GEOCHEMICAL DISPERSION HALOS OF LITHIUM PEGMATITES AS A POTENTIAL EXPLORATION TOOL: A PRELIMINARY COMPARATIVE STUDY IN FLORENCE COUNTY, NORTHEASTERN WISCONSIN, USA

As the demand for lithium (Li) is expected to increase over the next decades, the search for new sources is critical. In this study, we focus on Li and Li pathfinder mobility from the pegmatite system to its host rock. Constraining the geochemical transfer during the pegmatite emplacement would enable the use of dispersion halos as an exploration indicator. The ~10 km² Florence Co. pegmatite field contains several Li-rich pegmatites intruded in Proterozoic upper greenschist to amphibolite-facies metamorphic rocks. The King’s X2 (KX2) was documented in previous studies as a spodumene-bearing, 0.5 to 4 m thick, ~35 m long dike, emplaced discordantly in amphibolite. Here we introduce a newly discovered, 3 m thick, ~50 m long pegmatite, the Price Lake East (PE). We aim to 1) document the mineralogy of PE and surrounding amphibolite and 2) measure and compare the extent of the Li and rubidium (Rb) dispersion halos in the KX2 and PE host rocks using traverses of samples at a known distance from the contact.

The PE is dominated by quartz, K-feldspar, albite plagioclase, and white to purple mica. Tourmaline is the main accessory mineral. Unlike KX2, PE is not internally zoned and lacks Li minerals of economic interest such as spodumene or petalite, at least at outcrop observations. However, amblygonite-montebrasite, a rare accessory Li phosphate, was discovered in the lab, suggesting that PE is highly fractionated and may be mineralized.

Optical and scanning electron microscopies were used to document the mineral assemblages and textural features of pegmatites and their host rocks. Metasomatic minerals recognized at <1 m away from the contact include schorl tourmaline, K-feldspar, apatite, clinozoizite, ± holmquistite (a Li amphibole). Portable laser-induced breakdown spectroscopy (pLIBS) and X-ray fluorescence (pXRF) were used on rock samples to assess the extent of the dispersion halos. Among other pathfinders analyzed, Rb seems to correlate best with Li. At one PE traverse, the Rb contents of the host amphibolite drop exponentially from the contact levels of ~2700 ± 600 ppm to only 65 ± 57 ppm at 6 m away from the contact, reaching a background level (Rb <20 ppm) at 12 m from the contact. This is comparable to the KX2 trend of Rb declining from ~3500 ± 270 ppm Rb to the background levels at 7 m from the contact. Its well-developed halo suggests that PE may contain yet undiscovered Li mineralization.