Bald Mountain

Location: Wyoming, United States (44.793, -107.859 approx.)

Commodities: gold; titanium minerals; rare earth elements; thorium; zircon

Deposit model: alluvial paleoplacer · unclassified

Geology

“The locations given above encompass two separate sites where samples of the basal Middle Cambrian Flathead Formation (Deadwood conglomerate) contained at least 2 pounds of monazite per ton of rock (after Kline and Winkel, 1952; McKinney and Horst, 1953a, 1953b). The exact extent of these monazite-rich placers in the basal Flathead Formation is not known. Monazite-rich rocks might be present between sample locations, and extend in subsurface under thicker overburden to the east and north of the sample sites. Extensive areas of monazite-rich rock might underlie Bald Mountain and Rooster Knob, and possibly Burnt Mountain. This paleoplacer also contains gold and ilmenite. Because Precambrian tantalum- and niobium-bearing mafic dikes are present in the area (Harris, 1987), the paleoplacer might contain these elements as well. Wilson (1951) mentions that Quaternary monazite placers are present in the area (see following locality, 2 sup.).” (King (1991)) “In detail, zones of monazite enrichment within the basal Flathead Formation are not specifically confined to any lithology or horizon. The monazite-bearing rocks are easily recognized by their high radioactivity. Monazite is usually most abundant in poorly-cemented, iron-stained, irregularly distributed, lenticular, 2.5 to 1 O feet-thick conglomerates that lie on or near the underlying red or gray, Precambrian, granitic rock (Wilson, 1951; McKinney and Horst, 1953a). Preliminary isopach, elevation contour, and monazite-content contour maps of zones of monazite enrichment, based on the data in McKinney and Horst (1953b) and from the Bald Mountain 7-1/2 minute quadrangle map, support the irregular distribution and lenticular nature of the conglomerates as first noted by Wilson (1951). These data also indicate that these conglomerates might be stacked upon each other.” (King (1991)) “The potential monazite resources in secs. 22 and 23 are in a zone that was estimated to be about 6 feet thick, and overlain by O to 30 feet of overburden. This potential resource contains an estimated 4,447 tons of monazite in 674, 160 tons of rock, that averages 13.2 pounds of monazite per ton of rock. The rocks also contain about 22,000 tons of ilmenite. Lower grade resources are far more extensive and abundant. For example, the area in sec. 31 at the base of Bald Mountain contains about 7,800 tons of monazite in 3,500,000 tons of rock that averages about 4.45 pounds of monazite per ton of rock (McKinney and Horst, 1953b).” (King (1991)) “The Bald Mountain paleoplacer deposit has been extensively prospected for monazite and gold, but any production figures for monazite or gold have not been recorded. The paleoplacers were first examined for gold near the turn of the 19th Century, but gold concentrations proved to be unprofitable. Gold concentrations of about 0.1 oz/ton were reported (Darton, 1906; Darton and Salisbury, 1906). From six samples, McKinney and Horst (1953b) reported much lower gold concentrations of 0.001 to 0.005 oz/ton. These are probably lower than actual concentrations because they used composited samples from the entire depth of dry, rotary-drilled holes. Dense minerals like gold, monazite and ilmenite are incompletely recovered during air drilling at low pressures, and the holes were not entirely in conglomerate. Because dense minerals such as monazite commonly accumulate at the bottoms of drill holes during air drilling, this may also explain the small monazite grain-size in the cuttings, as reported by McKinney and Horst (1953a), compared to Wilson (1951 ). In the early 1950s, the deposit was examined for monazite as a source of thorium. At this time numerous pits and trenches were dug, and holes were drilled (about 2,000 feet). Numerous samples of drill cuttings were taken (about 27 tons) and analyzed. The potential resource of by-product ilmenite was also noted at this time (Kline and Winkel, 1952; McKinney and Horst, 1953a, 1953b).” (King (1991)) Gold in the basal Flathead Sandstone in the Bald Mountain area occurs as fine-grained, flat flakes with jagged edges that suggest a nearby source (Sutherland and others, 2013), and the paleoplacer also contains other heavy minerals, including ilmenite, magnetite, zircon, and monazite. The monazite attracted interest as a source for thorium in 1951. "Heavy-mineral concentrates from some of the higher grade samples contained 38.3 percent ilmenite, 8.7 percent monazite, 4.0 percent magnetite, 0.2 percent zircon, and 48.8 percent other impurities." (Sutherland and Cola (2016)) “McKinney and Horst (1953a, 1953b) had apparently assumed a sheetlike configuration for a zone of monazite enrichment, because the basal Flathead Formation was then considered to be a beach deposit. As shown later, recognition of the more complex fluvial nature of the monazite placer precludes any accurate determination of monazite resources from the extensive data in McKinney and Horst (1953b).” (King (1991)) “Wilson (1951) described the monazite concentrations as being pepper-sized, reddish-brown, monazite grains that are present at various locations within conglomerates that are cemented by limonite. McKinney and Horst (1953a) thought most monazite was restricted to poorly-cemented, deep red or yellowish red conglomerates that were overlain by well-cemented sandstones and finer-grained conglomerates. However, monazite concentrations greater than 5 pounds per ton of rock were shown by Kline and Winkel (1952) to be present in these "barren" overlying sandstones and well-cemented conglomerates. Rapid lateral changes are further demonstrated by samples from adjacent drill holes and test pits in the same horizon that do not contain anywhere near the same amounts of monazite (compare Kline and Winkel, 1952, with McKinney and Horst, 1953a, 1953b).” (King (1991)) “In the Bald Mountain area, the basal Flathead Formation is a poorly sorted, arkosic sandstone that contains irregularly-distributed, lenticular conglomerates. Low-angle bedding is quite visible in these rocks. The basal Flathead Formation is heterogeneous. It has: 1) variable textural maturity; 2) varies in color from pale buff to brown and deep red; 3) is poorly to well cemented; and 4) is from 10 to 50 feet thick. The conglomerates contain subrounded, quartz clasts that are usually pebble-sized and that appear to be derived from quartz veins. These clasts are in a matrix of sand-sized, angular to subangular, feldspar grains, with minor, sand-sized, rounded to subangular, quartz grains. The arkosic sandstones enveloping the conglomerates are very similar to the matrix material, except that the grain size in the enveloping sandstones may become coarser higher in the stratigraphic section (after Wilson, 1951; Cardinal, 1958; McKinney and Horst, 1953a; Middleton, 1980).” (King (1991)) ...a poorly cemented, limonitic layer in the basal Flathead Sandstone located at an unknown distance above the contact with Precambrian crystalline rocks. The sample was collected just below a coarse conglomeratic layer exposed on the west side of an old, north-trending, exploration trench about 6 m (20 ft) deep. LREE values were significant in this sample, and HREE and yttrium values were also elevated. (Sutherland and Cola (2016))

Published resource estimates

As compiled by USGS USMIN from the cited studies. Resource classes follow the source; "approximate" flags values USMIN reports as approximations. These are not current company reserve statements.

Exploration and site history

• Discovery — Discovered in 1951, 1951 (Jackson and Christiansen (1993))

Record imported from USGS USMIN ver. 4.0 (June 2019). Developments after that date appear on facility records, not here; see the methodology.