Roberts No. 2; McGinnis Glacier Prospect, Delta River Mining District, Southeast Fairbanks Census Area, Alaska, USAi

Regional Level Types
Roberts No. 2; McGinnis Glacier Prospect	Prospect
Delta River Mining District	Mining District
Southeast Fairbanks Census Area	Census Area
Alaska	State
USA	Country

Location: The Roberts No. 2 prospect is on a knife-like southeast -trending ridge at an elevation of about 7,200 feet. The prospect is about at the east end of the boundary between sections 19 and 30, T. 15 S., R. 8 E., Fairbanks Meridian. The location corresponds to locality N9 in table 2 of Nokleberg and others (1991); the Roberts No. 2 prospect corresponds to the McGinnis Glacier locality of Lange and others (1993). The location is accurate to within 1,100 feet.
Geology: The rocks for the Roberts No. 2 or McGinnis Glacier prospect are intensely deformed schists of Devonian or older age (Nokleberg and Aleinikoff, 1985; Aleinikoff and Nokleberg, 1985 [C 967]). They contain abundant mylonite zones and exhibit two periods of metamorphism and deformation: an older, lower amphibolite grade event, and a younger, lower greenschist grade event. At the prospect, massive sulfides occur in a 6-foot-thick zone that can be traced for about 30 feet. Boulder-size masses of pyrrhotite occur in a matrix of chlorite-calcite-quartz schist. The hanging wall of the zone is capped by a graphitic calcite-quartz schist. Other lithologies include metaconglomerate and metagraywacke; a 45-foot-thick layer of gray marble lies about 75 feet structurally below the sulfide zone (Lange and others, 1993). Pyrrhotite, along with lesser amounts of pyrite, chalcopyrite, sphalerite, and galena occur with quartz, chlorite, biotite, actinolite, and calcite (Lange and others, 1993, p. 350).
Workings: There are no extensive workings. The U.S. Geological Survey mapped and sampled the area in reconnaissance during the 1980's. One sample assayed 0.69 percent copper, 0.3 percent lead, 2.3 percent zinc, and 45.4 parts per million (ppm) silver (Lange and others, 1993). Sample 81NK226 of Nokleberg and others (1991) assayed 0.25 percent copper, 0.25 percent lead, 0.2 percent zinc, 50 ppm silver, and 30 ppm tin. The area was examined in the 1990's by American Copper and Nickel Company; their geologists concluded that the deposits were rich in pyrrhotite but poor in valuable metals (W.T. Ellis, unpublished data, 1993).
Age: Devonian, the protolith age of metamorphic host rocks.

Commodities (Major) - Cu, Pb, Zn; (Minor) - Ag, Sn
Development Status: None
Deposit Model: Kuroko massive sulfide (Cox and Singer, 1986; model 28a).

Other Databases

Link to USGS - Alaska:	MH010

Other Regions, Features and Areas containing this locality

References