The nature of the Cadillac – Larder Lake Fault: Implications for gold mineralization along the KerrAddison-Cheminis segment

Abstract

The Cadillac-Larder Lake Fault (CLLF) is a major fault system that controls the location of gold deposits in the southern Abitibi greenstone belt. Over a distance of 250 km from Matachewan in Ontario to Val d’Or in Quebec, it marks the contact between mafic-ultramafic volcanic rocks of the Piché/Larder Lake group and younger metasedimentary rocks of the Hearst, Timiskaming and Cadillac sedimentary assemblages. In the Larder Lake area, the contact between sedimentary rocks of the Timiskaming assemblage to the north and volcanic rocks of the Larder Lake group to the south defines the CLLF. This contact is typically sheared and deformed by two generations of folds but locally, in areas of low strain, an unconformity is preserved, expressed by a pebbly sandstone which youngs away from the underlying ultramafic rocks and contains detrital clasts of the ultramafic rocks. The sandstone stratigraphically overlies older volcanic rocks of the Larder Lake group suggesting that the unconformable contact between the Timiskaming assemblage and Larder Lake group is a primary feature that was later structurally modified. The structural history of the CLLF and surrounding rocks began prior to the deposition of the Timiskaming assemblage with the juxtaposition of the Larder Lake and Blake River groups during an early thrusting and imbricating D1 deformation event. Both the Larder Lake group and the Timiskaming assemblage were then deformed during a D2 deformation / mineralizing event, which produced regional F2 folds, a regional S2 cleavage, and ductile shearing along the contact between the Larder Lake group and the Timiskaming assemblage. Reactivation of this contact during and post the D2 event localized the distribution of gold deposits and formed the present day manifestation of the CLLF.