ANOMALOUS TA-ENRICHMENT IN PEGMATITES OF THE UPPER MIDWEST

Pegmatite populations exposed in the states of Wisconsin, Michigan and Minnesota, contain a characteristic late-stage tantalum-signature. Among the pegmatites populations investigated are the 1.5 Ga Wausau syenite complex (WSC) in Marathon Co., Wisconsin; pegmatites of the associated Wolf River batholith (WRB), Waupaca Co., Wisconsin; 1.4 Ga pegmatites near Waterloo, Jefferson Co., Wisconsin (WP); 1.8 Ga pegmatites in Fern Township, Florence Co., Wisconsin (FP); 1.8 Ga pegmatites in Dickinson and Marquette Cos., Michigan (DMP) and Neoarchean pegmatites around Orr, Saint Louis Co., Minnesota (OP).

Both NYF (Niobium-Yttrium-Fluorine) and LCT (Lithium-Cesium-Tantalum) type affiliations in the Upper Midwest show anomalously high Ta and Mn enrichments. The WSC and the associated WRB host clearly NYF-type pegmatites whereas the FP and WP show strong indicators for the LCT-types. The cases for the OP and the DMP are less clear but the trend is comparable to the other locations and further work is pending. The OP is of Archaean age and the DMP and all the other pegmatite locations of this study are Proterozoic in age and thus tectonic conditions have likely been different.

The LCT type pegmatites already have high Ta-species in the outer zones of the pegmatites which suggests that the S-type melts were unusually enriched in Ta.

Generally, NYF-type pegmatites contain low Ta- and Mn-concentrations, but these pegmatites contain unusual late-stage enrichments of Ta and Mn. Even though the bulk of the columbite-tantalite species identified are columbite-(Fe) and columbite-(Mn), a late-stage increase of Ta and Mn is present. We infer that this may be due to contamination of the melts as they passed through the overlying older Ta-enriched crustal material.

We suggest that the basement source rocks in the upper Midwest, which is part of the Canadian Shield, contain elevated Ta and thus impart that signature to pegmatites formed by partial melting of this source rock. Higher degree of fractionation for Ta and Mn as exhibited by the LCT affiliation will push the limits to near end-member composition in those pegmatites but even in NYF pegmatites, very late-stage fractionation produced near end-member tantalite-(Mn) compositions as well.