South Hill (near Potato Mountain)

Site Name

Primary: South Hill (near Potato Mountain)

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Commodity

Primary: Tin

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Location

State: Alaska
District: Port Clarence

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Land Status

Not available

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Prospect
Operation Type: Unknown
Years of Production:
Organization:
Significant:

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Physiography

Not available

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Mineral Deposit Model

Model Name: Sn veins
Model Name: Sn greisen

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Orebody

Not available

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Structure

Not available

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Alterations

Alteration Type: L
Alteration Text: Tourmaline +/- pyrite and arsenopyrite replacement of layering in the hornfels is common. Hydrothermal clay alteration is superimposed on mineralized and altered rocks throughout the system, even to depths of 1353 feet in diamond-drill hole PMD-1B.

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Rocks

Not available

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Analytical Data

Not available

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Materials

Ore: Cassiterite
Gangue: Tourmaline
Gangue: Quartz
Gangue: Pyrite
Gangue: Clay
Gangue: Arsenopyrite

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Comments

Comment (Exploration): Status = Inactive

Comment (Geology): Age = Late Cretaceous; the cassiterite mineralization at Potato Mountain is inferred to be similar in age to other tin sytems of western Seward Peninsula. These tin systems are interpreted to be linked to the evolution of associated granite intrusions that are 70 to 80 my old (Hudson and Arth, 1983, p. 769).

Comment (Deposit): Model Name = Quartz-cassiterite veins in hornfels. Related to both the tin vein (15b) and tin greisen (15c) models of Cox and Singer (1986)

Comment (Deposit): Model Number = 15b, 15c

Comment (Workings): Workings / Exploration = The USBM completed extensive detrital cassiterite mapping in the area and followed it up with many surface dozer trenches and related sampling (Mulligan, 1965). The USBM work also included the drilling of 5 short diamond-drill holes totalling 723 feet. Surface sampling and geophysical surveys for Anaconda Minerals Company were reported on by Hudson (1983) and McDermott (1982; 1983). Some of the later work was included by Hudson and Reed (1997). Two vertical diamond-drill holes (one to 57 feet and one to 1,353 feet) were completed in 1990 by Kennecott Exploration Inc. (Meyers, 1990).

Comment (Geology): Geologic Description = Potato Mountain is an isolated upland of hornfels. The protolith for the hornfels is a sedimentary sequence of laminated to thinly bedded mudstone, siltstone, sandstone and some impure limestone that dips moderately (30 to 40 degrees) and is of unknown but probable Paleozoic age. These rocks have a slaty cleavage but are everywhere thermally recrystallized in the uplands of the Potato Mountain area. Although only one granite dike is known at the surface, the presence of a granite stock at depth is suggested by the large area of hornfels and by gravity data. A gravity profile and model across the Potato Mountain upland (McDermott, 1982; Hudson and Reed, 1997, figure 5B) indicate that a buried granite stock has an irregular, perhaps block-faulted upper surface at a depth of about 1,300 feet. Significant block faulting is also suggested by discontinuities in magnetic data for the area (McDermott, 1983). The cassiterite mineralization at Potato Mountain is inferred to be similar in age to other tin sytems of western Seward Peninsula. These tin systems are interpreted to be linked to the evolution of associated granite intrusions that are 70 to 80 my old (Hudson and Arth, 1983, p. 769). Cassiterite occurs in quartz-toumaline +/- pyrite and arsenopyrite veinlets and replacements and as dessiminations in clay-altered zones (Mulligan, 1965; Hudson, 1983). The veinlets, up to a few inches in width, irregularly crosscut weakly to moderately tourmalinized hornfels, particularly near clay-altered zones. Small-scale replacement of mica-rich hornfels layers by very fine-grained dark tourmaline is common. Clay-rich, high-grade (to several per cent tin) zones extend to 300 feet along strike and are up to 20 feet wide. These high grade zones commonly have quartz and cassiterite within clay-rich alteration. All clay-rich surface samples (Mulligan, 1965) may have been affected by residual concentration of cassiterite during weathering. The density and orientation of the veinlets and clay-altered zones is not well defined but they seem to have steep to vertical dips. The Potato Mountain lode tin system that is presently exposed is geochemically characterized by a simple suite of elements; tin, boron, and arsenic are commonly anomalous in rock samples from throughout the area. Data for 36 grab and composite samples from the surface (Hudson, 1983, p. 10) show that tin ranges up to 6.3 %, arsenic in 18 samples is greater than 1,000 ppm, and boron contents are commonly several thousand to greater than 20,000 ppm. Fluorine has a range of 190 to 3,200 ppm and base metals, silver, and tantalum are not significant in the samples. In 1990, Kennecott drilled two vertical diamond-drill holes on this prospect (Meyers, 1990). They were located at 925 feet elevation on the crest of South Hill. One (PMD-1A) reached a total depth of 57 feet before being lost due to equipment problems. The second (PMD-1B) was a twin located 2.5 feet to the west of the

Comment (Geology): Geologic Description = first hole; it reached a total depth of 1,353 feet before encountering a clay-altered zone that could not be penetrated with the equipment available. The following data are taken from Meyers (1990). PMD-1A encountered one zone of steeply dipping quartz-tourmaline-cassiterite veins in the interval 8 to 16.5 feet with a true thickness of 2 feet. This zone contained 3.16% tin, 0.73 % arsenic, and 4.6 ppm silver. PMD-1B encountered three intervals with 0.1% tin or more; (1) the interval between 10 and 28 feet (true thickness of 4 feet ) contained 1.44% tin, 0.63% arsenic, 0.84% boron, 2,426 ppm fluorine, and 4.6 ppm silver; (2) the interval between 302 and 304 feet (true thickness of 1 foot) contained 0.125% tin, greater than 1% arsenic, greater than 1% boron, 1,322 ppm fluorine, and 3.9 ppm silver, and (3) the interval between 746 and 748 feet (true thickness of 1 foot) contained 0.2% tin, 0.08% arsenic, 0.21% boron, 1,140 ppm fluorine, and 0.2 ppm silver. The general character of the rocks changed with depth in PMD-1B. To a depth of 920 feet, the rocks were dominately hornfels with local quartz veins; from 920 to 1,277 feet, hornfels was more intensely developed and biotite-tourmaline bands and 'granitized' and skarn-like textures were present; and from 1,277 to 1,344 feet garnet and/or intense tourmaline with chlorite alteration was developed. From 1,344 to the bottom of the hole at 1,353 feet, a greenish sulfide-rich clay with 'skarn' clasts was encountered. These lithologic changes were reflected in the downhole geochemical profiles. Tin stayed at anomalous levels (between 50 and 200 ppm) though most of the hole but arsenic (increasing from 100 ppm or less to a few hundred ppm or more), fluorine (increasing from 800 ppm to 1,000 ppm levels to greater than 8,000 ppm), and silver (increasing from background levels of less than 0.1 ppm to background levels of 0.5 ppm) were clearly at higher concentrations in the part of the hole below 750 feet.

Comment (Reference): Primary Reference = Mulligan, 1965 (USBM RI 6587); Hudson, 1983; Meyers, 1990; Hudson and Reed, 1997.

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References

Reference (Deposit): McDermott, M.M., 1983, Investigation of the magnetic contact aureoles of the Khotol and Black Mountain granites, Alaska: Anchorage, Alaska, Anaconda Minerals Company internal report (Report held by Cook Inlet Region, Inc., Anchorage, Alaska).

Reference (Deposit): Hudson, T.L., and Arth, J. G., 1983, Tin-granites of Seward Peninsula, Alaska: Geological Society of America Bulletin, v. 94, p. 768-790.

Reference (Deposit): McDermott, M.M., 1982, Gravity profiles of Black Mountain and Potato Mountain, Seward Peninsula: Anchorage, Alaska, Anaconda Minerals Company internal report (Report held by Cook Inlet Region, Inc., Anchorage, Alaska).

Reference (Deposit): Cobb, E.H., and Sainsbury, C.L., 1972, Metallic mineral resource map of the Teller quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-426, 1 sheet, scale 1:250,000.

Reference (Deposit): Mulligan, J.J., 1965, Tin-lode investigations. Potato Mountain area, Seward Peninsula, Alaska: U.S. Bureau of Mines Report of Investigations 6587, 85 p.

Reference (Deposit): Hudson, T.L., 1983, Interim report on the Potato Mountain tin system: Anchorage, Alaska, Anaconda Minerals Company internal report (Report held by Cook Inlet Region, Inc., Anchorage, Alaska).

Reference (Deposit): Cobb, E.H., 1975, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Teller quadrangle, Alaska: U.S. Geological Survey Open-File Report 75-587, 130 p.

Reference (Deposit): Meyers, W.C., 1990, Report on 1990 exploration activities at the Potato Mountain tin prospect, Teller AMS sheet, Seward Peninsula, Alaska: Anchorage, Alaska, Kennecott Exploration Inc. internal report.

Reference (Deposit): Steidtmann, Edward, and Cathcart, S. H., 1922, Geology of the York tin deposits, Alaska: U.S. Geological Survey Bulletin 733, 130 p.

Reference (Deposit): Hudson, T.L., and Reed, B.L., 1997, Tin deposits of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 450-465.

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