Mineralogy and Petrology of Rare Element Pegmatites in The Eau Claire River Complex, Eau Claire County, WI

Abstract

Pegmatites are economically targeted for mining critical metals (Nb, Ta, Zr, Y, Th, U and REEs). The Eau Claire River pegmatite complex, originally mistaken as potassium-feldspar granite, are highly fractionated, garnet, two-mica, albite, quartz Nb, Y, F (NYF) pegmatite granites significantly enriched in a wide variety of high field strength (HFS) elements. The rare HFS elements occurs in a variety of mineral phases including REE-epidotes, phosphates (LREE-monazite and HREE-xenotime), REE-carbonates (parasite), oxides (uraninite, thorite, columbite group minerals, Zn spinel (gahnite)) and Hf and U-enriched zircon series minerals. The extreme fractionation of the alkalic magma that formed these pegmatites results in Zr/Hf ratios of significantly less than 10. Most of the Penokean age plutonic rocks in the Eau Claire region are tonalites, a product of decompression melting of basalts and gabbros. The Eau Claire River pegmatite complex mineralogy is not compatible with fractionation of Penokean age granitoids. Rare-element pegmatites of the NYF association are always associated with Na-rich anorogenic magmatic complexes. Rare NYF pegmatite occurrences are known in central Wisconsin and have ages consistent with Wolf River magmatism. Based on the NYF mineralogy, the Eau Claire pegmatites are most likely related to 1.4Ga anorogenic magmatism in central Wisconsin.