Upper Trench/ Lower Trench

Prospect, Active

Alternative names

Rob

Commodities and mineralogy

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

Geographic location

Quadrangle map, 1:250,000-scale BD
Quadrangle map, 1:63,360-scale B-1
Latitude 64.3747
Longitude -144.2168
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Upper Trench/ Lower Trench prospect is on the ridge south of Wolverine Creek, about 2.5 miles north of the north peak '5080' of Black Mountain. It is at an elevation of about 3,300 feet about 0.4 mile south-southeast of the center of section 20, T. 6 S., R. 18 E., of the Fairbanks Meridian. The location is accurate.

Geologic setting

Geologic description

Although several mines and prospects have been known since the 1930s within a mile of this prospect, the Upper Trench/ Lower Trench prospect was first described by Freeman in 2004. From 1995 to 1999, The Stone Boy Joint Venture (Sumitomo Metal Mining and WGM Inc.) 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 but their location is vague. They probably drilled one or more holes at the Upper Trench/Lower Trench prospect and did considerable trenching.
In 2002, Freegold Ventures optioned a large block of claims that covered this prospect and several other nearby deposits that are described separately, the Grizzly Bear (BD003), Michigan Lode (BD025), and Wolverine (BD057). Freegold Ventures explored them as a unit under the name Rob. They continued the work through early 2012 (Freegold Ventures Ltd., 2012). The work has included extensive geologic mapping, surface sampling, and geochemical and geophysical surveys. Through 2004, Freegold spent more than $1 million exploring the Rob property and in 2004 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) and Freeman (2004), this and the several other similar prospects in the area are near the contact of a large Cretaceous granodiorite stock that intrudes a large dome of Paleozoic biotite gneiss, feldspar-biotite augen gneiss, and quartzite. Locally the granodiorite is cut by hornblende andesite-porphyry dikes. The rocks are locally intensely sheared and this and the nearby Michigan Lode (BD025), Blue Lead (BD003), and Grizzly Bear (BD018) deposits are aligned along the northeast-trending Gray Lead fault. The Upper Trench/Lower Trench prospect is entirely within a Cretaceous granodiorite pluton which is cut by several Cretaceous andesite porphyry dikes.
The Upper Trench/Lower Trench prospect is probably similar to the other mines and prospects in the immediate area in that is is a gold -quartz vein along a shear zone. The quartz veins typically contain gold +/- base metal +/- silver and a variable combination of sulfides, including arsenopyrite, jamesonite, pyrite, and stibnite. The veins are commonly 2 to 3 feet in width but some are as wide as 8 feet (Thomas, 1970). There is little specific information on this prospect but Freeman (2004) and Flanders (2010) cite grab samples in the trenches that contain up to 2.04 ounces of gold per ton, 43.6 parts per million (ppm) silver, 1.35 percent arsenic and 17.6 percent antimony. Verification samples collected in 2002 contained up to 22.29 grams of gold per ton, up to 15.75 ppm silver, more than 10,000 ppm arsenic, up to 11.25 ppm bismuth, up to 1,925 ppm lead, more than 10,000 ppm antimony, and up to 1.0 ppm tellurium. Freeman (2004) and Flanders classifies the deposit as 'stockwork-shear style mineralization hosted in porphyritic intermediate to felsic intrusive rocks'.
Geologic map unit (-144.219125225407, 64.3743346085521)
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 pluton.
Alteration of deposit Not specified.

Production and reserves

Workings or exploration Although several mines and prospects have been known within a mile of this prospect since the 1930s, the Upper Trench/Lower Trench prospect was first publicly described by Freeman in 2004. From 1995 to 1999, The Stone Boy Joint Venture (Sumitomo Metal Mining and WGM Inc.) 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 this prospect and several other nearby deposits that are described separately: the Grizzly Bear (BD003), Michigan Lode (BD025), and Wolverine (BD057). Freegold Ventures explored them as a unit under the name Rob. The work has included extensive geologic mapping, surface sampling, and geochemical and geophysical surveys. Freegold continued their exploration into early 2012 (Flanders, 2010; Freegold Ventures Inc., 2012).
Indication of production None

References