Bearpaw Mountains-Eagle Buttes

Region West, Northwest
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Known REE mineralization in carbonatites of the Rocky Boy stock discovered during vermiculite mining along Big Sandy Creek. Carbonatite pegmatites, dikes, and veins composed of orthoclase, biotite, calcite, pyrrhotite, and pyrite occupy fractures in syenite, monzonite, and shonkinite. Accessory minerals include aegirine, apatite, zircon, U-rich pyrochlore, REE carbonates, barite, and ilmenite (Pecora, 1962). Assemblage of Sr, REE, Nb and Ba-bearing minerals in lens-shaped agpaitic pegmatites of Pegmatite Peak, part of the Elk Peak complex (Pecora, 1942; Chakhmouradian and Mitchell, 1999).
Identified resources Historical production of vermiculite (hydrobiotite).
Production Unknown.
Status At the Vermiculite or Bearpaw mine, several short adits were driven and at least two shafts sunk from 1908 to 1930 to explore for gold, silver, lead, copper, and zinc (Lindsey and others, 1977 and references therein). No significant precious or base metal deposits were found. In 1957, exploration geologists from Texas Instruments, Inc. collected samples from the old adits and drilled several diamond drill holes. Many veinlets and disseminations rich in burbankite were intersected in a drill hole at depths between 283 and 375 feet (Lindsey and others, 1977).
Estimated resources The Number One adit was driven through the selvage and two drifts were driven 10 and 15 feet along the contact. If the thickness of the selvedge at the north contact is assumed to average 4 feet over the known length and depth of the mineralized structure, it contains approximately 200,000 tons of rock with about 1,000,000 pounds of niobium and 500,000 pounds of uranium (Lindsey and others, 1977).
Geologic maps Bergantino and others (2002), scale 1:100,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Carbonatites and silicocarbonatites are exposed in the Rocky Boy stock exclusively in roadcuts, and prospecting trenches and pits along Big Sandy Creek. These rocks occur mostly as veins and dikes (commonly with a biotite-rich selvage) crosscutting the earlier-emplaced syenite, monzonite, and shonkinite. Agpaitic pegmatites enriched in Sr, REE, Nb Ba, Ti, and Zr are emplaced in nepheline syenites and earlier-crystallized monzonites, and locally occur as bodies extending from one type of country rock into the other at Pegmatite Peak.
Deposits Number One (MRDS dep_id: 10221174), Rocky Boy stock (MRDS dep_id: 10019736), Rocky Boy area (MRDS dep_id: 10148767).
Evidence from mineral occurrences MRDS.
Geochemical evidence Carbonatites along Big Sandy Creek: Burbankite, calkin site, ancylite, apatite, zircon, U-rich pyrochlore, REE carbonates, barite, and ilmenite. Pegmatite Peak: Zircon, rutile, thorite, betafite, loparite-(Ce), critchonite, ilmenite, pyrophanite, barytolamprophyllite, titanite, strontiochevkinite, chevkinite-(Ce), priderite, Ba-Fe “hollandite,” “metaloparite, bastnäsite-(Ce), ancylite-(Ce), fluorapatite.
Geophysical evidence None.
Evidence from other sources Eight distinct REE-carbonate minerals have been identified in the carbonatites and related potassic syenites. Ancylite-(Ce) [SrCe(CO3)2(OH)·H2O] is one of the most abundant REE carbonates in these rocks (Mitchell and Reguir, unpublished). The Rocky Boy ancylite is locally enriched in Nd relative to La and Ce. The Rocky Boy stock (carbonatite) is the type locality for burbankite [(Na,Ca)3(Sr,Ca,REE,Ba)3(CO3)5] (Pecora and Kerr, 1953). Burbankite crystals as large as 3 centimeters have been found (Lindsey and others, 1977). Burbankite is replaced by calkinsite.
Comments Known carbonatite-hosted REE mineralization occurs within the Rocky Boy's Reservation, Montana. More work is required to determine the abundance of several critical minerals. The types of intrusions in this district are known to be enriched in REE, Sr Ba, Nb, Ti, Zr, and Ta in other districts. The Rocky Boy's Reservation covers the southwest quarter of the mountain range.
Cover thickness and description Partially exposed at the surface and extends to unknown depth.
Authors Allen K. Andersen, Stanley L. Korzeb, Kaleb C. Scarberry, Jay A. Gunderson, Joshua M. Rosera.
New data needs High resolution geophysics, whole-rock geochemistry for all critical minerals, geologic mapping, lidar.
Geologic mapping and modeling needs Targeted 1:24,000 or 1:12,000 scale geologic mapping.
Geophysical survey and modeling needs High resolution aeromagnetic and aeroradiometric coverage.
Digital elevation data needs Lidar variable, with some complete, some planned/funded but much not covered.