Geology of the Lapa Orogenic Gold Deposit

Abstract

Abstract The Cadillac mining camp is known for its numerous, but relatively small, orogenic gold deposits, which are spatially associated with the Larder Lake-Cadillac fault zone. The Lapa deposit, with a total endowment of 36 t Au (1.15 Moz), represents the largest gold deposit of the Cadillac mining camp. The Lapa deposit main ore zones are mostly hosted in the Piché Group ultramafic to intermediate volcanic units that are strongly transposed and separated by subvertical, anastomosed high-strain corridors that are part of the Larder Lake-Cadillac fault zone. There are 12 ore zones that are stacked from north to south, forming a series of subparallel, E-striking (main foliation-parallel), steeply dipping south to subvertical “lenses.” The ore consists mainly of very fine-grained (≤1 mm), disseminated sulfides (arsenopyrite and pyrrhotite with traces of chalcopyrite, pyrite, and sphalerite), sulfosalts, native Au, and native Sb. Three amphibolite-grade metamorphosed proximal alteration assemblages are present at Lapa, namely bio-tite-bearing, sericite-bearing, and actinolite-bearing assemblages. The distribution of the three assemblages, defined by the most abundant mineral, is at least in part controlled by the primary host-rock composition. The proximal alteration facies give way to chlorite- (upper half of the deposit at <1,000 m) and hornblende-bearing (lower half of the deposit at >1,000 m) assemblages a few meters to a few decimeters away from the ore zones. The isograd defined by the presence of actinolite in the proximal alteration assemblage and hornblende in the distal assemblage below 1,000 m correlates with a shift from an Au-As association in the lowermost levels of the mine to an Au-Sb association at depth. This variation is thought to be due to varying heat and fluid flow regimes at different times and crustal levels in the fault, with the upgrading of early, “low-grade” Au during prograde and retrograde metamorphism. The Cadillac camp, including the Lapa deposit, is an excellent example of the camp to deposit to stope controls exerted by the structural and lithologic setting on the nature, style, and geometry of greenstone-hosted orogenic gold deposits.