Location: This prospect is at the south edge of the terminus of Herbert Glacier at an elevation of approximately 500 feet. It is in about the center of the S1/2 section 33, T. 38 S., R. 65 E. of the Copper River Meridian. The location is accurate.
Geology: The Herbert Glacier prospect was discovered in 1986 by Houston Oil and Minerals in outcrops recently exposed by retreating ice. Houston Oil and Minerals drilled the prospect in 1986 and Echo Bay Mines did additional drilling in 1988. The deposit consists of 2 parallel sets of east-northeast- trending quartz veins in quartz-diorite gneiss and andesitic metavolcanic rocks. The veins dip steeply to the north and are composed of quartz and subordinate calcite, and exhibit ribbon texture. The veins pinch and swell, varying in thickness from a few inches to 4 feet. The veins are in shear zones perpendicular to the regional foliation and are marked by slickensided cores. The quartz veins contain native gold, arsenopyrite, pyrite, galena, sphalerite, and scheelite. Fist-size clots of arsenopyrite form up to 20 percent of the principal veins and visible gold occurs in the footwalls of galena-rich parts of the veins. Potassic alteration in the footwalls extends several feet outward into the wallrock. There are two distinct sets of veins, called the north vein set and the south vein set. The south set can be traced for 1,200 feet along strike and 320 feet vertically. The north set is less extensive and contains less sulfide (Redman and others, 1989). U.S. Bureau of Mines samples of the north vein set contained up to 37.2 ppm gold, 186.7 ppm silver, greater than 1 percent lead, and 0.36 percent zinc. Samples of the south vein set contained up to 240.8 ppm gold, 126.9 ppm silver, greater than 1 percent lead, and 0.36 percent zinc (Redman and others, 1989). The Bureau collected a 240-pound metallurgical sample for analysis and beneficiation tests in 1988. A gravity separation test recovered 88.8 percent of the gold and 80.7 percent of the silver (Redman and others, 1989). This prospect is in the Juneau Gold Belt, which consists of more than 200 gold-quartz-vein deposits that have produced nearly 7 million ounces of gold. These gold-bearing mesothermal quartz vein systems form a zone 160 km long by 5 to 8 km wide along the western margin of the Coast Mountains. The vein systems are in or near shear zones adjacent to west-verging, mid-Cretaceous thrust faults. The veins are hosted by diverse, variably metamorphosed, sedimentary, volcanic, and intrusive rocks. From the Coast Mountains batholith westward, the host rocks include mixed metasedimentary and metavolcanic sequences of Carboniferous and older, Permian and Triassic, and Jurassic-Cretaceous age. The sequences are juxtaposed along mid-Cretaceous thrust faults (Miller and others, 1994). The sequences are intruded by mid-Cretaceous to middle Eocene plutons, mainly diorite, tonalite, granodiorite, quartz monzonite, and granite. Sheetlike tonalite plutons emplaced just east of the Juneau Gold Belt and undeformed granite and granodiorite bodies that are emplaced farther to the east are between 55 and 48 Ma (Gehrels and others, 1991). The structural grain of the belt is defined by northwest-striking, moderately to steeply northeast-dipping, penetrative foliation that developed between Cretaceous and Eocene time (Miller and others, 1994). The majority of the veins in the Juneau Gold Belt strike northwest. Isotopic dates indicate that the auriferous veins in the Juneau Gold Belt formed between 56 and 55 Ma (Miller and others, 1994; Goldfarb and others, 1997).
Workings: Houston Oil and Minerals drilled the prospect in 1986 and Echo Bay Mines undertook additional drilling in 1988.
Age: Isotopic dates indicate that the auriferous veins in the Juneau Gold Belt formed between 56 and 55 Ma (Miller and others, 1994; Goldfarb and others, 1997).
Alteration: Potassic alteration in the footwalls extends several feet outward into the wallrock.

Commodities (Major) - Ag, Au, Pb, Zn
Development Status: None
Deposit Model: Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a)

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Mineral List

8 valid minerals.