Basin shape and sediment architecture in the Gun Supersequence: A strike‐slip model for Pb–Zn–Ag ore genesis at Mt Isa

Abstract

Sequence‐stratigraphic correlations provide a better understanding of sediment architecture in the Mt Isa and lower McNamara Groups of northern Australia. Sediments record deposition in a marine environment on a broad southeast‐facing ramp that extended from the Murphy Inlier in the northwest to the Gorge Creek, Saint Paul and Rufous Fault Zones in the southeast. Depositional systems prograded in a southeasterly direction. Shoreline siliciclastic facies belts initially formed on the western and northern parts of the ramp, deeper water basinal facies occurred to the east and south. The general absence of shoreline facies throughout the Mt Isa Group suggests that depositional systems originally extended further to the east and probably crossed the Kalkadoon–Leichhardt Block. Fourteen, regionally correlatable fourth‐order sequences, each with a duration of approximately one million years, are identified in the 1670–1655 Ma Gun Supersequence. Stratal correlations of fourth‐order sequences and attendant facies belts resolve a stratigraphic architecture dominated by times of paired subsidence and uplift. This architecture is most consistent with sinistral strike‐slip tectonism along north‐northeast‐oriented structures with dilational jogs along northwest structures as the primary driver for accommodation. Although reactivated during deformation, the ancestral northwest‐ trending May Downs, Twenty Nine Mile, Painted Rocks, Transmitter, Redie Creek and Termite Range Fault Zones are interpreted as the principal synsedimentary growth structures. Sinistral strike‐slip resulted in a zone of long‐lived dilation to the north of the May Downs/Twenty Nine Mile and Gorge Creek Fault Zones and a major basin depocentre in the broad southeast‐facing ramp. Subordinate depocentres also developed on the northern side of the ancestral Redie Creek and Termite Range fault zones. Transfer of strike‐slip movement to the east produced restraining or compressive regions, localising areas of uplift and the generation of local unconformities. Northwest‐ and north‐northeast‐oriented magnetic anomalies to the south and west of Mt Isa, identify basement heterogeneities. Basement to the south and west of these anomalies is interpreted to mark intrabasin siliciclastic provenance areas in the Gun depositional system. Pb–Zn–Ag deposits of the Mt Isa valley are interpreted as occurring in a major basin depocentre in response to a renewed phase of paired uplift and subsidence in late Gun time (approximately 1656 Ma). This event is interpreted to have synchronously created accommodation for sediments that host the Mt Isa deposit, while focusing topographically and thermobarically driven basinal fluids into the zone of dilation.

Trace sulfate in mid-Proterozoic carbonates and the sulfur isotope record of biospheric evolution

“…The Paradise Creek Formation of the lower McNamara Group (ϳ300 m thick) is comprised primarily of interbedded, finely laminated dolomudstone and has been interpreted to represent deeper-water facies deposited mostly below wave base (Blake and Stewart, 1992;Southgate et al, 2000). In the present study, we analyzed stratigraphically controlled carbonate samples from the Paradise Creek Formation at Winston, 1990;Winston and Link, 1993;Luepke and Lyons, 2001 Luepke and Lyons (2001).…”

“…6) is represented by ϳ8500 m of primarily storm deposited and reworked sediments, intraclastic units, quartz and peloid silt and mud, stromatolitic dolomite, and laminated carbonates. Further details are provided in the sequence stratigraphy of Southgate et al (2000) and Sami et al (2000).…”

“…This characteristic Helena Formation locality contains the Baicalia-Conophyton stromatolite zone-a distinct lithostratigraphic marker that allows correlation of the Helena Formation over broad regions of the Belt basin (Winston and Link, 1993). The Ousel Page and Sweet (1998) and Southgate et al (2000).…”

“…Mid-Proterozoic carbonate-associated sulfate Oxide Creek, starting above the Fiery Creek Volcanics which form the base of the unit (see Southgate et al, 2000, for further details).…”

“…Map of northwest Queensland, Australia (afterSouthgate et al, 2000). Sample locality at Oxide Creek is within the Paradise Creek Formation of the lower McNamara Group.…”