Magmatic and Postmagmatic Processes in Tin-mineralized Granites: Topaz-bearing

The 1´57 Ga Eurajoki stock in southwestern Finland is composed been overemphasized at the expense of exsolution and re-of biotite±hornblende±fayalite granite, biotite granites and late-stage crystallization. topaz-bearing granite, as well as related topaz-bearing rhyolite (ongonite) dykes. Miarolitic cavities indicate the presence of a separate ¯uid 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 modi®ed the KEY WORDS: albitization; cassiterite; postmagmatic processes; rapakivi petrography and geochemistry of the topaz-bearing granite. The granite; topaz 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 INTRODUCTIONcomposition of cassiterite: the Nb2O5+Ta2O5 content of cassiterite Primary tin mineralization is commonly associated withis 4´8±8´7 wt % in granite, 2´3±3 % in pegmatite and 0´0±0´6% peraluminous leucocratic granites which represent latein greisen. Some of the rhyolite dykes contain topaz as phenocryst