Original paper

Chemical composition of coexisting columbite-group minerals and cassiterite from the Black Mountain pegmatite, Maine

Wise, Michael A.; Brown, Cathleen D.

Abstract

Detailed examination of hand samples and backscatter electron images of Nb-, Ta-, Sn-oxide minerals from the Black Mountain pegmatite has revealed the presence of discrete crystals, as well as cassiterite-hosted micrograins of columbite-group minerals (CGM). Micrograins and discrete grains of CGM show a wide range of Mn/(Mn þ Fe) values, but only the micrograins have limited Ta/(Ta + Nb) values. Trace amounts of Ti, Sc, Fe3+ and Sn are common in both generations of CGM, but their total rarely exceeds 1.0 oxide wt%. The chemistry of the host cassiterite is relatively homogeneous and pure; the total concentration of FeO, MnO, Nb2 O5, Ta2 O5 and TiO2 rarely exceeds 2.0 wt%. While discrete CGM grains show weak progressive zonation and minor replacement, micrograins display complex oscillatory zonation, resorption and/or replacement textures. Textural evidence suggests that the micrograins crystallized after the formation of discrete grains of columbite and cassiterite. On the whole, both discrete crystals and micrograins of CGM show increasing Mn/(Mn + Fe) with fractionation as commonly observed in F-enriched granitic pegmatites. Micrograins of CGM typically show higher Ta/(Ta + Nb) values compared to their discrete crystal counterparts. Textural characteristics of CGM micrograins are inconsistent with an origin by exsolution, and instead, indicate primary precipitation.

Keywords

cassiteritecolumbite-group mineralspegmatiteblack mountainmaine