Location: The Mex prospect extends into parts of five sections near the divide between Clearwater and Little Clearwater Creeks. The location for this record is at the approximate center of a tungsten-rich part of the prospect. It is at an elevation of about 4,800 feet, 500 feet southeast of the center of section 6, T. 20 S., R. 5 E., Fairbanks Meridian. The prospect corresponds to locality A31 of Kurtak and others (1992); the area includes localities 13, 14, 15, 18, and 19 of Clautice and others (1989). The Mex claims extend for about 2 miles to the north-northeast of the map site and include large parts of sections 29, 31, and 32, T. 19 S., R. 5 E. (G.A. Moerlein and R.A. Blakestad, written communications, 1980). This record describes only the claim block in and near section 6, T. 20 S., R. 5 E.
Geology: The rocks at the Mex prospect are in the footwall of a major low-angle thrust fault zone. The footwall rocks include Paleozoic or possibly lower Mesozoic greenstone, tuff, argillite, shale, and sandstone. These rocks are intruded by quartz diorite, quartz monzonite, and monzonite. Mineral deposits are spatially linked to fine-grained felsitic intrusions (Clautice and others, 1989; Kurtak and others, 1992; Nokleberg and others, 1991). Mineralization at the prospect is complex; base metal deposits are probably skarn deposits associated with granitic intrusives. Other mineralization seems more closely related to altered zones in and subparallel to the thrust faults. The complex juxtaposed metal suite and vuggy mineral textures suggest that some mineralization is epithermal. Private sector geologists who mapped and sampled the deposit in about 1980 did not recognize thrust faulting, but they mapped the Mex and related occurrences as crudely stratabound deposits that parallel the later recognized thrusts (G. A. Moerlein and R. A. Blakestad, written communications, 1980). The prospect contains a diverse metal suite. Minerals identified include primary scheelite, stibnite, pyrite, almost certainly galena and chalcopyrite, and secondary malachite. Zones particularly rich in tungsten (scheelite) occur at and near localities 18 and 19 of Clautice and others (1989). R.A. Blakestad found values of as much as 51 percent tungsten at locality 18 near the center of section 6 (T. 20 S., R. 5 E.). Scheelite also occurs in brecciated limestone and at a contact been shale and reddish carbonate rock at multiple localities 19 near the south boundary of section 6. Scheelite was panned from the headwaters of a drainage in the east part of adjacent section 1, T. 20 S., R. 4 E. Small veins of massive stibnite occur locally. Veins that assay about 27 percent antimony contain from 0.058 to 0.06 ounce of gold per ton and 0.13 to 0.36 ounce of silver per ton and are anomalous in mercury (samples 1590-1591, Kurtak and others, 1992, table A31). Arsenic is also locally anomalous. The presence of galena or a related lead oxide mineral is suggested by 1.67 percent lead contained in malachite-stained pyritic metasedimentary rocks (sample 1405). Discrete lead and copper minerals, such as galena and chalcopyrite, probably occur in a 9-foot-wide quartz vein on the south side of peak 5330 in the NE1/4 section 6, T. 20 S., R. 5 E., that assayed 240 parts per billion gold, 28 parts per millions (ppm) silver, 465 ppm arsenic, 0.27 percent copper, and 0.20 percent lead (Clautice and others, 1989).
Workings: Claims were first located about 1972. Geologic and geochemical work was done on the claims in 1980 by Mankomen Exploration Company partly on behalf of Occidental Minerals (Oxymin) (R.A. Blakestad and G.A. Moerlein, written communications, 1980). The area was examined in reconnaissance by Anaconda Minerals Company the same year (Ellis, 1980). In 1982, the claims were trenched and geophysically surveyed by magnetic, VLF, and EM methods. In 1983, the property was leased to Anshutz Mining Corp.; in 1988, it was briefly leased to Amax Exploration (Kurtak and others, 1992).
Age: Probably Late Cretaceous or early Tertiary, nearly synchronous with thrust faulting.
Alteration: Extensive quartz-carbonate type alteration; local skarn formation.

Commodities (Major) - Au, Sb, W; (Minor) - Ag, Cu, Hg, Pb, Zn
Development Status: None
Deposit Model: Complex mineral deposit; replacement lodes related to thrust zones; local skarn

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Mineral List

7 valid minerals.