Unnamed Occurrence (ARDF - TL019), Yentna Mining District, Matanuska-Susitna Borough, Alaska, USAi

Regional Level Types
Unnamed Occurrence (ARDF - TL019)	- not defined -
Yentna Mining District	Mining District
Matanuska-Susitna Borough	Borough
Alaska	State
USA	Country

This site is located within Denali National Park and Preserve.
Location: C.C. Hawley and Associates, Inc. (1978, localities A and B, Fig. 4.1-C(3)) show this occurrence in the northwest quarter of Section 10, T. 28 N., R., 17 W., of the Seward Meridian along a tributary on the north side of the west fork of the Yentna River at about 4600 feet within Denali National Park and Preserve.
Geology: C.C. Hawley and Associates, Inc. (1978) shown this occurrence at the west end of the Dall tactite region. Mineralization is associated with the early Tertiary Upper Yentna pluton, which consists of syenite, monzonite, lamprophyric peridotite and alkali gabbro dikes, that cuts the Permian Conglomerate of Mt. Dall (Reed and Nelson, 1980; Foley and others, 1997). Two types of mineralization have been noted. C.C. Hawley and Associates, Inc. (1978) describe small pods of sulfides consisting of pyrite, chalcopyrite, and possibly nickel arsenide within local zones of tactite and hydrothermally bleached conglomerate. Rock samples contain up to 1,200 ppm copper, 10,000 ppm nickel, and greater than 10,000 ppm arsenic; nickel and chromium are reported to be anomalous in some samples associated with the gabbro phase of the intrusive (C.C. Hawley and Associates, Inc., 1978). Foley and others (1997) describe very- fine- grained pyrrhotite and chalcopyrite abundantly disseminated in the lamprophyric peridotite and alkali gabbro dikes of the Upper Yentna pluton. These contain up to 0.14% combined copper and nickel, 928 ppm arsenic, 125 ppb gold, 22 ppb platinum and 11 ppb palladium along the northeastern margin of the pluton. The Upper Yentna pluton, one of the nine composite plutons (Tcp) described by Reed and Nelson (1980), sutures the fault contact between Mesozoic marine sedimentary rocks (KJs) and Paleozoic sedimentary rocks (Pzsu) (Reed and Nelson, 1980). Anomalous gold, platinum-group elements, copper, chrome, nickel and arsenic are reported from these plutons (Reed and others, 1978; Reed and Nelson, 1980; Nelson and others 1992). Gold and platinum group element placers have been worked at sites downstream from these bodies (Mertie, 1919; Cobb, 1973). Also see TL020, TL023, TL052, and TL053.
Workings: Reconnaissance mapping, stream silt and rock sampling are all that have been done here. Rock samples contain up to 1,200 ppm copper, 10,000 ppm nickel, and greater than 10,000 ppm arsenic; nickel and chromium are reported to be anomalous in some samples associated with the gabbro phase of the intrusive (C.C. Hawley and Associates, Inc., 1978). Foley and others (1997) describe very- fine- grained pyrrhotite and chalcopyrite abundantly disseminated in the lamprophyric peridotite and alkali gabbro dikes of the Upper Yentna pluton. These contain up to 0.14% combined copper and nickel, 928 ppm arsenic, 125 ppb gold, 22 ppb platinum and 11 ppb palladium along the northeastern margin of the pluton.
Age: Late Cretaceous/early Tertiary; mineralization is interpreted to be linked to the Upper Yentna pluton, one of the Tertiary composite plutons mapped by Reed and Nelson (1980).
Alteration: Hydrothermally bleached conglomerate and local zones of tactite are described by C.C. Hawley and Associates, Inc. (1978).

Commodities (Major) - Cu, Ni, Pt (?); (Minor) - As
Development Status: None
Deposit Model: Cu skarn deposits (Cox and Singer, 1986; model 18b).

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Link to USGS - Alaska:	TL019

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References