Sweet Grass Hills

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

Information leading to the delineation of this focus area

Basis for focus area Quartz-calcite-pyrite-fluorite stockworks in Eocene syenite intrusions and contact metamorphism between syenite and limestone (marble). Fluorite occurs as veins and replacement seams in the Madison Limestone with areas of marble and solution breccias. Presence of narsarsukite [Na2(Ti,Fe3+)Si4(O,F)11] in fine-grained alkaline igneous rocks. Gold and Te mineralization is confined to stockworks and contacts between intrusions and sedimentary rocks.
Identified resources Fluorspar occurrences with historical flotation tests. No known production.
Production None.
Status Past exploration for gold, silver, and copper resources.
Estimated resources Unknown.
Geologic maps Lopez (2002), scale 1:100,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Alkaline/peralkaline igneous rocks, shonkinite, syenite, trachyte, diorite, leucite phonolite, and minor lamprophyre occurring as stocks, dikes, sills, laccoliths, and irregular intrusions. Mineralized contacts between syenite and Mississippian limestones. Gold-mineralized contact between phonolite diatreme and clastic sedimentary rocks (Gavin, 1991).
Deposits Tootsie Creek prospect (MRDS dep_id: 10067313), Malvina claim (MRDS dep_id: 10216219).
Evidence from mineral occurrences MRDS.
Geochemical evidence Narsarsukite [Na2(Ti,Fe3+)Si4(O,F)11] is found in fine-grained, green alkaline igneous rocks at East Butte of Sweet Grass Hills (Stewart, 1959). Narsarsukite is a rare mineral also found in the Narsarsuk pegmatite in the Ilimaussaq intrusive complex of West Greenland and alkalic intrusive rocks of Mont Saint-Hilaire, Quebec. Possibly associated with minerals containing Sr, REE, Nb, Ba, Ti, Zr, Ba, and Be found in other agpaitic nepheline syenite pegmatites of Montana such as Gordon Butte (Chakhmouradian and Mitchell, 2002) and Pegmatite Peak (Chakhmouradian and Mitchell, 1999). Syenite-hosted quartz-calcite-pyrite-fluorite stockworks are documented from past drilling (Gavin, 1991). Calcite+fluorite veining is likely related to the interaction between local Mississippian limestones and fluids derived from alkalic magmas.
Geophysical evidence Unknown.
Evidence from other sources None.
Comments Fluorite is present in West, Middle, and East Buttes; East Butte fluorite described as only area to have potential commercial significance as vein and replacements in Madison limestone near alkalic syenite (Malvina claim; Tootsie Creek deposit).
Cover thickness and description Exposed at the surface or in historical workings.
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 1:24,000 scale geologic mapping.