Abstract

The Mesoproterozoic Upper McNamara Group forms the uppermost division of the Lawn Hill platform in the western Mount Isa block. Several zinc and lead occurrences, including the Century deposit, are hosted by clastic and carbonate sediments within this interval. The Upper McNamara Group is subdivided into four formations: the Shady Bore Quartzite, Riversleigh Siltstone, Termite Range Formation, and Lawn Hill Formation.The Shady Bore Quartzite comprises up to 460 m of sandstone and dolomitic siltstone, deposited in a marginal to shallow subaqueous environment. The Riversleigh Siltstone is up to 2,900 m thick in the southern Lawn Hill region, where four members, R 1 to R 4 , are recognized. Member R 1 to R 3 sandstone, carbonate, and shale were mostly deposited in mid- and outer-shelf environments, indicating a regional transgression above the Shady Bore Quartzite. Sediment transport during the R 1 to R 3 interval was mostly from the south. Member R 4 shale and sandstone represent deeper subaqueous deposits. Riversleigh Siltstone sedimentation focused on a depocenter in the southern Lawn Hill region. All members display an abrupt decrease in thickness and facies changes to the north of the Termite Range and Barramundi faults, which are interpreted as active structures during deposition.The Termite Range Formation is up to 1,300 m thick and comprises two fining upward sequences, T 1 and T 2 , both dominated by sandstone in the lower part and by shale in the upper part. The Termite Range Formation is sandiest in the north and thins to the south and east. Sandstone beds are interpreted as high-concentration turbidites, deposited in a deep subaqueous environment. T 1 and T 2 are interpreted as two major episodes of turbidite fan sedimentation, sourced from the northwest. Fault activity to the west of the Lawn Hill region at about 1640 Ma is inferred to account for deposition of sandstone and the major shift in provenance from the south to the northwest.The Lawn Hill Formation is 2,200 m thick in the southern Lawn Hill region where six members, H 1 to H 6 , are recognized. Member H 1 shale and sandstone were deposited in outer shelf and deeper environments. Member H 2 and H 3 tuffaceous sediments were deposited in a mid-shelf environment, indicating a regional regression. Member H 1 to H 3 sediments were sourced from the west. Member H 4 shale and siltstone were deposited at outer shelf and greater depths, indicating a regional transgression above member H 3 . Member H 5 sandstone beds are interpreted as high-concentration turbidites, deposited in prograding turbidite fans. Member H 1 to H 3 sedimentation occurred throughout the Lawn Hill region, but a major depocenter was located in the northeast at this time. Member H 1 H 2 , and H 3 sediments rapidly thicken to the north of the Termite Range fault, which is interpreted as an active structure during deposition of this interval. The northward shift of the depocenter with respect to the Riversleigh Siltstone can be attributed to a reversal in movement along the Termite Range fault. Member H 5 sandstone beds are widespread throughout the northern Lawn Hill platform, and their deposition indicates another significant change in the distribution and style of sedimentation. Deposition in response to uplift in the south can be interpreted by the age of the sandstone (1595 Ma), which may coincide with the onset of the D 1 phase of the Isan orogeny.This study provides a framework for the identification of prospective areas for base metal mineralization within the Upper McNamara Group, based on a knowledge of sediment architectures and their controls.

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