The Darby pluton is a relatively homogeneous, mostly quartz monzonite and minor granite. Locally there are segregations of quartz-deficient, potassium-rich rocks including syenite and alkali granite. The pluton is cut by rhyolite, aplite, and tourmaline-bearing pegmatite dikes. In to the Vulcan Creek and adjacent Rock Creek area, the Darby pluton is quartz monzonite containing biotite and lesser hornblende and cut by tourmaline aplite and a swarm of lamprophyre and apparently related variants of dark-colored sub-alkaline intermediate composition porphyritic dikes. The dikes contain euhedral biotite, amphibole, corroded quartz xenocrysts and, more rarely, clinopyroxenite or olivine in a groundmass of altered plagioclase and minor potassium feldspar. Generally, the Rock Creek and West Vulcan Creek area has higher radiometric levels than the surrounding region (Foley and Barker, 1986).
Rock Creek No. 6 consists of several silicified and iron-stained, northeast-striking shear zones exposed for 500 vertical feet in a steep, east-facing gully in a cirque wall. Steeply dipping quartz veins, containing banded jasper with hematite and narrow, parallel seams of pyrite-bearing fluorite greisen, occur in the altered quartz monzonite. Jasper bands up to 2 inches thick are concentrated in a 1 foot-wide central portion of the quartz veins, which pinch and swell to about 4 feet. Pitchblende was identified by the U.S. Bureau of Mines Reno Metallurgical Center in a high-grade sample of radioactive greisen that assayed 1,961 parts per million (ppm) uranium. Isolated float of highly radioactive jasper vein material was also found and samples assayed up to 0.76 percent uranium. Surrounding quartz monzonite cut by the shear zones is hydrothermally altered and contains chlorite and hematite with disseminated pyrite and clay minerals (samples assayed 253, 190, 565, and 18 ppm). Carbonate-altered biotite and olivine lamprophyre dike(s) underlie the vicinity with unknown orientations (Foley and Barker, 1986).Gangue minerals include chlorite, clay minerals, hematite, banded jasper, quartz, sericite. Epidote and plagioclase generally altered to white mica and carbonate occur with opaque minerals including magnetite and pyrrhotite.
|Alteration of deposit
||Greisen, locally argillic, sooty fluorite also noted at several sites. Banded jasper, pervasive hematitic staining, and corroded quartz after silicification, were also noted. Quartz stockwork and veining occurs with silicification in shear zones; also noted were discrete zones of sericite, epidote, and chlorite alteration. Clay minerals and iron oxides occur in all vein showings and adjoining quartz monzonite wall rock (Foley and Barker, 1986).