Michigan

Prospect, Active?

Alternative names

Michigan Lode
Michigan Lead
Rob

Commodities and mineralogy

Main commodities Ag; As; Au; Sb
Other commodities Bi; Cu; Pb; Te; W
Ore minerals arsenopyrite; covellite; digenite; gold; jamesonite; pyrite; stibnite
Gangue minerals quartz; sericite

Geographic location

Quadrangle map, 1:250,000-scale BD
Quadrangle map, 1:63,360-scale B-1
Latitude 64.3624
Longitude -144.1979
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Michigan prospect about 2.1 miles north-northwest of the north(5080) peak of Black Mountain and about 0.4 mile southwest of the center of section 28, T. 6 S., R. 18 E, of the Fairbanks Meridian. Although sometimes described as a separate deposit, it may be a continuation of the mineralization at the Blue Lead prospect (BD003) and Grizzly Bear Mine (BD018) just to the south. The location is accurate within 500 feet.

Geologic setting

Geologic description

The Goodpaster region was first explored for placer gold in 1915. In the early 1930s, gold-bearing quartz veins were discovered in the upper Tibbs Creek area. By the winter of 1936, the first underground workings were begun. A 450-foot tunnel was driven following a small vein, termed the Blue Lead Extension (BD003). After disappointing results, the work was stopped. In the summer of 1936, five men drove a 300-foot tunnel at the outcrop of the Blue Lead vein (Reed, 1937). During the winter of 1937, a 300-foot tunnel was driven at the nearby Grizzly Bear Mine (BD018) and a 50-ton mill was constructed. In the summer of 1938, the mill was moved to the Blue Lead Mine and operated for a year and a half until the fall of 1939 (Joesting, 1938). The Michigan Lode was also probably discovered about that time. No ore has been mined from the Michigan Lode (Thomas, 1970) and the only workings are trenches.
From 1995 to 1999, The Stone Boy Joint Venture (Sumitomo Metal Mining and WGM) spent more than $1.3 million exploring in the area; they did extensive surface mapping and sampling, surface and airborne geophysics, and 16,215 feet of diamond drilling. The joint venture drilled 26 holes but details on their location is largely lacking. Apparently no holes were drilled on the Michigan prospect. In 2002, Freegold Ventures optioned a large block of claims that covered the Michigan Lode and several other nearby deposits that are described separately: Gray Lead (BD017), the Grizzly Bear (BD018), Wolverine (BD057), Upper Trench, Lower Trench (BD058) and O'Reely (BD059). Through early 2012, Freegold has been exploring them as a unit under the name Rob (Flanders, 2010; Freegold Ventures Inc., 2012). The work has included extensive geologic mapping, surface sampling, and geochemical and geophysical surveys. In 2004, Freegold commissioned a comprehensive NI 43-101 report (Freeman, 2004) that summarized their and previous work. That report was updated in 2010 (Flanders, 2010).
As interpreted by Bailey (2001), Freeman (2004), and Flanders (2010) this and several other similar prospects in the area are near the contact of a large Cretaceous biotite granodiorite pluton that intrudes a mixed unit of Paleozoic biotite gneiss, feldspar-biotite augen gneiss, and quartzite that forms a large gneiss dome. The granodiorite is cut by hornblende, andesite porphyry dikes. The rocks are intensely sheared locally and this and the nearby Gray Lead (BD017), Blue Lead (BD003), and Grizzly Bear (BD018) deposits are aligned along the northeast-trending Gray Lead fault. This shearing is observed in the underground workings and at the surface as saddle-like depressions across the spurs separating the westward-flowing tributaries of Tibbs Creek.
The lode deposits in the area are gold-bearing quartz veins in shear zones in the metamorphic and igneous rocks. The quartz veins contain gold +/- base metal +/- silver and a variable combination of sulfides, including arsenopyrite, covellite, digenite, jamesonite, pyrite, and stibnite. Typically, the gold content decreases as sulfides increase. The veins commonly are 2 to 3 feet in width but some are as wide as 8 feet (Thomas, 1970). When gold is present, it is usually extremely fine grained. However, veins at the nearby Grizzly Bear mine (BD018) contain relatively coarse gold, which is easily visible in hand specimen.
Thomas (1970) describes the Michigan Lode as a vein. Samples from vein quartz with a blue hue contain 0.10 ounce of gold per ton; some Fe-stained quartz sampled contained 0.42 ounce of gold per ton and 0.08 ounce of silver per ton. A sample containing cryptocrystalline quartz with a blue cast contained 8.76 ounces of gold per ton and 3.26 ounces of silver per ton (Thomas, 1970). Freeman (2004) and Flanders (2010) classified the Michigan Lode as a 'stockwork-shear deposit with anomalous bismuth, tellurium, and tungsten, in a porphyritic, intermediate to felsic intrusion that may be genetically related to the deposit'. Flanders (2010) cites grab samples of vein and altered granite that contain up to 28.83 ounces of gold per ton, up to 9.50 parts per million (ppm) silver, more than 1 percent arsenic, up to 0.83 ppm bismuth, up to 40 ppm lead, up to 776 ppm antimony, up to 0.30 ppm tellurium, and up to 0.05 ppm tungsten.
Geologic map unit (-144.200224475634, 64.3620350642036)
Mineral deposit model Silver-gold-quartz veins and stockwork, +/- arsenic, copper, lead, antimony, tellurium, and tungsten (Cox and Singer, 1986; model 22c).
Mineral deposit model number 22c
Age of mineralization Probably genetically related to a nearby Cretaceous granodiorite intrusive.
Alteration of deposit Alteration varies from flood silica to intense sericite alteration that leaves the host rock soft and easily broken with bare hands (Flanders, 2010).

Production and reserves

Workings or exploration
The Goodpaster region was first explored for placer gold in 1915. In the early 1930s, gold-bearing quartz veins were discovered in the upper Tibbs Creek area. By the winter of 1936, the first underground workings were begun. A 450-foot tunnel was driven following a small vein, termed the Blue Lead Extension. After disappointing results, the work was stopped. In the summer of 1936, five men drove a 300-foot tunnel at the outcrop of the Blue Lead vein (Reed, 1937). During the winter of 1937, a 300-foot tunnel was driven at the nearby Grizzly Bear Mine (BD018) and a 50-ton mill was constructed. In the summer of 1938, the mill was moved to the Blue Lead Mine and operated for a year and a half until the fall of 1939 (Joesting, 1938). The Michigan Lode was also probably discovered about this time. No ore has been mined from the Michigan Lode (Thomas, 1970).
From 1995 to 1999, The Stone Boy Joint Venture (Sumitomo Metal Mining and WGM) spent more than $1.3 million exploring in the area; they did extensive surface mapping and sampling, surface and airborne geophysics, and 16,215 feet of diamond drilling. In 2002, Freegold Ventures optioned a large block of claims that covered the Michigan Lode and several other nearby deposits that are described separately: the Grizzly Bear (BD003), Gray Lead (BD017), Wolverine (BD057), Upper Trench, Lower Trench (BD058) and O'Reely (BD059) and explored them as a unit under the name Rob. The work has included extensive mapping, surface sampling, and geochemical and geophysical surveys. Through 2004, Ventures spent more than $1 million exploring the Rob property and in 2004 commissioned a comprehensive NI 43-101 report (Freeman, 2004); an update was done in 2010 (Flanders, 2010). Freegold drilled 3 holes in 2011.
The 2011 drilling was the first drilling done at Michigan; 2,984 feet was drilled in order to evaluate the bulk tonnage potential of the prospect. Highlights of the results of 2011 drilling are 57 gpt gold for over five feet, and 7 gpt gold for over 11.5 feet (Avalon Development Corporation, 2014).
Indication of production None

References