Grubstake

Prospect, Active

Commodities and mineralogy

Main commodities Au; Cu
Other commodities Fe; Pb; Zn
Ore minerals arsenopyrite; chalcopyrite; galena; magnetite; malachite; sphalerite
Gangue minerals barite; carbonate; quartz

Geographic location

Quadrangle map, 1:250,000-scale GU
Quadrangle map, 1:63,360-scale D-1
Latitude 62.7888
Longitude -144.0327
Nearby scientific data Find additional scientific data near this location
Location and accuracy The center of the Grubstake prospect is about 6.2 miles north-northwest of the community of Slana and about 0.5 mile west-northwest of peak 5509. It is about 0.6 mile east-southeast of the center of section 25, T. 12 N., R. 7 E., of the Copper River Meridian. The location is accurate within 1/4 mile.

Geologic setting

Geologic description

The upper Grubstake Creek area is underlain by highly altered, border-phase rocks of the Pennsylvanian to Permian, Ahtell Creek quartz monzonite pluton. The pluton intrudes the Pennsylvanian to Permian, Tetelna Formation, which is mainly volcanic rocks. Both the Ahtell pluton and the Tetelna Formation are cut by dikes and irregular bodies of a mid-Jurassic complex that varies from diorite to quartz diorite (Richter and Matson, 1968; Bull, Schneider, and Freeman, 1997; Bull, Freeman, and Schneider, 1999; Taylor 2010). Where the Tetelna rocks have been intruded by the diorite complex they have been converted to multicolored, biotite and hornblende hornfels.
Porphyry mineralization occurs mainly in the diorite complex and adjacent hornfels (Taylor, 2010). Various subtypes occur: 1) propylitic altered zones with as much as 10 percent pyrite are cut by quartz-carbonate veins that contain pyrite, chalcopyrite, and galena; 2) potassic altered zones containing secondary biotite and K-feldspar are cut by quartz-K feldspar veins, veinlets, and sporadic concentrations of as much as 15 percent pyrite, chalcopyrite, arsenopyrite, and magnetite; and 3) sericite/phyllic altered zones locally stained with malachite contain 5-30 percent sulfides. There are widespread localized zones of argillic and iron-carbonate alteration. The hornfelsed Tetelna Formation within about 100 feet of diorite also has several subtypes of porphyry mineralization including: 1) silicified zones containing as much as 15 percent pyrite; 2) quartz-carbonate veins with pyrite, chalcopyrite, arsenopyrite, and galena; and 3) chloritic-magnetite or chloritic-pyrite breccia cut by quartz-pyrite-chalcopyrite veinlets. The hornfels adjacent to chloritic zones typically is bleached. Reconnaissance mapping by Homestake Mining Company geologists (Bull, Scheider, and Freeman, 1997; Bull, Freeman, and Schneider, 1999; Taylor, 2010) indicates that the porphyry alteration and mineralization is crudely zoned, with potassic alteration at lower elevations and in upper Grubstake Creek. Discrete veins in the Grubstake area, such as the J. D. Lyons prospect (GU018), appear to be related to the hornfels phase of this porphyry deposit. As visualized by Taylor (2010), potassic alteration with potassium feldspar, biotite, and magnetite is nearest the mineralization. Outward, there is a shell of sodium-calcium alteration with albite, actinolite, and magnetite, followed by an outermost shell of propylitic alteration with pyrite, chlorite, epidote, and magnetite.
International Tower Hill Inc. staked a large block of claims in the area in 2006, carried out extensive geologic mapping, sampling, and geochemical surveys, and flew an aerial geophysical survey over the area. Corvus Gold Inc. (a spinoff of International Tower Hill) drilled 3 holes in 2010 that intersected extensive altered and veined rock (Corvus Gold, 2010). Notable intercepts in two of the holes were 199.3 meters with 0.08 percent copper and 0.06 grams of gold per tonne and 129.9 meters with 0.19 percent copper and 0.15 grams of gold per tonne. Taylor (2010) suggests that the mineralization is Cretaceous.
In 2010, Corvus (2011) discovered a series of en-echelon quartz-dolomite-barite veins 20 centimeters to 2 meters thick at Grubstake. This swarm of veins strikes north-northeast to northeast for a strike length of more than 600 feet. Nineteen rock samples averaged 7.38 grams of gold per tonne, 8.82 grams of silver per tonne, 0.91 percent lead, and 0.16 percent zinc; the best had 46.5 grams of gold per tonne, 29.8 grams of silver per tonne, 4.22 percent lead, and 1.22 percent zinc.
Geologic map unit (-144.034891741241, 62.7884103117813)
Mineral deposit model Porphyry Cu-Au (Cox and Singer, 1986; model 20c) and polymetallic barite veins.
Mineral deposit model number 20c
Age of mineralization Possibly related to the nearby Pennsylvanian to Permian Ahtell pluton or to other plutons, variously considered to be Jurassic or Cretaceous (Taylor, 2010).
Alteration of deposit Potassic alteration with potassium feldspar, biotite, and magnetite is nearest the mineralization. Succeeded by a shell of sodium-calcium alteration with albite, actinolite, and magnetite, followed by an outermost shell of propylitic alteration with pyrite, chlorite, epidote, and magnetite. Much hornfels locally (Taylor, 2010).

Production and reserves

Workings or exploration
Grubstake Creek has been explored extensively since the discovery of placer deposits in the area in 1934 (GU017). Work by Richter and his associates appears to have triggered exploration in the 1960s and 1970s (Richter, 1964,1966; Richter and Matson, 1968). Kirk Stanley was active at that time. More recently, the area has been studied by Cominco Exploration (St. George, 1992), and Homestake Mining Company (Bull, Schneider and Freeman, 1997; and Bull, Freeman, and Schneider, 1999). International Tower Hill Mines Inc. staked a large block of claims in the area in 2006, carried out extensive geologic mapping, sampling, and geochemical surveys, and flew an aerial geophysical survey over the area.
In 2007, based on favorable results in the southeast end of the belt, a helicopter magnetic survey was flown over the Grubstake target. In addition, an analysis of satellite imagery was undertaken in 2007 which produced an iron oxide and clay alteration map of the region outlining a number of new target areas which have yet to be explored.
In 2008 and 2009, ITH covered the bulk of the claim blocks with a stream sediment survey which identified multiple targets in the SE extension of the main Chisna claim block. In addition, a number of magnetic anomalies were investigated in the area southwest of the main claim blocks which returned copper mineralization from follow up prospecting and additional claims were staked.
Corvus took over the project from ITH in 2010 and will be managing the joint venture with Ocean Park (OCP). The Corvus-OCP joint venture is focused on testing the porphyry and precious metal potential of the Chisna belt using detailed airborne and ground geophysics to define drill targets which will be aggressively tested. A 2010 summer exploration program was carried out and included 2,926 meters (9600 feet) of diamond drilling, soil and rock sampling, airborne ZTEM surveying, and ground-based magnetotelluric (MT) with three-dimensional induced polarization chargeability-resistivity surveying and inversion modeling over selected target areas. Three of the holes drilled intersected extensive altered and mineralized rock (Corvus Gold, 2010).
In the summer 2012 exploration season, Corvus continued with surface prospecting and mapping. High-grade structurally controlled copper mineralization was found (Corvus Gold, 2014).
Indication of production None
Reserve estimates None.
Production notes None.

References

MRDS Number 10307479

References

Bull, Katharine, Freeman, Larry, and Schneider, Craig, 1999, Slana property summary report for Homestake Mining Company: 11 p., analytical appendices. (Report held by Ahtna Minerals Co., Anchorage, Alaska).
Bull, Katharine, Schneider, Craig, and Freeman, Larry, 1997, Summary report for Ahtna Corporation: 12 p., appendices, maps (Report held by Ahtna Minerals Co., Anchorage, Alaska).
Corvus Gold Inc. 2010, Corvus Gold Inc. announces initial drilling results from Grubstake target, Chisna coppe-gold project, Alaska: http://www.corvusgold.com/news/index.php?&content_id=38 (News release, Oct. 13, 2010).
Corvus Gold Inc., 2014, Chisna : http://www.corvusgold.com/projects/alaska/chisna/ (as of March 15, 2014).
St. George, Phil, 1992, 1992 Ahtna Lands; Gulkana quad reconnaissance: Cominco Alaska Exploration, 1 p., maps, scale 1:63,360. (Report held by Ahtna Minerals Co., Anchorage, Alaska).
Taylor, Chris, 2010, Technical report on the Chisna copper gold project, Chistochina mining district, south-central Alaska: Unpublished technical report for International Tower Hill Mines Ltd., 107 p. (posted on www.sedar.com, July 15, 2010).
Reporters W.T. Ellis (Alaska Earth Sciences), C.C. Hawley (Hawley Resource Group), and W.J. Nokleberg (U.S. Geological Survey); D.J. Grybeck (Contractor, USGS); V.C. Zinno (Alaska Earth Sciences, Inc.)
Last report date 5/30/2014