A.M.R. Neiva

Structural control of the chlorine content of OH-bearing silicates (micas and amphiboles)

Abstract Experimental studies of the incorporation of chlorine in trioctahedral biotite-like micas, belonging to the series phlogopite-annite, phlogopite-KCo3AlSi3O10(OH)2 and phlogopite-KNi3AlSi3O10(OH)2, were performed at 600°C and 2 kbars, with a duration of two weeks. The results confirm for the incorporation of an anion in a crystal structure, the fundamental role of the dimension of the anion site, as has been established for cations in previous works. In biotites, the dimension of (OH-Cl) site is mainly controlled by the rotation angle α of the tetrahedra around a direction approximately parallel to c ∗ . The experiments were performed using hydrothermal solutions with KCl≊ 0.5 M; under these conditions, the quantity of incorporated chlorine does not exceed ≊0300 ppm in the most receptive mica (annite) and is twenty times less in the less receptive ones (phlogopite, for example). These results are applied to natural biotites in porphyry copper deposits, metamorphic rocks and mafic rocks. We conclude that most natural biotites which have a chlorine content of 1000 ppm or more crystallized in equilibrium with a fluid phase with chloride contents of several molar (minimum 3 M). The consideration of micas applies in the same way to amphiboles. A clear correlation between the Cl content and XFe is observed which can be interpreted in terms of local structure of the minerals. The structural factors which favour the fixation of chlorine, a large anion are the same which favour the fixation of large alkali cations (replacement of Na by K). This explains the observed correlations between Cl and K in natural amphiboles.

Geochemistry of enclaves and host granites from the Nelas area, central Portugal

Abstract Tonalitic, granodioritic and monzogranitic enclaves occur in the Hercynian peraluminous porphyritic biotite granite and biotite–muscovite granite from the Nelas region. Some variation diagrams show linear trends, but others show dispersion. The enclaves generally have closely similar isotopic signatures to those of the host granites. They contain xenocrystic plagioclase with the same composition as phenocrysts of the host granite, and have biotite as the sole ferromagnesian phase with a composition typically similar to that of the host granite. However, some cores of enclaves have biotite slightly chemically distinct from that of rims. The modelling of major and trace elements of granodioritic enclaves and hybrid host granite indicate that they result from simple mingling/mixing between a tonalitic magma and the host granite magma. Microgranular enclaves are thus interpreted to be globules of a more mafic magma probably from an enriched mantle source. Partial equilibration has been achieved between these enclaves and the host granite.

Geochemistry of tourmaline (schorlite) from granites, aplites and pegmatites from Northern Portugal

Abstract The major, minor and trace element chemistry of schorlites from Hercynian granitoids in Northern Portugal indicates that the composition of schorlites varies systematically in comagmatic sequences of granite, aplite and pegmatite. The compositional variation appears to be the result of a magmatic fractionation process. A subsequent recrystallization has not appreciably altered the chemistry of these schorlites. Schorlite coexisting with biotite and muscovite contains more Ti, Mg, La, ∑Ce and less Fe 2+ + Fe 3+ than sehorlite coexisting with muscovite alone.

Geochemistry of the granitic rocks and their minerals from Serra da Estrela, Central Portugal☆

Abstract Several types of Hercynian peraluminous granitic rocks ranging from biotite-muscovite granodiorite to muscovite granite occur in the horst of Serra da Estrela. Variation diagrams of most major and trace elements of the rocks and biotites and Cr and V of muscovites show fractionation trends. However, Nb, Li, Rb, K Rb , Li Mg variation diagrams of the rocks and muscovites and also of Sn, Cs, Cs K of the muscovites indicate that the muscovite granite probably originated by another mechanism. Least squares analysis of major elements and modelling of trace elements indicate that the coarse grained porphyritic biotite-muscovite granite, granite porphyry and fine to medium grained muscovite-biotite granite were derived from the granodiorite magma by fractional crystallization of plagioclase, quartz and biotite. Emplacement of granodiorite magma took place at 3.5–3 Kb and 720°C: granites and granite porphyry probably originated about 690°C, but were completely crystallized about 520°C. ƒO2 was about 10−17 for the granodiorite and ƒ HCl ƒ HF ) was higher, whereas log ( ƒ h2o ƒ hcl ) was lower in the granodiorite than in the other granitic rocks of the fractionated series. The coarse-grained porphyritic biotite-muscovite granite was hydrothermally altered between 400–350°C and 260–230°C at about 1.5–1 Kb. The hydrothermal fluids were probably mainly meteoric in origin, but some fluids released during the late stage of granite solidification might also have been involved.

The mineralized veins and the impact of old mine workings on the environment at Segura, central Portugal

Abstract At Segura, granitic pegmatite veins with cassiterite and lepidolite, hydrothermal Sn–W quartz veins and Ba–Pb–Zn quartz veins intruded the Cambrian schist–metagraywacke complex and Hercynian granites. Cassiterite from Sn–W quartz veins is richer in Ti and poorer in Nb and Nb+Ta than cassiterite from granitic pegmatite. Wolframite from Sn–W quartz veins is enriched in ferberite component. The Sn–W quartz veins contain pyrrhotite, arsenopyrite, sphalerite, chalcopyrite, stannite, matildite and schapbachite and the Ba–Pb–Zn quartz veins have cobaltite, pyrite, sphalerite, chalcopyrite, galena and barite, which were analyzed by electron microprobe. The presently abandoned mining area was exploited for Sn, W, Ba and Pb until 1953. Stream sediments and soils have higher concentrations of metals than parent granites and schists. Sn, W, B, As and Cu anomalies found in stream sediments and soils are associated with Sn–W quartz veins, while Ba, Pb and Zn anomalies in stream sediments and soils are related to Ba–Pb–Zn quartz veins. Sn, W, B, As, Cu, Ba, Pb and Zn anomalies in stream sediments and soils are also related to the respective old mining activities, which increased the mobility of trace metals from mineralized veins to soils, stream sediments and waters. Stream sediments and soils are sinks of trace elements, which depend on their contents in mineralized veins and weathering processes, but Sn, W and B depend mainly on a mechanic process. Soils must not be used for agriculture and human residence due to their Sn, B, As and Ba contents. Waters associated with mineralized veins were analyzed by flame atomic absorption spectroscopy (FAAS) and ICP-AES have high As, Fe and Mn and should not be used for human consumption and agriculture activities. The highest As values in waters were all related to Sn–W quartz veins and the highest Fe and Mn values were associated with the Ba–Pb–Zn quartz veins. No significant acid drainage was found associated with the old mine workings.

Geochemistry of granitic aplite-pegmatite sills and petrogenetic links with granites, Guarda-Belmonte area, central Portugal

Granitic amblygonite-subtype and lepidolite-subtype, aplite-pegmatite sills intruded a biotite>muscovite granite (G1). Two other biotite>muscovite granites (G2 and G3) and a muscovite>biotite granite (G4) crop out in the area. Variation diagrams for major and trace elements of the Variscan rocks show fractionation trends for a) G1 and G4; b) G2, G3 and aplite-pegmatite sills. The two series are confirmed by the two trends defined by major elements of primary muscovite. The sills also contain Li-bearing muscovite, which has higher Mn, Li, F and paragonite contents and lower Al V1 content than primary muscovite from G2, G3 and sills. All sills have pure albite and P 2 O 5 content of K-feldspar and plagioclase increases in the series G2, G3 and sills. Beryl occurs in all sills, but lepidolite and a nearly pure petalite only occur in lepidolite-subtype sills, which are the most evolved sills. Primary topaz and amblygonite have a similar composition in all sills. Aplite-pegmatite sills contain cassiterite, which shows sequences of alternating darker and lighter zones. The former are richer in (Nb + Ta + Fe + Mn) than the latter. Manganocolumbite is common in all sills, but ferrocolumbite only appears in amblygonite-subtype sills and manganotantalite in lepidolite-subtype sills. The sills richest in Li contain reversely-zoned crystals with a homogeneous microlite core and a heterogeneous uranmicrolite rim. Least squares analysis of major elements shows that granite G3 and amblygonite-subtype and lepidolite-subtype aplite-pegmatite sills can be derived from granite G2 magma by fractional crystallization of quartz, plagioclase, K-feldspar, biotite and ilmenite. Modelling of trace elements shows good results for Sr, but magmatic fluids controlled the Rb and Ba contents of the aplite-pegmatite sills and probably also their Li, F, Sn and Ta contents and crystallization of lepidolite, cassiterite and Nb-Ta oxide mineral assemblage. Schorl from the lepidolite-subtype sills that cut granite G1 has higher Mg/(Mg + Fe) than schorl from metasomatised granite at sill walls and resulted from the mixing of magmatic fluids carrying B and some Fe with a meteoric fluid that has interacted with the host granite G1 and carried Fe and Mg. Schorl and dravite, respectively from metasomatised granite and micaschist at sill walls, were also formed from the mixing processes.

Geochemistry of tin-bearing granitic rocks

Abstract The use of Rb/Ba and Rb/Sr ratios as guides to tin mineralization has only local interest, because each of the correlations log Rb/Ba-log Sn and log Rb/Sr-log Sn of granitic series has a different slope in every area. The ternary relation between Rb, Ba and Cs is useful to distinguish between Sn-bearing granitic rocks and non-mineralized granitic rocks. The Sn-bearing granitic rocks are late differentiates of anatectic melts (S-type), with primary albite and muscovite, and greisenized granites with metasomatic albite and muscovite. They have high Sn, Rb contents, Rb/Ba and Rb/Sr ratios, and low Ba and Sr contents. The Portuguese Sn-bearing muscovite granites, aplites, pegmatites and hypothermal quartz veins have muscovites richer in Sn than those of greisenized granites. F and Sn of muscovites from Portuguese Sn-bearing granitic rocks are positively correlated. Cassiterite crystallization seems to be independent of f H 2 O f in HF and favoured by a decrease in fO2.

Petrogenesis of Tin-bearing Granites from Ervedosa, Northern Portugal: The Importance of Magmatic Processes

Abstract 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, K-feldspar, 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 {uscovite 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.

Geochemistry of granitic aplite-pegmatite sills and their minerals from Arcozelo da Serra area (Gouveia, central Portugal)

Granitic aplite-pegmatite sills intruded a granodiorite-granite and a biotite ≈ muscovite granite from Arcozelo da Serra (Gouveia, Portugal). A muscovite > biotite granite also crops out in the area. Variation diagrams of major and trace elements of the rocks show fractionation trends for a) granodiorite-granite and muscovite > biotite granite; b) biotite≈muscovite granite and aplite-pegmatite sills. REE patterns and δ 18 O of rocks, anorthite contents of plagioclases, Ba contents of potash feldspars, major elements and Li of biotites and muscovites confirm the two series. Least squares analysis of major elements and modelling of trace elements indicate that aplite-pegmatite sills were derived from biotite ≈ muscovite granite magma by fractional crystallization of quartz, plagioclase, potash feldspar and biotite. This mechanism is responsible for the Sn enrichment of aplite-pegmatite sills and Sn is retained in micas. Electron microprobe analyses of columbite-tantalite crystals from aplite-pegmatite sills show oscillatory, progressive and reverse zonings, which are characterized by the behaviours of eight elements and Mn/(Mn+Fe) and Ta/(Ta+Nb) ratios. Oscillatory zoning is mainly attributed to faster crystal growth than Nb, Ta, Fe and Mn can diffuse through liquid, while reverse zoning is due to nucleation and growth of evolved oxide cores and back-reaction of them with the more primitive bulk magma. Other samples of aplite-pegmatite sills show late zoned micas, consisting mainly of a Li-bearing muscovite core and a composition between zinnwaldite and trilithionite for the rim. However, alternating compositions of these two micas with relics of primary muscovite also occur. Late micas are derived from a phase melt enriched in F and Li.

Geochemistry of coexisting aplites and pegmatites and of their minerals from central northern Portugal

Abstract The major-, minor- and trace-elements chemistry of Hercynian granitoids and their minerals from the central area of northern Portugal indicates that the composition of the rocks and also of their minerals varies in the comagmatic sequence of granite, aplite and pegmatite. The compositional variation appears to be the result of a magmatic fractionation process and the minor and trace elements play an important role mainly in the trend aplite→pegmatite coexisting in the same vein, particularly for the minerals. A subsequent recrystallization of granites and aplites apparently did not alter the chemistry.