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Advanced Structural Analysis : Quantitative and Multiscale Analysis
No full textThis project is made possible with funding by the Government of Ontario and through eCampusOntario’s support of the Virtual Learning Strategy.This online course on Advanced Structural Geology is intended for senior undergraduate students and graduate students of solid earth sciences. The course consists of the following four Sessions covering modern developments of Structural Geology in the areas of: 1) Quantitative Geometrical Analysis, 2) Rheology and Deformation Mechanisms, 3) Quantitative Kinematic Analysis, and 4) Introduction to Micromechanics and Multiscale Analysis. Session 1 builds on traditional orientation analysis for linear and planar fabric elements covered in landmark textbooks of structural geology. We show that these fabrics can be readily dealt with by unit vectors. However, most fabrics in rocks including the shape and lattice orientations of the constituent elements and grains are defined by 3D elements. The orientation of such fabric elements must be defined by a set of Euler angles or equivalently an orientation matrix. Statistic analysis of 3D orientation data is introduced and associated numerical techniques are discussed using mathematics applications MATLAB and MathCad. Session 2 presents the fundamental concepts of mechanics at a level required for Sessions 3 and 4. This session also prepared the students for the research literature in solid earth sciences. The concepts of multiscale stress, multiscale strain, non-linear rheology, and related partitioning and homogenization are introduced at a primary level. Deformation mechanisms, lattice preferred orientation development, and rheological anisotropy are discussed. These concepts will be revisited in Sessions 3 and 4 at a more advanced level, Session 3 is devoted to the kinematics of continuous deformation. This provides the students with the knowledge and skill set necessary for understanding progressive, large-strain deformations of rocks that are responsible for the formation of fabrics. The two-state description of finite deformation is given first and the instantaneous description of flow second. The kinematic modeling work of tabular high-strain zones, from the seminal work of J.G. Ramsay and Graham in 1970 till the end of last century, is reviewed and summarized in this session. Session 4 is devoted to recent advances on multiscale analysis and a complete mechanics approach to Structural Geology. These advances tackles the major challenge in Structural Geology, namely how to bridge the scale gap between observations and the tectonic boundary conditions. As our observations are most commonly made on scales much smaller than the tectonic problem of interest, and that our observations are inevitably discontinuous, a multiscale approach is necessary. This session provides the basic theoretical background and numerical resource for a self-consistent multiscale approach based on micromechanics. The goal is to introduce the principles and encourage students to read the lecture notes and related papers if they tackle multiscale geology problems
The Effect of Shock Metamorphism on Zircon of the Huronian Supergroup in proximity to the Sudbury Impact, Ontario, Canada
Get PDFU-Pb geochronology of zircon is a widely used tool, but damage to the crystal lattice of zircon and loss of Pb by impact-related shock metamorphism can affect the interpretation of U-Pb isotope data. Five samples of the Huronian Supergroup (HSG), a transitional rift to passive margin sedimentary sequence of siliciclastic rocks deposited at 2.45 – 2.22 Ga, were collected for U-Pb detrital zircon geochronology from the Matinenda, McKim, Ramsay Lake and Mississagi Formations at varying distances from the Sudbury Igneous Complex (SIC). LA-ICP-MS is used to take repeat U-Pb analyses of individual zircon at varying distances from the edge of the remnant of the SIC. A sample of the Ramsay Lake Formation sampled 4.4 km away from the outer edge of the SIC, described by Menard (2017), contains an anomalously young detrital zircon population (n=13/49) 2590 – 2480 Ma. Individual grains from this population were reanalyzed with 2-12 data points analyzed per zircon. BSE and CL images of the grains did not show any evidence of differential growth events which could cause single grains to contain different age domains. U/Pb ages for these grains could not be reproduced and had elevated discordance, errors and mean square weighted deviation (MSWD) values, consistent with Pb-loss. Some grains appear to have been partially reset by the Sudbury impact, as discordant analyses trend along a discordia line with the impact event at 1850 Ma. Therefore, the 2590 – 2480 Ma population of grains represent Pb-loss ages between their crystallization ages and the impact event. These results imply that, deformation related to the Sudbury impact has preferentially affected the grains of this population, which have suffered more Pb mobility, leading to a false young 207Pb/206Pb ages.
The 2590-2480 Ma population is compared to the main population of the same sample. Data from the main population have lower MSWD values and ages remain reproducible within error of the analytical technique. This observation demonstrates that distance from the impact is not the sole factor affecting the reaction of individual zircon to shock metamorphism because within a sample not all of the zircon suffer the same degree of uncertainty.
Samples were taken from the Elliot Lake area, approximately 90 km away from the Sudbury Igneous Complex to control for potential effects of the Sudbury impact. As expected, the overall sample discordance, uncertainty and MSWD values for the zircon from Elliot Lake is much lower, indicating that as a first order of approximation, distance is the most dominant factor affecting Pb-loss in zircon on a kilometer scale. But, within the shatter cone limit of an impact, factors other than distance also affect the percent of discordant grains within a sample and Pb-loss within zircon. Overall, the effect of the Sudbury impact is more far reaching and may have disrupted U/Pb systems much farther than previously estimated
A ca. 273-261 Ma multi-component arc pluton in the southern area of the Beishan Orogenic Collage, NW China: a geochronological, geochemical, and petrogenetic analysis
Get PDFAlong the southern edge of the Central Asian Orogenic Belt lies the Beishan Orogenic Collage, bridging the Tianshan Orogen (west) to the Mongolia-Xing’anling Orogen (east). The Beishan Orogenic Collage is thought to represent the final stages of closure of the Paleo-Asian Ocean, and consequently the last tectonic events in the orogeny. The tectonic setting of the region has been a long-standing point of contention, and the timing and location of final ocean closure is highly debated. This study focuses on the geochemistry, geochronology, and petrogenesis of the multi-component Nanquan Quarry pluton located at the southernmost edge of the Beishan, providing new insight into the controversial and poorly understood tectonic history of the region. The pluton is hosted in the South Baidunzi Complex and consists of mingled quartz-diorite and quartz-monzonite, which are intruded by granodiorite. The quartz-diorite and quartz-monzonite both have geochemical signatures indicative of an arc setting, and the granodiorite has a post-collisional and adakitic signature. The mingled mixture of quartz-diorite and quartz-monzonite yielded SHRIMP U-Pb ages of ca. 273 Ma, while the granodiorite yielded a SHRIMP U-Pb age of ca. 261 Ma. This new data suggests subduction was still active in the Beishan Orogenic Collage between 273 – 261 Ma, ca. 10 Ma later than suggested by previous models. This study suggests that the Nanquan Quarry arc formed in either a north-dipping subduction zone between a terrane from the Dunhuang Orogenic Belt and the South Baidunzi Complex, or a south-dipping subduction zone between the North Baidunzi Complex and the South Baidunzi Complex. This new information about the most recent tectonic stage of the Paleo-Asian Ocean could help better define the history of the Central Asian Orogenic Belt
Stratigraphy, Structure and Geochronology of Archean Lode Gold Deposits in the Southeastern Rice Lake Greenstone Belt, Southeast Manitoba
No full textThe Central Manitoba mine trend is one of the most important lode gold camps in the Rice Lake greenstone-granitoid belt of the western Uchi Subprovince within the western Superior Province, Manitoba, Canada. Neoarchean host rocks consist of a south-facing volcano-sedimentary succession (2.75–2.73 Ga) intruded by voluminous gabbroic sills and tonalitic-granodioritic plutons (2.73–2.72 Ga), as well as late aplite dikes (2.73–2.72 Ga) and quartz-feldspar porphyry dikes (2.73–2.71 Ga). Five generations of deformation structures have been recognized through detailed geological mapping. The entire succession was folded during early deformation prior to rare late aplite dike emplacement. All fault-fill veins and extension veins cut all lithologic units, and are structurally governed by late conjugate shear zones. Main gold mineralization occurs within fault-fill veins hosted by west-trending steeply-dipping dextral brittle-ductile and ductile shear zones, which occur along or across contacts of metabasalt, metagreywacke and metagabbro or entirely within metagabbro. Microstructural and paragenetic analyses on main gold-bearing veins have revealed that gold is intimately associated with quartz, pyrrhotite and tellurobismuthite. Main gold introduction is interpreted to have taken place contemporaneously with pyrrhotite and tellurobismuthite deposition early during dextral shearing.
The Ogama-Rockland gold deposit consists of shear zone-associated quartz veins hosted by the Ross River pluton, a ca. 2728–2724 Ma tonalitic-granodioritic intrusion in supracrustal rocks (< ca. 2745– 2731 Ma) of the Bidou assemblage within the Rice Lake greenstone belt. The plutonic host rocks contain NE-to-ENE–trending, steep, early foliation that is overprinted by late conjugate sets of W-to-NW–trending dextral and N-to-NE–trending sinistral shear zones. Early gold introduction associated with folded veins and low grade planar extension veins occurred before shearing. Main gold mineralization and remobilization are associated with highly auriferous NW-trending steep shear veins. They were likely emplaced late during dextral shearing. Most of the native gold occurs in fractures and grain boundaries of quartz, pyrite and chalcopyrite-bornite in shear veins, which reflects a strong structural and mechanical control on the microscopic scale.
Early gold mineralization associated with low grade planar veins at the Ogama-Rockland deposit is constrained at ca. 2728 Ma, based on U-Pb zircon ages from two phases of host tonalite (2728.7 ± 0.7 Ma; 2728.2 ± 0.6 Ma) and one aplite dike (2727.9 ± 1.6 Ma) that cuts the early low grade veins. A Re-Os molybdenite age of 2727.4 ± 4.8 Ma was obtained from one vein that cuts aplite dikes and sheeted veins, which agrees well with the U-Pb zircon ages of the dated tonalite phases and the aplite dike. The dated tonalite phases, aplite dike and the dated vein are all cut by late ductile or brittle-ductile shear zones, indicating their emplacement, and possibly early gold mineralization, occurred before dextral shearing. The close time association and high Re concentration in the dated molybdenite suggest that granitoid magmas were the source of mineralizing fluids for early gold mineralization. High grade gold mineralization in dextral shear-zone–hosted veins represents a later gold introduction and/or remobilization during dextral shearing
Deformation History of the Black Bay Fault, Northwest Territories, Canada
Get PDFThe Black Bay Fault is a major Paleoproterozoic, NE-SW-trending, crustal-scale feature that separates different tectonometamorphic domains of the southern Rae craton in the Canadian Shield. This structure extends from the edge of Lake Athabasca northward for 100’s km into the Northwest Territories and was previously poorly constrained. Prior examination of the fault has been limited to small-scale studies in the Uranium City, SK vicinity. The Black Bay fault has previously been associated with rare-earth element and uranium mineralization in northern Saskatchewan, although this relationship is not fully understood. This thesis has focused on understanding the continuation of the Black Bay Fault into the Northwest Territories based on field mapping conducted as part of the GSC/NTGS GEM2 South Rae project. The observations are broken into three study areas: Tazin River, Insula-Labyrinth Lake and Dymond Lake.
The Black Bay Fault has had a cryptic polyphase history. Overall the fault is observed to be a steep, west-dipping structure associated with a strong NE/SW-trending fabric, but there are major disruptions to this trend by two NW-SE-trending segments of this fault. Field observations indicate it was impacted by four main deformational events, D1 to D4. D1 produced sinistral transpression along the fault, resulting in the uplift of the western domains relative to the eastern domains, a steeply west-dipping, NNE-SSW-trending gneissosity and shallowly NE-plunging lineations. D1 is inferred to have occurred ca. 1910 Ma in associated with the Snowbird Orogeny and the collision of the Hearne craton onto the eastern edge of the Rae Craton. D2 is a regionally limited event which modified the geometry of the fault rather than producing deformation along its length and resulted in the development of a left-stepping bend in the fault around Labyrinth Lake and another larger bend around Dymond Lake. D2 fabrics are not observed along the southern extent of the Black Bay Fault. This event remains very poorly constrained but is hypothesized to be associated with the indentation of the Slave craton into the Rae craton, further to the west, at ca. 1860 Ma. D3 was a dextral transpression event resulted in further west-side up movement on the primary Black Bay Fault trend and produced a steeply west-dipping, NE-SW-trending gneissic to mylonitic fabric along with shallowly SW-plunging lineation. D3 began by 1844.4 ±2.1 Ma continued until ca. 1830 Ma and was probably driven through far-field tectonic effects from the Trans-Hudson orogeny to the east. Differential uplift occurred during this event, with the fabrics becoming more ductile moving northward on the western side of the fault. D4 was a brittle-ductile to brittle deformation event associated with dextral movement along the fault and the development of a sinistral, NW-SE-trending conjugate fault/fracture system containing syn-tectonically emplaced mafic dykes. D4 was underway by ca. 1827 Ma and continued until at least ca. 1818 Ma. In addition, fault-associated rare-earth element and uranium mineralization previously discovered in northern Saskatchewan is observed to continue into the Northwest Territories, hosted primarily in S3 and S4 fabrics and possibly associated with the shift from a ductile regime to a brittle regime
A Structural Study of the Dogpaw Gold Deposit in the Rowan-Kakagi Greenstone Belt, Western Superior Province, Northwestern Ontario
Get PDFThe Dogpaw deposit is a structurally controlled gold deposit hosted in the Neoarchean, Rowan-Kakagi greenstone belt of the Western Wabigoon of the Superior Province. The Rowan-Kakagi greenstone belt consists of two metavolcanic terranes separated by the crustal-scale, Pipestone-Cameron fault zone: 1) the Kakagi Lake volcanic terrane, which hosts the Dogpaw gold deposit, and 2) the Rowan Lake volcanic terrane. A number of reported gold occurrences in the Rowan-Kakagi greenstone belt are spatially associated with the Pipestone-Cameron fault zone. Bedrock mapping along the trend of the Pipestone-Cameron fault zone in the Dogpaw-Flint Lake map area has revealed a network of faults which consists of a major, dominantly dextral, east-southeast striking deformation zone and a network of secondary faults with compatible kinematics. At the Dogpaw deposit the bulk of gold mineralization is expressed at surface as lodes of quartz-pyrite breccia veins and sheeted, quartz-carbonate veins hosted in a network of ductile to brittle-ductile, conjugate shear zones which occur at the upper succession of a layered mafic to ultramafic sill. These conjugate shear zones developed due to broadly, north-south directed compression during deformation and their distribution is controlled by the primary, textural heterogeneity of the hosting rock. These gold-hosting structures where subsequently overprinted by a subsidiary structure of the Pipestone-Cameron fault zone, the Dalby Bay shear zone, deforming the early geometry of the deposits. The Dalby Bay shear zone is interpreted as a secondary deformation zone of the Pipestone-Cameron fault zone based on compatible kinematics and identical style of deformation. Four samples were collected for LA-ICP-MS U-Pb analysis at the Dogpaw deposit. Weighted mean 207Pb/206Pb zircon ages were yielded from two samples collected from late-tectonic, quartz-feldspar porphyry dykes. These dykes crosscut both veins of high-grade gold and are themselves deformed within the boundaries of the Dalby Bay shear zone. Results from the analysis of sampled dykes provide a minimum age constraint on the timing of gold mineralization at the Dogpaw deposit at approximately 2694 to 2696 Ma. Results from the analysis of a sample collected from the hosting intrusion provides a maximum timing on gold mineralization at approximately 2717 Ma. These geochronology results, coupled with the U/Pb dating and mapping by previous workers, provides new constraints on gold mineralization, deformation and igneous activity in the Rowan-Kakagi greenstone belt
Numerical Modelling of Structural Patterns in Tectonic Flow with Applications to the Neoarchean Crustal Dynamics
No full textWe formulate a numerical framework, in both 2d and 3d, to model the structural patterns emerging from viscous tectonic flow by coupling a level set description of the material interface with a finite element flow solver. Our formulation has the advantage of straightforward extensibility to encompass complex rheology and versatile mesh geometry, as well as improved computational efficiency. A distinct novelty of our formulation is the capability to offer a fully dynamical approach to modelling structural patterns resulting from an inhomogeneous and non-steady tectonic flow. The model output, in the form of lithological distribution and deformation patterns, can be directly compared with the results of based geological mapping and structural analysis, thus offering the opportunity to ground-truth the abstract numerical models with concrete field observations.
As examples for the potential applications of our method, we apply our newly developed method to the modelling of the crustal dynamics of the Neoarchean granitoid-greenstone terranes in two case studies to shed light on the potential vertical- to horizontal-style tectonics. In the first case study, a field-based structural study is conducted in the Swayze greenstone belt in the Superior Craton and four generations (G1--G4) of ductile deformations are identified. Among them, G2 structures are associated with an oblique shearing kinematics with a granitoid-up/greenstone-down movement in the vertical direction and a dextral sense of shear in the horizontal direction, as well as an opposing plunge directions of L2 stretching lineation. The tectonic regime associated with G2 is interpreted to be the operation of vertical-style tectonism in the form of sagduction/diapirism under the backdrop of horizontal-style tectonism in the form of regional dextral simple shearing. To test this interpretation, an isothermal numerical model is constructed for the Swayze greenstone belt. The model replicates the observed lithological and deformation pattern, confirming the synchronous vertical and horizontal tectonism model as a viable regime to explain the observed structural patterns.
To further explore whether the synchronous vertical and horizontal tectonism is applicable to the Neoarchean crustal dynamics in general, a thermomechanical model is constructed in the second case study with both thermal and rheological conditions appropriate for the Neoarchean granitoid-greenstone terranes. As in the first case study, many aspects of the crustal architecture and structural patterns are comparable to the observations in Neoarchean terranes worldwide. Due to the high competency of the cold upper crust, the density-induced sinking of the supracrustal assemblages into the granitoid domes operates by active diachronous pointwise ``dripping'' at triple junctions or by sheet-like sagduction with higher rate of horizontal shearing. Furthermore, the transitional process of the crustal dynamics towards the modern-day conditions from the hotter Archean Earth is further explored by systematically decreasing the Moho heat flux, which results in a delayed initiation and a slower rate of the vertical tectonic process. The increase in the horizontal strain rate results in the promotion of sheet-like sagduction pattern, the elongation of granitoid domes, the preferential alignment of the high strain zones and the reduction of the rate of greenstone sagduction. It is therefore postulated that through the secular cooling of the crustal thermal condition, the role of density-induced sagduction/diapirism is suppressed, thereby completing the transition of the Earth's crustal dynamics from a dominantly vertical style in the early Earth's history to a present-day dominantly horizontal style, with the Neoarchean being the interlude period where both processes coexisted. Furthermore, taking into account that the Neoarchean lithospheric dynamics of the southeastern Superior Craton is interpreted to be terrane accretion under a plate tectonics-like regime, we conclude that the synchronous horizontal and vertical crustal dynamics is not contradictory to a framework of plate tectonics on a lithospheric scale. In fact, the Neoarchean transition in crustal dynamics may mirror a similar transition in lithospheric dynamics from the early stagnant-lid tectonics to the modern day plate tectonics
Paleozoic transpressional tectonics in the Beishan orogenic collage, Northwest China: Evolution of the Hongliuhe suture zone
No full textThe Beishan orogenic collage is a southern, central subset of the Central Asian Orogenic Belt, in Northwest China. It is an accretionary orogen that was active during the early Paleozoic, transitioning to convergent collisional tectonics in the late Paleozoic to early Mesozoic. It comprises two main early Paleozoic arcs built through Precambrian pericratonic fragments, the Hanshan and Dundunshan terranes. Detrital zircon geochronology indicates that they share a similar set of Precambrian ages with the Tarim craton. It also establishes that the Hanshan arc developed with magmatism from the Late Cambrian to Early Devonian, then re-initiated with a minor peak of magmatism in the Carboniferous–Permian. Magmatism in the Dundunshan arc follows an identical pattern, yet offset younger by roughly 50 m.y. in all aspects. We interpret this pattern to indicate a staged collision and accretion event, the docking of the arcs from north to south. The event initiated during a regional magmatic hiatus in the Devonian. Known ophiolite ages of genesis and emplacement respectively bracket the main arc-building magmatic period. This supports conclusions that an ophiolite belt in the Beishan does represent a suture.
Deformation in the Mazongshan terrane, the central Beishan suture and accretionary complex, is due to episodes of early Paleozoic accretion of arcs, oceanic units, and ophiolite emplacement. To further establish the sequence of tectonism we provide an age and tectonic setting of the Hongliuhe ophiolite, one of the oldest that now lies in the suture zone. It comprises cumulate ultramafic to mafic plutonic rocks formed in the Cambrian (520.3 ± 5.8 Ma, U-Pb, Gabbro). Volcaniclastic and arc-marginal sedimentary rocks overlie exposed ductilely deformed lower crustal ophiolitic plutonic rocks. Petrography, lithostratigraphy, and whole rock and mineral chemistry support a conclusion that the ophiolite developed in an extensional arc-marginal supra-subduction tectonic environment. This illustrates how ophiolites can be created during subduction rollback, not necessarily subduction initiation. The emplacement timing of the ophiolite is constrained by an undeformed Early Devonian granite (413.6 ± 3.5 Ma, U-Pb) that intruded through its tilted, folded sedimentary cover sequence.
Paleozoic deformation spanning the Mazongshan terrane can be differentiated into two events: pervasive Silurian–Devonian dextral transpression and Permian–Triassic sinistral transpression. We identify that a protracted regional accretion or collision event produced the early deformation. U-Pb zircon ages define a staged cessation of deformation from five late-syn-kinematic dykes in the Xingxingxia region (436.5 ± 2.6 Ma to 383.1 ± 4.6 Ma, Early Silurian–Middle Devonian). High strain zones distributed throughout the accretionary complex exhibit dextral kinematics, with transpressive fault dynamics. Narrow fault width and high variation of stretching lineation orientations within the foliation plane together correlate to earlier timing of fault cessation. Wide fault zones bounding the accretionary complex have focused lineation orientations and correlate to later cessation. This pattern of fault cessation indicates a progressive increase in localization of strain over time, due to a possible combination of uplift, cooling (cessation of magmatism), and waning tectonism; each of these mechanisms can be characteristically linked to collision. The dextral transpressive event is likely the collision of the northern and southern arc terranes of the Beishan and spanned the Silurian to Middle Devonian. The timing correlates to the emplacement of ophiolites within the suture.
To aid analysis of highly tectonized regions, we present a new detrital geochronology provenance analysis technique dating populations of variably tectonized conglomerate clasts. Bulk samples of clasts are sorted based on their degree of internal deformation into three subsets: undeformed, deformed, and intermediate, or slightly deformed. Zircon from each population is analyzed identically to a standard sandstone detrital geochronology sample. Our application of the method to the Hongliuhe Formation, a Permian syn-orogenic deposit overlying the Mazongshan accretionary units, compares clast and sandstone data sets at three stratigraphic levels. Clast population age spectra include a wider range of ages than sandstone. They indicate a minimal lag time, implying rapid exhumation rates. A dominant Ordovician magmatic peak is present in sandstone data, switching to a dominant Silurian–Devonian peak in clast data, indicating a younging of magmatism towards a proximal provenance. A hiatus of magmatism starts in the Devonian, correlating with the latest age of deformed clasts, interpreted as timing of collisional tectonics. The detailed age spectra provide regional tectonic context and interpretation of processes, as well as more robust provenance interpretation than could be determined from sandstone samples alone
Structural Controls and Deformation History of the Orogenic Island Gold Deposit, Michipicoten Greenstone Belt, Ontario
Get PDFIsland Gold is a currently producing mine on the high-grade orogenic Island Gold deposit in northern Ontario. It is located within the southern domain of the regional Goudreau Lake Deformation Zone (GLDZ), which trends east-west through the Michipicoten greenstone belt of the Wawa-Abitibi terrane.
The study area encompasses the Island Gold deposit and is located along the northern limb of the Goudreau Anticline, a regional-scale fold attributed to D1 deformation. D2 consists of regional greenschist-facies metamorphism, camp-scale F2 folds, associated steep axial-planar foliation S2, moderately to steeply east-plunging stretching lineation L2a, and sub-horizontal slickenside striations L2b. D3 structures are camp- and outcrop-scale F3 folding, which deforms S2 foliation into shallowly-plunging Z-folds, weakly developed axial-planar cleavage S3, and brittle reverse faults. The Island Gold deposit forms a mineralized corridor south of the trondhjemitic Webb Lake Stock intrusion. The main Lochalsh, Island, Island Deep, and Extension 1 and 2 Zones consist of steeply dipping, subparallel ore zones of laminated V1 quartz veins and V2 veinlets within a silicic-sericitic alteration package. V3 conjugate quartz-carbonate extensional veins cross-cut V1 and V2 ore veins. Offset to the north of the main zones is the Goudreau Zone, which contains both sub-vertical and sub-horizontal ore zones with VGD ore veins. All pre-existing vein sets and structures were overprinted by V4 tourmaline veins. U-Pb zircon geochronology analyses from this study place the age of the mineralized Webb Lake Stock at 2724.1±4.3 Ma and the age of the post-mineralization I2M intrusion at 2672.2±3.5 Ma, which constrain the upper and lower absolute limits on timing of mineralization. Youngest detrital zircon ages from the overlying Doré metasedimentary rocks, which show D2 greenschist-facies metamorphism, further constrain this timing to between 2680±3 Ma and 2672.2±3.5 Ma.
The GLDZ formed during D2 deformation along a major lithologic contact. The Island Gold deposit V1 and V2 ore veins were emplaced sub-parallel to S2 foliation along a strain shadow created by the Webb Lake Stock during D2 north-side-up, sinistral transpression. Subsequent D3 deformation folded and sheared the ore zones and V3 veins were emplaced in areas of high competency contrast
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