South Hill (near Potato Mountain)

Prospects, Inactive

Commodities and mineralogy

Main commodities Sn
Ore minerals cassiterite
Gangue minerals arsenopyrite; clay; pyrite; quartz; tourmaline

Geographic location

Quadrangle map, 1:250,000-scale TE
Quadrangle map, 1:63,360-scale C-6
Latitude 65.651
Longitude -167.553
Nearby scientific data Find additional scientific data near this location
Location and accuracy This is the most significant area of exposed lode cassiterite mineralization in the Potato Mountain area. It is a 1,200 by 1,500 area centered on a small rounded hill (South Hill, 900 feet elevation) between the headwaters of Iron Creek (TE022) and the West and Left forks of Buck Creek (TE020). It is the principal source of placer cassiterite in these adjacent drainages. Locality 2 of Cobb and Sainsbury (1972) is plotted in the approximate location of the Daisy and Eureka prospects (see Potato Mountain, TE024); they did not separately identify the South Hill lode prospect on their map. Cobb (1975) summarized relevant references under the name 'Potato Mtn.'.

Geologic setting

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 systems 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-tourmaline +/- pyrite and arsenopyrite veinlets and replacements and as disseminations 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 percent, 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 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 percent tin, 0.73 percent arsenic, and 4.6 ppm silver. PMD-1B encountered three intervals with 0.1 percent tin or more; (1) the interval between 10 and 28 feet (true thickness of 4 feet) contained 1.44 percent tin, 0.63 percent arsenic, 0.84 percent 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 percent tin, greater than 1 percent arsenic, greater than 1 percent 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 percent tin, 0.08 percent arsenic, 0.21 percent 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.
Geologic map unit (-167.555757462071, 65.6502280685466)
Mineral deposit model Quartz-cassiterite veins in hornfels. Related to both the tin vein (15b) and tin greisen (15c) models of Cox and Singer (1986)
Mineral deposit model number 15b, 15c
Age of mineralization 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).
Alteration of deposit 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.

Production and reserves

Workings or 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).
Indication of production None

References

MRDS Number 10308397

References

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).
Hudson, T.L., and Arth, J. G., 1983, Tin granites of Seward Peninsula, Alaska: Geological Society of America Bulletin, v. 94, p. 768-790.
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.
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).
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).
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.
Reporters Travis L. Hudson (Applied Geology)
Last report date 5/10/1998