The mineralized Calle Creek fault subcrops about 350 southeast of Rock Creek and can be traced, mainly in drill holes, for at least 1,200 feet nearly parallel to the creek. It strikes about N 45 E, dips steeply to the northwest, and ranges from 20 to 50 feet thick. It appears to be somewhat offset from, but parallel to, another fault locally called the Arsenic Hill fault (Kennecott Exploration Company, written communication, 1994, 1995).
Most of the drill holes across the Calle Creek fault are reverse circulation. A Placer Dome core hole (RR-88-032) on Kennecott Exploration Company section 3300NE bottomed in crushed and slickensided vein fault material that contained as much as 0.472 ounce of gold per ton. In general, assay gold values in the fault zone are relatively low. On Kennecott Exploration Company section 2500NE, a 25-foot section in Placer Dome RR-88-027, averaged 0.111 ounce of gold per ton; on the same section, a 15-foot section of a Kennecott Exploration Company core hole (RCC-94-11) assayed 0.105 ounce of gold per ton. The sheared structure of the vein zone and irregular assay results are consistent with a pre-mineral fault that was mineralized and subsequently sheared, as has been inferred for the Arsenic Hill fault. If this interpretation is correct, the Arsenic Hill (NM213), Calle Creek, and possibly Albion (NM211) faults could be fundamental controls on the Rock Creek mineralized system.
Southwest of the Calle Creek fault, a 40- to 65-foot-thick layer of graphitic quartz schist can be followed in drill holes for about 1,200 feet, rising slightly to the northeast. Cross sections suggest that the graphitic schist is warped into open folds trending northeast, generally in the region occupied by the main Rock Creek sheeted vein zone (NM207).Rocks exposed along Rock Creek in the series of hydraulic pits north of Sophie Gulch are mainly calc-mica schist as far northeast as Calle Creek, where a short spur road leaves Rock Creek to the south. To the north of this road, the calc-mica schist is overlain by a much more resistant quartz mica schist. Foliation in both schist units strikes north to north-northeast and dips gently to the east and east-southeast. Graphitic units are much less abundant southeast of the Calle Creek fault.