30-7 Lithium Pegmatite Mineral Resource Assessment of the United States with an Emphasis on the Black Hills, South Dakota Region

Session: Geological and Geochemical Investigations of Critical Minerals in New Mexico and Beyond, and Technological Advances in Extraction of Critical Minerals

Presenting Author:

Joshua Rosera

Authors:

Rosera, Joshua M.¹, Case, George N.², Holm-Denoma, Christopher S.³, Wintzer, Niki E.⁴, McCaffrey, Dalton M.⁵, Crocker, Kelsey E.⁶, Evart, Lucas L.⁷, Coyan, Joshua A.⁸

(1) U.S. Geological Survey, Geology, Energy & Minerals Science Center, Fairfax, VA, USA, (2) U.S. Geological Survey, Alaska Science Center, Anchorage, AK, USA, (3) U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver, CO, USA, (4) U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, Spokane, WA, USA, (5) U.S. Geological Survey, National Minerals Information Center, Las Vegas, NV, USA, (6) U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, Spokane, WA, USA, (7) U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, Spokane, WA, USA, (8) U.S. Geological Survey, Geology, Minerals, Energy, and Geophysics Science Center, Spokane, WA, USA,

Abstract:

Lithium (Li) is one of 60 critical mineral commodities that are vital to U.S. national economic health and security. The U.S. Geological Survey (USGS) is conducting a nationwide mineral resource assessment of Li deposits to characterize domestic undiscovered resources. The southern Black Hills of South Dakota contain numerous lithium-cesium-tantalum (LCT) pegmatites that were mined for Li and other commodities during the 20^th century. Here, we present an overview of the USGS method for mineral resource estimation applied to Li pegmatite deposits in this region.

Precambrian rocks in the southern Black Hills include polydeformed units associated with the accretion of juvenile terranes onto the Archean Wyoming craton during the Paleoproterozoic. Thousands of pegmatites and granitic pegmatite bodies, including the Harney Peak Granite, intruded during the late stages of the orogeny (ca. 1,715 Ma); however, the exact timing and nature of the relationship between granites and pegmatites remain unclear. Pegmatite bodies range from simple granite pegmatites through complex beryl-bearing to rare-element LCT pegmatites. Currently, one pegmatite deposit (Beecher) has an indicated resource of 2.2 million tons (Mt) at 1.05 wt% Li₂O. Globally, well-characterized Paleoproterozoic- to Mesoproterozoic-hosted Li pegmatite deposits have a median 6.5 Mt at 1.12 wt% Li₂O (n = 16), and are generally lower tonnage than those hosted in Archean to Archean-Paleoproterozoic basement domains (median 29.8 Mt, n = 38). These considerations are important for determining the appropriate grade and tonnage model to use for the Black Hills Li pegmatite assessment.

Permissive tracts for undiscovered Li pegmatite deposits are delineated and ranked during mineral prospectivity mapping. Known pegmatite-related mineral occurrences are essential for validating and/or training these models. We ranked and worked to verify the locations of hundreds of pegmatite-related mineral sites throughout the southern Black Hills. This approach highlights district-scale zonations and can guide mineral prospectivity modelling. Comparison of new and legacy location data reveals half of the mine-site records were inaccurate by more than 250 meters. Adjusting location data of historic mine records is essential for matching these data to high-resolution geologic map data (e.g., 1:100,000 scale, or ca. 50-meter resolution) for mineral prospectivity mapping. Grade and tonnage models, mineral sites, and permissive tracts are key components for completing quantitative mineral resource assessment.

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