|Quadrangle map, 1:250,000-scale||MG|
|Quadrangle map, 1:63,360-scale||C-6|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||The Vinasale prospect is near the top of Vinsale Mountain directly east of the Kuskokwim River about 18 miles south of McGrath. The location is centered on the area of a calculated resource of the Central zone near the western edge of section 8, T. 30 N., R. 34 W, of the Seward Meridian.|
Vinasale Mountain is underlain by a nearly circular composite, intrusive complex, about 2,500 meters in diameter (Bundtzen, 1986; DiMarchi, 1993; Adams and Giroux, 2011). The intrusive complex includes peraluminous, porphyritic quartz monzonite, rhyolite porphyry, shonkinite, and monzonite breccia. The intrusive phases cut and thermally alter flysch of the Late Cretaceous, Kuskokwim Group (Bundtzen and Miller, 1997). The stock has been dated by K-Ar methods at 69 Ma (Solie and others, 1991). The intrusive is marked by a magnetic high centered over Vinasale Mountain and a ring-shaped magnetic low that coincides with hornfels at the periphery of the intrusive complex.
During industry exploration work from 1989 to 1991 (DiMarchi, 1993), large coincident soil anomalies in gold, arsenic, antimony, lead, and molybdenum were found in three distinct zones named the Central, Northeast, and South zones on the top of Vinasale Mountain. The Central zone is approximately 457 by 610 meters in size and contains up to 2,470 parts per billion (ppb) gold. The Northeast and South zones contained up to 185 ppb and 335 ppb gold, respectively. The soil anomalies resulted in drilling that discovered significant gold-polymetallic mineralization.
The most common alteration types associated with gold mineralization consist of sericite-dolomite alteration and silicification. Strong dolomite alteration is generally indicative of higher gold values. Sericite-dolomite alteration occurs primarily as replacements of feldspars either in broad zones or in centimeter-scale alteration envelopes adjacent to sulfide-quartz veins. Silicification most often occurs as stockwork zones and veins up to 4 cm wide, although irregular quartz segregations and zones of pervasive flood silica can also be found locally. Broad zones of light-green sericite alteration are found adjacent to some zones of intense silicification. Weak propylitic alteration (mostly as incipient chlorite alteration of biotite) is widespread, generally occurring distal to gold mineralization relative to sericite-dolomite alteration and silicification. Minor amounts of chlorite or tourmaline alteration are found locally, and are typically associated with late shears and thin, late stage quartz veins (Abrams and Giroux, 2013).The mineralization consists of disseminated sulfides and sulfide-quartz veins and veinlets. Sulfide mineralization in the Central zone consists of disseminated pyrite and arsenopyrite in areas of silica flooding in quartz monzonite, sericitic alteration in monzonite breccias, and quartz-dolomite veins and segregations in all intrusive rocks. Subordinate veinlets and veins contain coarsely crystalline pyrite, galena, arsenopyrite, sphalerite, stibnite, pyrargyrite, and jamesonite, and microscopic native silver and minor gold (DiMarchi, 1993). The Northeast and South zones contain gold mineralization similar to but weaker than that in the Central zone.
|Geologic map unit||(-155.693544289749, 62.7101631381497)|
|Mineral deposit model||Intrusion-related gold (IRG) (McCoy and others, 1997).|
|Age of mineralization||Hydrothermal sericite from the Central zone has been dated at 68.0 +/- 1.7 Ma (DiMarchi, 1993).|
|Alteration of deposit||Sericite-dolomite alteration and silicification are the main forms of alteration observed with mineralization. Weak propylitic alteration occurs more distal to mineralization (Abrams and Giroux, 2013).|
|Workings or exploration||
In 1990, Placer Dome U.S. Inc. and Central Alaska Gold Mining Company initiated a diamond drill program, concentrating on the Central zone. They drilled 65 holes that totaled 11,260 meters through 1993 (DiMarchi, 1993). The two longest auriferous intercepts in the Central zone were 63.1 meters with 2.42 grams per tonne (g/t) gold and 71.9 meters with 2.27 g/t gold. Those intercepts averaged 338 parts per million (ppm) antimony, 108 ppm zinc, 5 ppm lead, 3 ppm bismuth, and 0.7 ppm silver. In mineralized core, the gold/silver ratio is about 2:1. Arsenic is most concentrated in the southern Central zone where it averages 0.80 percent. The Northeast zone averaged about 500 ppm barium.
ASA-Montague leased the property from Doyon, Limited in 1994, and collected 771 soil samples, 20 rock samples and drilled 26 core holes (5262.4 meters). Most of this work was focused on the Central zone to upgrade the quality of the resource but some drill holes tested the outlying geophysical and soil anomalies. In 1995 and 1996 ASA-Montague did metallurgical testing, an environmental and permitting review, pre-feasibility study, and a resource estimate (Abrams and Giroux, 2013).
In 2007, Freegold Ventures Limited (Freegold) (2007) signed an agreement with Doyon, Limited to explore the property. Their 2007 and 2008 work included staking an additional 12,000 acres northeast of the previous claims that covered about 128,000 acres, a rock and and stream sediment sampling program, an aerial geophysical survey over the property, and an induced polarization (IP) survey (Adams and Giroux, 2011).
Freegold drilled 6 holes totaling 6,396 feet in 2010 with the aim at expanding the historical resources as well as testing weaker geophysical anomalies to the northeast. Exploration by Freegold during 2011 was entirely focused on the Central zone. The program was aimed at expanding and upgrading the current NI 43-101 resource. This work included a diamond core drilling program and an IP/resistivity geophysical survey. The geophysical survey indicates that in addition to the Central zone where most of the drilling has been conducted that a significant geophysical anomaly is defined in the Northeast zone where limited drilling has indicated mineralization of similar character to the Central zone (Abrams and Giroux, 2013).In 2012 Freegold drilled 13 holes, 5 at the Northeast zone, 6 at the Central zone, and 2 between these two zones for a total of 3,425 meters. Highlights from the drilling include 85.3 meters at 1.49 g/t gold, 29.3 meters at 1.63 g/t gold, 12.2 meters at 1.34 g/t gold, 29.3 meters at 1.19 g/t gold, 9.1 meters at 1.10 g/t gold, and 33.7 meters at 1.09 g/t gold (Abrams and Giroux, 2013).
|Indication of production||None|
Adams and Giroux (2011) calculated the inferred resource of the Central zone at 15 levels by varying the cut-off grade from 0.10 to 2.00 grams per tonne (g/t) gold. At a cut-off grade of 0.5 g/t gold, the Central zone has an inferred resource of 37.260 million tonnes with a grade of 1.11 g/t gold (or 1,331,000 ounces of gold); at a cut-off grade of 1.0 g/t gold, it has an inferred resource of 17.070 million tonnes with a grade of 1.57 g/t gold (or 859,000 ounces of gold).The resource at Vinasale was updated again by Abrams and Giroux (2013). At a 0.5 g/t gold cut-off there is an indicated resource of 3.41 million tonnes averaging 1.48 g/t gold (162,000 ounces of gold) and an inferred resource of 50.2 million tonnes averaging 1.06 g/t gold (1,703,000 ounces of gold).
Additional commentsThe Vinasale prospect is located within Doyon, Limited conveyed land. For more information contact Doyon, Limited, Fairbanks, Alaska.
|Reporters||T.K. Bundtzen (Pacific Rim Geological Consulting); D.J. Grybeck (Contractor, USGS); N.V. King (Alaska Earth Sciences)|
|Last report date||2/25/2016|