Unnamed Prospect (ARDF - GU020; in upper Slope Creek), Chistochina Mining District, Valdez-Cordova Census Area, Alaska, USAi

Regional Level Types
Unnamed Prospect (ARDF - GU020; in upper Slope Creek)	- not defined -
Chistochina Mining District	Mining District
Valdez-Cordova Census Area	Census Area
Alaska	State
USA	Country

Upper Slope Creek contains part of a copper-gold porphyry that has only been examined in reconnaissance. Additional information can be obtained from Ahtna Minerals in Anchorage, Alaska.
Location: A copper-gold porphyry-type is exposed in upper Slope Creek. The center of the deposit is approximately 0.2 miles north of the center of section 25, T. 12 N., R. 7 E., Copper River Meridian. The deposit is approximately one-half mile long by one-third mile wide and is probably continuous with a similar porphyry system in upper Grubstake Creek (GU019). The location is approximate. It is upstream from a gold placer in Slope Creek (GU021).
Geology: The upper part of Slope Creek consists of highly altered volcanic rocks of the Pennsylvanian to Permian, Tetelna Formation. The Tetelna Formation is cut by by composite dikes and irregular bodies of a mid-Jurassic diorite to quartz diorite complex (Richter and Matson, 1968; Bull, Schneider, and Freeman, 1997; Bull, Freeman, and Schneider, 1999). Locally the diorite bodies trend north-northwest along the general regional trend. One diorite body is elongated east-northeast along a fault zone which appears to follow the north side of Slope Creek. The intensity of the mineralization and the distribution of the main masses of diorite indicate that a porphyry appears to underlie upper Slope Creek. 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 with up to 15% pyrite, chalcopyrite, arsenopyrite, and magnetite; and 3) sericite/phyllic altered zones locally stained with malachite contain 5-30 % sulfides. There are widespread local 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. The porphyry in upper Slope Creek has not been tested by drilling, but it has been mapped and sampled in reconnaissance by geologists affiliated with Homestake Mining Company. Soils collected by Homestake suggest that a broad area that covers both upper Grubstake and upper Slope Creeks contains more than 200 ppb gold. Eighty-four rock samples collected mostly in the Grubstake Creek-Slope Creek area contained a mean gold concentration of about 240 ppb gold. Rock samples collected in upper Slope Creek locally contained more than 2,000 ppb gold. Rock samples in the upper Slope Creek-upper Grubstake Creek area averaged about 350 ppm copper and locally exceeded 1%. The rocks in the area are so intensely deformed and altered that much more detailed work will be needed to accurately map the complex.
Workings: Slope Creek has been explored since the discovery of placer deposits nearby in Grubstake Creek in 1934. That discovery led to prospecting and the location of small placer mines in Boulder Creek (GU017) and Slope Creek (GU021). 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 the Ahtell and Grubstake Creek-Slope Creek areas. In more recent years the area has been studied by 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 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:	GU020

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References