B. Dube et J. Guha, FACTORS CONTROLLING THE OCCURRENCE OF FERRO-AXINITE WITHIN ARCHEAN GOLD COPPER-RICH QUARTZ VEINS - COOKE MINE, CHIBOUGAMAU AREA, ABITIBI GREENSTONE-BELT, Canadian Mineralogist, 31, 1993, pp. 905-916
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 gener
ally is not observed in association with auriferous zones. This paper
documents occurrences of ferro-axinite associated with gold-copper-ric
h quartz veins at the Cooke mine, near Chibougamau, in the Abitibi gre
enstone 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 brit
tle faults. Cross-cutting relationships and strain compatibility indic
ate that the late brittle faults and the gold-hosting structures were
formed in a relatively short period of time. The FeO content of the ax
inite is high (8%), whereas MgO is low (2%). Composition variations ar
e limited and do not seem to correlate with distinct environments of f
ormation, 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 hydroth
ermal fluid to form ferro-axinite rather than tourmaline. This reactio
n occurred early, as the ferro-axinite is commonly attached to the wal
ls of the veins, whereas the sulfides surround and partly replace the
ferro-axinite. The similarity in composition of the ferro-axinite asso
ciated with mineralized veins and with late brittle faults suggests th
at both structures were infiltrated by the boron-rich fluid. The forma
tion 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.