Shallow water and hypersaline features from the Middle Proterozoic Mt Isa Sequence

Abstract

Fundamental differences of opinion exist about the depositional environment of sediments which host the Middle Proterozoic¹ Mt Isa Cu and Pb–Zn deposits^2,3 of northwestern Queensland, Australia. The copper host has been repeatedly interpreted as an algal reef^2,4, and recently as a sabkha⁵, whilst models as diverse as saline lakes⁶ and deep oceanic basins^3,7–9 have been proposed for the deposition of both the Cu and Pb—Zn ore-bodies. We now report the discovery of stromatolites, halite casts, large cross-bedded channel deposits, and flat-pebble conglomerates in sediments closely associated with the ore deposit. These sedimentary structures provide evidence of shallow-water deposition with intermittent hypersaline and emergent conditions during deposition of the Upper Mt Isa Group^3,6 sequence. The assemblage of sedimentary structures strongly supports a shallow-marine or shallow-lacustrine depositional model. Either model is inconsistent with the previously postulated Red Sea-type submarine exhalative mineralization model^3,7–9 for Mt Isa. Instead we argue that Mt Isa ore formation took place in a shallow-water system, possibly aided by the action of evaporite-derived brines.