Sm-Nd isotope technique as an exploration tool:: Delineating the northern extension of the Thompson nickel belt, Manitoba, Canada

Application of Nd isotope mapping to aid mineral exploration has been tested successfully in areas north of the well-prospected, productive portion of the Thompson nickel belt in Manitoba, Canada. Economic nickel deposits in this main part of the belt are restricted to the Ospwagan Group cover sequence and ultramafic sills overlying Archean basement gneisses. Distinguishing some units ill the Ospwagan Group from its basement and less prospective Paleoproterozoic paragneiss of the Burntwood Group in the area is difficult based on lithology, stratigraphy, geochenristry, and structure alone. However, the Ospwagan Group has a unique Nd isotope signature that allows it to be identified with certainty. A compilation of Nd isotope data from the main portion of the Thompson nickel belt and a new dataset from drill core and reconnaissance mapping clearly support early exploration indications that basement gneiss and narrow belts of the Ospwagan Group typical of the belt extend northeast of the main part of belt for at least 100 km. This confirms the presence of a large new area for nickel exploration, the limits of which are still unknown. In this poorly exposed, northern area, crustal-residence Nd model ages (T-CR) are instrumental in clearly separating (1) the Ospwagan Group supracrustal rocks (T-CR = 2.8-3.2 Ga) that host the nickel deposits from (2) Neoarchean basement gneiss (T-CR = 3.2-3.5 Ga), (3) juvenile, overthrust Burntwood (TCR = 2.2-2.6 Ga), Grass River, and Sickle (T-CR = 2.0-2.4 Ga) Groups paragneisses, and (4) fragments of a Mesoarchean gneiss complex (T-CR = 3.5-4.1 Ga), the latter being the host of precious metal prospects. The variation in Nd model ages not only distinguishes the different supracrustal successions, but the variable inheritance of ancient crustal material in units of Archean gneiss reflected in Nd model ages serves as a tool to unravelling the complexly interleaved structure of this crust and aids in tracing the potentially nickel-hosting Ospwagan Group in narrow structural keels. The robustness of the Nd isotope system under the conditions of high-grade metamorphism and migmatization attained in the Thompson nickel belt makes this all ideal tool for distinguishing between units of economically prospective and barren gneiss in frontier areas where other methods have failed, are difficult to apply, or have led to erroneous interpretations.