Abstract

The Proterozoic Wausau syenite complex, exposed in central Marathon County, Wisconsin, is an A1-type within-plate granitoid consisting of four intrusive centers. The Nine Mile pluton is the youngest and most siliceous intrusion; the next oldest is the Rib Mountain pluton, then the Wausau pluton, and the Stettin complex is the oldest and most strongly alkaline of the four intrusive bodies. Pegmatites abound at all four intrusive centers and are generally rich in REE minerals. Locally, Be, Nb–Ta, and Zr minerals abound. The Nine Mile pluton is the best exposed of the four intrusives and has been studied in detail. Heterogeneity with respect to its mineralogy exists. The northern portion is rich in Be minerals and carbonates, but poor in fluorite. Carbonate likely formed a weak complex with Be, which sequestered it in the melt until it was released at the miarolitic cavity stage, forming phenakite and other Be minerals. Fluorite is much more abundant in the western and southern portions of the Nine Mile pluton, where it is associated with late-stage Ta and Mn enrichment in columbite–tantalite species and Hf enrichment in zircon. The high F content is likely responsible for the enrichment owing to the formation of coordination complexes, which then released higher Ta, Mn and Hf late in the crystallization history. The REE minerals are fairly constant in their abundance in all four intrusive centers.

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