|Quadrangle map, 1:250,000-scale||JU|
|Quadrangle map, 1:63,360-scale||C-3|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||This prospect is approximately 18.7 miles northeast of Juneau at the south edge of the terminus of Herbert Glacier at an elevation of approximately 500 feet. It is about 2.3 miles northeast of the center of Windfall Lake and about 0.5 mile south-southeast of the center of section 33, T. 38 S., R. 65 E., of the Copper River Meridian. The location site is centered in the area of drilling between 2010 and 2012 along where the Main vein outcrops.|
The Herbert Glacier prospect is near the Coastal Shear Zone that parallels the boundary between the Gravina belt to the west and the Taku terrane to the east. The Gravina belt comprises Upper Jurassic to Mid-Cretaceous marine argillite and greywacke, interbedded andesite to basaltic volcanic and volcaniclastic rocks, and plutons ranging from quartz diorite to peridotite (Gehrels and Berg, 1994). The Taku terrane contains older Permian to Triassic age marble, phyllite, pillow basalt, and flysch-related rocks, which are overlain by Upper Jurassic to Mid-Cretaceous greywacke and, likely, related to similar age rocks in the Gravina belt. Metamorphic grade ranges from greenschist to amphibolite facies and generally increases from west to east. Regional metamorphism and deformation, including the Coastal Shear Zone, are broadly linked to emplacement of multiple intrusive rocks in the Coast Mountains that have isotopic ages ranging from 10 to 55 Ma (Gehrels and Berg, 1994).
The Herbert Glacier prospect was discovered in 1986 by Houston Oil and Minerals in outcrops recently exposed by retreating ice (Redman and others, 1989). The deposit consists of five east-trending veins; from north to south these are the North vein, the Goat Creek vein, the Main vein, the Deep Trench vein, and the Floyd vein (Grande Portage Resources Ltd., 2010a; Hale and Hawley, 2011). The veins are in a northwest-trending, fault-bounded band of hornblende-biotite quartz diorite about 800 meters wide. The diorite is bounded to the northeast by gneiss and to the southwest by low-grade metasedimentary rocks.
The veins dip steeply to the north, are composed of quartz and subordinate calcite, and exhibit ribbon texture. They pinch and swell and vary from a few inches to 4 feet thick. The veins are in shear zones perpendicular to the regional foliation, are marked by slickensided cores, and contain native gold, arsenopyrite, pyrite, galena, sphalerite, and scheelite. Fist-size clots of arsenopyrite form up to 20 percent of the principal veins and visible gold occurs in the footwalls of galena-rich parts of the veins. Potassic alteration in the footwalls extends several feet into the wallrock. U.S. Bureau of Mines samples contained up to 37.2 parts per million (ppm) gold, 186.7 ppm silver, more than 1 percent lead, and 0.36 percent zinc (Redman and others, 1989). Samples of the south set of veins contained up to 240.8 ppm gold, 126.9 ppm silver, more than 1 percent lead, and 0.36 percent zinc. The Bureau of Mines collected a 240-pound metallurgical sample for analysis and beneficiation tests in 1988. A gravity separation test recovered 88.8 percent of the gold and 80.7 percent of the silver (Redman and others, 1989).In July 2016, Grande Portage Resources Ltd. announced it entered into an agreement, through its wholly-owned subsidiary GPG Alaska Resources Inc., with its’ joint-venture partner Quaterra Alaska, Inc., the wholly-owned subsidiary of Quaterra Resources Inc., to acquire Quaterra's 35 percent participating interest in the Herbert Gold project (Athey and Werdon, 2017).
|Geologic map unit||(-134.687288113328, 58.5305747396605)|
|Mineral deposit model||Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a).|
|Mineral deposit model number||36a|
|Age of mineralization||Isotopic dates indicate that the auriferous veins in the Juneau Gold Belt formed between 56 and 55 Ma (Miller and others, 1994; Goldfarb and others, 1997).|
|Alteration of deposit||Potassic alteration in the footwalls extends several feet into the wallrock (Redman and others, 1989). Sericite, chlorite, and carbonate-altered quartz diorite is observed up to 1 meter adjacent to veining (DuPre and Webb, 2013).|
|Workings or exploration||
Tenneco Minerals and its successor Echo Bay Mining Mining Company drilled at least 7 shallow holes from 1986 to 1988. Some notable intercepts were 1 foot that contained 3.85 ounces of gold per ton, 14 feet that contained 0.98 ounce of gold per ton, and 1 foot that contained 12.64 ounces of gold per ton (Grand Portage Resources Ltd., 2010a).
Grande Portage Resources Ltd. began exploring the Herbert Glacier deposit under an agreement with Quaterra Resources in June of 2010 (Grande Portage Resources Ltd., 2010a). In 2010, they drilled 19 holes in several veins (Grande Portage Resources Ltd., 2010b, 2010c). Some of the notable intercepts were 1.85 meters with 4.012 grams per tonne (g/t) gold; 1.51 meters with 4.84 g/t gold; 0.44 meter with 9.91 g/t gold; 1.14 meters with 17.1 g/t gold; and 0.71 meter with 11.35 g/t gold.
Grande Portage drilled 31 more holes in 2011 on the Goat Creek vein, the Ridge vein, the Main vein, and the Trench vein, and dug 4 trenches (Hale and Hawley, 2011; Grande Portage Resources Ltd., 2011). The tenor of the mineralization on the Main vein was confirmed. Some notable intercepts on the Deep Trench vein were 3.52 meters that averaged 11.53 g/t gold; 4.6 meters that averaged 4.18 g/t gold, and 1.36 meters that averaged 9.26 g/t gold.In 2012 Grande Portage Resources Ltd. drilled 62 more holes for a total of 8,805 meters (DuPre and Webb, 2013). Highlights of some of the assay results from drilling include 13.23 meters at 4.04 g/t gold, 2.05 meters at 79.41 g/t gold, 1.13 meters at 48.12 g/t gold, and 2.12 meters at 11.72 g/t gold (Grande Portage Resources Ltd., 2012a). The best intercept occurred over 0.71 meter and contained 428 g/t gold. The 2012 drilling help delineate mineralization along strike of the Deep Trench vein for over 400 meters (Grande Portage Resources Ltd., 2012b).
|Indication of production||None|
In 2013 DuPre and Webb completed an updated resource estimate for Herbert Glacier. The resource is based on eight veins, uses 2 grams per tonne (g/t) for a cut-off, and assumes an average gold price of $1,500 per ounce. The inferred resource is 51,611 tonnes grading 7.73 g/t gold yielding 12,819 ounces of gold and the indicated resource is 821,100 tonnes grading 6.91 g/t gold yielding 182,400 ounces of gold (DuPre and Webb, 2013).As recorded in their NI 43-101 technical report of April 10, 2013, the Herbert Gold project contains an indicated resource of 821,100 tonnes containing 182,400 ounces of gold at 6.91 grams of gold per tonne. Additionally, inferred resources include 51,600 tonnes containing 12,800 ounces of gold at 7.73 grams of gold per tonne (DuPre and Webb, 2013). Drilling has intersected the mineralized vein over a strike length of 370 meters and to a depth of more than 200 m. The vein can be traced at surface over a strike length of 1,000 meters (Athey and Werdon, 2017).
|Reporters||J.C. Barnett and L.D. Miller (Juneau, Alaska ); D.J. Grybeck (Contractor, USGS); N.V. King (Alaska Earth Sciences); M.B. Werdon (DGGS)|
|Last report date||8/26/2017|