11-7 Investigating the source for Mesoproterozoic lithium-cesium-tantalum pegmatites in northern WI: an integrated geophysical and geochemical approach

Session: Applications of Igneous Petrology, Geochemistry, and Geochronology to Understanding Earth and Other Planetary Systems (Posters)

Presenting Author:

Authors:

Abstract:

Lithium is an important global commodity for the development of reduced carbon emission technology like electric vehicles and battery storage. Lithium-cesium-tantalum (LCT) pegmatites, an important source of lithium, typically form as late-stage fractionation products of proximal (≤5 km) peraluminous granites or through partial melting of Li-bearing metamorphic rocks. Emplacement of LCT pegmatites in Florence County, WI, USA has been associated with regionally widespread granite emplacement between the end of the Penokean orogeny (~1835 Ma) and Yavapai arc accretion (~1750–1700 Ma). Field-geochemical models hypothesize the pegmatites formed either by fractional crystallization of the nearby ~1835 Ma Bush Lake granite or by partial melting of the metasedimentary/metavolcanic wall rock, the Paleoproterozoic Quinnesec Fm. However, new LA-ICP-MS U-Pb geochronology for magmatic apatite within the King’s X and Animickie Red Ace LCT pegmatites and neocrystallized titanite in the proximal wall rock yields Mesoproterozoic ages. These new data suggest LCT pegmatites formed episodically beginning around ~1450 Ma. The King’s X pegmatite crystallized at 1444 ± 6 [29] Ma, shortly followed by the Animickie Red Ace at 1432 ± 5 [29] Ma, which may have derived from partial melting or fractionation from an older LCT pegmatite – like the King’s X – at depth. Mesoproterozoic crystallization ages thus reveal that a previously unrecognized Mesoproterozoic thermal and fluid overprint in the region was responsible for LCT pegmatite emplacement and expand the set of possible source rocks for the pegmatites. New hypotheses to be tested for formation of the Florence County LCT pegmatites include 1) fractional crystallization from an unexposed Mesoproterozoic pluton at depth, 2) partial melting of the exposed rocks in the nearby area (Bush Lake granite or Quinnesec Fm.) during the Mesoproterozoic, or 3) far-traveled fluids/melts from the Wolf River batholith (exposed ≥24 km south) that utilize large-scale, potentially re-activated, structures like the south-dipping Niagara Fault zone for transport. We will explore these hypotheses through a combination of forward- and inverse-modeling of existing gravity and aeromagnetic data to better image the subsurface geology beneath the pegmatite outcrops, and whole rock major- and trace-element geochemical fractionation/melting models that consider a broader suite of source rocks (i.e., the ~1865-1835 Ma Dunbar gneiss dome suite, the ~1760 Ma intrusive rocks, and the ~1476 Ma Wolf River batholith suite) from northern Wisconsin.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

doi: 10.1130/abs/2026SE-12899

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