FLUID INCLUSION STUDY OF GOLD-BEARING QUARTZ VEINS IN THE SOUTHERN BLACK HILLS, SOUTH DAKOTA

This study describes fluid inclusions present in Echo, Luckybird and Rough Rider, 3 sub-economic gold prospects in the Berne Quadrangle, Black Hills, South Dakota. All are several-meter thick quartz veins. All the inclusions are secondary, lying along micro-fractures, and thus record the post-formation history of the vein. Three types of inclusion are present; type 1, regular to semi regular shapes with aqueous fluid + Carbonic fluid +/- carbonic vapor, type 2, small regular inclusions with aqueous fluid, type 3, highly irregular inclusions with aqueous fluid + vapor + halite. Type 1 inclusions have T_m dry ice (-56.7 to -56.5) suggesting the carbonic phase is nearly pure CO₂. Compositions were obtained using % fill (10-95) at T_m clathrate (7.0-12.0⁰C) and T_h CO₂ (8.6-21.7⁰C). They have between 2 and 3 wt% NaCl and have two groups of CO₂, 15-35 wt% and 70-80 wt%. The irregular shape of type 3 inclusions suggests they were decrepitated and healed. Their compositions were obtained using T_m ice (-25.7 to -25.3⁰C), T_h aqueous (124.0-158.2⁰C) and T_m halite (138.1-174.2⁰C). They have approximately 70 wt% H₂O, 18-20% NaCl and 9-11% CaCl₂. The Echo vein has significantly more vacant inclusions and high CO₂ inclusions than the other veins, and its high salinity inclusions are associated with sulfides.

In this regional metamorphic terrain, we assume the veins were in thermal equilibrium with the surrounding rocks and thus the intersection of the isochores with P-T paths gives P-T conditions of entrapment. Regional P-T histories from several published studies, and unpublished data all show similar clockwise trajectories through P-T space. The type 3 isochores intersect this path close to the end of the isobaric heating portion near the peak conditions. Their irregular shape suggests that they are relics of earlier, possibly primary, inclusions. All the other inclusions intersect the retrograde path during isothermal decompression. The low-CO₂ inclusions intersect at near-peak conditions and as wt% CO₂ increases, the pressure at intersection decreases. This means that CO₂ is becoming more abundant through time in this part of the path. This study suggests that these veins formed prior to peak metamorphic conditions and if any inclusions we observed evolved from the ore forming fluid, they would be the high salinity inclusions.