Abstract

We describe a unique occurrence of a world-class Proterozoic ore deposit, Century, affected by an Ordovician meteorite impact, the Lawn Hill impact structure. The meteorite excavated a complex crater with a Proterozoic core surrounded by an annulus filled with Cambrian carbonate rocks. The Century deposit is located at the southwestern edge of the crater and is bounded by postore faults that indicate an originally larger orebody. It is overlain by breccias, which contain evidence of impact-related textures that we interpret as fallback suevite. (Suevite is a rock consisting partly of melted material, typically forming breccia containing glass and crystal or lithic fragments, formed during an impact event.) Above the suevite are slumped Cambrian carbonate and Proterozoic shale, including a million-tonne block of Century ore detached from the main orebody and contained in the carbonate breccias. The Cambrian rocks were partially consolidated prior to impact, became fluidized on impact, and then resurged into the crater from surroundings areas. The resurge process resulted in a fivefold thickening of carbonates in the annulus, preserved to 600-m depth. Beneath the suevite, in the more competent orebody, fracturing and slab formation were the dominant responses to the impact. A restoration of the displacements on the impact-related faults suggests that the orebody was moved toward the crater. The inference is that, prior to impact, the deposit continued for several hundred meters beyond the current faulted northern boundary of the orebody and that this missing segment was displaced and may be buried within the annulus.

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