Reconstructing the Geochemical Signature of Sudbury Breccia, Ontario, Canada: Implications for Its Formation and Trace Metal Content

Abstract

The Sudbury breccia is an impact-generated pseudotachylitic breccia formed in the footwall of the 1.85 Ga Sudbury Igneous Complex in Ontario, Canada. The breccia is commonly developed in areas of contrasting country-rock types, and it represents a common host to footwall-style Cu-Ni-platinum group element sulfide mineralization, developed in the immediate footwall of large, contact-type, Ni-Cu–rich ore deposits at the base of the Sudbury Igneous Complex. Three models have been proposed for the genesis of the Sudbury breccia: (1) in situ frictional or shock melting of target rocks, (2) cataclasis of the target rocks, and/or (3) injection of allochthonous impact melt from the Sudbury Igneous Complex into the brecciated crater floor. This study reports new mineralogical and geochemical data for samples of Sudbury breccia and associated host rocks, both proximal and distal to mineralization, from six different locations around the Sudbury structure. A mixing model was used to construct the composition of the Sudbury breccia matrix, using a combination of country rock and quartz diorite from the offset dikes, which are believed to provide an estimate of the composition of the early melt sheet. We demonstrate that the breccia matrix can be successfully modeled from footwall lithologies with no requirement for a contribution from the impact melt sheet. Instead, we propose a dynamic, parautochthonous in situ melt model, where the matrix of the breccia was derived in part by high-temperature melting of footwall lithologies. These inclusion and crystal-laden melts were mixed and transported into tensile fracture zones that developed adjacent to weaknesses in the country rocks. Principal component analysis of Sudbury breccia indicates that the trace metal content within the breccia that developed distal from mineralization is associated with assimilation of mafic units in the footwall. In contrast, the matrix of Sudbury breccia developed closer to mineral zones has a contribution of metal derived from the fractionated sulfide melt and/or a contribution of hydrothermally remobilized metal. Thus, by comparing the trace metal content in Sudbury breccia with local footwall lithologies, baselines can be established that can help to identify anomalous metal contributions that may be derived from the periphery of a footwall mineral system.