The Geochemical Evolution of Tin-Bearing Granites in the Blue Tier Batholith, Tasmania

Abstract

Tin mineralization in NE Tasmania is associated with probable sheet-like bodies of medium-grained biotite-muscovite granite and associated leucogranites, aplites and pegmatites which are intruded into unmineralized porphyritic biotite graiiite/adamellite.Regional mapping, and chemical studies of the granites and their component biotites, has shown that the biotite-muscovite granite represents a highly fractionated rock that probably crystallized from the small volume of ultimate residual liquid of differentiation of a magma from which the less fractionated porphyritic biotite granite/adamellite previously crystallized. This residual liquid appears to have been relatively enriched in tin due to the preference of tin for the liquid phase during fractional crystallization. The high tin content of biotites from the biotite-muscovite granite appears to be due to a normal distribution of tin between liquid and crystals in this residual tin-rich liquid, and not to selective substitution of tin for particular cations within the biotite structure.The correlation of high Sn with high Li, Rb and F and low Fe, Mg, Ca and Ti in rocks and biotite in these high-tin granites may be a fortuitous one related to concentration (or depletion) of these elements in residual liquids as governed by the distribution of these elements between crystals and liquid, although there is the possibility of some modification of composition during slight deuteric attention of these granites late in their crystallization history.