Trio Occurrence, Tok Mining District, Southeast Fairbanks Census Area, Alaska, USAi

Regional Level Types
Trio Occurrence	Occurrence
Tok Mining District	Mining District
Southeast Fairbanks Census Area	Census Area
Alaska	State
USA	Country

The unpublished data that is cited can be seen by contacting Grayd Resources Inc. in Vancouver, B.C., Canada (www.grayd.com), or Northern Associates Inc. in Fairbanks, Alaska.
Location: 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.
Geology: 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as 17 percent combined copper, lead, and zinc were located at the Trio occurrence in 1976 (J.K. Muntzert and others, unpublished Resource Associates of Alaska, Inc. report, 1977).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).
Workings: 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).
Age: 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: Strong to intense sericite-quartz -CO3 +/- chloritoid, kaolin, kyanite, andalusite alteration (Lange and others, 1993; Dashevsky and others, 2003).

Commodities (Major) - Ag, Au, Zn; (Minor) - Cu, Pb
Development Status: None
Deposit Model: Kuroko massive sulfide (Cox and Singer, 1986; model

Other Databases

Link to USGS - Alaska:	MH333

Other Regions, Features and Areas containing this locality

References