Rock Creek No. 8

Occurrence, Inactive

Commodities and mineralogy

Main commodities U
Other commodities Bi; Mo; Pb
Ore minerals galena
Gangue minerals chlorite; clay minerals; hematite; magnetite; pyrrhotite; quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale SO
Quadrangle map, 1:63,360-scale D-1
Latitude 64.9562
Longitude -162.3577
Nearby scientific data Find additional scientific data near this location
Location and accuracy
Rock Creek prospect 8 occurs on the steep northwest-facing valley wall above the right limit of upper Rock Creek. Elevations in the area range from about 1,500 feet up to 2,800 feet at the top of the cirque wall. Rock Creek is one of several creeks that drain the east flank of the north-trending Darby Mountains and is a tributary to the Tubutulik River, which is located east of the report area. The occurrence site is in NE1/4 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 on this northwest-facing slope.
Accuracy of the location is about 1,500 feet.

Geologic setting

Geologic description

The Darby pluton is 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 the Vulcan Creek and adjacent Rock Creek area the rocks of the Darby pluton is quartz monzonite containing biotite and lesser hornblende and is cut by tourmaline aplite and a swarm of lamprophyre and apparently related variants of dark-colored sub-alkaline porphyritic dikes. The dikes contain euhedral biotite, amphibole, corroded quartz xenocrysts and, more rarely, clinopyroxenite or olivine in a dike 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. 8 consists of several silicified and iron-stained, shear zones about 2,000 feet along strike and exposed high on the northwest-facing slope above upper Rock Creek. Radiometric readings were up to 5 times background over hematite-rich quartz boulders from east- and southeast-striking zones aligned in shallow steep gullies best exposed between elevations of 1,600 and 2,500 feet. There are at least two systems of veins exposed that trend to the ridge crest where they are exposed in place; each exhibits closely spaced quartz veins, quartzose replacement lens, and leached hematitic lenses with relic pyrite. One system strikes N 85 E, dips steeply to the north, and is 12 feet wide with individual hematitic quartz lenses up to 10 inches thick. Two chip samples were analyzed; one contained 447 parts per million (ppm) uranium and the other 430 ppm uranium. A second hematitic quartz-biotite system is spaced 10 feet away; this zone is 12 to15 feet wide and soil assayed 229 ppm uranium. Samples of hematite-rich quartz boulders downslope assayed 270 and 1,000 ppm uranium (Foley and Barker, 1986).
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 showing banded jasper, pervasive hematitic staining, and corroded quartz is locally argillic. Quartz stockwork and veining forms in shear zones; discrete zones of sericite development, epidote, and chlorite alteration are apparent. Adjacent quartz monzonite exhibits propylitic alteration. Oxidation and clay minerals, after argillic alteration, infill between the vein and replacement lenses occurrences (Foley and Barker, 1986).

Production and reserves

Workings or exploration Radiometric readings were up to 5 times background over hematite-rich quartz boulders from east- and southeast-striking zones aligned in shallow steep gullies best exposed between elevations of 1,600 and 2,500 feet. There are at least two systems of veins exposed that trend to the ridge crest where they are exposed in place; each exhibits closely spaced quartz veins, quartzose replacement lens, and leached hematitic lenses with relic pyrite. One system strikes N 85 E, dips steeply to the north, and is 12 feet wide with individual hematitic quartz lenses up to 10 inches thick. Two chip samples were analyzed; one contained 447 parts per million (ppm) uranium and the other 430 ppm uranium. A second hematitic quartz-biotite system is spaced 10 feet away; this zone is 12 to15 feet wide and soil assayed 229 ppm uranium. Samples of hematite-rich quartz boulders downslope assayed 270 and 1,000 ppm uranium (Foley and Barker, 1986).
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