North Bradfield River

Prospect, Probably inactive

Alternative names

Ptarmigan

Commodities and mineralogy

Main commodities Cu; Fe
Other commodities Ag; Au; Mo; Zn
Ore minerals chalcopyrite; hematite; magnetite; malachite; pyrrhotite
Gangue minerals calcite; calc-silicate minerals

Geographic location

Quadrangle map, 1:250,000-scale BC
Quadrangle map, 1:63,360-scale B-5
Latitude 56.4012
Longitude -131.4084
Nearby scientific data Find additional scientific data near this location
Location and accuracy The coordinates mark the approximate center of a group of pods and lenses of mineralization that are scattered along a northwest-trending belt about 2 miles long and 0.5 mile wide. The center of the prospect is near the northeast corner of section 29, T. 63 S., R. 90 E., of the Copper River Meridian, about 9 miles southwest of Mount Pounder. Figure 33 of Still and others (2002) is a map of the deposit. The location is accurate.

Geologic setting

Geologic description

The North Bradfield Canal prospect was discovered by Ken Eichner in 1955 and by 1958 was covered by 41 claims (Eichner, 2002; Still and other, 2002). Takahashi, C.T., and Co. optioned the property until 1959; they mapped it, flew a aeromagnetic survey over it, and drilled 14 shallow holes. From 1960 to 1962, it was held by Utah Construction who did more surface work and drilled at least 460 feet of hole (Utah Construction, 1962). MacKevett and Blake (1963) of the U.S.G.S. mapped the geology in detail and Still and others (2002) of the U.S. Bureau of Land Management did some additional surface and geochemical sampling. Still and others (2002) revisited the prospect as part of a regional mineral assessment and did some sampling that largely confirmed the earlier work.
The North Bradfield River prospect (MacKevett and Blake, 1963, p. D1-D21) consists chiefly of metasomatic, magnetite-skarn deposits at the northwest end of a large roof pendant of gneiss, granulite, schist, marble, and skarn in quartz monzonite of the Coast Range Batholith, which in turn is cut by dikes of quartz diorite, aplite, and alaskite. The metamorphic bedded rocks are complexly folded. MacKevett and Blake interpret the general structure of the pendant as an overturned syncline that probably extends for many miles to the southeast. Sonnevil (1981, p. B117), on the other hand, interprets the dominant structure in the area as a homocline that dips northwest to northeast. Koch (1997, p. 24) reports that the pendant is marked by an aeromagnetic trough that roughly parallels its outcrop (U. S. Geological Survey, 1979). The deposit is in marble layers of the roof pendant and consists of calc-silicate skarn that is partly replaced by massive magnetite with interstitial pyrrhotite; the magnetite is cut by veinlets of chalcopyrite. This ore contains subordinate amounts of hematite, limonite, and malachite. The orebodies, of which at least 15 are exposed, are crudely stratiform and apparently discontinuous; they range in strike length from 50-350 feet and in thickness from 2-40 feet. They occur in a belt about 2 miles long and 0.5 mile wide. Koch (1997, p. 24-25) suggests that at least some of the metal concentration in these deposits is related to the emplacement of an Eocene quartz monzonite and granodiorite stock, near the contact with the Mesozoic or Paleozoic metamorphic bedded rocks that host the deposits (Elliott and Koch, 1981; Koch, 1996). Koch (1997, p. 24, 25) and Elliott and Koch (1981, p. 8, loc. 9) also report anomalous amounts of silver, gold, molybdenum, and zinc in rock samples collected at and near the North Bradfield River deposits.
As a result of their surface work and drilling, Utah Construction (1962) estimated that the 9 magnetite-copper bodies that they identified contain about 1,000,000 tons of proven and probable resources and 4,481,000 tons of possible resources with a estimated grade of 35 to 40 percent iron, 0.2 to 0.3 percent copper, and 3 to 4 percent sulfur.
Geologic map unit (-131.410102168075, 56.4008563678028)
Mineral deposit model Cu-Fe skarn (Cox and Singer, 1986; model 18d).
Mineral deposit model number 18d
Age of mineralization Probably Eocene based on a nearby intrusive that probably is genetically related to the deposit.
Alteration of deposit Calc-silicate skarn is developed in marble units adjacent to the orebodies.

Production and reserves

Workings or exploration The Bradfield Canal prospect was discovered by Ken Eichner in 1955 and by 1958 was covered by 41 claims (Eichner, 2002; Still and other, 2002. Takahashi, C.T., and Co. optioned the property until 1959; they mapped it, flew an aeromagnetic survey over it, and drilled 14 shallow holes. From 1960 to 1962, it was held by Utah Construction who did more surface work and drilled at least 460 feet of hole (Utah Construction, 1960). MacKevett and Blake (1963) of the U.S.G.S. mapped the geology in detail and Still and others (2002) of the U.S. Bureau of Land Management did some additional surface and geochemical sampling.
Indication of production None
Reserve estimates As a result of their surface work and drilling, Utah Construction (1962) estimated that the 9 magnetite-copper bodies that they identified contain about 1,000,000 tons of proven and probable resources and 4,481,000 tons of possible resources with a estimated grade of 35 to 40 percent iron, 0.2 to 0.3 percent copper, and 3 to 4 percent sulfur.

Additional comments

Elliott and Koch (1981) also apply the name 'Ptarmigan' to the North Bradfield River prospect. Still and others (2002) indicated that there were no active claims on the property at the time of their work.

References

References

Still, J.C., Bittenbender, P.E., Bean, K.W., and Gensler, E.G., 2002, Mineral assessment of the Stikine area, central southeast Alaska: Bureau of Land Management Technical Report 51, 560 p.
Reporters H. C. Berg (Fullerton, California); D.J. Grybeck (Port Ludlow, WA)
Last report date 3/4/2008