Deformation history and gold mineralization of the Cadillac Group north of the Larder Lake-Cadillac deformation zone, southern Abitibi greenstone belt, Quebec

Abstract

The Cadillac Group is located immediately north of the Larder Lake–Cadillac deformation zone (LLCDZ) along the southern margin of the Abitibi greenstone belt in the Archean Superior Province, Quebec. The LLCDZ is a major 250 km long structure known for hosting several gold deposits. It is generally east-trending over a distance of 170 km, and then it swings to the southeast over a distance of 35 km along the so-called Malartic segment, before resuming its easterly trend for another 45 km. With its thick sequence of <2687 Ma turbiditic sandstone interlayered with mudstone and polymictic conglomerate, the Cadillac Group provides favourable rocks for studying the structural history of the Malartic segment which, due to its unique trend, is important for understanding the overall tectonic evolution of the LLCDZ. Deformation of the Cadillac Group began with large-scale isoclinal folding and the formation of a regional cleavage parallel to the trend of the Malartic segment (D1 event). These structures overprint a 2675.5 ± 2 Ma quartzfeldspar-phyric (QFP) intrusion providing a maximum age for the D1 event and a minimum age for the deposition of the Cadillac sequence. The deformation of the Cadillac Group resumed with bedding-parallel sinistral shearing and the emplacement of gold-bearing extensional quartz veins (D2 event), and continued with bedding-parallel dextral shearing, Z-shaped folding, the formation of a cleavage oriented oblique (anticlockwise) to the trend of the Malartic segment, and by the rotation and segmentation of the gold-bearing veins (D3 event). Contrary to previous interpretations of the Malartic segment as an early pre-D1 accretionary structure, it is interpreted as the short limb of a broad Z-shaped flexure that formed during D3 folding of the LLCDZ and that postdates the emplacement of gold-bearing veins in the Cadillac Group. Mineralized veins at the nearby worldclass Canadian Malartic deposit were emplaced during the same D2 event but they differ in alteration mineralogy and geochemistry to the Cadillac veins suggesting different sources for the mineralizing fluids. Mineralization at the Lapa deposit is similar in sulphide mineralogy and metal association to the Cadillac veins but was emplaced earlier during the deformation history of the LLCDZ.