Stratigraphy and depositional setting of the Upper McNamara Group, Lawn Hill region, northwest Queensland

The Mesoproterozoic Upper McNamara Group forms the uppermost division of th e Lawn Hill platform in the western Mount Isa block. Several zinc and lead occurrences, including the Century deposit, are hosted by elastic and carbo nate sediments within this interval. The Upper McNamara Group is subdivided into four formations: the Shady Bore Quartzite, Riversleigh Siltstone, Ter mite Range Formation, and Lawn Hill Formation. The Shady Bore Quartzite comprises up to 460 m of sandstone and dolomitic s iltstone, deposited in a marginal to shallow subaqueous environment. The Ri versleigh 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 sandsto ne, carbonate, and shale were mostly deposited in mid- and outer-shelf envi ronments, indicating a regional transgression above the Shady Bore Quartzit e. Sediment transport during the R-1 to R-3 interval was mostly from the so uth. Member R-4 shale and sandstone represent deeper subaqueous deposits. R iversleigh Siltstone sedimentation focused on a depocenter in the southern Lawn Hill region. All members display an abrupt decrease in thickness and f acies changes to the north of the Termite Range and Barramundi faults, whic h 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 pa rt 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 interprete d as high-concentration turbidites, deposited in a deep subaqueous environm ent T-1 and T-2 are interpreted as two major episodes of turbidite fan sedi mentation, sourced from the northwest. Fault activity to the west of the La wn Hill region at about 1640 Ma is inferred to account for deposition of sa ndstone 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 w here six members, H-1 to H-6, are recognized. Member H-1 shale and sandston e 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 w est. Member H-4 shale and siltstone were deposited at outer shelf and great er 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 thr oughout the Lawn Hill region, but a major depocenter was located in the nor theast 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 str ucture during deposition of this interval. The northward shift of the depoc enter with respect to the Riversleigh Siltstone can be attributed to a reve rsal in movement along the Termite Range fault. Member H-5 sandstone beds a re widespread throughout the northern Lawn Hill platform, and their deposit ion indicates another significant change in the distribution and style of s edimentation. Deposition in response to uplift in the south can be interpre ted by the age of the sandstone (1595 Ma), which may coincide with the onse t 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.