Chemistry of magmatic fluids in the Harney Peak granite-pegmatite system, Black Hills, South Dakota

Magmatic fluid and crystallized melt inclusions trapped in minerals in the world famous Harney Peak Granite, Black Hills, SD and associated pegmatites shed a new light on the petrogenesis of the leucogranite-pegmatite complexes. Microthermometric and bulk-fluid chemical analyses on primary, CO₂-H₂O-dissolved salt fluid inclusions revealed that the fluid evolution was complex. It included gradual compositional changes and discrete unmixing episodes, and varied substantially amongst various intrusions in the granite-pegmatite system.; Based on microthermometric properties of fluid inclusions trapped under conditions of immiscibility, the zoned Tin Mountain pegmatite crystallized at 2.7 kbar and 400–350°C, some of the lowest known temperatures for a silicate melt in the crust. The magmatic character of the primary inclusions is established on the basis of their coexistence with crystallized melt inclusions in magmatic quartz samples. Chemistry of bulk-fluid leachates indicates that the magmatic inclusions contain significant amounts of dissolved Na, Li carbonates ± borates. The low temperatures of crystallization resulted from the combined fluxing effects of H₂O, Li, B, P, alkalis and carbonate ions in the pegmatitic melt. This research reports the first occurrence of dawsonite—NaAl(CO₃)(OH)₂—as an inclusion mineral in a pegmatite. The presence of this rare carbonate is consistent with the high alkalinity of the pegmatite fluid and confirms the important role played by carbonate complexes in pegmatite petrogenesis.