60-8 Rare-element Geochemistry of the Paleoproterozoic Pegmatites in the Marshfield Terrane of the Penokean Orogen, Wisconsin.

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

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Minerals, such as monazite and xenotime, are important sources of rare earth (La, Ce, Nd) and high field strength (Th, Nb, Zr) elements which are essential for modern energy, communication, and military technologies. These critical minerals are often sourced in pegmatites and are important exploration targets worldwide. The Archean Eau Claire Volcanic Complex (ECVC) in the Marshfield Terrane of the Penokean Orogen is intruded by granitic pegmatite dikes that postdate peak metamorphism, indicating they are unrelated to Penokean-aged orogenic events. The ECVC pegmatites are highly fractionated, garnet bearing, and contain a high concentration of U, Th, La, and other rare earth elements. Based on major and trace element associations, the pegmatites in the ECVC are classified as NYF family pegmatites that contain Nb>Ta, REE, U, Th, Zr, and are A- to I- types with peralkaline magmatic affinities.  

 This study collected bedrock samples from several locations across the ECVC (Little Falls, North Fork, Muskeg Creek). The pegmatite dikes can range in size from a few meters to 100m wide near the North Fork of the Eau Claire River. They mainly intrude foliated and metamorphosed Paleoproterozoic to Archean tonalites, amphibolites, and gneisses. Samples from these pegmatites were analyzed for whole rock and mineral chemistries. Whole rock geochemistry was analyzed on XRF and ICPMS whereas mineral chemistry was determined using SEM-EDS.  

The ECVC pegmatites have quartz, biotite and muscovite, albite with low quantities of orthoclase, and trace minerals with rare earth elements. Little Falls sample trace mineralogy includes Fe- and Mn-garnet, samarskite, columbite, and xenotime. Muskeg Creek sample trace mineralogy includes xenotime, monazite, and barite. North Fork sample trace mineralogy includes Fe- and Mn-garnets, monazite, xenotime, and thorite. The North Fork and Muskeg Creek samples have more barium-rich minerals than Little Falls, which may result from more fractionation of feldspars and plagioclase. Mn-rich garnets at Little Falls and North Fork indicate a higher degree of fractionation relative to Fe-garnets at Muskeg Creek. The North Fork is enriched in heavy rare earth elements, U, and Th. In comparison, Little Falls has more light rare earth elements. Muskeg Creek is also enriched in light rare earth elements and has an increased enrichment of heavy rare earth minerals. All three locations contain other metals such as Nb, Zr, and Hf. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7085

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