COMPLEX FLUID INCLUSION ASSEMBLAGES IN METAMORPHIC QUARTZ VEINS FROM THE BLACK HILLS, SOUTH DAKOTA

Graphite-bearing metasedimentary rocks that were involved in the Proterozoic Trans-Hudson Orogeny are exposed in the Black Hills of South Dakota and contain a record of carbon-bearing fluids. Fluid inclusions in syn-kinematic quartz veins in staurolite and sillimanite grade rocks near the Grand Junction fault were examined by microthermometry and Raman spectroscopy. Only secondary fluid inclusions are preserved in the veins and form planar arrays that are associated with fracture orientations. The veins contain diverse assemblages of inclusions that contain, in varying combinations and proportions, CO₂, H₂O, CH₄, N₂, H₂S, and HCO₃-. Two main groups of inclusions occur.

1. Inclusions with CO₂ as the dominant carbonic species. The CO₂-bearing inclusions may or may not contain an aqueous phase. In all but one assemblage the non-aqueous phase also contains minor amounts of N₂ (X_N2 < 0.05). With Raman spectroscopy, CO₂, N₂, H₂S, and CH₄ were detected in the non-aqueous phase of one sample, and the daughter mineral nahcolite (NaHCO₃) was identified in one CO₂-bearing inclusion. Isochores for CO₂-bearing inclusions indicate trapping temperatures of ~ 350-500°C at 3.5 kbars, constrained by geobarometry of country rocks.

2. Inclusions with CH₄ as the dominant carbonic species. CH₄-bearing inclusions also contain an aqueous phase in some cases but more frequently are composed of pure non-aqueous phases. The non-aqueous phases in these inclusions are primarily CH₄-N₂ mixtures, with relative mole proportions variable within a given inclusion array (e.g. X_N2 varies from 0.59 to 0.94 within a single array). Both microthermometric observations and the CH₄ peak position (n₁) of Raman spectra indicate that CH₄-N₂ mixtures are of very low (< 0.34 g/cm3) and variable density. Isochores for the highest density inclusion arrays indicate trapping temperatures of ~ 500°C at 3.5 kbars. Additionally, one sample containing inclusions of CH₄-N₂ mixtures (X_CH4 ~ 0.93) also contains additional hydrocarbons recognized by peaks in the Raman spectra. The presence of these hydrocarbons may be responsible for unusual homogenization temperatures between ~ -81°C and ~ -73°C. This fluid inclusion study reveals the complex composition and behavior of carbon-bearing fluids that are generated during metamorphism of graphitic metapelites.