West Vulcan Creek No. 1

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 64.9274
Longitude -162.2591
Nearby scientific data Find additional scientific data near this location
Location and accuracy
The West Vulcan Creek No. 1 consists of a group of mineral showings located on a north-facing promontory on the ridge that forms the south side of a tributary to Vulcan Creek. Elevation of the occurrence ranges from 1,500 to 2,000 feet.
Vulcan Creek is a headwater drainage of the Tubutulik River located east of the report area. The creek is one of several that drain the east flank of the Darby Mountains. The occurrence site is in the N1/2 sec. 35, T. 6 S., R. 18 W. of the Kateel River Meridian and the coordinates given above are the approximate midpoint of the east-northeast trend of alteration and mineral showings. Accuracy of the location is about 1,000 feet.

Geologic setting

Geologic description

The Darby pluton is a relatively homogeneous intrusion, mostly composed of quartz monzonite and minor granite. Locally there are segregations of quartz-deficient, potassium-rich rocks, including syenite and alkali granite. The pluton is further cut by rhyolite, aplite, and tourmaline-bearing pegmatite dikes.
In the Vulcan Creek and adjacent Rock Creek area, the Darby pluton is quartz monzonite containing include biotite and lesser hornblende and is cut by tourmaline aplite and a swarm of lamprophyre and apparently related variants of sub-alkaline dikes (Foley and Barker, 1986). These dark-colored, porphyritic 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 Vulcan Creek and Rock Creek area has higher radiometric levels than the surrounding region. At outcrop scale, mineralized zones will give radiometric responses of 2 to 10 times background.
At West Vulcan Creek no. 1, radiometric measurements up to six times background occur over frost boils along the ridge top. Downslope from these several groundwater seeps apparently emit radon. Altered shear zones and secondary quartz occurs along the ridge that descends east-northeast-oriented and is intruded by a dark-green fine-grained dike. Test pits encountered high levels of radiation and revealed hematitic mud, clay, and regolith containing quartz crystals variably altered to a sooty black color. Fragments of quartz-sericite greisen with boxwork after sulfides were found in several nearby test pits. Fourteen assayed samples ranged up to 1,290 parts per million (ppm) uranium. The highest uranium values are from test pit sites that expose decomposed, clayey, quartz monzonite that has a secondary matrix of varicolored earthy oxides. Several pits dug to 5.5 feet depth encountered residual stratified gruss with layers of varying radiometric response suggestive of localized secondary uranium. Distinctly higher radiometric measurements were recorded over hydrothermally altered veins and were structurally controlled by high-angle shear zones and breccia masses. Radiometric response locally exceeded the calibration limits of handheld scintillometers. These radiometric anomalies generally featured hematite, black iridescent manganese staining, quartz stockwork, ferruginous quartz, jasper, drusy and vuggy quartz. A sample from a nearby carbonate-altered, dark green porphyry contained 68 ppm uranium. The anomalous radiometric response apparently reflects uranium, as very little thorium was reported in any of the samples. Some samples containing elevated uranium values also contained anomalous lead (Foley and Barker, 1986).
Gangue minerals include chlorite, various clay minerals, hematite, secondary drusy and vein quartz, sericite, epidote, and plagioclase that is partly to mostly altered to white mica and carbonate and opaque minerals, which include magnetite, and pyrrhotite.
Geologic map unit (, )
Mineral deposit model Granite-hosted veins with uranium values generally much greater than thorium; minimal REE, tin; uranium values that do 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 with some subordinate granitic rocks (Miller and Bunker, 1975; Eakins and others, 1977; Wilson and others, 2015).
Alteration of deposit Greisen, locally argillic, pervasive hematitic staining and corroded quartz, wide zones of sericite, argillite, and chlorite alteration. Alteration is best exposed in avalanche gullies and/or some structurally controlled swales along exposed ridges (Foley and Barker, 1986).

Production and reserves

Workings or exploration Fourteen assayed samples ranged up to 1,290 parts per million (ppm) uranium. The highest uranium values are from test pit sites that expose decomposed, clayey, quartz monzonite that has a secondary matrix of varicolored earthy oxides. Several pits dug to 5.5 feet depth encountered residual stratified gruss with layers of varying radiometric response suggestive of localized secondary uranium. Distinctly higher radiometric measurements were recorded over hydrothermally altered veins and were structurally controlled by high-angle shear zones and breccia masses. Radiometric response locally exceeded the calibration limits of handheld scintillometers. These radiometric anomalies generally featured hematite, black iridescent manganese staining, quartz stockwork, ferruginous quartz, jasper, drusy and vuggy quartz. A sample from a nearby carbonate-altered, dark green porphyry contained 68 ppm uranium. The anomalous radiometric response apparently reflects uranium, as very little thorium was reported in any of the samples. Some samples containing elevated uranium values also contained anomalous lead (Foley and Barker, 1986).
Indication of production None
Reserve estimates No reserves.

Additional comments

The West Vulcan-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 in southern Death Valley about 6 miles to the north (Dickinson and others, 1987). No further work was performed; claims for sedimentary uranium are active in the Death Valley area.

References

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