Twin Mountain

Prospect, Probably inactive

Commodities and mineralogy

Main commodities Ag; Au; Pb; Zn
Ore minerals arsenopyrite; boulangerite; galena; gold; hematite; limonite; pyrite; sphalerite
Gangue minerals albite; ankerite; quartz

Geographic location

Quadrangle map, 1:250,000-scale NM
Quadrangle map, 1:63,360-scale C-1
Latitude 64.6769
Longitude -165.4948
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Twin Mountain prospect is at an elevation of 1,000 feet on the western headwater slope of Twin Mountains Creek valley, due west of the headwater drainage of Twin Mountains Creek. The prospect is about 700 feet south of hill 1134, in the SW1/4 section 28, T. 9 S., R. 34 W, Kateel River Meridian. The location is accurate within about 250 feet.

Geologic setting

Geologic description

Mineral deposits at Twin Mountain consist of nearly stratabound zones of sulfidized schist flanked to the north by zones of nearly horizontal white quartz veins. In plan, the deposit is at least 700 feet long, as much as 120 feet across in a north-south direction, and elongated about N80-85E. Drilling suggests that the zone is about 100 feet thick. Two other small deposits occur nearby. One is 25 by 50 feet in plan and 200 feet north of the west end of the main deposit. The other, also about 25 feet across, is about 300 feet east-northeast of the east end of the main deposit.
The sulfidized schist probably originally contained 5 to 10 percent arsenopyrite and pyrite, now mostly oxidized to hematite and limonite. Knots of almost massive sulfide occur locally in the schist. These knots are about 6 inches across and contain as much as several percent boulangerite, galena, sphalerite, pyrite, and arsenopyrite. Quartz is sparse in the sulfidized schist, but shallow-dipping quartz veins as much as 4 feet thick occur in the silicified zone adjacent to the metalliferous schist. The quartz is massive, white to creamy white, and generally contains 1 to 2 percent pyrite, arsenopyrite, and galena. Quartz veins appear to be partly controlled by tight recumbent folds.
The prospect was found by following up a soil geochemical survey by Kennecott Exploration Company in 1992 and 1993. The soil survey extended northerly from Monument Creek to Bangor Creek, a distance of about 6.5 miles, and was about 1 mile across. Soil samples collected in upper Twin Mountains Creek contained as much as 8,200 ppb gold and more than 8,000 ppm arsenic. The deposit is within a soil anomaly defined by greater than 100 ppb gold; the anomalous area is about 3,600 feet long, elongated west-northwest, and about 800 feet across. The gold-in-soil anomaly is within an arsenic soil anomaly defined by greater than 400 ppm arsenic. The anomalous area was soil covered; the only obvious unusual features were local concentrations of quartz boulders and an unusual abundance of poppies. Poppies were abundant enough that the anomalous area was called the Poppy anomaly.
The area was explored in 1993 by about 3,600 feet of backhoe trenches that ranged from 5 to almost 10 feet deep. The trenching exposed parts of three deposits. The main deposit was exposed in eight north-northeast-trending trenches and one north-northwest-trending trench through the strongest part of the deposit. Sulfidized schist mostly contains 1,500 ppb gold (0.04 ounce of gold per ton) and as much as 0.23 ounce of gold per ton. The quartz-veined zone mostly contains 200 to 600 ppb gold and as much as 0.18 ounce of gold per ton. Sulfide knots rich in galena and boulangerite contain as much as 15 ounces of silver per ton. A few reverse circulation holes drilled in 1994 and 1995 confirmed grades of a few hundredths ounce of gold per ton and appear to confirm a nearly stratabound occurrence localized by shallow-dipping schistosity and fold axes. The deposits at Twin Mountain are similar to those at Alpha Ridge (NM171) and Goodluck Gulch (NM202). Quartz, sulfide minerals, albite, and ankerite were introduced here in a ductile environment relatively early in the main period of gold deposition in the Nome district.
The Twin Mountain deposit is confined to a lithologic sequence less than 100 feet thick of weakly graphitic mica-schist. Graphitic quartz schist crops out south of the prospect, and a calc-mica schist unit occurs from 700 to 1,000 feet south of the prospect. These units are tentatively correlated with the 'mixed unit' schist of Till and others (1986). The prospect is about 250 feet south of and subparallel to an open antiform that trends nearly east-west. The deposit and antiform appear to be cut off to the east and north by a major thrust (?) fault that strikes west-northwest. The thrust (?) fault and mineralized rocks in turn abut against and are probably cut off by the major Rodine fault, which is about one-half mile southeast of the prospect (Bundtzen and others, 1994). Rocks that underlie hill 1134 north of the prospect are highly contorted pelitic calc-schist that could correlate with apparently older Solomon Schist units which generally underlie the Mixed Unit schist of Till and others (1986). The highly contorted schist is on the hanging wall of the thrust (?) fault that cuts off the deposit.
Geologic map unit (-165.497411051642, 64.676133862254)
Mineral deposit model Low-sulfide Au-quartz veins (Cox and Singer, 1986; model 36a); massive quartz-sulfide masses in schist.
Mineral deposit model number 36a
Age of mineralization Protolith ages of the enclosing schist is probably early Paleozoic (Till and others, 1986; Till and Dumoulin, 1994). Some fissure-controlled gold quartz veins on the Seward Peninsula formed at about 109 Ma (Ford and Snee, 1996), but more age determinations are needed to bracket the gold-mineralizing event near Nome.
Alteration of deposit Silicification, albitization, and some introduction of ankerite.

Production and reserves

Workings or exploration
The prospect was found by following up a major soil geochemical survey by Kennecott Exploration Company mainly in 1992 and 1993. The soil survey extended northerly from Monument Creek to Bangor Creek, a distance of about 6.5 miles, and was about 1 mile across. Soil samples collected in upper Twin Mountains Creek contained as much as 8,200 ppb gold and more than 8,000 ppm arsenic. The anomalous area was soil covered; the only obvious unusual features were local concentrations of quartz and an abundance of poppies. Poppies were abundant enough that the anomalous area was called the Poppy anomaly.
In 1993, the area was prospected by about 3,600 feet of backhoe trenches that disclosed a main zone containing quartz veins as much as 4 feet thick within strongly albitized and sulfidized schist. Shallow holes drilled in 1994 show that the deposit is thin and dips at a shallow angle.
Indication of production None

Additional comments

The Twin Mountain deposit is similar to those at Alpha Ridge (NM171) and Goodluck Gulch (NM202).

References

MRDS Number 10307989

References

Ford, R.C., and Snee, L.W., 1996, 40Ar/39Ar thermochronology of white mica from the Nome district, Alaska--The first ages of lode sources to placer gold deposits in the Seward Peninsula: Economic Geology, v. 91, p. 213-220.
Till, A.B., and Dumoulin, J.A, 1994, Geology of Seward Peninsula and St. Lawrence Island, in Plafker, G., and Berg, H.C., eds., The Geology of Alaska: Geological Society of America, DNAG, The Geology of North America, v. G-1, p. 141-152.
Reporters C.C. Hawley and Travis L. Hudson
Last report date 7/10/2000