Big Four

Mine, Probably inactive

Commodities and mineralogy

Main commodities Au
Other commodities Pb; Zn
Ore minerals arsenopyrite; galena; gold; limonite; pyrite; pyrrhotite; sphalerite
Gangue minerals carbonate minerals; chlorite; quartz; white mica

Geographic location

Quadrangle map, 1:250,000-scale VA
Quadrangle map, 1:63,360-scale A-7
Latitude 61.2213
Longitude -146.3615
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Big Four mine is locality 22 of Cobb and Matson (1972) and locality 19 of Winkler and others (1981 [OFR 80-892-B]). It is south of and below Johnson Glacier at an elevation of 3,200 feet, 1.1 miles west-northwest of Smith Mill on Mineral Creek. However, the USGS Valdez A-7 topographic map (1996) shows this location, marked by adit symbols, to be the Hercules mine. This record follows Cobb and Matson (1972). See also the Hercules mine (VA127).

Geologic setting

Geologic description

An iron-stained, gold-bearing quartz vein cuts metaflysch of the Valdez Group at this locality (Brooks, 1912). The vein is as much as 5 feet thick, trends northeast, dips steeply north, and is locally offset by northerly trending faults. It contains free gold, pyrite, galena, and sphalerite (Johnson, 1915). This vein was traced for at least 200 feet; it was developed by at least two adits with a total length of 200 feet by 1914 when a mill was installed at the mine. Additional work continued intermittently to 1939 (Cobb, 1972 [OFR 79-1241]); the reported production was 846 ounces of gold and 371 ounces of silver (Jansons and others, 1984). Four U.S. Bureau of Mines grab samples contained from less than 0.03 ppm gold to 3.98 ounces of gold per ton and a trace to 1.0 ounce silver per ton (Jansons and others, 1984).
This vein is probably similar to other gold-bearing quartz veins cutting metaflysch of the Valdez Group in the southern Valdez quadrangle. Data summarized by Goldfarb and others (1997) show that gold-bearing quartz veins in the Valdez Group commonly contain pyrite, arsenopyrite, carbonate minerals, chlorite, and white mica and formed from water-rich fluids with 5 to 15 mole percent CO2 and significant amounts of CH4, N2, and H2S. The vein-forming fluid salinities were less than 8 percent, vein formation temperatures ranged from 225 to 375 degrees centigrade, and emplacement depths varied from 3 to 10 kilometers. The vein-forming fluids were produced by metamorphic devolatilization reactions. Radiometric dating indicates that the veins formed from 57 to 49 Ma (Goldfarb and others, 1997, p. 171), when deep parts of the accreted Valdez Group flysch underwent high-grade metamorphism and partial melting (Hudson, 1994).
Geologic map unit (-146.363553939765, 61.2207813164468)
Mineral deposit model Low-sulfide Au-quartz veins (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization Radiometric dating indicates that the gold-bearing quartz veins in the Valdez Group formed from 57 to 49 Ma (Goldfarb and others, 1997, p. 171), when deep parts of the accreted Valdez Group flysch underwent high-grade metamorphism and partial melting (Hudson, 1994).
Alteration of deposit Oxidation; the quartz vein is iron-stained. Country rocks to gold-bearing quartz veins in Valdez Group metaflysch can be variably silicified, carbonitized, and sericitized (Goldfarb and others, 1997).

Production and reserves

Workings or exploration This vein was traced for at least 200 feet; it was developed by at least two adits with a total length of 200 feet by 1914 when a mill was installed at the mine. Additional work continued intermittently to 1939 (Cobb, 1972 [OFR 79-1241]).
Indication of production Yes; small
Production notes The reported production from this deposit was 846 ounces of gold and 371 ounces of silver (Jansons and others, 1984).

References

MRDS Number A011463

References

Goldfarb, R.J., Miller, L.D., Leach, D.L., and Snee, L.W, 1997, Gold deposits in metamorphic rocks in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 151-190.
Hudson, T.L., 1994, Crustal melting events in Alaska, in Plafker, G., and Berg, H. C., eds., The Geology of Alaska: Geological Society of America, DNAG, The Geology of North America, Vol. G-1, p. 657-670.
Reporters Travis L. Hudson
Last report date 12/14/2001