Edith M

Prospect, Active

Commodities and mineralogy

Main commodities Au; Cu
Ore minerals chalcopyrite; pyrite

Geographic location

Quadrangle map, 1:250,000-scale CR
Quadrangle map, 1:63,360-scale A-1
Latitude 55.06387
Longitude -132.1377
Nearby scientific data Find additional scientific data near this location
Location and accuracy
The Edith M prospect is just above the shoreline, about 0.5 mile southeast of the abandoned settlement of Niblack at the head of Niblack Anchorage. It is at 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 Edith M prospect and other deposits in the area are commonly grouped together under the name 'Niblack,' or 'Niblack project'.

Geologic setting

Geologic description

The Edith M prospect is one of several similar prospects (CR217-CR223) that cover about a square mile southeast of the old Niblack Mine (CR216). Little information has been published about the Edith M prospect since it was first described by Brooks (1902) as a 20-tunnel. The deposit consists of a zone about 1 foot thick in mineralized greenstone with chalcopyrite and pyrite. His samples contained up to $5 per ton in copper and gold (at 1902 metal prices). Brooks also noted several other such zones, including one 8 feet thick that contains pyrite. Maas and others (1992) collected several chip samples that contained 18-83 parts per billion gold, and up to 104 parts per million (ppm) copper and 225 ppm Zn. The Edith M prospect is on a large geophysical anomaly that has received considerable attention during recent exploration in the area (Maas and others, 1995). Recent geochemical and isotopic work by Slack and others (2002) confirm that this and nearby similar deposits are volcanogenic massive-sulfide deposits of Silurian or Ordovician age.
As described by Lac Minerals USA Inc. (1989) and Maas and others (1995), and from a cross-section by Abacus Minerals (www.abacusminerals.com/niblackxsec.htm; 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. Slack and others (2002) also demonstrate that the volcanogenic massive-sulfide deposits in the Niblack area are of Silurian or Ordovician age.
Geologic map unit (-132.139349413167, 55.0635216586004)
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. Brooks (1902) noted silicification adjacent to the gold-bearing veins.

Production and reserves

Workings or exploration The only working is an old 20-foot adit; it has been sampled and visited in recent years during mineral exploration in the surrounding area.
Indication of production None

References

MRDS Number A101013

References

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