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The Mesoproterozoic (1.1 Ga) Borden Basin contains extremely large, deep-water dolostone seep mounds (Ikpiarjuk Formation) whose distribution is controlled by faults. Four mounds were investigated along measured stratigraphic sections. Petrographic study revealed several depositional components, and a mixture of at least two distinct carbonate sources. Stable isotope data showed no significant methane contribution to the carbonate phases. Detritus-corrected REE+Y patterns, obtained using solution ICPMS, depict binary mixtures between basin-water-derived precipitates and seep-fluid-derived carbonate. The purest pelagic REE+Y signal is from mound tops, suggesting that mound accumulation ceased when the seep fluid waned. The REE+Y pattern of the pelagic precipitates resembles that of modern alkaline lake water. The shale-normalised pattern of the basin water is LREE-depleted, has a positive Ce anomaly and pronounced Y-excess, but lacks the La and Gd anomalies typical of seawater. The seep-fluid-related dolomite has flat shale-normalised REE+Y patterns, no Ce anomaly, and a negative Eu anomaly. This combination of characteristics points to circum-neutral (Ca and Mg-bearing?) fluids that interacted with the underlying basement before seeping into the lake bottom through faults. The chemostratigraphic patterns of the mounds result from the relative contribution of elements from the basin water vs. seep fluids. When combined with published geochemical data for coeval black shale surrounding the mounds, the new data suggest a lacustrine setting, surrounded by catchments with a preponderance of moderately to strongly weathered alkali basalt whose runoff drove the lake to alkalinity. Seep fluid was probably evaporatively concentrated basin water that acquired new geochemical characteristics both during evaporation and through water-rock interaction in the subsurface. The new understanding of this depositional stage of the Borden Basin highlights the importance of lacustrine d

The Arctic Bay Formation (Nunavut, Canada) represents a late Mesoproterozoic muddy terrigenous ramp and contains >200 m of black shale. The formation was studied in order to decipher the tectonostratigraphic and geochemical evolution of the basin, address the origin of metal enrichment, and determine whether this frontier basin has the potential to host sedimentary-exhalative or polymetallic black shale deposits. Samples were analysed in the laboratory for major and trace elements, total organic carbon (TOC), 4-step loss-on-ignition (LOI), and Pb isotopes. Non-calcareous black shale exhibits neither Ce nor Y anomalies, reflecting euxinia in the lower water column, whereas slightly dolomitic black shale has both Ce and Y anomalies, reflecting the dolomite's probable origin as a precipitate in the upper water column. The stratigraphic distribution of the rare earth elements (REEs) indicates an evolving sediment provenance, and Pb isotopic data indicate that the source of clay in the black shale was dominated by weathered, juvenile, mantle-derived material. Base metals and redox-sensitive metals, expressed as enrichment ratios relative to conservative lithophile elements, are elevated and exhibit coherent covariations in the black shale. Enrichment in the redox-sensitive elements, such as Mo and U, correlates with dolomite content of the shale, rather than with organic C or Fe-py. From a deep-time ocean evolution perspective, this important observation suggests that enrichment in these metals cannot necessarily be attributed to metal incorporation at an interface between sediment and euxinic water. Instead, in Arctic Bay Formation black shale, the metals were either scavenged onto dolomite as it precipitated in the water column, or secondarily re-distributed within the sediment according to its dolomite content. The base metals that are concentrated in the black shale (e.g., Zn) were probably sourced from diffuse hydrothermal venting, and although there is no evidence