Vinasale Prospect, McGrath District, Yukon-Koyukuk Borough, Alaska, USA
|Latitude & Longitude (WGS84):||62° 42' 36'' North , 155° 41' 23'' West|
|Latitude & Longitude (decimal):||62.71, -155.689722222|
Classified as a peraluminous granite porphyry-hosted gold-polymetallic deposit, an important deposit type in the Kuskokwim Mineral Belt (Bundtzen and Miller, 1997).
Location: The Vinasale deposit is located about 2,000 feet (610 m) from the summit of Vinasale Mountain at an elevation of 1,520 feet (463 m) in the NW1/4 sec. 8, T. 30 N., R. 34 W., of the Seward Meridian. The Vinasale deposit is about 1.5 kilometers due east of the Kuskokwim River and 29 kilometers (18 air miles) south of McGrath, Alaska. The reporter visited the site in 1977 and 1992.
Geology: Vinasale Mountain is underlain by a composite intrusive complex of Late Cretaceous age (Bundtzen, 1986; DiMarchi, 1993). The intrusive suite includes peraluminous, porphyritic quartz monzonite, rhyolite porphyry, shonkinite, and monzonite breccia. The intrusive phases cut and thermally alter clastic rocks of the early Late Cretaceous, Kuskokwim Group flysch (Bundtzen and Miller, 1997). The stock has yielded a K-Ar age of 69 Ma (Solie and others, 1991). Much of the following is based on work summarized by DiMarchi (1993) and Bundtzen and Miller (1997). During industry exploration work conducted from 1989 to 1991, (DiMarchi, 1993), large areas of coincident multi-element (Au, As, Sb, Pb, Mo) soil anomalies were found in three distinct zones named the Central, Northeast, and South zones surrounding the summit of Vinasale Mountain. The Central zone is approximately 457m by 610 m in area and contains maximum values in soil of 2,470 ppb Au. The Northeast and South Zones were similar in total area but contained weaker maximum soil anomalies of 185 ppb Au and 335 ppb Au respectively. The soil anomalies led to the subsurface exploration work that discovered significant Au-polymetallic mineralization. Silicification occurs in veins, segregations and silica flooded zones, and is frequently accompanied by sulfides. Sericite alteration in the Central Zone is characterized by progressive replacement of biotite and plagioclase by sericite and dolomite. and of K-feldspar by sericite, quartz, and dolomite. Propylitic alteration forms a broad halo around zones containing silica and sericite alteration, and is characterized by replacement of biotite by chlorite, and plagioclase by epidote, chlorite, and calcite. 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 lithologies. Subordinate veinlets and veins host coarse-grained pyrite, galena, arsenopyrite, sphalerite, stibnite, and jamesonite, and microscopic native silver and very minor gold. Practically all of the gold values appear to be concentrated in lattice structures of arsenopyrite, pyrite, and other sulfide minerals. The Northeast and South Zones contain similar but weaker manifestations of Au-polymetallic mineralization than that displayed in the Central Zone. The two longest drill intercepts from the Central Zone contain 63.1 m of 2.42 grams/tonne gold and 71.9 m containing 2.27 grams/tonne gold. The Central Zone is enriched in antimony (average=338 ppm), zinc (average=108 ppm), lead (average=75 ppm), bismuth (3 ppm), and silver (average=0.7 ppm). The gold/silver ratio averages about 2:1 in mineralized samples. Arsenic is most concentrated in the southern Central Zone where it averages 0.80 percent. High barium (average=500 ppm) occurs in the Northeast Zone. Based on 11,260 m of diamond drilling, the Vinasale Deposit is estimated to contain 10.3 million tonnes grading 2.40 g/tonne gold or about 24,540 kg (789,000 ounces) gold (Bundtzen and Miller, 1997).
Workings: Bundtzen (1986) first described the mineralized Vinasale pluton and associated gold-scheelite-bismuth vein mineralization on the south flank of Vinasale Mountain, and suggested that the lode mineralization was the source of placer gold mined in Alder Gulch. During 1990, Placer Dome U.S. Inc. and operator Central Alaska Gold Mining Company discovered significant gold anomalies in soils in the Central, Northeast, and South Zones, and initiated a diamond drill program, concentrating on the Central Zone (DiMarchi, 1993). By the end of 1991, a 5,182 m diamond drill program indicated that 11,566,000 tonnes of ore contained about 31,100 kg (1 million ounces) gold (Bundtzen and others, 1992). With additional work completed in 1992 and 1993, a revised resource estimate for the Vinasale deposit, based on a total of 11,260 m of subsurface drilling, is 10.3 million tonnes grading 2.4 g/tonne gold for a total in-place reserve of 24,540 kg (789,000 ounces) gold (J. DiMarchi, written communication, 1994; Bundtzen and Miller, 1997). An additional 6,562 kg (211,000 ounces) gold is regarded as an in-place resource.
Age: 68.0 Ma from sericite and 69.0 Ma from apatite, both from Central Zone mineralization.
Alteration: Silicification and dolomitic, sericitic, and propylitic.
Reserves: Drill-indicated gold resources are 24,540 kg (789,000 ounces); an additional 6,562 kg (211,000 ounces) gold in an inferred resource category. Byproduct metal content (Ag, Bi) has not been determined.
Commodities (Major) - Ag, Au; (Minor) - As, Bi, Mo, Pb, Sb, Zn
Development Status: No
Deposit Model: Porphyry gold-polymetallic stockwork (Cox and Singer, 1986; model 20c).
12 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
2.588 - 23.03 Ma
|Supracrustal; Sedimentary and/or volcanic: undivided|
Age: Neogene (2.588 - 23.03 Ma)
Description: Interior western Alaska, Yukon-Koyukuk Basin
Comments: Sedimentary basin; Gordey & Makepeace, 2003; Wilson & Hults, unpublished compilation, 2007-08
Reference: J.C. Harrison, M.R. St-Onge, O.V. Petrov, S.I. Strelnikov, B.G. Lopatin, F.H. Wilson, S. Tella, D. Paul, T. Lynds, S.P. Shokalsky, C.K. Hults, S. Bergman, H.F. Jepsen, and A. Solli. Geological map of the Arctic. doi:10.4095/287868. Geological Survey of Canada Map 2159A. 
|Maastrichtian - Tertiary|
|Felsic granitic rocks|