Faults in 1100 orebody Mount Isa Mines

Abstract

1100 orebody is a large copper orebody deep in the southern part of Mount Isa Mines, North-Western Queensland. Host rock is a recrystallised siliceous, dolomitic breccia enveloped by pyritic, dolomitic shale striking 340deg-160deg, dipping 65deg W. Within this orebody are faults which have a characteristic filling of calcite and gypsum. They are the last stage of shear fracturing in the mine area, possibly being a result of an E-W compression along an axis essentially normal to bedding, though their movement picture is not clear. The faults are as main faults, in a conjugate pair, and as auxiliary faults, whose development is influenced by the rock type, with an average orientation of 116deg-296deg, 86deg NE and 064deg-244deg, 70deg NW. Uplift of these rocks, with a relaxation of stress, has resulted in a N-S horizontal extension along bedding, opening of faults with subsequent filling of calcite and gypsum, and extension fracturing with fracture planes normal to bedding. Dolomite grains have been extended along an axis normal to bedding during a later stage of uplift.