|Quadrangle map, 1:250,000-scale||MG|
|Quadrangle map, 1:63,360-scale||B-2|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||The Dahl prospect is in a steep north-trending gulch, 4.0 kilometers west of the outlet of Smith Lake and one kilometer south of the junction of the headward tributaries of Sheep Creek. It is at elevation of about 3,600 feet near the middle of the boundary between sections 19 and 24, T. 26 N., R. 24 W., of the Seward Meridian.|
The Dahl prospect consists of several impressive zones of massive chalcopyrite-pyrite-sphalerite mineralization that replaces Lower Paleozoic carbonaceous shale in contact zones between a north-trending quartz-feldspar porphyry sill(?) and shale. The massive sulfide mineralization is exposed for over 90 meters along a narrow north-trending 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 sub-terrane (Bundtzen, Harris, and Gilbert, 1997).
The mineralized horizon consists primarily of massive pyrrhotite with variable amounts of chalcopyrite, and sphalerite, and traces of tetrahedrite; it is 2.0 to 4.5 meters wide in a 4.0 meter to 7.0 meter zone of siliceous 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 forms fine-grained masses up to 15 centimeters thick. Subordinate sphalerite, pyrrhotite, and pyrite accompany the chalcopyrite and form separate masses up to 5 centimeters thick. Minor quartz gangue occurs in the sulfide zones, which range from 1 to 5 meters thick.
The genesis of 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 exhalative, massive-sulfide deposit in lower Silurian tuffaceous(?) shale (Reed, 1982; Brewer and others, 1992). They based this interpretation on: (1) the stratigraphic section of the Dillinger sub-terrane is 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 contents, (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 syndepositional, submarine tuff. Sulfide-rich shale sections in drill core that were examined by T.P. Bundtzen (personal observation) in 1983 contained the graptolite Monograptus spiralis which confirms that the host rock 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. Isotopic 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 holes were drilled on 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 of silver per tonne. Diamond drill hole DP-D01 intersected 3.5 meters of semi-massive sulfides that averaged 4.0 percent copper, 0.3 percent lead, 1.0 percent zinc, and 370.1 grams of silver per tonne (Brewer and others, 1992). Bundtzen, Kline, and Clough (1982), Smith and Albanese (1985) and Bundtzen, Harris, and Gilbert (1997) reported that chip samples contained up to 5.40 percent copper, 3.48 percent zinc, and 165.0 grams of silver per tonne. A down hole geophysical survey showed that the DP-D01 drill-hole intercept is electrically continuous with the surface outcrops, suggesting that surface mineralization continues down dip for at least 150 meters. Additional surface conductivity surveys also suggest that the massive sulfide horizon extends to the north an additional 150 meters. No estimates of size and grade have been made (Reed, 1982; Brewer and others, 1992).In 2008, the Dahl prospect was one of the prominent deposits in a block of claims that covered more than 70 square miles and known collectively as the BMP project (International Tower Hill Mines, Ltd., 2008). Their mapping in 2007 suggests that the mineralization occurs along a fault developed in a north-trending anticlinal hinge that extends 5 km to the south through several prospects (MG057 and MG061) to the 6120 prospect (MG062) and about 3 kilometers to the north through several other prospects (MG049-051).
|Geologic map unit||(-153.814021419083, 62.3265867343701)|
|Mineral deposit model||Either polymetallic replacement or sedimentary exhalative lead-zinc (Cox and Singer, 1986; model 19a or 31a).|
|Mineral deposit model number||19a or 31a|
|Age of mineralization||Unknown; believed to be either Tertiary, based on presumed age of the associated quartz-feldspar porphyry sill or Silurian, based on fossil identification (Bundtzen, Harris, and Gilbert, 1997).|
|Workings or exploration||The Dahl prospect was discovered by Anaconda Minerals Company in 1980 or 1981 and two holes were drilled in 1982. In 2008, the Dahl prospect was one of the prominent deposits in a block of claims that covered more than 70 square miles and known collectively as the BMP project (International Tower Hill Mines, Ltd., 2008).|
|Indication of production||None|
Brewer, N.H., Puchner, C.C., and Gemuts, I., 1992, Farewell district, southwest Alaska Range: North Pacific Mining Company prospectus report, 21 p.
Bundtzen, T.K., Harris, E.E., and Gilbert, W.G., 1997, Geologic Map of the eastern McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-14, 34 p., 1 sheet, scale 1:125,000.
Bundtzen, T.K., Kline, J.T., and Clough, J.G., 1982, Preliminary geology of the McGrath B-2 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Open-File report 149, 22 p., 1 sheet, scale 1:40,000.
International Tower Hills Mines, Ltd., 2008, BMP: http://www.ithmines.com/s/BMP.asp (as of April 30, 2008).
Reed, Mark, 1982, The Tin Creek sector of the Farewell district, Alaska: Anaconda Minerals Company internal report, 44 p.
|Reporters||T.K. Bundtzen (Pacific Rim Geological Consulting); D.J. Grybeck (Port Ludlow, WA)|
|Last report date||6/5/2008|