Summary: Development of ideas on Sudbury geology, 1992-1998

Abstract

Evidence favoring an impact origin for the Sudbury Structure continues to accumulate, particularly the recent discovery in the Black Member of the Onaping Formation of shocked microdiamonds resembling those found at other impact structures. The 1990 Lithoprobe Vibroseis survey of the Sudbury basin and its subsequent interpretation indicated that the original diameter of the Sudbury Structure was of the order of 200 km, as opposed to previous estimates of 60-100 km. This new size estimate made it possible to interpret the Sudbury Igneous Complex (SIC) as an impact melt with no requirement for a mantle-derived contribution.The very felsic compositions of all of the exposed components of the SIC make it difficult to account for the origin of the dunite, harzburgite, wehrlite, websterite, and melanorite inclusions in the Sublayer at Sudbury, without calling upon their derivation from a preexisting layered intrusion in the target area. Recent U-Pb dating of baddeleyite and zircon from melanorite inclusions, giving a 1.85-Ga age, indicated that these are related to the development of the SIC, and are not from an older intrusion. However, new Re-Os isotopic data on variably mineralized melanorite inclusions indicate that the ore component of these inclusions has interacted with a dominantly crustal source with an average age of about 2.6 Ga, while the silicate component is younger. The solution to this enigma is probably that the mafic/ultramafic inclusion suite is composite, the more olivine-rich inclusions being from a preexisting source and the melanorites being of Sudbury age, with a major component of the mineralization also predating the SIC.Modeling the fractional crystallization of hypothetical SIC magmas indicates that the rock types, and the sequence of cumulus phases, is consistent with a fractionation origin, but that the relative proportions of the rock types is not; there is a great excess of granophyre relative to norite exposed at surfrace. The application of computer models designed to simulate the impact of large bolides to the Sudbury Structure leads to the conclusion that the resulting impact melt would have been superheated for the first 100,000-200,000 yr of its existence, as it cooled from about 1,700°C to its liquidus temperature (about 1,200°C). During this time a massive amount of the overlying basal breccia of the Onaping Formation would have been melted to accumulate at the top of the underlying impact melt. Sulfur solubility in silicates melts is strongly temperature-dependent, so that cooling through this 500°C temperature interval of superheat would also have favored liquation of a sulfide liquid if the initial impact melt had been anywhere close to sulfide saturation, and the very complete settling of this liquid to the base of the complex.It is proposed that the bolide impacted an area of the crust underlain in part by a preexisting mafic intrusion, and that the initial melt was more mafic than average Naldrett, A.

The mineral system approach applied to magmatic Ni–Cu–PGE sulphide deposits

“…Secondly, massive sulphide accumulations, characteristically with abundant silicate rock fragments, are almost exclusively found at the basal contact, and typically within trough-shaped footwall contact embayments. The key factor here is that the Sudbury body is widely held to have formed from a superheated melt sheet, such that accumulation of sulphide melt droplets took place in the absence of abundant co-precipitating silicate phases (Naldrett, 1999a). The association of the ore embayments and the offset dykes suggests that downward gravity-driven flow of sulphide liquid, essentially as density currents along the floor and into underlying cracks, may have played a major role at Sudbury .…”

The evolution of the Onaping Formation at the Sudbury impact structure

“…When the SIC was considered a product of endogenic igneous activity, it was compared to mafic layered intrusions (e.g., Naldrett et al. 1970; Naldrett and Hewins 1984; Naldrett 1999), which have roof and floor rocks that are finer grained than the main mass of the intrusion and record the initial composition of the magma. Chemical differentiation trends that characterize the bulk of the magma are, thus, reversed in the floor and, particularly, roof rocks (McBirney 1993).…”

Empirical and theoretical comparisons of the Chicxulub and Sudbury impact structures

“…Sudbury is one of the most studied terrestrial impact craters (e.g., Grieve et al 1991;Golightly 1994;Stöffler et al 1994;Deutsch et al 1995;Dressler and Sharpton 1999;Naldrett 1999;Grieve and Therriault 2000). Nevertheless, Table 1 and shown in Fig.…”