THE ORIGINS OF PEGMATITIC FLUIDS: INSIGHTS FROM THE AQUEOUS GEOCHEMISTRY OF FLUID INCLUSIONS

A preliminary comparative geochemical characterization of fluid inclusion leachates from several Li-bearing pegmatites (Black Hills, SD, Palermo, NH, and Yellowknife, NT, Canada) was undertaken to constrain the origin of rare-element, zoned pegmatites. In principle, the composition of magmatic fluid inclusions should mirror the evolving composition of the coexisting melt. Therefore, we used microthermometry to document the fluid-inclusion compositional types and to evaluate their trapping conditions, and Ion Chromatography and ICP-OES to determine the bulk chemistry of the fluids. Typical pegmatitic fluids are CO₂-H₂O solutions of low salinity. The major anions in the inclusions (average for Tin Mountain pegmatite, SD, n=22, in equivalents) are Cl 58%, carbonates 30%, B as B(OH)^-₄ 11%, with minor F, Br, and PO₄; the major cations are Na 80%, Ca 12 %, Li and K 3% each, with minor Mg and Sr.

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.