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

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

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 δ¹³C_carb 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 δ¹³C_carb 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

The Laird Lake property encompasses the tectonic contact between the Balmer (2.99 to 2.96 Ga) and the Confederation (2.74 to 2.73 Ga) assemblages on the south-western end of the Red Lake greenstone belt, Northwestern Ontario. The purpose of this study was to determine the tectonic setting in which the assemblages formed, and to characterize the controls on and nature of the gold mineralization associated with the tectonic contact between the Balmer and Confederation assemblages.

Detailed mapping of the area highlighted major differences between the two assemblages. The Balmer assemblage is typically composed of fine-grained, aphyric, locally pillowed mafic volcanic rocks, ultramafic intrusive and volcanic rocks with flow-breccia textures and local spinifex-bearing clasts, and banded-iron formations. In contrast, the Confederation assemblage consists of porphyritic (feldspar) or poikiloblastic (amphibole) mafic volcanic rocks intercalated with intermediate to felsic volcanic rocks that include crystal lapilli tuffs, crystal tuffs and tuffs. Syn-volcanic and syn- to post-D₂ intrusions commonly cross-cut the volcanic packages. A regional foliation (~E-trending) is present throughout the volcanic rocks and increases in intensity at the tectonic contact between the two assemblages where a shear zone no thicker than 100 m is present within the Balmer assemblage.

Whole-rock geochemical analyses were undertaken on 161 samples from the Laird Lake area. The Balmer assemblage is composed of tholeiitic mafic volcanic rocks with minor Al-undepleted komatiites, whereas the Confederation assemblage is composed of calc-alkalic mafic and intermediate to felsic volcanic rocks, which display FI, FII, and FIIIb rhyolite trends. Neodymium isotope analyses, in conjunction with trace element geochemistry, suggests that parts of the Balmer assemblage were weakly contaminated by an older intermediate basement. The data suggests both arc and back arc volcanism within the Confederation assemblage, with the arc rocks showing stronger a crustal component than the back arc rocks. U-Pb geochronology of volcanic and intrusive Confederation units yielded ages of 2741 ± 19 Ma (FI quartz-feldspar porphyritic crystal tuff) and 2737.68 ± 0.79 Ma (diorite). The geochemistry and age of the tuff correlates within error to the Heyson sequence of the Confederation, whereas the diorite is likely a syn-volcanic intrusion.

The Balmer assemblage is interpreted to represent an oceanic plateau formed by plume magmatism on the margins of the North Caribou Terrane whereas the Confederation assemblage was likely built in an oceanic arc setting where both arc and back arc volcanism were occuring simultaneously. The presence of xenocrystic zircons within the 2741 Ma quartz-feldspar porphyritic crystal tuff suggest that melts within the main arc incorporated xenocrystic zircons during ascent through a thin Mesoarchean crustal fragment. Juxtaposition of the Confederation assemblage onto the Mesoarchean assemblages likely occurred between 2739-2733 Ma.

Gold mineralization at the Laird Lake property is controlled by a D₂ shear zone within the Balmer assemblage at the tectonic contact between the Balmer and Confederation assemblages. The mineralization is commonly found associated with a mineral banded texture, accompanied by disseminated arsenopyrite, pyrrhotite, pyrite ± chalcopyrite, similar to the features observed at the nearby Madsen gold mine. Oxygen isotope data from early- to syn-D₂ gold-mineralized and barren quartz veins suggest both vein types originated from a similar source and overlap with δ¹⁸O_H2O values of metamorphic, magmatic and meteoric waters, over crystallizing temperatures ranging from 300 to 500°C. Evidence for a second gold event (post-2702 ± 1 Ma) is characterized by the presence of gold-rich shears and lamprophyres cross-cutting the regional foliation, and the lack of overprinting amphibolite facies mineralogy. The Laird Lake property likely represents the continuation of the same mineralized structure found at both the Madsen and Starrat-Olsen mines and was later displaced as far as 10 km west by the dextral Laird Lake fault post-2704 Ma. 

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4301

The 2.7 Ga Shebandowan greenstone belt in the Wawa-Abitibi terrane contains unusual ultramafic rocks. The two main assemblages present within the study area are the Greenwater and Shebandowan assemblages. The 2719.7 ± 1.0 Ma Greenwater assemblage is characterized by tholeiitic magmatism whereas the 2690-2680 Ma Shebandowan assemblage is characterized by calc-alkalic magmatism. Mapping of a 16 km2 area and 7 trenches has identified the following lithologies in the field area: orthocumulate ultramafic rocks, komatiites, pyroxenite, vesicular komatiites, serpentinite, ultramafic breccia, variolitic ultramafic rocks, mafic intrusive rocks, intermediate intrusive and extrusive rocks, felsic volcaniclastic rocks, monzonite, conglomerate, banded iron formation, argillite and chert. 

The ultramafic rocks are dark black in colour, but light green on surface as a product of serpentine alteration of olivine. They are highly magnetic, with magnetic susceptibility readings from 20 to 150 x10-3 SI. In some areas they occur as fine-grained, massive rocks with no distinct textures, and they also occur as medium-grained dark black rocks with a cumulate texture. The komatiites occur as fine-grained, light grey, highly silicified rocks. Several types of spinifex texture were observed throughout the field area, including: thin chaotic spinifex, thick chaotic spinifex, oriented spinifex, and pyroxene spinifex. Mafic intrusive rocks are massive, medium-grained, equigranular grey-blue gabbro with weakly disseminated pyrite. The intermediate intrusive rocks consist of a fine-grained, green-grey matrix with hornblende phenocrysts and red-pink autoliths. The intermediate extrusive rocks are fine-grained, light grey-blue metavolcanic rocks with medium-grained plagioclase phenocrysts. Felsic rocks in the field area are very fine-grained, light grey, siliceous rocks. They are massive with no flow textures observed in any of the outcrops and the weathered surfaces range in colour from beige to blue-grey.  A monzonite dyke occurs alongside an interpreted fault through the field area. The distinctly magnetic rock consists of a fine-grained, green-grey matrix with hornblende phenocrysts and red-pink autoliths. The conglomerate is a heterolithic pebble to boulder conglomerate consisting of a fine-grained matrix with clasts of basalt, monzonite, and jasper ranging in size from ~2 cm to 40 cm. The argillite is a very fine-grained dark black mudstone. It typically showed no bedding and locally contains graphite and abundant radial pyrite concretions up to 5cm in diameter. It is seen in the trenches intercalated with the hypabyssal komatiitic intrusions. 

The komatiitic rocks have SiO2 values of 47-67 wt. % and MgO values of 2-17 wt. %. Despite being outside the accepted values for komatiites, these rocks were identified as komatiites based on spinifex texture and spatial association with other ultramafic units. The abnormally high silica content in the ultramafic rocks was determined to be caused by silica alteration, and was not a primary feature of the rocks when they were emplaced. The rocks in the field area have undergone silica and carbonate alteration as well as greenschist facies metamorphism.

The Al2O3/TiO2 ratios of the ultramafic rocks are 17-29, and the CaO/Al2O3 ratios of the ultramafic rocks are 2-2.5. These ratios, combined with a flat pattern on a primitive mantle normalized multi-element diagram, identify the rocks as aluminium-undepleted komatiites. A distinct negative Nb anomaly is present in all samples, which could be caused by either the melt being derived from a heterogeneous deep-mantle source, where perovskite fractionation depleted the Nb, or crustal contamination. Sm-Nd isotope analysis was completed on six ultramafic samples from the field area. The εNd of all samples ranged from +2.34 to +2.83, which is not consistent with contamination by older continental crust. Consequently, melting of a heterogeneous deep mantle source is the favoured model for petrogenesis of the ultramafic rocks.

Based on the close spatial association of tholeiitic and calc-alkalic rocks as well as the presence of thick sequences of deep-ocean argillites, the geological environment was determined to be an oceanic-rifted arc through which a plume of heterogeneous deep-mantle melts ascended, that subsequently closed and then collided with the Superior Province. 

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4289

The Terrace Bay Batholith is a 25 km long oval shaped granitoid intrusion located in the western portion of the Schreiber-Hemlo greenstone belt, part of the larger Wawa-Abitibi terrane. The pluton was emplaced at 2689±1.1 Ma and intrudes circa 2720 Ma metavolcanic rocks of the Schreiber assemblage. The purpose of this study was to classify the Terrace Bay Batholith petrographically and geochemically in order to investigate the petrogenesis and tectonic setting in which the pluton formed, and to characterize the association with gold and base metal mineralization.

Detailed mapping of the pluton can separate the pluton into three mineralogically distinct lithologies:  granodiorite (typically consisting of medium to coarse quartz and feldspar phenocrysts with a groundmass of fine-grained amphibole, biotite,  disseminated magnetite, and sulphide minerals), a monzogranite (composed of medium-grained quartz and feldspar with increased amounts of potassium feldspar and amphibole relative to the granodiorite), and a diorite (composed of medium-grained amphibole and plagioclase with little to no quartz or potassium feldspar present). Two types of hydrothermal alteration are present in the pluton: a chlorite- epidote and a pervasive hematite alteration. These are present across the pluton, and always in proximity to cross-cutting regional scale faults or shears; however, no association was found between gold mineralization and regional structures.

Whole-rock geochemical analyses were undertaken on 147 samples from the Terrace Bay Batholith. Geochemically, the pluton is a homogenous calc-alkaline pluton, with minimal geochemical change between lithologies. The pluton exhibits trace element signatures that are characteristically arc-related signatures: fractionated heavy rare earth elements, negative high field strength element anomalies, enrichment of Th over light rare earth elements and enrichment of light rare earth elements. The fractionated heavy rare earth elements and the Th-Nb-La systematics are consistent with formation in a subduction zone at depths where garnet is stable. The Sr/Y and La/Yb signatures support formation within the garnet stability field and suggest small amount of slab-derived melt incorporated into the mantle wedge. The isotopic signature suggests that the pluton underwent minimal crustal contamination as shown by the depleted primitive mantle εNd values ranging from +2.16 to +2.49.

The emplacement of the pluton was determined to be through multiple injections in the intrusion from a single source. The pluton underwent prolonged fractional crystallization, creating subtle mineralogical lithologies with no geochemical differences. The homogeneous nature of the pluton suggests it is unlikely that there were numerous pulses because these would result in more variation across the pluton. 

Rhenium-Osmium isotope data were obtained on molybdenite to obtain an age of mineralization for the pluton, yielding a value of 2671±12 Ma. The molybdenum mineralization is spatially associated with gold mineralization in the pluton, suggesting that they were deposited from the same hydrothermal event. As is common in Archean cratons, this age of mineralization is syn- to post- both D2 and regional metamorphism, as well as postdating the emplacement of the pluton. The gold and molybdenum mineralization in the pluton is generally disseminated throughout with local occurrences hosted in quartz veins. Although these exhibit elevated gold and molybdenum values there is no distinct mineralization style characterized with gold deposits. These features can be explained by the magmatic vapor-dispersed system theory which suggests that when a pluton is emplaced at depth, the aqueous phase will remain dispersed throughout the pluton instead of concentrating in economic amounts.

URI
http://knowledgecommons.lakeheadu.ca/handle/2453/4542

The Mesoarchean Red Lake carbonate platform is the oldest (~2.93 Ga) known carbonate platform on Earth.  It is comprised of a variety of chemical sedimentary rocks including: limestone, dolostone, chert, oxide iron formation, and sulfide iron formation.  This study deals with its five different types of deeper water lithofacies, including four chemical sedimentary rocks and siliciclastic black slate, to delineate Mesoarchean ocean chemistry and comprehend the localized depositional settings.  Geochemical analysis of selected elements in the chemical sediments were used to constrain depositional processes.  Of these positive Eu anomalies in REE patterns suggests that the ocean was heavily influenced by hydrothermal activity and positive Ce anomalies in oxide iron formation indicate that a redoxcline existed in the depositional environment. Zirconium and hafnium in oxide facies iron formation were mostly derived from seawater, and their sorption was dependent on the rate of precipitation of iron hydroxide. Multiple sulfur isotope analyses revealed that mass-independent fractionation of sulfur was operating along with bacterial sulfate reduction, and the source of sulfur was diverse.  Organic carbon isotopes increase from ~ -27 ‰ to ~ -20 ‰ up-section towards the shallow portion of the carbonate shelf, possibly reflecting the presence of anoxic phototrophs in the shallows. Inorganic carbon isotopic ratios averaging ~ -1.5 ‰ fall within the range of Archean carbonate and reflect seawater values.  Also, δ18O data reveals that dolomitization occurred in a freshwater influenced environment. Evidence such as increasing Mn concentration towards the carbonate platform and positive Ce anomaly in oxide iron formation, as well as redox-sensitive element enrichment in the chemical sedimentary rocks, suggests that bacterial O2 production was somewhat active in the Mesoarchean sea. The interlayering of various types of chemical sediments at differing stratigraphic scales indicates that localized ocean chemistry changed repeatedly over both short and long time intervals.  Carbonates were perhaps formed in the slightly oxic shallow water environment, while magnetite and chert were precipitated in a suboxic environment distal to any venting fluids and iron sulfides accumulated in a reducing, anoxic environment during intervals of intense venting of hydrothermal fluids.  Shale was deposited as background sediment and accumulated during cessation of chemical sedimentation.  Finally,  the deeper water sediments repeatedly interlayered with the shallow water stromatolitic carbonate successions, which suggests multiple events of carbonate platform flooding.

URI

http://knowledgecommons.lakeheadu.ca/handle/2453/4351