3-D GEOLOGIC MODEL OF THE LARGE CAVITY AREA AT THE DEEP UNDERGROUND SCIENCE AND ENGINEERING LABORATORY, HOMESTAKE MINE, SOUTH DAKOTA

Preparation of a geologic model for the Large Cavity area on the 4850 Level of Homestake mine has progressed in two phases. The initial 3-D VULCAN model utilized incomplete drift maps (4100 and 4850 levels) and 376 m of exploration drill core prepared by Homestake mine geologists across several years before mine closure. In the second half of 2009, mapping of 1,394 m of drifts and logging of 1,363 m of drill core by several consulting groups have markedly increased understanding of geology of the 4850 Level and allowed a progression of geologic understanding there.

The area of the proposed Large Cavities lies west of the Vent Drift within the Yates member amphibolite. This area is along the axial portion of the large SSE-plunging, Precambrian-aged Lead anticline, as defined by foliation within the amphibolite and the trend of the upper amphibolite contact. A swarm of Tertiary rhyolite dikes (15% to 40% of the rock mass) crosses the Yates member-Poorman Formation contact. Along the drifts, the attitude of individual dikes, and of the 900 foot-wide swarm, changes from sub-vertical dips and a NNW strike in phyllite of the Poorman to northerly strikes and generally eastward dips, sub-parallel to the well developed fabric, within the amphibolite. The margins of the dikes are distinctly flow banded and annealed with the country rock. Recognition of this change in strike of the dike swarm required revision of the original geologic model which predicted a continuous, NNW strike.

The currently defined locations of the 55 m-diameter Cavities lie within the zone of combined rhyolite dikes and amphibolite. Minor faults and calcite-filled veins are abundant within the amphibolite member, but are annealed. Abundant systematic joint sets are also present.