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The Pontiac Subprovince is located in the Superior Province, south of the Abitibi Subprovince. The metasedimentary rocks of the Pontiac Group are characterized by a Barrovian metamorphic gradient increasing from north to south from biotite- through garnet- to staurolite-zone conditions. The Pontiac Subprovince has been interpreted as an accretionary wedge or alternatively as an exotic terrane that was tectonically docked to the Abitibi Subprovince during Archean subduction. However, few studies have attempted to characterize its pressure-temperature-time path to test this hypothesis and little is known about the timing relationship between regional metamorphism and the gold mineralization. An E-W lepidoblastic ductile micaceous foliation defines the regional fabric related to the second episode of deformation. Garnet and staurolite porphyroblasts are interpreted to be late- to post-kinematic to the regional fabric. Locally, cordierite or andalusite porphyroblasts fully pseudomorphed by a polymineralic assemblage of muscovite, feldspar and quartz are wrapped by the main foliation indicating that a potential low pressure - high temperature metamorphism (M1) preceded the main episode of deformation and Barrovian metamorphism (M2). Pressure and temperature (P-T) forward thermodynamic modelling on a water-saturated pseudosection yielded peak conditions of 550–600 °C and 5–6 kbar during a prograde, clockwise P-T path for M2. Lu-Hf dating of garnet from three different locations within the staurolite zone yielded a weighted average age of 2657.5 ± 4.4 Ma (95% confidence level) that is inferred to be representative of these conditions. The age of garnet growth calls into question the previous tectonic interpretation of the Abitibi and Pontiac Subprovinces and is coherent with the new rafted ribbon continent model of Bedard (2018) . Because the Canadian Malartic gold mineralization is older or synchronous with the garnet growth episode in the staurolite zone south of the ore body, this garnet age also provides an estimate of the minimum age of the mineralization event, previously dated at 2664 ± 11 Ma with Re-Os on molybdenite.

The Canadian Malartic deposit is a world class intrusion-related Archean gold deposit hosted in the Pontiac Subprovince, Superior Province, in Quebec, Canada. Laboratory petrophysical properties measurements were performed on 824 rock samples collected from the various rock types observed within the ore body and peripheral host rocks. The various rock types present in the Malartic District, mainly meta-sedimentary rocks, felsic-intermediate intrusive rocks and mafic dykes have contrasting grain densities and magnetic susceptibilities. Using support vector machines, it is shown that these two physical properties can be used to predict the rock type of a sample with an average precision and recall rate of 89%. Within the meta-sedimentary rocks class, variations in magnetic susceptibility are due to the changes in mineralogy associated with hydrothermal alteration. These are caused by the destruction of iron-bearing silicate minerals and magnetite in unaltered rocks (10−4 to 10−3 SI) to form pyrite, carbonates, K-feldspar and Fe-depleted hydrothermal biotite in altered rocks (10−5 to 10−4 SI). Within the felsic-intermediate intrusive rocks, grain densities below 2.7 g/cm3 and magnetic susceptibilities in the 10−6 to 10−4 SI range yield the highest probabilities that a rock has been submitted to carbonate and pyrite alteration. However, magnetic susceptibility and grain density of these intrusive rocks are also dependent on their Fe2O3/Al2O3 and TiO2/Al2O3 ratios, which are not related to the hydrothermal alteration footprint, but are rather due to distinct protolith compositions. Within the mafic dykes, grain density is the best indicator of hydrothermal alteration. Unaltered mafic dykes (2.95 to 3.10 g/cm3) are mostly composed of amphibole, whereas altered mafic dykes (2.70 to 2.95 g/cm3) have reduced amphibole contents and higher abundances of carbonates, pyrite, quartz, and biotite alteration. The support vector machine classifier is extended to predict if meta-sedimentary rocks, felsic-intermediate intrusive rocks and mafic dykes have undergone hydrothermal alteration with average F1 scores of 73%, 69% and 93%, respectively. In altered meta-sedimentary rocks, the integration of grain density and magnetic susceptibility allows the identification of altered but unmineralized samples. The classifier is further extended to predict if the gold content of meta-sedimentary rocks is above or below threshold values of 0.01, 0.1 and 1 ppm with average F1 scores of 83%, 80%, and 76%, respectively. Using conceptual models of the rock physical properties at the Malartic District scale, it is shown that ground magnetic surveys are the most promising geophysical tool for early-stage greenfield exploration of this type of deposit. However, depending on the scale at which the surveys are conducted, magnetic susceptibility contrasts between the various investigated rock types can overshadow the specific signatures of hydrothermally altered rocks. This in part explains why past airborne geophysical exploration campaigns for this type of deposit in the Malartic District were inconclusive. Finally, the machine learning process used in this case study can be applied in advanced exploration stages, during which drilling and either subsequent laboratory petrophysical analyses of core samples or downhole geophysical surveys produce large amounts of data that can be used to train prediction models.

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 ( 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.

Understanding the architecture of the West African craton at depth is essential to be able to reconstruct its evolution. Our study focuses on the crustal imaging of structures and geometries characterizing the crust of the Leo-Man shield with broadband and long period magnetotelluric data collected in southern Burkina Faso and covering a 220 km long profile. The resulting 3D resistivity crustal model highlights the distribution of the granite-greenstone assemblages with depth showing excellent correlation with mapped surficial lithologies. The whole crust of southern Burkina Faso is resistive, with lateral as opposed to vertical major resistivity contrasts, reflecting the location of major-scale shear zones characterizing this part of the Baoule-Mossi domain. Ground gravity data acquired along the same line as the MT data were also modeled and show relatively good correspondence with the resistivity model. The new resistivity and gravity models compared with results from joint inversion of gravity and aeromagnetic data highlight significant changes between the greenstone belts and granitoid domains along the profile. The comparison of the geophysics with the geology enables us to define new depth constraints of the main tectonic features in the area. The observed large scale dipping shear zones favour the model of crustal building through major parallel thrust faults.

The structural setting of the Pontiac Subprovince in the vicinity of the world-class Canadian Malartic gold deposit has been revisited by combining and reinterpreting airborne geophysical surveys together with a century of structural observations. Felsic-intermediate intrusive bodies are a key component of this deposit. Defining the regional and local favorable structural setting for intrusive rock emplacement within the clastic meta-sedimentary rocks of the Pontiac Group, south of the Cadillac Larder Lake Deformation Zone, may further assist gold exploration in similar tectonic environment. Three structural domains are interpreted in the area based on the geometry of the bedding, folds and structural fabrics related to the three major phases of deformation. During these events four phases of magmatism and one metamorphic episode occurred. The North domain, which hosts the Canadian Malartic deposit, is characterized by highly variable bedding orientations produced by the interference patterns of isoclinal F1 folds overprinted by open to tight, steeply dipping, F2 folds. The bedding in this domain is cut by a penetrative S2 biotite foliation, which possibly built on rheological changes enhanced by metasomatism in the footprint of the Canadian Malartic deposit. By contrast, the Central and South domains display homogeneous bedding orientations cross-cut by a discrete S2 biotite foliation and syn- to late- D2 metamorphic porphyroblasts. In all domains the L2 stretching lineation consistently plunges at about 60 degrees toward the east. D3 is a minor deformation event in the Pontiac Subprovince, which possibly correlates with late dextral transcurrent movement along the Cadillac Larder Lake Deformation Zone. Intrusive bodies were emplaced throughout the first and second deformation events. Phase 1 monzonite, quartz-monzodiorite and granodiorite plutons (ca. 2683–2680 Ma) intruded into consolidated Pontiac sedimentary rocks during D1. Phase 2 quartz-monzodiorite bodies (ca. 2679–2676 Ma) predominantly formed in the North Domain along F1 fold axial surfaces prior to or at the onset of D2. Phase 3 basic dykes (ca. 2675–2673 Ma) cross-cut earlier felsic-intermediate intrusions across all domains and subsequently developed an S2 metamorphic hornblende foliation. Phase 4 magmatism (ca. 2672–2662 Ma) is related to the Decelles Batholith S-type granite and pegmatite, which is interpreted to be contemporaneous with the peak of regional metamorphism. The Decelles Batholith may project at depth underneath the Canadian Malartic deposit and could be associated with magmatic-hydrothermal mineralizing fluids in the Canadian Malartic deposit. Gold mineralization at Canadian Malartic is spatially located on the contact of Phase 2 quartz-monzodiorite bodies. The proximity to the Cadillac Larder Lake Deformation Zone combined with the rheological contrast between steeply dipping clastic meta-sedimentary rocks and quartz-monzodiorite intrusions favored the protracted failure of the contacts between these two rock masses, thus forming favorable conduits for hydrothermal fluids (e.g., the Sladen Fault Zone). Such specific rheological behavior is demonstrated by domains of structural complexity, emphasized by the variance of the bedding dip. These structurally complex zones systematically host gold mineralization in the Canadian Malartic district of the Pontiac Subprovince proximal to the Cadillac Larder Lake Deformation Zone.

The Vazante deposit, which is the world's largest zinc silicate deposit, occurs in brecciated dolomite and comprises mainly willemite with various proportions of hematite, Fe-carbonate, minor franklinite and magnetite. Exploration for this type of deposit is more challenging than zinc sulfide deposits, as they do not exhibit similar geophysical anomalies. To improve the application of geophysical surveys to the exploration of hypogene silicate zinc deposits, data from 475 samples were investigated from drill holes representative of the various types of ore and host rocks as well as barren zones of known geophysical anomalies in the Vazante District. Lithogeochemical and mineralogical (optical, SEM and MLA) data were integrated with physical rock properties (density, magnetic susceptibility and K U Th gamma-ray spectrometry) to assist in exploring for this type of deposit. The most distinct physical property of the ore is density, compared with the host rocks due to high proportion of denser minerals (hematite and willemite). However, barren hematite breccias also have high densities. The zinc ore and hematite breccias yielded higher magnetic susceptibilities than the surrounding host rocks, with the highest values associated with greater proportions of franklinite and magnetite. The density and magnetic susceptibility contrasts are a result of hydrothermal fluids interacting with and altering the carbonate host rocks. Zinc ore also yielded elevated U concentrations relative to the various host rocks, yielding higher gamma-ray spectrometric values. The results of this investigation indicate that an integration of magnetic, gravimetric and radiometric surveys would be required to identify zinc silicate ore zones.

The West-African craton is defined by a combination of Archean and Palaeoproterozoic rocks that stabilised at ~2 Ga towards the end of the Paleoproterozoic Eburnean Orogeny, and therefore may reflect the transition from Archean to modern tectonic processes. Exploring its present lithospheric architecture aids further understanding of not only the craton’s stability through its history but also its formation. We investigate the lithospheric structure of the craton through analysing and modelling magnetotelluric (MT) data from a 500-km-long east-west profile in northern Ghana and southern Burkina Faso crossing part of the Baoule-Mossi Domain and reaching the Volta Basin in the south-eastern part of the craton. Although the MT stations are along a 2D profile, due to the complexity of the structures characterising the area, 3D resistivity modelling of the data is performed to obtain insights on the thermal signature and composition of the subcontinental lithosphere beneath the area. The thermal structure and water content estimates from different resistivity models highlight a strong dependence on the starting model in the 3D inversions, but still enable us to put constraints on the deep structure of the craton. The present‐day thermal lithosphere‐asthenosphere boundary (LAB) depth is estimated to be at least 250 km beneath the Baoule-Mossi domain. The area likely transitions from a cold and thick lithosphere with relatively low water content into thinner, more fertile lithosphere below the Volta Basin. Although the inferred amount of water could be explained by Paleoproterozoic subduction processes involved in the formation of the Baoule-Mossi domain, later enrichment of the lithosphere cannot be excluded

The Ashanti greenstone belt in southwest Ghana hosts many gold deposits distinguished by different timing and structural contexts. This study investigates the evolution of the Wassa system by integrating field and geophysical observations. This 4 million ounces (past production and current resources) gold deposit is interpreted to represent the oldest gold mineralization event in West Africa with gold-bearing pyrites aligned and stretched within the S1 ductile fabric. Mineralized quartz-carbonate veins were strongly deformed during the D1 deformation event. Three additional folding events are characterized by hectometer-scale tight to isoclinal folds, by a kilometer-scale synform fold centered on the mine and by a late recumbent metric-scale folds. Because of its early timing, the Wassa system represents a new poly-deformed deposit type in West Africa and highlights a potential for new discoveries in the underexplored meta-volcanic and meta-sedimentary Sefwi Group. Timing of the gold mineralization at the Wassa mine makes this deposit type a possible candidate for the source of the gold contained in the Tarkwa paleoplacer.

The Ashanti Belt in Ghana hosts numerous multi-million ounce gold deposits and is one of the most richly gold endowed Paleoproterozoic belts of the West African Craton. This work shows that the Wassa mineralized intrusion is part of the Sefwi Group. This unit at Wassa is strongly magnetic and show a distinctly high response in regional magnetic data sets compared to other units of equivalent age within the belt. The unit is inferred to be a lateral extension of an exposed fragment of what defines the substrate to the Tarkwa Basin sediments. The Wassa deposit, located in the eastern limb of the belt, is hosted within mafic to intermediate volcanic flows that are interbedded with minor horizons of volcaniclastics, clastic sediments. The clastic sediments include wackes and magnetite rich sedimentary layers, presumably derived from banded iron formations. The previously described sequence is intruded by syn-volcanic mafic intrusives and felsic porphyries rocks that are all part of the Birimian stratigraphy. Two new key SHRIMP II U–Pb ages were determined as part of this study: a new age of 2191 ± 6 Ma was determined on magmatic zircon grains of the Wassa porphyry host rock, which now represents the oldest known felsic intrusion hosting gold mineralization in the Ashanti Belt region. The Benso gold deposit system, which is located in the eastern limb of the Ashanti Belt approximately 38 km southwest of Wassa is hosted within a series of volcanic units intruded by mafic to intermediate units. A SHRIMP II U–Pb age of 2157 ± 5 Ma was determined from magmatic zircons obtained from a granodiorite of the G-Zone of the Benso deposit. This granodiorite is the main host rock for gold mineralization and thus the age provides an upper constraint for mineral emplacement. The newly determined ages provide an upper constraint for the gold mineralization within this region of the Ashanti Belt. They also support recent structural studies that have interpreted that the Wassa mineralization is related to the Eoeburnean period, defined to have occurred between 2191 and 2158 Ma (D1 event) prior to the main phase of gold mineralization. This new U–Pb age on the host rock provides an upper age limit to the Wassa mineralization of ca. 2191 Ma and is compatible with the proposed Eoeburnean early mineralization model for the Wassa deposit. The age of the intrusion along with the location of the deposit in the Sefwi Group give a unique characteristic that distinguishes Wassa from the other gold deposits along the Ashanti belt which are believed to have been the result of a mineralizing event between ca. 2090 and 2070 Ma.

Studies of mafic dyke swarms may simultaneously provide information on the mechanical, geochemical, geochronological and magnetic environments at the time of their formation. The mafic intrusive history of different cratons can also be potentially used to unravel their assembly into their current configuration. The identification and classification of dykes is a first step to all these studies. Fortunately, even in regions with poor outcrop, we can use the strong magnetic response of mafic dykes to identify and map their extent. In West Africa the first maps of mafic dyke distribution were made over 40 years ago, but there are still large areas where there are almost no published data. In this paper we present a significantly updated map of mafic dykes for the West Africa Craton based in large part on new interpretations of the regional airborne magnetic database. This map includes the locations of over three thousand dykes across the craton, which locally shows several orientation clusters that provide a minimum estimate for the total number of dyke swarms in this region. Whilst we will have to wait until systematic dating of the different swarms is completed, we can demonstrate that there is a long and complex history of mafic magmatism across the craton, with up to 26 distinct dyke swarms mapped based according to their orientation. The mapping and dating of these swarms will provide key constraints on the assembly of the fragments that make up the modern continents.

International audience; We estimate using gravity data the thickness of post-glacial unconsolidated sediment filling two major glacial valleys in northern Pyrenees: the Gave de Pau valley between Pierrefitte-Nestalas and Lourdes, and the Garonne valley between Saint-Béat and Barbazan. One hundred and eighty-four new gravity data complete 74 measurements obtained from the International Gravimetric Bureau database. Negative residual anomalies resulting from the presence of small-density unconsolidated sediment approach 4 mgal in both the Gave de Pau and the Garonne valleys. Estimating the sediment thickness requires knowing the density contrast between Quaternary sediments and the underlying bedrock. Supposing this density contrast is 600 kg/m3, the maximum estimated thickness of post-glacial sediment is ∼ 230 and 300 meters, and the volume of sediment is 2.1 and 3.2 km3 in the Gave de Pau and Garonne valleys, respectively. In both valleys, the depth of Quaternary sediment suddenly increases at the confluence between two major glacial valleys (Gave de Pau – Gave de Cauterets, and Garonne – Pique confluences). Overdeepened basins are less deep downstream when approaching terminal moraines (Lourdes and Barbazan area), illustrating that the efficiency of glacial erosion depends on the ice flux flowing through valleys.

Canadian Malartic is a large-tonnage, low-grade Archean gold deposit (16.3 Moz, 1.08 g/t Au) located in the Abitibi region of Quebec, Canada. A large part of the mineralization is hosted in the Pontiac Group metasedimentary rocks, which consist of mudstones to greywackes at upper greenschist to amphibolite facies. In exploration and production environments, these lithologies are challenging to characterize by conventional core logging methods, while in a research setting thin section-sized samples (2 cm × 4 cm) do not capture the full extent of mineralogical variability, which can extend from centimeters to meters away from mineralized zones. Here, high-resolution hyperspectral imagery (0.2–1.0 mm/pixel) in both shortwave infrared (SWIR, 1000–2500 nm) and longwave infrared (LWIR, 8000–12000 nm) is acquired for over two thousand meters of drill core, and is used to visualize changes in mineralogy and mineral chemistry related to metamorphism and hydrothermal alteration. Unaltered metasedimentary rocks contain metamorphic white mica with AlVI contents varying between 1.90 and 1.75 apfu (2195 to 2203 nm), depending on metamorphic grade. Hydrothermal alteration is characterized by white mica which becomes progressively more phengitic with increasing alteration intensity, with AlVI contents ranging from 1.70 to 1.50 apfu (2204 to 2212 nm). Phengitic white mica extends from meters to tens of meters away from major mineralized zones, and can be used as a vector towards mineralization in an exploration setting. White mica composition is correlated to Au content, and can be used to discriminate between unmineralized ( 0.3 g/t Au, 2205–2208 nm), and highly mineralized (>1.0 g/t Au, >2208 nm) samples, which is a simple metric that can directly be applied to sort ore in a production environment. The Mg# (molar Mg/[Mg + Fe]) of biotite, on the other hand, is unaffected by metamorphic grade, and consistently is Mg# 55–60 (2251–2250 nm) in unaltered samples. In mineralized samples, biotite is Mg-rich (Mg# >65

The process of building a 3D model necessitates the reconciliation of field observations, geophysical interpretation, geological data uncertainty and the prevailing tectonic evolution hypotheses and interpretations. Uncertainty is compounded when clustered data points collected at local scales are statistically upscaled to one or two points for use in regional models. Interpretation is required to interpolate between sparse field data points using ambiguous geophysical data in covered terranes. It becomes clear that multiple interpretations are possible during model construction. The various interpretations are considered as potential natural representatives, but pragmatism typically dictates that just a single interpretation is offered by the modelling process. Uncertainties are introduced into the 3D model during construction from a variety of sources and through data set optimisation that produces a single model. Practices such as these are likely to result in a model that does not adequately represent the target geology. A set of geometrical ‘geodiversity’ metrics are used to analyse a 3D model of the Gippsland Basin, southeastern Australia after perturbing geological input data via uncertainty simulation. The resulting sets of perturbed geological observations are used to calculate a suite of geological 3D models that display a range of geological architectures. The concept of biodiversity has been adapted for the geosciences to quantify geometric variability, or geodiversity, between models in order to understand the effect uncertainty has models geometry. Various geometrical relationships (depth, volume, contact surface area, curvature and geological complexity) are used to describe the range of possibilities exhibited throughout the model suite. End-member models geodiversity metrics are classified in a similar manner to taxonomic descriptions. Further analysis of the model suite is performed using principal component analysis (PCA) to determine important geometrical characteristics. The configuration of the model space is determined through identifying ‘outlier’ model examples, which potentially represent undiscovered model ‘species’.

Integration of regional geophysical datasets and detailed field observations provide new insights into the paleoproterozoic structural evolution of southwestern Ghana. The study area is dominated by three metavolcanic and metasedimentary packages known as the Sefwi Group, the Kumasi Group (Birimian) and the Tarkwa Group (Tarkwaian) that were intruded by abundant TTG granitoids during the Eoeburnean and Eburnean phases of an event termed the “Eburnean Orogeny”. This study identifies an Eoeburnean (pre-Tarkwaian) deformation event (D1) that produced significant deformation in the Sefwi Group metavolcanics. D1 is associated with N-S shortening manifested as regional scale folding in the southern Ashanti Belt. D1 synorogenic granitoids were intruded between 2187 Ma and 2158 Ma under greenschist metamorphic condition. Syn-D1 gold mineralisation associated with quartz veining could be the original source of Tarkwaian paleo-placers and/or remobilised gold concentrations along major shear zones. D2 represents an extensional phase associated with the Kumasi Group sedimentation (2154–2125 Ma) which could be related to activation of major structures such as the Ashanti Fault as low angle detachments that controlled the deposition of the Kumasi Group and the opening of the Kumasi and Akyem Basin. The Tarkwa Group (2107–2097 Ma) unconformably overlies the Birimian Supergroups and was deposited in response to D3 shortening. D3 resulted in the inversion of syn-D2 detachments faults within the Ashanti Belt. NW-SE D3 shortening produced regional scale folding within the Birimian and the Tarkwaian metasediments. D4 deformation corresponds with sinistral reactivation of D3 thrust faults, and is locally associated with macro-scale folding at Obuasi and Wassa gold mines. By the end of D4, the regional scale architecture was built and was only slightly modified by the two last events. D5 postdates the Eburnean metamorphic peak and corresponds to open recumbent folds associated with a subhorizontal crenulation cleavage. D6 is present as a subvertical crenulation cleavage and reverse faults associated with NE-SW shortening.