The Borden gold property is a multi-million-ounce deposit that is located 10 km east of Chapleau and 160 km southwest of Timmins.  It occurs within the Wawa Subprovince of the Archean Superior Province.  What is atypical about the Borden gold deposit is its location near the southern margin of the Kapuskasing Structural Zone (KSZ), a structurally controlled region of granulite and upper amphibolite facies metamorphic rock.  The deposit occurs within the Borden Lake Belt, an east- striking lithological assemblage, consisting of metasedimentary, felsic and mafic gneisses.  Gold mineralization is hosted by garnet-biotite gneiss (±sillimanite), amphibolite (±garnet) and deformed quartz veins.  Lithons of granulite facies rock are surrounded by foliated amphibolite facies gneisses and schists.  Structure and microstructure indicate polymetamorphism with retrograde amphibolite facies metamorphism after granulite facies metamorphism.  Garnet-biotite geothermometry based on the composition of unzoned almandine garnet, matrix biotite in the rock and biotite inclusions in garnet yields temperatures ranging from 411C to 933C 50C for metamorphism of the garnet-biotite schist at the Discovery Outcrop.  Although garnet compositions are fairly consistent, biotite compositions vary from inclusions in garnet cores to inclusions in garnet rims and to matrix biotite, yielding temperatures that increase from the garnet core towards the rim, recording prograde metamorphism from the upper amphibolite to granulite facies during garnet growth.  

To compliment metamorphic parameters and establish metamorphic geochronology, Lu-Hf geochronology of garnet was conducted.  Results suggest that peak granulite facies metamorphism associated with garnet growth took place at ca. 2629 Ma, consistent with earlier estimates of the age of granulite facies metamorphism.

Competency contrasts between the granulite and retrograde amphibolite facies lithologies created heterogeneous strain, ideal for gold mineralization, during ductile deformation at amphibolite facies metamorphic temperatures.  On the macroscopic scale, the relict granulite facies lithons behaved more competently than the reaction-softened retrograde amphibolite.  On the microscopic scale, competent relict orthopyroxene, garnet and pyrite provided an adjacent low-strain site for gold mineralization.  Gold is typically observed in competent lithologies with weakly developed foliation and also in competent units that are bordered by strongly foliated units.  Retrograde metamorphism is critical to the structural control of mineralization at this deposit.  Results indicate an important relationship between gold mineralization, retrograde metamorphism and deformation.  Understanding this relationship will benefit further exploration and development of the Borden gold deposit.

URI
https://knowledgecommons.lakeheadu.ca/handle/2453/5197

The carbon cycle is a key biogeochemical cycle with defined pools, where carbon accumulates, and fluxes, which describe the movement of carbon between pools. One of the most globally significant carbon pools is the terrestrial biosphere, specifically soils, which are the largest reservoir of organic carbon. Boreal regions, a globally significant biome, store approximately 30% of the global soil organic carbon (SOC) pool. The cool, wet conditions in boreal regions typically impede decomposition and promote accumulation leading to these large stores. Forest fires are a common disturbance mechanism of boreal regions, with combustion of biomass releasing massive amounts of carbon to the atmosphere and producing a large amount of wood ash. Globally, biomass burning for energy generation is becoming a more favorable alternative to reduce dependency on fossil fuels. Bioenergy production via combustion also produces wood ash. Wood ash can be used as a liming agent to increase the pH of soils, while also adding nutrients that are lost via disturbance, which can promote carbon accumulation through more productive vegetation communities. What is currently unknown is the effect that a change in soil pH can have on soil organic matter stores themselves and stabilization mechanisms. This study focuses on the effects of wood ash application on the distribution of carbon between chemically defined pools. Through a process of sequential extraction on the silt & clay size fraction of soils that were treated with different amounts of wood ash, the amount of carbon associated with specific stabilization mechanisms was determined. Wood ash application to the soil decreased the amount of carbon that was most weakly stabilized, which was recovered in a tetraborate extraction. Given the high specific surface area of wood ash, it is possible that dissolved carbon is being more tightly held on the surface of the ash. It was hypothesized that the amount of SOC stabilized via organo-metallic complexes (i.e., pyrophosphate extractable carbon) would decrease with increase in pH due to the solubility decrease of Fe and Al; however, this was not the case. It is likely that the increase in soil pH does not persist long enough to see a significant effect on stabilization via this organo-metallic complexing mechanism. Though there were changes in the tetraborate extractable fraction, it makes a small contribution to total carbon stores and implies that wood ash application whether via liming or forest fires would not greatly affect the way carbon is stabilized in similar boreal soils.

Abstract to follow.

The Escape intrusion is a tabular to bladed, mafic–ultramafic chonolith that hosts economic concentrations of PGE–Cu–Ni magmatic sulfide mineralization. The Mesoproterozoic intrusion is located about 50 km northeast of Thunder Bay, Ontario, and with the Current intrusion makes up the Thunder Bay North Intrusive Complex (TBNIC). The intrusive rocks of the TBNIC are part of the 1.1 Ga Midcontinent Rift System (MRS) of North America and were emplaced into the Quetico Basins during early stages of rift development. The fractionated HREE (Gd/Ybcn = 3.18–4.96) signature of the Escape rocks suggests magma derivation from a deep mantle source. Primitive mantle-normalized trace element patterns of the Escape intrusive rocks are similar to ocean-island basalt, as well as multiple MRS-related mafic–ultramafic intrusions (e.g., Hele, Disraeli, Kitto), which is consistent with the mantle-plume hypothesis of MRS formation.

The high-grade zone occurs within the (mostly wehrlitic) peridotite unit of the Escape intrusion, which lies below the weakly mineralized gabbro and hybrid units. The high-grade zone within the Escape intrusion is characterized by intercumulus sulfide mineralization that is net-textured at the core and disseminated at the margins. The primary sulfide mineralization within the net-textured ore is characterized by an assemblage which consists predominantly of pyrrhotite + chalcopyrite + pentlandite + platinum-group minerals (PGMs). The disseminated sulfides of the high-grade zone are composed of a variable assemblage that includes the primary sulfides as well as some or all of the following: native Cu, mackinawite, cubanite, native Ag/electrum, sugakiite, pyrite, and valleriite. In addition to interstitial sulfide mineralization at Escape, numerous centimetre-sized sulfide (± carbonate) veinlets crosscut the groundmass. The sulfide veinlets exhibit complex intergrowths often with mottled textures and variable composition. Phases commonly identified within the sulfide veinlets include cubanite, pyrrhotite, pyrite, pentlandite, chalcopyrite, and mackinawite.

Sulfide trace element compositions obtained via in situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) indicate that Pd is concentrated in pentlandite (around 100 ppm) and IPGE (Ir, Os, Ru) are concentrated in pyrrhotite within the net-textured ores. Sulfide minerals from the interstitial–primary assemblage exhibit S/Se ratios within the mantle domain (2850–4350). The high-grade PGE tenors of the deposit were likely the result of moderate magmatic enrichment of the Escape segregated sulfide liquid at R factors of ~7,500, based on numerical models derived from whole-rock, major and trace element geochemistry. Sulfide minerals within the disseminated style are depleted in PGE and S/Se (as low as 668) relative to the net-textured sulfides. These trace element signatures and the distinct sulfide assemblage identified in the disseminated ores are attributed to desulfurization and remobilization of metals during hydrothermal alteration. Sulfide minerals measured within veinlets from the crosscutting assemblage are depleted in PGE and strongly enriched in As (e.g., up to ~900 ppm in pentlandite) relative to the primary–interstitial assemblage, as well as elevated in S/Se (ranging up to 27,896), considerably outside of the mantle range. Various As-bearing PGM were identified within the sulfide veinlets but were not found within the net-textured ores. These features suggest that remobilized metals from the interstitial ores were transported as As-rich bisulfide complexes and emplaced as veinlets within small fractures between cumulus olivine during the later stages of hydrothermal activity.

In situ S-isotopes of Escape sulfides obtained via secondary ion mass spectrometry (SIMS) show that δ34S values range from -3.07 to -0.97‰, and all values for Δ33S and Δ36S fall within a range produced by mass-dependent fractionation (MDF). S-isotopes were also measured in pyrite from Quetico metasedimentary country rocks. There is overlap between Escape and Quetico S-isotopes both in and outside the mantle range. Results from numerical modelling of equilibration between the Escape sulfide liquid and pulses of fresh, uncontaminated melt suggest that the mass-independent fractionation (IDF) signal was erased due to isotopic exchange. It is inconclusive what drove the Escape parental magma to S-saturation, however, the origin of sulfur is likely to be a mixture of mantle source and country rock contamination.

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