Last Chance

Location: Montana, United States (44.938, -113.466 approx.)

Commodities: rare earth elements; thorium

Deposit model: REE-Th bearing veins · Th-REE veins

Geology

The veins in the area occur in faults that trend northwest. The veins are wall-rock replacements and fissure fillings along faults and shears and are composed dominantly of quartz with some barite, iron oxides and hydroxides, and lesser amounts of thorite. (Armstrong (1955)) Rare earths have been reported in analyses of samples from the vein, but no rare-earth minerals have been identified. The amount of rare earths reported in the analyses is such that all of it can be accounted for as substituting for thorium in the mineral thorite, and, therefore, rare-earth minerals need not be present. Analyses show that there is very little uranium in the vein. What uranium is present could also be accommodated by substitution for thorium in the thorite. (Armstrong (1955)) The Last Chance vein occupies a fault and has been emplaced by replacement of the brecciated wall rock and to a lesser extent by fissure filling. Even though replacement has played a large part in its formation, the contact of the vein with the wall rocks is sharp. The wall rocks have been intensely bleached for a distance of 3 to 5 feet from the vein, and locally they appear to be highly silicified. Numerous silicified breccia fragments of wall rock occur in and near the margins of the vein. The vein is intensely fractured from recurrent movement along the fault after the emplacement of the vein. Fracturing of the vein is most intense at points where abrupt changes in its strike occur. (Armstrong (1955)) The southern part of the Last Chance vein strikes about N. 45 degrees W., and the north end strikes about N. 60 degrees W. (fig. 3). The results of the diamond drilling indicate that the south third of the vein dips about 45 degrees SW.; a dip of 64 degrees NE. was observed on surface near the north end of the vein. The vein crops out for a distance of 500 feet and has been traced on the surface for a total distance of about 1,300 feet; it extends at least 290 feet down the dip (Sec. A-A'). Over the southern two-thirds of its length the thickness of the vein is between 10 and 15 feet; near the south end the vein has a maximum thickness of about 35 feet. The northern third of the vein progressively narrows and finally the vein dies out entirely. Several smaller veins, parallel or subparallel to the major vein, occur in the wall rocks. The largest of these, located about 35 feet northeast of the foot wall of the Last Chance vein, is 4 feet thick and is about midway between the ends of the major vein. (Armstrong (1955)) The vein consists of milky quartz and subordinate barite; barite is only locally abundant and in most places is much less abundant than quartz. The wide part of the vein about 550 feet from its north end (fig.3), however, is dominantly barite with only sparse quartz. Barite in white, finely crystalline masses and in pink veinlets replaces some of the quartz. The quartz and barite have been intensely fractured and later quartz and barite 5 and goethite, lepidocrocite, specular and earthy hematite have been deposited in the fractures as veinlets from less than an inch to several inches thick. In the barite-rich part of the vein a banding parallel to the vein walls results from alternate layers of barite and dark-red, hematite-impregnated layers of barite. A little malachite was also noted in the vein. The presence of iron oxides and hydroxides in the numerous fractures colors almost the entire vein red. (Armstrong (1955)) Under the D. M. E. A. contract the Last Chance vein was traced on surface for a distance of about 1,300 feet; the thickness ranges from about 35 feet to a few inches. Two diamond drill holes cut the vein 240 and 290 feet below the outcrop. (Armstrong (1955))

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

• Exploration — Assays, mapping, bulldozing and drilling - The Last Chance Group, Brown Bear, and Shady Tree Claims were explored for thorium under a Defense Minerals Exploration Administration Contract in 1951 and 1952, 1951–1952 (Armstrong (1955))
• Evaluation — Inferred resource estimate; Compliant diamond core drilling and sampling program; 12 channel samples; 8 core holes with depths rearching up to 520 feet, 2013 (U.S. Rare Earths Inc. (2015b))

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