Rock Creek No. 6

Occurrence, Inactive

Commodities and mineralogy

Main commodities U
Other commodities Pb
Ore minerals pitchblende
Gangue minerals banded jasper; chlorite; clay minerals; epidote; hematite; magnetite; plagioclase; pyrrhotite; quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale SO
Quadrangle map, 1:63,360-scale D-1
Latitude 64.9554
Longitude -162.3553
Nearby scientific data Find additional scientific data near this location
Location and accuracy
Rock Creek No. 6 occurs in steep, narrow, northwest-oriented gullies incised in the ridge that forms the northwest-facing slope above the upper right limit of Rock Creek. Elevations in the area of the occurrence range from about 1,500 feet up to 2,800 feet at the top of the cirque wall. Rock Creek is a tributary to the Tubutulik River, which is located east of the report area. Rock Creek is one of several creeks that drain the east flank of the north-trending Darby Mountains. The occurrence site is in W1/2NE1/4 sec. 20, T. 6 S., R. 18 W. of the Kateel River Meridian and coordinates are the approximate midpoint of several vein and alteration features.
Accuracy of the location is about 1,500 feet.

Geologic setting

Geologic description

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.
Geologic map unit (, )
Mineral deposit model Granite-hosted veins with uranium generally much greater than thorium; minimal REE and tin; uranium does not concentrate in resistant minerals as indicated by heavy mineral survey (Foley and Barker, 1986); mineralized shear zones with structural control, spatial association with lamprophyre dikes; no clear comparable examples available.
Age of mineralization Quartz monzonite of the Darby pluton is reported to be Late Cretaceous (Miller and Bunker, 1975; Eakins and others, 1977; Wilson and others, 2015).
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).

Production and reserves

Workings or exploration 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 (Foley and Barker, 1986). 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).
Indication of production None
Reserve estimates No reserves.

Additional comments

The West Vulcan and Rock Creek area was selected for study by the U.S. Bureau of Mines in 1979 as part of ongoing mineral land assessments in Alaska and land designations proposed by Congress, and because recent (1970s) private sector exploration had discovered sedimentary uranium in the Boulder Creek basin of southern Death Valley about 6 miles to the north (Dickinson and others, 1987). No further work was performed.

References

Reporters J.C. Barker
Last report date 12/15/2016