Timing and controls on Ni-Cu-PGE mineralization within the Crystal Lake Intrusion, 1.1 Ga Midcontinent Rift

Abstract

North America's 1.1 Ga Midcontinent Rift (MCR) hosts a diverse range of ultramafic and mafic intrusions, many of which contain Ni-Cu-PGE mineralization. The gabbroic, sulphide-bearing Crystal Lake Intrusion was emplaced between 1095-1091 Ma during the 'main-stage' of rift evolution. The Ni-Cu-PGE mineralization shows a close spatial association with contaminated, Cr-spinel-bearing, vari-textured gabbros. Despite the MCR having a rich legacy of research, the temporal and spatial controls on ore formation, in addition to the processes critical for ore genesis, remain poorly constrained. Here we present new field, petrographic, and mineralogical observations, with preliminary U-Pb zircon and baddeleyite isotope dilutionthermal ionization mass spectrometry (ID-TIMS) results for various phases of the Crystal Lake Intrusion to resolve questions regarding the timing of emplacement and its relationship to other MCR magmatic events, such as the Mount Mollie dyke, the Pigeon River dykes, and the Cu-Ni-bearing Duluth Complex. Results suggest that the sulphide ores at Crystal Lake were formed under low confining pressures from a magma saturated in volatiles through contamination. The addition of crustal S prior to emplacement, by assimilation and melting of wall rocks and xenoliths entrained in the magma, is considered critical for the genesis of the ores. The local addition of S in situ, as indicated by S/Se ratios, likely contributed only minor S into the system and is not considered essential for S saturation. Reworking of the magmatic system is indicated within the taxitic units by local brecciation, 'ripped' sulphide clasts and large (>1 cm) globular sulphides, which may indicate minimal transportation of the ores and the potential of a deeper sulphide pool/source. The pronounced magnetic anomaly of the southern limb may suggest the presence of a feeder dyke in the south. Palladium (91-99%) and Pt (>99%) reside primarily as platinum group minerals (PGMs) in association with sulphides. The PGM assemblage, which is largely controlled by the availability of semi-metals through contamination, is dominated by Pt arsenides, Pd-Bi-Sb}Te phases, Pd-Sn}Sb phases, Pd bismuthides, and Pd antimonides. The similarity of the PGM assemblage throughout the Crystal Lake Intrusion indicates crystallization from compositionally similar magmas. The distribution of elements is consistent with the fractionation of a sulphide liquid, with later low-temperature alteration locally remobilizing elements at the micro-scale. The preferential partitioning of Mo, As, Bi, Re, and Pd into discrete parallel linear features and the development of a micro-fabric within unaltered sulphide globules suggests localized remobilization of select elements post-sulphide fractionation.