Mount Hurst

Occurrence, Undetermined

Commodities and mineralogy

Main commodities Cr; Pt
Ore minerals chrome spinel; chromite; spinel

Geographic location

Quadrangle map, 1:250,000-scale OP
Quadrangle map, 1:63,360-scale A-2
Latitude 63.2325
Longitude -156.9343
Nearby scientific data Find additional scientific data near this location
Location and accuracy Mt. Hurst is a 2882-foot-high peak approximately 25 miles northwest of Ophir. The site corresponds to location 10 of Roberts (1984), in section 15, T. 26 S., R. 10 E, Kateel River Meridian. It is on a northeast-trending ridge, approximately 1 mile northeast of the summit of Mt. Hurst. The location is accurate within 1/2 mile.

Geologic setting

Geologic description

Mt. Hurst is composed of chromite- and chrome spinel-bearing ultramafic rocks surrounded by lower Paleozoic crystalline limestone, schist, slate, phyllite, quartz, and chert. A nearby upper Paleozoic sequence of greenstone, metavolcanic rocks, tuff, and chert-bearing sedimentary rocks suggests a possible ophiolitic origin for the ultramafic rocks (Roberts, 1984).
The ultramafic rocks at Mt. Hurst are crudely layered; certain layered zones are traceable for more than 2 miles. Rock types include dunite, wehrlite, harzburgite, lherzolite, and clinopyroxenite. One outcrop of dunite has been altered to carbonate, quartz, and talc. At least two minor occurrences of basalt and one outcrop of gabbro are also present (Roberts, 1984).
Chromitite samples collected from the Mt. Hurst area are anomalous in platinum (0.014 - 0.026 ounce per ton) and palladium (0.003 ounce per ton). Wehrlite samples also contained minor amounts of platinum. Altered dunite samples contained elevated concentrations of mercury, antimony, arsenic, and silver. Coalescent or banded chromite is present almost exclusively within dunite layers in wehrlite (Roberts, 1984). Heavy-mineral concentrates panned from three separate drainages on the flanks of Mt. Hurst respectively contained 1474 ppb platinum, 1508 ppb platinum, and 2674 ppb gold (Roberts, 1984).
The largest chrome spinel occurrence in the Mt. Hurst area is at this site (Roberts, 1984, locality 10). It is in a chromite band in a light orange-brown dunite lens in well-layered, light-green wehrlite. The chromite band trends northeast for about 30 feet; the southern end disappears beneath vegetation, soil and talus. It pinches and swells from 6 to 30 inches thick over 30 feet. Concentrations of chrome spinel within the chromite band vary from 30 to 80% of the rock. Chromitite samples from this locality contain about 30% chromic oxide (Roberts, 1984).
The chrome spinel occurrences on Mt. Hurst appear to be very sparse and variable in size and quality. The presence of detectable platinum and palladium in sediments shed from the ultramafic rocks at Mt. Hurst (see Boob Creek, OP013) suggests a potential for paleo-placers (Roberts, 1984).
Geologic map unit (-156.936672948288, 63.2318610130986)
Mineral deposit model Podiform chromite (Cox and Singer, 1986; model 8a)
Mineral deposit model number 8a

Production and reserves

Workings or exploration The U.S. Bureau of Mines first investigated Mt. Hurst as a potential source of chromic oxide and platinum-group metals in 1981. Since then, there have been staking and minor exploration programs by private interests.
Indication of production None

Additional comments

See also Boob Creek (OP013).

References

MRDS Number A015019; A015020; A015021; A015022; A015023; A015024; A015025

References

Roberts, W.S., 1984, Economic potential for chromium, platinum and palladium in the Mount Hurst ultramafics, west-central Alaska: U.S. Bureau of Mines Open-File Report 22-84, 52 p.
Reporters C.E. Cameron
Last report date 8/7/2001