MHC

Prospect, Active

Alternative names

Mount Henry Clay

Commodities and mineralogy

Main commodities Ag; Au; Ba; Cu; Pb; Zn
Ore minerals barite; bornite; chalcopyrite; galena; gold; pyrite; sphalerite
Gangue minerals calcite; chlorite; epidote; quartz

Geographic location

Quadrangle map, 1:250,000-scale SK
Quadrangle map, 1:63,360-scale B-4
Latitude 59.3944
Longitude -136.4674
Nearby scientific data Find additional scientific data near this location
Location and accuracy The MHC prospect consists of a 1/2-mile-long arcuate pattern of mineralized float below a small hanging glacier. Most of the mineralized float is between about 4,200 and 4,400 feet elevation in a narrow septum of rock and talus between glaciers. It is about 1 mile north-northeast of Mt. Henry Clay in section 3, T. 29 S., R. 53 E., of the Copper River Meridian. Figure 11 of Greig and Giroux (2010) is a geologic map of the prospect. The location is accurate within 500 feet.

Geologic setting

Geologic description

The MHC prospect was found at the base of a retreating glacier by Merrill Palmer in 1984 as numerous high-grade massive-sulfide boulders (Greig and Giroux, 2010).
The rocks in the region are part of the Upper Triassic Hyd Group that hosts similar volcanic massive sulfide deposits the length of southeastern Alaska (Newberry and others, 1997). The rocks are predominantly massive and pillow basalts and fragmental basalt and andesite(?), intercalated with siltstone, tuff, and rare rhyolite flows and dikes. They have been subject to greenschist-facies metamorphism; folding and faulting locally repeats the stratigraphy in places, and the rocks are highly altered.
Still (1984) reported that more than 30 high-grade sphalerite-barite-pyrite-chalcopyrite boulders up to 6 feet in diameter had been found in float over an area about 800 meters in diameter. Assays of the boulders varied. The highest grade contained 20 to 44 percent zinc, 5 percent barium, and several percent copper. No ore-grade mineralization was found in place but altered andesite in the area contained elevated zinc, copper, barium, lead, silver, and gold. None of 13 holes drilled by Kennecott, Granges, and Rubicon cut mineralization as high grade as the boulders (Greig and Giroux, 2010). Several of the drill holes intersected lower-grade mineralization in broad quartz-sericite-pyrite-chlorite alteration zones. The better intercepts were 49.1 meters with 0.19 percent copper, 10.7 meters with 0.44 percent copper, and 36.6 feet with 0.29 percent copper. Drilling by Stryker Resources and Freeport Resources nearby in Canada showed similar values (Still and others, 1991; Rosenkrans and Jones, 1985).
There have been several published studies of the mineral deposits in the area by government and academia (MacKevett and others, 1974; Still, 1984, Still and others, 1991; Newberry and others, 1997; Greig and Giroux, 2010) and unpublished studies by industry. The consensus from early on was that they are Late Triassic, stratiform volcanogenic massive-sulfide deposits.
Geologic map unit (-136.469268500471, 59.3940580198612)
Mineral deposit model Kuroko- or Besshi-type volcanogenic massive-sulfide deposit (Cox and Singer, 1986; models 28a or 24b)
Mineral deposit model number 28a or 24b
Age of mineralization Late Triassic by analogy with the many other similar deposits in Hyd Group rocks of that age spread along the length of southeastern Alaska and into Canada (Greig and Giroux, 2010).
Alteration of deposit Intense quartz-sericite-pyrite and chloritic alteration near mineralization (Greig and Giroux, 2010).

Production and reserves

Workings or exploration
The MHC prospect was found by Merrill Palmer in 1984 at the base of a retreating glacier as numerous high-grade massive-sulfide boulders. Subsequently, a succession of companies explored in the general area including Bear Creek Mining from 1983 to 1985; Newmont Exploration from 1987 to 1989; Granges Exploration Ltd. in 1989; Cominco Alaska from 1990 to 1993; Kennecott Minerals from 1993 to 1997, Rubicon Minerals from 1998 to 2000; and Toquima Minerals Corp. in 2004 (Greig and Giroux, 2010). Kennecott Exploration drilled 7 holes at MHC; Ganges Exploration Ltd. drilled 4 holes, and Rubicon Minerals Corp. drilled 2 holes. The holes totaled 2,957 feet. The area was also mapped and sampled by the U.S. Geological Survey (MacKevett and other, 1974) and the U.S. Bureau of Mines (Still, 1984) and the Alaska Division of Geological and Geophysical Surveys (Still and others, 1991). In 2006, Constantine Metal Resources Ltd. acquired the property under a long-term lease and began actively exploring it. Their work has included detailed geologic mapping and sampling, ground geochemical and geophysical surveys (Greig and Giroux, 2010). In 1985, Stryker Resources drilled 5 holes totaling 2,787 feet in length on extensions of this prospect on the Canadian side of the border (Still and others, 1991; Rosenkrans and Jones, 1985).
In 2013, Constantine conducted a two day massive sulfide boulder survey where the glacier had retreated; 311 samples were counted. These boulders were newly exposed and 'upstream' from the samples collected by the US Bureau of Mines in 1985 where the average of 26 boulders sampled were 19.3 percent zinc, 1.0 percent copper, 0.22 gram of gold per tonne and 38.2 grams of silver per tonne (individual boulders up to 6 feet in diameter) (Constantine Metal Resources Ltd., 2014).
Indication of production None
Reserve estimates None.
Production notes None.

References

MRDS Number A013093

References

Constantine Metal Resources Ltd., 2014, Corporate Presentation February 2014: http://constantinemetals.com/_resources/presentations/CEM_Presentation.pdf (as of Sept. 16, 2014).
Greig, C.J., and Giroux, G.H., 2010, Palmer VMS project, southeast Alaska; Mineral resource estimation and exploration update: Technical l report for Constantine Metal Resources Ltd. 82 p. (posted on www.sedar.com, Mar. 8, 2010) http://www.sedar.com/GetFile.do?lang=EN&docClass=24&issuerNo=00023827&fileName=/csfsprod/data104/filings/01543498/00000001/C%3A%5CSEDAR%5CCEM_Filings%5CPalmer_43-101_Rept_CEM_2010.pdf (as of December 24, 2014).
Newberry, R.J., Crafford, T.C., Newkirk, S.R., Young, L.E., Nelson, S.W., and Duke, N.A., 1997, Volcanogenic massive sulfide deposits of Alaska, in Goldfarb, R.J. and Miller, L. D., eds., Mineral deposits of Alaska: Economic Geology Monograph 9, p. 120-150.
Rosenkrans, D.S., and Jones, B.K., 1985, Jarvis Glacier project, 1985 annual progress report: Kennecott Alaska Exploration report, 30 p. (Unpublished material available at the Juneau Mineral Information Center, U.S. Bureau of Land Management, Juneau, Alaska).
Reporters T.C. Crafford (T. Crafford & Associates, Anchorage); D.J. Grybeck (Contractor, USGS); V.C. Zinno (Alaska Earth Sciences, Inc.)
Last report date 12/24/2014