FACTORS CONTROLLING THE OCCURRENCE OF FERRO-AXINITE WITHIN ARCHEAN GOLD COPPER-RICH QUARTZ VEINS - COOKE MINE, CHIBOUGAMAU AREA, ABITIBI GREENSTONE-BELT

Boron is commonly associated with gold mineralization, as suggested by the widespread occurrence of tourmaline in gold-bearing quartz veins. Axinite, a calc-silicate of boron, is rarer than tourmaline and generally is not observed in association with auriferous zones. This paper documents occurrences of ferro-axinite associated with gold-copper-rich quartz veins at the Cooke mine, near Chibougamau, in the Abitibi greenstone belt. At the Cooke mine, pinkish violet ferro-axinite commonly occurs in auriferous quartz veins, in barren calcite-quartz extension-related veins, in hydrothermal breccias, and within or near late brittle faults. Cross-cutting relationships and strain compatibility indicate that the late brittle faults and the gold-hosting structures were formed in a relatively short period of time. The FeO content of the axinite is high (8%), whereas MgO is low (2%). Composition variations are limited and do not seem to correlate with distinct environments of formation, despite the different modes of occurrence. The formation of ferro-axinite is considered to occur where carbonatization is limited, so that the available Ca combines with B, Fe, and Si from the hydrothermal fluid to form ferro-axinite rather than tourmaline. This reaction occurred early, as the ferro-axinite is commonly attached to the walls of the veins, whereas the sulfides surround and partly replace the ferro-axinite. The similarity in composition of the ferro-axinite associated with mineralized veins and with late brittle faults suggests that both structures were infiltrated by the boron-rich fluid. The formation of ferro-axinite rather than tourmaline is also strongly promoted by a Ca- and iron-rich environment. The study illustrates the strong influence of the composition of the host rock and of the hydrothermal fluid on the formation of ferro-axinite.