Geology and regional hydrothermal alteration of the crater-fill, Onaping Formation: Association with zinc lead copper mineralization, Sudbury Structure, Canada

This study of the impact crater-fill sequence (Onaping Formation) in the Sudbury Structure reveals significant features critical to our understanding of modification processes and water-rock interaction in large (∼200 km) impact structures. The crater-fill sequence has a distinct mappable stratigraphy and consists of three informal members, the Garson, Sandcherry and Dowling, that have different vitric morphologies, percentage of matrix and lithic fragments and depositional characteristics. A distinct, coeval set of aphanitic dykes occupy syn-depositional faults and fractures.; Similarities of trace, REE and isotopic data of least altered andesitic Onaping Formation melt, felsic norite and quartz diorite of the Sudbury Igneous Complex (SIC) supports the interpretation that they represent an initial melt composition. The Onaping Formation was emplaced early, prior to differentiation and/or emplacement of the SIC in its present position.; Stratigraphy, morphology, structural controls and geochemistry of the Onaping Formation constrains the timing and evolution of the Sudbury Structure. A two stage history of crater-fill is recorded. Stage 1 involves, the emplacement of Garson and Sandcherry member units as products of slumping and fallback followed by intrusion of andesitic melt into crater floor-fractures. Stage 2 involves, large-scale, crater wall collapse along superfaults outside of the transient crater followed by melt-water interaction that resulted in “phreatomagmatic” explosive eruptions to produce the Dowling member. The basal intrusion was then injected along pre-existing fractures formed in Stage 1.; The crater-fill succession records a major impact-induced, hydrothermal system at 1848.4 +3.8/−1.8 Ma that produced low temperature, (<250°C) carbonate-hosted Zn-Cu-Pb subseafloor replacement-type deposits within the crater-fill. Mineralogical, geochemical and mapping data show a complex hydrothermal history controlled by the structural, magmatic and stratigraphic history of the impact crater. Alteration zones include a regional, upper calcite zone, transition zone, chlorite zone, albite zone and lower silicification zone. Field, textural, stable and radiogenic isotope data for the regional carbonate zone are compatible with magmatic CO₂ diluted by Proterozoic seawater. The Sudbury Structure is the largest, well preserved example of a terrestrial impact crater and records a distinct history of crater-fill emplacement and a productive hydrothermal system that is unparalleled in other impact craters on Earth.