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A high‐resolution seismic reflection transect was acquired over a hard‐rock geological setting along an existing roadway in the Larder Lake area of the Superior Craton of Canada for the Metal Earth project in 2017. This profile, as well as other Metal earth transects, primarily aims to enhance the knowledge and to better understand the subsurface geology of the Abitibi Greenstone Belt within the Canadian Shield. The complex geological settings of the study area as well as the tribulations caused by the survey geometry have made the imaging and velocity field estimation more challenging. A recently introduced 2.5D multifocusing stacking method is one potential solution for processing crooked‐line seismic data with a poor signal‐to‐noise ratio. The 2.5D multifocusing approach offers more realistic modelling of the zero‐offset wavefield by explicitly accounting for the midpoint dispersion and cross‐dip effects. The main practical problem of the 2.5D multifocusing implementation is the simultaneous determination of the optimal wavefield parameters for each image point and time location. We address this optimization problem using a multidimensional constrained differential evolution global optimization algorithm, as this improves the efficiency and accuracy of the estimation. We have also designed an efficient processing sequence for multifocusing seismic imaging. The performance of the 2.5D multifocusing procedure has been examined on a synthetic model, generated using the same real acquisition geometry. Numerical tests demonstrate that the 2.5D multifocusing technique can produce a more focused stack with the primary reflections appearing at their poststack correct locations, and the procedure can also provide reliable estimates of interface dips. Due to the importance and difficulty of imaging the data, several conventional and advanced processing strategies have been attempted on the transect, specifically: 2D phase‐shift time migration of a dip moveout corrected stack; 2D prestack Kirchhoff time migration; swath 3D poststack migration; and our 2.5D multifocusing imaging algorithm. We found that applying the 2.5D multifocusing stacking algorithm followed by a poststack time migration approach improved the resolution of the image significantly compared to all the conventional and advanced methods and identified new reflections. The 2.5D multifocusing method also focused the steeply dipping reflections more coherently, which resolved ambiguities in geological architecture by understanding the location and continuity of structures. The method also accurately extracts 3D structural information and results in an improved signal‐to‐noise ratio. [ABSTRACT FROM AUTHOR]

• Geochemical subdivision of mafic-intermediate volcanic rocks based on immobile element ratios. • Blake River Group geochemical diversity is mostly caused by crustal interaction with TTG-like contaminant. • VMS deposits in the Blake River Group show a potential relationship with crustal assimilation. • Composition of mafic to intermediate volcanic rocks has potential for VMS vectoring. Volcanogenic massive sulfide (VMS) deposits, prevalent since the Archean, constitute a well-explored ore deposit type. Nonetheless, the factors contributing to variations in fertility among volcanic centers, assemblages within greenstone belts, or entire greenstone belts remain unclear. For instance, the Blake River Group (BRG) of Ontario and Quebec, Canada, hosts almost half of the total VMS tonnage of the entire Abitibi greenstone belt, yet represents less than 10 % of its map surface. This study utilizes a large dataset (n = 2271) of whole-rock geochemistry for mafic to intermediate volcanic rocks to understand the BRG petrogenesis, and its implications for VMS endowment in the Abitibi. Principal Component Analysis shows that Ti, Th, and Yb are the primary contributors to the dataset's variation. This yields a two-ratio diagram (Th/Yb versus Zr/Ti) revealing seven geochemical groups with coherent map patterns referenced as tholeiitic basalts, tholeiitic intermediate rocks, medium-Th basalts, transitional intermediate rocks, high-Th basalts, calc-alkaline intermediate rocks and low-Zr basalts. A least altered subset of the data is used to analyze major element patterns. BRG mafic to intermediate magmas show variable degrees of Fe-Ti enrichment/depletion during their evolution, with simultaneous Al 2 O 3 depletion or lack thereof. Utilizing a single parental mafic tholeiitic magma, the Magma Chamber Simulator enables the modeling of major and trace element compositions for each geochemical group. Fractional crystallization (FC) models with variable initial water contents can explain the range of the tholeiitic basalt and tholeiitic intermediate compositions. Assimilation fractional crystallization (AFC) models successfully replicate most of the geochemical groups using a TTG average composition for the assimilant. While mafic-intermediate host rock geochemical signatures are not the sole factor influencing VMS endowment in volcanic belts, regional analysis reveals that the vast majority of volcanic rocks in geological formations bearing VMS deposits in the BRG are crustally contaminated lacking tholeiitic geochemical groups. Conversely, poorly endowed formation consistently contain samples from these groups. Moreover, the Bousquet formation, which hosts several world-class gold-rich VMS deposits, displays the highest rates of crustal assimilation, further suggesting that petrogenesis of mafic-intermediate volcanic rocks is linked to VMS metallogenesis in the BRG. [ABSTRACT FROM AUTHOR]

In 2017, the Metal Earth multi‐disciplinary exploration project acquired a total of 921 km of regional deep seismic reflection profiles and 184 km of high‐resolution seismic reflection profiles in the Abitibi and Wabigoon greenstone belts of the Superior province of Canada. The Abitibi belt hosts several world‐class mineral deposits, whereas the Wabigoon has sparse economic mineral deposits. Two high‐resolution surveys in the Swayze area, a poorly endowed part of the western Abitibi greenstone belt, served as pioneer surveys with which to better understand subsurface geology and design a strategy to process other surveys in the near future. Swayze seismic data were acquired with crooked survey geometries along roads. Designing an effective seismic processing flow to address these geometries and complex geology required straight common midpoint lines along which both two‐dimensional prestack dip‐moveout correction and poststack migration processing were applied. The resulting seismic sections revealed steeply dipping and subhorizontal reflections; some correlate with folded surface rocks. An interpreted fault/deformation zone imaged in Swayze north would be a target for metal endowment if it extends the Porcupine–Destor structure. Because of the crooked line geometry of the surveys, two‐dimensional /three‐dimensional prestack time migration and swath three‐dimensional processing were tested. The prestack time migration algorithm confirmed reflections at the interpreted base of the Abitibi greenstone belt. The swath three‐dimensional images provided additional spatial details about the geometries of some reflections, but also had less resolution and did not detect many reflectors observed in two dimensions. Geological contacts between felsic, mafic and ultramafic greenstone rock layers are thought the main cause of reflectivity in the Swayze area. [ABSTRACT FROM AUTHOR]

The Red Lake greenstone belt is situated in northwestern Ontario within the Uchi Subprovince, Superior Province. Most gold deposits therein are associated with major deformation corridors; the east-west oriented 'Mine trend' hosts most of the large deposits and the northeast-southwest 'East Bay trend' hosts several small deposits and showings. Gold along the East Bay trend typically occurs in quartz replacement veins that were emplaced into pre-existing quartz-carbonate veins. Gold can occur as free gold or along vein margins associated with pyrite and pyrrhotite. Most primary fluid inclusions, preserved in relatively undeformed portions of veins, are carbonaceous with lesser quantities of aqueous inclusions. The average homogenization temperature of aqueous fluids is ~250 °C; however, the abundance of three-phase inclusions, variation in liquid-vapor ratios, and a wide range in homogenization temperatures indicate that immiscibility, effervescence, and fluid mixing are mechanisms associated with gold deposition. The age (~2550 Ma) of alteration minerals in the Abino area is considerably younger (by ~100 Myr) than alteration minerals in other deposits in the Red Lake district, indicating that the mineralizing fluid history was more protracted than previously thought. Along the East Bay trend, barren veins generally have lower δO values (0.0 to 8.5‰) relative to auriferous veins (9.6 and 13.1‰). Consequently, the oxygen isotopic composition of quartz could be used as a vector for gold mineralization. The genetic model for the East Bay trend involves several stages of vein formation. Auriferous veins formed near the upper boundary of the mesozonal regime (depth of ~5-6 km). [ABSTRACT FROM AUTHOR]

[Display omitted] • Mineral prospectivty mapping for Au. • Random forest algorithm for integrating geoscience data. • Knowledge-based algorithms. • Superior tectonic province. This paper uses the random forest (RF) algorithm to produce two maps of orogenic gold prospectivity (MPM) across the entire Superior geologic province of Ontario and Quebec, Canada. One MPM of this entire study area is based on a compilation of mapped faults while the other is based on a manual interpretation of lineaments from airborne magnetic data. In addition, we have created four MPMs over the Abitibi region as the MPM map of the entire Superior is characterized by a mapping bias due to more intense mapping of faults over the Abitibi portion of the study area with respect to the rest of the Superior province. There are three RF maps generated over the Abitibi region based on faults, fault density and fault intersection density, faults + magnetic data and faults + gravity + magnetic data. To reduce the effect of the fault mapping bias, we developed a fourth map based on a knowledge driven (weighted sum technique). Statistically the best MPM is based on the faults, gravity and magnetic data. The major predictors of gold are NW-SE, NE-SW, EW trending faults, fault intersection density between EW and NW trending faults and to a lesser extent fault density and a magnetic vertical gradient image. In addition, we compare two greenstone belts in three dimensions with respect to their potential for gold exploration. We conclude that the Larder Lake greenstone belt is more fertile with respect to gold mineralization than the Swayze greenstone belt due to deeper penetrating faults which have greater potential for tapping gold mineralized fluid from the mantle or lower crust. Our final MPM map has identified prospective areas that contain known gold mines as well as areas that do not contain any known gold mines. These prospective areas may be prime for orogenic gold exploration. [ABSTRACT FROM AUTHOR]

• Red Lake Greenstone Belt contains Earth's oldest known, thick, carbonate platform. • Outcrops and multiple cored drill holes provide excellent stratigraphic control. • Interlayering of lithofacies suggests short to long term relative sea level changes. • Precipitation rates of all types of chemical sediments varied temporally. • Archean seawater REE chemistry is similar in shallow and deep areas of the shelf. The > 200 m thick Red Lake carbonate platform of the 2.940-2.925 Ga Ball Assemblage (Red Lake Greenstone Belt, Superior Province, Canada) is the oldest known carbonate platform preserved on Earth. This study examined surface outcrops and multiple industry cored drill holes comprising a diverse assemblage of chemical and siliciclastic sedimentary rocks of the deeper water lithofacies associated with the shallow water carbonates. The offshore lithologies, underlying, interlayered with, and overlying the mainly stromatolitic carbonates, consist of alternating packages of sandstone, siltstone, carbonaceous slate, pyritic carbonaceous slate, chert, and oxide facies iron formation (OFIF). These interlayer with carbonate in transitional slope environments, which consist in places of carbonate with magnetite laminae above slumped carbonate debris in iron formation and/or carbonaceous slate. Chert and OFIF were deposited further offshore in a suboxic environment, with lithology dependent on concentrations of seawater Fe(II) and silicic acid. The iron sulfides were deposited in a reducing, anoxic, and organic-rich bottom water and muddy pore water environment not conducive to Fe hydroxide or chert precipitation. Carbonaceous mud was the background sediment, accumulating during intervals when the precipitation of chert, Fe hydroxides, or massive layers of Fe sulfides was inhibited. The interlayering of differing assemblages of chemical and siliciclastic sedimentary rocks indicates temporal and spatial fluctuations in the chemistry of local paleo-seawater and variable sedimentation rates on the margins of the Red Lake carbonate platform. Post-Archean Australian Shale (PAAS)- normalized rare earth element (REE) systematics of carbonate and OFIF show positive La and Gd anomalies, super-chondritic Y/Ho ratios, and depleted light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), similar to modern seawater, while the positive Eu anomalies in the carbonate and OFIF samples reflect hydrothermally influenced Archean seawater. The chemical sedimentary rocks exhibit enrichment of redox-sensitive elements (i.e., Cr, Mo, V, and U) compared to their siliciclastic associates. This, combined with concentrations of MnO up to 6.3 wt% and positive Ce anomalies in the iron formation directly below the carbonate platform, all indicate the likely presence of some O 2 ca 2.93 Ga. [ABSTRACT FROM AUTHOR]

Assimilation by mafic to ultramafic magmas of sulfur-bearing country rocks is considered an important contributing factor to reach sulfide saturation and form magmatic Ni-Cu-platinum group element (PGE) sulfide deposits. Sulfur-bearing sedimentary rocks in the Archean are generally characterized by mass-independent fractionation of sulfur isotopes that is a result of atmospheric photochemical reactions, which produces isotopically distinct pools of sulfur. Likewise, low-temperature processing of iron, through biological and abiotic redox cycling, produces a range of Fe isotope values in Archean sedimentary rocks that is distinct from the range of the mantle and magmatic Fe isotope values. Both of these signals can be used to identify potential country rock assimilants and their contribution to magmatic sulfide deposits. We use multiple S and Fe isotopes to characterize the composition of the potential iron and sulfur sources for the sulfide liquids that formed the Hart deposit in the Shaw Dome area within the Abitibi greenstone belt in Ontario (Canada). The Hart deposit is composed of two zones with komatiite-associated Ni-Cu-PGE mineralization; the main zone consists of a massive sulfide deposit at the base of the basal flow in the komatiite sequence, whereas the eastern extension consists of a semi-massive sulfide zone located 12 to 25 m above the base of the second flow in the komatiite sequence. Low δFe values and non-zero δS and ΔS values of the komatiitic rocks and associated mineralization at the Hart deposit is best explained by mixing and isotope exchange with crustal materials, such as exhalite and graphitic argillite, rather than intrinsic fractionation within the komatiite. This approach allows tracing the extent of crustal contamination away from the deposit and the degree of mixing between the sulfide and komatiite melts. The exhalite and graphitic argillite were the dominant contaminants for the main zone of mineralization and the eastern extension zone of the Hart deposit, respectively. Critically, the extent of contamination, as revealed by multiple S and Fe isotope systematics, is greatest within the deposit and decreases away from it within the komatiite flow. This pattern points to a local source of crustal contamination for the mantle-derived komatiitic melt and a low degree of homogenization between the mineralization and the surrounding lava flow. Coupled S and Fe isotope patterns like those identified at the Hart deposit may provide a useful tool for assessing the potential of a komatiitic sequence to host Ni-Cu-(PGE). [ABSTRACT FROM AUTHOR]

The finding of Fe-rich picritic basalts on Mars has stimulated interest in Fe-rich magmatism on the early Earth. Rare vestiges of Fe-rich magmatism have been identified in remnants of old crust, but many other lithologies reported as ‘enriched’ or ‘Al-depleted’ komatiites in Archean terranes may also be derived from Fe-rich primitive magmas. Although absent today and rare in the Phanerozoic, Fe-rich magmatism may thus have been widespread on the early Earth. Knowledge of the mineralogy and geological setting of Archean Fe-rich igneous rocks may reveal the nature of volcanic activity on the early Earth and possibly Mars. We conducted a detailed petrological study of some of the best exposed of these terrestrial Fe-rich igneous rocks, which occur in the Pulpwood-Playter Harbours sequence of the Heron Bay volcanics (∼2.6–2.7 Ga) in the Wawa oceanic plateau association of the Superior Province (Canada). Rocks from these Fe-rich igneous bodies range from serpentinized dunite and clinopyroxenite to differentiated gabbro. They followed the crystallization sequence: olivine ± chromite, clinopyroxene, Fe–Ti oxide and plagioclase. Whole-rock major and trace element analyses show that these Fe-rich bodies are mildly alkaline, and are enriched in incompatible trace elements. The parental magma that we calculated for the Fe-rich bodies is a strongly Fe-rich (∼19 wt.% FeO tot ) alkaline magma, with at least ∼9 wt.% MgO. It represents the most Fe- and trace element-enriched end-member of the Wawa greenstone belts and may be the Archean equivalent of modern alkaline basalt. Magmatic clinopyroxene compositions indicate that crystal fractionation occurred in a small closed-system under low-pressure conditions without significant recharge of magma. Our crystal accumulation models confirm that a closed body of this ferropicritic alkaline magma can undergo progressive crystal accumulation and differentiate to produce the Fe-rich rocks of Pulpwood-Playter Harbours. The observed range of trace elements is mainly controlled by increasing proportions of inter-cumulus liquid retained in the rocks during their crystallization and the gabbros represent residual liquid portions of thick lava flows or sub-volcanic sills undergoing extensive closed-system differentiation (∼80% crystal fractionation). Detailed mapping of the Fe-rich bodies revealed that they are underlain by tholeiitic lava and overlain by some volumetrically minor Fe-rich lava. These upper Fe-rich lavas are more enriched in Fe and incompatible trace elements compared to the lower tholeiites and correspond to mixtures between a tholeiitic magma and the Fe-rich alkaline magma parent to the cumulate rocks. The close spatial association between the tholeiites and alkaline ferropicritic bodies at Pulpwood-Playter Harbours suggests a mantle source in which local regions enriched in Fe- and trace elements melted to produce volumetrically minor, mildly alkaline ferropicritic magmas. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Esker intrusive complex and associated chromitite deposits are modeled in 3D. • Shear zone refinement suggests late dextral apparent movement with ∼ 1.5 km offset. • A pre-shear reconstruction reveals multiple intrusive centers fed by faults. • The intrusive centers inflated and merged into a single complex with time. • 3D ore models suggest mechanical sorting contributed to chromitite mineralization. A 3D geological model of the Archean Esker ultramafic–mafic intrusive complex of the Ring of Fire intrusive suite of northwestern Ontario (Canada) is presented, providing insight into the nature of its magmatic plumbing system and allowing for speculation on the formation of its valuable Ni-Cu-(PGE) and chromitite deposits. The 3D model was constrained by exploration drillholes and high-resolution aeromagnetic data, which allowed the interpretation of the broad internal structure of the intrusive complex and two shear zones, which define for a major part of their strike extent the contacts between ultramafic and overlying mafic intrusive rocks. Restoring the post-emplacement component of strike-slip displacement and subsequently rotating the intrusive complex back from its tilted to sub-horizontal orientation of emplacement, provides a three-dimensional rendition of a 14 km-long dominantly tabular-shaped intrusive complex. The complex is defined by intact basal intrusive contacts with localized keel-shaped ultramafic promontories parallel to a nearby blade-shaped magmatic conduit. The keels extend in a subparallel orientation to depths of up to 1 km into the underlying tonalitic rocks and coincide with significant offsets of the basal intrusive contact, leading to speculation that their intrusion may have been guided by pre-existing normal faults. Two of these magmatic conduits host Ni-Cu-(PGE) deposits/prospects that formed from gravitational segregation of sulfides. The chromitite deposits, hosted in ultramafic rocks at higher stratigraphic levels, were modeled based on drill hole assay data. Zones of dominantly massive to semi-massive chromitite were defined by the ≥ 35% Cr 2 O 3 threshold, while intercalated sequences of chromitite in dunite/peridotite were defined by the ≥ 15% to ≤ 35% Cr 2 O 3 percentage range. The 3D modeled surfaces fitted to these constraints exhibit ore zones with lateral extents from several hundreds of meters for massive bodies up to a maximum of 2 km for intercalated chromitite ore that are dominantly conformable to the layering of the ultramafic host rocks. The lack of lateral continuity of chromitite ore zones beyond this scale, suggests that the chromitite layers are not continuous along the entire strike extent of the Esker intrusive complex, which is consistent with bifurcations of the 3D modeled ore shells signifying pinch-outs of both the massive chromitite layers and the dunitic interlayers. This discontinuous distribution of chromitite ore resembles that in the stratiform deposits of the Stillwater Complex and may support recent models of mechanical segregation of chromitite from cotectic chromite-olivine slurries. Based on our 3D model, we propose that the Esker intrusive complex evolved as a series of individual intrusions that were emplaced during multiple magmatic pulses that eventually coalesced to form a composite ultramafic–mafic complex prior to being dissected and partly dismembered by post-ore shear zone deformation. [ABSTRACT FROM AUTHOR]

Geophysical exploration for concealed orogenic gold deposits is challenging because petrophysical contrasts between the gold lodes and their host rocks are normally subtle. Most greenstone-hosted lode‑gold deposits do not contain strong conductors as they normally contain <5 vol% of poorly conductive disseminated arsenopyrite and pyrite. Where there is no outcrop, reliance must be placed on lithological and structural interpretations from aeromagnetic data. Airborne Induced Polarization (AIP) Cole-Cole parameters from helicopter time-domain electromagnetic (HTEM) surveys are demonstrated to be effective in geophysical exploration for concealed gold deposits in the Kabinakagami Lake greenstone belt in Ontario, Canada. Interpretation reveals that most AIP positive responses are associated with clays and all known gold deposits in the study area are in resistive rock formations. Hence, resistivity-scaled chargeability (RSC) can be used to detect weakly chargeable zones within these resistive units. Deep Neural Network (DNN) predictive targeting analysis is utilized to integrate the AIP apparent resistivity, resistivity-scaled chargeability (RSC), EM induction time-constant, and magnetic data. The DNN results imply that the known gold occurrences coincide with high DNN probabilities, and hence exploration targets can be identified based on DNN analysis. From a geological perspective, gold prospects also occur at triple-point junctions between granite intrusions and at probable intersections between potentially hosting shear zones and oblique cross faults or dykes, both common structural geometries for orogenic gold deposits. Consequently, DNN predictive targeting results based both on HTEM and magnetic data, and consistent with expected structural geometries, are proposed as a vectoring tool in the generation of drill targets during exploration for concealed orogenic gold deposits in greenstone belts such as those of the Superior Province, Canada. Deep neural network top 12% target probabilities and the new suggested exploration targets T1 to T4, labelled from southwest to northeast. [Display omitted] • Magnetic, EM and AIP data provide vectors to concealed gold mineralization • Targets at the Kabinakagami greenstone belt are marked by resistive EM anomalies • Template for the interpretation of structural geometry and geophysical datasets [ABSTRACT FROM AUTHOR]

Highlights: [•] We report a detailed deformation history of a Mesoarchaean greenstone belt. [•] Structural tests can discriminate between competing models for Archaean tectonics. [•] Polydeformation of greenstone belt preserved along clockwise P–T path. [•] Structural and metamorphic results comparable to subduction settings. [ABSTRACT FROM AUTHOR]

The Hemlo gold deposit (~700 t Au; northwestern Ontario, Canada) is hosted in a shear zone in the Hemlo greenstone belt, which is a synclinal keel between two granitoid domes. The dome-and-keel structure formed by diapirism and sagduction (vertical tectonism) that was synchronous with regional horizontal shearing (horizontal tectonism). Mineralization occurred during shearing, and mineralizing fluids probably had a magmatic source that is interpreted to be related to syntectonic mantle-derived granodiorite (sanukitoid) intrusions. The gold deposit, the intrusions, and the hosting shear zone all developed within a regime characterized by synchronous vertical and horizontal tectonism. They were all possibly linked to a range of processes associated with the accretionary growth and stabilization of the craton, in particular slab breakoff and the associated extensional orogenic collapse following terrane accretion. [ABSTRACT FROM AUTHOR]

Time-domain airborne surveys gather hundreds of thousands of multichannel, multicomponent samples. The volume of data and other complications have made ID inversions and transforms the only viable method to interpret these data, in spite of their limitations. We have developed a practical methodology to perform full 3D inversions of entire time- or frequency-domain airborne electromagnetic (AEM) surveys. Our methodology is based on the concept of a moving footprint that reduces the computation requirements by several orders of magnitude. The 3D AEM responses and sensitivities are computed using a frequency-domain total field integral equation technique. For time-domain AEM responses and sensitivities, the frequency-domain responses and sensitivities are transformed to the time domain via a cosine transform and convolution with the system waveform. We demonstrate the efficiency of our methodology with a model study relevant to the Abitibi greenstone belt and a case study from the Reid-Mahaffy test site in Ontario, Canada, which provided an excellent practical opportunity to compare 3D inversions for different AEM systems. In particular, we compared 3D inversions of VTEM-35 (time-domain helicopter), MEGATEM II (time-domain fixed-wing), and DIGHEM (frequency- domain helicopter) data. Our comparison showed that each system is able to image the conductive overburden and to varying degrees, detect and delineate the bedrock conductors, and, as expected, that the DIGHEM system best resolved the conductive overburden, whereas the time-domain systems most clearly delineated the bedrock conductors. Our comparisons of the helicopter and fixed-wing time-domain systems revealed that the often-cited disadvantages of a fixed-wing system (i.e., response asymmetry) are not inherent in the system, but rather reflect a limitation of the 1D interpretation methods used to date. [ABSTRACT FROM AUTHOR]

The archetypical komatiite-hosted Alexo Ni-Cu-(PGE) deposit occurs in the 2,720-2,710-Ma Kidd-Munro Assemblage of the western Abitibi greenstone belt in Dundonald Township, Ontario. Detailed mapping of a 200-m long glacially polished outcrop provides unequivocal evidence that the host komatiite flow thermomechanically eroded footwall andesites: (1) the contact between komatiite and andesite is very sharp but delicately scalloped, marked by a <1-cm-thick selvedge of black aphanitic komatiite and clearly transgresses pillow structures and interpillow breccias in the andesite without any evidence of a regolith, shearing, or folding, producing multiple nested embayments on scales from hundreds of meters to a few centimeters; (2) the andesites have been contact metamorphosed and altered along the entire length of the outcrop and the degree of metamorphism/alteration is thicker and more intense around embayments; (3) xenoliths of andesite in komatiite are more common within embayments; (4) komatiitic dikes penetrate downward into underlying andesites, primarily along the lateral margins of embayments; and (5) many of the dikes and marginal rocks exhibit geochemical evidence of contamination. This physical and geochemical evidence for thermomechanical erosion, combined with S isotopic evidence for a major component of non-magmatic country-rock S in the ores, provides additional support for the roles of thermomechanical erosion and incorporation of country-rock S in the genesis of komatiite-associated Ni-Cu-(PGE) deposits. The detailed mapping also reveals that the stratigraphy of the ore zone is considerably more complex than previously reported, indicating that the sulfides were emplaced in several stages, confirming the dynamic nature of the ore emplacement process in komatiite-associated Ni-Cu-(PGE) deposits. [ABSTRACT FROM AUTHOR]

The Red Lake mine trend, a deformation zone in the Archean Red Lake greenstone belt that hosts the world-class Campbell-Red Lake gold deposit, is characterized by abundant foliation-parallel iron-carbonate ± quartz veins with banded colloform-crustiform structures and cockade breccias overprinted by silicification and gold mineralization. There is an apparent incompatibility between the cavity-fill structures of the veins and breccias (typically developed at shallow crustal depths) and the upper greenschist to lower amphibole facies metamorphic conditions recorded in the host rocks (indicating relatively deep environments). This, together with the development of veins along the foliation plane, represents an enigmatic problem that may be related to the interplay between fluid dynamics and stress field. We approach this problem through systematic study of fluid inclusion planes (FIPs) in the vein minerals, including the orientations of the FIPs and the pressure-temperature conditions inferred from fluid inclusion microthermometry. We find that fluid inclusions in the main stage vein minerals (pregold mineralization ankerite and quartz and syn-ore quartz) are predominantly carbonic without a visible aqueous phase, whereas many inclusions in the postore stage contain an aqueous phase. Most FIPs are subvertical, and many are subparallel to the foliation. High fluid pressure coupled with the high wetting angles of the water-poor, carbonic fluids may have been responsible for the abundance of brittle deformation features. The development of subvertical FIPs is interpreted to indicate episodic switching of the maximum principal compressive stress (σ1) from subhorizontal (perpendicular to the foliation) to subvertical (parallel to the foliation) orientation. The subvertical σ1 is favorable for the formation of foliation-parallel veins, as fractures are preferentially opened along the foliation in such a stress regime, the origin of which may be linked to the fluid source. [ABSTRACT FROM AUTHOR]

The archaeological community of northern Minnesota and adjacent Ontario has long recognized the prehistoric use of lithic materials sourced from Knife Lake, Minnesota, and Lake of the Woods, Ontario. The strong geologic link between these two has not been fully explored in the archaeological literature to date. This article will not only explore the links between these two well-recognized "greenstone belt" lithics, but also explore broader horizons drawing connections with potential lithic sources scattered throughout the Canadian Shield. [ABSTRACT FROM AUTHOR]

Abstract  Spinifex-textured sills (i.e., veins) characterized by komatiitic magmas that have intruded their own volcanic-piles have long been recognized. For instance, in the early 1970s, Pyke and coworkers, in their classic work at Pyke Hill in Munro Township, noted that not all spinifex-bearing ultramafic rocks formed as lava flows, rather some were clearly emplaced as small dikes and sills. Several hypotheses have been proposed to explain spinifex-textured sills: intrusion into a cold host, filter pressing, or drainage of residual liquid. However, these do not satisfactorily explain the phenomenon. Field and petrographic observations at Pyke Hill and Serpentine Mountain demonstrate that spinifex-bearing komatiite sills and dikes were emplaced during channel inflation processes when new magma was intruded into a cooler, semi-consolidated but permeable cumulate material. Komatiitic liquids were intruded into the olivine cumulate rocks near the boundary between the spinifex and the cumulate zones of well-organized to organized komatiite flows. Spinifex-textured sills are generally tabular in morphology, stacked one above another, with curviplanar contacts sub-parallel to stratigraphy. Some sills exhibit complex digitated apophyses. Thinner sills typically have a random olivine spinifex texture similar, though generally composed of coarser crystals, to that of komatiite lava flows. Thicker sills exhibit more complex organization of their constituent crystals characterized by zones of random olivine spinifex, overlying zones of organized coarse spinifex crystals similar to those found in lava flows. They have striking coarse dendritic spinifex zones composed of very large olivine crystals, up to several centimetres long and up to 1 cm wide that are not observed in lava flows. Typically, at the sill margins, the cumulate material of the host flow is composed of euhedral to subhedral olivine crystals that are larger than those distal to the contact. Many of these margin-crystals have either concentric overgrowth shells or dendritic olivine overgrowths that grew from the cumulate-sill contact toward the sill interior. The dendrites grew on pre-existing olivine cumulate at the contact in response to a sharp temperature gradient imposed by the intrusion of hot material, whereas the concentric overgrowths formed as new melt percolated into the unconsolidated groundmass of the host-flow cumulate material. Spinifex-textured sills and dikes occur in well-organized to organized flows that are interpreted to have formed by “breakouts” above and peripheral to lava pathways (channels/conduits) as a result of inflation that accompanied voluminous komatiitic eruptions responsible for the construction and channelization of komatiitic flow fields. The spinifex-textured dikes and sills represent komatiitic lava that was originally emplaced into the channel roof during periods of episodic inflation that resulted in lava breakouts and was subsequently trapped in the “roof rocks” during periods of channel deflation. Accordingly, the occurrence of spinifex-textured sills and dikes may indicate proximity to, and aid in the identification and delineation of lava channel-ways that could potentially host Ni–Cu–(PGE) mineralization within komatiitic lava flow-fields. [ABSTRACT FROM AUTHOR]

Abstract: Supracrustal rocks of the North Spirit Lake (NSL) greenstone belt comprise three stratigraphic units. The basal unit (Unit 1) consists of pillowed, massive and tuffaceous, tholeiitic basalts and komatiitic basalts (approximately 3.1Ga) with minor interbeds of greywacke and black pelite displaying characteristics of turbidites and mass flow deposits. REE patterns indicate basalt genesis at comparatively shallow depths (spinel-lherzolite field). Although REE patterns and other trace element characteristics are similar to N-MORBs, concentrations of REE''s and Groups IV and V incompatible elements are somewhat lower than in average N-MORB, suggesting comparatively high degrees of partial melting of a fertile mantle. The evidence for an active hydrothermal system (black smoker and “Lost City” types) during the late stages of subaqueous volcanism, the lack of calc-alkaline characteristics for the NSL volcanic rocks, and the absence of geological evidence of a convergent margin setting, together suggest that the rocks of Unit 1 originated at a spreading centre, and represent ocean floor, generated at about 3Ga. Whether derived from a mantle plume or through upwelling mantle at a spreading centre, the source was more fertile than depleted MORB mantle. Sedimentary rocks of Units 1 and 2 provide evidence of a mixed compositional source that included intra-basinal basaltic material and an exotic tonalitic component. Sedimentary rocks of Unit 3 are derived almost exclusively from a tonalitic source, with derived sediments ranging from highly mineralogically mature to very immature. In spite of the highly active tectonic setting (i.e., rapid denudation rates), alumina-rich pelites are present in all the three Units, implying exceptionally rapid chemical weathering rates in the (tonalitic) source lands. Rapid weathering may be explained by a high CO2 content in the Archean atmosphere. There is about a 300Ma interval between Unit 1 volcanism at approximately 3.1Ga, and igneous activity represented by the Bijou Point Complex which intrude rocks of Unit 3 at about 2.7Ga. The basal volcanic rocks resemble MORBs whereas the young volcanic rocks are potassic and of calc-alkaline affinity. Although the rocks of the two basal units appear to have formed in or near a spreading center, the rocks of Unit 3 likely were produced in a subduction environment such as those found in island arcs or some continental margin settings. Together, the three units resemble products of a Wilson Cycle, operating between 3.1 and 2.7Ga, with the calc-alkaline Bijou Point Igneous Complex representing terminal volcanism. Proof or rejection of this hypothesis requires additional detailed mapping, geochemical and geochronological investigations. [Copyright &y& Elsevier]

Abstract: Supracrustal rocks in the north-central part of the North Spirit Lake greenstone belt may be divided into three major stratigraphic units. The basal unit is a thick succession of mafic and ultramafic subaqueous lavas with minor tuffaceous and sedimentary rocks. Overlying the lava-dominated succession, there is a tuffaceous unit that includes a variety of chemical sedimentary rocks such as bedded pyrite-chert deposits, bedded carbonate-cherts, large carbonate mounds, and magnetitic iron-formation. The tuffs and chemical sedimentary rocks are overlain by an extensive and thick cover of siliciclastic rocks displaying evidence of emplacement by mass flows and turbidity currents. The contact between the tuffaceous and siliciclastic units appears to be conformable but it is generally poorly exposed. Some of the siliciclastic rocks are quartz-rich and contain clasts indicating the existence of an older cratonic source land. The siliciclastic unit displays striking lateral variation from a conglomerate-dominated facies in the north to turbidites and submarine channel-fill deposits in the south and southeast of the study area. Stratigraphic and sedimentological data support geochemical evidence presented elsewhere [Nesbitt, H.W., Young, G.M., Bosman, S.A., Longstaffe, F.J. (in press). Oceanic spreading center-generated basaltic crust and associated sulfidic and carbonate-rich hydrothermal deposits in the Archean (∼3Ga), North Spirit Lake greenstone belt, Ontario, Canada. Geological Society of America Bulletin] suggesting that the lower part of the succession represent part of an ancient ocean floor that was buried beneath a thick siliciclastic succession under highly unstable tectonic conditions. A synthesis of U–Pb geochronological and Sm–Nd isotopic data suggests that volcanic rocks on the north shore of North Spirit Lake, in the core of the Prospair anticline, in the Peridotite Bay area, and to the east, may all have formed at around 3.1Ga. Succeeding hydrothermal chemical deposits are conformable with and genetically related to the volcanic rocks. Much younger (∼2.77Ga) crystal tuffs and associated subvolcanic intrusive rocks may represent a separate, much younger volcano-plutonic cycle. If the siliciclastic unit is part of this younger cycle (a question that can only be resolved by further geochronological investigations), then the time between ocean opening and closure must have been on the order of 270Ma, casting doubt on the commonly held notion that Archean plate tectonics involved small, rapidly moving plates. [Copyright &y& Elsevier]

Abstract: The Red Lake greenstone belt is one of the foremost Au mining camps in Canada and hosts the world-class Campbell-Red Lake Au deposit. Belt-scale hydrothermal alteration is characterized by proximal ferroan dolomite zones associated with Au mineralization surrounded by distal calcite zones, both being accompanied by potassic alterations (sericite, muscovite, and biotite). At the Campbell-Red Lake and Cochenour deposits Au mineralization (in particular high-grade ore) is associated with silica and sulfides (especially arsenopyrite) that replace carbonate±quartz veins and the host rocks. The prevalence of carbonic fluid inclusions and rare occurrence of aqueous-bearing inclusions in carbonate–quartz–Au veins in the Campbell-Red Lake deposit, and the consistency of homogenization temperatures of carbonic inclusions within individual fluid inclusion assemblages (FIA), have been interpreted to indicate that H2O-poor, CO2-dominated fluids were responsible for the carbonate veining and Au mineralization. Further studies of fluid inclusions in carbonate–quartz veins within and outside the deformation zone hosting the Campbell-Red Lake deposit (the Red Lake Mine trend) including the Cochenour Au deposit, the Redcon Au prospect, and outcrops in the distal calcite zone also reveal the dominance of carbonic fluid inclusions in vein minerals. These studies indicate that CO2-dominated fluids were flowing through fractures during carbonate vein formation and Au mineralization both within and outside major structures. The carbonic fluid may have been initially undersaturated with water, or it may have resulted from phase separation of an H2O–CO2–NaCl fluid. In the latter case, phase separation modeling indicates that the initial fluid likely had values larger than 0.8. Calculations based on hydrothermal mineral assemblages indicate values in the host rocks from 0.025 to 0.85, reflecting a change from CO2-dominated fluids in the fractures (veins) to H2O-dominated fluids in the host rocks away from the fractures. The CO2-dominated fluids were likely advected from granulite facies in the deeper crust, whereas the H2O-dominated fluids were derived from the ambient host rocks of amphibole to greenschist facies. Calculations based on CO2 requirements and source constraints indicate that the mineralizing fluids were likely two orders of magnitude more enriched in Au than the commonly assumed values of a few μg/L, which may explain why the Campbell-Red Lake deposit has a very high-grade of Au (average 21g/t for the whole deposit and 81g/t for the Goldcorp High-Grade zone). Fluid inclusion data suggest that the carbonate veining and Au mineralization likely took place at depths from 7 to 14km. The development of crustiform–colloform structures in the carbonate±quartz veins, which was previously interpreted to indicate relatively shallow environments, may alternatively have been related to extremely high fluid pressures and the CO2-dominated nature of the fluids, which could have enhanced the brittle properties of the rocks due to their high wetting angles. [Copyright &y& Elsevier]

Abstract: Neoarchean (ca. 2700Ma) Wawa greenstone belts, Superior Province, contain two lithologically and geochemically distinct volcanic associations: (1) a plume-derived oceanic plateau association; and (2) a subduction-derived oceanic island arc association. The oceanic plateau association is composed of three groups: (1) voluminous tholeiitic basalts with near-flat REE patterns; (2) volumetrically minor moderately LREE-depleted tholeiitic basalts and Al-undepleted komatiites; and (3) volumetrically minor moderately to strongly LREE-enriched transitional to alkaline basalts and Al-depleted komatiites. This geochemical diversity is comparable to those of Phanerozoic oceanic plateaus and ocean islands. The oceanic island arc association is characterized mainly by variably LREE-enriched and Nb-depleted (relative to Th and La) tholeiitic to calc-alkaline basalts, andesites, dacites and rhyolites, with minor adakites, magnesian andesites, and Nb-enriched basalts. All these lavas are intruded by subduction-derived late- to post-tectonic gabbros, tonalites, trondhjemites, granodiorites, and monzonites. Within the Wawa subprovince the Schreiber–Hemlo greenstone belt is dominated by plume-derived oceanic plateau volcanic rocks (60–80%), whereas the Winston Lake and Manitouwadge greenstone belts are composed dominantly of subduction-derived (70–90%) volcanic rocks. New major and trace element, and Sm–Nd isotope data are presented for volcanic rocks from the Schreiber–Hemlo, Winston Lake, and Manitouwadge greenstone belts, to address their petrogenetic origin. In addition, new major and trace element data are reported for subduction-derived gabbros from the Schreiber–Hemlo greenstone belt. The geochemical diversity of the oceanic plateau association is consistent with a compositionally heterogeneous mantle plume source. Depleted initial Nd isotopic compositions (ɛ Nd =+1.7 to +2.2) indicate that transitional to alkaline basalts were derived from a long-term LREE-depleted mantle source. It is shown that the origin of transitional to alkaline lavas can be explained by mixing of LREE-depleted and LREE-enriched melts in an upwelling mantle plume. Depleted to enriched LREE patterns in the plume-derived association are consistent with variable degrees of melting that took place over a range of temperatures and pressures as the plume ascended towards the surface. On the basis of field relationships and geochemical data, it is suggested that the geodynamic evolution of the Schreiber–Hemlo greenstone belt began with the initiation of a subduction zone at the margin of an oceanic plateau at about 2725Ma, forming an intra-oceanic island arc. In response to plate reorganization and subducted slab rollback, the island arc underwent extension leading to the opening of a backarc basin. Hydrothermal alteration of backarc oceanic crust resulted in the formation of volcanogenic massive sulphide (VMS) deposits in the Winston Lake and Manitouwadge greenstone belts. [Copyright &y& Elsevier]

Abstract: Nd isotope and geochemical data are presented for biotite tonalite, hornblende tonalite, hornblende granodiorite, biotite granite and peraluminous granite from the Archean Berens River subprovince of the Superior province in northwestern Ontario, Canada. The older tonalitic suites were synchronous with greenstone belt volcanism and were followed by granodioritic and granitic intrusions. The suites vary from 57 to 74% SiO2, are light rare-earth element (REE) enriched, with variable Eu and Nb anomalies and heavy REE depletion among the tonalites. The Nd isotope compositions for the granitoids vary from depleted values of ɛ Nd =+1.5 (tonalites) to enriched values of −1.5 (peraluminous granite). The range of isotopic composition supports a model where the granitoids were emplaced into, contaminated by, and locally melted an older pre-existing Archean crust (ca. 3.0Ga). The Nd isotope compositions of the granites and the tonalites decrease towards the north (increased crustal contamination) away from the greenstone belts that form the southern margin. This implies a thickening of the crust towards the north and is consistent with an Andean-style convergent margin in which oceanic crust subducted northwards beneath an older Archean crustal plate. Crustal contamination increased heavy REE contents in the tonalites and decreased the Sr/Y ratios for all granitoids. Failure to account for this crustal contamination can lead to errors in estimating the degree of partial melting based on the Sr/Y ratio. Compositional differences between the biotite tonalites and hornblende tonalites likely reflect different degrees of partial melting of a garnet-bearing amphibolite source. [Copyright &y& Elsevier]

At the Dundonald Beach South Ni-Cu occurrence in the western Abitibi subprovince, Ontario, Canada, komatiite-hosted magmatic sulfide mineralization (pentlandite-dominated; 0.5-7 ppm Pt + Pd) contains millimeter- to centimeter-size blebs of polycrystalline gersdorffite and nickeline. The gersdorffite-nickeline blebs are concentrated (∼6 vol %) in a 10- to 20-cm-thick by 10-m-long lens that occurs at the base of a massive sulfide horizon. The lens is significantly more enriched in Pt and Pd (15-50 ppm Pt + Pd) than typical sulfide horizons in which sulfarsenide and arsenide phases occur only as trace to minor (up to 0.15 vol %) constituents. In the platinum group element (PGE)-rich lens, ∼29 and ∼74 percent of the bulk rock Pd and Pt, respectively, are dissolved in the sulfarsenide and arsenide minerals. Almost all the remaining Pd and Pt in the lens occur in inclusions of sperrylite, michenerite (PdBiTe), and an unidentified Pd-Bi-Te phase coexisting with inclusions of tellurobismuthite (Bi2Te3), altaite (PbTe), and electrum that fill former open spaces (vugs) in gersdorffite and nickeline. In the surrounding sulfide mineralization, PGE mineral inclusions are 4 to 10 times less abundant, and the bulk PGE grade is an order of magnitude lower. Overall, PGE grade on a deposit scale may be controlled by the abundance of gersdorffite and nickeline. Even in minor abundances (e.g., ∼ 1 wt %), sulfarsenide and arsenide minerals may produce economically favorable bulk PGE grades if dispersed throughout barren (PGE-poor) sulfide horizons. The PGE-rich lens may represent the accumulation of either crystallized droplets of an immiscible sulfarsenide melt phase or sulfarsenide phenocrysts that separated from a silicate or sulfide liquid. Arsenic-rich melt or mineral phases extracted (partitioned) the trace metals Pt, Pd, Au, Bi, Te, Sb, Pb, Zn, Ag, and Sn from non-As-bearing phases (silicate liquid, Fe-Ni-Cu sulfide liquid, or monosulfide solid solution [mss]) with inferred values of DAs-phase/sulfide on the order of ∼10 to 100. Textural, mineralogical, and geochemical observations highlight the importance of As-rich liquids or mineral phases as scavengers of the PGE and other chalcophile metals in komatiite-associated magmatic sulfide environments as well as the potential for high-grade PGE mineralization to occur where komatiites were contaminated by As-rich sediments or metasedimentary rocks. [ABSTRACT FROM AUTHOR]

Recent advances in the use of Geographic Information Systems (GIS) software and analysis can be used in conjunction with traditional geoscience data sets to determine effective predictors for gold mineralization, from which mineral prospectivity maps can be generated that highlight potential exploration targets on a regional scale. In this paper, key components of the Archean lode gold deposit model for the Red Lake belt are selected and modeled using weights of evidence (WofE) analysis and logistic regression, leading to the creation of gold prospectivity maps. The best predictors for past and present gold producers in the Red Lake camp, according to WofE analysis include (1) elevated trace elements, Au, As, and Sb; (2) a number of alteration indices calculated from oxide geochemical data; (3) alteration characterized by pervasive and vein-style ferroan carbonate and elevated Au, As, Sb, and S anomalies; (4) proximity to the Mackenzie Island stock and diorite phases of the Dome stock; and, (5) tholeiitic basaltic flows and associated gabbroic rocks of the Balmer assemblage. Gold prospectivity maps produced by logistic regression using binary evidence maps highlight anomalous localities within known and highly prospective areas in the district (Madsen – Red Lake corridor, Balmertown – Cochenour – East Bay). In addition, a number of localities not known to contain significant deposits were also identified as prospective. [ABSTRACT FROM AUTHOR]

Seismic reflection line 2B was shot across the Archean Red Lake greenstone belt and Sydney Lake fault zone that marks the northern boundary of the English River metasedimentary belt, as part of the Western Superior Lithoprobe transect. Three-dimensional tomographic inversion of first arrival traveltimes recorded in this survey delineate the subsurface to depths as great as 1.5 km around this crooked two-dimensional seismic line. Within the Red Lake greenstone belt, P-wave velocities of 6.2–7.0 km s–1 occur at 500 m depth in the Mesoarchean Balmer assemblage, clearly distinguishable from the lower velocities of 5.1–6.1 km s–1 of the Neoarchean Confederation assemblage. Although the overall range of velocities in the metasedimentary rocks of the English River subprovince is similar to that found in the Confederation assemblage, lower velocities of 5.1–5.4 km s–1 are found in the upper 300 m of the metasedimentary rocks. In particular, two 2–3 km wide, east-northeast-striking zones of low velocity are associated with the Sydney Lake fault zone and the Pakwash Lake fault zone. Correlation of the velocities with the coincident reflection section suggests that these two faults delineate a fault-bounded block in the hanging wall of a more northerly fault zone that crops out within the Uchi subprovince. Anomalous regions of low velocity, which occur at the boundary between the Confederation and Balmer assemblages, and within the Balmer assemblage, may also be related to shear zones that have minimal near-surface expression, felsic lithologies, or hydrothermal alteration of the basalts. [ABSTRACT FROM AUTHOR]

Structural information about that portion of the economically important Destor–Porcupine Deformation Zone and its associated faults, which lie buried under many metres of Clay Belt sediments in the region north of Matheson, Ontario, must rely principally on interpretation of geophysical data. In this area, the Archean greenstones of the Abitibi are crosscut by at least three generations of strongly magnetic diabase dykes. The predominant dyke set, the Matachewan, trend north–south, orthogonal to structures in the greenstones. Directional horizontal derivative analysis of the total magnetic field, which has a much sharper angular cutoff than that provided by standard Fourier-based filters, provides a method for separating the magnetic contributions from these two sources. This analysis revealed the existence of a previously unidentified fault (the Winslow fault) located to the north of, and parallel to, the economically significant Pipestone Fault. We also define a second new fault, the Carr fault, that is intimately associated with the existence of a buried felsic intrusive complex in Carr Township. Possible associations between these faults and previously known faults defined from surface mapping programs are explored.On doit se fier surtout à des interprétations de données géophysiques pour obtenir des informations structurales sur la portion d'importance économique de la Zone de déformation Destor–Porcupine et ses failles associées qui se trouvent enfouies sous plusieurs mètres de sédiments du Clay Belt dans la région au nord de Matheson, en Ontario. Dans cette région, les roches vertes archéennes de l'Abitibi sont recoupées par au moins trois générations de dykes de diabase fortement magnétiques. L'ensemble de dykes prédominant, le Matachewan, a une direction N–S orthogonale aux structures dans les roches vertes. L'analyse directionnelle de la dérive horizontale du champ magnétique total, qui a une coupure angulaire beaucoup plus abrupte que celle des filtres standard basés sur les séries de Fourier, fournit une méthode pour séparer les contributions magnétiques de ces deux sources. Cette analyse révèle l'existence d'une faille antérieurement non identifiée (la faille Winslow) située au nord de, et parallèle à, la faille Pipestone, une faille d'une grande importance économique. Nous définissons également une seconde nouvelle faille, la faille Carr, qui est intimement associée à l'existence d'un complexe intrusif felsique enfoui dans le canton de Carr. Nous étudions aussi les associations possibles entre ces failles et d'autres failles connues antérieurement alors que définies par des programmes de cartographie de surface.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Presents high precision trace element data for mafic to intermediate volcanic rocks from the ...2.7 Ga Confederation assemblage of the Birch-Uchi greenstone belt in Ontario. Discussion on Archaen greenstone belt volcanic sequences; Methodology; Results and discussion; Implications.

With the increasing size and complexity of geoscientific databases, statistical tools and machine learning algorithms are needed to highlight subtle patterns within dense data clouds and reduce human-related bias in geological mapping. This study presents a new tool that maps structural complexity using circular variance and spherical dispersion in two Archean greenstone belts near Dryden and Timmins in the Superior Province, Ontario. Structural complexity quantitatively highlights areas where structures are folded, juxtaposed, and/or parallel. Bedding measurements and autodetected aeromagnetic lineaments are used as inputs to test if structural complexity can refine or add confidence to existing geological interpretations. Linear, low variance trends frequently delineate the location and extent of deformation zones, which likely reflect parallel transposed fabrics in high strain corridors. Areas of high variance spatially correlate with fold traces. Sharp boundaries between high and low variance anomalies are interpreted as faults and/or shear zones that juxtapose structural blocks. The near continuous and representative nature of autodetected magnetic lineaments was effective in capturing structural complexity at a regional scale; however, spherical dispersion captured the highest resolution of structural complexity at local scales (<1:30000). Key considerations when performing structural complexity analysis are scale, data density, and optimization of input parameters (e.g., neighborhood radius, lineament length, minimum neighborhood populations). Gold deposits are found <~1700 m from low (< μ - 1σ) and < ~1500 m from high circular variance anomalies (> μ + 1σ). Additionally, the gold grade of large deposits (> 1 Moz) increases with proximity to high variance anomalies. When paired with expert knowledge, these techniques will increase repeatability in future exploration endeavors, making exploration a more rigorous process with increased confidence. • Statistically quantify structural complexity from bedding and geophysical data • Deformation zones and fold traces indicated by variance anomalies • Orogenic Au mineralization adjacent to variance anomalies • Auto-detected aeromagnetic lineaments most effective for regional-scale analyses • Dispersion of bedding vectors most effective for deposit-scale/brownfields settings [ABSTRACT FROM AUTHOR]

• Translating the orogenic mineral system of the Swayze greenstone belt into mappable expressions. • Support vector machines and artificial neural network were used to predict gold prospectivity. • Stratified k-fold cross validation resulted in a mean accuracy of > 87% after k =10 folds. • The best predictor layers were proximity to D 2 and D 3 high strain zones, lithological contacts, and D 2 folds. • New deposit targets lie proximal to faulting, EM conductivities, and along chemically reactive rocks. Exploration for new mineral deposits has become increasingly difficult as new discoveries are being found under progressively deeper cover. To better understand and predict orogenic gold mineralization in the Archean Swayze greenstone belt, the essential ingredients of a mineral system are considered: 1) the source of gold and transport fluid ligands, 2) fluid pathways, 3) traps, and 4) the processes responsible for gold precipitation. The aim of this study is to use a mineral systems approach to help generate exploration targeting models using a spatial statistical method - weights of evidence (WofE) and data-driven machine learning tools, namely radial basis function neural networks (RBFNN) and support vector machine (SVM). The mineral prospectivity maps generated using the RBFNN and SVM machine learning methods were trained using the K-Fold cross-validation approach whereby 10 subsets of the data were used to train and test the model performance. The mean area under receiver operator curve after 10-fold cross-validations were 91% and 94% for the RBFNN models, and the SVM models obtained accuracies of 91% and 87%. Feature importance estimations obtained from both methods indicate that D 2 and D 3 high-strain zones, lithological contacts and D 2 folds (i.e., synclines and anticlines) were found to be important predictor layers for targeting potential prospective zones of gold mineralization. The machine learning algorithms used in this study are novel and pragmatic methods that use the full potential of geoscience datasets in mapping orogenic gold prospectivity. [ABSTRACT FROM AUTHOR]

(A) AIP relaxation time-constant data superimposed on the RTP data from the Sandy Lake study area (SLGB) and (B) from the Favourable Lake study area (FLGB). • We propose that AIP relaxation time-constant highs be used as vectors in exploration for concealed orogenic gold deposits, especially where they are spatially associated with concealed structural geometries. • Our method is documented at two study areas in the Sandy Lake and Favourable Lake greenstone belts, Superior Province, Canada. • Our study can potentially be used as a template for the integration of structural geometry and multiple geophysical methods combined with follow-up geochemistry for multi-disciplinary exploration of concealed orogenic gold deposits in Archean greenstone belts. Since 2000, gold discovery rates have not reflected the record levels of exploration expenditure, with those discoveries that are made being predominantly in areas of post-mineral cover. Hence, gold exploration has been increasingly focused on concealed exploration targets where geophysics takes on an increasingly important role in the exploration process. Archean orogenic gold deposits are attractive exploration targets that represent a major proportion of global gold resources. Orogenic gold deposits consistently form late during the tectonic and structural evolution of their host terranes, normally during a transition from compressional to transpressional tectonic stress regimes. Thus, for target generation, the late timing of gold mineralization within the structural history of a prospective orogenic belt is crucial, as it means that current geological maps and sections reflect the geometry of the gold deposits at the time of formation unless there has been subsequent orogeny. Lithological contacts between granitic intrusions and volcano-sedimentary greenstone sequences, as well as between volcanic and sedimentary sequences, are important rheological and geochemical trap sites. Lithosphere-scale faults and dilational jogs in second-order faults act as ore-fluid conduits with anticlinal folds as structural traps for ore-fluid infiltration and gold deposition given suitable host rocks and fluid seals. Geophysical exploration for concealed orogenic gold deposits is challenging due to normally subtle petrophysical contrasts between the gold lodes and their host rocks. Most greenstone-hosted lode-gold deposits do not contain strong conductors as they normally contain <5 vol% of disseminated arsenopyrite and pyrite, which are poor conductors, and rarely contain significant quantities of strong conductors such as pyrrhotite. However, pyrrhotite may be a component of rock sequences such as sulfidic shales or cherts adjacent to the orogenic gold lodes. It is proposed that the combination of HTEM and magnetic surveys be used to explore for concealed orogenic gold deposits, using the Favourable Lake and Sandy Lake greenstone belts in the Superior Province, Canada, as a case study. AIP Cole-Cole time-constants, combined with magnetic and EM data, can efficiently detect disseminated pyrite/magnetite at shallow depths, indicating their potential use as vectoring tools for concealed Archean orogenic gold deposits. [ABSTRACT FROM AUTHOR]

• Evaluation of gold deposit distribution at various scales in the Swayze greenstone belt. • Gold deposits are spatially correlated with ESE-WNW- and ENE-WSW-striking faults. • Ripley's K-function cluster analysis suggests gold deposit occurs within 2 to 4 km of each other. • Fractal analysis indicates that gold deposit distribution is bifractal at scales <8 km and >8 km. • Over 96% of all gold deposits are found within 3 km distances from faulting. Understanding the spatial controls on mineral deposits is crucial in mineral exploration targeting. This paper uses a combination of spatial data analysis techniques, such as the distance distribution method, Fry, nearest neighbor index, Ripley's K-function multi-distance, fractal, and kernel density analysis, to better define controls on the distribution of gold deposits in the Swayze greenstone belt, Ontario, Canada. According to results obtained with the distance distribution method, D 2 structures were critical in controlling the spatial distribution of the gold mineralization, with 97% of the gold deposits occurring within 7 km of these structures (and 65% within 4 km). Fry analysis confirmed this spatial relationship with a close agreement between the orientations of the D 2 structures and gold deposit alignment. The outcome of Ripley's K-function cluster analysis suggests that gold deposits occur within 4 km distances of other gold deposits. These findings are consistent with the results from the fractal analysis, showing that gold deposit distribution is bifractal and that different geological features occurred at local (<8 km) versus regional (>8 km) scales. We conclude that higher-order D 2 structures and lower-order faults connected to the D 2 structures acted as pathways to focus gold-bearing hydrothermal fluids, thus leading to the observed concentration and distribution of gold deposits at the regional to local scales. [ABSTRACT FROM AUTHOR]

In Archean greenstone belts, magmatism is dominated by intrusive and volcanic rocks with tholeiitic affinities, as well as tonalite- and granodiorite-dominated large-volume batholiths, i.e., tonalite–trondhjemite–granodiorite (TTG) suites. These intrusions are associated with poorly documented mineralization (Cu-Au porphyries) that, in the Neoarchean Abitibi Subprovince (>2.79 to ~2.65 Ga), Superior Province, Canada, are associated with diorite bearing plutons, i.e., tonalite–trondhjemite–diorite (TTD) suites. The importance of TTG versus TTD suites in the evolution of greenstone belts and of their magmatic-hydrothermal systems and related mineralization is unconstrained. The aim of this study was to portray the chemistry and distribution of these suites in the Abitibi Subprovince. The study used data compiled by the geological surveys of Québec and Ontario to evaluate the chemistry of TTG and TTD suites and uncovered two coeval magmas that significantly differentiated (fractional crystallization mostly): 1) a heavy rare earth elements (HREE)-depleted tonalitic magma from high pressure melting of an hydrated basalt source; and 2) a hybrid HREE-undepleted magma that may be a mixture of mantle-derived (tholeiite) and tonalitic melts. The HREE-depleted rocks (mostly tonalite and granodiorite) display chemical characteristics of TTG suites (HREE, Ti, Nb, Ta, Y, and Sr depletion, lack of mafic unit, Na-rich), while the other rocks (tonalite and diorite) formed TTD suites. Tonalite-dominated magmatism, in the Abitibi Subprovince, comprises crustal melts as well as a significant proportion of mantle-derived magmas and this may be essential for Cu-Au magmatic-hydrothermal mineralizing systems. [ABSTRACT FROM AUTHOR]

One of the major consequences of photosynthesis is locally increased carbonate alkalinity, which stimulates the precipitation of carbonate minerals in aqueous environments. On the modern Earth, reef–type ecosystems occupy the most productive end of the oxygen– and carbonate–producing spectrum. However, prior to about 700 million years ago, photosynthetic bacteria fulfilled this role, building reef–like microbialite (e.g., stromatolite) structures, and eventually whole carbonate platforms, out of the carbonate minerals they help precipitate as a consequence of their alkalinizing effect. The most ancient examples of these systems, dating back to ca. 3.0 Ga, remain little explored. The EARTHBLOOM project examines in detail the stratigraphy, major element and trace element geochemistry of the 2,940 +/- 2 Ma to 2,925+/- 3 Ma Ball assemblage, Red Lake Greenstone belt, N. Ontario, Canada (dates from Corfu and Wallace, 1986, Can. J. Earth. Sci. 23, 27–42). This deposit contains over 400m of metasedimentary rocks that include over 200m of stromatolitic calcite and dolomite, making it the earliest known significant accumulation of carbonate on the planet. Field studies and examination of multiple industry drill cores reveal that in the east, deep-water facies (BIF, chert) are overlain by carbonates with pervasive microbial fabrics, while in the west, carbonates showing few microbial textures were deposited on and interbedded with abundant siliciclastics. Trace element data indicate general anoxia during precipitation of carbonate lithofacies. Relatively abundant pyrite and potential pseudomorphs after gypsum suggest evaporative concentration of sulfate, and sulfur isotope studies are currently underway. Carbonate C and O isotope compositions are consistent with precipitation from Archean seawater. Carbonate C isotopes, however, do show some variation between facies, with lighter values in deeper water facies. Organic carbon isotopes show a significant range, with values spanning from -30 to as heavy as -10 permil. The statistical distribution of organic carbon isotope data appears to suggest different organic carbon sources in near-shore carbonate-dominated environments versus off-shore cherty or shaley environments. A diagenetic origin of this signal appears unlikely based on C- and O- isotope systematics. The ensemble of data available to date indicates that the Red Lake carbonate platform represents an evaporitic setting, perhaps with locally concentrated sulfate, and remarkably, with little evidence for the presence of cyanobacteria during carbonate precipitation. [ABSTRACT FROM AUTHOR]