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quartz exhalite that is chloritically altered and associated sphalerite and galena (R.A. Blakestad and others, unpublished Resource Associates of Alaska, Inc. report, 1978).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10.5 percent zinc, 0.187 ounce of silver per ton, and less than 0.004 ounce of gold per ton (R.A. Blakestad and others, unpublished Resource Associates of Alaska, Inc. report, 1978).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). |
