Mafic intrusions in the footwall of the Sudbury Igneous Complex: Origin of the Sudbury impact melt sheet and its associated ore deposits

Abstract

The majority of the mafic intrusions and volcanic rocks in the footwall of the Sudbury Igneous Complex (SIC), which is believed to have formed from a melt sheet produced by the impact of a very large bolide, belong to one of two large mafic igneous suites, viz: the ∼2450 Ma East Bull Lake (EBL) suite of intrusions or the ∼2200 Ma Nipissing Diabase suite of intrusions. This has been established by U-Pb LA-ICP-MS dating of zircons from the newly recognized Frood intrusion, which hosts the Frood and Stobie ore bodies, and two of the Sudbury Gabbro intrusions, and comparison of the geochemistry of these intrusions with that of the EBL intrusive suite and the Nipissing Diabase suite. The age of the Frood intrusion (2421 ± 32 Ma) falls within error of other EBL-type intrusions whereas the ages of the Nairn (2203 ± 11 Ma) and Totten (2168 ± 11 Ma) intrusions fall within error of published ages for the Nipissing Diabase. Moreover, the primitive mantle-normalized patterns of the petrogenetic trace elements on extended spidergrams of the Nairn and Totten Sudbury Gabbros are similar to those of the Nipissing Diabase as are those of the Frood intrusion to other members of the EBL intrusive suite.The Ni-Cu-PGE sulfide deposits at Sudbury are associated with the lower contact of the SIC, where a discontinuous inclusion-rich norite contains much of the mineralization at the contact, and an inclusion-bearing quartz diorite is directly associated with the mineralization of the radial and concentric offset dykes. The Sublayer rocks and the overlying unit of melanocratic norites provide strong evidence for a relationship between the ore deposits and both inclusions and melts of mafic-ultramafic target rocks. The most common pre-impact mafic-ultramafic target rocks are intrusions of the EBL and Nipissing Diabase suites, although there are also Archean-aged mafic-ultramafic enclaves in the gneisses beneath the North Range of the SIC. The Totten orebody in the Worthington Offset and the Kelly Lake ore body in the Copper Cliff Offset are directly associated with footwall rocks comprising Sudbury Gabbro and Nipissing Gabbro, respectively, whereas the Frood-Stobie Offset and the associated Frood and Stobie ore deposits are sandwiched in a breccia belt containing large rafts of amphibole megacrystic gabbro belonging to the extensively broken-up Frood intrusion. Olivine-bearing mafic and ultramafic rocks, some of which exhibit shock metamorphic features, but others do not, are common inclusion types and occur in diverse Sublayer environments. The unshocked melanorite and olivine melanorite inclusions share common geochemical characteristicsde, indicating that they were derived from a similar source. Their geochemistry is also similar to that of the Frood Intrusion as well as the East Bull Lake intrusion; this indicates that they may have been sourced from East Bull Lake-type intrusions that were incorporated into the melt sheet at the time of melt generation.Mass balance calculations indicate that the felsic country rocks which account for 77% of the total volume of the SIC and contain 18 ppm Ni and 21 ppm Cu together with the sulfide-poor mafic country rocks which account for 23% of the SIC and contain 133 ppm Ni and 92 ppm Cu have too little Ni and Cu to account for the Ni and Cu contents of the quartz diorite, the composition of which provides an indication of that of the initial melt sheet.Calculations using the R-factor equation of Campbell and Naldrett (1979) indicate that the magmas that formed the norites and the ore deposits had 137 ppm Ni and 121 ppm Cu. Mass balance calculations have been used to determine if the country rocks have sufficient Ni and Cu in them to generate the ore deposits. The weighted median Ni content of the sulfide poor (<200 ppm Cu) mafic country rocks is 133 ppm and hence very similar to that of the ore-forming magma. However, the weighted median Cu contents of these rocks is only 90 ppm and hence too low to provide all of the Cu required by the ore-forming magma. What the mass balance calculation shows is that a contribution of Cu (and Ni) in the ore-forming magma was derived from pre-existing Ni-Cu-PGE sulphides associated with the mafic target rocks. This contribution was diluted by the lower concentrations of Ni and Cu from the Proterozoic metasedimentary rocks and the Archean granitoids and gneisses. For this reason, a contribution from magmatic sulfides in the mafic target rocks is required, and this may have been similar to the known disseminated concentrations of Cu-PGE-Ni sulfides present in mafic intrusions in the Sudbury area.