Dichotomous Damage Zones: Lithologic Controls on Fracture Densities Surrounding Late Faults at the Lac Des Iles Mine, ON, Canada

Noah Phillips, University of Southern California
Jordan Peterzon, Lakehead University
Pete Hollings, Lakehead University
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Abstract

Host rock lithology plays a primary role on fault zone architecture, however, damage zone structure in mafic lithologies remain poorly characterized relative to felsic lithologies. In this contribution, we use drillholes at a Pt-Pd mine which intersect two reverse faults (the Camp Lake and Offset faults) juxtaposing mafic gabbronorites against felsic tonalites to test the role of host lithology on damage zone structure. Chlorite geothermometry demonstrates these faults were active at ~210 °C and displaced contacts and ore shells reveal displacements of 560 and 280 m for the Camp Lake and Offset faults, respectively. Fracture densities and fracture decay rates are consistently higher in the tonalites than the gabbronorites, irrespective of whether the tonalites are in the hanging wall or footwall. Mineralized fractures tend to be composed of chlorite or epidote in the gabbronorites, while quartz-rich fracture fill is more common in the tonalites. We hypothesize that the lower tensile strength of fracture fill in the mafic lithologies allows for repeated reactivation of fractures while the higher tensile strength of the quartz-rich fracture fill promotes new fracture formation. This leads to thinner damage zones with lower fracture densities in more mafic protoliths relative to felsic protoliths.

Session

Deformation in the upper crust