Geochemistry of the Panasqueira tungsten-tin deposit, Portugal

Abstract

Major tin-tungsten deposits in Portugal are related to intrusions of the Younger Series (300-280 Ma) of Hercynian granitoids. Mineralized granites are 'specialized' by a specific increase or decrease of major, minor and trace element contents in comparison with non-mineralized occurrences. Component analysis is used to make distinctions between the various geochemical types of barren and mineralized granites (Chapter 2). A one-component model is characterized by positive loadings for Rb, Sn, Cs, Li, Nb, F, Ta, Zn, Wand P20S and negative loadings for Zr, Ba, Ti02 and Sr. In this one-component model the component scores increase from barren to highly specialized granites. In a two-component model component 1 (F, Li, Cs, Rb, Zn, Sn, P20S, Nb and W) expresses a hydrothermal influence upon the granites, whereas component 2 (Ti02, Ba, Zr, Sr, -Ta, -Nb, -Rb and -Sn) possibly represents the magmatic differentiation trend. In this model the composition of the individual granites concentrate in one of several fields according to their loadings by the element parameters. Muscovite granites, albitized and greisenized granites deviate clearly in composition from the more common granites. The geochemical variability of granites, greisens and schists of the Panasqueira tungsten-tin deposit is investigated in great detail (Chapter 3). The trace element contents of the Panasqueira cupola are compared with those of other granite occurrences in the surroundings. Only ore-bearing granites show a geochemical differentiation concurrent with mineralization. The major elements illustrate the general chemical evolution from muscovite-albite granite to greisenized granite; trace elements, however, show in addition distinct groups of granitoids. The general trend in the granitoids of Panasqueira is an increase in Rb, Li, F, Sn, Cs, W, Fe203' P20S, Zn, As, Nb, Ta and Cu contents while Na20, CaO, MgO, Sr, Zr, Ba and Ti02 contents decrease. Rare earths are strongly depleted in the greisen. This alteration in composition is the result of metasomatic and hydrothermal processes; closed in an early stage but thereafter opened due to the development of sub-horizontal joints. The schist along the hydrothermal ore veins at Panasqueira is affected as well by the mineralizing fluids in the veins; tourmaline and sericite alteration zones contain elevated amounts of the orerelated elements Rb, Li, Cs, Sn, Wand Na20 and K20. Schist samples were also taken at the surface and used for litho-geochemical exploration purposes. The same elements, when plotted in contour maps, outline the outcropping ore zones very well and can be used as a tool in geochemical exploration (Chapter 4). A major part of this investigation is dedicated to the physical conditions under which the mineralization of Panasqueira was formed (Chapter 5). Homogenization temperatures of fluid inclusions in the minerals of Panasqueira point to an interval between 325 and 2500 C for the early stages; salinities are between 5 and 10 wt% NaC1. In the final hydrothermal stage homogenization temperatures dropped to about 100oC; the salinity is also lower, down to 3 wt% or less. Due to trapping of C02-rich fluids against an impermeable granite-schist contact, fluid pressures point to an 'overpressure' of about 1 kbar before opening of the joint system. Opening of this joint system caused vigorous boiling of the fluids in the ore veins at a depth of about 1800 m. Gas chromatographic analyses show next to C02 the presence of CH4 and N2' The presence of CH4 suggests a reducing environment during ore deposition. The origin of the CH4 and N2 is possibly found by assimilation of schist containing organic matter. Stable isotopes of carbon, oxygen and nitrogen indicate a homogeneous isotopic source for the early fluids. High overpressures in the earliest stage and about constant oxygen, carbon and nitrogen isotopes strongly favour a 'magmatic' origin for the initial mineralizing fluids. Oxygen and carbon isotopes of the fluids differ strongly in the latest stage and point to a large influx of meteoric water.