The Composition of Mica from the Salmi Batholith Granites as an Indicator of the Formation of Nb–Ta Mineralization

Abstract

Successive differentiates of acid rocks from the Salmi batholith are represented by the following series: biotite–amphibole rapakivi granite, biotite granite, and highly differentiated granites (Li-siderophyllite and Li–F zinnwaldite varieties). There is a gradual increase in the content of Ta and Nb and a decrease in the Nb/Ta ratio in this rock series. A regular change in the Ta and Nb partitioning coefficients between mica and the bulk rock composition is observed as well. Comparison of Li-siderophyllite granite and Li–F granite shows that the latter has a sharp decrease in the partitioning coefficients K_{pTa, Nb}. This is due to a decrease in the content of these metals in zinnwaldite from Li–F granite and associates with the appearance of columbite in these rocks. The study of rock samples from the Salmi batholith suggests that such change of the Ta and Nb content in the mica from deeply differentiated granite and the corresponding decrease in the K_p value of these metals may be diagnostic evidence of the presence of columbite mineralization in the rock.