Paper No. 3
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 Tm
dry ice (-56.7 to -56.5) suggesting the carbonic phase is nearly pure CO2
. Compositions were obtained using % fill (10-95) at Tm
clathrate (7.0-12.0⁰C) and Th
(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 Tm
ice (-25.7 to -25.3⁰C), Th
aqueous (124.0-158.2⁰C) and Tm
halite (138.1-174.2⁰C). They have approximately 70 wt% H2
O, 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 Regional P-T histories from several published studies, and unpublished data all show similar clockwise trajectories through P-T space.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.