Arnold

Prospect, Active

Commodities and mineralogy

Main commodities Au
Other commodities Cu; Mo; Pb; W
Ore minerals anglesite; chalcopyrite; galena; gold; hematite; magnetite; malachite; molybdenite; pyrite; pyrrhotite; scheelite
Gangue minerals calcite; iron carbonate; limonite; quartz

Geographic location

Quadrangle map, 1:250,000-scale RM
Quadrangle map, 1:63,360-scale D-8
Latitude 61.83
Longitude -161.896
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Arnold prospect is in the northeast headwaters of Willow Creek. It is at an elevation of about 1,250 feet and 1 mile east-southeast of the summit of Mount Okumiak. The map site is in the SW1/4 sec. 12, T. 20 N., R. 70 W., of the Seward Meridian. This is locality 1 of Hoare and Cobb (1972, 1977).

Geologic setting

Geologic description

The Arnold prospect was first staked in 1914 (Harrington, 1918). It has been explored with surface trenches and pits at several times since, but mining has not occurred. The deposit is quartz veins, quartz vein stockworks, and quartz-cemented breccia in pyrite-bearing mafic volcanic rocks intruded by quartz porphyry. As mapped by Turner (1987), the breccia is commonly in quartz porphyry but also includes greenstone fragments. The largest stockwork and (or) breccia zone is about 200 feet wide and 700 feet long, but exposure is mostly slope rubble. Most geologic units trend northwest, including well-developed shears and small faults; contacts are commonly slickensided. The quartz veins contain as much as 2 percent sulfide minerals, including chalcopyrite, galena, molybdenite, pyrite, pyrrhotite, and sphalerite. Other minerals in the veins include magnetite, hematite, anglesite, malachite, limonite, scheelite, and calcite. The best gold grades are in quartz veins. Selected quartz veins contain as much as 2.72 ounces of gold per ton but altered greenstone with or without quartz veins commonly contains a few tens to a few hundreds parts per billion gold (Turner, 1987). Homestake collected 78 rock, 5 stream sediment, and 70 soil samples in the general area, but almost all anomalous samples were from the Arnold prospect (Bull and Schneider, 1997). The anomalous rock and soil samples mostly contained a few tens to hundreds of parts per billion gold, although one quartz vein with visible gold contained 63.5 grams of gold per ton. Copper, in the 100 to 835 ppm range, was the element most commonly anomalous in these samples. Molybdenum, as much as 126 ppm, was locally anomalous, and lead, zinc, silver, antimony, and arsenic were present in low amounts in most of the samples. A reconnaissance examination of the prospect for radioactive minerals did not find any material containing more than 0.001 percent equivalent uranium (West, 1954). The general area is one where mafic volcanic rocks are intruded by a variety of intermediate to felsic igneous rocks (Hoare and Coonrad, 1959; Turner, 1987; Bull and Schneider, 1997). Mineralization appears to be dominantly associated with felsic intrusive rocks.
Geologic map unit (-161.898374681923, 61.8292570688484)
Mineral deposit model Polymetallic veins (Cox and Singer, 1986: model 22c)
Mineral deposit model number 22c
Age of mineralization Cretaceous or Tertiary. The altered and mineralized host volcanic rocks are part of the Gemuk Group that includes rocks as young as Early Cretaceous (Hoare and Coonrad, 1959). The felsic intrusive rocks in the prospect area may be part of a Upper Cretaceous and Lower Tertiary suite of igneous rocks that are widespread in southwest Alaska (e.g., Box and others, 1993).
Alteration of deposit Silicification, oxidation, sericitization, iron-carbonate development, and potassic replacement(?).

Production and reserves

Workings or exploration The Arnold prospect has been explored with surface trenches, pits, and dozer cuts several times since it was first staked in 1914. Drilling has not been reported. Surface mapping and sampling was completed for Calista Corporation in the 1980s (Turner, 1987), and additional sampling, including a soil grid, was completed in 1997 (Bull and Schneider, 1997).
Indication of production None
Production notes A test shipment of high-grade material returned $80 per ton in 1915.

References

MRDS Number A013406; D002724

References

Reporters Travis L. Hudson and Madelyn A. Millholland
Last report date 6/10/2001