Trio

Occurrence, Active

Commodities and mineralogy

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

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-1
Latitude 63.2616
Longitude -144.0395
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Trio occurrence is about 4 miles southeast of the confluence of the Robertson River and Rumble Creek. It is at an elevation of about 5,400 feet about 0.6 mile south-southeast of the center of section 10, T. 17 N., R. 7 E., Copper River Meridian. The location is accurate.

Geologic setting

Geologic description

Massive sulfide boulders and rubble crop that contain as much as 17 percent combined copper, lead, and zinc were located at the Trio occurrence in 1976 (J.K. Muntzert and others, unpublished Resource Associates of Alaska, Inc. report, 1977). More typical values are 0.5 to 1.5 percent copper, 2 to 8 percent lead, 4 to 7 percent zinc, 30 to 125 grams of silver per ton, and 0.5 to 3 grams of gold per ton. Intensive programs of geological mapping, ground geophysical and geochemical surveys, and drilling were carried out from 1976 to 1984 by Resource Associates of Alaska and in 1990 by Phelps Dodge. Eight core holes were drilled at the occurrence by 1990, but the intersections of the massive sulfides did not correlate well with surface mapping or from hole to hole. Additional mapping and prospecting by American Copper and Nickel Company from 1994 to 1998 determined that the sulfide showings and the fairly shallow drill holes were all within an area of landslide debris. Three deeper, more widely spaced holes drilled in 1998 outside the landslide mass intersected an intensely altered quartz-sericite-pyrite layer but found no significant sulfide mineralization (S.S. Dashevsky, written communication, 2003).
The Trio prospect is in the lower Lagoon unit of Devonian age and is similar to the nearby high-grade SC East occurrence (MH332). 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 for the basal section are immature sediments or wackes, mudstone, quartz arenite, and lesser calcareous arenite and carbonates. 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 prospect (MH 332). 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 volcanogenic massive sulfide occurrences in the 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 (-144.041751571455, 63.2612257836259)
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 Strong to intense sericite-quartz -CO3 +/- chloritoid, kaolin, kyanite, andalusite alteration (Lange and others, 1993; Dashevsky and others, 2003).

Production and reserves

Workings or exploration Massive sulfide boulders and rubble crop that contain as much as 17 percent combined copper, lead, and zinc were located at the Trio occurrence in 1976 (J.K. Muntzert and others, unpublished Resource Associates of Alaska, Inc. report, 1977). More typical values are 0.5 to 1.5 percent copper, 2 to 8 percent lead, 4 to 7 percent zinc, 30 to 125 grams of silver per ton, and 0.5 to 3 grams of gold per ton. Intensive programs of geological mapping, ground geophysical and geochemical surveys, and drilling were carried out from 1976 to 1984 by Resource Associates of Alaska and in 1990 by Phelps Dodge. Eight core holes were drilled at the occurrence by 1990, but the intersections of the massive sulfides did not correlate well with surface mapping or from hole to hole. Additional mapping and prospecting by American Copper and Nickel Company from 1994 to 1998 determined that the sulfide showings and the fairly shallow drill holes were within an area of landslide debris. Three deeper, more widely spaced holes drilled in 1998 outside the landslide mass intersected an intensely altered quartz-sericite-pyrite horizon but found no significant sulfide mineralization (S.S. Dashevsky, written communication, 2003). A total of 6,385 feet of drilling in 12 drill holes has taken place in the Trio area that includes the Trio West (MH336) and Trio East (MH337) prospects (E. Hunter, unpublished American Copper and Nickel, Inc. report, 1998).
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 (www.grayd.com), or Northern Associates Inc. in Fairbanks, Alaska.

References

References

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 W.T. Ellis (Alaska Earth Sciences) and A.W. Wyatt and S.S. Dashevsky (Northern Associates Inc.)
Last report date 3/20/2003