Isotopic evidence for distinct crustal sources of North and South Range ores, Sudbury Igneous Complex

Lead isotope ratios were measured on Fe-Cu-Ni sulphide ores and associated silicate phases from nine mines on the North and South Ranges of the Sudbury Igneous Complex (SIC). These data are compared with the lead isotope signatures of country rocks and shock-melted Sudbury Breccias around the complex. Both ore and feldspar data from the South Range fall along a 1.85 Ga reference line (the age of the igneous complex) and indicate a magmatic origin for lead in the ore. Data from the North Range are also consistent with a magmatic origin, but are displaced to lower Pb-206/Pb-204 and Pb-207/Pb-204 ratios. These two suites fall close to the lead isotope composition of country rocks adjacent to the South and North ranges, respectively. Furthermore, there is a close correspondence between lead signatures of the SIC and Sudbury Breccia on each side of the complex. The evidence suggests an origin for the SIC by shock melting of different crustal units on the two sides of the complex. This is consistent with the impact of a meteorite on the southward-thickening apron of Huronian strata which overlay the margin of the Superior craton at 1.85 Ga.