Fractionation trends in mica and tourmaline as indicators of pegmatite internal evolution: Bob Ingersoll pegmatite, Black Hills, South Dakota

Abstract

Pegmatitic micas from the Bob Ingersoll No. 1 Dike, a large, internally zoned, rare element pegmatite body, have been analyzed for trends of compositional variation that indicate the sequence of crystallization and record the progressive compositional differentiation of the pegmatite. The dike consists of six primary zones and one replacement assemblage. 2M 1, muscovite occurs in the border and wall zones and outer parts of the first intermediate zone. Intermediate-Li micas of mixed polytype, but with a dominant 2M 1 component, occur in inner parts of the pegmatite, including the core and areas of contact between intermediate zones and the core. 2M 2 lepidolite occurs as a replacement of K-feldspar along margins between the first intermediate zone and core. Fractionation trends of the micas include increasing Li substitution, decreasing Rb Cs and increasing Ta (Nb + Ta) in a general sequence from the outer wall zone to the pegmatite core, but with considerable overlap among intermediate zones. When compared to coexisting tourmaline, overlapping compositional trends of the interior zones suggest overlap in timing of crystallization and interzonal geochemical communication. Border zone mineral compositions do not fit major variation trends due to the combined effects of wallrock contamination, replacement of early microcline by lithian mica during differentiation of the pegmatite, and possibly rapid, disequilibrium crystallization. Strong zoning of mica and tourmaline crystals reflects disequilibrium conditions as crystalline pegmatite reacted with compositionally evolving fluids. Lithian mica and lepidolite that replaced early-formed K-feldspar may have been generated by fluids that were in equilibrium with a residual silicate melt in the pegmatite core. Aqueous fluids exsolved from silicate melt appear to have leaked from the pegmatite throughout much of its crystallization history, and the late stage Li, Rb, and Cs-enriched fluids contributed to the exomorphic halo surrounding the pegmatite.