THE ORIGINS OF PEGMATITIC FLUIDS: INSIGHTS FROM THE AQUEOUS GEOCHEMISTRY OF FLUID INCLUSIONS
There are extreme compositional variations within each pegmatite pluton, zone, and even sample. Moreover, cation ratios in the pegmatite fluids greatly depart from compositions predicted by equilibrium partitioning coefficients possibly because of (1) poorly known partition coefficients, (2) non-equilibrium crystallization, and (3) subsolidus processes altering the magmatic signature of the fluid. However, fluid data from the Black Hills and New Hampshire, follow continuous compositional trends in the system Na-Cl-Br-Li that may indicate similar petrogenetic processes. For instance, Li varies continuously within a wide range concomitant with only small changes in Cl/Na and Br/Na ratios that are highly correlated. These relations suggest a similar halogen source of the magmatic fluids in the Black Hills and New Hampshire pegmatites, but various degrees of Li fractionation. The Yellowknife samples have extremely high Br concentrations but relatively low Li values, which may indicate the post-magmatic addition of evaporated seawater or an oilfield-brine component. The combined analysis of major and minor cations and anions in fluid inclusions in granitic pegmatites may help constraining the origin and evolution of pegmatitic melts and possible influence of external fluids.