scholarly journals The role of enigmatic deep crustal and upper mantle structures on Au and magmatic Ni-Cu-PGE-Cr mineralization in the Superior Province

2021 ◽  
Author(s):  
L B Harris ◽  
P Adiban ◽  
E Gloaguen
Keyword(s):  
Machine Learning ◽  
Fluid Flow ◽  
Lithospheric Mantle ◽  
Lower Crust ◽  
Numerical Models ◽  
Regional Metamorphism ◽  
Shear Zones ◽  
Superior Province ◽  
Upper Crust ◽  
Pge Mineralization

Aeromagnetic and ground gravity data for the Canadian Superior Province, filtered to extract long wavelength components and converted to pseudo-gravity, highlight deep, N-S trending regional-scale, rectilinear faults and margins to discrete, competent mafic or felsic granulite blocks (i.e. at high angles to most regional mapped structures and sub-province boundaries) with little to no surface expression that are spatially associated with lode ('orogenic') Au and Ni-Cu-PGE-Cr occurrences. Statistical and machine learning analysis of the Red Lake-Stormy Lake region in the W Superior Province confirms visual inspection for a greater correlation between Au deposits and these deep N-S structures than with mapped surface to upper crustal, generally E-W trending, faults and shear zones. Porphyry Au, Ni, Mo and U-Th showings are also located above these deep transverse faults. Several well defined concentric circular to elliptical structures identified in the Oxford Stull and Island Lake domains along the S boundary of the N Superior proto-craton, intersected by N- to NNW striking extensional fractures and/or faults that transect the W Superior Province, again with little to no direct surface or upper crustal expression, are spatially associated with magmatic Ni-Cu-PGE-Cr and related mineralization and Au occurrences. The McFaulds Lake greenstone belt, aka. 'Ring of Fire', constitutes only a small, crescent-shaped belt within one of these concentric features above which 2736-2733 Ma mafic-ultramafic intrusions bodies were intruded. The Big Trout Lake igneous complex that hosts Cr-Pt-Pd-Rh mineralization west of the Ring of Fire lies within a smaller concentrically ringed feature at depth and, near the Ontario-Manitoba border, the Lingman Lake Au deposit, numerous Au occurrences and minor Ni showings, are similarly located on concentric structures. Preliminary magnetotelluric (MT) interpretations suggest that these concentric structures appear to also have an expression in the subcontinental lithospheric mantle (SCLM) and that lithospheric mantle resistivity features trend N-S as well as E-W. With diameters between ca. 90 km to 185 km, elliptical structures are similar in size and internal geometry to coronae on Venus which geomorphological, radar, and gravity interpretations suggest formed above mantle upwellings. Emplacement of mafic-ultramafic bodies hosting Ni-Cr-PGE mineralization along these ringlike structures at their intersection with coeval deep transverse, ca. N-S faults (viz. phi structures), along with their location along the margin to the N Superior proto-craton, are consistent with secondary mantle upwellings portrayed in numerical models of a mantle plume beneath a craton with a deep lithospheric keel within a regional N-S compressional regime. Early, regional ca. N-S faults in the W Superior were reactivated as dilatational antithetic (secondary Riedel/R') sinistral shears during dextral transpression and as extensional fractures and/or normal faults during N-S shortening. The Kapuskasing structural zone or uplift likely represents Proterozoic reactivation of a similar deep transverse structure. Preservation of discrete faults in the deep crust beneath zones of distributed Neoarchean dextral transcurrent to transpressional shear zones in the present-day upper crust suggests a 'millefeuille' lithospheric strength profile, with competent SCLM, mid- to deep, and upper crustal layers. Mechanically strong deep crustal felsic and mafic granulite layers are attributed to dehydration and melt extraction. Intra-crustal decoupling along a ductile décollement in the W Superior led to the preservation of early-formed deep structures that acted as conduits for magma transport into the overlying crust and focussed hydrothermal fluid flow during regional deformation. Increase in the thickness of semi-brittle layers in the lower crust during regional metamorphism would result in an increase in fracturing and faulting in the lower crust, facilitating hydrothermal and carbonic fluid flow in pathways linking SCLM to the upper crust, a factor explaining the late timing for most orogenic Au. Results provide an important new dataset for regional prospectively mapping, especially with machine learning, and exploration targeting for Au and Ni-Cr-Cu-PGE mineralization. Results also furnish evidence for parautochthonous development of the S Superior Province during plume-related rifting and cannot be explained by conventional subduction and arc-accretion models.

Deep fluid release beneath arcs from delayed breaching of the slab lower crust

2020 ◽  
Author(s):  
Matthijs Smit ◽  
Philip Pogge von Strandmann
Keyword(s):  
Fluid Flow ◽  
Lower Crust ◽  
Subduction Zones ◽  
Shear Zones ◽  
Channel System ◽  
Deep Fluid ◽  
Chronometric Analysis ◽  
Strong Control ◽  
Insight Into ◽  
Fluid Release

<p>Slabs in subduction zones with geotherms of 7 K km<sup>-1</sup> or higher are expected to dehydrate effectively in the forearc. Nevertheless, large amounts of water are released from these slabs at and beyond subarc depth, indicating that H<sub>2</sub>O remains slab-bound to much greater depth than expected. It is possible that this reflects a transient sealing effect exerted by the subducting lower crust—a section of the lithosphere that typically undergoes delayed recation and is effectively impermeable until then. To test this concept, we investigated gabbros that were partially transformed to hydrous eclogite along shear zones during subduction. The rocks were subjected to a textural, petrological and Li-chronometric analysis. The observations characterize the progressive stages of transformation, and provide detailed insight into the governing feedbacks among fluid flow, deformation, and reaction. Lithium chronometry indicates that it took only a few weeks for the shear zone network to develop and for the externally derived fluids to traverse this network and drive eclogitization; the switch in these rocks—going from strong to weak and from impermeable to sustaining long-range fluid flow—thus was essentially instanteneous on subduction time scales. The re-equilibration of the rocks occurred well beyond equilibrium at c. 90 km depth, which is where large fluid-filled channel system typically emanate from warm slabs. Our data suggest that the fluids that are produced in the slab mantle throughout the forearc accumulate beneath the Moho until the lower crust is breached by dynamic fluid vents and commences its delayed transformation. The subducting lower crust may thus be a exert a strong control on H<sub>2</sub>O and element budgets, and the rheology of slabs in warm subduction zones.</p>

The origin and history of the Kapuskasing structural zone, Ontario, Canada

1980 ◽  
Vol 17 (7) ◽  
pp. 866-875 ◽  
Author(s):  
Janet Watson
Keyword(s):  
Lower Crust ◽  
Bouguer Anomaly ◽  
Shear Zones ◽  
Superior Province ◽  
Dike Swarm ◽  
History Of ◽  
Dike Swarms ◽  
Relationship Of ◽  
The Relationship ◽  
Geochronological Evidence

Reexamination of geological, geophysical, and geochronological evidence suggests three principal conclusions. Firstly, the Kapuskasing line came into existence very shortly after the stabilization of the Superior Province. The upfaulting of strips of granulites and associated rocks, and probably the redistribution of densities in the lower crust responsible for the regional positive Bouguer anomaly, took place before the emplacement of the Matachewan dike swarm at ~2690 Ma. Secondly, relative movements along the line during this tectonic stage had an important sinistral transcurrent component and sinistral movements continued during the emplacement of the Matachewan swarm. Thirdly, displacements of various kinds took place on the Kapuskasing line during a number of later events spaced out over a period of more than 2000 Ma; some of their effects can be deduced from the relationship of regional dike swarms and of cratonic cover formations. These findings suggest that the Kapuskasing line did not originate as a rift structure. Analogies are seen with deep transcurrent shear zones of other continents.

scholarly journals Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden

Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 547-578 ◽  
Author(s):  
Joel B. H. Andersson ◽  
Tobias E. Bauer ◽  
Edward P. Lynch
Keyword(s):  
Fluid Flow ◽  
Iron Oxide ◽  
Structural Geology ◽  
Hydrothermal Alteration ◽  
Structural Model ◽  
Regional Metamorphism ◽  
Spatial Relationship ◽  
Shear Zones ◽  
Geological Mapping ◽  
Supracrustal Belt

Abstract. An approximately 90 km long Palaeoproterozoic supracrustal belt in the northwestern Norrbotten ore province (northernmost Sweden) was investigated to characterize its structural components, assess hydrothermal alteration–structural geology correlations, and constrain a paired deformation–fluid flow evolution for the belt. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West, and Fjällåsen–Allavaara) indicates two major compressional events (D1 and D2) have affected the belt, with each associated with hydrothermal alteration types typical for iron oxide–apatite and iron oxide Cu–Au systems in the region. Early D1 generated a regionally distributed, penetrative S1 foliation and oblique reverse shear zones that show a southwest-block-up sense of shear that formed in response to NE–SW crustal shortening. Peak regional metamorphism at epidote–amphibolite facies broadly overlaps with this D1 event. Based on overprinting relationships, D1 is associated with regional scapolite ± albite, magnetite + amphibole, and late calcite alteration of mafic rock types. These hydrothermal mineral associations linked to D1 structures may form part of a regionally pervasive evolving fluid flow event but are separated in this study by crosscutting relationships. During D2 deformation, folding of S0–S1 structures generated F2 folds with steeply plunging fold axes in low-strain areas. NNW-trending D1 shear zones experienced reverse dip-slip reactivation and strike-slip-dominated movements along steep, E–W-trending D2 shear zones, producing brittle-plastic structures. Hydrothermal alteration linked to D2 structures is a predominantly potassic–ferroan association comprising K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides. Locally, syn- or post-tectonic calcite is the main alteration mineral in D2 shear zones that intersect mafic rocks. Our results highlight the importance of combining structural geology with the study of hydrothermal alterations at regional to belt scales to understand the temporal–spatial relationship between mineralized systems. Based on the mapping results and microstructural investigations as well as a review of earlier tectonic models presented for adjacent areas, we suggest a new structural model for this part of the northern Fennoscandian Shield. The new model emphasizes the importance of reactivation of early structures, and the model harmonizes with tectonic models presented by earlier workers based mainly on petrology of the northern Norrbotten area.

Lateral crustal flow along discrete flat-lying lower crustal shear zones during Archean continent construction

2020 ◽  
Author(s):  
Graham Hill ◽  
Eric Roots ◽  
Ben Frieman ◽  
Jim Craven ◽  
Richard Smith ◽  
...  
Keyword(s):  
Gravitational Collapse ◽  
Lower Crust ◽  
Plate Tectonics ◽  
Shear Zones ◽  
Transitional Period ◽  
Superior Province ◽  
Near Surface ◽  
Low Resistivity ◽  
Crustal Shear Zones ◽  
Lower Crustal

<p>The nature of lithospheric evolution and style of the driving ‘tectonic’ processes occurring during Archean continent construction remain enigmatic. A significantly hotter thermal regime characterised the early Earth and was pervasive for much of the Archean. This resulted in construction of continents that were significantly weaker and unable to support the thick crustal sequences and topographies common to modern orogens. Gravitational collapse of these early continents may have occurred when deeper material became less dense by heating or partial melting and created a density contrast beyond the crustal competence and/or due to post-orogenic relaxation. Such a collapse could result in large scale horizontal spreading within the middle to lower crust and the development of lateral crustal flow along flat-lying shear zones producing fluid-deposited graphitic and metallic sulphide films at these depths, which, if preserved would produce broad scale quasi-horizontal mid-lower crustal low resistivity anomalies. Here we show 3D magnetotelluric resistivity models of the Archean Superior Province of Canada that reveal these types of anomalies that could represent lateral crustal flow in the middle to lower crust. Further, the magnetotelluric model shows narrow sub-vertical zones of low resistivity extending from the mid crust to the near surface, interpreted to represent remnant fluid pathways that potentially formed prior to gravitational collapse. These sub-vertical low resistivity features correlate spatially with crustal-scale deformation zones that potentially are host to hydrothermal ore deposits and abundant metasomatic mineral assemblages. The well preserved record of primary crustal amalgamation within the Superior Province of Canada with both features expected of autochthonous vertical ‘drip’ tectonics (sub-vertical fluid pathways) and allochthonous horizontal plate tectonics (flat-lying lower crustal shear zones) regimes, suggests a potential transitional period of tectonic evolution might have characterised the region during the late Archean.</p>

scholarly journals Vertical strain partitioning in hot Variscan crust: Syn-convergence escape of the Pyrenees in the Iberian-Armorican syntax

2017 ◽  
Vol 188 (6) ◽  
pp. 39 ◽  
Author(s):  
Bryan Cochelin ◽  
Dominique Chardon ◽  
Yoann Denèle ◽  
Charles Gumiaux ◽  
Benjamin Le Bayon
Keyword(s):  
Lower Crust ◽  
Shear Zones ◽  
Crustal Thickening ◽  
Upper Crust ◽  
Strain Partitioning ◽  
Kinematic Data ◽  
Mechanical Coupling ◽  
Gneiss Domes ◽  
Lateral Escape ◽  
Lower Crustal

A new structural map of the Paleozoic crust of the Pyrenees based on an extensive compilation and new kinematic data allows for the evaluation of the mechanical coupling between the upper and lower crust of the abnormally hot foreland of the Variscan orogen of SW Europe. We document partitioning between coeval lower crustal lateral flow and upper crustal thickening between 310 and 290 Ma under an overall dextral transpressive regime. Partitioning also involved syn-convergence transtensional gneiss domes emplacement during this period. Late orogen-normal shortening of the domes and strain localization in steep crustal-scale transpressive shear zones reflects increasing coupling between the lower crust and the upper crust. The combination of dextral transpression and eastward flow in the Pyrenees results from the shortening and lateral escape of a hot buoyant crust along the inner northern limb of the closing Cantabrian orocline at the core of the Iberian-Armorican arc between ca. 305 and 295 Ma. Delamination or thermal erosion of the lithosphere enhanced orocline closure and explains (1) the switch from crust- to mantle-derived magmatism in the Iberian-Armorican arc and (2) the abnormally hot and soft character of the Pyrenean crust that escaped the closing syntax.

In-situ reaction-replacement origin of hornblendites in the Early Cretaceous Laiyuan complex, North China Craton, and implications for its tectono-magmatic evolution

2021 ◽  
Author(s):  
Jia Chang ◽  
Andreas Audétat ◽  
Jian-Wei Li
Keyword(s):  
Partial Melting ◽  
North China Craton ◽  
Lithospheric Mantle ◽  
Lower Crust ◽  
North China ◽  
Ultramafic Rocks ◽  
Magmatic Evolution ◽  
Mafic Magmas ◽  
Felsic Rocks

Abstract Two suites of amphibole-rich mafic‒ultramafic rocks associated with the voluminous intermediate to felsic rocks in the Early Cretaceous Laiyuan intrusive-volcanic complex (North China Craton) are studied here by detailed petrography, mineral- and melt inclusion chemistry, and thermobarometry to demonstrate an in-situ reaction-replacement origin of the hornblendites. Moreover, a large set of compiled and newly obtained geochronological and whole-rock elemental and Sr-Nd isotopic data are used to constrain the tectono-magmatic evolution of the Laiyuan complex. Early mafic‒ultramafic rocks occur mainly as amphibole-rich mafic‒ultramafic intrusions situated at the edge of the Laiyuan complex. These intrusions comprise complex lithologies of olivine-, pyroxene- and phlogopite-bearing hornblendites and various types of gabbroic rocks, which largely formed by in-situ crystallization of hydrous mafic magmas that experienced gravitational settling of early-crystallized olivine and clinopyroxene at low pressures of 0.10‒0.20 GPa (∼4‒8 km crustal depth); the hornblendites formed in cumulate zones by cooling-driven crystallization of 55‒75 vol% hornblende, 10‒20 vol% orthopyroxene and 3‒10 vol% phlogopite at the expense of olivine and clinopyroxene. A later suite of mafic rocks occurs as mafic lamprophyre dikes throughout the Laiyuan complex. These dikes occasionally contain some pure hornblendite xenoliths, which formed by reaction-replacement of clinopyroxene at high pressures of up to 0.97‒1.25 GPa (∼37‒47 km crustal depth). Mass balance calculations suggest that the olivine-, pyroxene- and phlogopite-bearing hornblendites in the early mafic‒ultramafic intrusions formed almost without melt extraction, whereas the pure hornblendites brought up by lamprophyre dikes required extraction of ≥ 20‒30 wt% residual andesitic to dacitic melts. The latter suggests that fractionation of amphibole in the middle to lower crust through the formation of reaction-replacement hornblendites is a viable way to produce adakite-like magmas. New age constraints suggest that the early mafic-ultramafic intrusions formed during ∼132‒138 Ma, which overlaps with the timespan of ∼126‒145 Ma recorded by the much more voluminous intermediate to felsic rocks of the Laiyuan complex. By contrast, the late mafic and intermediate lamprophyre dikes were emplaced during ∼110‒125 Ma. Therefore, the voluminous early magmatism in the Laiyuan complex was likely triggered by the retreat of the flat-subducting Paleo-Pacific slab, whereas the minor later, mafic to intermediate magmas may have formed in response to further slab sinking-induced mantle thermal perturbations. Whole-rock geochemical data suggest that the early mafic magmas formed by partial melting of subduction-related metasomatized lithospheric mantle, and that the early intermediate to felsic magmas with adakite-like signatures formed from mafic magmas through strong amphibole fractionation without plagioclase in the lower crust. The late mafic magmas seem to be derived from a slightly different metasomatized lithospheric mantle by lower degrees of partial melting.

A structural reappraisal of the Beardmore–Geraldton Belt at the southern boundary of the Wabigoon subprovince, Ontario, and implications for gold mineralization

2004 ◽  
Vol 41 (2) ◽  
pp. 217-235 ◽  
Author(s):  
Bruno Lafrance ◽  
Jerry C DeWolfe ◽  
Greg M Stott
Keyword(s):  
Gold Mineralization ◽  
Shear Zones ◽  
Axial Plane ◽  
Superior Province ◽  
Southern Boundary ◽  
Gold Mines ◽  
Southern Margin ◽  
Ore Zones

The Beardmore–Geraldton Belt occurs along the southern margin of the Archean Wabigoon subprovince, Superior Province, Ontario. The belt consists of shear-bounded interleaved metasedimentary and metavolcanic units. The units were imbricated from 2696 to 2691 Ma during D1 thrusting and accretion of the Wabigoon, Quetico, and Wawa subprovinces. Post-accretion D2 deformation produced regional F2 folds that transposed lithological units parallel to the axial plane S2 cleavage of the folds. During D3 deformation, the folds were overprinted by a regional S3 cleavage oriented anticlockwise of F2 axial planes, and lithological contacts and S2 cleavage were reactivated as planes of shear within dextral regional shear zones that generally conform to the trend of the belt. D3 is a regional dextral transpression event that also affected the Quetico and Wawa subprovinces, south of the Beardmore–Geraldton Belt. Gold mineralization at the Leitch and MacLeod-Cockshutt mines, the two richest past-producing gold mines in the Beardmore–Geraldton Belt, is associated with D3 shear zones and folds, overprinting regional F2 folds. The plunge of the ore zones is parallel to F3 fold axes and to the intersection of D3 shear zones with F2 and F3 folds.

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