FLUID INCLUSION AND CARBON ISOTOPE STUDIES OF QUARTZ-GRAPHITE VEINS, BLACK-HILLS, SOUTH-DAKOTA, AND RUBY-RANGE, MONTANA

Fluid inclusions and graphite are intimately associated in quartz veins that cut high grade metamorphic rocks in the Black Hills, South Dakota, and at the Crystal Graphite Mine in the southwestern Ruby Range, Montana. Measured fluid inclusion compositions and volumetric properties were compared with calculated compositions of graphite-saturated fluids and with estimates of metamorphic P-T conditions and carbon isotope ratios of graphite were measured to evaluate possible sources of carbon in veinforming fluids. Fluid inclusions from the two areas contrast markedly in their reliability as recorders of metamorphic fluid compositions and metamorphic conditions. The δ13C of graphite associated with the veins indicates that the source of carbon was also different in the two areas. In the Black Hills veins, fluid inclusions are dominantly H2OCO2 mixtures with 24-96 mol% CO2 and a maximum of ∼5 mol% N2 and ∼ 13 mol% CH4. Isochores for the highest density inclusions pass near estimated peak metamorphic conditions (550°-600°C, 4.5-6.5 kbar) and fluid inclusion compositions are compatible with thermodynamic predictions for fluids in equilibrium with graphite in the stated P-T range at geologically reasonable f{hook}O.Graphite in a 12-cm wall-rock alteration zone adjacent to one of the veins has uniform δ13C of -20.8 ± 0.2%., indicating that carbon in the vein-forming fluid was derived largely from reduced organic carbon. In the Ruby Range, peak metamorphic conditions were higher- ∼750°-850°C, 5-8 kbar. In contrast to the Black Hills veins, fluid inclusions are almost all CO2CH4 mixtures (with unknown N2 content). Many contain > 20 equivalent mol% CH4 and mixed H2OCO2 inclusions were observed in only one sample. Inclusions in one vein have ∼ 84-97 mol% CH4. Virtually all inclusion compositions are incompatible with computed graphite equilibria and inclusion isochores likewise do not pass through estimated metamorphic conditions. The density and composition of most, if not all, inclusions have been modified subsequent to original trapping, possibly through H2O loss. The range of δ13C values of vein graphites (-5.8 to -8.6%.) is nearly indistinguishable from values for graphite in dolomitic marble near the veins (-4.8 to -7.1%.). Carbon was probably mobilized through devolatilization reactions in the marble and precipitated as 13C-rich graphite in the veins at fairly constant temperature and from fluid of fairly constant composition. © 1990.