Adrian Art's MSc Thesis Abstract

Thesis Title: 
Macro- and Micro-Structure Comparison of Ancient and Modern Iron-Rich Stromatolites
Adrian
Arts
MSc
2015

The Gunflint and Biwabik Formations comprise the middle units of the Paleoproterozoic Animikie Group that crop out near the northwestern shore of Lake Superior.  Stromatolitic units were investigated and sampled at 9 outcrop sites, 4 mine sites and in 12 drill-cores.  Controversy exists as to whether the stromatolitic units that are now composed of chert were deposited as a silica gel or represent carbonate replaced by chert.  There is also debate in the literature as to whether the iron oxides that coat many microfossils are primary or deposited during later diagenesis.  Younger examples of siliceous stromatolites are very rare and iron hydroxides coating bacteria are only known for limited modern environments.  Thus, understanding the primary composition of the Gunflint microbialites will provide information on ocean chemistry during this interval.

Three stromatolitic members have previously been described within these formations: the Lower Stromatolitic Member containing stromatolites that grow directly on the peneplained Archean basement or on the conglomerate which forms the base of the Animikie Group, a Middle Member 45 to 50 meters above the base, and the Stromatolitic Limestone Member at the top of the formation directly beneath the Sudbury impact layer.  Based on this work, a new Upper Stromatolitic Member 50 meters below the Limestone Member in the Gunflint Formation was recognized.  This member contains abundant iron-silica-carbonate-manganese oncolites, and is correlative to stromatolitic units described from the Biwabik.  The oncolites represent a primary-to-very early authigenic, precipitated mineral assemblage.  The accretion of lacustrine ferromanganese nodules at the sediment-water redox boundary provides a modern analogue for the development of the oncolites described in this work.

The presence of exceptionally well-preserved, hematite coated microfossils, encased in chert in the relatively unmetamorphosed Middle Stromatolitic Member together with positive cerium and europium anomalies indicates that these sediments were deposited at a redox boundary in ocean waters near saturation with respect to silica and iron.  Conversely, precipitates, containing carbonaceous filaments in the Lower Stromatolitic Member contain a distinct negative cerium anomaly, indicating deposition in an oxidized environment.

Alternating manganese- and iron-rich laminae within the Upper Stromatolitic Member indicate a fluctuating redoxcline and oxygen levels in the shallow subtidal to peritidal environments within the Animikie Basin.  The fluctuation reduces the Fe:Mn ratio, allowing for the less readily oxidized manganese to precipitate.  The presence of dehydration cracks which cross-cut original ooid and stromatolitic laminae indicate that deposition of the silica and iron most probably occurred as amorphous gels of opal-A and iron hydroxides.

This study strongly indicates the Gunflint and Biwabik stromatolites were originally siliceous and formed by a different precipitation mechanism than that of Proterozoic carbonate stromatolites or modern agglutinated forms.

 

URI
https://knowledgecommons.lakeheadu.ca/handle/2453/4126

Christopher Yip MSc thesis abstract

Thesis Title: 
Sedimentology and geochemistry of regressive and transgressive surfaces in the Gunflint Formation, Northwestern Ontario
Christopher Ira
Yip
MSc
2016

The 1.878 Ga Gunflint and equivalent iron formations of the Animikie Basin were deposited during the period after the rise in atmospheric oxygen during the Great Oxidation Event, which began at approximately 2.4 Ga. The atmospheric composition, and especially oxygen content, during this period after the rise in atmospheric oxygen, is not well known. The Gunflint and correlative formations, having been deposited in a shallow-marine setting, might provide information on oxygen levels present in the atmosphere-ocean system at this time. This is further made possible as the rocks that make up these formations are excellent records of transgression and regression of the sea. This record is reflected in up-section changes in lithology of the rocks that make up these formations. Further, the strata show evidence of exposure indicative of maximum regression. This exposure would have allowed the sediments and rocks to interact chemically with the atmosphere and meteoric water at the time. 

Stromatolites developed in the near-shore deposits and the rocks upon which the stromatolites grew showevidence of exposure during the repeated relative sea level low stands. These rocks have an alteration pattern that depends on their lithology. The rocks that comprise the Archean crystalline basement below the Gunflint show an intense alteration pattern that includes the formation of large corestones as well as increases in Fe and Mn content. Their geochemistry is largely the result of alteration by basin derived fluids. The exposure surfaces in the grainstones that make up the Gunflint and equivalent formations exhibit cementation and brecciation of the grainstones directly beneath the stromatolites.

Geochemical analysis performed on these rocks targeted the concentration of redox sensitive oxides and elements; Fe2O3, MnO, V, Cr, Mo and U.  Also considered were the Ce and Eu anomalies present. These oxides and elements react differently in oxidizing vs. reducing fluids. Subsequently, this analysis showed a shift in the Ce anomaly throughout the stratigraphy. The change from negative Ce anomalies in the basal samples to the positive Ce anomalies in the middle and upper lowstand layers indicate a shifting redox boundary, where the lowest stromatolite layers had produced enough oxygen to balance the flux of Fe2+- rich seawater in a position away from them, whereas the middle and upper stromatolite areas had redox boundary at the surface of the stromatolite themselves. Enrichment in Cr and V, as well as depleted Eu anomalies in strata associated with lowstands, points to the mixing of oxic freshwater,  probably a combination of surface runoff from the craton and diffusion of groundwaters, and the anoxic seawaters filling the basin.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/758

Monica McCullough MSc thesis abstract

Thesis Title: 
Sedimentology and Paleogeographic Reconstruction of the Strata in and Adjacent to the Sudbury Impact Layer in the Northern Paleoproterozoic Animikie Basin
Monica M.
McCullough
MSc
2016

The Sudbury Impact Layer (S.I.L.) is dated at 1850 Ma and is located between the underlying Gunflint Formation, with an age of 1878±1 Ma that was obtained from zircons in a tuffaceous zone approximately 105 meters below the S.I.L., and the overlying Rove Formation that has an U-Pb zircon age of 1832 Ma which was obtained from tuffs 5-6 meters above the S.I.L.  There is an 18 Ma hiatus between the Sudbury Impact Layer and from where the zircon was extracted from the overlying Rove Formation, and a 46 Ma hiatus between the Sudbury Impact Layer and the underlying Gunflint Formation.  These large age anomalies associated with the hiatuses, along with little sedimentation between the tuffs that supplied the ages, suggest that periods of non-deposition and sub-aerial exposure eroded the land, resulting in a lack of sedimentation in the allotted age gaps.

The time interval was investigated in a number of outcrops and cored drill-holes in the northern portion of the basin.  Sedimentological aspects of the rocks were noted and samples collected for geochemical studies.  The upper portion of the Gunflint Formation contains grainstones that were deposited in shallow water along with chemical sediments precipitated from Paleoproterozoic seawater.  Positive Ce anomalies indicated oxygen production by stromatolites in the inter-tidal to very shallow sub-tidal lead to some oxygenation of the shallow nearshore.  The chemical sediments in the limestone that overlies the Gunflint ankerite and chert had their calcite cement formed in meteoric phreatic conditions, with extremely elevated contents of vanadium and large negative cerium anomalies indicating these waters were significantly oxic.  The overlying Sudbury Impact Layer shares these characteristic, though in the southeast it was probably deposited in a very wet, likely marine, setting.   Ankeritic grainstones overlying the Sudbury Impact Layer refute the idea that the impact caused an end to iron formation deposition and show a transition from flooding and sub-tidal deposition to extensive sabkha development.  The common occurrence of gypsum is indicated by the presence of its pseudomorphs forming bladed crystals, desert roses and vein systems.  The Rove sea flooded over this surface after lithification.

 

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4252

Ayait Baig MSc thesis abstract

Thesis Title: 
Characterization of the MAX porphyry Mo deposit using trace element geochemistry in hydrothermal alteration minerals, Trout Lake, B.C.
Ayat
Baig
MSc
2016

Over the last few decades it has become increasingly difficult to discover new ore deposits and as existing deposits are exhausted, techniques for improving exploration success are becoming vital. In this study I investigated the application of trace element mineral geochemistry, in the context of well-defined petrology and field mapping, to the identification and characterization of hydrothermal alteration of a small porphyry Mo deposit located in southeastern B.C., Canada.  Detailed geochemical analysis of the MAX Mo-porphyry deposit was undertaken and the use of trace element geochemistry signatures of hydrothermal alteration minerals were established. Ninety-six samples were collected from surface, underground and drill core and graphic core logging was carried out on five holes. Whole rock geochemistry was obtained for all samples in order to observe any general alteration trends. The trace element concentrations of quartz, chlorite and epidote were analysed using laser ablation inductively coupled plasma mass spectrometry. Cathodoluminescence was used to show textural differences and growth zoning in quartz and to observe different generations of quartz. The results indicated that in hydrothermal quartz Ti, Al and Li showed systematic increases in concentration towards the porphyry center and could be used to infer the temperature of formation. The concentrations of Ti, Al, Li, Ge, Mg and Fe could be used to distinguish between different types of quartz. Hydrothermal quartz showed the most variance when it came to concentrations of individual trace elements, whereas regional metamorphic quartz that was unaffected by hydrothermal alteration had the lowest concentration of trace elements and a uniform trace element chemistry regardless of proximity to the deposit center. Igneous quartz was depleted in Ge relative to hydrothermal and metamorphic quartz.

For chlorite it was found that Ti, V and Sr concentrations varied depending on distance from the porphyry center. Both Ti and V decreased in concentration away from the deposit center, whereas Sr peaked around 350 - 400 meters and then decreased away from the deposit center. The trace element alteration vectors identified in this study combined with subtle, but identifiable, field criteria may assist exploration companies in the search for hidden deposits.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4283

Douglas Nikkila MSc thesis abstract

Thesis Title: 
The Mineralogy and Petrology of Layered Series Nepheline Syenite within Centre II of the Coldwell Complex
Douglas
Nikkila
MSc
2017

The Coldwell Complex is situated within the Archean Schreiber-White River metavolcanic and metasediment of the Superior Province. Spanning over 25km in diameter, it is the largest alkaline intrusion in North America. The 1108 ± 1 Ma age of the Coldwell Complex and its close spatial proximity supports a strong relationship to the magmatism of the Keweenawan Midcontinent Rift. Early studies define three magmatic centers of the Coldwell Complex, which in order of intrusion are Center I, Center II, and Center III. Center I consists of an oldest phase gabbro, which borders a ferroaugite syenite to the east and north. Center II includes a nepheline-bearing biotite-gabbro and several intrusions of nepheline syenite, and Center III is composed of four syenites which in order of intrusion are: magnesiohornblende syenite, contaminated ferroedenite syenite, ferroedenite syenite, and quartz syenite. This study evaluated the formation of the layered series nepheline syenite in Center II of the Coldwell Complex. Field mapping and sampling were completed along the shoreline of Neys Provincial Park, where extensive exposures of massive, hybrid, and layered syenite; together with xenolith-rich zones, biotite-gabbro, lamprophyre dykes and pegmatitic syenite of diverse composition are preserved. In the layered series, perthitic K-feldspar with secondary plagioclase forms the cumulus phase. Feldspar observed through cathodoluminescence (CL) imaging demonstrated multiple feldspar species within single crystals. K-feldspar ranged in orthoclase component from 87-100%, whereas plagioclase exsolution and alterations were characterized as albite, with anorthite components ranging from 0-3%. Amphibole (classified as ferro-pargasite) and minor apatite represent post-cumulus phases, forming interstitially to feldspar laths or in amphibole-rich laminae at the base of individual layers. Biotite, also a post-cumulus phase, was classified as the iron-rich end member annite. Fluorapatite is the most abundant accessory mineral, hosting light rare earth elements (LREE), along with less abundant britholite, wöhlerite, pyrochlore, titanite and other minor accessory minerals hosting incompatible elements. Layers commonly display modal grading from amphibole-dominant laminae, to a mixture of amphibole and feldspar, followed by a section of ‘normal syenite’ (dominantly feldspar with disseminated amphibole). Hydrodynamic processes are favoured for the origin of the layered series, specifically surge-type density currents and separation and reattachment vortex cells, are proposed to have formed the modally graded cumulate layers. In conjunction with graded layers, other magmatic “sedimentary” features including slumping, scour channels, flame structures, load casts, and various stages of hybridization and deformation of mafic xenoliths produced during hydrodynamic processes, indicate a strong convecting current operating during the formation of these rocks. The formation of the layered series was a product of fractional crystallization, varying deposition mechanisms, and reworking through erosion or deformational processes.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4212

Sean O'Brien MSc thesis abstract

Thesis Title: 
Petrology of the Crystal Lake Gabbro and the Mount Mollie Dyke, Midcontinent Rift, Northwest Ontario
Sean
O'Brien
MSc
2018

The Crystal Lake Gabbro (CLG) is a Y-shaped, up to 750 m wide, layered mafic intrusion with a 5 km long northern limb and a 2.75 km long southern limb, with localized Cu-Ni and Cr mineralization. The Mount Mollie Dyke (MMD) is an arcuate, 60 to 350 m wide, macrodyke that lies on trend east of the CLG and extends for 35 km toward Lake Superior. Both intrusions are part of the 1.1 Ga Midcontinent Rift (MCR) and were emplaced into the Paleoproterozoic Rove Formation of the Logan Basin, approximately 50 km south of Thunder Bay. Current U-Pb age determination implies a ~10 m.y. age difference with CLG being formed at ~1100 Ma and the MMD being formed at ~1109 Ma. However, this age difference is at odds with both intrusions being normally polarized (an attribute of MCR rocks younger than 1102 Ma) and their being on trend with each other. This study seeks to determine whether the two intrusions may be petrogenetically linked by evaluating the petrography, geochemistry, mineral composition, and sulphur isotopes of samples collected from drill core.

The CLG profiled in a drill core from its southern limb can be broadly divided into Upper, Main, and Lower Zones with further subdivisions of the Main and Lower Zones based largely on geochemistry. The Lower Zone occurs between two xenoliths of an early MCR (~1115 Ma) plagioclase porphyritic Logan Sill diabase. The Lower Zone consists of subophitic to ophitic troctolite, augite troctolite, and olivine gabbro and can be subdivided into an upper and basal marginal subzone as well as an interior subzone. Both marginal subzones host disseminated sulphides with the basal margin also containing Cr-spinel seams. An overall bottom-up-directed fractional crystallization of the Lower Zone is suggested by the progressive decrease in Fo content of olivine, Mg# of clinopyroxene, and whole-rock MgO upsection. Above the upper Logan Sill xenolith, the Main Zone similarly consists of subophitic to ophitic troctolite, augite troctolite, olivine gabbro, and gabbro. Petrography, lithogeochemistry, and mineral composition was used to subdivide the Main Zone into five subzones: a basal marginal subzone, upper margin subzone, and three interior cycles that display cryptic variations indicative of fractional crystallization and magma recharge events.  Like the margins of the Lower Zone, the Upper Zone as well and the basal marginal subzone of the Main Zone contain disseminated sulphides and Cr-spinel, and are characterized by relatively high Fo content olivine and low incompatible trace element concentrations.  These mineralized zones are interpreted to have crystallized from the same initial pulse of magma into the CLG, which was sulfide- and Cr-spinel-saturated. Cyclical cryptic variations in the internal subzone of the Main Zone are interpreted to indicate upward directed fractional crystallization, interrupted by emplacement of additional magma pulses into the core of the intrusion. All rocks of the Main Zone are olivine and plagioclase orthocumulates indicating that fractional crystallization was not particularly efficient (i.e., did not experience a strong segregation of cumulus minerals from the parental magma).  The lack of Cr-spinel in the interior and upper marginal subzones of the Main Zone further indicates that subsequent magma pulses either were more evolved than the original  parental  magma or were volumetrically subordinate to  the evolved magmas that resided in the chamber. Throughout the evolution of the CLG, the differentiation of the magma was limited as it did not result in clinopyroxene and Fe-Ti oxide becoming cumulus phases. This was likely due to magmatic recharge and inefficient fractional crystallization.

Texturally and geochemically, the MMD can be broadly divided into an Upper and Main Zones, with a subdivision of the Main Zone into an upper and lower sequence and a pegmatitic segregation subzone. The Upper Zone consists of ferrodiorite and likely represents the end product of extensive fractionation. The Main Zone is characterized by troctolite, augite troctolite, olivine gabbro, and gabbro with MgO, CaO, Al2O3, and Ni concentrations decreasing upwards and SiO2, TiO2, K2O, Na2O, P2O5, and incompatible trace element concentrations increasing, consistent with bottom-up fractional crystallization. Strong differentiation of the MMD magma is indicated by the habit change of clinopyroxene from ophitic (intercumulus) to granular (cumulus), which is the basis for the subdivision of the lower and upper sequences. The lower sequence of the Main Zone also hosts a 24 m thick interval containing 1 to 2 m wide gabbroic pegmatite layers. These pegmatites are interpreted to be the result of localized enrichment of magmatic volatiles.

The presence of an evolved core in the MMD surface expression, coupled with the mineral composition of olivine, plagioclase, and clinopyroxene, remaining at relatively constant Fo, An, and Mg# values, respectively, below the pegmatitic layers suggests that there was some degree of lateral crystal fractionation as well as bottom up fractionation. The well-defined fractionation sequence as well as an absence of abrupt geochemical changes suggests that the MMD fractionally crystallized from a single pulse.

Liberation of external sulphur from the surrounding Rove Formation, is suggested by the greater than mantle S/Se values as well as δ34S values between +4.0 and +21.0‰ of the sulphides within the CLG.  The addition of external sulphur evidently resulted in sulphur saturation during initial emplacement of the CLG magmas.  Primitive mantle normalized multi-element diagrams and trace element ratios provide supporting evidence for a localized shallow level of crustal contamination, as well as a deeper more widespread contamination component of both the CLG and MMD magmas.

The estimated parental magma compositions and average primitive mantle normalized trace element concentrations of the CLG and MMD suggest that they shared similar, if not the same, magma source. The CLG parental magma was slightly more evolved than the MMD suggesting that the magmas were sourced from a fractionating staging chamber. The estimated parental magma compositions of the CLG and MMD closely resemble those of the Layered Series intrusions of the Duluth Complex, supporting previous speculation that the CLG may be a satellite intrusion of the Duluth Complex. Despite current geochronology data to the contrary, the results of this study strongly suggest that the CLG and the MMD are petrogenetically linked, if not parts of the same intrusive system.

 

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4177

Brittany Ramsay MSc thesis abstract

Thesis Title: 
Environmental Control of Seawater Geochemistry in a Mesoarchean Peritidal System, Woman Lake, Superior Province
Brittany
Ramsay
MSc
2020

The 2.857±5 Ga (this study) carbonate platform at Woman Lake, Ontario, Canada, presents a unique opportunity to fill a 130 million year knowledge gap on early carbonate sedimentology and ocean chemistry between similar platform occurrences at Steep Rock Lake (2.80Ga) and Red Lake (2.93Ga). Woman Lake carbonates are among the few very early and thick carbonate platforms to develop in the Mesoarchean. Field, petrographic, and geochemical investigations were performed on the limestone sequence to better understand the paleoenvironmental context of this understudied, 90-meter-thick succession. At the base of the carbonate platform, lying atop felsic subaerial Archean tuff, are stratiform stromatolites interbedded with thin beds of massive carbonate grainstone, followed by laterally linked low domal stromatolites, which gradually become larger domes, then bioherms with walled pseudocolumnar stromatolites. They are overlain by cross-stratified and parallel laminated carbonate grainstones and more pseudocolumnar stromatolites. A variety of fenestral microbialites overly this unit, including thrombolites, stromatactis-bearing low domal stromatolites, and narrow isolated columnar stromatolites. This is followed by a cyclic succession of low domal stromatolites alternating with microbial carbonate and carbonate grainstone. Three main stromatolitic morphologies exist and represent a range of low to moderate current energies from upper intertidal to subtidal environments. They are: 1) low relief stratiform to undulating stromatolites 2) laterally linked low domal and pseudocolumnar stromatolites, and 3) isolated to locally isolated domes and narrow columnar stromatolites. Evidence here supports mainly peritidal environments on a carbonate platform with fluctuating sea-level and water energies in an overall deepening succession. The diverse carbonate facies are comprised of geochemical features reminiscent of both Archean and modern signatures in shale normalized REE patterns. Trace elements indicate that the carbonates precipitated from a mixture of two different fluids: anoxic seawater that carried a positive Eu anomaly, and oxygenated waters that imparted significant negative Ce anomalies. On a microscopic scale, using LA-ICP-MS, there is less compositional contrast between carbonate phases, which indicates that dissolution and precipitation on a small spatial scale homogenized localized areas, but did not affect changes on a metric scale. Geochemical trends paired with stratigraphic depth show decameter cycles of gradual declines in Mg, Fe, Mn, Ba and Sr substitution into the calcite lattice followed by sharp increases throughout the platform’s deposition, possibly reflecting changing accommodation space effecting precipitation rate. Typical Archean values for δ 13Cranging from -3.83‰ to 1.30‰, with an average of 0.53‰ (±0.59, n=31) occur with Y/Ho ratios ranging from 27 to 117 and 87Sr/86Sr isotopic values from 0.700346 to 0.711313 (±0.00098 (1σ)). The observed trends suggest that the precipitating carbonates were able to record and retain the effects of an evolving water column had in the local environment. Importantly, the Woman Lake carbonate platform provides context for, and evidence of, free oxygen approximately 500 million years before the Great Oxygenation Event, during a relatively undocumented period in time.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4731

 

Johnathan Savard MSc thesis abstract

Thesis Title: 
Petrology of Ijolites from the Prairie Lake Carbonatite Complex
Johnathan
Savard
MSc
2019

This study investigates the major and trace element composition of minerals of the ijolite series rocks occurring at the Prairie Lake carbonatite complex, northern Ontario, together with comparative data for ijolites from the Fen complex, Norway. Trace element data (Sr, Zr, REE) were collected by LA-ICP-MS for clinopyroxene, garnet, and apatite. and in conjunction with the major element data are used to develop a petrogenetic model for Prairie Lake.

The ijolites and calcite ijolites (hollaites) of Prairie Lake carbonatite complex have been formed by magma mixing, crystal settling, solid-state deformation, and deuteric alteration. The complex represents at least three stages of intrusion by melts of differing composition. The initial stage is predominantly biotite pyroxenites and associated coarse carbonatite veins. The second stage is primarily members of the ijolite series together with solid state deformation creating meta-ijolites, with differentiation forming malignites (potassic nepheline syenites). The third major stage is the intrusion of the CII carbonatites derived from different batches of magmas. These latter rocks contain xenoliths of ijolite suite rocks, wollastonite apatitite and phoscorite.

Pyroxene compositions show an evolutionary trend from diopside in biotite pyroxenites to Fe-enriched diopside-augite in ijolites to aegirine in malignites. Clinopyroxene major and trace element data shows that clinopyroxene cores from the biotite pyroxenites formed at depth and were emplaced as part of a later event. These data are used to show that a continuously filled fractionating magma chamber was not present at Prairie Lake and that the complex formed as result of small intrusions of nephelinite magma into pre-existing ijolites. A similar style of petrogenesis is suggested for the Fen complex.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4487

Sophie Kurucz MSc thesis abstract

Thesis Title: 
Paleoproterozoic Snowball Earth? Sedimentology and Geochemistry of a Huronian Glacial Cycle
Sophie
Kurucz
MSc
2019

The study discusses new sedimentological and geochemical results for the Paleoproterozoic Bruce glacial cycle, which represents the second of three glacial cycles within the Huronian Supergroup. This Bruce glacial event is unique for its cap carbonate, the Espanola Formation, which directly overlies glacial diamictites of the Bruce Formation. Cap carbonates are ubiquitous overlying glacial sediments in Neoproterozoic successions and are believed to be precipitated as a direct result of deglaciation. An investigation of the Bruce glacial cycle has led to a summary of the depositional history, wherein the Bruce ice sheet was grounded as it entered the Huronian basin and overrode outwash of the Mississagi Formation. It quickly delaminated, and deposited a uniform blanket of diamict before retreating, followed by the development of a glacial rebound sequence within the Espanola and Serpent Formations.

The upper Mississagi Formation directly underlies the glacial diamictites of the Bruce Formation and consists predominantly of planar and trough cross-stratified sandstones. These sandstones were deposited in a distal braided fluvial to deltaic system immediately prior to the advent of the local Bruce ice sheet advance. The Bruce ice sheet was grounded as it entered the basin, which is indicated by the typically sharp contact with the Mississagi Formation and the erosional uptake of Mississagi sands into the base of the Bruce. Delamination of the Bruce ice sheet and development of a floating ice shelf is illustrated by the dominance of massive, laterally continuous diamictites and evidence for current winnowing and iceberg rafting. These units, which constitute the majority of the Bruce Formation, were deposited beneath floating ice. Partial digestion of a carbonate-rich laminated dropstone lithofacies in the uppermost Bruce Formation for δ13Ccarb analysis produced highly negative values that are derived from a methane seepage signature. These same samples also have REE patterns with consistent negative Eu anomalies that are the result of remobilizing reduced Eu2+ via the highly reducing fluids sourced from methane seepage.

The Espanola Formation, the cap carbonate which overlies the Bruce Formation glacial deposits, is typically separated from the underlying Formation by a 0.5-3m thick laminated rain-out siltstone unit. Overlying this unit, the stratigraphy of the Espanola Formation records a gradual shallowing upwards sequence, with the lower, offshore facies being dominated by laminated carbonate and siltstone with abundant contortion and slumping, and the middle and upper stratigraphy dominated by ripple and hummocky cross-lamination and intraformational conglomerate beds deposited in a higher energy nearshore environment. An upward trend, from ~-4‰ to -2.5‰ over approximately 110m, occurs in the δ13Ccarb of the carbonate fraction of the middle and upper stratigraphy in drill hole E150-2. These samples produce patterns that have a ‘hat-shaped’ distribution with middle REE-enrichment that is similar to patterns of some Neoproterozoic cap carbonates. These patterns are suggested to be a reflection of influence by both seawater and freshwater signatures.

The stratigraphy of the upper Espanola Formation gradually transitions into the lower Serpent Formation, another sandstone-dominated Formation that represents the completion of the glacial rebound cycle. Environments of deposition for the lower Serpent Formation are variable, but range from storm- tide- and wave-influenced nearshore settings to fluviodeltic with varying tidal influence. This sequence concludes the glacial rebound cycle of the Bruce ice sheet in the Huronian Supergroup.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4503

Simon Dolega MSc thesis abstract

Thesis Title: 
Geochemistry of Shallow and Deep Water Archean Meta-Iron Formations and their Post-Depositional Alteration in Western Superior Province, Canada
Simon
Dolega
MSc
2018
One purpose for studying banded meta-iron formations is to determine the chemical composition of seawater in the Archean ocean and the oxygen content of the Archean oceanic-atmospheric system. Geologists use the geochemistry of meta-iron formations to make interpretations on the chemical conditions in the Archean. However, post-depositional alteration can affect the element geochemistry preserved in the meta-iron formations. This thesis explores the role of post-depositional mechanisms and determines element provenance in four Archean banded meta-iron formations. The four different locations hosting Archean metamorphosed meta-iron formations chosen for this study are: meta-iron formations from the Beardmore/Geraldton greenstone belt of the Eastern Wabigoon Domain, Lake St. Joseph greenstone belt of the Uchi Domain, North Caribou greenstone belt of the North Caribou Terrane and Shebandowan greenstone belt of the Wawa Subprovince. The metairon formations from the Beardmore/Geraldton and Lake St. Joseph greenstone belts are interpreted to have been deposited in a shallow water setting, while meta-iron formations from the North Caribou and Shebandowan greenstone belts are interpreted to be deposited in deeper water environments. This thesis also investigated element and oxygen ocean stratification by comparing the geochemistry of shallow and deep meta-iron formations. The main source of iron and silica to the oceans was hydrothermal venting fluids. Iron and silica precipitated out of seawater as iron oxyhydroxides and amorphous silica. Elements dissolved in the Archean ocean were adsorbed onto iron oxyhydroxides and silica during deposition. Crystallization of quartz, magnetite and hematite occurred during diagenesis and magnetite continued to grow during progressive metamorphism. The lack of cerium anomalies, absence of significant Y/Ho anomalies and deficiency of authigenic chromium supplied to the ancient suggests that the oceans were anoxic. Therefore, oxygen stratification did not occur between shallow and deeper water environments in the Archean. Significantly most of the elements were derived from multiple sources, including the siliciclastic phase, seawater or hydrothermal venting fluids, at various proportions. Al2O3, TiO2, Th, V, Nb, U, REEs and Y were determined immobile during post-depositional alteration. Mobility during diagenesis is clearly exhibited by sodium and potassium in the meta-iron formation samples from the Beardmore/Geraldton, Lake St. Joseph and North Caribou greenstone belts. Diagenetic modification mobilized sodium in the hematite-, jasper- and chert-dominated samples, while potassium was mobilized in the magnetite-dominated samples. Element stratification occurred in the Archean due to the source provenance. Deeper oceans were more enriched in Cs, Na2O, CaO, MnO, Cr and HREEs relative to shallow waters. Shallow oceans were more enriched in K2O, Rb and LREEs relative to deeper waters. This indicates that the Archean oceans were heterogeneous.

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