Unnamed Occurrence (ARDF - TL077; on Portage Creek), Yentna Mining District, Matanuska-Susitna Borough, Alaska, USAi

Regional Level Types
Unnamed Occurrence (ARDF - TL077; on Portage Creek)	- not defined -
Yentna Mining District	Mining District
Matanuska-Susitna Borough	Borough
Alaska	State
USA	Country

Location: This occurrence is located in Portage Creek at about 3100 feet in the southwest quarter of Section 11, T. 22 N., R. 20 W., of the Seward Meridian.
Geology: The area is underlain by Mesozoic marine sedimentary rocks (KJs) that are intruded by the granodiorite of Mount Estelle (Te) (Reed and Nelson, 1980). At this locality the granodiorite has a leucocratic equigranular phase and a porphyritic phase with potassium feldspar phenocrysts up to 10 millimeters in size (Cominco American Incorporated, unpublished report). Cominco American Incorporated (unpublished report) describes chalcopyrite, pyrrhotite, arsenopyrite, molybdenite and pyrite as joint coatings, in veins, and as disseminations adjacent to northwest-striking mineralized joint sets. Sulfide-bearing rock samples from this site contain gold which ranges from 60 ppb to greater than 1.0 oz/ton. Silver values are generally below detection (less than 0.4 ppm); however isolated values of up to 11 ppm were recorded. The granodiorite of Mt. Estelle probably reflects the same plutonic event as the composite plutons (Tcp) to the north described by Reed and Nelson (1980). Altered granodiorite float about 2 miles S72W of this location location contains gold-bearing veinlets of arsenopyrite and pyrite (TL063). Curtin, Karlson, O'Leary, Day, and McDanal (1978) report pan concentrate samples with visible gold from Quaternary gravels about one mile downstream from this locality at site TL052. Similar gold-sulfide veins are present in the Estelle pluton (TY019) to the south in the Tyonek quadrangle.
Workings: Rock chip sampling and mapping at 1:12,000 scale by Cominco American Incorporated was conducted in the late 1980's (unpublished report). Reconnaissance mapping, stream silt and pan concentrate sampling was done in the area by the U.S. Geological Survey (Reed and Elliot, 1970; Curtin, Karlson, O'Leary, Day, and McDanal, 1978; Reed and Nelson, 1980). Sulfide-bearing rock samples from this site contain gold which ranges from 60 ppb to greater than 1.0 oz/ton. Silver values are generally below detection (less than 0.4 ppm); however isolated values of up to 11 ppm were recorded (Cominco American Incorporated, unpublished report).
Age: Late Cretaceous/early Tertiary; mineralization is interpreted to be linked to the granodiorite of Mount Estelle (Te) for which Reed and Nelson (1980) give hornblende ages of about 65 and 66 m.y.

Commodities (Major) - Au, Cu, Mo; (Minor) - As
Development Status: No
Deposit Model: This occurrence does not specifically fit any of those defined by Cox and Singe

Other Databases

Link to USGS - Alaska:	TL077

Other Regions, Features and Areas containing this locality

References