Dahl Prospect, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Dahl Prospect	Prospect
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

The Dahl prospect is not typical of other deposits in the Farewell Mineral Belt. The mineralized zone appears to be controlled by a regional structure that extends toward the Crash (MG049) and Little Bird (MG047) prospects.
Location: The Dahl prospect is located in a steep north-trending gulch 4.0 kilometers due west of the outlet of Smith Lake and one kilometer south of the confluence of the headward tributaries of Sheep Creek at an elevation of 3,600 feet (1,097 m) near the boundary between sections 19 and 24, T. 26 N., R. 24 W., of the Seward Meridian. The reporter visited the site in 1981 and in 1997. Note that the Dahl prospect is not accurately located in Bundtzen and others (1997; prospect # 14).
Geology: The Dahl prospect consists of several impressive zones of massive chalcopyrite-pyrite-sphalerite mineralization that replaces Lower Paleozoic carbonaceous shale, and in contact zones between a north-trending quartz-feldspar porphyry sill (?) and shale. The massive sulfide mineralization at the Dahl prospect is exposed for over 90 meters along a narrow north-south ravine between two unnamed tributaries of upper Sheep Creek. The quartz-feldspar porphyry sill is parallel to the contact between the Lower Ordovician to Lower Silurian Post River Formation and mid-Silurian Terra Cotta Mountains Sandstone; both are units of the Dillinger subterrane (Bundtzen, Harris, and Gilbert, 1997). The massive sulfide horizon consists primarily of mobilized massive pyrrhotite with variable amounts of chalcopyrite, sphalerite, and traces of tetrahedrite over widths of 2.0 to 4.5 meters within a 4.0 meter to 7.0 meter wide zone of siliceous foliated breccia. This breccia is localized in the faulted contact between the Ordovician-Lower Silurian Post River Formation and the mid to Upper Silurian Terra Cotta Sandstone. Chalcopyrite occurs as fine grained masses up to 15 centimeters thick. Subordinate sphalerite, pyrrhotite, and pyrite occurs with the chalcopyrite and in separate masses up to 5 centimeters thick. Minor quartz gangue occurs in the sulfide zones, which range from 1 to 5 meters thick. Ore genesis for the Dahl prospect is controversial. The Anaconda Minerals Company, which explored the deposit in 1982 with a diamond drill, regarded the Dahl prospect as a shale hosted, sedimentary exhalitive, massive sulfide deposit hosted in lower Silurian tuffaceous(?) shale (Reed, 1982; Brewer and others, 1992). They based this interpretation on : (1) the stratigraphic section of the Dillinger subterrane is very similar to that in the Selwyn basin in Yukon Territory, Canada, which hosts significant 'sedex' mineralization of the same age; (2) the wallrocks at the Dahl prospect contain framboidal pyrite with high lead, zinc and copper backgrounds; (3) pyrite has been altered to pyrrhotite and 'buckshot' textures similar to that observed at Faro orebody, in Yukon; and (4) the whitish (?) layer interlayered with the sulfides was believed to be a syngenetic, submarine tuff. Sulfide-rich shale sections in drill core that were examined by the reporter in 1983 contained the graptolite Mongraptus spiralus which confirms that the host rock for mineralization is Lower Silurian in age. However, Smith and Albanese (1985) observed crosscutting relationships of sulfides and host sediments and sulfide replacements in the younger (?) quartz-feldspar porphyry sill (?). The quartz-feldspar porphyry was also observed to cut bedding in host sediments. Del S34 values from pyrrhotite average -0.2, which suggests derivation from plutonic sources (T.K. Bundtzen, written communication, 1989). Hence, Smith and Albanese (1985) and Bundtzen, Harris, and Gilbert (1997) classified the Dahl prospect as an epigenetic, sulfide replacement deposit. Two drill holes tested the Dahl prospect. Drill hole DP-W01 intersected 5.5 meters of massive sulfide mineralization that averaged 0.9 percent copper, 1.0 percent lead, 6.0 percent zinc, and 177.3 grams/tonne silver. Diamond drill hole DP-D01 intersected a true width of 3.5 meters in semi-massive sulfides that averaged 4.0 percent copper, 0.3 percent lead, 1.0 percent zinc, and 370.1 grams/tonne silver (Brewer and others, 1992). Bundtzen, Kline, and Clough (1982), Smith and Albanese (1985) and Bundtzen, Harris, and Gilbert (1997) reported that mineralized chip-channel surface samples contained up to 5.40 percent copper, 3.48 percent zinc, and 165.0 grams/tonne silver. No estimates of size and grade have been made or are available (Reed, 1982; Brewer and others, 1992).
Workings: The Dahl prospect was discovered by Anaconda Minerals Company in 1980 or 1981 and drilled from a precipitous platform in the steep 'discovery' gulch during the 1982 season. Drill hole DP-W01 intersected a true width of 5.5 meters of massive sulfide averaging 1.0 percent lead, 0.9 percent copper, 6.0 percent zinc, and 177.3 grams/tonne silver. Diamond drill hole DP-D01 intersected semi-massive sulfides with a true width of 3.5 meters that averaged 4.0 percent copper, 0.3 percent lead, 1.0 percent zinc, and 370.1 grams/tonne silver (Brewer and others, 1992). Bundtzen, Kline, and Clough (1982), Smith and Albanese (1985) and Bundtzen, Harris, and Gilbert (1997) reported that mineralized chip-channel surface samples contained up to 5.40 percent copper, 3.48 percent zinc, and 165.0 grams/tonne silver. A downhole mise-a-la-masse survey showed that the DP-D01 intercept is electrically continuous with the surface outcrops, confirming a downdip dimension of at least 150 meters. Additional surface conductivity surveys also suggest that the massive sulfide horizon extends to the north for an additional 150 meters.
Age: Unknown; believed to be either Tertiary, based on presumed age of the associated quartz-feldsar porphyry sill or Silurian based on fossil control (Bundtzen, Harris, and Gilbert, 1997).
Reserves: Although the deposit was drilled, no reserves were calculated or are available.

Commodities (Major) - Ag, Cu, Zn; (Minor) - Pb
Development Status: No
Deposit Model: Either polymetallic replacement (?) or sedimentary exhalitive lead-zinc (Cox an

Other Databases

Link to USGS - Alaska:	MG053

Other Regions, Features and Areas containing this locality

References