Abstract

The Sudbury Breccia is an impactite that formed in the target rocks of the 1850 +1.3/–2.4 Ma Sudbury impact structure in Ontario, Canada. The breccia is interpreted to have formed during crater excavation or modification at the time of the impact event. Copper-Nickel-Platinum Group Elements sulfide melts moved away from contact mineralization at the base of the melt sheet to form veins and stockworks in the footwall. The distribution of both Sudbury Breccia and the later sulfide melts is partially controlled by pre-existing weaknesses in the footwall, such as lithological contacts; as a result, the two are often spatially associated. A combination of early magmatic-hydrothermal and late metamorphic fluids modified the sulfide mineralization to create broad haloes of metalliferous hydrous silicate minerals proximal to the Sudbury Breccia and in the footwall rocks. This study examines variations in the trace-element geochemistry of the breccia-matrix mineral assemblage developed adjacent to footwall mineralization in the North Range (Coleman Mine) and the South Range (Creighton Mine). The Sudbury Breccia is widely considered to have formed in a single event, contemporaneous with the meteorite impact, therefore it provides a relatively consistent baseline for studying subsequent metamorphic and hydrothermal processes, unlike the Archean Superior Province and Paleoproterozoic Southern Province, which experienced pre-impact alteration associated with tectonic and igneous activity.

This study focused on examining the textural, chemical, and petrological changes recorded in biotites, amphiboles, chlorites, and Fe-oxides produced by late magmatic and post-magmatic sulfide mineralization and U-Pb age dates of titanite. Titanite-chlorite assemblages in the North Range footwall of the SIC overprint metalliferous assemblages adjacent to the McCreedy East 153 footwall ore body. They yielded an age of 1358 ± 78 Ma, which is interpreted to be associated with the waning effects of the 1450 ± 0.15 Ma Chieflakian orogeny. In the South Range at the Creighton Mine, the Sudbury Breccia hosting the Creighton Deep footwall ore body records a shift from ferro-hornblende to ferro-tschermakite amphibole. When coupled with variations in Ca/Ti in titanite and a U-Pb titanite age of 1616 ± 33 Ma, the assemblage is interpreted to reflect increasing temperature-pressure gradient towards shear zones that were active during the Mazatzalian orogeny, during which time the sulfide mineralization was remobilized. At both locations, biotite exhibits an increase in Tl content that relates to the Ni content of the host rocks and proximity to sulfide mineralization. This relationship may be produced by remobilization of metals during interaction between sulfide mineralization and hydrothermal fluids. Although not directly associated with sulfide emplacement, these signatures provide evidence for a larger geochemical halo around the mineral zone that may provide the basis for ranking the prospectivity of the footwall and possibly vectoring towards mineral zones.

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