Evolution of the Palaeoproterozoic Prize, Gun and lower Loretta Supersequences of the Surprise Creek Formation and Mt Isa Group

Abstract

Sequence‐stratigraphic interpretations of the 4200 m‐thick Palaeoproterozoic (1700–1650 Ma) Mt Isa Group and underlying Surprise Creek Formation identify three unconformity‐bounded packages termed the Prize, Gun and Loretta Supersequences. Siliciclastic rocks of the Surprise Creek Formation and Warrina Park Quartzite comprise the Prize Supersequence. Rapid facies changes from proximal, conglomeratic fluvial packages to distal, fine‐grained and deep‐water, rhythmites characterise this supersequence. Conglomeratic intervals in the Mt Isa area reflect syndepositional movement along basin‐margin faults during the period of supersequence initiation. A major unconformity, which extends over a period of about 25 million years, separates the Gun and Prize Supersequences. In the Leichhardt River Fault Trough uplift and incision of Prize sedimentary rocks coincided with emplacement of the Sybella Granite (1671 ± 8 Ma) and Carters Bore Rhyolite (1678 ± 2 Ma) and the removal of an unknown thickness of Prize Supersequence section. Deep‐water, turbiditic rhythmites of the Mt Isa Group dominated the Gun and Loretta Supersequences. Tempestites are present over discrete intervals and represent times of relative shallowing. High accommodation and sedimentation rates at the base of the Gun Supersequence resulted in the deposition of transgressive nearshore facies (uppermost Warrina Park Quartzite) overlain by a thick interval of deep‐water, siltstone–mudstone rhythmites of the Moondarra Siltstone and Breakaway Shale. With declining rates of siliciclastic sedimentation and shallowing of the succession, calcareous sediments of the Native Bee Siltstone prograded over the deeper water deposits. Two third‐order sequences, Gun 1 and 2, characterise these lower parts of the Gun Supersequence. An increase in accommodation rates near the top of the Native Bee Siltstone in Gun 3 time, resulted in a return to deep‐water sedimentation with deposition of dolomitic rhythmites of the Urquhart Shale and Spear Siltstone. The Pb–Zn–Ag ore‐ hosting interval of the Urquhart Shale is interpreted to occur in progradational highstand deposits of the Gun 3 Sequence. In the Leichhardt River Fault Trough the Loretta Supersequence boundary forms a correlative conformity. Coarser grained and thicker bedded sediments of the Kennedy Siltstone comprise lowstand deposits at the base of this cycle. These sediments fine up into the transgressive, deep‐water, siliciclastic facies of the Magazine Shale, which in turn are truncated against the Mt Isa Fault.

Basin inversion and supercontinent assembly as drivers of sediment-hosted Pb–Zn mineralization in the Mount Isa region, northern Australia

“…Moreover, even though contacts between the Loretta and Gun supersequences in this part of the Leichhardt River Fault Trough are conformable with little or no direct evidence that one or both units were undergoing deformation at the time they were being deposited, sedimentation patterns nevertheless change quite dramatically across this depositional boundary. Indeed, beginning at the very top of the Gun Supersequence, and continuing through Loretta into the overlying River Supersequence, bedding thickness, grain size and the amount of quartz sand entering the basin all increased (Domagala et al, 2000). Growing tectonic instability and the emergence of a new source region have been invoked as the most likely explanation for these changes (Southgate et al, 2000b) and find support in observations made farther north the Lawn Hill Platform and Murphy Ridge (Fig.…”

Basin inversion and structural architecture as constraints on fluid flow and Pb-Zn mineralisation in the Paleo-Mesoproterozoic sedimentary sequences of northern Australia

“…3). These broadly conform with the sedimentary units or supersequences previously identified at the top of each superbasin (Bradshaw et al, 2000;Bradshaw et al, 2018;Domagala et al, 2000;Krassay et al, 2000a;Krassay et al, 2000b;Southgate et al, 2000a) and have an important bearing on basin evolution, and more particularly on the timing and duration of the basin inversion events that brought successive basin cycles to a close. These and other differences with previously published interpretations can be illustrated with a few well-chosen survey lines and there is no need to include interpretations of the full seismic dataset.…”

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

“…The disturbed spectra have been attributed to thermal overprinting, excess 40 Ar, 39 Ar recoil, and postprecipitation alteration (Spikings et al, 2002). Therefore, 40 Ar/ 39 Ar ages are reported as either (1) plateau ages that correspond to a region on the 40 Ar/ 39 Ar spectrum that is concordant and represents ≥50 percent of 40 Ar/ 39 Ar released (McDougall and Harrison, 1999;Spikings et al, 2002), (2) pseudoplateau ages, which are those that incorporate at least two steps of comparable age on the 40 Ar/ 39 Ar spectrum and represent >40 percent of 40 Ar/ 39 Ar Notes: Isotopic compositions of the fluid from which the dolomite precipitated are calculated using temperature calculations that (1) predict D 1 dolomite formed at shallow depths, <2,000 m, (2) use depth estimates based on sequence stratigraphic data in the Mount Isa basin (Domagala et al, 2000;Southgate et al, 2000b) and temperature constraints published by Choquette and James (1990) for D 2/3 saddle dolomite, and (3) use fluid inclusion homogenization temperatures for D 4 dolospar and veins; oxygen isotope fractionation factors used are those suggested by O'Neil et al (1969) and Sheppard and Schwarcz (1970) for water-dolomite; carbon isotope fractionation factors used are those suggested Chacko et al (1991) and Sheppard and Schwarcz (1970) for dolomite-CO2 and Chacko et al (1991) for calcite-CO2; abbreviations: Form. temp.…”