Fine-grained pyrite within the Mount Isa (Enterprise) copper system, NW Queensland; geological relationships, modelled distribution and links to reactive ground

Abstract

The Mount Isa Cu and Pb-Zn-Ag deposits are located in North-West Queensland within the Western Fold Belt of the Mount Isa Inlier. The genesis of these deposits has been debated since mining began in the 1920s. The most abundant sulphide within the host unit is fine-grained pyrite, which is present in high concentrations within the mining area. There is debate on the origin and timing of fine-grained pyrite and how it relates to the silica-dolomite alteration halo, and copper mineralisation. This thesis is presented as a broad multi-focussed project, with the overarching aim to characterise the fine-grained pyrite mineralisation at Mount Isa. The relationship of fine-grained pyrite to the ore system and to reactive ground is also investigated. In this thesis, hand sample observation, petrographic analysis, SEM analysis and MLA analysis are used to interpret the geology of pyritic shales. Additionally, Leapfrog geological modelling of diamond drill-hole data was used to visualise the large-scale distribution of fine-grained pyrite. In order to test the relationship between fine-grained pyrite and reactive ground, 63 pyritic shale samples from underground were tested for reactivity with explosive products. Results indicate that fine-grained pyrite has been deposited over a protracted period spanning over deformation periods, and its formation is related to the early stages of the main hydrothermal system. Fine-grained pyrite appears to act as a rheological inhibitor to hydrothermal fluids on both the micro and macro scales, affecting the distribution of subsequent alteration, brecciation, and location of Cu mineralisation. Rare magnetite mineralisation has been observed and is interpreted to have formed at a later stage to fine-grained pyrite. Ore textures within pyritic shales often show economic base metal sulphides (Cu, Pb and Zn) with paragenetic relationships indicating co-precipitation. Reactive ground testing provided evidence that the concentration of fine-grained pyrite alone does not directly control reactivity. Samples rich in fine-grained pyrite hence do not necessarily pose a direct risk to drilling and blasting. Within the context of the deposit, fine-grained pyrite was an important pre-ore mineralisation component of the Urquhart Shale which has influenced the location of the later copper orebodies. The rheological variations of the variably pyritic sedimentary layers, distribution of silica/dolomite alteration and orientation of the Paroo Fault were also significant factors enabling the unique circumstances for the deposition of the Cu-Pb-Zn ore system at Mount Isa.