Structural controls on development and localization of syntectonic copper mineralization at Mount Isa, Queensland

Three major deformation events were responsible for the structural relationships in the Mount Isa mine where steeply west-dipping siliceous and dolomitic sediments of the Mount Isa Group, which host the Ag-Pb-Zn ores as well as the epigenetic Cu orebodies, are truncated against the underlying greenschists. The Cu orebodies and their alteration envelope, the silica dolomite, developed during the third deformation (D 3 ) within the dolomitic Urquhart Shale, adjacent to this greenschist contact.This contact is a fault that formed during thrusting in D 1 and was rotated and overturned by the combined effects of D 1 and D 2 into a shallowly dipping attitude on the western limb of a regional-scale upright D 2 antiform. Consequently it lay at a high angle to the steeply plunging stretching direction established during the third deformation (D 3 ). This fault was a plane of considerable anisotropy and competency contrast because the rocks above were dolomitic siltstones whereas those below were greenschists, with bedding respectively at a high angle and subparallel to the fault.Folding during D 3 generated opposed senses of shear and hence differential movement across the fault, especially at fold hinges. This, in combination with the differential anisotropy and competency contrast across the fault, and extension at a high angle to it, resulted in localized fracture and opening of the contact and consequently a rapid drop in fluid pressure, possibly on several occasions in different locations along the greenschist contact. The large imbalance in fluid pressure across the walls of these localized openings caused explosive fracturing and local brecciation, especially of the dolomitic rocks on the fault walls into the space created. This provided access for the syndeformation hydrothermal fluid to all faces of the jostled blocks of dolomitic shales enabling their alteration to form the silica dolomite and the Cu ores.