Trace element constraints on pegmatite genesis: Tin Mountain pegmatite, Black Hills, South Dakota

Abstract

The Tin Mountain pegmatite is a small, zoned granitic body that is extremely enriched in Rb and Li, but has moderate concentrations of Sr and Ba. These trace elements are modelled using granitic distribution coefficients in order to test the potentials of partial melting of metasedimentary rocks and fractionation of a less-evolved granitic melt to have produced the parental liquid to the Tin Mountain pegmatite. Batch melting of any reasonable metasedimentary source rock would likely have yielded melts that were either insufficiently enriched in Rb and Li to be the parental liquid, or that had Sr and Ba concentrations that were much higher than those estimated for the parental liquid. The modelling of simple fractional crystallization and equilibrium crystallization of a granitic melt within the compositional range of the spatially associated Harney Peak Granite gives calculated melt compositions with either lower Sr and Ba concentrations or inadequate Rb and Li enrichments, to be the parent liquid of the pegmatite. At least two variants from simple crystal-liquid fractionation models can, however, successfully account for the derivation of the parent liquid: 1) generation of a Rb-, Li-, Ba- and Sr-rich granitic melt (outside of the compositional range of the sampled portions of the Harney Peak Granite complex) by low degrees of partial melting of metasedimentary rocks found in the Black Hills, followed by moderate extents of fractional or equilibrium crystallization, 2) derivation from Harney Peak granite via a complex, multi-stage crystal-liquid fractionation process, such as progressive equilibrium crystallization.