Unnamed (near Lacuna Glacier)

Occurrence, Inactive

Commodities and mineralogy

Main commodities Cr; Fe
Ore minerals chromite; magnetite; pyrrhotite

Geographic location

Quadrangle map, 1:250,000-scale TL
Quadrangle map, 1:63,360-scale D-4
Latitude 62.785
Longitude -151.515
Nearby scientific data Find additional scientific data near this location
Location and accuracy Reed and others (1978, locality 30) locate this occurrence west of the Lacuna Glacier at about 4300 feet elevation in the southwest quarter of Section 10, T. 31 N., R. 12 N., of the Seward Meridian, approximately 3500 feet northeast of TL 055. This occurrence is within Denali National Park and Preserve.

Geologic setting

Geologic description

This is one of several chromite occurrences in a 25-mile long belt of alpine-type ultramafic bodies that are discontinuously exposed from the Dall Glacier northeast to the Lacuna Glacier which is part of the Dall Trend, described by C.C. Hawley and Associates, Inc. (1978, Fig. 4.1-(C)3 and Fig. 4.0-B). These chromite- and magnetite-bearing dunite and peridotite sills are shown by Reed and Nelson (1980) in a narrow belt of middle to upper Paleozoic sedimentary rocks that are exposed between the middle Tertiary (38 m.y.) Foraker pluton and the northwest trending fault which places the Paleozoic rocks over Mesozoic marine sedimentary rocks (KJs). Foley and others (1997, p. 431) suggest that these ultramafic bodies may be genetically related to the composite plutons (Tcp) of Reed and Nelson (1980) in the upper Yentna River.
Reed and others (1978) describe these occurrences as magmatic segregations of podiform and disseminated chromite hosted in dunite sills. At this site a grab sample of chromite contained 0.02 ppm each ruthenium and iridium. Chromite occurs as: (1) disseminated rounded grains 1-3 mm in diameter, (2) streaks and lenses, (3) irregular pods up to 6 feet long and (4) lens-like bodies up to 6 feet thick and 60 feet long throughout the dunite. Chromium content of 0.7 to 1% from typical dunite is given in Reed and others (1978). The average of 3 microprobe analyses of one sample was 58.4% Cr2O3, 21.1% FeO, 8.9% MgO, and 9.7% Al2O3.
Other chromite occurrences in this trend are described in TL 003, TL008 - 012, and TL055.
Geologic map unit (-151.517263407793, 62.7844643396951)
Mineral deposit model Podiform chromite (Cox and Singer, 1986; model 8a).
Mineral deposit model number 8a
Age of mineralization Mesozoic and (or) Paleozoic (Reed and Nelson, 1980) or Late Cretaceous/early Tertiary (?) (Foley and others, 1997).

Production and reserves

Workings or exploration Reconnaissance mapping, stream silt and rock sampling by the U.S. Geological Survey (Reed and others, 1978; Reed and Nelson, 1980) have been done here. Chromium content of 0.7 to 1% from typical dunite is given in Reed and others (1978). The average of 3 microprobe analyses of one sample was 58.4% Cr2O3, 21.1% FeO, 8.9% MgO, and 9.7% Al2O3.
Indication of production None

Additional comments

Mineral occurrences in the Dall Trend are all within Denali National Park and Preserve.

References