Tim McIntyre's Honours Thesis Abstract

Located in the western area of the Uchi subprovince of Superior Province Mesoarchean carbonates and iron formation that extend from western Red Lake to Wallace Lake are the remnants of a large carbonate platform that formed at some time between 2940Ma and 2925Ma.  The platform is dominated by peritidal depositional structures overlain by basin transitional lithofacies which give the impression that the platform had a flat topped and relatively steep sided geometry.  The peritidal lithofacies assemblage consists of tidal flat to subtidal structures that are common in the Archean (i.e. laterally linked and unlinked domal stromatolites, crystal fans, and herring bone calcite), and has REE characteristics of carbonate precipitated from water with near neutral pH, high pCO₂, and an oceanic REE reservoir dominated by hydrothermal input.  The peritidal lithofacies assemblage has δ¹³C_PDB=0.42‰ +/- 0.53‰ which is consistent with other shallow water carbonates of the Meso- and Neoarchean.  The basin transitional lithofacies assemblage consists of slump structures and carbonate associated oxide facies iron formation (IF).  The REE characteristics and depositional structures are the result of down-welling water from a saline, semi-restricted, peritidal depositional environment and mixing with basin waters.  The carbon isotopes of the carbonate associated oxide facies IF show basin transitional values  δ¹³C_PDB  as low as -3.07‰ reflecting the mixing of shallow water with positive δ¹³C_PDB  values and basin water with more negative δ¹³C_PDB values.  More distal lithofacies consist of chert-oxide facies IF.  The chert-oxide facies IF shows positively sloping shale normalized FEE patterns and strong positive Eu anomalies characteristic of the precipitation of iron hydroxides from ocean waters with a pH~5 and REE characteristics that are strongly influence by hydrothermal alteration of oceanic crust.

Negative Ce anomalies in the peritidal carbonates occur consistently in the crystal fans and rarely in other facies suggesting precipitation from water that was relatively oxic.  The fans also contain consistently elevated basin derived elements such as P, Fe, Sr, and Ba.  Storm events and/or small scale transgressive-regressive cycles probably episodically added open ocean water to the semi-restricted platform providing nutrients for photosynthesising organisms.  Blooms of photosynthesising organisms would remove CO₂ from the platform water and cause an increase in pH resulting in the precipitation of fans.  The Red Lake-Wallace Lake carbonate platform is the oldest known carbonate platform in a series of geographically scattered platforms contributing to a gradual change in oceanic processes in the Mesoarchean and Neoarchean including the gradual oxidation of the atmosphere by the initial production of oxygen in these semi-restricted, shallow water settings.  The evidence for oxygenic photosynthesis presented here predates the Great Oxidation Event by 400 my.