Rock Creek No. 4 (ARDF #SO182) U

Rock Creek No. 4

Occurrence, Inactive

Commodities and mineralogy

Main commodities	U
Other commodities	Pb; Zn
Gangue minerals	chlorite; clay minerals; epidote; hematite; magnetite; plagioclase; pyrrhotite; sericite

Geographic location

Quadrangle map, 1:250,000-scale	SO
Quadrangle map, 1:63,360-scale	D-1
Latitude	64.9371
Longitude	-162.3755
Nearby scientific data	Find additional scientific data near this location
Location and accuracy	The Rock Creek No. 4 consists of a group of mineral showings located on a prominent north-facing buttress above an upper cirque near the head of Rock Creek. Elevations in the area of the occurrence range from about 1,500 feet up to 2,400 feet at the top of the buttress. 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 W1/2SW1/4 sec. 29, T. 6 S., R. 18 W. of the Kateel River Meridian and is the approximate midpoint of several vein and alteration features along the 2,000-foot north trend of alteration and mineral showings across the top of the buttress. Accuracy of the location is about 1,000 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. In to the Vulcan Creek and adjacent Rock Creek area the Darby pluton is quartz monzonite containing biotite and lesser hornblende. The pluton is also cut by lamprophyre, rhyolite, aplite, and tourmaline-bearing pegmatite dikes. These later dikes contain euhedral biotite, amphibole, corroded quartz xenocrysts and, more rarely, clinopyroxenite or olivine in a groundmass of altered plagioclase and minor potassium feldspar (Foley and Barker, 1986). Generally, the Rock Creek and West Vulcan Creek area has higher radiometric levels than the surrounding region. At this occurrence numerous quartz veins and abundant vein quartz rubble occur along a high cirque wall for 1,000 feet and at several sites of alteration on the top of the buttress. In bedrock showings, the veins are leached and the wall rock variably altered to hematitic clay residue and drusy quartz. Veins dip vertically and range from less than an inch up to several feet thick. Veins are contain secondary drusy quartz, relic pyrite and traces of other sulfide minerals. A grab sample from a hematitic quartz vein contained 153 parts per million (ppm) uranium and 236 ppm thorium. No radiometric readings were collected due to weather conditions at the time. Random samples of altered granitic and vein rock contained 10 to 53 ppm uranium (Foley and Barker, 1986). Gangue minerals include chlorite, clay minerals, hematite, and sericite. Epidote and plagioclase, are generally altered to carbonate and white mica, occurs with opaque minerals including pyrrhotite and magnetite. Generally the entire area of the occurrence is variably hydrothermally altered and discolored. Fine-grained epidote and chlorite impart a light green color to the less altered rocks. Locally, the granitic rocks contain quartz stockwork and veins, cryptocrystalline silica coatings and segregated masses, abundant hematite, clay minerals, carbonate, and sericite. More highly altered rocks are various shades of red due to iron oxide minerals.
Geologic map unit	(, )
Mineral deposit model	Granite-hosted veins with uranium values 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 by 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, pervasive hematitic staining and corroded quartz after silicification, quartz stockwork and veining; discrete zones of sericite development, epidote, and chlorite alteration (Foley and Barker, 1986).

Production and reserves

Workings or exploration	A grab sample from a hematitic quartz vein contained 153 parts per million (ppm) uranium and 236 ppm thorium. No radiometric readings were collected due to weather conditions at the time. Random samples, of altered granitic and vein rock contained 10 to 53 ppm uranium (Foley and Barker, 1986).
Indication of production	None
Reserve estimates	No reserves.
Additional comments The West Fork 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