The Paroo Fault and the Mount Isa copper orebodies; a revised structural and evolutionary model, Mt Isa, Queensland, Australia
Long, Ryan David (2010) The Paroo Fault and the Mount Isa copper orebodies; a revised structural and evolutionary model, Mt Isa, Queensland, Australia. PhD thesis, James Cook University.
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Abstract
Numerous studies on the Mount Isa copper orebodies have assumed the role of the Paroo Fault, which has juxtaposed the Mount Isa Group against the Eastern Creek Volcanics, and forms a footwall to the copper and lead-zinc-silver orebodies. The copper orebodies have largely been considered to have formed during east northeast-west southwest shortening, late during the 1590-1500 Ma Isan Orogeny. This thesis examines the Paroo Fault from the km-scales to the sub mm-scales in order to understand its development and role in the mineralisation of the copper orebodies. Recent techniques such as Gaussian Curvature analysis, fold profile analysis, dilation and slip tendency analysis, spanned length analysis as well as established techniques such as thickness analysis and geological mapping have been used to examine the km-scales geometry of the Paroo Fault. This has lead to a new proposed timing for initial reactivation of the Paroo Fault during the final stages of deposition of the Mount Isa group, followed by folding during east-west shortening and refolding during east northeast-west southwest shortening. Detailed geological mapping of the Paroo Fault Zone at 10s of m-scales supports the conclusions drawn from the macro-scale observations and suggests a new interpretation of the timing for the formation of the copper orebodies, before or early in the east-west shortening event, at Mount Isa. Re-examination of the mapping database collected by mine geologists (MIM and Xstrata Copper) has demonstrated that at the hunreds of metres scale, folds formed during the east northeast-west southwest shortening bend around the copper orebodies, indicating that the copper orebodies formed earlier than this event, consistent with conclusions drawn from the mapping of the Paroo Fault at the tens of m scale. Previously unrecognised east-west orientated folds which have no relationship to the copper orebodies and appear to predate them have also been recognised, indicating a phase of north-south shortening prior to ore genesis and prior to the east-west shortening. Halogen systematics on fluid inclusions from the copper orebodies, Paroo Fault Zone filling, silica-dolomite halo and basement rocks demonstrate that the ore system penetrated into the footwall, thus predating the sealing of the Paroo Fault Zones quartz-rich infill. The copper orebodies have a bittern brine source for halogens, which in the previous model for late metamorphic ore genesis, posed some problems because of the high Br/Cr ratios requiring an exotic (unknown) basin to have been placed above the Mount Isa Group during ore genesis. In the revised model here, such basinal brines are inferred to be residual evolved diagenetic brines within the Mount Isa Group or broader sedimentary package, which mixed with metamorphic fluids (with lower Br/Cl) during ore genesis, early in the inversion and shortening history.
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