EARTH-MRI IN IDAHO: GEOLOGIC MAPPING, GEOCHEMICAL CHARACTERIZATION, AND MAGNETIC SUSCEPTIBILITY OF THE IDAHO COBALT BELT AND MINERAL HILL REE DISTRICT

The Idaho Geological Survey (IGS) has mapped and refined the Mesoproterozoic stratigraphy along the Idaho cobalt belt (ICB) near Salmon, Idaho, as part of the USGS Earth-MRI Program. In addition, we have mapped Mesoproterozoic intrusive rocks thought to be related to mineralization. The objective of our work is to provide detailed geologic maps, analysis of samples, and surface measurement of magnetic susceptibility (MS) and radiometric U-Th-K, to better understand the distribution, age, and controls of mineralization. New Earth-MRI funding in 2023 expanded our mapping efforts to the Mineral Hill rare earth element (REE) district immediately northeast of the ICB.

The IGS has published new geologic maps and a map compilation along the ICB. Additional sampling and geologic mapping in two additional quadrangles will be completed by September 2023 and a new mapping project will commence. The IGS collected stream-sediment samples and rock samples that provide baseline compositional data for Cu-Co and REE concentrations. Historic stream-sediment sampling results have also been digitized and published.

The IGS has conducted various analyses of a recent cooperative USGS-Industry airborne geophysical survey in this area, including statistical clustering techniques and band ratios of potassium to thorium concentrations. The results inform geologic mapping and identify areas where the radiometric response is discordant with known lithology and may represent alteration, mineralization, or regional-scale contact metamorphism.

Important results include: 1) revised and simplified regional stratigraphy correlates strata in the Yellowjacket area in the west with the Lemhi Group, Swauger Formation, and Apple Creek Formation eastward into the Lemhi Range; 2) outcrop MS measurements indicate that the Lemhi Group has at least one interval of elevated MS values and the lower part of the Apple Creek Formation has the highest values; 3) the banded siltite member of the Apple Creek Formation which hosts most of the Co mineralization has low magnetic susceptibility; 4) in most places Th concentrations are elevated in the Mesoproterozoic granitic rocks relative to the Belt Supergroup strata; and 5) Several areas identified as anomalous from the airborne survey share a similar radiometric signature to known REE occurrences within the area and represent target for further investigation.