GEOCHEMICAL ANALYSIS OF WEATHERING PRODUCTS AS AN EXPLORATION TOOL OF LCT PEGMATITES: PRELIMINARY RESULTS FROM FLORENCE COUNTY, WISCONSIN

As sustainable energy becomes increasingly reliant on lithium (Li), a critical component in rechargeable batteries, innovative exploration methods must be developed. Lithium-cesium-tantalum (LCT) pegmatites are an important source of Li and other rare metals. However, the weathering of LCT pegmatite minerals and the paths taken by Li and LCT-pathfinder elements from parent to alteration minerals, saprolite, soil, and vegetation remain under-explored. This study focuses on the geochemistry of weathering products as a potential exploration indicator for LCT pegmatites.

Samples were collected at two LCT pegmatites from Florence County, WI: Animikie Red Ace (ARA) rich in Li silicates – lepidolite and spodumene and King’s X2 (KX2) rich in spodumene and Li phosphate – amblygonite. The ARA and KX2 are subparallel 1-4 m thick, ~600 m long dikes intruded in schists and amphibolites, cropping out in a heavily wooded, glaciated terrain. We sampled soil horizons H and A at ≈ 13 cm depth, that formed 1) directly on pegmatite, along strike and 2) on host rock, along transects perpendicular to the strike, consisting of points 1.2 to 2.7 m apart. Representative rock samples and moss growing on bedrock were also collected to explore whether moss carries the geochemical signature of its substrate.

The soil was dried, passed through a 2 mm sieve to eliminate large plant remains, pulverized, and homogenized. Soils – water mixtures at a 1:4 ratio, were shaken for 24 hours and centrifuged. The soil leachates were filtered and analyzed using ion chromatography (IC) to quantify the soluble ions. The soils formed on amblygonite-rich KX2 had the highest contents of phosphate (PO₄^3-) and fluoride (F^-) of ~1900 and 200 ppb, respectively, whereas the soils formed on lepidolite-rich ARA had ~250 and 110 ppb, respectively. Acid-digested moss, pegmatite, host rock, and soil samples were analyzed using ion-coupled plasma optical emission spectroscopy (ICP-OES) to determine bulk-chemistry correlations between the parent material and the weathering products. Optical and scanning electron microscopy (SEM) was used to characterize the primary minerals and their alteration products in polished thin sections. Also, unpolished fragments were imaged in back-scattered electrons (BSE) to capture the interface between moss and its mineral substrate.