The 1093 Ma Crystal Lake Intrusion: A nickel-copper mineralized intrusion emplaced during the younger southwest–northeast rift phase of the Midcontinent Rift (North America)

Abstract North America’s 1.1 Ga Midcontinent Rift preserves a remarkably complete record of Proterozoic continental rifting and large-scale magmatism. Here, we present new, high-precision U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) ages for the Ni-Cu-platinum group elements (PGE)–bearing Crystal Lake Intrusion and its feeder, the Mount Mollie dike. Our new data constrain the emplacement of the Crystal Lake Intrusion to 1092.9 ± 0.8 Ma and the Mount Mollie dike to 1094.0 ± 1.1 Ma. These ages place both within the final stages of a 1097–1092 Ma magmatic episode that followed the emplacement of the 1099 Ma Duluth Complex. This magmatic episode corresponds to a younger phase of rifting, marked by a pronounced reorganization of the rift axis from a northwest–southeast to a southwest–northeast orientation and the spatial focusing of magmatism into a narrow, structurally controlled corridor along the present-day northwestern shoreline of Lake Superior. This corridor, termed the North Shore Magmatic Feeder Zone, includes the Pigeon River and Mount Mollie dikes as well as the Beaver Bay Complex. It is interpreted as a deep-seated, magmatic pathway that fed the final major phases of Midcontinent Rift volcanism. The temporal overlap between this 1097–1092 Ma episode and the widespread magmatism of the Southwestern Laurentia large igneous province in the southwestern United States supports a broader geodynamic linkage across Laurentia during Mesoproterozoic plume activity. Our data indicate that the Crystal Lake Intrusion represents a dynamic, multiphase system formed by repeated pulses of magma. It was fed laterally by the large, composite Mount Mollie dike. Sulfide mineralization occurred during early volatile- and sulfur-saturated pulses, producing globular ores above older Logan sill rafts in the southern limb and massive sulfides in the northern limb via in situ contamination of sulfur-rich sediments. Distinct Cu/Zr and rare earth element patterns shared by both intrusions support derivation from a common, shallow, depleted-mantle source. This distinguishes them from earlier Midcontinent Rift intrusions. These findings emphasize that mineralization during the 1097–1092 Ma episode was largely restricted to intrusions emplaced into sulfur-bearing sedimentary basins, with the transition from dikes to sill-like geometries emerging as an important control on sulfide accumulation. The refined genetic and temporal framework presented here offers an improved basis for targeting Ni-Cu-PGE mineral systems in rift-related provinces. Moreover, our results reinforce the interpretation that late-stage Midcontinent Rift magmatism formed part of a broader, continent-scale phase of plume-influenced activity across Laurentia consistent with the timing of the Southwestern Laurentia large igneous province.

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