Abitibi Subprovince Research Articles

Characterization of pegmatites in the La Motte Batholith area, Preissac-La Corne Plutonic Suite, Abitibi Subprovince and its implications for exploration

This project focuses on the study of pegmatite characteristics in the La Motte Batholith (LMB) sector in the Abitibi Subprovince, located between the cities of Rouyn-Noranda and Val d'Or, and approximately 30 km west of the North American Lithium mine (101.9 Mt @ 1.06 % Li2O; Sayona Québec). The study area hosts mineralizations in critical and strategic minerals, most notably the Authier deposit, which are exclusively associated with Li-Cs-Ta pegmatites. The aim of the study is to characterize the mineralogical and geochemical properties of the pegmatites in the sector to better target lithium exploration. This characterization enables the subdivision of various families of pegmatites in the LMB sector, ranging from barren pods to spodumene-rich dykes. Within the LMB, the pegmatitic facies exhibit chemical characteristics similar to those of intersecting pegmatite dykes, suggesting a genetic link between the La Motte Batholith monzogranite and the different pegmatite families despite a possible age difference of several million years. The results also confirm that elemental ratios from whole-rock analyses (K/Rb, K/Cs, Nb/Ta, Mg/Li, and Zr/Hf) within lithium-bearing dykes show the lowest values compared to other dykes, regardless of the sampling location within the same dyke. Potassium feldspar and muscovite microprobe analyses from different pegmatites also reveal K/Rb and K/Cs ratios similar to whole-rock analyses, thereby strengthening the assessment of the fractionation degree of the LMB pegmatites. These ratios thus allow for targeting sectors hosting highly fractionated dykes even where spodumene has not yet been identified.

Natural metal contents and influence of salinization in deep Canadian Shield groundwater: Base level versus mineral deposit enrichment halos

Groundwater interacts with mineral deposits, making this secondary environment a medium of choice for geochemical exploration. With increasing depth, groundwater in Precambrian shields typically matures and increases in salinity through prolonged water–rock interactions and mixing with geologically old saline fluids. As the focus of mineral exploration is gradually shifting toward deeper targets, the influence of salinity on pathfinder background levels and anomalous thresholds must be considered. We compiled a deep background database (Deep Abitibi Groundwater database; DAGW) containing groundwater samples between depths of 0 m and 690 m from barren exploration boreholes in the Abitibi subprovince, Quebec, to evaluate the natural elemental levels at depth. The DAGW provides valuable knowledge of the base levels of a wide spectrum of elements and of the changes in these base levels along with the increasing salinity in bedrock aquifers of the deep Canadian Shield. This can serve to estimate background values during hydrogeochemical exploration campaigns. The regional Abitibi base level for Zn (median of 7.5 μg/L), a key pathfinder for VMS mineralization, is unaffected by the increasing salinity levels down to depths of 690 m. Hence, down to this depth, the Zn concentrations measured may be used directly for mineral exploration, and no further correction for the effects of salinization is necessary. We conducted a case study near an undisturbed VMS-type deposit (Daniel 25) and a Zn enrichment halo was observed. Within the halo, Zn concentrations increased with depth, indicating that Zn mobility was positively affected by the geochemical conditions of the deeper aquifer. Anomalous Zn thresholds representing the proximal (50–600 m from the mineralization) and direct contact (within 50 m of the mineralization) footprints of the Daniel 25 deposit were estimated at 50 μg/L and 380 μg/L, respectively. An anomalous threshold was also estimated for Co, a secondary pathfinder, at 0.5 μg/L. Unlike Zn, several other elements, which could be used to trace mineral deposits, are affected by salinization. As a final step, we applied a correction to the Daniel 25 data, based on elemental baseline trends observed in the DAGW. The aim was to remove the effects of salinity and enhance the statistical correlations caused by the presence of the mineralized body. This correction led to the emergence of new spatial correlations, notably with Na and Sr, that are coherent with the increasing hydrothermal alteration in proximity to the ore.

Portable XRF applied to regional bedrock mapping in Quebec, Canada

Whole-rock geochemistry yields better geological maps of the bedrock, since it allows, for example, visually similar lithologies to be distinguished. However, conventional laboratory geochemistry is not available in the field during a mapping campaign. Portable X-ray fluorescence (pXRF) analyzers can produce fit-for-purpose data rapidly, and constitute a useful complement to conventional geochemistry in a bedrock mapping project. The pXRF data can be employed (i) while still in the field, to orient the mapping campaign; (ii) to prepare a preliminary geological map back in the office; and (iii) to interpolate between conventional whole-rock analyses on the final map. The latter application requires a matrix-matched secondary calibration of the pXRF data to improve its accuracy. Attention must also be given to precision issues, as well as potential instrument drift and analytical interferences.This paper summarizes recent efforts to systematically integrate quantitative pXRF data in government mapping projects within the Province of Quebec (Canada). We first demonstrate how an Olympus Vanta series M pXRF analyzer was commissioned and calibrated. We then show how the pXRF data was systematically acquired and utilized during a 1:50,000 to 1:20,000 mapping campaign in a Precambrian greenstone belt setting (Abitibi Subprovince). Before the mapping began in 2021, volcanic rocks in the study area were supposed to consist of only two mafic tholeiitic formations, called Obatogamau and Bruneau. Integrating pXRF into this project meant that while still in the field and immediately afterwards during preparation of the preliminary map: (1) a marker horizon representing the top of the Obatogamau Formation was chemically identified and then traced laterally; (2) the felsic Waconichi Formation was recognized despite the high metamorphic grade and also traced laterally just above the marker horizon; (3) the Bruneau Formation was shown to be much more continuous than previously known and to have Mg-tholeiites at its base, like in the type locality; (4) the Blondeau Formation was shown to occur above the Bruneau Formation. Within the volumetrically dominant Obatogamau Formation, Fe-tholeiites were also distinguished from visually similar Mg-tholeiites, which allowed these subunits to be traced while still in the field, leading to a more detailed map. Later, after the conventional laboratory chemistry had been received, the final geological map was prepared, but it was very similar to the preliminary map, confirming the satisfactory performance of the pXRF method for this application. However, we do note some issues and limitations even with the latest generation of analyzers.

Density and magnetic susceptibility of major rock types within the Abitibi greenstone belt: a compilation with examples of its use in constraining inversion

Geophysical inversions give non-uniqueness solutions and unless constrained by appropriate initial values and geological constraints can give unrealistic results. One of the critical constraints can be the physical property values of different lithologies. We have compiled a density and magnetic susceptibility database consisting of thousands of measurements collated from different organisations and/or projects across the Abitibi greenstone belt. Statistical tools (histograms, quantile-quantile probability plots and boxplots) are applied to characterise systematically major and minor lithologies. We observed that the magnetic susceptibility frequently has a bimodal distribution, while density is typically unimodal. Our results are summarized in a table that includes the representative mean (or median) and a range of acceptable values. These values can be used to better understand the regional geology, but in this paper, we used the tabulated properties in a geophysical/petrophysical inversion of gravity data from the Chicobi area in the Abitibi subprovince to show the level of improvements that the petrophysical constraints can add to an unconstrained model. When our density database is used to seed the initial guess in a gravity inversion, an anomalous zone becomes apparent that was less evident on an unconstrained inversion.

Random forest classification for volcanogenic massive sulfide mineralization in the Rouyn-Noranda Area, Quebec

Random forest (RF) classification was applied to 37 predictor maps (vectors to mineralization) producing a Mineral Prospectivity Map (MPM) for volcanogenic massive sulfide (VMS) mineralization in the Noranda District, Abitibi subprovince, which is host to ∼ 20 VMS deposits and numerous subeconomic occurrences. The predictor maps were created using geological, geochemical, and geophysical data, and the known VMS deposits were used to train the RF classifier. The RF model was applied on two regions of interest (ROI) to investigate the effect of different sized ROIs on the results. Five sets of balanced and unbalanced training data were used in the experiments to examine the sensitivity of RF to different sets of training data. The probability/prospectivity maps showed very high success rate of classification with regard to training data particularly for the training sets with higher ratio of non-deposits/deposits. Accuracy assessment of classification maps using cross-validation also showed very high accuracy when assessed by the training data in all experiments again giving a higher accuracy for the training sets with higher ratios of non-deposits/deposits, however the later showed very low accuracy when applying k-fold cross-validation or when assessed by 15 VMS showings used as the test data indicating an overfitting problem with the unbalanced data. In most of the experiments, principal component 4 (PC4) of geochemical analysis, proximity to synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, lithology, and sericite alteration maps showed higher predictive power. The importance of individual variables changed when the ROI was changed to a smaller area suggesting that RF is sensitive to the study area.

Paleoproterozoic (Huronian) valley-controlled deglacial-fluvial sedimentation, northern Cobalt Basin, Ontario, Canada

Detailed sedimentological and stratigraphic analysis of pre-vegetation sedimentary systems that deposited across global-scale climate change episodes is key to fully understanding past feedback responses in Earth surface systems and the details of their paleoenvironments. The Cobalt Basin, recording the upper Huronian Supergroup, captures a transition from glacial (Gowganda Formation) to post-glacial (Lorrain Formation) deposystems in parallel with Earth's Great Oxidation Event and is ideal for sedimentological/stratigraphic analysis due to its limited post-depositional deformation. Stratigraphic measurements and correlations from the northern Cobalt Basin reveal that the Gowganda and Lorrain formations are preserved within a paleovalley, interpreted as a fjord. Sub-glacial deposits of the Coleman Member (lower Gowganda Formation) are preserved at the base of the paleovalley, unconformably above Archean volcanic and granitic rocks of the Abitibi subprovince. Subsequently, the fjord was filled by glaciomarine strata sourced from a grounded tidewater icesheet prior to climate warming. Above the glaciomarine deposits, deltaic deposits of the Firstbrook Member (upper Gowganda Formation) represent a transition to nonglaciogenic deposition under a warm and humid climate. The deltaic deposits comprise three facies associations: (1) delta front/slope strata dominated by horizontally stratified siltstone; (2) interdistributary bay deposits composed of symmetrically rippled and thinly laminated siltstone; (3) distributary mouth bar deposits dominated by very fine-grained arenite. Delta front/slope deposits occur directly above glaciomarine strata, while interdistributary bay and distributary mouth bar deposits occur above delta front/slope strata and are laterally discontinuous across the study area. Prodelta deposits are not preserved in the study area, although they are reported elsewhere in the basin. At the flanks of the paleovalley, alluvial fan deposits, confined by Archean rocks, interfinger with deltaic strata. Alluvial fan sedimentation, triggered by enhanced weathering in an unvegetated warm and humid setting, led to flashy sheetflood deposition due to significant topographic variations at the flanks of the valley setting. In some instances, sandstone beds of the overlying Lorrain Formation, comparable to distal to medial reaches of the modern South Saskatchewan River, cut down through delta strata. Up-section, the braidplain strata coarsen to gravel-rich facies representing Donjek River-type deposits. Overall, the stratigraphic succession in the northern Cobalt basin study area represents a single progradation from glaciomarine through deltaic deposition, into fluvial braidplain deposition. The succession is preserved at the cratonward limit of a passive margin, suggesting that progradation was likely influenced by relative sea-level fall linked to glacio-isostatic uplift, similar to fjord-filled sedimentary successions that have developed since the Last (Pleistocene) Glacial Period.

Geologic and Geochemical Features of the World-Class Archean Windfall Intrusion-Related Au Deposit, Abitibi Subprovince, Canada

Abstract The Neoarchean Windfall gold deposit, hosted in the Urban-Barry greenstone belt of the Abitibi subprovince (Quebec, Canada), represents an emerging and significant Au deposit with a resource of 7.4 Moz of Au. It is hosted in 2717 Ma bimodal volcanic rocks that are cut by several generations of calc-alkaline quartz-feldspar porphyry dikes separated into (1) a 2697.6 ± 2.6 Ma group spatially related to Au mineralization and (2) a 2697.6 ± 0.4 Ma group that truncates the earlier dikes and the Au mineralization. The Au zones are structurally controlled and localized to faults and fractures proximal to the contacts of the early quartz-feldspar porphyry dikes; these zones form thin, subvertical, and elongate lenses plunging 35° east-northeast. Gold mineralization, present as both free gold and inclusions in pyrite, occurs (1) in gray quartz veins and stockworks with pyrite and subordinate carbonate and tourmaline and (2) in pervasive to patchy sericite-silica-pyrite-carbonate ± tourmaline ± fuchsite alteration zones. The Au mineralization and associated hydrothermal alteration, along with all the host rocks that include postmineralization intrusions, are overprinted by D2 deformational features that include a penetrative fabric, shear zones, and associated folds. The spatial and temporal association of the quartz-feldspar porphyry intrusions with the Au mineralizing event at the Windfall gold deposit, along with its elemental association (Ag, As, Sb, S, Se, Bi, Te, ± Zn, Cu, Pb, Mo, W), suggests an intrusion-related model and contrasts with the more abundant orogenic gold deposits in the Abitibi greenstone belt. This interpretation has important implications both locally and regionally for Au exploration in Archean greenstone terranes.

Genesis of the Island Gold Deposit, Ontario, Canada: Implications for Gold Mineralization in the Wawa Subprovince of the Superior Province

Abstract The Island Gold deposit represents an uncommon example of known economic mineralization in the Wawa subprovince, which has been tectonically correlated with the neighboring Abitibi subprovince that hosts worldclass orogenic gold deposits. The Island Gold deposit is hosted in dacite, gabbro, and tonalite-trondhjemite, and accompanied D2 deformation. The main ore zone dips steeply toward the south and consists of early shear-hosted laminated quartz veins and late extensional veinlets; both vein sets host gold mineralization. A set of shallowly dipping extensional quartz veins in the Goudreau zone located north of the main ore zones also host economic gold mineralization. Multiple sulfur isotope analysis of pyrite associated with gold-bearing alteration envelopes in the main ore zone indicates no involvement of sulfur affected by mass-independent fractionation, which rules out sulfur (± gold) sourced from nearby banded iron formation or metasedimentary material. However, a single analysis of pyrite from an auriferous Goudreau zone vein indicates the involvement of sulfur that underwent mass-independent fractionation, which suggests a different fluid source. Zircon U-Pb geochronology of pre- and postmineralization rock samples at Island Gold restricts the timing of mineralization to between ca. 2724 and 2672 Ma. Including previous results of detrital zircon geochronology, gold mineralization occurred between ca. 2680 and 2672 Ma. This age range is similar to the timing of gold mineralization at the Hemlo deposit in the Wawa subprovince but is slightly older than the bulk of orogenic gold mineralization in the neighboring southern part of the Abitibi subprovince. Multiple sulfur isotopes indicate that gold mineralization at Island Gold results from fluids of igneous affinity associated with second-generation transpressional deformation. The ore-controlling structures were developed in bends in regional shear zones focused around relatively competent premineralization plutons.

U–Pb vein xenotime geochronology constraints on timing and longevity of orogenic gold mineralization in the Malartic-Val-d’Or Camp, Abitibi Subprovince, Canada

U–Pb vein xenotime geochronology constraints on timing and longevity of orogenic gold mineralization in the Malartic-Val-d’Or Camp, Abitibi Subprovince, Canada

Crustal architecture of the south-east Superior Craton and controls on mineral systems

Crustal and lithospheric architecture provide a first-order control on Earth evolution, including mineral systems. The ability to understand the internal nature and margins of crustal and lithospheric blocks, in both space and time, is vital in order to use continent architecture as a predictive tool in mineral exploration. In this study, we use U-Pb-Hf-O-trace element (TE) geochemical data from zircon grains in felsic magmatic rocks to isotopically map the south-east Superior Craton, producing a time-constrained architecture of the Archean crust in this area. We then assess the localization of volcanogenic massive sulphide (VMS), komatiite-hosted Ni-Cu-PGE, and syn- to post-tectonic Au systems within that architecture, constraining the first-order crustal-scale controls on these mineral systems. In terms of zircon data, at ca. >2750–2695 Ma, the central and north-west Abitibi subprovince has more juvenile εHf, light to mantle-like δ18O, lower (Eu/Eu*)/Y*10000 (drier/shallower crust), reduced ΔFMQ, less continental initial-U (Ui)/Yb, and more mantle-like Ui/Nb, relative to surrounding crust. The syn-volcanic mineral systems are localised in this juvenile zone. At ca. 2704–2695 Ma, there is a marked transition in multiple datasets, including increases in δ18O, (Eu/Eu*)/Y*10000, ΔFMQ, Ui/Yb and Ui/Nb data, and a decrease in εHf. These data represent a more evolved continental signature and the transition to a relatively homogenous regional Hf-O-TE architecture. World-class orogenic gold mineralization occurring at ca. <2680 Ma is predominantly localized into the same region as the syn-volcanic deposits, demonstrating the profound role of early architecture on the localisation of later mineral systems.

Structural and geochronological constraints on orogenic gold mineralization in the western Wabigoon subprovince, Canada

Some of the world’s largest orogenic gold deposits are hosted within granite-greenstone subprovinces of the Superior Province of Canada. The known gold endowment of the western Wabigoon subprovince is much less than that of the Abitibi subprovince, despite both Neoarchean subprovinces sharing similar structural and metallogenic histories. Further, the relationship between regional deformation and orogenic gold mineralizing events in the western Wabigoon subprovince is poorly constrained in comparison with other greenstone belts within the Superior Province. New structural mapping of the Dryden area of the western Wabigoon subprovince indicates that it experienced a N–S shortening event (D1), circa 2705–2695 Ma, which reactivated, or initiated the formation of, the E–W-trending Wabigoon and Mosher Bay – Washeibemaga deformation zones (Wdz and MBWdz, respectively). Following the D1 event, after 2695 Ma, a protracted period of NNW–SSE shortening (D2) localized transpression along the Manitou Dinorwic deformation zone (MDdz), which is spatially associated with orogenic gold systems, including the Goldlund deposit (∼1.7 Moz Au) and Kenwest prospect (∼0.3 Moz Au). During D2, the MDdz experienced NW-over-SE sinistral transpression, while the Wdz and MBWdz experienced N-over-S dextral-sense movement. U–Pb geochronology of vein-hosted xenotime from the Goldlund deposit and Kenwest prospect indicate that the main hydrothermal event, and likely gold mineralization, occurred syn- to late-D2 deformation around 2664 ± 8.3 Ma. A second population of xenotime, and overgrowths, recorded a younger hydrothermal overprint between ∼2590 and ∼2580 Ma. These ages are comparable with other absolute geochronology data on hydrothermal events in the gold-rich Abitibi subprovince. The structural evolution and timing of orogenic hydrothermal events in the western Wabigoon subprovince are broadly similar to the Abitibi subprovince. Therefore, we propose that the structural evolution and timing of individual greenstone belts is not the primary controlling factor on their bulk gold endowment. Nevertheless, orogenic gold deposits are spatially associated with major D2 deformation zones and linked to widespread hydrothermal event(s), spanning 2680–2660 Ma across the southern Superior Province.

Magmatic, hydrothermal and ore element transfer processes of the southeastern Archean Superior Province implied from electrical resistivity structure

Although magmatic- and metamorphic- derived fluids are widely recognized mineralizing agents, the role of crustal architecture in defining source and sink zones within the middle to lower crust and upper mantle of ancient orogens remains enigmatic. The globally largest and best-preserved Archean greenstone belts lie in the Superior Province, Canada. They provide an ideal location to investigate the influence of igneous construction and subsequent syn-deformational plutonism and metamorphism on the localization of metal-rich melts and fluids throughout the crustal column. Integration of three-dimensional magnetotelluric modelling and seismic reflection sections across the Abitibi subprovince reveals details of a 'whole-of-crust' magmatic and hydrothermal system. East-west low resistivity structures broadly underlie the surface traces of the major deformation zones that are host to significant gold endowment (>200 Moz), while mid-crustal cross trends suggest mineralized fluids flowed along 'pipes' within fault planes. Most low resistivity structures are inferred to represent domains containing interconnected zones of graphite and/or sulfide. These delineate relict mantle source/transit domains and crustal pathways enriched by the flow of magmas or metamorphic fluids genetically related to a late-stage pulse of ore-bearing magmatism, possibly as a result of slab break-off or delamination. Using the combined MT and seismic data, we develop a 3-D crustal-scale model which highlights how evolving orogenic architecture-controlled mass transfer and metallogenic processes developed.

Variable Modes of Formation for Tonalite–Trondhjemite–Granodiorite–Diorite (TTG)-related Porphyry-type Cu ± Au Deposits in the Neoarchean Southern Abitibi Subprovince (Canada): Evidence from Petrochronology and Oxybarometry

Abstract Most known porphyry Cu ± Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite–trondhjemite–granodiorite–diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ~2·7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu ± Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ~2·74 Ga, ~2·70 Ga, and ~ 2·69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t) ± SD values of 4·5 ± 0·3, 4·2 ± 0·6, and 4·3 ± 0·4, and δ18O ± SD values of 5·40 ± 0·11 ‰, 3·91 ± 0·13 ‰, and 4·83 ± 0·12 ‰, respectively. These isotopic signatures indicate that, although these magmas are mantle-sourced with minimal crustal contamination, for the St-Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids. Primary barometric minerals (i.e. zircon, amphibole, apatite, and magnetite–ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite–magnetite–quartz buffer as ΔFMQ, using zircon geochemistry indicates that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from ΔFMQ –0·3 ± 0·6 to ΔFMQ +0·8 ± 0·4 and to ΔFMQ +1·2 ± 0·4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of ΔFMQ +1·6 ± 0·3 and ΔFMQ +2·6 ± 0·1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate that amphibole may overestimate the fO2 of intrusive rocks by up to 1 log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e. S6+/ΣS) in primary apatite yielded ≥ΔFMQ −0·3 and ΔFMQ +1·4–1·8 for St-Jude and Clifford, respectively. The magnetite–ilmenite mineral pairs from the Clifford tonalite yielded ΔFMQ +3·3 ± 1·3 at equilibrium temperatures of 634 ± 21 °C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0·1 wt%) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results to indicate that the deposits at Côté Gold and Clifford formed from mildly (~ΔFMQ +0·8 ± 0·4) to moderately (~ΔFMQ +1·5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (~ΔFMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu–Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias.

Crustal architecture and structural evolution of a Neoarchean sedimentary basin: geological and geophysical evidence from Metal Earth Chicobi transect in the Abitibi Subprovince, Superior Province, Quebec, Canada

Our understanding of crustal architecture and tectonic evolution has advanced due to integrated geological and geophysical research on Precambrian cratons. However, it is still a heated debate whether Archean crust structure developed by an allochthonous model similar to modern plate tectonics or autochthonous model emphasizing in-situ evolution of supracrustal belts. The < ca. 2696 Ma Chicobi belt is a sedimentary basin in the central Abitibi Subprovince of the Superior Province, and as such provides a unique opportunity to contribute to this debate. It comprises both Porcupine-type turbidite and Timiskaming-type polymictic conglomerate and cross-bedded sandstone, and unconformably overlies ca. 2728–2711 Ma metavolcanic rocks, including the Amos Group to the south. The metasedimentary rocks were deposited after an early (D1 > ca. 2696 Ma) deformation event, which is expressed by internally foliated clasts in conglomerate. The Chicobi belt was regionally folded and cut by shear zones during a second (D2) deformation event, and was re-folded by Z-shaped folds during a third (D3) dextral shearing event. The D2 event is constrained between ca. 2696 Ma, the pre-existing age of detrital zircon in the Scapa Group along strike of the Chicobi belt, and ca. 2681 Ma, the new age of the cross-cutting syenitic Gémini-Saint-Éloi Pluton.New high-resolution seismic reflection and magnetotelluric surveys were conducted along the same transect across the Chicobi belt. The seismic reflection survey imaged subhorizontal to shallowly north-dipping reflective bands at depths of 3–7 km. Regional-scale inversion of new and pre-existing gravity data indicates that these reflective bands are linked to the Beauchamp Fault south of the Amos Group. Magnetotelluric inversion model shows high resistivity (>1000 Ω·m) zones under metasedimentary rocks, as well as a synform defined by low resistivity (10–100 Ω·m) zones, which aligns with folded metavolcanic rocks south of the Chicobi belt.Integration of the geophysical data with surface mapping suggests that the upper-middle crustal architecture of the Chicobi belt is dominated by D2 upright folds overlying a fault zone. We suggest that deformation of the Chicobi belt is dominated by thrusting and folding, a deformation style compatible with allochthonous growth of the Abitibi Subprovince.

Synsedimentary rifting and basaltic-komatiitic volcanism in the Pontiac subprovince, Superior craton (Canada): Implications for Neoarchean geodynamics

New field observations from the Pontiac subprovince, Superior Province, supported by whole-rock geochemistry provide strong evidence for extensional magmatism and volcanism during deposition of the ca. 2685 Ma Pontiac sediments. Contacts between basaltic and komatiitic volcanic rocks and sedimentary rocks are concordant and weakly deformed, inconsistent with an allochthonous emplacement model proposed for volcanic successions in the Pontiac subprovince. Structural observations and aeromagnetic imagery show that the volcanic rocks are present as thin (<500 m), laterally continuous (>30 km) conformable successions within the turbiditic succession and were emplaced prior to the earliest deformation of the Pontiac subprovince. Sediment-matrix breccias with fluidal and blocky igneous clasts identical to the compositions of the mafic–ultramafic rocks occur along intrusive contacts with Pontiac Group wacke. These textures are interpreted as peperite, representing the interaction of magma or lava with wet, unconsolidated sediment. Peperite was observed along the margins of tholeiitic-komatiitic rocks and calc-alkaline lamprophyre intrusions, indicating that these magmas were broadly synchronous with the deposition of the Pontiac sediments. The volcanic rocks are characterized as tholeiitic basalt and Al-depleted to Al-undepleted komatiite, have high Nb/Th, flat to positive light rare earth element (REE) slopes, and flat heavy REE slopes, consistent with partial melting of a mantle plume. The calc-alkaline lamprophyre intrusions have low Nb/Th and prominent negative light and heavy REE slopes, consistent with partial melting of a previously metasomatised mantle. The presence of autochthonous komatiitic and tholeiitic rocks is consistent with the Pontiac subprovince developing as an extensional basin within the Abitibi subprovince in response to an upwelling mantle plume ca. 2.7 Ga. This model is incompatible with uniformitarian evolutionary models for the Superior craton interpreting the sedimentary subprovinces as accretionary prisms that formed during subduction processes. Non-uniformitarian models invoking stagnant-lid tectonics predict the formation of extensional basins and ascent of depleted mantle-derived magmas associated with upwelling mantle plumes, but the ability to produce source-metasomatism magmas without subduction is highly contested. The evolution of the Pontiac basin may thus reflect a transitional period in the Neoarchean where stagnant-lid and proto-subduction styles of tectonism were operating intermittently.

Inversion of Magnetic Data Acquired with a Rotary-Wing Unmanned Aircraft System for Gold Exploration

A hexacopter unmanned aircraft system instrumented with a caesium vapour magnetometer recorded total magnetic intensity over a 5.0 km2 prospective gold area in the Abitibi Subprovince of the Canadian Precambrian Shield. The survey also included a N–S repeatability line which showed that the unmanned aircraft system was very stable in flight with average standard deviations from nominal altitude and easting being 1.8 m and 0.7 m, respectively. The total magnetic intensity map revealed the structural framework of the banded iron formations present in the survey area and showed that the gold ore zones are not directly associated with magnetic highs but rather with steeply dipping faults. The total magnetic intensity data was inverted in 3-D using unconstrained and constrained approaches with 12.5 m (northing) × 12.5 m (easting) × 5 m (depth) cells and a maximum of 20 iterations. The processed (after diurnal corrections, heading correction, and tie-line levelling) total magnetic intensity data was input directly in the unconstrained inversion algorithm. Initial model building for the constrained inversion was a much more laborious process involving the inclusion of 15 synthetic structures based on borehole magnetic susceptibility measurements and knowledge of the local geology. The results of both inversion approaches were very similar. They revealed the presence of near-vertical thin sheets, individually resolvable down to approximately 400 m. In this particular case, the straightforward unconstrained inversion yielded a realistic and detailed model of the subsurface in approximately 1 h of runtime. Unmanned aircraft system total magnetic intensity data could therefore be processed and inverted almost immediately after acquisition and have an impact on decisions made in the field while a survey is still in progress.

An Archean Porphyry-Type Gold Deposit: The Côté Gold Au(-Cu) Deposit, Swayze Greenstone Belt, Superior Province, Ontario, Canada

Abstract The Archean low-grade, large-tonnage Côté Gold Au(-Cu) deposit is the first large gold deposit discovered in the Swayze greenstone belt, Ontario, Canada. The deposit is hosted by the Chester Intrusive Complex, a low-Al composite, subvolcanic intrusion composed of tonalite, quartz diorite, and diorite that was previously constrained to ca. 2741 to 2739 Ma (U-Pb zircon). Presented here is the first detailed study of the mineralization and related alteration, along with the relative and absolute age (U-Pb, Re-Os) constraints on gold mineralization. The earliest hydrothermal stage is represented by rare Au-bearing amphibole-rich veins and breccias. The main ore stage consists of biotite-rich alteration centered on an Au(-Cu)–bearing magmatic-hydrothermal biotite breccia body with spatially related disseminated biotite and veins of both sheeted and stockwork type. Extensive fracture-controlled and replacement-style Au ± Cu-bearing muscovite alteration overprints biotite-altered rocks in the core of the deposit. Barren fracture-controlled and disseminated epidote alteration is localized to the north of the deposit and above the magmatic-hydrothermal biotite breccia. Late, texturally destructive albite alteration overprints the mineralized hydrothermal alteration in the deposit core. U-Pb isotope dilution-thermal ionization mass spectrometry and laser ablation-inductively coupled plasmamass spectrometry ages for hydrothermal titanite from amphibole (2745 ± 3 Ma) and albite (2737.5 +2.2/–1.8, 2745 ± 9, and 2736 ± 7 Ma) alteration assemblages constrain hydrothermal activity to ca. 2740 Ma. The timing of gold and sulfide mineralization is also constrained by two Re-Os molybdenite ages of 2736.1 ± 11.4 (biotite alteration) and 2746.8 ± 11.4 Ma (muscovite alteration). These new ages overlap with the ca. 2741 to 2739 Ma magmatism for the Chester Intrusive Complex, thereby suggesting a synintrusion, magmatic-hydrothermal origin for the gold mineralization and related alteration. This is significant, as it represents a new gold metallogenic event in the Abitibi subprovince, for which regional importance remains to be defined. Considering the spatial association of the deposit with a dioritic intrusion and the temporal overlap of igneous activity with alteration (i.e., amphibole, biotite, sericite) and mineralization (i.e., breccias, veins, disseminations), the deposit is interpreted as an Archean magmatic-hydrothermal ore system sharing analogies with Phanerozoic Au-Cu porphyry deposits. It suggests that Archean porphyry-type deposits can form in low-Al composite intrusions, which are known to host Cu-Mo-Au breccia, vein, and disseminated mineralization and underlie temporally and genetically related felsic to intermediate volcanic rocks that host volcanogenic massive sulfide deposits.

Petrogenesis and economic potential of the Obatogamau Formation, Chibougamau area, Abitibi greenstone belt

Unravelling the petrogenesis and stratigraphy of Archean mafic lava flows is essential to our comprehension of the early geodynamic evolution and economic potential of greenstone belts. This study focuses on one of the oldest and thickest sequences of lava flows observed in the Neoarchean Abitibi Subprovince (greenstone belt), that is, the Obatogamau Formation. The undated formation extends for more than 100 km in an E–W direction and consists mostly of aphyric and feldspar megacryst-bearing basaltic-andesite lava flows. These lava flows are tholeiitic, mostly Fe-rich, and have nearly homogeneous chemistry. Petrogenetic modelling carried out using MELTS software points to limited magmatic differentiation as most samples of mafic lava flows did not reach Fe-Ti-oxide saturation. Zircon U-Pb dating establishes a crystallization age of 2726.2 ± 1.6 Ma for a felsic unit located at an intermediate stratigraphic position in the sequence of lava flows. Constraints from stratigraphically overlying volcanic units suggest that the Obatogamau Formation was likely emplaced rapidly, possibly within a few million years and as a consequence of frequent replenishment of shallow magma accumulations. High eruption rates are consistent with short episodes of volcanic quiescence deduced from field observations, indicating non-optimal conditions for volcanogenic massive sulfide systems. The pressure and temperature of peak metamorphism deduced from amphibole chemistry, however, points to favorable conditions for the release of metamorphic fluids. The study area may thus be prospective for orogenic gold mineralization, provided that fluids had access to a source of gold and that structural conduits allowed for the channeling of hydrothermal fluids.

Structural analysis and metamorphism of the Barlow Fault Zone, Chibougamau area, Neoarchean Abitibi Subprovince: implications for gold mineralization

Faults and deformation zones are important features of Archean terranes because of their significance for structural evolution and the formation of large gold districts. In the Chibougamau area, northeastern portion of the Abitibi Subprovince, an Apogee Metal Earth seismic reflection survey identified an association between an exceptional shallow-dipping subsurface reflector zone and the Barlow Fault Zone visible at the surface. This fault zone is located adjacent to the Opatica Subprovince and shares many characteristics with well-documented major gold-bearing faults in the more gold-endowed southern portion of the Abitibi Subprovince. This study aimed to reconstruct the kinematic evolution of the Barlow Fault Zone, determine its position within the structural setting of this section of the Abitibi Subprovince, and evaluate its importance for gold potential in the northern part of the Chibougamau area. The fault zone can be divided into three segments that differ markedly in dip. Structural reconstruction and field observations are compatible with a reverse south-over-north movement related to a ductile north–south shortening event, which culminated with amphibolite metamorphism. A late normal movement is deduced at the final stage. Geothermobarometers indicate peak metamorphism conditions of 550 ± 50 °C and 6 ± 1.2 kbar. Results from this study suggest that amphibolite facies metamorphism covers a much wider area within the Chibougamau region than previously documented. The Barlow Fault Zone shares similar geometric characteristics and evolutionary history with other gold-bearing structures in the Abitibi Subprovince, but ultimately it was unable to provide optimal conditions for channelling fluid and precipitating gold. High-grade metamorphism and the limited crustal extent of the fault likely prevented the tapping of deep metamorphic fluid and magma reservoirs. The Barlow Fault Zone is interpreted as a back-thrust fault that belongs to a more extensive south-dipping fault system encasing juxtaposed tectonic slivers. This system, with amphibolite facies metamorphism, is a defining feature of the northern portion of the Chibougamau area and developed during the accretion between the Opatica and Abitibi subprovinces.

Oxygen Fugacity and Volatile Content of Syntectonic Magmatism in the Neoarchean Abitibi Greenstone Belt, Superior Province, Canada

Neoarchean syntectonic intrusions from the Chibougamau area, northeastern Abitibi Subprovince (greenstone belt), may be genetically related to intrusion related gold mineralization. These magmatic-hydrothermal systems share common features with orogenic gold deposits, such as spatial and temporal association with syntectonic magmatism. Genetic association with magmatism, however, remains controversial for many greenstone belt hosted Au deposits. To precisely identify the link between syntectonic magmas and gold mineralization in the Abitibi Subprovince, major and trace-element compositions of whole rock, zircon, apatite, and amphibole grains were measured for five intrusions in the Chibougamau area; the Anville, Saussure, Chevrillon, Opémisca, and Lac Line Plutons. The selected intrusions are representative of the chemical diversity of synvolcanic (TTG suite) and syntectonic (e.g., sanukitoid, alkaline intrusion) magmatism. Chemical data enable calculation of oxygen fugacity and volatile content, and these parameters were interpreted using data collected by electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry. The zircon and apatite data and associated oxygen fugacity values in magma indicate that the youngest magmas are the most oxidized. Moreover, similar oxygen fugacity and high volatile content for both the Saussure Pluton and the mineralized Lac Line intrusion may indicate a possible prospective mineralized system associated with the syntectonic Saussure intrusion.