Rock Creek No. 7

Occurrence, Inactive

Commodities and mineralogy

Main commodities U
Other commodities Bi; Mo; Pb
Ore minerals galena
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 64.9578
Longitude -162.3491
Nearby scientific data Find additional scientific data near this location
Location and accuracy
Rock Creek No. 7 occurs on the crest of the northwest-facing valley wall above the right limit of upper Rock Creek. Elevations in the area of the occurrence range from near 1,000 feet up to 1,700 feet at the top of the ridge. 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 NE1/4, 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), but inclement weather prevented radiometric measurements at this occurrence.
This poorly exposed occurrence is a silicified hematitic and iron-stained, northeast-striking shear zone exposed near the crest of the northwest-facing slope above upper Rock Creek. The shear zone cuts propylitically-altered, medium-grained quartz monzonite. Because of the poor exposure, the dimensions of the occurrence are unknown. Hematite-rich quartz boulders occur on the slope and appear to be derived from the shear zone(s). Samples assayed 7 to 155 parts per million uranium (Foley and Barker, 1986).
Gangue minerals include chlorite, clay minerals, hematite, quartz, sericite were noted. Plagioclase generally altered to white mica, epidote, and carbonate occur with opaque minerals including magnetite and pyrrhotite.
Geologic map unit (, )
Mineral deposit model Granite-hosted veins that contain generally much greater uranium than thorium and have minimal REE and tin. Uranium does not concentrate in resistant minerals as indicated by heavy mineral survey (Foley and Barker, 1986). Occurs as mineralized structurally controlled shear zones. There is a spatial association with lamprophyre dikes. No clear comparable deposit model examples are 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 A greisen quartz hematitic shear zone, locally argillic altered, is apparently present under surficial cover. 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 Samples assayed 7 to 155 parts per million 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