Golden Zone Mine, Valdez Creek Mining District, Matanuska-Susitna Borough, Alaska, USAi

Regional Level Types
Golden Zone Mine	Mine
Valdez Creek Mining District	Mining District
Matanuska-Susitna Borough	Borough
Alaska	State
USA	Country

Location: The Golden Zone mine is located at the headwaters of Bryn Mawr Creek, a tributary of the West Fork Chulitna River. The map site is at an elevation of about 3,500 feet, on the north bank of the creek, and about 2,000 feet southwest of the Golden Zone mine symbol on the topographic map. Access is via dirt road from Colorado Station along the north side of the West Fork Chulitna River to the junction with Bryn Mawr Creek. The road crosses the West Fork Chulitna River and climbs southwesterly to the mine site at an approximate elevation of 3,500 feet. The site is in the NW1/4 of sec. 3, T. 20 S., R. 11 W., of the Fairbanks Meridian. The location is accurate to within 1,000 feet.
Geology: The country rocks in the area of the Golden Zone mine include Devonian to Triassic clastic, carbonate, volcanic, and volcaniclastic strata, which are intruded by an Upper Cretaceous (70-65 Ma) biotite-quartz diorite porphyry plug (Swainbank and others, 1977). The Golden Zone is the most significant deposit in the southwest portion of the Healy quadrangle. The deposit consists mostly of a single breccia pipe that measures 250 by 300 feet at the surface and thins downward. The contacts are steeply dipping except on the northern side where the dips appear to flatten to the north. Drilling has confirmed that the pipe reaches a depth of 650 feet and possibly as deep as 1,500 feet. The pipe is almost entirely contained within a biotite-quartz diorite porphyry plug measuring 600 by 1,000 feet. The breccia pipe was produced by magmatic devolatilization either from the diorite porphyry or from a related intrusion at depth. Both the breccia and the porphyry have been dated at 65-70 Ma (Swainbank and others, 1977). Major faults in the area trend northeast-southwest and have imparted a strong northeast trending fabric to the host rocks. Differential motion along these faults has localized the subsequent emplacement of the biotite-quartz diorite porphyry and mineralizing fluids. Postmineral northwest-trending faults cut the breccia (unpublished report by Addwest Minerals International, Ltd, 1997). Early brecciation produced angular clasts cemented by vuggy, pale-gray quartz and minor amounts of arsenopyrite, pyrite, pyrrhotite, chalcopyrite and sphalerite. Several younger, sulfide-dominated events have overprinted the earlier, relatively barren, quartz-dominated brecciation, producing breccias containing over 10% sulfides. These younger breccias can be subdivided into arsenic-rich zones (earliest) and copper-rich zones (late); the former contains better gold grades - in places up to 5 ounces of gold per ton.
Workings: There are both surface and underground workings at the Golden Zone mine. Exploration between 1936 and 1996 included 54,326 feet of drilling in 137 drill holes (102 core and 35 reverse circulation holes). Numerous trenches and geochemistry samples have been collected. 1,900 feet of underground development has accumulated on the 100, 200 and 500 foot levels. Close-spaced helicopter aeromagnetic and EM geophysical surveys have been flown over the mine, along with some ground-based IP work (unpublished report by Addwest Minerals International Ltd., 1997). The State of Alaska sponsored an aeromagnetic survey in 1996 that included the mine site in 1996 (Burns, 1997). Sulfide-dominated events have overprinted the earlier, relatively barren, quartz-dominated brecciation, producing breccias containing over 10% sulfides. These younger breccias can be subdivided into arsenic-rich zones (earliest) and copper-rich zones (late); the former contains better gold grades - in places up to 5 ounces of gold per ton.
Age: The Golden Zone is a magma-driven breccia pipe. Both the intrusion and the breccia minerals are Late Cretaceous (70-65 Ma) in age (Swainbank and others, 1977).
Alteration: Alteration and mineral zonation is present around the Golden Zone pipe. A distal lead and zinc halo is reported up to 2 miles from the pipe; the gold/silver ratios in arsenopyrite and chalcopyrite show a systematic variation; and red-bed sediments show widespread bleaching (C.C. Hawley, oral communication, 1999). Propylitic haloes consisting of epidote-carbonate-chlorite are present at the margin of the quartz diorite stock, and hornfels-skarn zones are in carbonate-rich country rock. Sericite is the dominant alteration mineral in the breccias, and argillic overprinting (?), quartz flooding and Fe-carbonates are also reported, but their paragenesis is as yet unclear.
Production: The mine produced 49,169 grams of gold, 267,990 grams of silver, and 19 tonnes of copper between 1941 and 1942 (Hawley and Clark, 1974).
Reserves: The proven and probable reserves of the breccia pipe and surrounding prospect are 8 million tons containing 0.1 ounce of gold per ton (at a cutoff of 0.02 ounce of gold per ton), or about 800,000 ounces of gold.

Commodities (Major) - Ag, Au; (Minor) - Cu, Pb, Zn
Development Status: Yes; small
Deposit Model: Polymetallic vein and Au-Ag breccia pipe or Cu-Au porphyry (Cox and Singer, 198

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Link to USGS - Alaska:	HE043

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References