Boundary Prospect, Fortymile District, Southeast Fairbanks Borough, Alaska, USA

Location: The Boundary prospect covers about 2 square miles; it is bounded to the east by the Canadian border, to the south by the Top of the World Highway, to the west by Arkansas Creek, and to the north by the ridge north of Brophy Creek. The coordinates are the approximate center of the drillholes, in the NE1/4 section 26, T. 27 N., R. 22 E., of the Copper River Meridian; the location is accurate.
Geology: The Boundary prospect is interpreted to be a lead-zinc-copper-silver, volcanogenic massive-sulfide deposit. The prospect is in greenschist-facies metamorphic rocks, including quartz-white mica schist, and underlying quartz-chlorite-white mica schist and minor quartz-chlorite schist. Foliation and compositional layering generally dip 20 to 30 degrees to the north and northwest (Resource Associates of Alaska, 1977). These rocks are part of the Klondike Schist (the Klondike series of McConnell, 1905). The Klondike Schist has mid-Permian protolith ages determined by uranium-lead zircon dating (Mortensen, 1999). Because the Klondike Schist hosts mineralization that is probably syngenetic with its host rocks, the stratabound sulfide mineralization is probably mid-Permian. A galena sample taken from drill core from the Boundary prospect was analyzed for common lead values in two separate dissolutions. Both analyses fall within the middle of the field for galenas from other Permian Klondike Schist-hosted syngenetic occurrences in the area (J.K. Mortensen, written commun., 2002). Klondike Schist near the Boundary prospect overlies carbonaceous schist and quartzite to the south that are part of the Nasina Series, which has a Mississippian protolith age (McConnell, 1905; Resource Associates of Alaska, 1977; Mortensen, 1999). Along the north side of the Boundary prospect, serpentinized ultramafic rocks (including an ultramafic body more than 1 square mile in area along Hall Creek) crop out along a thrust contact with the overlying Nasina Series rocks and extend across the border into Yukon Territory (Mortensen, 1988; R.L. Flynn, unpub. data, 2000). Two base-metal sulfide prospects in Klondike Schist are located across the border in the Yukon Territory: the Baldy prospect is 2 miles to the east, and the Pub prospect is 5 miles to the east of the Boundary prospect (Mortensen, 1988). At the Boundary prospect, the Klondike Schist contains abundant quartz lenses; locally abundant iron oxides replace pods of pyrite and occur along the foliation (R.L. Flynn, unpub. data, 2000). Surface oxidation is extensive to depths of more than 300 feet in drillcore, and there are local gossan zones. Where sulfides are present in outcrop, they are generally found as thin laminations and include pyrite and minor galena, sphalerite, and covellite and possibly chalcocite (Resource Associates of Alaska, 1977). The sulfides most commonly occur in felsic schist. Sulfides in drillcore are found to below 300 feet and consist primarily of pyrite, with rare galena, sphalerite, and chalcopyrite (Resource Associates of Alaska, 1977). Whole-rock analyses of some felsic schists indicate that they have high magnesium and iron contents, suggesting chlorite alteration; chlorite alteration is visible in some drillcore samples (Smit, 2000). A lead-zinc-copper-silver geochemical anomaly occurs along the contact between felsic schists and mafic schists at the Boundary prospect (Smit, 2000). Rock samples from gossan zones commonly contain 3,000 to 5,000 ppm lead, 300 to 500 ppm zinc, 300 to 800 ppm copper, and 7 to 14 ppm silver; gold is below detection limits. Zinc is more dispersed than lead; a soil geochemistry anomaly approximately 6,000 by 2,000 feet in extent typically has 100 to 300 ppm lead, 300 to700 ppm zinc, and 75 to100 ppm copper, with less than 2 ppm silver. Stream sediments generally contain less than 50 ppm lead and copper, but they typically contain 400 to 900 ppm zinc (Resource Associates of Alaska, 1977). Several geochemically anomalous intervals were found in drillcore; a 48-foot section of pyritic chlorite schist has 241 ppm copper, 709 ppm lead, 1,933 ppm zinc, and 6.4 ppm silver (Smit, 2000). The area between Arkansas Creek (see EA150) and Brophy Creek was first identified as a potential lead-zinc deposit by R.R. Asher of the Alaska Division of Mines and Geology (Asher, 1970). Asher observed a gossan zone with limonite replacing pyrite, and he reported that several samples contain anomalous lead, zinc, and copper. In 1977, Resource Associates of Alaska conducted regional and detailed geologic mapping, sampled soil on a grid over about 2 square miles, and drilled four diamond drill holes totaling 1,560 feet near Brophy Creek. Recoveries were poor; only two of the drillholes reached target depth, and massive sulfides were not intersected. Most of this drillcore is now stored at the University of Alaska Museum, Fairbanks, Alaska. Resource Associates of Alaska (1977) suggested that the drillholes may have been collared in the footwall of a potential massive-sulfide deposit and the deposit may be to the north of Brophy Creek, down-dip and up-section. However, recent mapping (R.L. Flynn, unpub. data, 2000) suggests that near Brophy Creek, Nasina Series rocks have been thrust over the Klondike Schist that hosts the Boundary prospect. A horizontal-loop-EM (electromagnetic) survey with a penetration depth of about 140 feet was run on six grid lines by Resource Associates of Alaska in 1977, but no conductors were found. Regional exploration was conducted by Anaconda in the Boundary area in 1978; about 70 square miles was mapped at a reconnaissance scale (Wiltse, 1978). In 1979, two drill holes totaling 1,004 feet were drilled by American Copper and Nickel Company in the same area, but they also failed to intersect massive-sulfide mineralization (Smit, 2000); most of this drillcore is still at the drill hole collars, but information concerning the orientation of the holes has been lost. Limited work on the Boundary prospect in 1999 by Grayd Resource Corporation did not produce new exploration targets (Smit, 2000). Most streams in the Boundary area have had placer gold production; placer gold-producing streams draining the Boundary prospect area include Arkansas Creek (EA150), Canyon Creek (EA143), Davis Creek (EA155), and Walker Fork (EA156). The lode sources of the placer gold in the Boundary area have not been identified, but because the gold content of the Boundary deposit is so low, it is unlikely to be the source of the placer gold (Resource Associates of Alaska, 1977).
Workings: A lead-zinc-copper-silver geochemical anomaly occurs along the contact between felsic schists and mafic schists at the Boundary prospect (Smit, 2000). Rock samples from gossan zones commonly contain 3,000 to 5,000 ppm lead, 300 to 500 ppm zinc, 300 to 800 ppm copper, and 7 to 14 ppm silver; gold is below detection limits. Zinc is more dispersed than lead; a soil geochemistry anomaly approximately 6,000 by 2,000 feet in extent typically has 100 to 300 ppm lead, 300 to 700 ppm zinc, and 75 to 100 ppm copper, with less than 2 ppm silver. Stream sediments generally contain less than 50 ppm lead and copper, but they typically contain 400 to 900 ppm zinc (Resource Associates of Alaska, 1977). Several geochemically anomalous intervals were found in drillcore; a 48-foot section of pyritic chlorite schist has 241 ppm copper, 709 ppm lead, 1,933 ppm zinc, and 6.4 ppm silver (Smit, 2000). The area between Arkansas Creek (see EA150) and Brophy Creek was first identified as a potential lead-zinc deposit by R.R. Asher of the Alaska Division of Mines and Geology (Asher, 1970). Asher observed a gossan zone with limonite replacing pyrite, and several samples contain anomalous lead, zinc, and copper. In 1977, Resource Associates of Alaska conducted regional and detailed geologic mapping, sampled soil on a grid over about 2 square miles, and drilled four diamond drill holes totaling 1,560 feet near Brophy Creek. Recoveries were poor; only two of the drill holes reached target depth, and massive sulfides were not intersected. Most of this drillcore is now stored at the University of Alaska Museum, Fairbanks, Alaska. Resource Associates of Alaska (1977) suggested that the drillholes may have been collared in the footwall of a potential massive-sulfide deposit and the deposit may be to the north of Brophy Creek, down-dip and up-section. However, recent mapping (R.L. Flynn, unpub. data, 2000) suggests that near Brophy Creek, Nasina Series rocks have been thrust over the Klondike Schist that hosts the Boundary prospect. A horizontal-loop-EM (electromagnetic) survey with a penetration depth of about 140 feet was run on six grid lines by Resource Associates of Alaska in 1977, but no conductors were found. Regional exploration was conducted by Anaconda in the Boundary area in 1978; about 70 square miles was mapped at a reconnaissance scale (Wiltse, 1978). In 1979, two holes totaling 1,004 feet were drilled by American Copper and Nickel Company in the same area, but they also failed to intersect massive-sulfide mineralization (Smit, 2000); most of this drillcore is still at the drill hole collars, but information about the orientation of the holes has been lost. Limited work on the Boundary prospect in 1999 by Grayd Resource Corporation did not produce new exploration targets (Smit, 2000).
Age: Because the Klondike Schist hosts mineralization that is probably syngenetic with its host rocks, the stratabound sulfide mineralization is probably mid-Permian. A galena sample taken from drillcore from the Boundary prospect was analyzed for common lead values in two separate dissolutions. Both analyses fall within the middle of the field for galenas from other Permian, Klondike Schist-hosted syngenetic occurrences in the area (J.K. Mortensen, written commun., 2002).
Alteration: Oxidation is extensive to depths of more than 300 feet, and there are local gossan zones (Resource Associates of Alaska, 1977). Whole-rock analyses of some felsic schists indicate that they have high magnesium and iron contents, suggesting chlorite alteration; chlorite alteration is visible in some drillcore samples (Smit, 2000).

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

Mineral List

8 entries listed. 7 valid minerals.

References