Rock Creek No. 5

Occurrence, Inactive

Commodities and mineralogy

Main commodities U
Other commodities Pb
Gangue minerals 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 64.9426
Longitude -162.3461
Nearby scientific data Find additional scientific data near this location
Location and accuracy
The Rock Creek No. 5 occurs in a steep, narrow gully above a northwest-facing cirque on the right limit of upper Rock Creek. Elevations in the area of the occurrence range from about 1,700 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 E1/2NE1/4 sec. 29, 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 1,000 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.
Rock Creek No. 5 consists of several closely spaced, parallel, silicified shear zones that are poorly exposed in a steep, narrow avalanche chute above the northwest-facing cirque on the right limit of upper Rock Creek. The zones strike west-northwest with a steep northerly dip. At one outcrop within the chute, a 1.5 foot-wide silicified zone with hematite, chlorite, and secondary quartz veins contains disseminated pyrite and rare chalcopyrite. Other outcrops and float are leached of sulfide minerals. The presence of boulder-sized rubble in the chute indicates that silicified shear zones range in width from 1 to 4 feet or more and are bordered by narrow zones of clay and fault gouge. Generally the leached and silicified zones yield radiometric values of 2 times background. Two chip samples of leached hematitic quartz vein rubble contained 140 and 170 parts per million (ppm) uranium and anomalous lead concentrations (Foley and Barker, 1986).
Gangue minerals include chlorite, clay minerals, hematite, quartz, and sericite. Epidote and plagioclase generally altered to white mica and carbonate occurs with opaque minerals including magnetite and pyrrhotite.
Geologic map unit (, )
Mineral deposit model Granite-hosted veins with uranium concentrations 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, pervasive hematitic staining and corroded quartz after silicification, quartz stockwork and veining with silicification formed in sheared clay and crushed granitic host rock zones; discrete zones of sericite development, epidote, and chlorite alteration were observed (Foley and Barker, 1986).

Production and reserves

Workings or exploration Generally the leached and silicified zones yield radiometric values of 2 times background. Two chip samples of leached hematitic quartz vein rubble contained 140 and 170 parts per million (ppm) uranium and anomalous lead concentrations (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 (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