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A biotite granodiorite and seven Sn-bearing two-mica granites crop out in the Gouveia area, central Portugal. A SHRIMP U-Th-Pb zircon age from the granodiorite, and monazite ages from four of the two-mica granites, show that they are of Early Ordovician (~480 Ma) and Permo-Carboniferous, i.e. Variscan (~305 and 290 Ma) age respectively. The Variscan two-mica granites are late-and post-D3. Major and trace element variation in the granitic rocks and their biotite and muscovite indicate mainly individual fractionation trends. The granitic rocks are mostly depleted in HREE relative to LREE. The biotite granodiorite is probably derived from igneous lower crust, as evidenced by low initial 87 Sr/ 86 Sr (0.7036), high εNd T (+ 2.5) and moderate δ 18 O (8.8‰). The two-mica granites are probably derived by partial melting of heterogeneous mid-crustal metasediments, mainly metapelite and some metagraywacke, as evidenced by their high initial 87 Sr/ 86 Sr (0.7076-0.7174), δ 18 O (10.7-13.4‰) and major element compositions. However, variation diagrams for major and trace elements from two of the muscovite N biotite granites and their micas define fractionation trends. Rb-Sr whole-rock analyses from the two granites are perfectly fitted to a single isochron and the rocks have subparallel REE patterns; the younger granite is derived from the older by fractional crystallization of quartz, plagioclase, biotite and ilmenite (tested by modelling major and trace elements). Most of the Sn-bearing granites are derived from distinct magma batches. They result from partial melting of a heterogeneous midcrustal metasediment. They do not represent a crustal anomaly in tin. Fractional crystallization is responsible for the increase in the Sn contents of the granites and their micas. Muscovite has a higher Sn content than coexisting biotite and is the principal host mineral for Sn in these rocks.

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Three Hercynian highly peraluminous tin-bearing granites define a sequence ranging from muscovite-biotite granite to muscovite granite. Tin-bearing quartz veins are genetically related to this sequence. Variation diagrams of most major and trace elements of granites, biotite and muscovite show fractionation trends. Least squares analysis of major elements and modelling of trace elements indicate that the muscovite-biotite granite M2 and the muscovite granite M3 were derived from the slightly porphyritic muscovite-biotite granite magma M1 by fractional crystallization of plagioclase, Kfeldspar, biotite and quartz. The granite magma M1 was originated by partial fusion of peraluminous metasedimentary crustal materials. The magmatic fractionation was responsible for the increase in Sn contents of granites and their micas. Biotite has higher Sn content than coexisting muscovite. However, muscovite retains a higher percentage of the total granite Sn content, up to 99 % of the muscovite granite. The very rare magmatic cassiterite present in muscovite granite M3 confirms the tin enrichment of magma. In the sequence, the melt temperature decreases from 765 to 735 °C, P H 2 O decreases from 4 to 3 kb, and the F content in melt increases. Feldspars reequilibrated at 567-329 °C.

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The Ervedosa Mine, in northeastern Portugal, has Sn-bearing quartz veins containing cassiterite and sulphides that cut Silurian schists and a Sn-bearing muscovite granite. These veins were mined for Sn and As 2 O 3 until 1969. Cassiterite, the main Sn ore, has alternate lighter and darker growth-zones. The darker zones are richer in Fe, Nb, Ta and Ti, but poorer in Sn than the adjoining lighter zones. Exsolution blebs of ferrocolumbite, manganocolumbite, Ti ixiolite, rutile, ilmenite and rare wolframite were found in the darker zones. Arsenopyrite is the most abundant sulphide and contains inclusions of pyrrhotite, bismuth, bismuthinite and matildite. Other sulphides are pyrite, sphalerite, chalcopyrite and stannite. Secondary solid phases consisting mainly of hydrate sulphate complexes of Al, Fe, Ca and Mg (aluminocopiapite, copiapite, halotrichite, pickeringite, gypsum and alunogen, meta-alunogen) occur at the surface of the Sn-bearing quartz veins and their wall rocks (granite and schist), while oxides, hydroxides, arsenates and residual mineral phases (albite, muscovite and quartz) occur in mining tailings. Toxic acid mine waters (acid mine drainage AMD), which have high conductivity and significant concentrations of As, SO 4 and metal (Cu, Zn, Pb, Fe, Mn, Cd, Ni and Co), occur in an area directly affected by the mine. Surface stream waters outside this area have low conductivity and a pH that is almost neutral. Metal and As concentrations are also lower. Stream waters within the impact area have an intermediate composition, falling between that of the AMD and the natural stream waters outside impact area. Waters associated directly with mineralised veins must not be used for human consumption or agriculture.

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The tin-bearing muscovite granite from Ervedosa contains unzoned primary muscovite. This Hercynian S-type granite was hydrothermally altered at the stanniferous quartz vein walls and contains three types of muscovite: (1) very small unzoned muscovite replacing albite; (2) small unzoned hydrothermal muscovite replacing K-feldspar and quartz; and (3) zoned subhedral muscovite.In the zoned muscovite, the core has a composition similar to that of magmatic muscovite from the unaltered granite, while the rim has a composition similar to that of hydrothermal muscovite replacing K-feldspar and quartz in the altered granite. The rim corresponds to a late overgrowth richer in the celadonitic component than the core. Infiltrated mineralizing fluids reacted with biotite and K-feldspar of the unaltered granite. We interpret the rim of muscovite to have precipitated from these solutions.