EARTH MRI IN IDAHO: GEOLOGIC MAPPING, GEOCHEMICAL CHARACTERIZATION, AND MAGNETIC SUSCEPTIBILITY OF MESOPROTEROZOIC STRATA IN THE IDAHO COBALT BELT

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. The ICB consists of strata-bound Fe-Co-Cu-Au-Bi-Y-REE deposits that have a complex history. 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, for use by industry, state, and federal geologists to better understand the distribution, age, and controls of mineralization.

The IGS has published new geologic maps of three 7.5' quadrangles along the ICB and one new geologic map compilation for the area around the Blackbird mine. Additional sampling and geologic mapping in two additional quadrangles are ongoing. In addition, the IGS has collected stream-sediment samples and mineralized and unmineralized rock samples; these provide baseline compositional data for Cu-Co and REE concentrations. Historic stream-sediment sampling results have also been digitized and posted on the IGS website. A cooperative USGS-Industry aeromagnetic and radiometric survey in this area will be released by the USGS in 2022.

Important results include: 1) revised and simplified regional stratigraphy correlates strata in the Yellowjacket mine area in the west with the Lemhi Group, Swauger Formation, and Apple Creek Formations 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; mudcracks in these rocks are evidence of subaerial exposure during deposition; 3) the overlying banded siltite member of the Apple Creek Formation hosts most of the Co mineralization, lacks mudcracks, and contains small amounts of carbon in the fine-grained layers; 4) an early (Mesoproterozoic) style of Cu-Co mineralization may have involved preferential deposition from metal-bearing brines at the upper and lower contacts of the banded siltite member; and 5) later vein mineralization and remobilization of sulfides (particularly chalcopyrite) have obscured the earlier Cu-Co mineralization in rocks at higher metamorphic grade, but remnants may be present at the Iron Creek deposit in the southeast part of the ICB.