ARCHAEAN IOCG DEPOSIT IN A FLUIDIZED DILATIONAL JOG (CHIBOUGAMAU, ABITIBI, CANADA)

Iron Oxide Copper Gold deposits (IOGC) are composed of ferruginous breccias associated with major faults and volcanism, such as the giant Olympic Dam deposit (South Australia).

In the Abitibi greenstone belt, the Chibougamau district hosted several copper deposits associated with late granite of Neoarchaean age. In the southern zone of the district, the Chibougamau tonalitic intrusion dated at 2718 Ma hosts the Queylus breccias. These breccias are associated with a large magnetic anomaly with a circular shape of 6 km in diameter. Several shear-zones show evidence of transtensional structures, forming pull-apart zones of more than 100 m in length. Large polymicts breccias zone displays fragments of coarse grain tonalite (85%), fine gain tonalite (20%), meladiorite (3%) and quartz and magnetite veins. Matrix is composed of crushed granite rich in sericite, with a late hydrothermal imprint composed of tourmaline, chlorite, albite, apatite, allanite, titanite, magnetite, pyrite (chalcopyrite-calcite-barite). Five episodes of alteration indicate interactions with hydrothermal fluid at temperatures in excess of 250°C. This association recall strongly the IOGC style.

Two styles of brecciation occur, showing contrasting particle size distribution and clast rounding: (1) early tectonic breccias, and (2) late fluidized breccias. In the latter style, breccias show a central zone, without any fragments organisation and a fractal distribution of their size, and lateral zones where vertical pockets of distinct particle size distribution may be distinguished. Fine-grain laminations composed of quartz fragments (25-75 µm) are present. This organization implied a size-sorting process of transportation that is characteristic of fluidization after tectonic comminution and opening in a dilational jog.

The IOGC Queylus occurrence shows that breccia-pipes often related to subvolcanic process may also be generated structurally in a transtension context.