Unnamed Prospect (ARDF - GU019; in headwaters of Grubstake Creek), Chistochina Mining District, Valdez-Cordova Census Area, Alaska, USAi

Regional Level Types
Unnamed Prospect (ARDF - GU019; in headwaters of Grubstake Creek)	- not defined -
Chistochina Mining District	Mining District
Valdez-Cordova Census Area	Census Area
Alaska	State
USA	Country

Upper Grubstake Gulch contains part of a copper-gold porphyry that only has been examined in reconnaissance. Additional information can be obtained from Ahtna Minerals in Anchorage, Alaska.
Location: A copper-gold porphyry is exposed in upper Grubstake Creek. The location is the approximate of the porphyry system. It is about two-thirds of the way south along the section line between 25 and 26, T. 12 N., R. 7 E., Copper River Meridian. The deposit is approximately three-quarters of a mile long by one-half mile wide and is probably continuous with a similar porphyry in upper Slope Creek (GU020). The location is accurate.
Geology: The upper part of Grubstake Creek consists mainly of highly altered, border phase rocks of the Pennsylvanian to Permian, Ahtell Creek quartz monzonite. The Ahtell Creek rocks are intruded into the Pennsylvanian to Permian,Tetelna Formation which is mainly volcanic rocks. Both the Ahtell pluton and the Tetelna Formation are cut by composite 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). Where the Tetelna rocks have been intruded by the diorite complex they have been converted to multicolored, biotite and hornblende hornfels. Porphyry-type mineralization occurs mainly in the diorite complex and adjacent hornfels. Various subtypes occur: 1) propylitic altered zones with as much as 10 % 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 disseminated concentrations of as much as 15% pyrite, chalcopyrite, arsenopyrite, and magnetite; and 3) sericite/phyllic altered zones locally stained with malachite contain 5-30 % 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% 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 is typically bleached. Based on reconnaissance mapping by Homestake Mining Company geologists (Bull, Scheider, and Freeman, 1997; Bull, Freeman, and Schneider, 1999) the porphyry alteration and mineralization is crudely zoned, with potassic alteration occurring 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. The rocks in the area are deformed and so extensively altered that much more detailed work will be needed to accurately map the complex. The porphyry in upper Grubstake has not been tested by drilling, but it has been mapped and sampled by Cominco geologists (St.George, 1992) as well as the geologists affiliated with Homestake Mining Company. St. George found soils that are highly anomalous in gold along reconnaissance lines through upper Grubstake Creek. Soils collected by Homestake Mining Company indicate that a broad area that covers much of upper Grubstake and upper Slope Creeks contains more than 200 ppb gold. Eighty-four rock samples collected mostly in the Grubstake area contained a mean gold concentration of about 240 ppb gold; copper in these samples averaged about 350 ppm, and locally exceeded 1%.
Workings: Grubstake Creek has been explored extensively since the discovery of viable placer deposits in 1934 (GU017). Work by Richter and his associates appears to have triggered exploration in the 1960's and 1970's (Richter, 1964 and 1966; Richter and Matson, 1968). Kirk Stanley was active at this time. In more recent years 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). Ahtna Minerals Company has recently consolidated the geologic data as much of the prospect area lies on Ahtna regional Native corporation land.
Age: The porphyry deposit is probably related to the emplacement of a Jurassic diorite complex.
Alteration: Potassic alteration with the introduction of K-feldspar and biotite occurs in the core of the system. Locally the rocks are sericitized and phyllically altered. Higher in the system, the rocks are silicified, altered to clay, and carbonatized. Bull and her co-authors (1997 and 1999) indicate that galena is more abundant distally.

Commodities (Major) - Au, Cu; (Minor) - Fe, Pb, Zn
Development Status: None
Deposit Model: Porphyry Cu-Au (Cox and Singer, 1986; model 20c)

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Link to USGS - Alaska:	GU019

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References