Phase I Activities of the Earth Mapping Resources Initiative (Earth MRI) in the Western Kentucky Fluorspar District

The Western Kentucky Fluorspar District (WKFD), which comprises the Kentucky portion of the Illinois–Kentucky Fluorspar District, is well known for the occurrence of strategically important critical mineral resources, such as fluorite (fluorspar) and rare earth elements (REEs). Due to technological advances, demand has increased for critical minerals used in cell phones, magnets in wind turbines, batteries for electric vehicles, smart weapons, and other devices and systems. The overall objective of the Earth Mapping Resources Initiative (Earth MRI) program, directed by the U.S. Geological Survey (USGS), is to gather information that will help better understand the distribution of critical minerals in the United States. This report details the Phase I Earth MRI activities conducted by the Kentucky Geological Survey (KGS), including a review of more than 1,400 mineral and rock specimens that resulted in the inventory of 486 specimens relevant to future Earth MRI work. Furthermore, 3,590 digital files were reviewed and indexed with keywords and other attributes to create an upgraded online map service, the Kentucky Mineral Resources Information Map Service, with advanced search functions. Thirty-nine probable igneous dikes that may be an economically viable source of REEs were identified using a filtered aeromagnetic dataset provided by the USGS, and these features were incorporated into a 1:50,000-scale geological map that covers the WKFD. The total REE content (including yttrium and scandium) in 66 analyzed dike samples range from 152 ppm to 620 ppm with an average of 341 ppm. The various types of igneous dikes that have been identified—alnöite, aillikite, and rocks in which carbonate alteration predominates—have overlapping but somewhat distinct concentrations of REEs with alnöite samples having the highest, the carbonate alteration predominant samples the lowest, and the aillikite samples intermediate average total REE values. The relatively high REE content in one massive calcite vein (280 ppm) suggests remobilization of REEs and warrants further study of fluid–rock interactions to better understand the mineral system of the WKFD