Geology and gold enrichment at the Horne 5 Archean volcanogenic massive sulphide deposit, Abitibi greenstone belt, Quebec: a synthesis

Abstract

The Archean Horne 5 volcanogenic massive sulphide deposit contains total resources of 172.6 t Au (5.6 Moz; 112.7 Mt at 1.53 g/t Au) with significant Ag, Cu, and Zn as by-products, one of the world's largest volcanogenic massive sulphide-associated gold resource. The deposit consists of stacked massive to semi-massive sulphide lenses that alternate with extensive zones of disseminated and stringer sulphides enclosed within steeply dipping felsic volcaniclastic and shallow synvolcanic intrusive units of transitional to calc-alkaline magmatic affinity. Massive sulphide clasts are locally abundant in the volcaniclastic rocks at several stratigraphic positions. Gold is interpreted to be synvolcanic and is spatially associated with an assemblage primarily composed of pyrite, sphalerite, and chalcopyrite. Gold distribution in the Horne 5 deposit is largely controlled by the higher porosity and permeability of select host lithologies (fragmental felsic units), which facilitate hydrothermal fluid circulation in the subseafloor environment. The presence of felsic synvolcanic intrusions and fine-grained tuffs overlying auriferous zones also influences the distribution of the mineralization by acting as impermeable cap rocks to ascending fluids. Although part of the mineralization formed below the seafloor, the presence of massive sulphide clasts indicates that sulphide mounds also accumulated on or close to the seafloor. The architecture of the Horne 5 deposit is characterized by along-strike zonation of two dominant metal assemblages, i.e., Zn±Au and Cu-Au, with the Cu-Au assemblage likely representing higher temperature discharge sites. However, the original geometry of the deposit was modified when it was tilted and transposed along the east-west foliation during deformation and metamorphism. Remobilization of metals due to metamorphism and deformation likely occurred at the micro-scale and resulted primarily in the exsolution of gold and its associated trace elements from recrystallized pyrite. This study provides further evidence for a major synvolcanic gold event in the southern Abitibi. It indicates that the Horne deposit is associated with calc-alkaline felsic volcanic rocks and is consistent with the preferential association between Archean synvolcanic gold and the early phases of rifting of a thickened arc crust towards the end of the southern Abitibi greenstone belt volcano-magmatic construction.