Réal Daigneault

ARCHEAN TERRANE DOCKING : UPPER CRUST COLLISION TECTONICS, ABITIBI GREENSTONE BELT, QUEBEC, CANADA

Abstract The northern (NVZ) and southern volcanic zones (SVZ) of the Abitibi greenstone belt are separated by the major E-trending Destor-Porcupine-Manneville fault zone (DPMFZ). The DPMFZ is interpreted to be the locus of Archean terrane docking between the older diffuse volcanic arc of the NVZ (2730-2710 Ma) and the younger arc segments of the SVZ (2705-2698 Ma). Two distinct evolutionary phases can be documented along the DPMFZ of the Abitibi greenstone belt and include (1) arc-arc collision occurring between 2697 and 2690 Ma, and (2) arc fragmentation between 2689 and 2680 Ma. Identification of these two events along the DPMFZ is based on detailed structural studies, sedimentary basin analysis, and precise UPb age determinations. The thrusting event, representative of the arc-arc collision phase, is characterized by shallow north-dipping foliations (20–40°) and dip-parallel stretching lineations in the eastern Manneville segment of the DPMFZ. Local overturned mafic pillowed units suggest recumbent folding. Late strike-slip or transcurrent movement displayed in the late-orogenic sedimentary Duparquet Basin records the arc fragmentation phase. Basin geometry, E-trending en-echelon folds, shallow E-plunging stretching lineations and a late NE-striking cleavage cross-cutting the folds support a dextral shear sense along the western Destor-Porcupine segment of the DPMFZ. The sedimentary facies observed in the basin are consistent with those of modern strike-slip basins located along the East Anatolian fault, Turkey (Hazar Lake) and the Hope fault, New Zealand (Hanmer Basin). Precise UPb zircon age determinations from porphyry stocks located at the northern and southern limits of the Duparquet Basin, yielded 2681 ± 1 Ma and 2689+3.2−2.9 Ma, respectively. These ages constrain the rapid change from thrusting to transcurrent movement. It is apparent that once thrusting ceased the response to oblique subduction continued in the form of strike-slip displacement. Modern fold and thrust belts commonly show this evolution. The deformation pattern is the result of oblique convergence. The Abitibi greenstone belt is considered to be an Archean analogue of modern subducting oceanic plates such as those found in the western Pacific.

Oblique Archean subduction: accretion and exhumation of an oceanic arc during dextral transpression, Southern Volcanic Zone, Abitibi Subprovince Canada

Abstract The Archean Abitibi Subprovince, divided into Northern (NVZ) and Southern (SVZ) Volcanic Zones, is characterized by the crustal-scale Destor-Porcupine-Manneville Fault Zone (DPMFZ), which links the SVZ and NVZ, and the Cadillac-Larder-Lake Fault Zone (CLLFZ), which separates the Abitibi and Pontiac Subprovinces. The DPMFZ and CLLFZ represent major deformation zones that record over 60 million years of Archean deformation. The dextral transpressional phase in the NVZ, which represents incipient SVZ deformation, occurred between 2700 and 2692 Ma and is characterized by Southeast (SE)-trending dextral faults (e.g. Macamic Fault). Synorogenic flysch sedimentation is focussed at the interface between the SVZ–NVZ and Pontiac–Abitibi Subprovinces. The SE-trending Parfouru fault, linking the two major trench fault systems, is interpreted as a reactivated leaky transform fault and is associated with synorogenic flysch sedimentation. The early monzodioritic suite of the Preissac-Lacorne batholith was related to DPMFZ thrusting (ca. 2692–2690 Ma). The change from early thrusting to transcurrent motion is recorded in the Duparquet strike-slip basin, which formed between 2690 and 2680 Ma along the DPMFZ, and the Granada pull-apart basin, which evolved between 2680 and 2670 Ma along the CLLFZ. The timing of these basins indicates a diachronous evolution. Renewed thrusting affected the Granada basin but not the Northern Duparquet strike-slip basin, so that a southward migrating deformation front is inferred. Late exhumation resulted in extension along both fault zones and was responsible for the juxtaposition of medium- and low-grade metamorphic rocks. The monzogranitic suite of the Lamotte pluton, which occurred between 2660 and 2642 Ma is linked to exhumation. Final dextral transpression post-dating exhumation produced shearing and folding. The SVZ of the Abitibi Subprovince shows the salient attributes of modern oblique orogenic collisions, with alternating phases of thrusting and strike-slip movement along crustal-scale faults during dextral transpression.

Evolution of a submerged composite arc volcano: volcanology and geochemistry of the Normétal volcanic complex, Abitibi greenstone belt, Québec, Canada

Abstract The 4 km-thick Archean Normetal volcanic complex (NVC), composed of basaltic andesite, dacite, and rhyolite, is represented by five distinct volcanic phases and one sedimentary phase. Initial volcanic construction features effusive mafic volcanism characterized by massive, pillowed and pillow breccia flows and local massive dacite (phases 1 and 2a). Prominent felsic volcanism of phase 2 commences locally with tuffs, lapilli tuffs and lapilli tuff breccias derived either from hydroclastic or autoclastic fragmentation processes (phase 2b). The principal constructive phase of the NVC (phase 2c) is composed of pillowed andesite, massive dacite, and dominant massive, flow banded and lobate rhyolite flows. Autoclastic or hydroclastic brecciation of the former have produced rhyolitic tuff, lapilli tuff and lapilli tuff breccia. Rhyolitic volcanism continued with eruption of lava flows (phase 3) and the intrusion of dykes and felsic endogenous domes (phases 3 and 4). A subsequent 20–70 m-thick sedimentary unit, composed of volcaniclastic turbidites and pelagic background sediments, constitutes a marker horizon indicating volcanic quiescence. Renewed volcanism of phase 5 is characterized by mafic to felsic turbiditic lapilli tuffs and tuffs, and mafic to felsic flows or intrusions. The felsic lapilli tuffs, tuffs and flows host the Normetal VMS deposit. The geometry and volcanic stratigraphy of the NVC suggests emission of viscous, phenocryst-rich felsic flows from three principal centers, including a parasitic western vent, the major central 6 km-wide cauldron structure and an eastern vent. Voluminous viscous felsic lava over a large area supports the inference of numerous vents whereby individual centers coalesced to produce a composite or complex stratovolcano. Proximal to distal facies changes, variable rhyolitic unit and lobe closures argue for multiple conduits. The VMS deposits are located at the western edge of the central cauldron. Geochemical analyses show two complete compositional spectrums (phases 1, 2, 4 and 5) from basaltic andesite to rhyolite. The Zr/Y and LaN/YbN ratios of phases 1, 2, 4 and 5 show a transitional affinity whereas phase 3 is tholeiitic to slightly transitional. Multi-element diagrams suggest that all phases are consistent with subduction-related processes. The mafic-felsic NVC, a composite volcano that formed upon a shield type volcano, displays subaqueous effusive dominant volcanic construction at depth below storm wave base, as indicated by pillowed flows, turbiditic and pelagic sedimentary rocks, and massive sulphide deposits. Geochemistry and physical volcanology of the NVC are consistent with construction of an immature arc volcano. The submerged Izu-Bonin arc volcanoes may be modern analogues.

Flat vein formation in a transitional crustal setting by self-induced fluid pressure equilibrium — an example from the Géant Dormant gold mine, Canada☆

Abstract Gold-bearing veins grossly define a bimodal distribution within the Earths crust as demonstrated by epithermal- (0 to 2 km) and mesothermal-type (∼5 to 15 km) deposits. Vein formation in the epithermal and mesothermal environments is commonly attributed to the suction pump and the fault-valve mechanisms, respectively. Characteristics of the Geant Dormant gold-bearing vein network are compatible with neither mechanism. In this paper, vein morphology and geometry, alteration styles, and host rock characteristics are used to constrain the following empirical parameters: tectonic regime, fluid flow vectors, crustal depth, host rock permeability, fluid pressure, mineral precipitating conditions and scale of filling processes (zoning). Based on these parameters, a three-stage model for vein formation is envisaged. The self-equilibrating mechanism involves the formation of mostly flat veins stacked along cross-stratal dikes within an impermeable volcanic pile. The dikes served as flow-restricted fluid feeders and as conduits for fluid discharge to the paleosurface. During the stable prefailure stage, the dike conduits acted as dampers in controlling the fluid discharge rate and in keeping fluid pressure at a constant level needed for the opening of preexisting fractures for vein formation (Pf≈σ1+0.3T) at specific vertical intervals. The formation of veins corresponds to an equilibrating process that releases differential fluid pressure (ΔP) built up vertically in the flow-restricted conduits. The ΔP is induced by the decrease of the lithostatic pressure as long as the hydrothermal fluids move upward at a low velocity. At a critical state, when the deepest veins cannot physically absorb more fluid pressure accumulation, the excess fluid pressure (ΔP) is then transferred upwards along the QFP dikes, leading to the failure of the equilibrium process for vein formation at the network-scale (failure stage). The postfailure stage involves draining to the paleosurface of the underlying pressurized hydrothermal reservoir. At an advanced state, hydrothermal self-sealing leads progressively to the restoration of the initial, prefailure, flow-restricted conditions of the dike conduits. The proposed model involves a crustal depth of 2–5 km and a near-neutral tectonic regime. These characteristics are intermediate to those involved for the suction pump and the fault-valve mechanisms and suggest that each tectonic regime has an optimal crustal depth for the formation of gold-bearing veins.

Geostatistical analysis of fractures in shear zones in the Chibougamau area: applications to structural geology

Abstract A geostatistical approach is used to analyze the spatial distribution of fracture density in shear zones. The methodology proposed is illustrated with data obtained from the mapping of drift walls at the Henderson mine in the Chibougamau mining district, Quebec. Fracture sets are defined using statistical characterization of fracture orientation and genetic parameters of fractures. Fracture density is defined as a function of the frequency, length and width of fracture traces measured on rock exposures. The spatial distribution of each set is estimated using three variables based on structural interpretation of fracture density ranges: the absence of fracture, the presence of clusters of small fractures with negligible thickness and the presence of fractures with measurable thickness. Results are presented in the form of variograms and of kriged contour maps. A positive relationship is shown between the range of the variogram and the type of fracture development. Furthermore, indicator maps provide insight into the spatial relationship between the major and the minor fractures in the investigated shear zone.

Characterization of general and singular features of major aquifer systems in the Saguenay-Lac-Saint-Jean region

The hydrogeology of the municipalized territory (13,210 km2) of the Saguenay-Lac-Saint-Jean (SLSJ) region has been studied as part of the groundwater knowledge acquisition program entitled Programme d’acquisition de connaissances sur les eaux souterraines (PACES) launched in 2008 by the Government of Quebec, Canada. This study involved the collaboration of numerous contributors (municipal policymakers, government agencies, watershed organizations and universities) to meet the program’s multiple and wide-ranging requirements. The key deliverables included a numerical geodatabase, 38 regional-scale maps and a scientific report, all elaborated after 4 years of data gathering and compilation, fieldwork and information synthesis. In addition, numerous collateral research projects were undertaken by undergraduate and graduate students. The results of the SLSJ-PACES project provided new insights into regional groundwater resources and led to a generalized conceptual model of regional hydrostratigraphic features and groundwater quality. This paper summarizes the particular aspects of the major aquifers in the SLSJ region as uncovered by PACES, and presents the emerging challenges for updating and improving the region’s hydrogeological knowledge and ensuring the sustainable management of regional groundwater resources.

Modelling Seismically Induced Mesothermal Goldfields along the Deep-Rooted Cadillac-Larder Lake Fault, Abitibi, Canada

Gold deposits are not uniformly distributed along major faults due to complex (and long-debated) interactions between seismicity, hydrothermalism, and structural heterogeneities. Here, we use static stress modelling (SSM) to quantitatively investigate these interactions, by exploring the role of Cadillac-Larder Lake Fault (CLLF) Archean seismicity in the genesis of the regional goldfields. Various rheological factors are evaluated for optimizing the models’ ability to reproduce known gold occurrences, regarded as the fossil primary markers of synkinematic hydrothermal systems. We propose that the marked structural heterogeneities of the CLLF induced persistent seismic segmentation and recurrent ruptures of the same fault windows that arrested on robust node points. These ruptures favour repeated occurrences of seismically triggered hydrothermalism along long-existing fluid pathways having an enhanced permeability and iterative ore formation into supracrustal discharge zones by means of episodic drops and build-ups of pressure. Two-dimensional SSM permits the predictive mapping of these high-potential zones. These modelled zones correlate positively with the actual observed gold distribution. We demonstrate that the ruptures along the Joannes Segment arresting on the Davidson Fault and Lapa’s bend can explain the occurrence and location of the Rouyn and Malartic goldfields; the models’ validity is improved by implementing regional geological constraints; and the distant gold occurrences from the CLLF, including the Bourlamaque field, can be explained by doublet seismic events along the Riviere-Heva and Lapause subsidiary faults. Our results provide new perspectives from a fundamental standpoint and for exploration purposes.