Geochemistry of cassiterite and wolframite from tin and tungsten quartz veins in Portugal

Abstract

Cassiterite and wolframite compositions from Sn N W-and W N Sn-bearing quartz veins in Northern and Central Portugal are compared to provide evidence on fluid compositions. In Sn N W-bearing quartz veins, euhedral cassiterite shows sequences of alternating parallel darker and lighter zones. The darker zones are pleochroic, oscillatory zoned, exhibit exsolutions of columbite and ixiolite and are richer in Nb, Ta and Fe than the lighter zones which consist of nearly pure SnO 2 . Cassiterite from W N Sn-bearing quartz veins is usually zoned, with homogeneous and slightly pleochroic darker zones, which are chemically similar to lighter zones. Both zones have inclusions of rutile and rare ilmenite. The darker zones of cassiterite from the former veins are richer in Nb, Ta and Fe contents and poorer in Ti than the darker and lighter zones of cassiterite from the latter veins. This is attributed to differences in the composition of magmatic hydrothermal fluids.Wolframite compositions from Sn N W-and W N Sn-bearing quartz veins do not show any significant distinction, because they precipitate from relatively similar magmatic hydrothermal fluids. In some deposits, most wolframite crystals are homogeneous, but others are heterogeneous. Inner patches, rich in a hübnerite component, rarely occur in crystals from the Filharoso and Panasqueira deposits. Zoned crystals, showing an increase in Fe and a decrease in Mn from core to rim, were found in the Vale das Gatas deposit. Complex oscillatory zoned crystals occur. In the Carris deposit, later wolframite contains inclusions of scheelite, partially replaces it and is richer in Fe and poorer in Mn than earlier wolframite. Wolframite from Sn N W-bearing quartz veins in the Argozelo deposit and W N Sn-bearing quartz veins from Vale das Gatas and Panasqueira deposits has significant Nb content. This does not depend on the Fe and Mn content of the wolframite, but W content is negatively correlated with Nb content. Only very rare single crystals of wolframite show an increase in W and a decrease in Nb from core to rim. SnN W-bearing quartz veins contain wolframite poorer in Nb than the darker zones of cassiterite, which exsolved columbite and ixiolite. In W N Sn-bearing quartz veins from Panasqueira and Vale das Gatas, the wolframite has a higher Nb content than the cassiterite, which contains rutile inclusions enriched in Nb, because cassiterite and wolframite are derived from two distinct magmatic hydrothermal fluids of different age. The fluid responsible for wolframite precipitation will have a similar composition to that resulting from the evolution of the fluid responsible for cassiterite precipitation in the SnN W-bearing quartz veins.

Oxide minerals in the granitic cupola of the Jálama Batholith, Salamanca, Spain. Part II: Sn, W and Ti minerals in intra-granitic quartz veins

“…Generally, much of Nb, Ta and Fe present in cassiterite analyses should be attributed to submicroscopic inclusions and exsolutions, impossible to be detected by EMPA. However, the (Ta + Nb)/(Fe + Mn) ratios in all the cassiterite crystals are less than 2, indicating that the Nb content is not exclusively controlled by the inclusions and exsolved blebs of columbite-group minerals (Möller et al 1988;Neiva 1996). Thus, the (Nb + Ta) versus the (Fe + Mn) plot for cassiterite from the intra-granitic quartz veins (Fig.…”

“…Gouanvic and Babkine 1985;Giuliani 1987;Černý and Ercit 1989;Neiva 1996Neiva , 2008Haapala 1997;Lerouge et al 2007;Pal et al 2007). Additionally, Nb-Ta oxide minerals containing variable amounts of W are common in this type of rare-metal granites and related granitic pegmatites (e.g.…”

“…These pegmatite bodies generally belong to the LCT family of Černý and Ercit (2005). In contrast, much less attention has been paid to the Nb-Ta oxide minerals in hydrothermal quartz veins with cassiterite and/or wolframite (Amichba and Dubakina 1974;Beddoe-Stephens and Fortey 1981;Tindle and Webb 1989;Neiva 1996).…”

“…Recent contributions showed the broad mineralogical and chemical variability of these deposits, which is not restricted to the Sn and W ores. For example, abundant Fe-, Mn-, Sr-, Li-, Ca-and Mg-bearing phosphates have been documented from the granites, the pegmatite dikes and the mineralized quartz veins (Llorens and Moro 2007, 2008, 2012aMoro and Llorens 2008). These phosphates have been useful for determining the magmatic, hydrothermal and supergene evolution of the Jálama Batholith.…”

Chemistry of cassiterite in rare metal granitoids and the associated rocks in the Eastern Desert, Egypt

“…Thus, during the panning sur- 4+ for Nb, Ta -oxides (from Cerny and Ercit, 1985). The field boundaries of cassiterite and rutile are from Neiva (1996). Symbols similar to those in Figure 3. vey, which is largely applied to rare metal mineralizations in general and cassiterite deposits in particular (where cassiterite is essentially dispersed in mechanical aureoles), the pleochroism of any cassiterite present indicates the nature of the primary mineralizations (i.e., magmatic or metasomatic).…”