HIDDEN ORE: USING THERMAL METAMORPHISM TO DETERMINE SHORTENING ON THRUST FAULTS CUTTING THE SUDBURY IGNEOUS COMPLEX

Ni-Cu ore in the North Range of the 1.85 Ga, ~250 km diameter Sudbury impact structure occurs within footwall rocks and offset dykes adjacent to the Sudbury igneous complex (SIC). The SIC is currently exposed as an elliptical 60 x 30 km, 2.5 km thick remnant of the original impact melt sheet. The North Range footwall rocks of the SIC are composed predominantly of Archean Levack gneiss and Cartier granitoid. The Levack gneiss is in direct contact with the rocks of the SIC. It forms a continuous layer along the North Range and it probably extends at depth beneath the 30 degree southward-dipping SIC. A discontinuous zone of Footwall Breccia is located in the contact footwall rocks immediately beneath the SIC. This rock is a mixture of clasts of local footwall rocks in an igneous matrix. It is within Footwall Breccia that ore bodies can be located. Ore is also commonly developed where the radial offset dykes meet the SIC in so-called embayment structures (troughs or channels in the floor of the impact melt sheet). Surrounding the SIC, out to a radius of 1.5 km, is a metamorphic aureole that comprises an innermost pyroxene hornfels facies zone; followed by hornblende hornfels facies; and an outer zone of plagioclase and quartz recrystallisation that equates to the albite-epidote hornfels facies. There is retrogression to greenschist facies in the highest grade rocks, including the Footwall Breccia. Field mapping and sampling in the North Range reveals faults subparallel to the footwall /SIC contact. Petrography and analytical electron microscopy of samples from these faulted transects show that in these areas some, or all, of the isograds are missing. Field evidence suggests that a number of thrust faults are developed at, or close to, the footwall/SIC contact. Aureole mapping indicates that fault displacements of up to 1.5 km have occurred. As a result, any ore-bearing footwall contact rocks may have been overthrust and so would occur hidden below the overlying impact melt sheet. This has particular significance where thrust faults occur in the vicinity of radial offset dykes. It is possible that as yet undiscovered embayments exist beneath the SIC. This could result in the discovery of new ore deposits in the important Sudbury mining camp.