- © 2017 Society of Economic Geologists.
The Sudbury Igneous Complex is the preserved remnant of a 1.85 Ga impact-induced crustal melt sheet that eventually crystallized into a coherent magmatic stratigraphy. At its base, the complex hosts world-class Ni-Cu-platinum group element (PGE) sulfide mineralization, distributed around the basin. Previous studies have shown strong Pb and Sr isotope contrasts between the North and South ranges of the Sudbury Igneous Complex. This study presents an extensive dataset of Pb isotopes and trace elements of hand-picked feldspars and sulfides from the complex, Ni-Cu-PGE mineralization, and underlying country rocks that reveals the finer details of isotopic decoupling between the North and South ranges.
In terms of mean Pb isotope compositions, the new data confirm the previously established picture that North Range Main Mass rocks and associated Ni-Cu-PGE sulfide deposits are more juvenile than South Range Main Mass and associated mineralization, but there are several findings that point to a more complex evolution, with considerable implications for the source of sulfide in the Sudbury Igneous Complex. First, North Range sulfide ores and Main Mass silicates are more radiogenic than local footwall rocks. When expressed as per mil deviation in 207Pb/204Pb from a model contemporaneous mantle isochron (Δ207Pb/204Pb), the North Range Main Mass ores range from +130 to +421 and North Range ores range from +160 to +518 (primarily +160 to +260). This indicates that the Huronian Supergroup cover constituted a significant target rock type also in the North Range of the Sudbury Igneous Complex. Second, there is significant variability in the Pb isotope compositions between Ni-Cu-PGE sulfide deposits in both the North and South ranges and systematic differences between the average silicate compositions and sulfide compositions of Ni-Cu-PGE deposits. In the South Range Main Mass rocks, the Δ207Pb/204Pb values range from +360 to +494, and associated ores range from +150 to +481 (primarily +400 to +480). Third, North Range mafic norite is much less radiogenic than North Range Main Mass, indicating that it may be contaminated by or derived from less radiogenic Ni-Cu-PGE-bearing mafic-ultramafic rocks. Finally, the Pb isotope variations between Sudbury Igneous Complex ores are corroborated by similar systematic variations in rare earth element patterns likely inherited by assimilation of local footwall rocks.
Importantly, the considerable variations in the Pb isotope compositions of the ores between different deposits and the intermediate compositions between overlying Main Mass and footwall rocks suggest that the sulfides originally equilibrated with Main Mass, but acquired significant amounts of Pb (and therefore likely S and other metals) from local footwall rocks, most likely during thermomechanical erosion of the footwall rocks by dense molten sulfides. Potential S and metal sources are the Nipissing gabbros and the East Bull Lake intrusions, both of which are mineralized and whose Pb isotope compositions help to explain the variability in Sudbury Igneous Complex Ni-Cu-PGE sulfide ores. Small-scale lateral variations in Pb isotopes previously identified in the South Range of the Sudbury Igneous Complex are also present in the North Range. This indicates that the laterally extensive (200-km wide × 2.5-km thick) superheated Sudbury Igneous Complex melt sheet developed multiple chemically and isotopically isolated convection cells that continued to assimilate isotopically distinctive footwall rocks after impact. Lateral variations are much greater at lower levels in the magmatic stratigraphy than in the upper part of the stratigraphy. This resulted from a combination of three factors: (1) the overall depletion in Pb of the proto-Sudbury Igneous Complex (likely due to volatile element loss upon impact) made it more susceptible to contamination in this element than in more refractory elements; (2) assimilation of heterogeneous footwall into the base vs. incorporation of more homogeneous fallback breccias into the roof of the Sudbury Igneous Complex; and (3) possible hydrothermal roof alteration. Lead isotopes and trace element data for samples of the footwall collected and analyzed as part of this study used in tandem with data from previous work confirm that the Sudbury Igneous Complex can be reproduced by mixing the currently exposed target lithologies and indicate that no mantle or lower crustal component is required.