Minerals commonly incorporate trace elements into their crystal structure from the fluids that precipitate them.  The concentration of the trace element in the mineral is proportional to the concentration of the trace element in the fluid.  If the proportionality constant (i.e., the "partition coefficient") for the trace element's concentration in water and the mineral can be found, then the concentration of the trace element in a fluid that has long ago disappeared from a mineral deposit site can be predicted from the trace element's concentration in the mineral, which tends to be relatively easy to measure.  We are conducting experiments to determine partition coefficients for base metals (Zn, Pb, Cu) incorporated into dolomite precipitated from brine at temperatures and pressures typical of sedimentary basins.  The results of the experiments should provide a way to predict base metal concentrations in fluids that formed many sediment-hosted base metal deposits.   

The Lemhi Pass and Diamond Creek districts are part of an approximately 135 km northwest-trending belt of thorium and rare earth element (REE) mineralization near the central Idaho-Montana border.  The Lemhi Pass district represents the largest known thorium occurrence and the 5th largest known REE occurrence in the U.S.  The Diamond Creek district is much smaller but geologically similar to Lemhi Pass.  The mineralization consists chiefly of quartz, K-feldspar, and iron oxide veins hosted by Proterozoic quartzites and siltites, where the thorium and REE's are carried mainly by monazite and thorite.  Our studies of fluid inclusions and their host minerals are revealing that the deposits precipitated from oxidizing thorium-, REE-, and base metal-enriched brines.