Exposures of the Leta Arm (LA) group and southwestern Hewitt Lake (HL) group volcanic rocks in the northeast-trending 2670-2629 Ma Indin Lake Supracrustal Belt (ILSB) of the western Slave Craton were studied to understand their composition, petrogenesis and tectonic setting. This was achieved by integrating petrology, mineral chemistry, whole-rock geochemistry, U-Pb zircon geochronology, and Sm/Nd isotope analyses. Samples were collected through a combination of surface geological mapping and sampling of historic drill core from different lithologies within the central ILSB volcanic rocks. 

The ≤2670 Ma HL group is amphibole-rich and comprises mafic and intermediate rocks of basaltic and andesitic composition whereas the 2668-2673 Ma LA group is pyroxene-rich and contains mafic through felsic units ranging from basalt to rhyolite. Alteration was mainly the result of metamorphic processes based on the abundance of ferro-hornblende, actinolite, carbonates, sericite, epidote and minor biotite, quartz, rutile and Fe-Ti oxides. Metamorphic conditions vary from greenschist to amphibolite facies and revealed an interplay of prograde and minor retrograde metamorphism based on amphibole chemistry and the textural relations with primary phases in the rocks. Metamorphism was regional and accompanied by plastic deformation resulting in the formation of high temperature microstructures. 

Mafic tholeiites of the western HL and LA groups likely formed in an oceanic plateau based on their flat REE on primitive mantle-normalized spider plots, positive to minor Nb anomalies and low La/SmN values of <2. The eastern LA group basalts and felsic to intermediate rocks are characterized by LREE enrichment (La/SmN up to 5.47), positive Th anomalies, negative Ti and Nb anomalies as well as both positive and negative Zr-Hf anomalies. These characteristics are typical of arc-type rocks formed by metasomatism of a mantle wedge above a subducting slab. Low Gd/YbN values < 2.80 in both groups suggest melting at shallow depths in the absence of garnet, possibly suggesting an oceanic rather than continental arc setting. Positive ƐNd values suggest that the rocks did not interact significantly with older crust. The variable negative Nb anomalies in the arc rocks could be the result of crustal contamination or a subduction zone signature as slab-derived fluids stabilize Nb-bearing phases in the slab, leading to depletions of Nb in the metasomatized mantle. The Coterill tonalites may have been the source of contamination and the broadly similar age to the volcanic rocks would allow for modification of the whole rock signature without modifying the isotopic make-up of the LA and HL rocks. 

Data from this study was used to revise the existing tectonic model for the ILSB volcanic rocks. The new model places the initial formation of the western basalts in an oceanic plateau which collided with a west-moving crustal block and clogged the subduction zone due to its buoyant nature with a new subductions zone forming behind the plateau. Arc magmas from the subducting slab erupted through and onto the plateau to form primitive arc tholeiites and the intermediate to felsic rocks observed in the ILSB. 

The Archean Lac des Iles suite is located just north of the Wabigoon-Quetico boundary, approximately 90 kilometers north of Thunder Bay in Northwestern Ontario. The suite includes discrete mafic and ultramafic complexes associated with sanukitoids, which were emplaced along deep-seated regional faults. Of these, only the Lac des Iles Complex hosts an economically significant palladium deposit at the Lac des Iles mine. The complex is divided into two parts: North Lac des Iles and South Lac des Iles. The North Lac des Iles mainly comprises ultramafic rocks such as websterite, clinopyroxenite, wehrlite, lherzolite, dunite, and peridotite. In contrast, the South Lac des Iles Complex is primarily composed of mafic rocks including gabbro, gabbronorite, norites, and melanorite and is the main host of the Roby, Offset, and Camp Lake zones which are the main economically important mineralized zones.

This study focused on the Camp Lake zone, the deepest part of the deposit, recently identified by exploration drilling. The host rocks of the Camp Lake mineralization primarily consist of gabbronorite and norite, exhibiting preserved cumulate textures with plagioclase and pyroxenes as the main primary phases. The pyroxenes predominantly comprise orthopyroxene with minor clinopyroxene, which are partially to completely replaced by amphiboles (chlorite, actinolite, and tremolite). The plagioclase is weakly to moderately altered by sericite and generally retains its original habit. 

Magmatic sulfide contents in Camp Lake rocks have modal abundances from 0.5% to 3%, and are dominated by pyrrhotite, pentlandite, and chalcopyrite, with minor pyrite. Sulfide minerals often occur as blebs or disseminated grains intergrown with silicate minerals. Palladium enrichment is interpreted as magmatic, driven by sulfide saturation and high metal-silicate partitioning efficiency, with platinum group minerals (PGMs) hosted primarily in pentlandite and chalcopyrite. A new zircon U-Pb age was acquired for the mineralized Camp Lake rocks, yielding an emplacement age of 2690.56 ± 0.80 Ma, similar to that of the Roby and Offset deposits. Geochemical signatures, including depleted incompatible elements and positive εNd values, support a mantle-derived origin with minor crustal input. The Camp Lake Zone rocks are enriched in LREE (La/Smn ranging from 1.29 to 7.75, with a median of 2.90), have unfractionated HREE (Gd/Ybn ranging from 0.56 to 1.49, with a median of 0.82), and negative Nb anomalies. These values are similar to those of the Roby and Offset zones and are consistent with a subduction zone setting. Similar to the Roby-Offset deposits, PGE values in Camp Lake range between 1.0 g/t and 3.0 g/t, increasing with Cu and Ni content. However, Camp Lake is distinguished by higher proportions of pyrrhotite compared to chalcopyrite and lower Pd/Pt and Cu/Pd ratios than the other zones. 

The Camp Lake Zone has low δ34S values, ranging from (-1.1‰ to +0.3‰), whereas the Roby and Offset zones show wider variations ranging from (-0.37 to +3.28‰). This suggests that the sulfur in the Camp Lake Zone is of mantle origin. Sm–Nd isotopic compositions (εNd = +0.30 to +1.12) for the Camp Lake Zone is also slightly more positive than the Roby and Offset zones (εNd = -4.1 to 0.384). Overall, when compared with the more contaminated Roby-Offset zones, Camp Lake represents a transitional domain with greater geochemical uniformity, suggesting a continuum of magmatic processes and metallogenic evolution within the Lac des Iles Intrusive Complex.

The Moss Au deposit is an orogenic-style gold deposit hosted in felsic to intermediate rocks of the western Shebandowan Greenstone Belt, close to the terrane boundary between the Wawa-Abitibi terrane and the Quetico metasedimentary basin, ~120 km west of Thunder Bay. The deposit has an inferred mineral estimate of 140.07 Mt of ore averaging 1.09 g/t Au, which yields 4.91 Moz (Goldshore, 2024). The majority of the gold is hosted within diorite and dacite and is localized by shear zones and an array of quartz-carbonate-pyrite veins. The central-felsic metavolcanic belt of the Shebandowan Greenstone belt comprises felsic to intermediate units surrounded by late granitic intrusions, such as the Burchell Lake and Moss Lake stocks. This study focused on characterizing the alteration and mineralization at the Moss deposit and investigating any correlation between alteration and gold mineralization. A combination of petrography, geochronology, geochemistry, and mineral chemistry was used to achieve the objectives of this study.

Alteration occurs in different styles and intensities but generally comprises albite, biotite, sericite, chlorite, carbonate, and epidote alteration. Sulfide minerals are dominated by pyrite with minor chalcopyrite, sphalerite and molybdenite. Sulfide abundance is commonly 2 – 10% of the samples but can be up to 15% within sulfide-rich veins. Disseminated and vein-hosted pyrite are the two main textures in which pyrite occurs within the host rocks. A total of 12 vein types were observed, with quartz and carbonate being the most dominant veins occurring together in five of the vein types. Using the observed textural and crosscutting relationships of the alteration, sulfides, and veins, a paragenetic sequence was developed, highlighting the secondary processes associated with the formation of the Moss Lake deposit. Deformation textures were observed in early and late alteration phases, suggesting a long deformation history that was broadly coeval with mineralization.

Quartz-carbonate-pyrite ± sericite ± chlorite ± epidote veins are host to most of the observed gold occurrences, and are common within or in proximity to shear zones. Gold was rarely associated with disseminated pyrite away from veins. Gold grains occurred as inclusions in

pyrite, on the rims of pyrite grains and in the groundmass around pyrite grains within the host vein and are genetically related to pyrite.

A Re/Os age (2708 ± 12 Ma) from molybdenite from a Type 3 quartz-carbonate-pyrite vein is interpreted to be the age of mineralization. This age, when compared with the ages of the host rock from the Skimpole Lake area (2721 ± 4 Ma; Corfu, 1998), and the nearby Burchell Lake stock (2680 ± 3 Ma; Corfu, 1983), constrains the gold mineralizing event between 2725 Ma – 2694 Ma. The age of gold mineralization overlaps with the age of the Central Felsic Metavolcanic belt (CFB) and indicates that the mineralizing event is older than the nearby intrusions.

Hyperspectral data showed the presence of different species of white mica and chlorite. The white mica with a spectral range of 2208 – 2216 nm is associated with most of the high gold values and corresponds to white mica with a mixed phengite-muscovite composition. Chlorite with a spectral range between 2242 – 2249 nm is associated with gold-bearing samples.

Mineral chemistry of chlorite and white mica varies with proximity to the center of the deposit. For white mica, Mg was highest in the samples proximal to the center of the deposit and lowest in the more distal samples. For chlorite, Mg and Si are highest in proximal samples and lower in distal samples, whereas Fe and Al are lower in the proximal samples and higher in the distal samples. These compositional changes in white mica and chlorite composition could be linked to the Tschermak substitution reaction, where Al is replaced by Si in the tetrahedral sites, while Mg or Fe is incorporated into the octahedral sites. This reaction can be attributed to temperature changes linked to interaction with mineralizing fluids during deposit formation. In general, the alteration intensity varies across the deposit with no clear vector towards mineralization. However, the mineral chemistry and spectral features of white mica and chlorite show a trend that can be used as a vector to ore, if properly applied.

Faults and their associated damage zones are important geologic structures that serve as permeable pathways through the upper crust. The development of fault cores and damage zones is typically controlled by the strength and composition of the protolith, conditions of deformation, and fluid chemistry. The effect of host lithology on fault core development and damage zone structure is currently poorly constrained. This study uses the Lac des Iles palladium mine in northwestern Ontario, Canada as a natural laboratory to study how faults behave in different lithologies. The Lac des Iles mine is hosted in the 2.689 Ga ± 1.0 Ma Lac des Iles mafic-ultramafic intrusion with current reserves of ~5 Moz of 3E (Pd+Pt+Au) at an average grade of 2.6 g/t. The intrusion hosts numerous mineralized zones, most notably the Roby, Offset, and Camp Lake zones, divided by faults. The Camp Lake and Offset faults are two major structures that offset blocks of the ore body within the Lac des Iles mine, with displacements of ~500 and ~275 m, respectively. The faults crosscut gabbronorite and tonalite, often with these rock units in fault contact with each other.

We studied the variation in fracture density surrounding the Camp Lake and Offset faults to quantify how damage zone structure changes with respect to the host lithology. Fracture density decay rates within damage zones (with distance from the fault core) show that fractures in tonalite decay at a faster rate than gabbronorites, irrespective of whether they are in the hanging wall or footwall. The fault cores in gabbronorites are characterized by chlorite-rich gouges whereas tonalite fault cores are composed of silica-rich cataclasites. It is hypothesized that the development of a frictionally weak, chlorite-rich fault core impeded the development of a more fracture-dense damage zone in the gabbronorite. Results from whole-rock geochemistry show variations in major elements that correlate with the largest zones of visible alteration and deformation within the damage zone, and no significant rare earth element variation that can be attributed directly to the faulting. Electron microprobe analysis of chlorite was conducted on fault core and host rock samples to constrain the temperatures of deformation. Three notable clusters in the temperature data were observed, interpreted to represent periods of chlorite formation that were pre-, syn- and post-faulting (290°C, 236°C, and 110 – 175°C, respectively). 

A combination of structural and geochemical data shows that the Camp Lake and Offset faults have undergone multiple deformation events, with pulses of hydrothermal fluids altering the mineralogy and geochemical signature of the surrounding rocks in the Lac des Iles mine. Palladium mineralization is depleted within the fault core and damage zones. It is hypothesized that hydrothermal fluids associated with faulting are the cause of stripped palladium mineralization.

This research highlights the interplay of the geological processes associated with faulting and fluid migration and how they have affected the distribution of palladium within the Lac des Iles mine. Understanding the relationship between faulting and economic mineralization can improve exploration strategies and guide mining efforts for the future.

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.