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-homblende-fayalite granite, biotite granites and late-stage lopaz-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 the 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 re-crystallization.