Complex hydrothermal alteration system postdates deposition of the Mount Isa Group and the lead-zinc-silver orebodies. Silica dolomite results from differential progressive metasomatic alteration of the entire range of microfacies constituting the lead-zinc sequence. The main components of this alteration are phyllosilicate development, dolomitization, and silicification. This study concentrates on the latter two components and their role in breccia host formation. All stages from incipient bedding and vein-controlled alteration to total pervasive brecciation can be related to a deformational history of folding, cleavage development, and faulting. A regional folding event (D 2 ) formed the fold limb, which constitutes the ore setting. Dolomite replacement veins and pseudobreccia complexes developed with varying intensity. In favorable zones combinations of bedding and fracture-controlled replacement continued until total dolomitization of the former shales occurred. All forms and stages of dolomitic pseudobreccias are preserved by pseudomorphic quartz replacement in silicification zones which form the core of the orebody systems. The separation in timing and mode of formation of lead-zinc and copper orebodies is significant for exploration in the Mount Isa inlier. The lead-zinc mineralization appears to accumulate either syngenetically or at a very early stage in diagenesis, but the copper has a structural control and a basic igneous association, and is estimated to have been introduced at least 150 m.y. later.--Modified journal abstract.

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