Emplacement of the Whistle Dike, the Whistle Embayment and Hosted Sulfides, Sudbury Impact Structure, Based on Anisotropies of Magnetic Susceptibility and Magnetic Remanence

Abstract

The Sudbury impact structure is one of the few multi-ring meteorite impact structures recognized on Earth. The meteorite impact occurred ca. 1850 Ma, resulting in the production of impact melts derived from the crust, possibly with a mantle component. The impact melts formed the Sudbury Igneous Complex and also were emplaced as radial and concentric dikes within the target rocks. The Whistle dike, one of the radial “offset” dikes in the Sudbury impact structure, is connected to the Sudbury Igneous Complex by the Whistle embayment structure, which is composed of noritic cumulates. We present a study of the emplacement flow patterns in the Whistle dike and the embayment, using magnetic anisotropy methods to measure the petrofabrics that record emplacement. Petrographic and scanning electron microscope observations, combined with thermal demagnetization measurements, and tests of the acquisition and demagnetization of anhysteretic magnetic remanence indicate that the primary sources of the magnetic signals in the sample suite are coarse-grained, soft coercivity magnetite and pyrrhotite that are interpreted to be primary igneous phases. Some samples also contain hard coercivity phases of possible metamorphic origin. The anisotropy of magnetic susceptibility (AMS), the anisotropy of an-hysteretic remanent magnetization (AARM) and the anisotropy of partial anhysteretic remanent magnetization (ApARM) were measured for the samples collected at 20 sites within the Whistle dike and the embayment structure, including an exposed chalcopyrite-rich massive sulfide body. The AMS method provided well-defined, interpretable fabrics for many of the sample sites. However, the AMS and AARM methods were unsuccessful in providing interpretable fabric measurements at some sample sites due to contributions from the hard coercivity ferromagnetic minerals. For those samples, the ApARM method was successful in isolating the sub-fabric defined by the primary magnetite ± pyrrhotite crystals. The fabric maps are consistent with lateral (horizontal) injection of the Whistle offset dike and subsequent sinking of molten massive sulfides within the still ductile dike. Sinking of the massive sulfides is recorded by steeply plunging magnetic lineations. The magnetic fabrics in the cumulate norites of the embayment structure show that the norites were deposited in a dynamic environment that resulted in a well-oriented lineation, which may record flow of gravity or thermal currents, channelled along the embayment axis. In the embayment, the lineations plunge toward a massive sulfide body, suggesting the flow that led to the formation of the lineation may also have led to emplacement of the massive sulfide ore. It is suggested that steeply plunging lineations elsewhere in the Whistle offset dike, and possibly in other Sudbury offset dikes, might indicate the trails of sunken massive sulfides in the subsurface. It is also suggested that magnetic fabrics might be useful to locate massive sulfide bodies hosted in embayment structures.