Prospect, Active

Commodities and mineralogy

Main commodities Ag; Au; Cu; Zn
Ore minerals chalcopyrite; pyrite; sphalerite

Geographic location

Quadrangle map, 1:250,000-scale CR
Quadrangle map, 1:63,360-scale A-1
Latitude 55.05953
Longitude -132.13577
Nearby scientific data Find additional scientific data near this location
Location and accuracy
The Broadgauge prospect is at an elevation of about 700 feet, about 0.6 mile east-northeast of hill 2230 and about 0.3 mile south-southeast of the center of section 34, T. 78 S, R. 88 E.
From the late 1970s to the present (2004), there has been nearly continuous exploration of several geologically similar deposits in an area of about a square mile southeast of the old Niblack Mine (CR216). Those deposits are described separately (CR217-223), but in the recent literature, the Broadgauge prospect and other deposits in the area are commonly grouped together under the name 'Niblack,' or 'Niblack project'.

Geologic setting

Geologic description

The Broadgauge prospect was not identified by early workers in the area, but was described by Maas and others (1992, 1995). The workings consist of three short adits, the longest about 30 feet long. The deposit at the Broadgauge prospect presumably is similar to the other volcanogenic massive sulfide deposits in the area (CR216-CR223), but the only public information specifically about it is limited sampling by Maas and others (1992). Their samples contained up to 209 parts per billion gold, 0.6 part per million (ppm) silver, 230 ppm copper, and 760 ppm Zn. Recent geochemical and isotopic work by Slack and others (2002) demonstrates that this and the similar nearby prospects nearby are volcanogenic massive-sulfide deposits of Silurian and Ordovician age.
Beginning in the mid-70s, a succession of companies, including Cominco-Alaska, Inc., Anaconda Minerals, Noranda Exploration, Houston Oil and Minerals, Long Lac Minerals, and, beginning in 1995, Abacus Minerals, have carried out extensive exploration on several deposits in the Niblack Mine area (CR217-223), testing the now widely accepted theory that they are volcanogenic, stratabound, massive-sulfide deposits. Those deposits are commonly grouped with the old Niblack Mine under the name 'Niblack' or 'Niblack Project'.
The exact location of most of the recent exploration in the Niblack area, including several generations of drilling, is not well documented in the public literature. Anaconda Minerals drilled at least two holes in 1978. Green and others (1989) note that Noranda and Lac Minerals drilled 24,000 feet on the deposits in the Niblack area through 1988, and Abacus Minerals drilled 39,000 feet in 1996 (1997?) (Swainbank and others, 1998). The exploration has been mentioned frequently in the annual reports of the Alaska Division of Geological and Geophysical Surveys since the early 80s. In their most recent annual report (Swainbank, and others, 2002), the deposits are grouped under the name 'Niblack'. The area currently is being explored by Abacus Minerals, and information about their work can be found on their web site (; Jan. 26. 2004).
As described by Lac Minerals USA Inc. (1989) and Maas and others (1995), and from a cross-section by Abacus Minerals (; April 18, 2000), the area features several large folds that trend west-northwest. The folds consist of a layered sequence of rhyolitic flows and volcaniclastic rocks that host the ore deposits, a hanging wall of mafic flows and sedimentary rocks, and a foot wall of amygdaloidal mafic flows. All of the rocks are regionally metamorphosed to greenschist grade. Maas and others (1991) describe three types of deposits at the Lookout prospect (CR221). It is the best- known deposit and probably typifies the other deposits in the area, including the one at the Niblack Mine. The three types are: 1) volcanogenic massive-sulfide bodies up to 20 feet thick with values of up to 4.9 percent copper, 8.0 percent zinc, 0.265 ounce of gold per ton, and 4.6 ounces of silver per ton; 2) stringer-type sphalerite mineralization in lithic tuffs in the footwalls of the massive sulfide bodies; and 3) auriferous, pyrite-bearing volcaniclastic rocks and polylithic breccias that typically contain about 0.05 ounce of gold per ton, 0.5 to 1.0 ounce of silver per ton, and 1 percent combined copper-zinc across widths of more than 50 feet.
The age of the rocks in the area has been variously interpreted. Eberlein and others (1983) mapped the strata as locally metamorphosed graywacke of Silurian or Ordovician age, near a large Paleozoic or Mesozoic granitic intrusion. Gehrels (1992) and Maas and others (1995) mapped them as pre-Ordovician metamorphosed volcanic and sedimentary rocks near a Silurian or Ordovician granitic intrusion. Brew (1996) called them Late Proterozoic and Cambrian Wales Group schist, phyllite, and marble, near a Tertiary granitic intrusion of intermediate composition. Most recently, Slack and others (2002) and S.M. Karl (oral communication, 2003) mapped the strata as Silurian and Ordovician, low-grade, regionally metamorphosed sedimentary and volcanic rocks.
Geologic map unit (-132.137418621068, 55.0591821312441)
Mineral deposit model Besshi-type volcanogenic Ag-Au-Cu massive-sulfide deposit (Cox and Singer, 1986; model 24b).
Mineral deposit model number 24b
Age of mineralization Silurian or Ordovician.
Alteration of deposit Not specifically noted, but probably typical of volcanogenic massive-sulfide deposits.

Production and reserves

Workings or exploration The only workings on the prospect consist of three short adits that probably date to before WW I.
Indication of production None



Brewer, N.H., 1989, Geology of the Niblack massive sulfide property, Prince of Wales Island, Alaska (abs.): Alaska Miners Association, Juneau Branch, Conference Juneau, Abstracts of Professional Papers, p. 34-35.
LAC Minerals (USA) Incorporated, 1989, Niblack Project prospectus, Prince of Wales Island, Alaska: LAC Minerals (USA) Incorporated, 16 p. (Unpublished report held by the Bureau of Land Management, Mineral Information Center, Juneau, Alaska).
Peek, B.C., 1975, Geology and mineral deposits of the Niblack Anchorage area, Prince of Wales Island, Alaska: Fairbanks, University of Alaska, M.Sc. thesis, 50 p.
Roppel, Patricia, 1991, Fortunes from the earth: Manhattan, Kansas, Sunflower University Press, 139 p.
Slack, J.F., Shanks, W.C. III, Karl, S.M., Ridley, W.I., and Bittenbender, P.E., 2002, Geochemical and sulfur isotope compositions of Late Proterezoic and early Paleozoic volcanogenic massive sulfide deposits, Prince of Wales Island and vicinity, southeastern Alaska (abs.): Geological Society of America, Abstracts with Programs, v. 34 (6), p. 113.
Reporters D.J. Grybeck (Applied Geology)
Last report date 5/1/2004