The Good Hope carbonatite is located northwest of Marathon, Ontario (49° 02’ N, 86°43’ W). It occurs along the northwest margin of the Prairie Lake complex. The objectives of this research are to characterize the carbonate and other minerals of the Good Hope carbonatite, use mineral compositions and textural associations to discuss the petrogenesis, and compare the mineralogy of the Good Hope and Prairie Lake carbonatites to assess potential relationships.

The major rock-forming minerals in the Good Hope carbonatite are calcite, ferroan dolomite, and apatite. The apatite occurs predominantly as elongated aggregates and clasts that define heterogeneous banding within the carbonatite. Syenite xenoliths are centimeter scale, entrained within the carbonatite, composed of K-feldspar, biotite-phlogopite, magnesio-arfvedsonite, aegirine, quartz, and carbonate. The minor minerals within the carbonatite include dolomite, K-feldspar, quartz, chlorite, magnetite, barite, biotite – phlogopite, magnesio-arfvedsonite, aegirine, pyrochlore, albite, and fluorite. The accessory minerals include pyrite, synchysite-(Ce), rutile, siderite, strontianite, thorite, parisite-(Ce), bastnaesite-(Ce), burbankite, and zircon.

The sequence of formation began with the crystallization of the syenite from an unknown alkaline parent magma, which was followed by the first pulse of carbonatitic magma that disaggregated and entrapped the syenite xenoliths and began crystallizing the Ca-Na pyrochlore and apatite. Subsequent evolution of the initial carbonatitic magma resulted in the alteration of the pyrochlore as well as the dissolution-reprecipitation of the apatite. The second pulse of carbonatitic magma resulted in the resumption of magmatic crystallization of the Ca-Na pyrochlore and apatite together with the crystallization of the alkaline silicates, aegirine and magnesio-arfvedsonite. The apatite + pyrochlore ± magnesio-arfvedsonite accumulate and are subsequently disaggregated with the clasts being deformed by turbulent flow of the third carbonatitic magma pulse. Groundmass calcite and dolomite with increasingly Fe-rich compositions were formed as crystallization progressed. The system becomes increasingly influenced by hydrothermal and carbothermal processes with the crystallization of the late ferroan dolomite with associated late-hydrothermal apatite ± pyrochlore. The precipitation of the hydrothermal phases and Fe-overgrowths are the last stages of crystallization.

The close spatial association with the Prairie Lake carbonatite complex and the crystallization sequence consistent with other alkaline rock-carbonatite complexes may support a genetic association. However, the mineralogy of Good Hope is distinctly different from that of Prairie Lake and as yet there is insufficient mineralogical or geological evidence to permit formulation of any simple genetic relationships between the two complexes. 

URI
https://knowledgecommons.lakeheadu.ca/handle/2453/5069

The Wabigoon subprovince is a 900 km long by 150 km wide Archean-aged granite greenstone belt. Large, synvolcanic batholiths with smaller late to post-tectonic stocks cut the numerous greenstone belts of the subprovince. One of the post-tectonic stocks is the Taylor Lake Stock, located within the western Wabigoon subprovince with a late Archean crystallization age that has been interpreted to infer the cessation of regional tectonics in the area. However, granitoids are highly competent and dry rocks that are difficult to deform, leading them to appear undeformed/unmetamorphosed in the field even though they may have undergone ductile deformation, brittle deformation and associated hydrothermal/ metasomatic alteration. In this study, field mapping and sampling, petrographic analysis, mineralogical compositional analysis, stable isotope geochemistry and cathodoluminescence imaging of twelve granitoid plutons (including the Taylor Lake Stock) across the subprovince are used to constrain the relationship between the brittle deformation, ductile deformation and alteration of the plutons to provide insight into the tectonic history of the Wabigoon subprovince.

The twelve plutons record chlorite and/or epidote infilled steeply dipping shear fractures with sub-horizontal lineations that indicate an oblique strike-slip displacement, characteristic of transpression. The strike of the infilled shear fractures varies across the individual plutons, possibly as a result of non-coaxial strain in which the rigid and competent granitoid bodies have undergone a component of rigid body rotation, resulting in a shifting trend of the maximum elongation direction. Evidence for this stems from the shape of the Ottertail pluton in the western Wabigoon subprovince that resembles a porphyroclast entrained within a dextral shear zone. Adjacent to the chlorite and/or epidote infilled shear fractures, wall-rock alteration common to all plutons includes white mica (mostly phengite) ± epidote alteration of the feldspar and chloritization of biotite. Epidote, chlorite and sphene are also commonly seen along microfractures within the host rock. Alteration of the plutons is noted to be lesser in the low strain areas of the plutons where brittle deformation is not as pervasive. Cathodoluminescence imaging of the feldspars from the various strain zones supports lower degrees of alteration in lower strain zones, as feldspars are more consistent with their color hues in the lowest strain samples.

Each of the twelve plutons also record solid-state deformation microstructures within quartz and feldspar, demonstrating dislocation creep was active in both mineral phases. As dislocation creep does not become an effective process within feldspar until temperatures reach ⁓450°C, these solid-state deformation microstructures provide evidence for amphibolite facies metamorphism of the plutons. Ductilely overprinted quartz veins and the presence of chlorite and/or epidote shear fractures within shear zones demonstrates brittle deformation during regional ductile deformation and associated alteration, which is seen in six of the plutons studied (Sabaskong batholith, Dryberry batholith, Revell batholith, Ottertail pluton, Irene-Eltrut batholithic complex and the Croll Lake Stock), providing evidence that regional-scale transpression consisted of coeval brittle-ductile deformation and associated alteration within the granitoid plutons. The δDfluid and δ18Ofluid values of the hydrothermal fluid calculated from measured δD and δ18O values of chlorite infilled shear fractures from six samples in two plutons range from -30 to -45‰ and 5.6 to 7.1‰, respectively, recording a metamorphic water signature that likely stems from the devolatilization of the surrounding host greenstone during regional Archean metamorphism.

XRD qualitative mineral phase analysis of two chlorite infilled shear fractures also shows the presence of the illite 2M1 polytype associated with the vein, suggesting that some of the hydrothermal fluid circulation occurred at temperatures within the realm of the illite 2M1 stability field (lower than roughly 300°C). This coupled with the presence of a chlorite infilled cataclasite, which represents a lower temperature brittle feature, suggests that at least a component of the brittle deformation and associated hydrothermal fluid flow occurred post peak metamorphism, likely into exhumation of the granitoid plutons.

Furthermore, the alteration and deformation of the twelve plutons studied demonstrates evidence for coeval brittle-ductile deformation and associated hydrothermal fluid flow in the amphibolite facies of metamorphism, with brittle fracturing and alteration continuing into exhumation. The late Archean crystallization age date from the Taylor Lake Stock should not be used to mark the cessation of regional tectonics within the western Wabigoon subprovince.

URI
https://knowledgecommons.lakeheadu.ca/handle/2453/4959

Archean precipitates such as crystal fans can be used to investigate the difference between Archean ocean and modern-day seawater chemistry. The globose radiating masses of crystal fans in the Hogarth Member of Steep Rock, the crystal fan fabric in the Elbow Point member of Steep Rock, and the vertically stacked acicular crystal fans from Red Lake carbonates were included in this study. These vertical crystals which grew off the substrates precipitating from anoxic Archean seawater are suggested to be primarily aragonite or gypsum. By analyzing Sr and Ba in the Steep Rock and Red Lake samples and comparing them with gathered literature data of old and modern-day carbonates, the primary mineralogy of these crystal fans is investigated. Sr and Ba show preferential substitution in aragonite, calcite and dolomite with high preservation of Sr concentrations in aragonite. Steep Rock fans have significantly higher Sr concentrations relative to older and dolomitized Red Lake fans representing tidal flats, while the Red Lake atikokania showed a relatively higher Sr value representative of deeper waters. Dolomite samples from both study areas showed a notable distinction in their Sr value from calcite samples. However, Sr concentrations for Steep Rock and Red Lake are significantly lower when compared to modern aragonites. This result concurs with the Sr loss that happens during recrystallization over time, i.e., as the mineral transforms from aragonite to calcite to dolomite. The primary mineralogy of these crystal fans is inconclusive due to this significant Sr loss, however, it is most likely that they were carbonates than gypsum or anhydrite.

The boundary zone between the Quetico and Wabigoon subprovinces is a complex zone of deformation and metamorphism that is marked by a fault, change in metamorphic grade and lithology. The Quetico-Wabigoon subprovince boundary zone is exposed along Highway 527 within a roughly 23km stretch of highway. At the south end of this zone the DeCourcey Lake outcrop is a strongly foliated, mylonitic rock containing quartz, feldspar, garnet, sillimanite, muscovite, and biotite with pegmatites and boudinaged quartz veins and is classified as part of the Quetico subprovince. The north end of the zone is marked by a weakly foliated conglomerate that displays primary sedimentary textures and is classified as part of the Wabigoon subprovince. Catacalsis was discovered 9.8km north of the DeCourcey Lake outcrop and marks a sharp change between the high-grade amphibolite to granulite facies Quetico rock to the south and sub-greenschist to greenschist facies Wabigoon rock to the north. This cataclasis is evidence for a fault that has not been recorded in previous studies. The fault is mapped parallel to the foliation of the cataclasite. This fault is interpreted to be a boundary fault marking the boundary at this location between the Quetico and Wabigoon subprovince.

This research was focused on constructing a depositional model for the thick, massive sandstone layers present in the Pass Lake and Outan Island Formations of the 1.4Ga Sibley Group Delta, east of Thunder Bay, Ontario. Massive sandstone beds are not typically deposited in a distributary-mouth bar environment and are anomalous in this instance. To develop a depositional model, field mapping and core logging was conducted, and thin sections of samples were used to ascertain grain orientation in the massive sandstone. The results were compared to established literature on sediment gravity flows and prevegetated delta environments in order to discern the nature of the depositional mechanisms that resulted in the formation of the thick massive sandstone layers. It is likely that these layers were deposited by high-energy flood events that created turbulent flows of sediment and water, ranging from high density to moderate density. These flows would have deposited the sediment rapidly from suspension and consolidated quickly, creating largely ungraded deposits with poorly oriented grains. Understanding the Sibley Group massive sandstone may assist in identifying other types of flow deposits in both Precambrian and extraterrestrial environments.

Abstract to follow.