Ear Mountain

Prospect, Inactive

Alternative names

North Hill
Winfield shaft

Commodities and mineralogy

Main commodities Ag; Cu; Sn; Zn
Other commodities Au
Ore minerals cassiterite; chalcopyrite; sphalerite
Gangue minerals arsenopyrite; axinite; calc-silicates; fluorite; pyrrhotite; pyrite; quartz; sericite; tourmaline; zinnwaldite

Geographic location

Quadrangle map, 1:250,000-scale TE
Quadrangle map, 1:63,360-scale D-3
Latitude 65.945
Longitude -166.206
Nearby scientific data Find additional scientific data near this location
Location and accuracy Ear Mountain is an isolated upland cored by a granite stock (Sainsbury, 1972) that reaches a maximum elevation of 2,329 feet in the north-central Teller D-3 quadrangle. The Ear Mountain prospect is located in the north contact zone of the Ear Mountain granite stock. This contact is locally irregular but trends approximately east -west across North Hill, a flat-topped hill (elevation of 1,642 feet) on the northeast flank of the Ear Mountain upland. The Ear Mountain prospect is centered on the old Winfield shaft (Mulligan, 1959). This is locality 14 of Cobb and Sainsbury (1972) and Cobb (1975) summarized relevant references under the name 'Ear(s) Mtn.'.

Geologic setting

Geologic description

Ear Mountain is cored by a 2 x 2 mile, Late Cretaceous (76.7 +/- 2.8 my; Hudson and Arth, 1983, p. 769), composite biotite granite stock. Country rocks to this stock are an impure and schistose carbonate sequence, with some metapelitic rocks, of unknown but probable Paleozoic age.
The Ear Mountain stock is mostly porphyritic biotite granite with an aplitic groundmass. Medium- to coarse-grained seriate biotite granite is present along the southern margin and medium-grained equigranular alaskite forms small intrusive bodies in the vicinity of the northern contact. Dikes and sills of alaskite and fine-grained granite porphyry are present in the country rocks. A few mafic dikes are also locally present in granite and country rocks. The country rocks, mostly impure and schistose carbonate rocks but also including some fine-grained, dark metapelitic rocks, are variably converted to tactite and hornfels around the granite stock (Knopf, 1908, p. 28-29). Skarn is well-developed in parts of the Ear Mountain prospect for about 7,000 feet of strike along the north contact of the Ear Mountain granite (Mulligan, 1959). Skarn minerals include idocrase, garnet, wollastonite, diopside, spinel, salite, and forsterite. A later hydrous skarn stage has been reported by Bond (1982) that includes idocrase, quartz, tourmaline, fluorite, calcite, magnetite, pyrrhotite, pyrite, stannite, chalcopyrite, and sericite or muscovite. Much of the hydrous alteration is vein-controlled and overprinting assemblages (pyrrhotite and chalcopyrite-pyrite veins in magnetite) are present.
Greisen-like alteration is present within equigranular granite north of the main pluton contact, distinct linear zones cutting coarse porphyritic granite inward from the main pluton contact, and an irregular zone in granite at the north contact of the pluton. Most of the greisen-like alteration is quartz-tourmaline replacement that is widespread in the Ear Mountain area as veins that are developed along joints or fractures in granite. Selvages adjacent to mineralized fractures and joints, in places a few feet wide, are characterized by tourmaline replacement of feldspar. In porphyritic granite with coarse feldspar phenocrysts, tourmaline aggregates that pseudomorph the feldspar crystals are common. Fluorite-white mica-quartz-tourmaline alteration with some disseminated cassiterite is present at the surface east of the Winfield shaft (trench 3E of Mulligan, 1959) and high-grade samples from the Winfield shaft dump (see below) are rusty, arsenopyrite-fluorite-quartz-tourmaline rocks. The protolith of the high-grade rocks from the Winfield shaft dump is not known but mapping and sampling by Mulligan (1959) in the underground workings of the Winfield shaft suggests that they may have been metasedimentary.
There is a strong stratigraphic control to mineralization in the Winfield shaft workings (Mulligan, 1959). Specific 'altered limestone' beds peripheral to an unaltered alaskite sill were mapped and sampled separately by Mulligan (1959, p. 16). Mineralized layers within 7.5 feet stratigraphic thickness (above and below, 15 feet total stratigraphic thickness) of an 0.8 foot thick alaskite sill average 1.09 percent tin. Layers that overlie the tin-rich beds above the alaskite sill, aggregating almost 12 feet of additional stratigraphic thickness, contain lower tin values (0.1 to 0.2 percent) but significant copper (0.72-3.0 percent), zinc (0.8 to 1.9 percent) and silver (0.36 to 3.41 opt). Mulligan (1959, p. 43-44) also reports a trace or slightly more gold in some samples from the Winfield shaft; one sample contained a highly anomalous 2.58 opt gold. Sample results reported by Hudson (1983), including several from the Winfield shaft dump, contain gold in the less than 5 ppb to 105 ppb range. The mineralogic character of mineralized beds in the Winfield shaft workings have not been specifically described but composite samples of altered limestone adjacent to the alaskite sill contain 'quartz, less fluorite, some feldspar and tourmaline, relatively small amounts of clinopyroxene, amphibole, siderite, arsenopyrite, chlorite, limonite, epidote, sericite, pyrite, biotite, pageite, pyrrhotite, and cassiterite. Very small amounts of rutile, garnet, magnetite, pyrolusite, and gold are also observed' (Mulligan, 1959, p. 49). Other mineralized limestone samples from the Winfield shaft contain 'feldspar with amphibole, fluorite, quartz, clinopyroxene, tourmaline, and relatively small amounts of pyrite, pyrrhotite, siderite, limonite, scapolite, arsenopyrite, chlorite, pageite, and sericite. Also very small amounts of cassiterite, pyrolusite, rutile, and chalcopyrite' (Mulligan, 1959, p. 49). Samples from the Winfield shaft dump contain 'quartz, fluorite, axinite, zinnwaldite, chalcopyrite, arsenopyrite, sphalerite and relatively small amounts of pyrrhotite, tourmaline, hematite, limonite, cerussite, talc, and pyrite. Also very small amounts of malachite, galena, hypersthene, calcite, cassiterite, and scheelite' (Mulligan, 1959, p. 49).
Geochemical data for rock samples from the Ear Mountain prospect area are included in Hudson (1983). Nine samples of arsenopyrite-quartz-tourmaline-fluorite rocks from the Winfield shaft dump contain 1.10 to 5.0 percent tin. One tactite, one hornfels, and three quartz-tourmaline rocks contain 0.19 to .88 percent tin. All other rocks had 1,000 ppm tin or less. Tungsten was highly anomalous in two quartz-tourmaline rocks (400 and 1,800 ppm), tantalum is not present in amounts greater than 40 ppm, and fluorine and boron are commonly present in highly anomalous amounts (greater than 20,000 ppm and 10,000 ppm respectively). Arsenic exceeds 1,000 ppm in many samples with higher tin contents and base metals were at low to strongly anomalous concentrations (5 to 4,600 ppm copper, 4 to 1,010 ppm lead, and 3 to 385 ppm zinc). None of these samples appear to have represented base metal-rich mineralization like that encountered by Mulligan (1959) in the Winfield shaft workings.
Anaconda Minerals Company completed an airborne magnetometer survey (0.25 mile flight spacing) over the Ear Mountain area in 1979 (Hudson, 1983). These data indicate local magnetic highs in the contact zone surrounding the Ear Mountain granite stock, and a magnetic low over the granite itself. A broader magnetic gradient to the north of the granite supports the interpretation of a shallow dipping granite contact in this area (Knopf, 1908, p. 30) but direct links between known mineralization and the magnetic character of the area are not obvious.
Geologic map unit (-166.208753669432, 65.9442641686118)
Mineral deposit model Greisen, skarn, and sulfide-rich replacement in carbonate rocks. Tin greisen (15c), tin skarn (14b), and possibly replacement tin (14c) models after Cox and Singer (1986).
Mineral deposit model number 14b, 14c, 15c
Age of mineralization The mineralization is assumed to be related to evolution of the Ear Mountain granite stock and therefore Late Cretaceous in age (76.7 +/- 2.9 my, Hudson and Arth, 1983, p. 769).
Alteration of deposit Skarn and hornfels development in country rocks; quartz-tourmaline-fluorite-white mica veining and replacement of granite; and sulfide-bearing replacement of country rocks with or without quartz, fluorite, and boron-bearing silicates.

Production and reserves

Workings or exploration Fourteen dozer trenches, a 29 foot shaft (Winfield shaft), a 35 foot winze, and about 100 feet of irregular drifts have been completed in the prospect area (Mulligan, 1959). There has been no diamond drilling to date.
Indication of production None
Reserve estimates Not defined but significant thickness and grade are present in the altered carbonate rocks of the Winfield shaft.

References

References

Reporters Travis L. Hudson (Applied Geology)
Last report date 5/10/1998