The Journal of Petrology, Volume 38, Issue 12: December 1997.

Magmatic and postmagmatic processes in tin-mineralized granites: topaz-bearing leucogranite in the Eurajoki rapakivi granite stock, Finland

I Haapala

Department of Geology, PO Box 11, FIN-00014 University of Helsinki, Finland. E-mail: ilmari.haapala@helsinki.fi

The 1.57 Ga Eurajoki stock in southwestern Finland is composed of biotite-hornblende-fayalite granite, biotite granites and late-stage topaz-bearing granite, as well as related topaz-bearing rhyolite (ongonite) dykes. Miarolitic cavities indicate the presence of a separate fluid phase during late stages of crystallization of topaz-bearing granite, and greisen-type Sn-Be-W-Zn mineralization is closely associated with it. Subsolidus reactions have modified the petrography and geochemistry of the topaz-bearing granite. The presence of magmatic topaz is indicated by petrographic features and crystallized melt inclusions entrapped in topaz. The magmatic origin of accessory cassiterite in the topaz-bearing granite (average 80 ppm Sn) is indicated by mode of occurrence and chemical composition of cassiterite: the Nb₂O₅ + Ta₂O₅ content of cassiterite is 4.8-8.7 wt % in granite, 2.3-3% in pegmatite and 0.0-0.6% in greisen. Some of the rhyolite dykes contain topaz as phenocrysts and as small prismatic crystals in the groundmass showing locally fluidal texture. These features indicate that the late-stage magma was highly enriched in F and Sn. Strong exsolution of alkali feldspar has produced intergranular albite rims and grain rows. De-anorthitization of primary plagioclase has produced turbid albite with inclusions of topaz, fluorite and quartz. Metasomatic albitization of K-feldspar and K-feldspathization of plagioclase have only locally been important. In greisenized granite, metasomatic addition of topaz and quartz is common. The topaz-bearing granite had originally anomalous geochemistry and mineralogy, and its anomalous character was further increased during postmagmatic fluid-rock interaction. The petrographic and geochemical peculiarities of the topaz-bearing granite of Eurajoki are characteristic of many other late-stage intrusive phases of rapakivi complexes and for many tin granites of different ages. In interpreting the origin of such highly evolved granites, the role of Na-metasomatic albitization has often been overemphasized at the expense of exsolution and recrystallization.

Key words: albitization; cassiterite; postmagmatic processes; rapakivi granite; topaz

Pages 1645-1659