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Abstract:

The Ikpiarjuk Formation consists of a group of very large (kms long and hundreds of metres thick), isolated, deep-water, vent-related dolostone mounds that are unlike any documented example of vent-related carbonate rocks in the geological record. The mounds lack ‘normal’ features of Proterozoic carbonates: the mounds contain thrombolites (microbially clotted carbonate), formed below the photic zone, and lack the stromatolitic fabric characteristic of most Proterozoic reef structures. The sedimentology, stratigraphy, geochemistry, and diagenetic history of the mounds were investigated through field work, standard petrographic methods, and various geochemical and other analytical techniques.<br/><br/>Shale-normalised rare earth element patterns of various depositional components of the mounds depicts binary mixing between basin-water-derived precipitates and seep-fluid-derived carbonate. Basin fluid has characteristics that are consistent with deposition in an alkaline lake, and seep fluid features are consistent with circum-neutral groundwater that had interacted extensively with basement blocks before seeping into the lake bottom through faults.<br/><br/>The sedimentology and mineralogy of the mounds indicates that they were formed by thrombolites that were made up of a microbiota that was not photosynthetic or aerobic. The Ikpiarjuk Formation thrombolite microbiota was probably dominated by an anaerobic chemoautotrophic or chemoheterotrophic metabolism, and records visible preservation of a community that has never before been described from the Mesoproterozoic.<br/><br/>The diagenetic history of the Ikpiarjuk Formation is complicated. Dolomitisation occurred early during diagenesis, and subsequently, five different fluids flowed through primary pore space in the mounds and precipitated successive generations of cement. One of the cements contains sulphides inclusions and may correlate with base-metal bodies elsewhere in the basin.