Hydrothermal processes and shifting element association patterns in the W—Sn enriched granite of Regoufe, Portugal

Combined research in geochemistry and mineral chemistry of the hydrothermally altered W-Sn specialized granite of Regoufe and its derivatives in Portugal was undertaken to gain insight in the mineralogical changes associated with hydrothermal processes within a single granite cupola. Over 1000 unpolished rock sections were analyzed by automated X-ray fluorescence spectrometry (XRF). On the basis of the XRF data, a small number of these same sections was selected for investigation by electron probe microanalysis. The study focuses on fourteen elements of interest that are measurable with the chosen techniques. Major pervasive alteration within the Regoufe granite is virtually contemporaneous with mineralization in the form of Sn- or W-bearing quartz veins. Two phases of hydrothermal activity are discerned, characterized by different element associations. Fluids of the first phase were especially rich in Sn, Cs and F, whereas the second phase was marked by a W-Ta-Nb-Rb association and presumably carried less F. Phosphorus probably was an important fluid component in both phases. The fluids are inferred to have fractionated from a related granitic magma at depth.Tin, W, Nb and Ta are mainly found as substitutions or inclusions in biotite in the least altered part of the Regoufe granite. Tantalo-niobian rutile is an important control for the distribution of Nb. Tin occurs in rutile and rarely as cassiterite. Muscovitization caused leaching of Ti and Zr from the granite. Tin, supplied or mobilized by the hydrothermal fluids, behaves differently from W, Nb and Ta. In the most altered rocks, muscovite hosts significant amounts of Sn. Whereas Sn is still related to Cs and Ti, elements that probably represent altered biotite, W, Ta and Nb are related to newly formed Rb-rich muscovite. Columbitetantalite was detected embedded in late muscovite. In contrast to Sn, the fluid-supplied W was predominantly deposited as wolframite in quartz veins and the altered granite is not enriched in W compared to the relatively unaltered rocks. Strontium is preferentially hosted by K-feldspar in the least altered granite; Ca in this rock is still partly contained in albite. With increasing albitization and muscovitization, Sr and Ca were released and are partly bound in newly formed apatite. The primary magmatic apatites have near-ideal formula compositions, but mobilization of P during hydrothermal activity resulted in the formation of Mn-rich apatite in all parts of the granite, and Sr- and probably Li-rich varieties in the most altered rocks. Eosphorite, and scorodite as the oxidation product of arsenopyrite, were also formed as a result of P mobilization. In the least altered rocks, Cs is mainly contained in biotite. In the most altered granite and aplitic rocks, enrichment of Cs and Rb is evenly distributed over K-feldspar and micas. The processes that lead to increased Rb are partly independent of Cs enrichment, and apparently related to the W-Nb-Ta mineralization event, separate from the preceding Sn mineralization.