Magnetic fabric constraints on friction melt flow regimes and ore emplacement direction within the South Range Breccia Belt, Sudbury Impact Structure

Abstract

Magnetic fabric analysis and petrography have been carried out on ∼1100 samples from the South Range Breccia Belt (SRBB), a 45-km-long arc of Huronian Supergroup breccia fragments and crystallized friction melt (pseudotachylyte), sub-concentric to the 1.85 Ga Sudbury Impact Structure. Optical petrography and analytical SEM analyses show the pseudotachylyte to consist of fine-grained quartz, biotite and ilmenite, with minor plagioclase, apatite, zircon, pyrite and pyrrhotite. Alternating field and thermal demagnetization spectra indicate that the principal magnetic carrier in the matrix is pyrrhotite, while clasts can contain either pyrrhotite or magnetite. Mean magnetic susceptibilities in the matrix are generally on the order of 10 −4 SI. Two AMS (anisotropy of magnetic susceptibility) fabrics have been determined in the matrix. The most common is an oblate, sub-vertical foliation, sub-parallel to the local strike of the SRBB, with anisotropies ( P) ranging from 1.08 to 1.20. The second AMS fabric is a weakly to strongly prolate sub-vertical lineation with generally higher P values of 1.2 to 1.55. Both fabrics are coaxial to the petrofabric in the matrix. Huronian clasts exhibit scattered, heterogeneous AMS fabrics, distinct from those found in the SRBB matrix. The strongly prolate ( T = −0.73) and anisotropic ( P = 1.55) AMS fabric found directly over the giant Frood-Stobie NiCuPGE deposit, hosted within the SRBB, suggests that sulfide droplets were injected and/or squeezed into the matrix above the deposit. Samples containing pyrrhotite from the edges of the deposit do not exhibit this magnetic signature. Similar prolate, moderately anisotropic lineations are also found further to the east of the Frood-Stobie Mine, and appear to be associated with a magnetotelluric conductor at depth. It is suggested that these lineations are due to injection of material from depth into the SRBB during failure of the inner rim of the Sudbury Impact Structure during the crater modification stage. The oblate magnetic fabric found within the SRBB is consistent with frictional comminution and melting of the wall rock lithologies during collapse of the transient cavity and subsequent slumping of the crater walls. However, subsequent deformation of the Sudbury Structure by late Penokean folding and thrusting to the NW could produce a similar oblate fabric. Tight clustering of the maximum principal axes of the magnetic susceptibility could be interpreted as being consistent with tectonometamorphic transposition of the matrix petrofabric, as well as generation by slumping and friction melt flow during failure of the hanging wall. Further work is needed on undeformed impact structures of similar size to Sudbury in order to constrain the petro- and magnetic fabrics produced during rim collapse. Additional work in the westerly trending SRBB along the edge of the Creighton Pluton would help to determine if the dominant NE-SW-trending matrix fabric is regional in origin, or is controlled by the current orientation of portions of the SRBB.