Geophysical characterization of the Elk Creek Carbonatite based on 3D joint inversion and geology differentiation

The Elk Creek Carbonatite located in southeastern Nebraska hosts the largest known niobium deposit in the United States. Most of the known niobium is hosted within magnetite-dolomite carbonatite, a dense and highly magnetized unit within the carbonatite. The shallower lithology of the carbonatite has been well explored by boreholes, but the deeper lithology remains poorly understood. Three-dimensional joint inversion of airborne gravity gradiometry and magnetic measurements was performed, producing a structurally coupled density and susceptibility model. Geology differentiation, a process of classifying the recovered subsurface models into distinct units, was then carried out to develop a 3D quasi-geology model. Physical property measurements based on drill core samples and analysis of inverted physical property values in the spatial domain were used for geology differentiation. The resulting quasi-geology model, an approximation of the subsurface geology, shows the spatial distribution of various geological units in 3D, and includes units at greater depths than previous studies on the region. I identified 11 geological units with each characterized by a distinct combination of density and susceptibility values. These units include the country rock surrounding Elk Creek, various carbonatites, mafic rocks, the niobium target zone, and additional distinct geological units which have not been previously classified. Geology differentiation also identifies the known niobium ore zone and indicates the existence of a significant volume of dense and strongly magnetized rocks below the deepest boreholes. These rocks are likely to be associated with unexplored niobium mineralization. This thesis work is the first attempt at constructing a 3D quasi-geology model in the study area based on airborne geophysical measurements, and demonstrates the added value of 3D geophysical inversions and geology differentiation when it comes to mineral exploration under thick sedimentary overburden