Unnamed (south tributary to Fielding Lake)

Occurrence, Active

Commodities and mineralogy

Main commodities Fe; Ti
Other commodities Cu; Ti
Ore minerals ilmenite; titaniferous magnetite

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale A-4
Latitude 63.108
Longitude -145.7733
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence is along the principal south tributary of Fielding Lake at an elevation of 3,300 feet. It is in the NW1/4SE1/4 section 11, T. 21 S., R. 10 E., Fairbanks Meridian. It corresponds to locality 1 in Rose (1966 [ADMM GR 19]).

Geologic setting

Geologic description

At this site rusty-weathering gabbro contains as much as 28 percent titaniferous magnetite and ilmenite (Rose, 1966 [ADMM GR 19]). The magnetite-bearing gabbro appears to be a layer that dips 42 N. It is 100 to 150 feet thick and is exposed for a length of 500 feet in the valley bottom before it becomes covered with glacial deposits. The base of the magnetite-bearing layer grades within a few feet into gabbro containing minor orthopyroxene. The overlying rock is mafic gabbro, but the contact is covered. The magnetite zone is probably a differentiate of the gabbro magma.
These rocks are part of the Tangle ultramafic-mafic intrusive complex of Late Triassic age, which may be a lopolithic feeder of the Nikolai basalt flows of the Wrangellia Terrane (Nokleberg and others, 1991; W.T. Ellis, oral communication, 2001). The Tangle complex is a north-dipping layered intrusion of dunite, peridotite, and gabbro that varies from less than 2,000 feet thick to more than 4,000 feet thick and extends for more than 12 miles in length (W.T. Ellis, oral communication, 2001). The complex intrudes the Tangle Formation of Permian age (Nokleberg and others, 1991). It may connect with the Fish Lake complex at depth below the Amphitheater synform. Magmatic mineralization is synchronous with emplacement of Tangle complex, which is part of a 120-mile-long belt of mafic-ultramafic and associated rocks in the east-central Alaska Range.
Geologic map unit (-145.775536033325, 63.1075785690267)
Mineral deposit model Ni-Cu-PGE in differentiated mafic-ultramafic sill.
Age of mineralization Late Triassic.
Alteration of deposit Oxidation of iron mineral(s)

Production and reserves

Workings or exploration
A chip sample across the magnetite-bearing zone assayed 22.1 percent acid soluble iron and 0.15 percent copper (Rose, 1966 [ADMM GR 19]). A magnetic concentrate separated from the sample contained 47.1 percent total iron, 11.1 percent TiO2, 0.16 percent sulfur, and 0.02 percent potassium. The high titanium content suggests that about 20 percent of the magnetic concentrate is ilmenite.
The occurrence is on active claims of MAN Resources.
Indication of production None

Additional comments

Thin sections of the country rock show that they are gabbro, norite, and magnetite gabbro (Rose, 1966 [ADMM GR 19]).



Reporters W.T. Ellis (Alaska Earth Science), C.C. Hawley (Hawley Resource Group), and W.J. Nokleberg (USGS)
Last report date 6/17/2002