Las mineralizaciones filonianas de F-Zn-PB de Las Marmoleras (Pirineo occidental, prov. Huesca): caracterización geológica, mineralògica y geoquímica

Abstract

The Marmoleras F-Zn-Pb mineralizations occur as vein developed in the facies Larrue limestones or in the contact between the former and the Sia facies; the quoted facies, Upper Devonian in age, suffered contact metamorphism. These veins are located in the Valle de Tena (Spanish Western Pyrenees). Some of the veins crosscut diabasic dykes of permian age. Two different phases of ore deposition have been distinguished: phase 1, definite sulphide asemblage, sphalente + galena + pynte + chalcopynte; phase 11, sphalente + galena + tetrahedryte + green fluonte + white fluonte. Sencite and chlonte were formed as hydrotherrnal alteration in the diabasic dykes. Fluid inclusions in sphalerite 11 homogenize at 272.2OC and have salinities of 9.2 wt %eq NaC1. Inclusions in fluorite homogenize at 167.3'C and have salinities of 14.9 wt %eq NaC1. Eutectic temperatures in both minerals are lower than -21.1°C, indicating that the trapped fluids are polysaline. There is a decrease in homogenization temperatures with increasing salinity from sphalente 11 to fluorite. Moreover, salinity decrease from green fluonte to white one. Mineral assemblages combined with fluid inclusion and geochemical data indicate that sulphur and oxygen fugacities decrease with increasing paragenetic time. Under these conditions, and as suggested by available experimental data, metals were most likely transported as chloride complexes while F did as NaP o CaF+. The sulphur isotopic compositions of disseminated pynte in the upper Devonian metasediments is ca +12 per mil. These values are similar to those f ron~pa iaeozoic metasediments of the eastem Pyrenees.

The 634s values df ore sulphides have a wide range between -1.0 and +8.8 per mil. The calculated 634SLvalues of ore fluid vary from +1.1 to +11.1 per mil, Guggesting a mixing process which involves sulphur from the palaeozoic sedimentary rocks and sulphur from an igneous source. Rare earth distribqtion studies allow us to define two different types of fluorite on the ba(;is of their total REE content. The REE content of the first type is cal 100 ppm and its chondrite normalized curves illustrate strong LREEenrichrnent, suggesting an early crystallization. Type 2 fluorites dispby ligand nch REE normaiized curves produced by an enrichment in EiREE. The Tb/Ca and Tb/La ratios display an horizontal trend what it can be interpreted as a remobilization trend. In short, geochemical investigations on the fluontes from Las Marmoleras allow us to deduce that type 1 fluorites can be consider as a precursor for the type 2 ones.