The Sigma deposit is a large network of Au-mineralized veins that crosscut a sequence of andesitic volcanics and volcanoclastics intruded by porphyritic diorite and feldspar porphyry dikes of the Archean Abitibi greenstone belt of Canada. The network is mainly made of shear zone-hosted, steeply dipping veins and subhorizontal extension veins filled for the most part by quartz and tourmaline. Hydrothermal alteration around these veins is made of coeval albite-rich and tourmaline-rich visible haloes, and formed during multiple stages of shearing of the host rocks, opening, growth, and interconnection of vein segments, and during gold precipitation within veins and wall rocks.
Appropriate scale mapping of vein outcrops located within a 1.3 km-deep section of the deposit is used here to gather quantitative geometrical data on the vein network and the alteration haloes developed at the vein walls. These data show that the albite- and tourmaline-rich haloes have no preferential distribution within the deposit, form patterns that are asymmetrical along and across vein strike, and are mainly controlled by the local three-dimensional arrangement of the veins. Vein gold grades do not correlate with metric properties of veins like thickness and depth.
Results suggest that the hydrothermal alteration at Sigma developed mainly from diffusive transport of mass through the wall rocks. They also suggest that the control of local topology of vein arrays on alteration patterns, underestimated by previous studies of Sigma, may be important in other structurally complex vein network deposits. Scale mapping of both mineralized veins and hydrothermal alteration envelopes has proved useful in constraining the structural relations between vein growth and fluid-rock interaction and in determining locations of most intense wall-rock metasomatism, which provides an objective guide to the choice of samples for geochemical studies.