Magmatic and Postmagmatic Processes in Tin-mineralized Granites: Topaz-bearing Leucogranite in the Eurajoki Rapakivi Granite Stock, Finland

Abstract

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.