Abstract:

The Sudbury Igneous Complex (SIC) is the preserved remnant of a 1.85 Ga meteorite impact-induced crustal melt sheet and hosts world-class Ni-Cu-PGE mineralization. The melt sheet eventually crystallized into a magmatic stratigraphy that can be traced along the entire impact basin. Previous studies have shown that there is a strong isotopic contrast between the North and South ranges of the SIC, which has traditionally been attributed to assimilation of distinct source rocks on the two sides of the complex. Pb isotopes and trace elements of feldspars and sulfide separates analyzed in this study expand the isotopic decoupling between the North and South ranges of the SIC: North Range Main Mass lithologies range 130-421 A207Pb/204Pb, North Range ores range 160-518 A207Pb/204Pb, South Range Main Mass range 360-494 A207Pb/204Pb, and South Range ores range 150-481 A207Pb/204Pb (where A207Pb/204Pb = 1,000 x 207Pb/204Pb _sample- 207Pb/204Pb_isochron). North Range ores and Main Mass are more radiogenic than Levack Gneiss, suggesting that North Range Main Mass assimilated significant amounts of the Huronian Supergroup, indicating that it extended much further to the north at the time of impact. The large systematic differences between North Range Main Mass and ores, and South Range Main Mass and ores, indicate extensive local contamination. The smaller but significant systematic differences between the Main Mass and the ores in both the North and South Ranges indicate that the latter are not derived from the Main Mass, but probably from footwall Nipissing gabbros and East Bull Lake intrusives, both of which are mineralized. North Range Mafic Norite, which occurs only very locally (typically above mineralization), is significantly less radiogenic than North Range Main Mass, suggesting that it may also have a local origin. There are also systematic variations in REE contents of feldspars between North and South Range lithologies. Together, the data suggest a much larger role for footwall rocks as sources for S and potentially also metals than has been appreciated in the past. The small scale lateral variations in Pb isotopes that have been previously indentified in the South Range of the SIC are also present in the North Range. These variations indicate that the laterally-extensive (200 km wide x 2.5 km thick) superheated SIC developed multiple geochemically- and isotopically isolated convection cells that continued to assimilate isotopically distinctive footwall lithologies after impact. Lateral variations are greater at lower levels in the magmatic stratigraphy and disappear in the upper magmatic stratigraphy. This likely results from assimilation of more homogeneous fallback breccias above the SIC and more heterogeneous individual target lithologies below the SIC. Lead isotopes and trace elements of samples of the footwall collected and analysed as part of this study used in tandem with data from previous work confirm that the SIC can be reproduced by mixing the currently exposed target lithologies and that no mantle or lower crustal component is required.