins are spatially associated with the Barruecopardo granite, a syntectonic two-mica alkaline granite. The granite crosscuts regional metamorphic isograds and estimates of its emplacement age range from 311 to 326 M.a. Gold-tungsten-bearing veins crosscut the Barruecopardo granite and the surrounding country rocks. The vein mineralogy can be subdivided into five paragenetic stages. Gold deposition is associated with stage I (gold included in arsenopyrite) and stage II (gold in cracks in arsenopyrite). Stage I muscovites are dated (K-Ar method) at 277.9 ± 5.6 and 257.0 ± 5 M.a. Two main different compositional types of primary fluid inclusions were observed in quartz: type (l) CO2-H2O inclusions, and type (2) CO2-rich inclusions. Type 1 and type 2 inclusions are related to stage-I gold deposition and are thought to represent immiscible fluids because they are primary and the homogenization temperatures for both types (H2O-rich and CO2-rich) are in the same range, with a maximum homogenization temperature of 428oC. The calculated temperature of immiscibility ranges from 355 to 435-C and pressures from 1400 to 2100 bars. δ34S arsenopyrite values suggest a constant source of sulphur with values close to O per mil. Calculated δDfluid values are all quite D-enriched, but show significant variability (-32 to -2‰ SMOW), whereas δ18Ofluid values show small variation ( from + 7 to + 10.3 ‰ SMOW). These values for the fluids are consistent with interaction between magmatic fluids and metamorphic rocks (from 0.1 to 0.01 fluid/rock ratios). The most likely mechanism for stage-I gold deposition is the loss of H2S during fluid immiscibility.