Carli Nap HBSc thesis abstract

NASA’s Opportunity rover landed on Meridiani Planum in the summer of 2004 with the intention of studying a rich concentration of hematite in much finer detail than what the preliminary images from the orbiting Mars Global Surveyor could possibly allow. Small, diagenetic, 4mm spherules composed primarily of hematite were discovered embedded in sand blasted bedrock, arming the scientific community with further evidence for a past presence of water on Mars (NASA, 2012). This thesis is an attempt at providing a terrestrial analogue for the formation of Martian spherules by using hematite-rich concretions observed in the minimally metamorphosed, 1.8Ga Gunflint formation as a proxy.

Approximately 60km eastbound of Thunder Bay at intersections with Mirror Lake (ML) and West Loon (WL) road lie two recently exposed, iron-oxidized grainstone outcrops standing ~2-3m vertically. Original deposition occurred in the Paleoproterozoic at 1,878Ma in a storm-dominated shallow shelf. The grainstone has a typical grey-green ankeritic to white chert colour with spherical to rhombic hematite-rich concretions averaging <2mm to 2cm in diameter present in thin layers oriented parallel to the shallowly dipping, lensy bedding or randomly distributed within lenses as independent concretions or hematitic masses. Three fundamental questions occur in response to these outcroppings: Why are these rocks so iron stained when compared with the upper cherty and shaley Gunflint at outcrops of nearby Pass Lake (PL) road?; How do these concretions form?; Is the mechanism responsible for the formation of these concretions a reasonable analogue for those observed on Mars? A scaling down approach to these queries consisted of site mapping and stratigraphic sectioning, qualitative and quantitative petrographic and SEM analysis, and quantitative geochemical analysis using data acquired from ICP-MS, ICP-AES and XRD techniques.    

It is well known that the Gunflint is high in iron that precipitated out of seawater solution as an insoluble chemical precipitate. In the studied upper cherty member, individual grainstone grains have fine, iron-rich laminae that had precipitated onto the grain surface itself or were accumulated by rolling over an Fe-rich substrate. An implicit, unconformable upper contact with the Sibley group, present at one time above the Gunflint outcrop at large would have allowed for iron-rich and potentially oxidative fluid migration into the underlying Gunflint. Large, centimeter scale, iron-rich fracture sets as well as very fine, micrometer scale capillary networks provide evidence for fluid migration. Variability in the red colouration, from blood red to maroon, can be differentiated on concentric layers of individual concretions, as well as overprinting masses, and is suggestive of multiple phases of redox fluid front migrations. It is by some combination of intrinsic and extrinsic iron combined with oxidation that gave these rocks their ultimate red colouration.

Hematite concentrations within the ML and WL outcrops are always associated with carbonates that are at varying stages of decay. These hematite-bearing carbonates have been identified through geochemical, XRD and SEM analysis to be of ferroan dolomite to ankerite in composition. They are often found nucleating on or within siliceous and hematite altered grains as rhombs, and commonly mimic the entire grain. Spherical concretions occur when several altered carbonate grains are enclosed by the growth of successive poikilitic carbonate and rarely display a distinguishable nuclei: the appearance of framboidal pyrite central to a select group of concretions and scattered within the groundmass strongly suggests the influence of bacterial sulfate reduction (BSR) on carbonate growth. The variation in size, morphology and distribution of iron-bearing carbonates within individual grainstone lenses as concretionary spheres or masses, as well as the presence of framboidal pyrites, suggests that primary, hematite-poor carbonate concretions formed in a shallow phreatic, anaerobic environment at some time after cementation.

Gunflint hematite concretions differ in several ways to those observed on Mars. Negative weathering, original iron-carbonate composition, the promotion of growth by BSR and a lesser random to common bedding parallel stratigraphic distribution define the concretions observed at the terrestrial site whereas positive weathering, jarosite-hematite-alunite composition, spherulitic growth by supersaturation of Fe-rich fluids, and non-conformable growth over all stratigraphic units define the concretions at the Martian sites (Morris et al, 2010). Concretions preserve fluid chemistry and are blueprints for flow regimes and are as such important to the evolution of water on Mars and early Earth.