Textural and compositional variation of tuff marker beds at the Mount Isa and George Fisher deposits; implications for origin and mineralisation

Abstract

World-class resources of Cu, Pb and Zn are deposited at Mount Isa Mines and George Fisher Mine, North West Queensland, hosted within the Urquhart Shale unit (1655 Ma) of the Proterozoic Mount Isa Inlier. Numerous beds of tuffaceous metasediments are also intercalated within the carbonaceous units and have been relied on for constraining sequence stratigraphy. Tuff Marker Beds (TMBs) are described as cherty beds with cross-fractures at a high angle to bedding, and as having a highly potassic composition (Croxford, 1964). They provide the only indication of proximal volcanism associated with sedimentation; a line of evidence used to support a syngenetic sedimentary exhalative model of ore formation. The origin of potassic enrichment has previously been interpreted to be related to the composition of the original detrital sediments (including possible ash-fall tephra) and an unconstrained hydrothermal or diagenetic component. TMB samples for this study, collected from Mount Isa Copper Operations (MICO) and George Fisher Mine (GFM), indicate potassic enrichment is not confined to TMBs. Additionally, characteristic fining-upwards sequences were not observed, which is consistent with the interpretation that some of the potassic enrichment is a hydrothermal alteration product, formed either as part of the diagenesis or from later mineralisation-related fluid events. TMB mineralogical and geochemical compositions are closely related to base metal sulphide mineralisation with textural evidence of several episodes of fluid migration, including potassic enrichment. Results indicate that the K-feldspar rich beds identified at MICO are not genuine air-fall tuffs. Textural investigations of mineralised TMBs support a post-sedimentation and deformation paragenesis of ore emplacement of the Urquhart Shale, providing further evidence for an epigenetic Cu and Pb-Zn mode of ore formation at Mount Isa and George Fisher. It is proposed that TMBs record volcanogenic sedimentation and provide a record of potassic and mineralisation-related alteration events but are unrelated to ore genesis.