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

Cassiterite and wolframite compositions from Sn > W- and W > Sn-bearing quartz veins in Northern and Central Portugal are compared to provide evidence on fluid compositions. In Sn > 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 SnO2. Cassiterite from W > 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 > W- and W > 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 > W-bearing quartz veins in the Argozelo deposit and W > 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. Sn > W-bearing quartz veins contain wolframite poorer in Nb than the darker zones of cassiterite, which exsolved columbite and ixiolite. In W > 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 Sn > W-bearing quartz veins.