Bounty Gold Mine : deformation history and the development of ore fluid pathways within an iron formation host, Western Australia

The Bounty orebody has a steep plunge and is bound within strata parallel shear zones which are developed in an iron formation horizon. A footwall ultramafic volcanic sequence and a hanging wall gabbro bound the mineralized iron formation which dips steeply west. Shearing occurred during deformation event Dn and a peak contact metamorphic grade of lower amphibolite fades. Deformation in pre-Dn times was dominated by strong east-west directed compression which resulted in complex, upright folding and thrusting of the supracrustal sequence. With cessation or relaxation of the compression batholiths, plutons and porphyritic stocks, dykes and sills of granite to granodiorite composition intruded the folded greenstone sequence and broad contact metamorphic aureoles were formed. A period of vertically oriented, maximum compressive stress (Dn) succeeded the intrusive event. Resulting strain was focused along the footwall and hanging wall boundaries of the iron formation and shear zones with a normal movement sense developed. The hanging wall boundary of the iron formation, which was locally rotated west of north during intrusion of a pre-Dn pluton, underwent down dip or steep oblique shear movement with a normal sense. Sheared lithological boundaries striking about 4° to 8° west of north were dilated, developing ore fluid pathways with a steep plunge. The ore fluid pathways are controlled by the rheological contrast and original shape of the iron formations hanging wall boundary during Dn. The original shape was influenced by intrusion of a pre-Dn pluton proximal to the deposit.