PETROGENESIS OF THE CROSS LAKE PEGMATITE FIELD, CENTRAL MANITOBA

The identification of groups of cogenetic granitic pegmatites within a large pegmatite field is important for targeting rare metal prospects and for better understanding the process by which these deposits form. A genetic relationship between spatially related pegmatites is commonly inferred from structural and paragenetic relations, and from trends in minor or trace element content of major minerals (feldspars, micas, oxides). However, such links are typically unconstrained by absolute geochronology and initial radiogenic isotopic compositions. In the Cross Lake field, central Manitoba, dikes of two granitic pegmatite series occur within separate east-west trending fault zones. In the two series simple quartz-feldspar pegmatites crop out in the west and rare-element enriched pegmatites in the east. Minerals of the two series display separate, parallel trends of rare alkali contents. The most highly evolved pegmatites within each series are distinguished by the presence of lithium mineralization in the southern series, and by the abundance of phosphate minerals and the absence of Li mineralization the northern series. With the extant data, two petrogenetic models are equally plausible: the series are the products of distinct granitic sources, or the series are comagmatic and developed geochemical distinctions because of the heterogeneity of the parent melt. To test the hypothesis that the two series were derived from different sources, we are investigating the Sm-Nd isotopic systematics of fluorapatite crystals from different pegmatites in the northern and southern series. Apatite concentrates the lanthanides in these pegmatites and is likely to preserve magmatic isotopic signatures undisturbed by subsequent open system behavior. Such an approach provides an independent means of assessing pegmatite sources, particularly in complex systems such as the Cross Lake field. We shall discuss the derivation of pegmatites in the Archean Cross Lake greenstone belt in light of the assembled trace element data and new Sm-Nd isotopic measurements.