EXPLORING FOR LITHIUM-CESIUM-TANTALUM (LCT) PEGMATITES: WEATHERING AND DISPERSION AUREOLES ASSESSED THROUGH SOIL GEOCHEMISTRY

As sustainable energy increasingly relies on lithium, novel exploration methods must be developed. The focus of this study is weathering of LCT pegmatites and surrounding host rocks and the paths taken by Li and LCT-pathfinder elements from the pegmatite 1) to the host rock within the contact aureole and 2) to weathering products (alteration minerals, saprolite, and soil). The two pegmatites studied are in the Florence County, WI pegmatite field, Animikie Red Ace (ARA), and King’s X2 (KX2). These pegmatites intruded in micaschists and amphibolites, are 1-4 m thick and 100s m in length, and outcrop in the forested Pine-Popple Wild River Preserve. The ARA is a highly-differentiated pegmatite with lepidolite, spodumene, and Li-rich tourmaline. KX2 is rich in spodumene and amblygonite–montebrasite (Li phosphates).

Soil samples included 1) soils formed directly on pegmatite outcrops, 2) soils collected along transects perpendicular to the exposed dikes extended into the host rock, and 3) soils collected along blind transects across hypothetical extensions of dikes completely covered by soil. Representative rock samples were also collected to examine how the LCT-pathfinder elements diffuse from the pegmatite to the host rock and how the pegmatite’s signature is mirrored in the overlying soil cover along the soil transects.

Ion-coupled plasma optical emission spectroscopy (ICP-OES) analysis was used to quantify the soluble pathfinders released by soil leaching after shaking samples for 24 hrs in nano pure water and the bulk composition of the soils and rocks after microwave-assisted acid digestion. Scanning electron microscopy – energy dispersive spectroscopy (SEM-EDS) was used to characterize the alteration minerals at the weathering surface and along superficial fractures. Biologically induced chemical weathering was studied at the microscopic interface between Li minerals and lichens.

The compositional transects indicate that soil geochemistry is a promising exploration tool for LCT-pegmatites. Dispersion–aureole soils are enriched in Li, B, P, S, Rb, and Sn and depleted in Fe and Ti. For example, soil concentrations ranged from < 340 and < 1650 ppm of Li and Rb, respectively, when formed on the ARA pegmatite, to as low as < 10 and < 50 ppm of Li and Rb, respectively, at 20 m away from the pegmatite.