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Isotopic proxies such as Hf, Nd and Pb are widely used to understand the evolution of Earth’s crust and mantle. Of these, Pb isotopes are particularly sensitive to crustal influences, and the extraction of mantle melts. We present a global compilation of Pb isotope data from syngenetic Volcanogenic Massive Sulphide (VMS) deposits, which allow us to track the evolution of Pb isotopes in deposits that are associated with dominantly back-arc and extensional oceanic settings through time.Unradiogenic Pb isotope signatures, specifically low model source µ (238U/204Pb) values, in some Archean cratons have long been recognised, yet their origin remains elusive. For example, sulphides from the c. 2.7 Ga Abitibi Belt in the Superior Province of Canada require long-lived (> 500 my) evolution of a source component to generate the Pb isotope signatures observed. Other isotope systems, such as Lu-Hf and Sm-Nd, show relatively juvenile signatures for the Abitibi Belt, suggesting decoupling of the different systems. Low µ values are evident in ore deposits and rocks from the Archean to modern settings but are most prominent in Archean settings because of their associated low 207Pb/204Pb values, unlike for younger times.Pb isotope data at a global and broad temporal scale show that periods with distinct low µ values have a marked cyclicity that coincides with the supercontinent cycle. We propose that during supercontinent assembly, portions of older unradiogenic, Pb-rich mantle are tapped and incorporated into VMS deposits. Pb, possibly enriched in sulphides, can explain the apparent decoupling of Pb from silicate-controlled isotope systems like Hf and Nd. We suggest that the source of this unradiogenic mantle component formed during the previous supercontinent cycle when large volumes are extracted from the mantle to form (radiogenic) crust and an unradiogenic residue, which most likely resides in the lithospheric mantle although some may also be present as discrete ‘pods’ in the circulating mantle. This process provides a mechanism to explain isolation of source regions for several hundred million years, as required to generate the low µ values, until later tapping during a subsequent supercontinent amalgamation cycle.The low µ values in the c. 2.7 Ga Abitibi Belt represent the best-known Archean occurrence of this signature, indicating that their unradiogenic source relates to a major mantle extraction event that would have occurred at least 500 my earlier, i.e. at about 3.2 Ga.

Mesoproterozoic orogenesis is well established on the western and southern flanks of Laurentia in the well-known Racklan-Forward and Mazatzal Orogens, but its significance within the previously assembled interior of the supercontinent Nuna has not been established. We examine regional isotopic and structural evidence for Mesoproterozoic deformation in the ca . 1.7-1.63 Ga Hornby Bay, Elu, Thelon and Athabasca intracontinental basins, and present evidence for Mesoproterozoic reactivation of Paleoproterozoic structures in Wopmay and Trans-Hudson orogens. Racklan-Forward Orogeny in the interior of north Laurentia comprises north-south trending, high angle, east-vergent folds and thrusts that occur across a region 1660 kilometers wide and over 1000 kilometers long, stretching from the Yukon to near Hudson Bay and from Banks Island to below the Western Canada Sedimentary Basin. The structures progress from ductile amphibolite and greenschist facies in the Racklan type area to subgreenschist facies and ultimately brittle or brittle-ductile in the far foreland, showing a predominant thick-skinned style typical of many intracontinental orogens. We present compiled low-temperature thermochronologic data, including ages of spatially associated uraninite mineralization, to characterize the scope of reactivation of basement structures in the Archean Rae craton in Nuna's interior. We compare the nature of widespread far-field reactivation in Racklan-Forward Orogen with other orogens of Nuna's assembly to show it is unusual for Nuna's peripheral margin. We suggest that ca. 1.6 Ga continent-continent collision of North Australia with northwest Laurentia propagated stresses far into the interior as a result of combined favourable pre-existing structural grain and a weak subcontinental lithosphere mantle in Rae craton due to repeated episodes of refertilization across 500 Ma of accretion and intrusion. Cratons that experience the complex, two-sided collision and protracted upper plate setting during supercontinent assembly noted herein may be particularly susceptible to extensive foreland propagation of peripheral orogens.

The package `PbIso` is a free and open R toolbox for commonly used calculations and plots of Pb-Pb isotope data and for generating Pb evolution models. In this paper, we review Pb isotope systematics and the calculations that are commonly used, such as model age, model source mu (238U/204Pb), time-integrated kappa (232Th/238U), and initial Pb isotope ratios. These complicated equations are implemented into simple R functions in the package `PbIso`. In addition, functions are provided for generating Pb evolution models, isochron lines, and time-dependent slopes. This allows users to apply calculations to their data in a straightforward way, while providing transparency and flexibility of the calculations used. We have implemented some basic features of the `PbIso` package into an online shiny application (https://shereearmistead.shinyapps.io/PbIsoApp/), which makes it easy for users without any R experience to use these calculations with their own data and to generate plots.

Palaeoproterozoic orogenesis within the Archean southeastern Rae craton is related to the initial amalgamation of Laurentia. Characterizing the accompanying tectonic processes during this time has been complicated due to polymetamorphism, which results in the obscuring of the age record of the terranes involved. To improve the knowledge of the tectonic evolution of the South Rae Craton, petrologic and structural analyses are applied in conjunction with in situ trace element chemistry, inclusion barometry, U–Pb monazite and titanite, and Lu–Hf garnet chronology. The data robustly constrain Palaeoproterozoic pressure–temperature–time paths of major deformational events along the southeastern Rae craton margin. D1 occurred between 1.94 and 1.93 Ga in the Dodge-Snowbird domain, which included prograde burial of metasedimentary rocks, deposited at 2.2–2.0 Ga, and the development of migmatitic layering and east-southeast trending folds (S1, F1). Peak metamorphism is recorded in metasedimentary units at c. 1.93 Ga when rocks reached conditions of 9.0–10.5 kbar and 810–830°C. Within the Dodge-Snowbird domain, D2 imparted north-northeast trending open folds and associated axial planar cleavage (S2, F2) between 1.93 and 1.90 Ga during east-west compression that appears to have been synchronous with cooling and exhumation. Later D2 deformation, localized within the Wholdaia Lake shear zone (WLsz; ST1), developed in the footwall of this thrust-sense structure at 1,873 ± 5 Ma at conditions of 9.5–11.0 kbar and 820–850°C. The hangingwall Dodge-Snowbird domain had already cooled to below 300°C by then, indicating a significant structural and metamorphic break across the domain's western boundary. A new phase of unroofing (D3) involved pervasive amphibolite- to greenschist facies extensional shearing (ST2) within the WLsz, which overprinted ST1 foliations between 1.87 and 1.86 Ga. Continued greenschist facies shearing younger than 1.86 Ga likely ended by c. 1.83 Ga when lamprophyre dykes cut the structure, which was followed by cooling until c. 1.80 Ga. This work highlights the utility and application of multiple chronometers (zircon, monazite, titanite, garnet) along with structural and petrologic analysis that together can resolve precise orogenic cycles in polymetamorphic terranes that may otherwise be undetected. The time-resolved P–T–D histories derived here enable more robust interpretations regarding the nature and evolution of 1.9 Ga tectonism along the southeast Rae craton margin, which may be used to refine models for Laurentian terrane amalgamation.

Recent discoveries of basement-hosted uranium deposits in the Patterson Lake corridor in the southwestern Athabasca Basin of Canada have brought vigorous exploration interest to the region. New lithostratigraphic constraints, geochronology and airborne geophysical surveys have dramatically improved the understanding of the host basement geology, warranting a re-examination of the remote predictive mapping and geophysical responses of the buried basement rocks. This study took a two-step approach to examine the regional basement geology and architecture. First, a mosaic of the long-wavelength response of potential field (gravity and magnetic) datasets was examined to divide the basement into regional domains based on bulk physical property variations. The interpretive geological model was then refined using textural and lineament analysis of new airborne gravity and magnetic datasets, geological drill hole logs and magnetic susceptibility measurements. The new basement map identifies and updates major features including a crustal-scale structure that separates the southern Tantato Domain from the newly defined eastern Taltson Domain. This structure may have played a role in localizing fluid flow in the Patterson Lake corridor, defining the spatial extents of structurally controlled buried felsic intrusions, and redefines the boundaries of the Taltson, Clearwater and Tantato Domains. In addition, the potential field enhancements delineated significant regional faults that controlled the geometry of Paleoproterozoic cover sequences and have implications for understanding the crustal architecture of the southern Rae Province. These new interpretations shed light on the tectonic history of the region to support on-going exploration activities and delineate regionally prospective areas in this understudied area of the Canadian Shield. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

The world’s largest Ni–Cu–Platinum group element (PGE) deposits are dominantly hosted by ultramafic rocks within continental extensional settings (e.g., Raglan, Voisey’s Bay), resulting in a focus on exploration in similar geodynamic settings. Consequently, the economic potential of other extensional tectonic environments, such as ocean ridges and back-arc basins, may be underestimated. In the northeastern portion of the ca. 2.7 Ga Yathkyed greenstone belt of the Chesterfield block (western Churchill Province, Canada), the Ni–Cu–Co–PGE Ferguson Lake deposit is hosted by >2.6 Ga hornblenditic to gabbroic rocks of the Ferguson Lake Igneous Complex (FLIC), which is metamorphosed up to amphibolitic facies. The FLIC has a basaltic composition (Mg# = 31–72), flat to slightly negatively sloped normalized trace element patterns (La/YbPM= 0.7–3.5), and negative Zr, Ti, and Nb anomalies. The FLIC rocks are geochemically similar to the 2.7 Ga back-arc basin tholeiitic basalts from the adjacent Yathkyed and MacQuoid greenstone belts (Mg# = 30–67; La/YbPM= 0.3–3.0), but the Ferguson Lake intrusions appear to be more crustally contaminated. We interpret the FLIC to have formed in an equivalent back-arc basin setting. This geodynamic setting is rare for the formation of Ni–Cu–PGE occurrences, and only few examples of this tectonic environment (or variations of it, e.g., rifted back-arc) are found in other Proterozoic and Archean sequences (e.g., Lorraine deposit, Quebec). We suggest that back-arc basin-derived mafic rocks within the Yathkyed and other Neoarchean greenstone belts of the Chesterfield block (MacQuoid and Angikuni) could represent important targets for future mineral exploration.

Deformed and metamorphosed mafic to ultramafic intrusive rocks are common along and adjacent to the Snowbird Tectonic Zone, a major Paleoproterozoic structure separating the Rae and Hearne cratons. On the Rae side of this major lithospheric structure, we investigated a series of mafic bodies that intrude high-grade paragneiss and are associated with Ni–Cu ± PGE mineralization. By linking field observations, geochronology and metamorphic petrology, our results indicate Archean (∼2.63 Ga) crystallization ages for these mafic intrusions over a minimum along-strike extent of ∼500 km and a later high-temperature (∼820 °C and ∼7.5 kbar), Paleoproterozoic metamorphic overprint at t

The Big Lake shear zone (BLsz) is a 60 km long deep crustal structure adjacent to the northern segment of the Snowbird Tectonic zone (STZ: Nunavut, Canada) in a region characterized by intensely sheared high-pressure granulite-facies rocks. The units exposed here provide an exceptional record of the lower crust that preserves crucial evidence bearing on interpretation of superimposed Neoarchean and Paleoproterozoic tectono-metamorphic events. Deformation along the BLsz postdates 2650 Ma, the age of a mylonitized monzogranite, and predates 2190 Ma, the age of cross-cutting MacQuoid mafic dykes. Metamorphic assemblages and P–T modeling suggest crustal thickening in the Neoarchean with peak conditions of 800 °C and 14–15 kbar at ca. 2530 Ma. Mylonitic fabrics developed at ca. 2505 Ma (T ~ 800 °C, P 10 °C/Ma) and partial exhumation post-ca. 2505 Ma followed by re-burial at 1900 Ma, rather than slower isobaric cooling at depth. These results indicate that the long-term residence models proposed by several authors for domains in the central STZ are not applicable to the BLsz region, and are more compatible with a ca. 1.9 Ga collisional setting.

The synoptic analysis of coeval orogens and sedimentary basins is crucial to our understanding of Earth’s crustal and surface processes. The Nonacho Group (Northwest Territories, Canada) is a major and yet enigmatic clastic sedimentary unit part of a basin system preserved over >2300 km along its original continental-sloping direction. This basin system records crustal sagging, relaxation, and flexure at ~2.3–1.9 Ga in the Rae Craton, one of the main building blocks of ancestral North America. Correlations between the Nonacho Group and coeval supracrustal assemblages in the Rae Craton has posed challenges. The group has been hitherto related to continental deposition at ~1.9 Ga, in a regime of sinistral strike-slip linked to the collision of the Rae and adjoining Slave cratons. We reappraise the Nonacho Group by means of facies analysis, stratigraphic logging, and palaeocurrent-data collection at key sites. Record of marine deposition is inferred based on the cumulative thickness of shoreface strata, an inference corroborated by sedimentological indicators such as wave- and bimodal-ripple, hummocky-cross, and flaser stratification. The alluvial to marine stratigraphy of the lower and middle parts of the Nonacho Group records basin inception and expansion in a regime of crustal relaxation. Topographic rejuvenation and deposition of younger alluvial to marine strata floored by an unconformity (upper Nonacho Group) followed in a regime of crustal flexure and renewed extension. These stratigraphic trends are tentatively related to broadly contemporaneous tectono-depositional events in the Rae Craton, including crustal thickening and ensuing extensional collapse of the bounding Thelon-Taltson and Snowbird orogens. By these means, we provide new testable hypotheses of a pan-continental tectono-sedimentary system developed during the Palaeoproterozoic amalgamation of the Canadian Shield.

New mineral deposit discoveries are required to meet the forecasted demand for some critical raw materials. Governments are responding to that challenge by investing in mineral systems research and by by making government geoscience datasets freely available to the public and explorers. However, translating conceptual mineral system models to mappable geological, geochemical, and geophysical proxies is difficult with incomplete data of variable quality from modern and legacy surveys. Herein we address those knowledge gaps and propose a new open source workflow in R for prospectivity modelling using public geoscience datasets. We focus on the largest footprints of magmatic Ni (±Cu) sulphide mineral systems and their critical raw materials (±Co ± PGE). Multiple prospectivity models are presented, including data-driven methods (e.g., weights of evidence, gradient boosting machines) that use the features of known mineral occurrences as a training set and a hybrid method that also incorporates conceptual mineral system criteria. All models are validated using data from northern Canada (i.e., north of 60° latitude) as a test set. Statistical analysis of the prospectivity results suggests that rock types and geological ages are two of the most important predictive datasets, which correspond to the sources and drivers within the mineral system framework, respectively. Variable importance plots further suggest that geological boundaries (e.g., horizontal gradient magnitude of the gravity data and multi–scale edges) and the close spatial association between areas of high mineral potential and the edges of thick continental crust represent prospective ore-forming pathways. Model performance and the best combination of predictors and hyperparameters for each model are based on the receiver operating characteristics (ROC) plots, which yield a range of area under the curve (AUC) from 0.846 to 0.923 for the spatially independent test set. Most Canadian geological provinces , possibly with the exception of the Grenville orogen for the hybrid and weights of evidence methods (AUC = 0.716–0.726), yield comparable model performance, suggesting that the heterogeneous spatial distribution of different mineral system sub-types (e.g., komatiite-associated, rift-related, Alaskan-type, and hydrothermal awaruite) have a relatively minor impact on the prospectivity results. Monte Carlo-type simulations further suggest that the expert weightings used in the hybrid method (AUC = 0.843) are only slightly better than an average model constructed from random combinations of weightings (AUC = 0.809). The general agreement between different methods and multiple iterations of the same model demonstrate that public geoscience datasets can effectively reduce the search space to support mineral exploration targeting (i.e., less than 8% of map pixels contain more than 80% of the known Ni mineralization). However, vast segments of the Canadian landmass have not undergone systematic geological surveying or data acquisition. Prospectivity modelling can thus also be used by governments and academia to prioritize areas for future targeted geoscience research.

The Trans-Hudson Orogen (THO), formed from the convergence between the Superior craton and the composite Churchill Upper Plate (CUP), is one of the best-preserved examples of a collisional orogen in the Paleoproterozoic. Similar to modern collision systems such as the Himalayan orogen, it is characterized by a composite upper plate in which terrane accretion established a continental plateau that was tectonically and magmatically active for >100 myr. Our study presents new petrological and geochronological data for four samples collected in three lithotectonic domains of the south Rae craton (one of the CUP terranes). The results presented here allow us to re-define the previously proposed extent of THO reworking in the CUP and afford the opportunity to study and compare the evolution of various fragments that illustrate differing levels of a collapsed plateau in the CUP hinterland. The new data indicate that the south Rae craton locally preserves evidence for burial at 1.855–1.84 Ga with peak metamorphic conditions at approximately 790 °C and 9.5–12.5 kbar followed by rapid cooling and decompression melting (P These results, which provide important and so far missing Pressure-Temperature-time (P-T-t) constraints on the evolution of the south Rae craton in the Northwest Territories at Trans-Hudson time, coupled with existing regional geochronological and geochemical data, are used to propose an updated model for the post-1.9 Ga THO collision and extensional collapse. Our results reveal that: i) initial thickening in the upper plate started at Snowbird time (ca. 1.94 Ga), then continued via Sask collision (with high-grade metamorphism recorded in the south Rae craton, ca. 1.85 Ga), and ended with Superior collision (ca. 1.83 Ga); ii) the extent of the THO structural and metamorphic overprint in the SW CUP is much broader across strike than previously recognized, and iii) T-t data in the south Rae are indicative of relatively fast cooling rates (8–25 °C/Ma) compared to other known Precambrian orogens. We suggest that the Paleoproterozoic THO represents the first record of a major ‘modern-style’ orogenic plateau collapse in Earth’s history.

Detailed analysis of gravity and aeromagnetic data covering the southwest Thelon Basin, Northwest Territories, Canada, has provided insight into basement geology that has significance to exploration for uranium and possibly other economic metals in a remote frontier region. Interpretation of basement geology has been constrained by the calibration of gravity and magnetic signatures with Precambrian geology adjacent to the basin and sparse seismic data within the basin, creating the first basement geologic map of the southwest Thelon Basin. The basement to the overlying sedimentary units is dominated by magnetic felsic and mafic bodies variably overlying and intruding the gneissic crystalline basement. Supracrustal belts located outside the basin margins are interpreted to continue below the basin fill. Major structures have been delineated geophysically including the Howard Lake Shear Zone and the Bathurst and McDonald fault systems. Northwest-trending structures forming part of the Bathurst fault system appear to control the unconformity surface morphology and the location of basin depocenters. The geologic interpretations are corroborated by joint gravity and magnetic modeling of profiles that reveal the deepest part of the Thelon Basin reaches depths of [Formula: see text] in an area of subdued magnetic and gravimetric response to the north. The basin is a focus of active exploration for uranium, and we have found that areas along the south and eastern margins underlain by U-rich granitoid rocks may have significant potential where intersected by reactivated faults.

The Flin Flon mining district is part of a greenstone belt, the Flin Flon-Glennie Complex, in the Paleoproterozoic Trans-Hudson orogen. Its tectonic history began prior to 1872 Ma with the development of regional folds—the faulted F1 Burley Lake syncline and F2 Hidden Lake syncline—during D1 and D2 intraoceanic accretion of the 1888 Ma Flin Flon arc to other volcanic terranes. Amalgamation of the Flin Flon to Glennie terrane, possibly during D3, produced a W-propagating thrust-fold belt and basins in which fluvial sedimentary rocks were deposited between 1847 Ma and 1842 Ma. As the fold-thrust belt migrated westward, these rocks were incorporated into a stack of E-dipping thrust sheets bounded by NNW-striking thrust faults (1920 fault) and internally folded by W-verging folds (Pipeline, Mud Lake, and Grant Lake synclines). Subsequent D4 collision of the Flin Flon-Glennie Complex with the Archean Sask microcontinent was broadly coeval with but outlasted the emplacement of 1840 Ma Phantom Lake dikes. D4 produced a second truncating fold-thrust system characterized by N-directed thrust faults (Club Lake and Railway faults) and E-trending folds (Flin Flon Creek syncline). These folds were overprinted by two regional cleavages, and the thrust faults were reactivated as oblique-slip shear zones, either late during the same collisional event (D5) or during terminal collision (D6) of the Sask craton and Flin Flon-Glennie Complex with the Superior craton at 1.83 to 1.79 Ga. The Flin Flon volcanogenic massive sulfide ore system was thrust-imbricated during D3 and D4, and ore lenses were stretched parallel to a regional, SE-plunging, stretching lineation that formed during D4 and was later modified during D5.

Three-dimensional geologic modeling by integrated analysis and interpretation of drill core, 2-D and 3-D seismic, mine survey, and geologic map data provides new insights into the structural setting of the 85.5 million tonne (Mt) Flin Flon-Callinan-777 volcanogenic massive sulfide (VMS) ore system hosted in the Paleoproterozoic Flin Flon belt of Manitoba and Saskatchewan, Canada. The resultant camp-scale 3-D geologic model was, in addition to the densely drill intersected VMS-hosting mine horizon, constrained by lithostratigraphic reference drill holes intersecting the footwall and hanging-wall rock successions of the ore system, which were established by relogging 52 drill holes and systematically reconciling the lithofacies classes in their log descriptions with the lithofacies units of the 1:10,000-scale Flin Flon mining district geologic map. The lateral persistence and large stratigraphic range of these drill hole markers supported seismic interpretation and allowed tracing lithostratigraphic horizons and structures in areas with low drill hole density, expanding 3-D subsurface insight from the local to the more regional structural setting of the ore system. The overall 3-D modeled geometry and topological relationships between lithostratigraphic surfaces and multiple generations of thrust faults suggest that the Flin Flon mining district is underlain by an E-dipping stack of W-vergent thrust imbricates that formed during precollisional and collisional stages of the 1.9 to 1.8 Ga Trans-Hudson orogeny. The imbricate stack was subsequently deformed by E-trending ductile thrust faults that internally imbricated the Flin Flon arc assemblage and the molasse cover rocks of the Missi Group and brought up rocks of the former over the latter in a northerly direction. The VMS-hosting Millrock Member has been stacked on at least four structural levels, enhancing the VMS potential in the footwall and hanging wall of the known ore deposits where both thrust systems intersect.

A 17-km2 3-D seismic survey was conducted in 2007 within the Flin Flon volcanogenic massive sulfide (VMS) mining camp as part of the Targeted Geoscience Initiative-3. The resultant 3-D images show strong reflectivity associated with the ore-hosting mine horizon and prominent reflections attributed to the known ore zones. Structural repetition of the mine horizon is clearly identified in the seismic images with at least three levels identified. The 3-D seismic images have been used in conjunction with detailed geologic and geophysical drill hole logs to identify new exploration drill targets. Results from the first three seismic-based exploration drill holes are presented. The drill holes encountered reflective geologic contacts at depths within 3 to 8% of those predicted from the 3-D seismic images. In the first case, a gabbro of no economic significance was encountered at the target depth. In the second case, a highly altered mafic volcanic panel with stringer mineralization was identified sitting blind beneath surface exposures of sandstone. In the third case, the target turned out to be an additional structural level of the mine rhyolite, which provides a new horizon for further exploration. A small massive sulfide ore lens was discovered during follow-up drilling around the third target. These results demonstrate that seismic methods can be successfully applied for mineral exploration in areas of existing infrastructure and cultural noise.

Till geochemical datasets from archived samples in west-central Manitoba are interpreted using multivariate data analytical techniques such as principal component analysis and pathfinder element index methods, combined with a spatial representation of the data in a geographic information system (GIS). Results show that carbonate till derived from Hudson Bay, as expressed by the (Ca + Mg)/Na ratio, is found predominantly east of Leaf Rapids and extends south into the Northeastern Kisseynew Domain. A 30- to 70-km-wide zone is documented beyond the limit of dissolvable carbonate dispersal that contains till of Labradorean provenance with low Ca-Mg-Mn-Te concentrations, noncalcareous till of Keewatin provenance, and silty calcareous till at depth. The variation in till provenance, together with the predominance of southwesterly ice-flow indicators, suggests that carbonate dispersal occurred prior to deglaciation, and that surface tills in the zone of confluence between Labradorean and Keewatin ice are likely the result of recycling of calcareous tills and/or carbonate-leached sandy tills, mainly by Keewatin ice. The newly defined limits of carbonate and Ca-rich till, typically used to assess provenance and glacial transport, provide a new framework for evaluating the significance of till geochemical patterns when drift prospecting over the Trans-Hudson orogen. Several regional- to local-scale dispersal trains from geochemically distinct bedrock sources are discussed in terms of elemental association, glacial transport characteristics, and potential bedrock sources. The multivariate statistical approach combined with GIS mapping also highlights new areas with small multi- or single-element enrichments that may be of further interest for volcanogenic massive sulfide exploration or for potentially more detailed drift sampling.

The Trout Lake deposit (1878 ± 1 Ma) is a rhyolite-hosted, bimodal-mafic, volcanogenic massive sulfide (VMS) deposit. It is one of the largest VMS deposits in the Flin Flon mining district, containing 21.8 Mt of Cu-Zn rich ore. The deposit underwent greenschist facies metamorphism and polyphase synvolcanic to postvolcanic hydrothermal alteration. Footwall and hanging-wall lithostratigraphic units of the Trout Lake deposit occur as an upright, steeply NE dipping, homoclinal succession of thrust-repeated volcanic and siliciclastic sedimentary units. The deposit consists of several massive sulfide ore lenses hosted within a felsic volcanic unit that represents a localized felsic eruptive center composed of flows, synvolcanic sills, and volcaniclastic lithofacies with an FIIIb-type tholeiitic to transitional rhyolite composition (Zr = 213–385 ppm; Ybcn = 47–100; La/Ybcn = 0.97–2.33). The felsic unit occurs along strike of an andesitic unit comprising an intercalation of high-MgO andesite (MgO > 5.0 wt %) and Nb-enriched andesite (Nb = 10–13 ppm) flows that are underlain by a footwall basaltic unit composed of flow lithofacies with a low-Ti tholeiitic basaltic composition (TiO2 < 0.48 wt %; Zr = 15–17 ppm; Al2O3/TiO2 = 37–45). Postvolcanic gabbros with back-arc basin basalt composition intrude the volcanic units, which are unconformably covered by a younger argillite unit deposited at ca. 900°C during partial melting of mafic lower crust. Collectively, the occurrence of high-temperature melts indicates that the Trout Lake VMS deposit formed in a hot, intraoceanic, extensional-arc geodynamic setting. Intra-arc rifting, asthenospheric upwelling, and subduction of young and hot oceanic crust are mechanisms that explain the high-temperature magmatism and are features consistent with the formation of the massive sulfide ore lenses. The back-arc basalt geochemical signatures of the postvolcanic gabbros suggest that the tectonic environment of the Trout Lake area evolved from an intra-arc rift, during the deposition of the ore lenses, to an incipient back-arc basin during the intrusion of the gabbros.

Paleoproterozoic volcanogenic massive sulfide (VMS) deposits are hosted in greenstone belts that formed during assembly of Earth’s first supercontinent, Nuna, at 2.1 to 1.7 Ga. These deposits account for a significant component of the world’s VMS tonnage and, thus, are an important global exploration target. Among these Paleoproterozoic belts, the 1.92 to 1.80 Ga Trans-Hudson orogen of Canada is the largest and best endowed in Laurentia, with over 29 deposits totaling 320 Mt of Cu-Zn (±Au, Ag, Pb) ore. Study of its deposits, districts, and tectonostratigraphic evolution enables greater understanding of metallogenic and geodynamic processes during the Paleoproterozoic. We briefly overview the Trans-Hudson orogen, its VMS deposits, and their exploration history, and illustrate that various distinct types of volcanogenic massive sulfide deposits form in predictive tectonostratigraphic sequences associated with both pre- and postaccretion volcanism. We present results from a four-year program to investigate the deposits and evolution of the Trans-Hudson orogen, highlighting various approaches at district and belt scales to improve exploration models for other structurally complex and covered Precambrian terranes.

Archean greenstone belts host a significant proportion of the world's gold, typically in deposits that formed late during greenstone belt formation and cratonization. However, this is not always the case and, in the multiply reworked western Churchill Province (wCP), orogenic lode gold deposition post-dates greenstone belt formation by nearly one billion years. The spatial link between Proterozoic gold and Archean greenstone belts in the wCP is thus particularly striking although its significance is still not fully understood. The Meliadine gold district (2.8 Moz contained Au in reserves, plus an indicated and inferred resources of 5.8 Moz Au) represents an important example of this deposit style and is hosted within the Rankin Inlet greenstone belt (RIGB), which occupies a critical, but controversial position along the largely inferred boundary between the Hearne craton and the Chesterfield block. RIGB felsic volcanic rocks (ca. 2.66 Ga) are structurally intercalated, and broadly coeval, with mafic volcanic and volcaniclastic rocks (2.66–2.64 Ga), turbidite (≤2.66–2.64 Ga), argillite, auriferous banded iron formation successions and syn-volcanic granodioritic to tonalitic intrusions (2.67–2.64 Ga). Neoarchean basaltic to andesitic volcanic rocks possess calc-alkaline to primitive arc-like tholeiitic magmatic affinities along with lesser MORB-like basaltic compositions. Geochemically evolved lavas yield depleted 144Nd/143Nd ratios (ɛNd2.66 Ga = −1.1 to +1.6) that reflect variable interaction with an evolved and hitherto undocumented Meso- to Neoarchean basement underlying the RIGB, whereas transitional, arc-like primitive tholeiitic and MORB-like basaltic samples overlap with the Nd isotopic composition of depleted mantle at ca. 2.66 Ga (ɛNd2.66 Ga = +1.6 to +2.7). These Neoarchean volcano-sedimentary panels represent the main auriferous rock package within the Meliadine gold district and are intercalated with deformed Paleoproterozoic conglomerate (≤2.50 and ≤2.155 Ga). The latter are, in turn, unconformably overlain by a geochemically distinct pillowed-basalt sequence and a unique carbonate-siliciclastic package that presumably represent the remnants of Paleoproterozoic basins and are not known to host gold. The geological setting of gold deposits thus likely reflects this favourable Neoarchean lithostratigraphy in addition to metamorphism and fluid focusing along the reactivated faults during the collision of the Hearne and combined Chesterfield block-Rae craton at 1.90–1.85 Ga.

The Nonacho Group comprises six formations of continental clastic rocks that were deposited between 1.91 and 1.83 Ga. The Nonacho Group is part of a broader assemblage of conglomerate and sandstone that was deposited atop the Rae craton in response to the amalgamation of Laurentia and supercontinent Nuna, but the details of its tectonic setting are contentious. This paper documents an outlier of Nonacho Group rocks ∼50 km east of the main Nonacho basin. Field observations and LA-ICPMS (laser ablation inductively coupled plasma mass spectrometry) U-Pb detrital zircon geochronology are integrated with previous studies of the main basin to better understand the group’s depositional history, provenance and tectonic setting. The lithology and detrital zircon age spectra of the outlier allow for its correlation to the upper two formations of the Nonacho Group. CA-ID-TIMS (chemical abrasion isotope dilution thermal ionization mass spectrometry) analyses of two fragments of the youngest detrital zircon provide a maximum depositional age of 1901.0 ± 0.9 Ma. A felsic volcanic cobble dated at ca. 2.38 Ga provides evidence of volcanism during the Arrowsmith orogeny. Detrital zircon dates recovered from the outlier (ca. 3.4–3.0, 2.7, 2.5–2.3 and 2.0–1.9 Ga) are consistent with derivation from topography of the Taltson and/or Thelon orogens on the western margin of the Rae craton. Taltson-Thelon (2.0 to 1.9 Ga) aged detritus is only abundant in the upper two formations of the Nonacho Group, marking a change in provenance from the lower formations. This change in provenance may have coincided with a period of renewed uplift and the unroofing of Taltson-Thelon plutons. The detrital zircon provenance and depositional age of the Nonacho Group is consistent with models that link its deposition to the Taltson and/or Thelon orogens. However, tectonism associated with the 1.9 to 1.8 Ga Snowbird and Trans-Hudson orogens to the east could also have affected basin formation or the change in provenance from the lower to upper Nonacho Group. This study highlights the importance of CA-ID-TIMS in establishing accurate and precise maximum depositional ages for sedimentary successions.

The northern Canadian Shield is comprised of multiple Archean cratons that were sutured by the late Paleoproterozoic to form the Canadian component of supercontinent Nuna. More than 2000 combined K-Ar and 40Ar/39Ar cooling ages from across the region reveal a stark contrast in upper and lower plate thermal responses to Nuna-forming events, with the Churchill Province in particular revealing near complete thermal reworking during the late Paleoproterozoic. We review the detailed cooling history for five regions that span the Churchill Province and Trans-Hudson orogen (THO): Thelon Tectonic Zone, South Rae, Reindeer Zone, South Hall Peninsula, and the Cape Smith Belt. The cooling patterns across Churchill Province are revealed in two >1500 km transects. At the plate scale, Churchill’s cooling history is dominated by THO accretionary and collisional events, during which it formed the upper plate. Cooling ages generally young from west to east across both southern and central Churchill, and latest cooling in the THO is 50 myr older in southernmost Churchill (Reindeer Zone) compared to eastern Churchill (Hall Peninsula), indicating diachronous thermal equilibration across 2000 km strike length of the THO. Churchill exhibits relatively high post-terminal THO cooling rates of ~4 °C/myr, which support other geological evidence for widespread rapid exhumation of the THO upper plate following terminal collision, potentially in response to lithospheric delamination.

The gold endowment of the western Churchill Province is conventionally attributed to reworking and attendant metamorphism during the Trans-Hudson Orogeny (1.9–1.8 Ga). The Meliadine gold district is regarded as a type example of this inferred Paleoproterozoic gold metallotect and also represents one of Canada’s largest emerging orogenic greenstone- and banded iron formation (BIF)-hosted gold districts (2.8 Moz contained Au in reserves and total resource of 5.8 Moz Au). The largest gold deposits are cospatial with the Pyke fault and associated quartz (± ankerite) veining, which cuts Neoarchean (ca. 2.66 Ga) plutonic and supracrustal rocks comprising the Rankin Inlet greenstone belt. Meliadine gold occurs as inclusions within idioblastic arsenopyrite crystals, at sulfide crystal boundaries, and/or as sulfide fracture fills in hydrothermally altered, sulfidized, and veined BIF. Clusters of gold, Bi-Mo telluride, chalcopyrite, and galena inclusions characterize high-grade ore zones and are spatially associated with sieve-textured arsenopyrite domains. These microtextural relationships suggest that incipient sulfide recrystallization liberated gold, which, along with other precious and base metals, was redistributed during late, fluid-assisted and deformation/metamorphic-driven remobilization. Late precious and base metal enrichment is also demonstrated by in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) arsenopyrite mapping. Hydrothermal xenotime crystals occur with gold in low-strain microtextural sites and yield U-Pb ages at ca. 1.86 Ga, suggesting that remobilization was concomitant with Trans Hudson orogenesis. New Re-Os arsenopyrite model ages range from 2.3 to 1.8 Ga and document a hitherto unrecognized, pre-1.86 Ga hydrothermal activity. The range of Re-Os model ages tends to support partial open-system behavior and/or mixing of disparate arsenopyrite generations that are evident from microtextural observations and in situ LA-ICP-MS element mapping. Replicate analyses of the two most rhenium rich arsenopyrite samples yield reproducible Re-Os model ages at ca. 2.27 and 1.90 Ga. These rhenium-rich samples are also gold poor and likely yield ages that predate gold remobilization and subsequent enrichment along arsenopyrite crystal boundaries and fractures. We suggest that pre-1.86 Ga hydrothermal activity was likely critical to the gold endowment of the district, but is obscured by reworking during the later stages of the Trans-Hudson Orogeny at 1.86 Ga. If correct, these new ages imply that the Paleoproterozoic gold metallotect, which is recorded across the western Churchill Province, in fact comprises multiple, temporally distinct gold events. Differentiating between disparate Paleoproterozoic gold events represents a critical step for effective mineral exploration in the western Churchill Province and at other reworked Archean terranes.

The link between observed episodicity in ore deposit formation and preservation and the supercontinent cycle is well established, but this general framework has not, however, been able to explain a lack of deposits associated with some accretionary orogens during specific periods of Earth history. Here we show that there are intriguing correlations between styles of orogenesis and specific mineral deposit types, in the context of the Nuna supercontinent cycle. Using animated global reconstructions of Nuna's assembly and initial breakup, and integrating extensive databases of mineral deposits, stratigraphy, geochronology and palaeomagnetism we are able to assess spatial patterns of deposit formation and preservation. We find that lode gold, volcanic-hosted-massive-sulphide and nickel–copper deposits peak during closure of Nuna's interior ocean but decline during subsequent peripheral orogenesis, suggesting that accretionary style is also important. Deposits such as intrusion-related gold, carbonate-hosted lead-zinc and unconformity uranium deposits are associated with the post-assembly, peripheral orogenic phase. These observations imply that the use of plate reconstructions to assess orogenic style, although challenging for the Precambrian, can be a powerful tool for mineral exploration targeting.

The early Proterozoic Era between 2.45 and 2.2 Ga is well known for a distinct minima in juvenile magmatism and detrital zircon abundance, an intriguing observation given its coincidence with many fundamental changes in Earth processes. A recent hypothesis seeks to explain this Siderian ‘Quiet Interval’ as the result of a plate tectonic shutdown in which extended tectonic quiescence is due to widespread lithospheric stagnation in an episodic mantle overturn regime. The model suggests that this period characterizes a ‘pre-modern’ geodynamic style and has profound implications for many geodynamic processes. We use spatially-linked chronostratigraphic and paleomagnetic databases to assess the major predictions of the model and find six of its key predictions are not supported by current data. The quiet interval includes a greater extent of contractional orogenesis and a broader range of paleopressures than previously known and is not characterized by LP–HT metamorphism proposed to have been related to higher upper mantle temperatures from decreased upper mantle cooling. Glacial conditions do not appear to have been triggered by the coincidence of the onset of magmatic shutdown with the end of mass-independent sulphur isotope fractionation and oxygenation of the atmosphere, as the initial glacial episodes predate this time. The glacial record, moreover, requires four episodes of climatic amelioration during the proposed shutdown, for which a mechanism appears lacking. A purported gap in Large-Igneous-Province formation, related to decreased mantle vigour, is not apparent. Quiet interval magmatism includes juvenile, arc-type and TTG magmatism, supporting significant crustal additions on a number of cratons. The prediction of negligible plate velocities during shutdown is not borne out by the well-constrained Superior Province paleomagnetic record. We suggest that plate tectonics did not shut down but that the Siderian Quiet Interval represents overall diminished tectonic activity during peripheral orogenesis, as is known for other relatively quiet periods following supercontinent or supercraton amalgamation.

Available geochronological (U-Pb and Ar/Ar) and geological data indicate that the Corner Brook Lake block of the Humber zone in the western Newfoundland Appalachians has unique characteristics. Grenvillian ages (ca. 1.0 Ga), which are typical for the Laurentian Appalachian margin, are absent in the crystalline basement to the Corner Brook Lake block. This makes it unlikely that the block is underlain by true parautochthonous Humber margin basement. However, the lithological makeup of its late Neoproterozoic–Early Cambrian sedimentary cover and detrital zircon populations indicate a Laurentian-type provenance. In addition, there is no geochronological or other geological evidence for a Middle Ordovician tectono-thermal event in the Corner Brook Lake block, suggesting that it escaped the penetrative tectono-thermal effects of the Taconic collision present elsewhere in the Laurentian realm. Instead, the block underwent strong regional deformation and associated peak metamorphism during the Silurian Salinic orogeny. Combined evidence, including the absence of Grenvillian ages, the presence of late Neoproterozoic ca. 600 Ma granitoid plutons, the regionally distinct early Paleozoic tectonic history of the Corner Brook Lake block, and its fault-bounded nature, implies that the block represents an allochthonous terrane. Available data indicate that significant orogen-parallel movement of the block (possibly 400 km or more) could have taken place during the Appalachian orogeny. The possibility of large-scale strike-slip tectonics, in addition to the well-documented convergent motions, has significant implications for the tectonic interpretation of the early Paleozoic evolution of the Newfoundland Appalachians.

Studies of the evolution of the Earth require a better understanding of regional geological geology, particularly for earth scientists who do not have broad, domain-specific knowledge. The Palaeoproterozoic geology of North America and Greenland has been summarized at domain level using the database system developed for the IGCP 509 project “Palaeoproterozoic Supercontinents and Global Evolution”, with additional geochronological information from the DateView database. A series of diagrams illustrate and contrast the geochronology and isotopic evolution of the Trans-Hudson Orogen and its bounding older crustal domains. These are accompanied by several time-space correlation charts and time-slice tectonic reconstructions showing the lithostratigraphy of the region. The origin and setting of uranium mineralization in Canada is discussed in order to illustrate the knowledge potential captured within these databases.

New insights into the assembly of the Nuna supercontinent are gained from detailed mapping, structural analysis, and geochronology which constrain the tectonometamorphic evolution of the Rae craton, Woodburn Lake area, Nunavut. The earliest Paleoproterozoic fabric, SP1, is a subhorizontal to shallowly dipping schistosity that helps to define a regional fold and thrust belt bracketed in age between 1.95 and 1.83 Ga. The probable ca. 1.9 Ga age and southerly vergence of this belt are consistent with formation during the 1.9 Ga collision of the Rae and Hearne cratons. The other dominant regional fabric, SP2, is related to a northwest-vergent, thick-skinned, fold-thrust belt involving Paleoproterozoic cover and Archean basement. In situ SHRIMP monazite data for three amphibolite-facies metasedimentary samples yield statistically indistinguishable ages regardless of textural location, most precisely dated at 1834 ± 5 Ma from monazite inclusions in garnet that are aligned with the external SP2 foliation. The chemical and textural features suggest synchronous, syn-DP2 monazite and garnet growth at 540–560 °C on a clockwise P–T path constrained by thermobarometry and calculated phase relationships. The extent, orientation and vergence of the tectonic thickening event indicated by these observations is most consistent with the ca. 1870 Ma collision of Meta Incognita–Sugluk block with the southeastern flank of the Rae craton, during early stages of Manikewan ocean closure. Three subsequent phases of deformation are related to final stages of amalgamation of Nuna. The widespread extent, yet contrasting times, of mid-Paleoproterozoic interior orogenic reworking recognized within the western Churchill Province negates the use of its interior Trans-Hudson-related orogens as piercing points for Nuna reconstruction.

Geochronological and thermobarometric data for two widely spaced and previously unstudied areas of the Rae craton, Canada provide important temporal and spatial links between previously investigated areas in the northern and southern Rae craton that had been interpreted to have been reworked during the ca. 2.5–2.3 Ga Arrowsmith orogeny. On southern Boothia Peninsula, upper amphibolite- to granulite-facies metamorphism, dated by in situ SHRIMP U–Pb analysis of monazite, occurred at ca. 2.64–2.60, 2.54, 2.50–2.49, 2.44, and 2.37–2.34 Ga, with retrograde monazite growth at 2.27 Ga. SHRIMP U–Pb ages of zircon in two metasedimentary rocks provide a further indication of metamorphism at ca. 2.54 and 2.50 Ga, which also corresponds to the age determined for a K-feldspar augen granite. Textural relationships indicate that the regional foliation formed between ca. 2.54 Ga and 2.50–2.49 Ga. Peak metamorphic conditions of ∼700 °C and 5–6 kbar are interpreted to have been reached during the 2.37–2.34 Ga event. In the McCann–McArthur Lake region of the southwestern Rae craton, upper amphibolite- to granulite-facies metamorphism is dated by in situ SHRIMP monazite ages of ca. 2.53, 2.47, and 2.37–2.28 Ga. Textural relationships indicate deformation initiated by ca. 2.37 Ga and terminated by ca. 2.30 Ga. Metamorphic conditions of ∼5 kbar and 700–735 °C were quenched at ca. 2.3 Ga following peak conditions of ∼7 kbar and 800 °C. The evidence for regional deformation at 2.54–2.49 Ga on southern Boothia Peninsula supports an Andean-type accretionary margin setting for the Arrowsmith orogeny. A collisional event may have terminated high-grade metamorphism in the northern region by ca. 2.34 Ga while metamorphism associated with continental arc magmatism continued in the south until at least ca. 2.28 Ga. These combined data demonstrate the continuity and broad extent of the Arrowsmith orogen along the western flank of the Rae craton, extending ∼1500 km from northern Saskatchewan to northern Baffin Island. The broad extent of this globally rare orogenic event provides an important piercing point for supercontinent reconstructions which are discussed in a companion paper. The inferred convergent tectonic activity between ca. 2.54 and 2.28 Ga is interpreted to have occurred on the flank of a growing supercontinent, a setting that could account for an apparent slowdown, but not shutdown in global tectonics.

The basal Paleoproterozoic supracrustal succession of the Churchill Province in northern Canada comprises sedimentary basins overlying the Archean Rae and Hearne domains. The Rae domain supracrustal succession (e.g. Amer, Ketyet River, Chantrey and Montresor groups) comprises four lithotectonic assemblages which include, in ascending stratigraphic position: (1) shallow marine-fluvial quartzarenite with mainly Neoarchean detrital zircons of local provenance; (2) shallow marine quartzarenite with mainly local provenance that locally includes carbonates ± mafic volcanic flows; (3) abrupt but conformable transition to basinal dark shales and overlying platformal carbonates that pass upward into siltstones and sandstones that record gradual emergence of the platform. Sandstones within this assemblage display a substantially different detrital zircon provenance signature dominated by early Paleoproterozoic zircons, with grains as young as 1.95 Ga; (4) unconformably to conformably overlying immature marine sandstones, including turbidites, with a significant component of 2.0–1.9 Ga detritus probably derived from a combination of local and distal sources. The thickness and relative compositional maturity of the basal lithotectonic assemblage 1 records a protracted period of enhanced weathering on a slowly subsiding but stable supercontinent (e.g. Kenorland). This was followed by mafic volcanism and marine transgression, consistent with partial rifting of the Rae domain and crustal extension attending supercontinent break-up (lithotectonic assemblage 2). Strata of lithotectonic assemblage 3 record basin flooding and deepening followed by platformal sedimentation along the margins of extended continental blocks. Assemblage 4 is interpreted to represent deposition in foreland basins during early amalgamation of Laurentia (part of Nuna) at ca. 1.9 Ga. The stratigraphy and detrital zircon geochronology of the Hurwitz Group, which overlies the Hearne domain, is broadly similar to the supracrustal sequence of the Rae domain, but differs through the presence of basal diamictites and lack of 2.5 Ga detritus, a major local source in the Rae. Our data suggest that the Rae and Hearne domains were assembled together by max. 1.91 Ga (i.e. max. age of deposition of the uppermost Hurwitz and Amer groups). The Paleoproterozoic stratigraphic assemblage framework for the Churchill Province compares well with that of other cratons derived from break-up of Kenorland and ultimately amalgamated in Nuna. Examples from Laurentia include supracrustal rocks that overlie the southern margin of the Superior Province (Southern Province, e.g. Huron Supergroup, Animikie Group, Marquette Range Supergroup, Whitewater Group and Mistassini-Otish Group). The Paleoproterozoic assemblages of the Southern and Churchill provinces exhibit first-order stratigraphic similarities that indicate a shared history of stable continental sedimentation followed by rifting, basin opening and basin closure.

Analysis of metallogenic data, including grade and tonnage, host-rock succession, ore and alteration mineralogy, and lead and sulfur isotope data, indicates significant secular changes in the character of volcanic-hosted massive sulfide (VHMS) deposits, which appear to be related to changes in tectonic processes, tectonic cycles, and changes in the composition of the hydrosphere and atmosphere. The distribution of these deposits, whether measured in number of deposits, tons of ore, or tons of metal, is episodic, with major peaks at 2740 to 2690, 1910 to 1840, 510 to 460, and 390 to 355 Ma. These peaks correspond to the assembly of major continental land masses, including Kenorland, Nuna, Gondwana, and Pangea, respectively. Periods when fewer VHMS deposits formed correspond to periods of supercontinent and/or supercraton stability. The VHMS deposits do not form during supercontinent and/or supercraton breakup; rather, these intervals are associated with deposition of clastic-dominated sediment-hosted zinc-lead deposits. The main exception to these generalizations is the amalgamation of Rodinia, which was not accompanied by significant VHMS formation. Rodinian amalgamation may have been dominated by advancing accretionary orogenesis, whereby the overriding plate did not go into extension. In this case, slab rollback and the associated extension to form back-arc basins would not have been common, a setting typically conducive to the formation and preservation of VHMS deposits. Large ranges in source 238U/204Pb (μ) that characterized VHMS deposits in the Archean and Proterozoic indicate early (Hadean to Paleoarchean) differentiation of the Earth. A progressive decrease in μ variability may indicate homogenization with time of these differentiated sources. Secular increases in the amount of lead and decreases in 100Zn/(Zn+Pb) relate to an increase in felsic rock-dominated successions as hosts to deposits and to an apparent absolute increase in the abundance of lead in the crust with time. The increase in the abundance of barite and other sulfate minerals in VHMS deposits, from virtually absent in the Mesoarchean and Neoarchean to relatively common in the Phanerozoic, relates to the progressive oxidation of the atmosphere and hydrosphere. The total sulfur content of the oceans also increased, resulting in the enhanced importance of seawater sulfur in VHMS ore fluids with time. In Archean to Paleoproterozoic deposits, the bulk of the sulfur was derived by leaching rocks underlying the deposits, with little contribution from seawater, resulting in uniform, near-zero per mil values of δ34Ssulfide. In contrast, the more variable δ34Ssulfide values of younger deposits reflect the increasing importance of seawater sulfur in the hydrothermal systems. Unlike Mesoarchean and Neoarchean deposits, Paleoarchean deposits contain abundant barite. This sulfate is inferred to have been derived from photolytic decomposition of atmospheric SO2 and does not reflect overall oxidized oceans. Archean and Proterozoic seawater was significantly more saline than that in the Phanerozoic, particularly upper Phanerozoic seawater. The VHMS ore fluids reflect this, being on average more saline in Archean and Proterozoic deposits. This variability introduces uncertainty into genetic models advocating brine pools or magmatic- hydrothermal contributions based on high-salinity ore fluids.

The Meadowbank gold deposits are hosted by a series of polydeformed and metamorphosed auriferous iron formations located in the Neoarchean Woodburn Lake group, western Churchill province, Nunavut. The supracrustal rocks in the Meadowbank area consist of a thick sequence of intermediate-composition volcanic rocks with intercalated iron formation, ultramafic volcanic rocks, and quartzite to quartz arenite. U-Pb geochronology gives an interpreted age of 2711 ± 3 Ma for the host strata at Meadowbank. The intermediate volcanic rocks are predominately volcaniclastic and have a geochemical composition that is consistent with an active continental margin setting. Iron formation deposition was coeval with the intermediate volcanism, as indicated by incorporation of volcanic detritus in the chemical precipitate. The geochemistry of the iron formation indicates that it precipitated from a fluid that at one time was at high temperature (>250°C), reducing, and likely acidic. Intercalated ultramafic rocks rarely show spinifex textures and have trace element ratios that are consistent with mantle plume-related undepleted mantle asthenosphere, likely recording episodic mantle upwelling associated with rifting in an active continental margin. The contact between the ultramafic volcanic rocks and the overlying quartz arenite is a disconformity marked by a quartz pebble conglomerate. However, similar geology on the structural hanging wall and footwall of the contact suggests that the conglomerate does not represent a significant hiatus in deposition, and is more likely a prograding terriginous siliciclastic unit. Three metamorphic events are recognized. The first, M1, is a cryptic greenschist facies event that is pre-D2. The second event, M2, is a mid-greenschist to amphibolite facies, syn-D2 event. M3 is the last event recognized, and is a post-tectonic greenschist facies event that is regional but inhomogeneous in extent, possibly reflecting thermal aureoles around post-tectonic 1.7 to 1.8 Ga Nueltin granites. The structural geology of the area is complex with four phases of deformation recognized, two of which had a significant effect on the geometry of the deposit. All of these regional events are interpreted as Paleoproterozoic in age. Relationships between deformation fabrics and mineralization, as well as the overall geometry of the mineralized envelopes, suggest that the deposit formed during syn- to late-D2. Superimposed on the mineralization are D4 structural elements that postdate gold mineralization. The main control on gold mineralization is replacement of magnetite by pyrrhotite and pyrite in high-strain corridors. The composition of amphiboles and chlorites associated with gold mineralization is remarkably consistent and shows no spatial or temporal variation, suggesting that it was buffered by the iron-rich nature of the host rocks. The bulk metasomatic effect on the intermediate volcanic rocks is characterized by the addition of K2O and the loss of CaO, Na2O, and MgO, with little variation in the total iron content. The elemental losses are likely the result of destruction of feldspars, stilpnomelane, and ferroactinolite, which are relatively common in the unmineralized rocks, and the formation of sericite, chlorite, and grunerite. Textural and timing relationships suggest that the mineralization is syn- to late-D2, and by inference M2. Fluid inclusions, chlorite, and arsenopyrite geothermometry all suggest that mineralization took place at about 325° to 375°C and 1.3 kbar pressure. These are lower P-T conditions than is indicated by M2 mineral assemblages, suggesting that the deposit formed during the waning stages of metamorphism. Strain partitioning due to the mechanical contrast of the iron formation preferentially localized dilational settings during D2 deformation. This allowed fluid influx, sulfidation of the iron formation, and resulting precipitation of gold to be concentrated in high-strain corridors. Geologic relations and geochronology collectively suggest that a Paleoproterozoic (ca. 1.8–1.9 Ga) deformation event was responsible for the introduction of gold into the Neoarchean supracrustal sequences.

Seven accretionary sutures, formed between 1.16 and 1.03 Ga, have been identified by different authors in the Ontario–Quebec–Adirondack (OQA) segment of the Mesoproterozoic Grenville orogen in Canada. With one exception, the inferred accretionary terrane boundaries lie within, or at the margins of the Central Metasedimentary Belt (CMB), located between the Central Gneiss Belt and the Adirondack Highlands (Central Granulite Terrane). However, geological, geochronological, and petrological data suggest that the Grenville orogen on both sides of the proposed terrane boundaries (sutures) preserves a common 1.4–1.03 Ga tectonomagmatic history, inconsistent with its origin as a post-1.4 Ga collage of exotic tectonic blocks. Features which straddle the proposed 1.16–1.03 Ga ‘sutures’, from the Central Gneiss Belt, via the Adirondack Highlands, to the Mauricie area, include: (1) Mesoproterozoic continental crust (1.5–1.4 Ga) forming the host and/or basement to younger magmatic and supracrustal suites. (2) A 1.35–1.3 Ga continental arc, remnants of which occur from the CMB boundary zone (CMBBZ) in Ontario to the Appalachians in the United States, built on the 1.5–1.4 Ga continental crust. (3) Intrusions of 1.17–1.13 Ga age in the Central Gneiss Belt (mafic suite), and the Adirondack Highlands and their Quebec extension (AMCG suite, i.e. anorthosite massifs and related granitoids). (4) Relics of 1.18–1.14 Ga sedimentary basins in the northwestern CMB and the Mauricie area. We propose that an alternative model can adequately account for the observed geology of this part of the Grenville orogen wherein, the rocks of the OQA segment were part of an Andean-type margin between 1.4 and 1.2 Ga. At 1.35–1.3 Ga, a continental magmatic arc was built upon the southeastern margin of Laurentia represented by the 1.5–1.4 Ga Mesoproterozoic continental crust. The arc split at 1.3 Ga forming an ensialic back arc basin, relics of which now occur in the northwestern part of the CMB, and the back arc basin was flanked to the southeast by an active 1.28–1.25 Ga arc. Collision between the Laurentian margin and another continent (Amazonia?) occurred at 1.2 Ga, resulting in closure of the back arc basin and initiation of thrusting along the CMBBZ. Post-collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere, upper mantle melting, and extension of the overlying crust, resulting in widespread magmatic activity at 1.17–1.13 Ga, including emplacement of the AMCG massifs. Crustal extension generated sedimentary basins now represented by the St Boniface sediments in the Mauricie area (1.18 to between 1.15 and 1.09 Ga), and the penecontemporaneous Flinton Group in the northwestern CMB. Renewed, post-collisional, granulite facies shortening commenced at 1.12 Ga, manifested as nappes in the Central Gneiss Belt, and thrusting in the Mauricie area. Continued post-collisional shortening at 1.08–1.05 Ga was more localised, resulting in reactivation of thrusting in the CMBBZ, and initiation of the kinematically compatible Tawachiche shear zone along the eastern border of the Quebec extension of the Adirondack Highlands. The characteristics of the OQA segment of the Grenville orogen can all be accounted for in the context of: (1) a 1.4–1.2 Ga, southeast facing Andean-type margin to a Laurentian upper plate, associated with northwest dipping subduction; (2) continental collision at 1.2 Ga; and (3) subsequent, continued, post-collisional shortening, without invoking accretion of exotic terranes between 1.4 and 1.0 Ga.

U-Pb baddeleyite geochronology for two Borden diabase dykes of northern Baffin Island gives an intrusion age of ca. 720 Ma, coeval with the age established elsewhere for the Franklin igneous event. Thus, the Borden dykes belong to the Franklin dyke swarm, rather than forming a separate swarm that intruded at ca. 950-900 Ma, as has been suggested previously on the basis of paleomagnetism and K-Ar ages. As a result, the paleopole from the Borden dykes can no longer be utilized to help constrain the ca. 1050-850 Ma Grenville Loop of the North American polar wander path. Reevaluation of paleomagnetic data for the dykes of northern Baffin Island suggests that Borden dyke magnetizations resulted from superposition of a steeply directed component of chemical remanent magnetization on normal and reversed primary Franklin components. The overprint direction is consistent with a Cretaceous-Tertiary age and is likely related to normal faulting and graben development during the opening of Baffin Bay.

The Raglan gabbro belt of the Ontario Grenville Orogen is coincident with the top of the Central Metasedimentary Belt boundary thrust zone, a major mid-crustal shear zone separating the Central Gneiss Belt in the footwall from the Central Metasedimentary Belt in the hanging wall. It has been suggested that the gabbros making up the belt are coeval, that they formed in a marginal basin within the Central Metasedimentary Belt, and that they formed a horizon of Theologically stiff material that controlled the localization of the top of the boundary thrust zone during its initiation as the marginal basin closed at ca. 1190 Ma. U–Pb zircon dating of plutons within the Raglan gabbro belt was undertaken to test the coeval nature of intrusions in the belt. Magmatic crystallization ages for three of the gabbros fall in the range 1246–1227 Ma, and a fourth yields a minimum age of ca. 1175 Ma. The results are permissive of a common origin for the gabbros and allow that the Raglan gabbro belt may have been related to the marginal basin, at least with respect to the later stages of its evolution. Inherited 1440–1301 Ma zircons in the gabbros suggest interaction with underlying Central Gneiss Belt crust during magmatism and support an ensialic marginal-basin model, as opposed to an island-arc model, for the evolution of the northwestern part of the Central Metasedimentary Belt.

The ∼2800-km-long Snowbird tectonic zone is one of the most controversial tectonic features of the Canadian shield. Metamorphic and in situ geochronologic data reported here reveal that a 1.9 Ga medium- to high-pressure belt extends along most of this tectonic zone. In contrast to recent interpretations, a collisional origin is indicated by the length of this metamorphic belt, tectonic thickening documented in parts of it, geological contrasts across it, and subduction-type microdiamonds within it. This collisional event marks a pre–1.865 Ga phase of the Hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of Laurentia.

The Anton terrane of the western Slave Province has been interpreted as a remnant of a meso-Archean (3500–2800 Ma) microcontinent. The largest coherent entity of the terrane is a previously unrecognized 50 by 250 km granulite complex, herein termed the Snare domain. The granulite facies mineral assemblages give the domain a characteristic magnetic signature that is distinct from areas of exposed meso-Archean basement in the Slave Province. The domain consists of interlayered 3100–2624 Ma orthogneisses and paragneisses and 2600–2585 Ma granitoid rocks; its structure was largely established during 2610–2590 Ma orogeny throughout the province that culminated in ca. 2590 Ma granulite facies metamorphism. Pressure-temperature ( P-T ) determinations yield peak conditions of 775–875 °C at 6–7 kbar. Shortly thereafter, 2–4 kbar of decompression occurred before the domain cooled below 450 °C. Decompression and retrogression textures are widespread but are most pervasive adjacent to late extensional detachment faults. U-Pb zircon age data suggest that the Snare domain was partially exhumed from mid-crustal levels ca. 2585–2580 Ma. We attribute exhumation to tectonic denudation of an overthickened crustal welt in the western Slave Province. Localization of the crustal welt in this part of the Slave Province may be related to the presence there of meso-Archean crust.