Crystal chemistry of tourmaline from Variscan granites, associated tin-tungsten- and gold deposits, and associated metamorphic and metasomatic rocks from northern Portugal
Neiva, Ana M.R.; Silva, M. Manuela V.G.; Gomes, M. Elisa P.
published: Jul 31, 2007
ArtNo. ESP154018401005, Price: 29.00 €
Tourmalines from Variscan granitic rocks, associated Sn, W and Au mineralizations, and metasomatic rocks were analyzed by electron microprobe to look for distinct zoning and compositional differences between magmatic and hydrothermal tourmalines and among hydrothermal tourmalines associated with different types of mineralization in order to explain their formations. Magmatic tourmaline in S-type Variscan granitic rocks from Vieiros, Rebordelo, Jales-Gralheira and Paredes da Beira-Penedono is essentially chemically homogeneous and reflects the bulk chemical composition of the host rock. At Jales-Gralheira, tourmaline compositions record the differentiation sequence. Schorl from aplite and pegmatite is chemically zoned, with increasing Fe/(Fe+Mg) from core to rim.At Rebordelo, a peraluminous hybrid granite contains metasomatic anhedral dravite, which is commonly oscillatory zoned; some crystals have dravite cores and Mg-foitite rims. This tourmaline formed by the assimilation of metasediments in a differentiated granitic magma.Hydrothermal tourmaline in quartz veins, generally close to the vein walls and in metasomatized micaschist adjacent to these veins and granitic rocks resulted from the mixing of magmatic-hydrothermal fluids carrying B and some Fe with a meteoric fluid that has interacted with Fe-Mg-bearing pelitic country rock. Tourmaline composition depends on the composition of the magmatic-hydrothermal fluid and the degree of hydrothermal alteration of the pelitic country rock. Schorl and rare foitite in W±Sn-bearing quartz veins containing wolframite (Panasqueira) are generally richer in Fe/(Fe+Mg) than schorl and dravite in similar scheelite-bearing veins (Rebordelo), and than dravite and Mg-foitite in Au-quartz veins (Jales-Gralheira). Tourmaline from these Au-quartz veins has the lowest Fe content and Fe/(Fe+Mg) ratio of the vein tourmalines. Tourmaline in micaschist at quartz vein walls (Panasqueira; Jales-Gralheira) has a composition similar to that of the tourmaline in their veins. Most hydrothermal tourmaline displays fine-scale oscillatory zoning, but in Au-quartz veins and micaschist at the vein walls it shows an increase in Al and a concomitant decrease in Fe and Mg from core to rim. Metasomatic tourmaline-rich rock adjacent to W±Sn-bearing quartz veins (Panasqueira) contains crystals with F- and dravite-rich cores and F- and schorl-rich rims, showing increase in F and Fe/(Fe+Mg) from core to rim.The comparison of schorl-dravite, foitite and Mg-foitite from the granite-related deposits with published data from other similar deposits shows: a) the importance of magmatic differentiation in the compositions of tourmaline from granitic rocks and in the zoning of individual crystals; b) magmatic tourmaline is either unzoned or shows an increase in Fe/(Fe+Mg) and a decrease in Mg from core to rim, while hydrothermal tourmaline is either oscillatory zoned or shows an increase in Al and a decrease in Fe and Mg from core to rim; c) hydrothermal tourmaline is lower in Fe/(Fe+Mg) than magmatic tourmaline; d) magmatic tourmaline reflects the bulk composition of the host rock; e) the Sn-W-quartz veins mainly contain schorl, while Au-quartz veins mainly have dravite.