The geochemistry and petrogenesis of the Blue Draw Metagabbro

The Blue Draw Metagabbro (BDM) in western South Dakota, is an 800m thick, layered intrusion, which is interpreted to have been intruded as a rift-related subvolcanic sill during the Palaeoproterozoic. The age and tectonic setting of the BDM are similar to those recorded by the East Bull Lake Suite of layered intrusions in Southern Ontario. These similarities have led previous authors to suggest that the two sets of intrusions are cogenetic. The East Bull Lake Suite intrusions are relatively well studied and are known to host significant contact-type Ni–Cu–PGE sulphide mineralisation, however, prior to this work, relatively little was known of BDM both in terms of its geochemistry and mineralisation potential. Chemostratigraphic profiles through the BDM show that the intrusion is the product of at least two magma pulses which fractionated to produce a sequence of rocks which grade from peridotitic at the base of the intrusion to gabbronoritic at the upper margin. Closed-system fractionation following the intrusion of the second magma pulse caused the magma to become saturated in sulphur and precipitate Ni–Cu–PGE bearing sulphides—now preserved in a low-grade 50m thick zone near the top of the intrusion. Petrological modelling shows that the parental melt of the BDM was a low-Ti tholeiite, with a trace element chemistry defined by enrichments in large-ion lithophile and light rare-earth elements and prominent negative Nb, Ta, and Ti anomalies. This ‘arc-like’ geochemistry recorded by the BDM parent magma is shared with parent magmas of the East Bull Lake Suite and may suggest that the BDM and its potential Canadian relatives share a common magmatic source. However, the relative ubiquity of such geochemical signatures in Archaean–Palaeoproterozoic intracontinental magmatic rocks coeval with the BDM suggests that the geochemical similarities recorded by the BDM and East Bull Lake Suite are non-unique and hence, are not definitive evidence of a genetic link between the two sets of intrusions. Instead, this geochemical signature that is common to many ancient igneous provinces may indicate the presence of a transient and (currently) poorly understood Archaean–Palaeoproterozoic mantle reservoir which was a globally significant magma source.