Trio West

Occurrence, Probably inactive

Commodities and mineralogy

Main commodities Zn
Other commodities Ag; Cu; Pb
Ore minerals arsenopyrite; chalcopyrite; galena; gold; magnetite; pyrite; pyrrhotite; sphalerite
Gangue minerals actinolite; carbonate; chlorite

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-1
Latitude 63.2604
Longitude -144.0536
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Trio West occurrence is located about 3.8 miles southeast of the confluence of the Robertson River and Rumble Creek. It is at an elevation of about 5,300 feet elevation about one-half mile north-northeast of the center of section 15, T. 17N., R. 7 E., Copper River Meridian. The location is accurate.

Geologic setting

Geologic description

The Trio West occurrence is a 2- to 3-foot-thick massive sulfide body that crops out discontinuously on a talus-covered hillside for approximately 60 feet. The massive sulfide contains quartz (exhalite?) fragments as much as 8 inches long; it has an upper pyrrhotite-chalcopyrite-rich zone and a lower pyrrhotite-sphalerite-galena-rich zone. The massive sulfide is everywhere underlain by an 8-inch- to 1-foot-thick quartz exhalite that is chloritically altered and associated sphalerite and galena (R.A. Blakestad and others, unpublished Resource Associates of Alaska, Inc. report, 1978). The quartz exhalite overlies a gabbroic body of undetermined attitude and size (C. Schaefer, written communication, 2003). Three chip samples had average values of 0.49 percent copper, 0.22 percent lead, 10.5 percent zinc, 0.187 ounce of silver per ton, and less than 0.004 ounce of gold per ton (R.A. Blakestad and others, unpublished Resource Associates of Alaska, Inc. report, 1978).
The Trio West prospect is in the lower Lagoon unit of Devonian age. The lower Lagoon unit has a basal section of banded, medium- to coarse-grained quartz-sericite(-chlorite) schists and carbonaceous schists. The upper section is finer grained schist and phyllite. Protoliths of the basal section are immature sediments or wackes, mudstone, quartz arenite, and lesser calcareous arenite and carbonate units. Thin, gray to white and pale-green interbedded metavolcanic members of the lower Lagoon unit typically are rhyolite and rhyodacite but include rare andesite and basalt. A prominent graphitic member in the lower section also hosts the nearby SC East (MH 332)and Trio (MH333) prospects. The graphitic member serves as a stratigraphic marker near the lower contact and can be traced in float and by electromagnetic surveys as an extensive low-resistivity zone. A less prominent, but distinctive chloritoid-kyanite assemblage within the graphitic member forms a discontinuous but identifiable horizon for 3 miles along strike that is spatially related to the VMS occurrences in lower Lagoon unit. This mineral assemblage has delineated a metamorphosed advanced-argillic alteration zone, that has been associated with high-sulfidation volcanogenic massive sulfide deposit environments elsewhere (Dashevsky and others, 2003).
Geologic map unit (-144.055851407258, 63.2600254764378)
Mineral deposit model Kuroko massive sulfide (Cox and Singer, 1986; model 28a).
Mineral deposit model number 28a
Age of mineralization The lower Lagoon unit, which is part of the metamorphic sequence that includes the rocks at this deposit, has been dated as Devonian on the basis of one SHRIMP U-Pb zircon age of 372 +/- 6 Ma at the LZ East prospect (MH328) (Dashevsky and others, 2003).
Alteration of deposit Intense chlorite +/- carbonate alteration.

Production and reserves

Workings or exploration Surface sampling, mapping, and geophysical surveys were conducted from 1976 to 1984 by Resource Associates of Alaska.
Indication of production None

Additional comments

The unpublished data that is cited can be seen by contacting Grayd Resources Inc. in Vancouver, B.C., Canada (, or Northern Associates Inc. in Fairbanks, Alaska.



Lange, I. M., Nokleberg, W.J., Newkirk, S. R., Aleinikoff, J.N., Church, S.E., and Krouse, R.H., 1993, Devonian volcanogenic massive sulfide deposits and occurrences, southern Yukon-Tanana terrane, eastern Alaska Range, Alaska: Economic Geology, v. 88, p. 344-376.
Reporters Ellis, W.T., (Alaska Earth Sciences), A.S. Wyatt and S.S. Dashevsky (Northern Associates, Inc.), and W.J. Nokleberg (USGS)
Last report date 3/20/2003