Archean Superior Province Research Articles

Mantle plume trail beneath the ca. 1.1 Ga North American Midcontinent Rift revealed by magnetotelluric data.

Whilst the 1.1 Ga North American Midcontinent Rift (MCR) system is formed in association with the Keweenaw mantle plume, the absence of a northern third rift arm or aulacogen (a general characteristic of mantle plumes) has previously not been well understood. To help clarify this unusual plume-rift relationship and to better establish the region affected by the Keweenaw mantle plume, we present the first electrical resistivity model of the MCR derived from 3D inversion of EarthScope USArray and Lithoprobe magnetotelluric (MT) data, extending northwards into the Archean Superior Province. Our model shows a prominent highly conductive anomaly trending NW-SE at the base of Western Superior's cratonic lithospheric mantle, cross-cutting and extending for over 300km on both sides of the western rift branch. We propose that this anomaly reflects the ancient signature of a plume trail, resulting from metasomatism and/or partial melting of the sulfide-rich basal lithospheric mantle during impingement of the Keweenaw mantle plume.

Oblique lateral extrusion during dextral transpression along the Beardmore-Geraldton belt, Canada

The Beardmore-Geraldton belt (BGB) is a boundary zone between the Wabigoon and Quetico subprovinces of the Archean Superior Province, Canada. It consists of interleaved metavolcanic and metasedimentary panels that were imbricated during an early D1 thrusting event and regionally folded during a D2 event. D1 and D2 steepened stratigraphy and produced contact-parallel shear zones and a regional bedding-parallel cleavage, which controlled the development of a D3 dextral transpression zone. During D3, the panels deformed by pure shear and were overprinted by a second regional cleavage. Bedding and the early cleavage acted as subvertical planes of anisotropy that were reactivated during D3 dextral shearing. Z-shaped drag folds and pre-existing D1 and D2 folds are parallel to a shallowly-plunging (20°–30°) stretching lineation, which together with the fold axes rotated towards the extensional flow apophysis during D3 shearing. As the lineation and fold axes are oblique to the sub-horizontal shear direction expressed by the intersection between synthetic shear bands and the early cleavage, the reactivated shear zones are transpression zones with shallowly-plunging lateral extrusion and triclinic symmetry. Thus, the shear zones are transpressive within a regional tectonic transpression zone that extends across the BGB and over 500 km along the Quetico-Wabigoon boundary.

Comparative orotomy of the Archean Superior and Phanerozoic Altaid orogenic systems.

We compare and contrast the materials and mechanisms of continental crustal growth in the largest preserved regions of Phanerozoic and Archean juvenile additions to the crust, to test for similarities or differences in the formation of continents through time. We accomplish this through a comparison of map patterns, lithological contents, and structural and metamorphic evolution of the Phanerozoic Altaid orogenic system of Asia, with the Archean Superior Province of the North American Craton, using a method termed comparative orotomy. Both orogenic systems consist of collages of curvilinear belts of eroded arcs, some older continental slivers, and vast tracts of former subduction/accretionary complexes. These contain numerous shreds of portions of the ophiolite suite, slivers of island and continental arcs, and accreted oceanic plateau, all intruded by multiple magmatic suites during or between multiple deformation events, then sliced by large transcurrent fault systems and bent into large oroclinal structures. We make this comparison because the Superior Province is a typical Archean craton that was later, in the Paleoproterozoic, incorporated into the larger North American Craton, and has occupied a central position in several supercontinents (e.g. Kenorland and Nuna, which then formed the core of Columbia, Rodinia, Laurentia and Pangea) during its longevity. Since it is the largest single fragment of Archean continental cratonic lithosphere preserved on Earth, the Superior Province is widely regarded as a testing ground for how Earth's continental crust was formed. Likewise, the Altaids encompass the largest region of crustal growth for the Phanerozoic. Our comparison with the Altaids is needed, as in recent years many myths about how the planet may have responded to higher heat production and flow in the Archean have emerged, because of trends in the science where regional geology is ignored in favor of numerical models, isotopic proxies for assumed models of chemical behavior for crust-forming or tectonic processes, or comparisons with other-worldly bodies that bear little resemblance to our hydrous Earth. Thus, we return to the geological record, and here describe the map patterns, lithological associations, structural patterns and evolution of both the Altaids and Superior Province, showing how comparative tectonics, orotomy, is useful in the absence of meaningful paleomagnetic or biostratigraphic data. We pay particular attention to the style of preservation of disaggregated members of the ophiolite suite (ophirags) and their relationships with other tectonic units, and to the widespread but largely overlooked role of late-stage major transcurrent motions and structural slicing of both Archean and Phanerozoic orogenic systems in defining the present-day architecture of both orogenic systems.

Structural geology of the Cadillac Group along the Malartic segment of the Larder Lake Cadillac deformation zone, Quebec, and implications for gold mineralization

The gold-endowed Larder Lake Cadillac deformation zone (LLCDZ) in the Abitibi greenstone belt of the Archean Superior Province generally trends east but swings to the southeast along the Malartic segment in Quebec. Immediately north of the LLCDZ along this segment, deformation of the less than ca. 2687 Ma metasedimentary Cadillac Group began with the formation of isoclinal F1 folds, which are overprinted by post-2676 ± 2.0 Ma regional F2 folds with an axial planar S2 cleavage oriented parallel to the trend of the Malartic segment. Auriferous quartz veins strike east–west (085°) anticlockwise to bedding and S2. They are folded into sigmoidal S-shaped folds due to sinistral shear parallel to bedding. Later dextral shearing segmented the veins and folded the F2 folds into Z-shaped F3 folds. The veins are similar in structural chronology, fluid isotopic composition (calculated [Formula: see text] value between –9.5‰ and –3.4‰ and [Formula: see text] values of 12‰), and isotopic re-equilibration temperatures (469 and 498 °C, quartz–biotite oxygen thermometer) to the veins at the nearby Canadian Malartic deposit to the east. They possess comparable Au–As–Sb association and sericite-arsenopyrite alteration halo, as other Cadillac-Group-hosted veins at the Lapa mine to the west. Collectively, they constitute a vein field that extends across the entire length of the Malartic segment. Contrary to recent interpretations of the Malartic segment as an early accretionary structure that controlled the emplacement of gold deposits, its southeast trend is interpreted as the short limb of a broad Z-shaped flexure that formed during later F3 folding of the LLCDZ.

The Abitibi-Opatica transition, Superior Province, Quebec, Canada: Structural analysis, 40Ar/39Ar thermochronology and implications for Archean tectonics

We present a structural and metamorphic study of the Abitibi greenstone belt (AGB) and Opatica Plutonic Belt (OPB) of the Archean Superior Province. The AGB-OPB contact is considered as an archetype example of Archean subduction, based on a LITHOPROBE seismic profile showing a North-dipping lithospheric-scale reflector interpreted as the vestige of subduction. Our mapping indicates that the AGB overlies the OPB, and that the AGB-OPB contact does not show evidence of significant shear deformation, as expected for a major upper plate-lower plate boundary. There is no metamorphic break at the AGB-OPB transition but rather a progressive increase of metamorphic grade toward the OPB. Therefore, we think that the OPB simply exposes the deepest part of a composite AGB-OPB sequence. 40Ar/39Ar ages suggest that the AGB and OPB rocks were exhumed from amphibolite-facies conditions at ∼ 2678 Ma and ∼ 2668 Ma, respectively, and then that both terranes share the same succession of thermal disturbances at c. 2650 Ma and 2600 Ma, the latter corresponding to shearing along the Nottaway River Shear Zone. We suggest that progressive cooling of both assemblages was accompanied by strain localisation along strike-slip shear zones and occurred when lateral flow of the lower crust became predominant over the convective overturn and vertical crustal material transfer. Comparison with adjacent areas suggests that regional metamorphism has been coeval over a large region, which is consistent with pervasive deformation and slow cooling as expected for vertical tectonic models and diapiric magmagenesis and ascent during the Archean.

Early depositional and magmatic history of the Beardmore-Geraldton Belt: Formation of a transitional accretionary belt along the Wabigoon-Quetico Subprovince boundary in the Archean Superior Craton, Canada

The Beardmore-Geraldton Belt (BGB) is a major gold camp along the boundary between the granite-greenstone eastern Wabigoon subprovince and the metasedimentary Quetico subprovince of the Archean Superior Province, Canada. New detrital zircon geochronology suggests that the metasedimentary rocks of the BGB and northern Quetico subprovince were derived from the erosion of the eastern Wabigoon subprovince that is dominated by ca. 2900–2680 Ma rocks but also comprises crust as old as > 3200 Ma. Uplift of the eastern Wabigoon subprovince as a result of convergence and compression along its southern margin exposed those rocks to erosion and shed sediments in the Quetico basin to the south. Sedimentation in the Quetico basin is bracketed between ca. 2700 Ma, the age of the youngest detrital zircon population, and 2694.0 ± 1.0 Ma, the crystallization age of cross-cutting feldspar-quartz porphyries (FQP). Continued convergence facilitated the thrust-imbrication of the metavolcanic rocks of the eastern Wabigoon subprovince with the metasedimentary rocks of the Quetico basin resulting in the assembly of the BGB. This resulted in crustal thickening, the introduction of hydrated mafic rocks at the base of the crust, and partial melting of these rocks to form TTG suite melts at depths >50 km, as suggested by the geochemical characteristics of the FQP intrusions. Imbrication and thrusting ended with the emplacement of the stitching, 2690 ± 1 Ma Croll Lake Stock, which has a high transition elements and ferromagnesian oxides signature of sanukitoid suite rocks. It formed during delamination of the lower crust or slab break-off from magmas generated by mixing of hot, mantle material with TTG suite melts at depths > 50 km. The early geological evolution of the Geraldton gold camp is broadly similar to that of other major Archean gold camps associated with major faults and/or terrane boundaries, where deposition of fluvial conglomerate and turbiditic sandstones in fault-bounded basins overlying older volcanic rocks is accompanied by magmatic activity and the upward migration of gold-bearing hydrothermal fluids, while differences between the Geraldton and other camps may be explained by craton-scale variations in geodynamic processes.

Forging isotopically juvenile metamorphic zircon from and within Archean TTG gneiss: Whole-rock Sr-Nd-Pb and zircon U-Pb-Hf-REE constraints

The Winnipeg River terrane is one of three plutonic-gneiss terranes that contains the oldest rocks in the Archean Superior Province and is integral to understanding the evolution of Earth's largest Archean craton. We evaluate the evolution of the Winnipeg River terrane using whole-rock Sr-Nd-Pb isotope data for a suite of 17 samples of the Cedar Lake gneiss, along with U-Pb-Lu-Hf isotopes and trace elements in zircon grains from one sample. Most whole-rock Sr-Nd-Pb isotope data trend along 3.25 Ga reference isochrons, which overlap with the dominant population of ca. 3.25 Ga zircon, and are interpreted to mark the igneous crystallization of the protolith to the Cedar Lake gneiss. An older population of zircon aged ca. 3.5–3.4 Ga indicate either Hadean or Eoarchean crust was reworked at ca. 3.25 Ga. At ca. 2.7 Ga, new zircon grew with limited dissolution of older grains resulting in near-chondritic time-integrated Hf isotope signatures and low Th/U ratios relative to the older grains. As a result of the overlap between whole-rock reference isochrons at 3.25 Ga and the dominant zircon population ca. 3.25 Ga, the radiogenic Hf isotope signature of the ca. 2.7 Ga zircon grains imply Hf was recycled from non-zircon constituents within the rock rather than from external input. Additional support is drawn from elevated Nb and Ta concentrations in ca. 2.7 Ga zircon, hornblende and biotite. Our data highlights a complexity in the LuHf isotope systematics of zircon, where metamorphic zircon inherited radiogenic Hf isotope compositions as a result of internal redistribution of Hf from non-zircon phases in the rock rather than juvenile input.

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.

Synsedimentary deformation structures within the Paleoproterozoic Espanola Formation, Huronian Supergroup, Canada: Implications for basin tectonism and paleodepositional conditions

The Espanola Formation represents part of the lower Huronian Supergroup (2.45–2.2 Ga), which is interpreted to have been deposited in a restricted, fault-bound, intracontinental rift basin during Early Proterozoic episodes of crustal stretching and contemporaneous faulting along the southern margin of the Archean Superior Province. Field examination of Espanola Formation outcrops exposed north of Elliot Lake, Ontario, Canada, revealed widespread synsedimentary deformation structures, which can be broadly grouped into three main categories: (1) forceful-injection structures, (2) slump structures, and (3) soft-sediment deformation structures. Examples of these structures include clastic dykes, large-scale slump structures, intraformational conglomerate, load casts, ball-and-pillow structures, convolute laminae, and dish-and-pillar structures. Most of the reported deformation structures are restricted to discrete stratigraphic horizons, are laterally continuous on an outcrop scale, are abundant and widespread across the studied region and are confined between undisturbed strata of similar lithology. These characteristics strongly indicate an origin by liquefaction and fluidization mechanisms that were triggered by syndepositional rift-related seismic activity, inferred to have been induced by movement along bounding normal faults during subsidence of the lower Huronian basin. Autogenic triggers such as overloading by rapid sedimentation, slump emplacement, inverse density gradient, gravity-driven sediment transport and mobilization, and storm waves may have also locally contributed to the development of the reported soft-sediment deformation structures.

Altered Mantle Fabric Beneath the Mid‐Continent Rift

AbstractWe present new, densely sampled shear‐wave splitting results from southern Minnesota and adjacent areas of neighboring states, sampling the southwestern limit of the Archean Superior Province and straddling the Proterozoic Mid‐Continent Rift (MCR). The new measurements include data from the Earthscope Transportable Array (TA) as well as the Superior Province Rifting Earthscope Experiment (SPREE), yielding 99 new station‐averaged measurements. The split times show a consistent decrease from 1.1 s in the NE to 0.2 s in the SW, with the lowest values being associated with the Minnesota River Valley Terrane (MRVT). From modeling and other geophysical constraints, we interpret the split time variations to represent variations in fabric strength within a thick lithosphere, rather than lithospheric thinning or a multi‐layered effect, and propose that the weak fabric of the MRVT is associated with a different mechanism of formation than elsewhere in the Superior Province. The fast directions we measure range from NNE‐SSW to E‐W and vary on a shorter length scale than the split times, with a pattern of NE‐SW splits that closely follows the axis of the MCR. We interpret this as a perturbation of the net fast direction due to anisotropy in an underplate along the rift.

Geophysical insights into Paleoproterozoic tectonics along the southern margin of the Superior Province, central Upper Peninsula, Michigan, USA

The southern margin of the Archean Superior Province in the central Upper Peninsula of Michigan was a nexus for key Paleoproterozoic tectonic events involved in the ~2.1 Ga rifting of proposed Archean supercraton Superia and subsequent assembly of Laurentia. Interpretations of the region’s tectonic history have historically been hampered by extensive Pleistocene glacial and Paleozoic sedimentary cover and a lack of appropriate geophysical data. These rifting and orogenic events formed geologic effects that are readily mappable with modern geophysical methods. New aeromagnetic and gravity data provide a critical means of mapping and interpreting the complex geological framework through cover, allowing development of significantly richer geographical and process-based perspectives on all these tectonic events. Interpretations of Precambrian contacts and structure are here, for the first time, carried >30 km eastward under Paleozoic cover. Effects of ~2.1 Ga rifting are strongly expressed geophysically, including the Dickinson Group, perhaps a unique record of the progression of rift-related sedimentation and magmatism, shown here to be a geographically extensive and largely concealed tectonic feature of the southern Superior Province. The geophysical evidence for plausible ~2.1 Ga rift-related intrusive magmatism includes a previously unrecognized swarm of northeast-striking mafic dikes cutting Archean rocks and gravity lows produced by granites. Effects of the ~1.87–1.83 Ga Penokean orogeny include gravity and magnetic gradients and pattern breaks along the Niagara fault zone suture, abundant evidence for thin-skinned thrusting and folding in the Menominee iron district, and speculative emplacement of an allochthonous sedimentary sequence in the Calumet trough. Numerous east–west trending structures imaged geophysically likely originated, or were significantly reactivated by, post-Penokean deformation. Metamorphic events at ~1.76 Ga and ~1.65 Ga may correspond to orogenies involving younger, outboard Paleoproterozoic crustal provinces recognized in southern Laurentia. For example, the previously unrecognized West Branch fault, separating the Dickinson Group from Archean rocks, is shown to be a major structure in the region, and is a proposed expression of ~1.76 Ga thick-skinned deformation. Oblique disruptions of crudely east–west striking structures have robust geophysical expressions and are speculatively connected to transpressive deformation at ~1.65 Ga. These new geophysical observations and interpretations collectively help illuminate a critical period in the tectonic evolution of Laurentia, as it transitioned from a disparate array of Archean cratons to a more coherent, growing continent.

Thermal properties of carbonatite and anorthosite from the Superior Province, Ontario, and implications for non-magmatic local thermal effects of these intrusions

Igneous intrusions are important to the thermomechanical evolution of continents because they inject heat into their relatively cold host rocks, and potentially change the distribution of radiogenic heat production and thermal properties within the crust. To explore one aspect of the complex evolution of the continental crust, this paper investigates the local thermal effects of two intrusive rock types (carbonatites and anorthosites) on the Archean Superior Province of the Canadian shield. We provide new data on their contrasting properties: rock density near 298 K, thermal diffusivity, and heat capacity up to 800 K (which altogether yield thermal conductivity), plus radiogenic element contents. The volumetrically small carbonatites have widely varying radiogenic heat production (2–56 µW m−3) and moderate thermal conductivity at 298 K (~ 1 to 4 W m−1 K−1) which decreases with temperature. The massive Shawmere anorthosite has nearly negligible radiogenic heat production (< 0.002 µW m−3) and low, roughly temperature-independent thermal conductivity (~ 1.6 W m−1 K−1). Steady-state thermal structures within and around these intrusions, which have quite different shapes and physical properties, were modeled using a pipe geometry for carbonatite and a tabular geometry for anorthosite. We found that the thermal aureoles of these intrusion types persist for hundreds of millions of years after the magmatic heat advected by the intrusions has dissipated. Longevitity of aureoles is due to the high radiogenic element concentrations of the small carbonatite intrusions, and to the low thermal conductivity of the Shawmere anorthosite. Our findings apply to other anorthosite bodies, which vary little in composition and mineralogy, whereas results for carbonatites depend on variations in their radiogenic content.

On Archean craton growth and stabilisation: Insights from lithospheric resistivity structure of the Superior Province

The nature of lithospheric evolution and style of the driving tectonic processes during the growth and stabilisation of continental crust in the Archean remain enigmatic. The hotter, rheologically weak Archean crust would be unable to support thick orogens. Thus, gravitational collapse likely occurred when continental fragments became overthickened and/or when far-field stresses were relieved during the terminal stages of orogenesis. 3D magnetotelluric resistivity models of the Archean Superior Province, reveal the presence of low resistivity zones in the mid-lower crust that reflect a protracted history of magmatic-hydrothermal activity contemporaneous with construction and collapse. These include sub-vertical zones of low resistivity in the mid-upper crust, inferred to represent corridors of paleo-fluid flow along crustal-scale structural networks developed in response to terrane amalgamations. Subsequent orogenic collapse resulted in widespread lateral flow within the lower crust accommodated by sub-horizontal shear zones and included magmatic refertilisation. Thus, the preserved low-resistivity anomalies in the mid-lower crust represent an amalgamation of magmatic-hydrothermal and deformational processes that occurred during construction, peak orogenesis, and collapse in the Archean.

Timing and duration of Archean orogenic gold deposits in the Bourlamaque pluton, Val d’Or mining camp, Abitibi, Canada

In the Val-d’Or mining district of the Archean Superior Province, the relationship between regional metamorphism/deformation and the timing of orogenic vein-type gold mineralization is debated; some authors arguing that gold mineralization was coeval with the regional D2 deformation event and broadly synchronous with metamorphism, and others suggesting that it is 60 to 100 Ma younger than peak metamorphism. Using the 40Ar/39Ar method and the compilation of U-Pb or Pb-Pb ages of ore-related minerals, this contribution presents a geochronological study on the timing and the inferred duration of the formation of vein-type gold deposits hosted by the Bourlamaque pluton in the Val d’Or mining district. We present an extensive set (49 samples) of 40Ar/39Ar CO2 laser probe data acquired on amphibole and white mica single grains from three major gold deposits, the Lac-Herbin, Beaufor and Beacon-2 mines. In each deposit, gold mineralization is host by shear and extensional quartz veins associated to steeply south-dipping networks of ductile shear zones. Barren north-dipping faults also occur at each deposit and are attributed to post-mineralization structural event(s). Amphiboles 40Ar/39Ar analyses yield magmatic crystallization ages clustering around 2704 Ma and preserve evidences for at least two subsequent disturbances linked to probable hydrothermal circulation events at c. 2667 Ma and c. 2652 Ma. White micas 40Ar/39Ar age spectra from auriferous quartz veins and hosting mylonitic shear zones are typical of long-lasting and sequential dynamic recrystallization and suggest that Au-bearing hydrothermal circulation was establish, at least, around c. 2650 Ma, and followed by a succession of hydrothermal pulses at c. 2597, 2575, 2551, 2500 and 2452 Ma. 40Ar/39Ar ages distribution among the different deposits as well as between the auriferous quartz veins and hosting shear zones are consistent with fluids production and circulation along an active network of seismically active fault zones.

Geochemistry of the Paleoproterozoic Espanola Formation, Bruce Mines—Elliot Lake area, Ontario, Canada: implications for provenance, paleo-weathering, and tectonic setting

The Espanola Formation of the Paleoproterozoic Huronian Supergroup is a mixed carbonate-siliciclastic succession that is well exposed in the Bruce Mines-Elliot Lake Area, Ontario, Canada. The siliciclastic portion of the formation is mainly dominated by shale with subordinate content of wake and arkose. Elemental geochemical analysis of the rocks was conducted in order to decipher their provenance, paleo-weathering conditions, and plate tectonic setting. The analyzed Espanola Formation samples display steep REE distribution patterns with high LaN/YbN ratios ranging from 3.68 to 20.83 (average LaN/YbN = 16.4), and are characterized by consistently negative Eu anomalies (average Eu/Eu* = 0.69). Furthermore, on the La-Th-Sc ternary model, the samples cluster around the average granodiorite and tonalite-trondhjemite-granodiorite (TTG) compositions with a clear linear trend toward felsic sources. A similar distribution and trend are also displayed on the Co/Th and La/Sc ratio plot. Such geochemical characteristics indicate derivation from an exposed upper continental crust of granodioritic and TTG compositions, namely the Archean Superior Province to the north, with minor contributions from mafic volcanic and/or sedimentary rocks, as further demonstrated by provenance discriminant models (La-Th-Sc and Co/Th-La/Sc). The source rocks were subjected to relatively low to moderate chemical weathering as reflected by their generally low chemical index of alteration values (average CIA = 62.90). Tectonic discriminant models (K2O/ Na2O-SiO2 and SiO2/Al2O3-K2O/Na2O) indicate significant contributions of siliciclastic sediments from sources within a stable passive continental margin, with minor contributions from an active continental margin and continental island arcs. The Zr/Sc-Th/Sc model further confirms that the composition of the analyzed Espanola Formation siliciclastic samples was mainly controlled by tectonic setting and provenance with minimal influence of sediment recycling.

The use of lithogeochemistry in delineating hydrothermal fluid pathways and vectoring towards gold mineralization in the Malartic district, Québec

The world-class, oxidized intrusion-related Canadian Malartic gold deposit, with reserves estimated at 5.56 Moz Au grading 1.10 g/t Au, and a total geological endowment of 16.3 Moz Au, is one of the largest gold deposits in the Archean Superior Province of Canada. The gold mineralization is hosted predominantly by Pontiac Group metasedimentary rocks, Piché Group metavolcanic rocks, and quartz monzodiorite to granodiorite porphyritic intrusions. The ore takes the form of a low-grade envelope of disseminated pyrite (0.35 to 1 g/t Au) grading inwards into higher grade (>1 g/t Au) stockwork and breccia zones. Hydrothermal alteration in the metasedimentary rocks is zonally distributed around the fluid pathways. Proximal alteration is characterized by a microcline±albite-quartz replacement-type assemblage, with lesser phlogopite, calcite±Fe-dolomite, pyrite and rutile. The distal alteration assemblage comprises biotite, microcline±albite, phengite, quartz, calcite, pyrite and rutile.In this study, we assess the magnitude and distribution of fluid-rock interaction in the metasedimentary rocks of the Malartic district. The metaturbidites are separated into four lithotypes based on grain-size to reduce the effects of primary depositional processes on mass change calculations. Despite the variability in protolith composition, the metasedimentary rocks define a geochemically consistent, cogenetic sequence. The results of the mass transfer calculations indicate progressive gains in CO2, S, K2O and LOI, as well as Au, Te, W, Ag, As, Be, Sb, Bi, Mo and Pb, from background, to distal and proximal alteration zones (relative to the least-altered samples). Molar element ratio analysis (alkali/aluminum) indicates an increase in alkali metasomatism (K and Na) adjacent to the main hydrothermal fluid pathways, which is manifested by the progressive stabilization of microcline and albite at the expense of oligoclase, biotite and white mica.Ore-associated pathfinder elements delineate broad enrichment patterns around the deposit, and are used to understand hydrothermal fluid circulation in the Malartic district. A statistical approach based on a comparison of the mass change results with the background composition provides robust constraints on the magnitude and extent of the lithogeochemical haloes. Generally, the alteration forms envelopes that extend along the S2 fabric, with the largest lithogeochemical anomalies (e.g., Au, W, Te and Ag) reaching up to 10 km in length, and 2 km in width. The results of this study demonstrate that whole-rock lithogeochemistry can provide a valuable tool with which to define vectors toward gold mineralization in a regional exploration context.

Growth, overprinting, and stabilization of Proterozoic provinces in the southern Lake Superior region

New geochronologic data in the southern Lake Superior region provide key information on the timing and nature of tectonic activity that pre-and post-date initial Paleoproterozoic growth of Laurentia during the geon 18 Penokean orogeny. The obducted Pembine ophiolite formed along the edge of a Paleoproterozoic ocean basin at least 30 m.y. prior to Penokean island arc/microcontinent accretion beginning at 1860 Ma. Following Penokean orogenesis, intrusion of mafic dikes at 1817 ± 2 Ma indicate a period of extension that coincided with a 30 m.y. gap in orogenic felsic magmatism at 1835–1805 Ma (between the Penokean and Yavapai orogenies) and likely represents relaxation of Penokean compression and a tectonic switch to intra-arc extension related to initiation of Yavapai subduction. Subsequent Yavapai arc accretion (1750–1720 Ma) resulted in pervasive ductile deformation of the dikes and host rocks at temperatures of ~700 °C, previously attributed to Penokean deformation. Geon 16 Mazatzal overprinting of the accreted Penokean and Yavapai provinces was widespread but of overall lower metamorphic grade (greenschist facies), and the thermal effects of the 1476–1470 Ma shallow level Wolf River batholith was limited to a 10–15 km wide contact zone surrounding the intrusion.In contrast to the Archean Superior Province to the north, Paleoproterozoic terranes in the southern Lake Superior area experienced widespread low-temperature reheating and cooling of shallow crustal levels at ca. 1.1–1.0 Ga attributed primarily to magmatic underplating with little subsequent Neoproterozoic exhumation. In the southern Lake Superior region widespread magmatic underplating likely thickened, strengthened, and stabilized Proterozoic Penokean-Mazatzal lithosphere but destabilized Archean cratonized Superior Province lithosphere to the north.

Topological characterization of a polyphased gold-bearing vein network

It is challenging to evaluate the connectivity of metal-bearing hydrothermal vein systems, i.e. the degree to which fractures or veins are connected within a network. Moreover, many ancient ore-bearing vein networks have been deformed and metamorphosed, which hinders the reconstruction and quantification of their primary characteristics. Here, we explore the reliability and limitations of the network distribution and topologic (classification of vein terminations into I, Y and X nodes) approaches to study deformed and metamorphosed mineral deposits. We focus on the Cheechoo gold deposit located in the Eeyou Istchee James Bay area of Quebec, Canada (Archean Superior Province). This gold deposit shows a complex hydrothermal and magmatic polyphased gold-bearing vein system hosted in a granodioritic-tonalitic intrusion. The Cheechoo deposit is composed of quartz-feldspar-diopside (Qtz-Fsp-Di) and quartz (Qtz) veins that underwent ductile deformation and amphibolite facies metamorphism.The high-grade metamorphism led to the modification of vein shape, relative position and spacing, making these parameters unreliable for the characterization of the Cheechoo vein system and similar metamorphosed networks. However, the number of veins per length unit and topology are reliable tools to characterize such networks. In the gold-bearing Cheechoo system, the gold mineralization appears to be mainly controlled by (i) the number of Qtz and Qtz-Fsp-Di veins per length unit and by (ii) the connectivity of these networks as shown by the increase of both parameters with the gold grade.Furthermore, we introduce the Y/I ratio as a simple topological parameter to quantify the connectivity of two dimensional (I-Y dominated) systems. This parameter is positively correlated with the number of connections per branch (CB) used to quantify network connectivity, and ultimately represent the development of a fault or fracture zone.

The Formation of Continental Crust from a Physics Perspective

The generation of crustal material and the formation of continental crust with a thickness of ≈40 km involve different physical mechanisms operating over different time-scales and length-scales. This review focusses on the building of a thick crustal assemblage and on the vertical dimension where the consequences of gravity-driven processes are expressed most clearly. Continental crustal material is produced by a sequence of crust and mantle mlelting, fractionation of basaltic melts and sinking of dense mafic cumulates. The repeated operation of these mechanisms over tens of million years leads to a thick stably stratified crust. We evaluate the main mechanisms involved from a physics perspective and identify the key controls and constraints, with special attention to thermal requirements. To form magma reservoirs able to process significant magma volumes and to allow the foundering of mafic cumulates, melt must be fed locally at rates that are larger than that of average crustal growth. This requires the temporary focussing of magmatic activity in a few centers. In some cases, foundering of dense cumulates does not go to completion, leaving a deformed residual body bearing tell-tale traces of the process. Crust must be thicker than a threshold value in a 30–45 km range for mafic cumulates to sink into the mantle below the crust. Once that threshold thickness has been reached, further additions lead to increase the proportion of felsic material in the crust at the expense of mafic lithologies which disappear from the crust. This acts to enhance radiogenic heat production in the crust. One consequence is that crustal temperatures can be kept at high values in times of diminished melt input and also when magmatic activity stops altogether, which may lead to post-orogenic intracrustal melting and differentiation. Another consequence is that the crust becomes too weak mechanically to withstand the elevation difference with neighbouring terranes, which sets a limit on crustal thickening. The thermal structure of the evolving crust is a key constraint on the overall process and depends strongly on radiogenic heat production, which is surely one of the properties that make continental crust very distinctive. In the Archean Superior Province, Canada, the formation of juvenile continental crust and its thermal maturation 2.7 Gy ago can be tracked quite accurately and reproduced by calculations relying on the wealth of heat flow and heat production data available there. Physical models of magma ascent and storage favour the formation of magma reservoirs at shallow levels. This suggests that crustal growth proceeds mostly from the top down, with material that gets buried to increasingly large depths. Vertical growth is accompanied by lateral spreading in two different places. Within the crust, magma intrusions are bound to extend in the horizontal direction. Deeper down, lateral variations of Moho depth that develop due to the focussing of magmatic activity get relaxed by lower crustal flow. This review has not dealt with processes at the interface between the growing crust and the mantle, which may well be where dikes get initiated by mechanisms that have so far defied theoretical analyses. Research in this particular area is required to further our understanding of continental crust formation.

Strain variations across the Proterozoic Penokean Orogen, USA and Canada

Strata in the Huron (2.5–2.0 Ga) and Animikie (2.2–1.85 Ga) basins were deposited on the southern margin of the Archean Superior province. These rocks were deformed during the Penokean orogeny (∼1850 Ma) followed by subsequent accretionary orogens to the south at 1750 Ma (Yavapai) and 1630 Ma (Mazatzal). Strain patterns are unique to each orogenic belt with no far-field effect: Archean Wawa terrane rocks in the Penokean foreland preserve deformation associated with Archean accretion with no younger Penokean, Yavapai or Mazatzal strain overprint. The Penokean orogeny deformed Huron-Animikie basin sediments into a north-vergent fold-and-thrust belt with no Yavapai or Mazatzal strain overprint. Yavapai orogen strains (SW-NE margin-parallel shortening) are unique when compared to the younger Mazatzal shortening (N20°W) shortening, with no strain overprint.Penokean deformation is characterized by shortening from the south including uplifted Archean gneisses and a northerly thin-skinned fold-and-thrust belt, with north-vergent nappes and a gently-dipping foreland. Our study of finite and calcite twinning strains (n = 60) along (∼1500 km) and across (∼200 km) the Penokean belt indicate that this orogeny was collisional as layer-parallel shortening axes are parallel across the belt, or parallel to the tectonic transport direction (∼N-S). Penokean nappe burial near the margin resulted in vertical shortening strain overprints, some of which are layer-normal. The Sudbury impact layer (1850 Ma) is found across the Animikie basin and provides a widespread deformation marker with many local, unique strain observations. We also report new geochronology (U-Pb zircon and apatite) for the gneiss-mafic dike rocks at Wissota (Chippewa Falls, WI) and Arbutus (Black River Falls, WI) dams, respectively, which bears on Penokean-Yavapai deformation in the Archean Marshfield terrane which was accreted during the Penokean orogen. Pseudotachylite formation was common in the Superior province Archean basement rocks, especially along terrane boundaries reactivated by contemporaneous Penokean, Trans-Hudson, Cape Smith and New Quebec deformation. In the hinterland (south), the younger Yavapai orogen (1750 Ma; n = 8) deformation is preserved as margin-parallel horizontal shortening (∼SW-NE) in Yavapai crust and up to 200 km to the north in the Penokean thrust belt as a strain and Barrovian metamorphic overprint. Mazatzal deformation (1630 Ma; n = 16) is preserved in quartzites on Yavapai and Penokean crust with layer-parallel and layer-normal shortening strains oriented N20°W.