ZONED REE-ENRICHED DRAVITE FROM A GRANITIC PEGMATITE IN FORSHAMMAR, BERGSLAGEN PROVINCE, SWEDEN: AN EMPA, XRD AND LA-ICP-MS STUDY

摘要

Green to grayish green tourmaline crystals (up to 10 cm across), with distinct optical zoning, occurs with quartz, blocky albite and muscovite in the Forshammar granitic pegmatite, central Bergslagen province, Sweden. Tourmaline contains inclusions of zircon and xenotime-(Y), and it is cut by veinlets of muscovite and hydroxylbastnasite-(Ce). Microanalytical and structural data (from the rim) indicate that the tourmaline can be classified as a dravite with moderate Al-Mg disorder at the Y and Z sites. Tourmaline displays chemical zoning that reflects the distribution of Fe, Mg, Al, Ca and Na. The Mg/(Mg+Fe) value is high; it decreases from core (~0.85) to intermediate zone (0.76-0.79), but increases in the rim and vein dravite (0.93). The core has the highest proportion of X-site vacancy and Al content, whereas the intermediate zone is the most enriched in Fe and Na. The rim is slightly depleted in Al and has the highest Na compared to inner zones. Tourmaline veins crosscut the pre-existing tourmaline and are relatively more enriched in Na and Ca. The main compositional variations are driven by Al~x□Mg_(-1)Na_(-1) and AlOMg_(-1)(OH)_(-1) substitutions. The Forshammar dravite shows the highest known concentrations of REE from pegmatite tourmaline, <1200 ppm REE, ≤210 ppm La, ≤670 ppm Ce; the chondrite-normalized patterns reveal high La_N/Yb_N (32 to 464) values and strongly negative Eu anomalies (Eu/Eu~* = 0.005 to 0.05). The contents of Ti, Mn, Y and REE generally increase at the boundary of the intermediate zone and rim, whereas the contents of Zn, Ga and Sn decrease from the core to the rim. The core is likely a product of an early magmatic process during the late Svecofennian pegmatite formation (～1.8 Ga) as suggested by oscillatory zoning of trace elements. The intermediate zone, rim and tourmaline veins originated during the late magmatic to hydrothermal stage. Hydroxylbastnasite-(Ce) and muscovite are apparently the final products of the hydrothermal process.