GEOPHYSICAL IMAGING OF THE BURIED EXTENTS OF SELECTED LAYERED MAFIC INTRUSIONS AND RELATION TO PLATINUM GROUP ELEMENT EXPLORATION

Layered, ultramafic-to-mafic intrusions such as the Stillwater, Bushveld and Duluth Complexes are economically important because they contain concentrations of chromium, nickel, copper, titanium, vanadium, and critical platinum-group elements (PGE’s). Large igneous systems are more likely to have PGE-enriched deposits because of the large volumes of silicate melt that required to form these deposits. Geophysical models indicating thick stratigraphic sections of cumulates and the presence of ultramafic rocks could be used to prioritize exploration activity. Gravity, magnetic, and seismic reflection data help determine the lateral extent and thickness of layered mafic intrusions. Gravity highs characterize the Stillwater, Duluth, and Bushveld Complexes. A 3D gravity model, constrained by magnetic and seismic reflection data, of the Stillwater Complex, indicates that it extends 30 km north and 40 km east of its outcrop beneath Phanerozoic cover and varies in thickness from 3,000 to 10,000 m, depending on the density contrast of the model. Two newly re-processed seismic lines cross portions of the gravity model related to buried Stillwater Complex and show layering in the upper to mid-crust (to depths of ~18 km) that may relate in part to the Stillwater Complex and underlying Archean basement. Inferred mid-crustal faults may reflect the Archean tectonics that originally displaced the Stillwater Complex. The 3D gravity and magnetic models of the Bushveld Complex north of the Thabazimbi-Murchison Lineament (TML), indicates that it is ~4 km thick, 160 km x ~125 km, and underlies ~1-2 km of cover. Modeled thick spots in the TML may represent feeders to the Bushveld Complex. Seismic reflection data from the Bushveld Complex image mineralized strata. The Duluth Complex magnetic data, when reduced to the measured paleomagnetic pole, show magnetic highs over normally magnetized intrusive and volcanic sequences and lows over reversely magnetized volcanic rocks. Archean granites below the low reflectivity Duluth Complex produce clear reflectors, providing constraints on the maximum thickness of the complex on its western end. Volcanic rocks overlying the complex near Lake Superior produce disrupted but laterally traceable reflectors, providing constraints on the depth of the top of the Complex in this region.