Trio East

Occurrence, Inactive

Commodities and mineralogy

Main commodities Ag; Pb; Zn
Other commodities Cu
Ore minerals galena; pyrite; pyrrhotite; sphalerite
Gangue minerals andalusite; kaolin; quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-1
Latitude 63.2525
Longitude -144.0175
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Trio East occurrence is about 5 miles southeast of the confluence of the Robertson River and Rumble Creek at an elevation of about 6,200 feet. The location is the area that was drilled by Phelps Dodge in 1990, about 0.1 mile south-southwest of the center of section 14, T. 17 N., R. 7 E., Copper River Meridian. Additional exposures of the mineralized layer or unit occur about one-half mile to the north at an elevation of about 5,900 feet. The location is accurate.

Geologic setting

Geologic description

The Trio East occurrence follows an intermittently exposed siliceous (metavolcanic or exhalative) pyritic schist unit with semimassive sulfide mineralization; it is in a sequence of carbonaceous metasedimentary rocks. Local blocks of semimassive to massive sulfide as much as 5 feet thick are found in talus near this altered unit (C. Schaefer, oral communication, 2003). Most samples were only moderately enriched in metals and contain as much as 0.1 percent copper, 0.2 percent lead, 0.5 percent zinc, no silver, and 0.2 part per million gold (S.S. Dashevsky, unpublished data, 2003). An analysis of massive sulfide samples reported by Resource Associates of Alaska showed 0.85 percent copper, 9.2 percent lead, 4.5 percent zinc, and 130.6 grams of silver per ton (Lange and others, 1993).
Geologic map unit (-144.019751373512, 63.2521258507994)
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 +/- kaolin +/- andalusite alteration (Lange and others, 1993).

Production and reserves

Workings or exploration The Trio East area was covered by a number of geological and geophysical surveys in the late 1970s and in 1990. Phelps Dodge drilled one hole near this occurrence in 1990; it intersected extensive quartz-pyrite-sericite alteration but no significant values in base or precious metals (R. Cheff, unpublished Phelps Dodge Exploration, Inc. report, 1990).
Indication of production None



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), A.S. Wyatt and S.S. Dashevsky (Northern Associates, Inc. ), and W.J. Nokleberg (USGS)
Last report date 3/20/2003