A non-marine depositional setting for the northern Fortescue Group, Pilbara Craton, inferred from trace element geochemistry of stromatolitic carbonates

Abstract

The depositional environment of sedimentary rocks from the Neoarchaean Fortescue Group (2.78–2.63 Ga) in the Hamersley Basin, Pilbara Craton, Western Australia, is controversial, with both lacustrine and shallow-marine settings proposed. Stromatolitic carbonates occur throughout the stratigraphic section at various levels, and their trace element geochemistry is used herein to examine ambient water from which these carbonates precipitated. Specifically, rare earth elements (REE) and yttrium were analysed to understand whether: (i) Fortescue carbonates display a marine or lacustrine geochemical signature, and (ii) to identify temporal trends in the geochemistry consistent with a transition in the depositional environment. Detailed inspection reveals that Fortescue stromatolites from the northern part of the Hamersley Basin are devoid of trace element compositions that are characteristic of marine carbonates. For instance, the carbonates lack distinct La (expressed as La/[3Pr–2Nd] in shale-normalised diagrams) and Gd ([Gd/(2Tb–Dy)] shale) anomalies and supra-chondritic Y/Ho ratios, which are well-developed in seawater-derived carbonates formed throughout Earth history. The Neoarchaean Fortescue Group stromatolites also lack depletion of the light REE relative to the middle and heavy REE when shale-normalised, in contrast to modern seawater and marine carbonates, which are typically HREE-enriched. Importantly, the absence of a clear temporal trend in the geochemistry suggests that the depositional environment remained unchanged from 2.8 to 2.6 Ga. It is concluded that Neoarchaean stromatolitic carbonates throughout the Fortescue Group in the northern part of the Hamersley Basin were deposited in a lacustrine environment, or a very shallow lagoonal environment dominated by freshwater influx from rivers. Our study demonstrates that trace element geochemistry is a powerful tool to constrain depositional environments of ancient carbonate rocks. This study is the first to document the geochemical characteristics of carbonates formed through microbial activity in an ancient lake, or shallow lagoonal, setting.