FLUID-INCLUSION AND STABLE-ISOTOPE STUDIES OF GOLD TUNGSTEN BEARING HYDROTHERMAL DEPOSITS, SAUCELLE-BARRUECOPARDO AREA, SPAIN

The Saucelle-Barruecopardo mineralized (Au-W) area is located in the n orthwest of Salamanca province, Spain, within the Centroiberian zone ( the most internal part of Hercynian Cordillera System). The quartz vei ns are spatially associated with the Barruecopardo granite, a syntecto nic two-mica alkaline granite. The granite crosscuts regional metamorp hic isograds and estimates of its emplacement age range from 311 to 32 6 M.a. Gold-tungsten-bearing veins crosscut the Barruecopardo granite and the surrounding country rocks. The vein mineralogy can be subdivid ed into five paragenetic stages. Gold deposition is associated with st age 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 pr imary fluid inclusions were observed in quartz: type (1) CO2-H2O inclu sions, and type (2) CO2-rich inclusions. Type 1 and type 2 inclusions are related to stage-I gold deposition and are thought to represent im miscible fluids because they are primary and the homogenization temper atures for both types (H2O-rich and CO2-rich) are in the same range, w ith a maximum homogenization temperature of 428 degrees C. The calcula ted temperature of immiscibility ranges from 355 to 435 degrees C and pressures from 1400 to 2100 bars. delta(34)S arsenopyrite values sugge st a constant source of sulphur with values close to O per mil. Calcul ated delta D-fluid is values are all quite D-enriched, but show signif icant Variability (-32 to -2 parts per thousand SMOW), whereas delta(1 8)O(fluid) values show small variation (from + 7 to + 10.3 parts per t housand SMOW). These values for the fluids are consistent with interac tion between magmatic fluids and metamorphic rocks (from 0.1 to 0.01 f luid/rock ratios). The most likely mechanism for stage-I gold depositi on is the loss of H2S during fluid immiscibility.