LITHIUM-CESIUM-TANTALUM (LCT) PEGMATITE EXPLORATION IN FLORENCE COUNTY, WISCONSIN, U.S.A.: FOLLOWING THE LITHIUM FROM BEDROCK TO SOIL

The lithium (Li) demand for electric batteries exceeds the known available resources. Consequently, it is vital to develop new exploration methods for LCT pegmatites, the main hard rock Li resource. This study focuses on Li transfer processes from the parent LCT pegmatite to the metamorphic host rock and their soil cover. The Florence Co. pegmatite dikes are 1 – 7 m thick, > 50 m in length, and outcrop in glaciated and forested terrain. The pegmatites’ host rocks are mafic and felsic metavolcanics of the Quinnesec Formation, metamorphosed during the Penokean Orogeny (1,875-1,835 Ma). The dikes are syn- to post-tectonic. Although they contain Li minerals of economic interest such as spodumene, petalite, Li-tourmaline, lepidolite, and amblygonite, they are relatively unexplored. We aim to 1) understand the mineralogical and chemical changes occurring in the metasomatic aureole, as hydrothermal fluids infiltrated the host rock during magma emplacement and 2) use soil geochemistry to explore for LCT pegmatites.

Rock samples included pegmatites, host rocks in the contact aureole, and remote schist and amphibolite country rock. Optical microscopy and SEM-EDS were used to identify metasomatic minerals formed in the pegmatite’s endo- and exo-contacts. We recorded the first Wisconsin occurrence of holmquistite (Li-amphibole) in the amphibolite host of an LCT dike.

The H or A soil horizons were sampled at depths of <0.5 m. We collected soils formed on 1) pegmatite bedrock, 2) host rock (transects perpendicular to the exposed dikes), and 3) hypothetical extensions of dikes completely covered by soil (blind transects). The bulk geochemistry of soils and rocks was analyzed using ICP-OES after microwave-assisted acid digestion. Maximum contents of LCT pathfinders in soils formed on LCT pegmatites and their host rock (LCT) far exceed the contents in soils on unmineralized granite and pegmatites (simple).

ppm	LCT	Simple
Lithium	1250	40
Rubidium	1660	110
Boron	3080	220

If glacial till is avoided, the geochemistry of soil cover is a promising indicator of the bedrock type and can reveal concealed LCT pegmatites. The positive geochemical anomalies defined by the LCT pathfinders (Li, Rb, B, ±Sn) extend 10 to 30 m away from the dikes. This approach could be used to expand known LCT pegmatite occurrences with similar weathering environments.