GAS-FLUID EVOLUTION AND THE FORMATION OF RHYOLITE DIKE-ASSOCIATED BRECCIA MARGINS, HOMESTAKE MINE, LEAD, SOUTH DAKOTA

A north-trending swarm of six to ten rhyolite dikes (individuals up to several meters wide) extends from the surface Open Cut mine to the 2440 m (8000 ft) mine level in underground workings of the former Homestake Mine. Associated breccia dikes consist of angular (chiefly rhyolite)-to-extremely rounded (black schist and rhyolite) clast-supported xenoliths (to 10 cm) in a matrix of black, disarticulated, fine-grained schist fragments. In the unique mine setting breccias dikes are exposed along the crests and margins, especially the footwalls, of the east-dipping rhyolite dikes and extend upward into the Precambrian or Cambrian country rock. Such dikes vary in width from a maximum of 10 m in the upper workings to a few centimeters at their greatest depth of 1950 m (6400 Level) where they are located at the upper tips of small rhyolite dikes.

Fluid inclusions in Tertiary veins are saline (up to 10% halite and sylvite inclusions) at depths below the present 880 m (2900 ft) mine level indicating a magmatic fluid origin, as δ¹⁸O values of 11.4 to 16.9 per mil for vein quartz. Above the 880 mine level fluid inclusions contain CO2 indicating evolution of a gas phase. Regional stratigraphic reconstructions show that about one kilometer of Phanerozoic strata covered the mine area at the time of dike emplacement. It is interpreted that the explosive release of magmatic-hydrothermal fluids from the magma occurred at depths to approximately three kilometers and that the release of CO2 gas, which may have added to the force of explosive behavior, occurred at about two kilometers. The explosions drove the breccia upward into the country rock and rounded and reduced the size of clasts by the process of autogenous milling. The resulting breccias served as hydrothermal passageways, were strongly annealed and, locally, were mineralized with combinations of fluorite, pyrite, molybdenite, lead and gold. Larger breccia dikes at shallower depths suggest either that explosive forces produced more clasts there or that breccias carried upward from lower levels coalesced into larger bodies.