Crustal-Scale Geology and Fault Geometry Along the Gold-Endowed Matheson Transect of the Abitibi Greenstone Belt

2021 ◽  
Author(s):  
Rasmus Haugaard ◽  
Fabiano Della Justina ◽  
Eric Roots ◽  
Saeid Cheraghi ◽  
Rajesh Vayavur ◽  
...  
Keyword(s):  
Lower Crust ◽  
Greenstone Belt ◽  
Hydrothermal Fluids ◽  
Gravity Models ◽  
Fault System ◽  
Upper Crust ◽  
Magnetotelluric Data ◽  
Middle Crust ◽  
Low Resistivity ◽  
Abitibi Greenstone Belt

Abstract Gold in the Abitibi greenstone belt in the Superior craton, the most prolific gold-producing greenstone terrane in the world, comes largely from complex orogenic mineralizing systems related to deep crustal deformation zones. In order to get a better understanding of these systems, we therefore combined new magnetic, gravity, seismic, and magnetotelluric data with stratigraphic and structural observations along a transect in the Matheson area of the Abitibi greenstone belt to constrain large-scale geologic models of the Archean crust. A high-resolution seismic transect reveals that the well-known Porcupine Destor fault dips shallowly to the south, whereas the Pipestone fault dips steeply to the north. Facing directions and gravity models indicate that these faults are thrust faults where older mafic volcanic rocks overlie a younger sedimentary basin. The depth of the basin reaches ~2 to 2.5 km between these two faults, where it is interpreted to overlie mafic-dominated volcanic substrata. Regional seismic and magnetotelluric surveys image the full crust down to 36-km depth to reveal a heterogeneous architecture. Three crustal-scale layers include a resistive (104–105 Ωm) upper crust of granite-greenstone rocks, a low-resistivity (~10–50 Ωm) middle crust dominated by granitic plutons for which low resistivity is attributed to the presence of graphite, and a low to moderately resistive (50–1,000 Ωm) and seismically homogeneous lower crust interpreted as granulite gneisses. The significant resistivity transition between upper and middle crust is interpreted to be the result of interconnected micrographite grain coating, precipitated from carbon-bearing crustal fluids emplaced during Neoarchean craton stabilization. A major subvertical, seismically transparent, and extremely low resistive (<10 Ωm) corridor connects the lower and middle crust with the upper crust. The geometry of this low-resistivity feature supports its interpretation as a deep-rooted extensional fault system where the corridor acted as a regional-scale conduit for gold-bearing hydrothermal fluids from a ductile source region in the lower crust to the depositional site in the brittle upper crust. We propose that this newly discovered whole crustal corridor focused the hydrothermal fluids into the Porcupine Destor fault in the Matheson region.

Crustal velocity models for the Archean Abitibi greenstone belt from seismic refraction data

1995 ◽  
Vol 32 (2) ◽  
pp. 149-166 ◽  
Author(s):  
Gilles Grandjean ◽  
Hua Wu ◽  
Donald White ◽  
Marianne Mareschal ◽  
Claude Hubert
Keyword(s):  
Lower Crust ◽  
Greenstone Belt ◽  
Velocity Structure ◽  
Complex Structure ◽  
Seismic Refraction ◽  
Seismic Energy ◽  
Crustal Layer ◽  
Middle Crust ◽  
Velocity Models ◽  
Abitibi Greenstone Belt

We present velocity models for two seismic wide–angle-refraction profiles across the Archean Abitibi greenstone belt and the Pontiac Subprovince. The seismic profiles are 210 and 220 km long. Traveltime inversion and amplitude forward modelling were used to obtain two-dimensional velocity structure and interface geometry. The main features of the velocity models include (1) three crustal layers; (2) variable velocities (5.6–6.4 km/s) in the upper crust (~0–12 km), with the higher velocities generally associated with mafic metavolcanics and the lower velocities with metasediments and granitic plutons; (3) a relatively uniform middle crust (~12–30 km) with velocities ranging from 6.4 to 6.6 km/s; (4) a velocity increase of 0.3 km/s across the middle crust–lower crust boundary; (5) a lower crust (~30–40 km) with velocities increasing from 6.9 km/s at the top to 7.3 km/s at the base; (6) an average upper mantle velocity of 8.15 km/s; (7) depth to Moho of about 40 km in the north-central Abitibi belt, decreasing southward to 37 km beneath the Pontiac Subprovince; and (8) observed attenuation of seismic energy propagating through the Casa–Berardi deformation zone, suggesting a complex structure in this fault zone. The velocity model is generally consistent with seismic reflection interpretations that suggest that the shallow supracrustal assemblages form an allochthonous veneer, overlying a mid-crustal imbricate sequence of metaplutonic and metasedimentary rocks. The uniform-velocity structure below 12 km depth indicates that the tectonic zones juxtaposing disparate crustal blocks may have limited depth extent. The 40 km thick crust and 10 km thick high-velocity lower crustal layer exceed the thicknesses observed in other studies of Archean crust.

Chapter 32: Gold Deposits of the Archean Abitibi Greenstone Belt, Canada

2020 ◽  
pp. 669-708
Author(s):  
Benoît Dubé ◽  
Patrick Mercier-Langevin
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Greenstone Belt ◽  
Large Scale ◽  
Sedimentary Basins ◽  
Fault Zones ◽  
Gold Deposits ◽  
Hydrothermal Fluids ◽  
Abitibi Greenstone Belt ◽  
High Level

Abstract The Neoarchean Abitibi greenstone belt in the southern Superior Province has been one of the world’s major gold-producing regions for almost a century with >6,100 metric tons (t) Au produced and a total endowment, including production, reserves, and resources (measured and indicated), of >9,375 t Au. The Abitibi belt records continuous mafic to felsic submarine volcanism and plutonism from ca. 2740 to 2660 Ma. A significant part of that gold is synvolcanic and/or synmagmatic and was formed during the volcanic construction of the belt between ca. 2740 and 2695 Ma. However, >60% of the gold is hosted in late, orogenic quartz-carbonate vein-style deposits that formed between ca. 2660 and 2640 ± 10 Ma, predominantly along the Larder Lake-Cadillac and Destor-Porcupine fault zones. This ore-forming period coincides with the D3 deformation, a broad north-south main phase of regional shortening that followed a period of extension and associated crustal thinning, alkaline to subalkaline magmatism, and development of orogenic fluvial-alluvial sedimentary basins (ca. <2679–<2669 Ma). These sedimentary rocks are referred to, in the southern Abitibi, as Timiskaming-type. The tectonic inversion from extension to compression is <2669 Ma, the maximum age of the D3-deformed youngest Timiskaming rocks. In addition to the quartz-carbonate vein-style, stockwork-disseminated-replacement-style mineralization is hosted in and/or is associated with ca. 2683 to 2670 Ma, early-to syn-Timiskaming alkaline to subalkaline intrusions along major deformation corridors, especially in southern Abitibi. The bulk of such deposits formed late-to post-alkaline to subalkaline magmatism and the largest deposits are early- to syn-D3 (ca. 2670–2660 Ma), whereas the bulk of the quartz-carbonate vein systems formed syn- to late-D3 and metamorphism. At belt scale, these illustrate a gradual transition, as shortening increases, in ore styles in orogenic deposits throughout the duration of the D3 deformation event along the length of the Larder Lake-Cadillac and Destor-Porcupine faults. The sequence of events, although similar in all camps, was probably not perfectly synchronous at belt scale, but varied/migrated with time and crustal levels along the main deformation corridors and from north to south. The presence of high-level alkaline/shoshonitic intrusions, which are spatially associated with Timiskaming conglomerate and sandstone, large-scale hydrothermal alteration, and numerous gold deposits along the Larder Lake-Cadillac and Destor-Porcupine faults indicates that these structures were deeply rooted and tapped auriferous metamorphic-hydrothermal fluids and melts from the upper mantle and/or lower crust, late in the evolution of the belt. The metamorphic-hydrothermal fluids, rich in H2O, CO2, and H2S were capable of leaching and transporting gold to the upper crust along the major faults and their splays. Although most magmatic activity along the faults predates gold, magmas may have contributed fluids and/or metals to the hydrothermal systems in some cases. This great vertical reach explains why the Larder Lake-Cadillac and Destor-Porcupine fault zones are very fertile structures. The major endowment of the southern part of the Abitibi belt (>8,100 t Au) along the corridor defined by the Larder Lake-Cadillac and Destor-Porcupine faults may also suggest that these faults have tapped particularly fertile upper mantle-lower crust gold reservoirs. The concentration of large synvolcanic and synmagmatic gold deposits along that corridor supports the idea of gold-rich source(s) that may have contributed gold to the ore-forming systems at different times during the evolution of the belt.

scholarly journals Crustal architecture of a metallogenic belt and ophiolite belt: implications for mineral genesis and emplacement from 3-D electrical resistivity models (Bayankhongor area, Mongolia)

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
Matthew J. Comeau ◽  
Michael Becken ◽  
Alexey V. Kuvshinov ◽  
Sodnomsambuu Demberel
Keyword(s):  
Electrical Resistivity ◽  
Crustal Structure ◽  
Three Dimensional ◽  
Suture Zone ◽  
Fault System ◽  
Magnetotelluric Data ◽  
Low Resistivity ◽  
Ophiolite Belt ◽  
Geological Processes ◽  
Metallogenic Belt

AbstractCrustal architecture strongly influences the development and emplacement of mineral zones. In this study, we image the crustal structure beneath a metallogenic belt and its surroundings in the Bayankhongor area of central Mongolia. In this region, an ophiolite belt marks the location of an ancient suture zone, which is presently associated with a reactivated fault system. Nearby, metamorphic and volcanic belts host important mineralization zones and constitute a significant metallogenic belt that includes sources of copper and gold. However, the crustal structure of these features, and their relationships, are poorly studied. We analyze magnetotelluric data acquired across this region and generate three-dimensional electrical resistivity models of the crustal structure, which is found to be locally highly heterogeneous. Because the upper crust (< 25 km) is found to be generally highly resistive (> 1000 Ωm), low-resistivity (< 50 Ωm) features are conspicuous. Anomalous low-resistivity zones are congruent with the suture zone, and ophiolite belt, which is revealed to be a major crustal-scale feature. Furthermore, broadening low-resistivity zones located down-dip from the suture zone suggest that the narrow deformation zone observed at the surface transforms to a wide area in the deeper crust. Other low-resistivity anomalies are spatially associated with the surface expressions of known mineralization zones; thus, their links to deeper crustal structures are imaged. Considering the available evidence, we determine that, in both cases, the low resistivity can be explained by hydrothermal alteration along fossil fluid pathways. This illustrates the pivotal role that crustal fluids play in diverse geological processes, and highlights their inherent link in a unified system, which has implications for models of mineral genesis and emplacement. The results demonstrate that the crustal architecture—including the major crustal boundary—acts as a first‐order control on the location of the metallogenic belt.

Archean terrane docking: upper crust collision tectonics, Abitibi greenstone belt, Quebec, Canada

1996 ◽  
Vol 265 (1-2) ◽  
pp. 127-150 ◽  
Author(s):  
W.U Mueller ◽  
R Daigneault ◽  
J.K Mortensen ◽  
E.H Chown
Keyword(s):  
Greenstone Belt ◽  
Upper Crust ◽  
Abitibi Greenstone Belt ◽  
Collision Tectonics

Lower crustal low-resistivity zones caused by compaction-induced fluid localization and stagnation – recent results from electromagnetic data in an intracontinental setting

2021 ◽  
Author(s):  
Matthew Joseph Comeau ◽  
Michael Becken ◽  
James A. D. Connolly ◽  
Alexander Grayver ◽  
Alexey V. Kuvshinov ◽  
...  
Keyword(s):  
Electrical Resistivity ◽  
Transition Zone ◽  
Hydrodynamic Model ◽  
Lower Crust ◽  
Dislocation Creep ◽  
Magnetotelluric Data ◽  
Low Resistivity ◽  
Vertical Extent ◽  
Brittle Ductile Transition ◽  
Lower Crustal

&lt;p&gt;We investigate how a conceptual hydrodynamic model consisting of fluid localization and stagnation by thermally activated compaction can explain low-resistivity anomalies observed in the lower crust (&gt;20 km depth). Electrical resistivity models, derived from magnetotelluric data collected across the intracontinental Bulnay region, a subset of a larger regional array across central Mongolia, are generated. They reveal low-resistivity (3 - 30 &amp;#937;m) domains with a width of ~25 km and a vertical extent of &lt;10 km in the lower crust, with their tops ~5 km below the brittle-ductile transition zone. In 3-D these features appear as laterally extended (tube-like) structures, 300 km long, rather than disconnected ellipsoids. The features are oriented parallel to the adjacent Bulnay fault zone segments and perpendicular to the far-field compressive tectonic stress (i.e., northward motion from China and Tibet). These low-resistivity domains are consistent with the presence of saline metamorphic fluids. Deeper features imaged with the data include a large upper mantle conductor that we attribute to an asthenospheric upwelling, and thin lithosphere, related to intraplate surface uplift and volcanism, in agreement with recent geodynamic modelling of lithospheric removal in this region.&lt;/p&gt;&lt;p&gt;Based on the observed thermal structure of the crust, and assuming the mean stress at the brittle-ductile transition is twice the vertical load, the hydrodynamic model predicts that fluids would collect in zones &lt;9 km below the brittle-ductile transition zone, and the zones would have a vertical extent of ~9 km, both in agreement with the resistivity models across the Bulnay region. The hydrodynamic model also gives plausible values for the activation energy for viscous creep (270 - 360 kJ/mol), suggesting that the mechanism is dislocation creep.&lt;/p&gt;&lt;p&gt;From the electrical resistivity models, the lower crustal viscous compaction-length is constrained to be ~25 km - in this region. Within the conceptual model, this length-scale is entirely consistent with independent estimates for the specific hydraulic and rheological properties of this region. In fact, this can be used to independently constrain acceptable ranges for the lower crustal effective viscosity, which is found to be low (on the order of 10^18 Pas). Accordingly, the results indicate that low-salinity fluids (likely 1 - 0.01 wt% NaCl), and correspondingly low porosities (likely 5 - 0.1 vol%), are the most plausible. These key findings suggest partial melts are not favoured to explain the anomalies. Overall, the results of this contribution imply that it is tectonic and compaction processes that control lower crustal fluid flow, rather than lithological or structural heterogeneity.&lt;/p&gt;

Imaging the magmatic system beneath the Krafla geothermal field, Iceland: A new 3-D electrical resistivity model from inversion of magnetotelluric data

2019 ◽  
Vol 220 (1) ◽  
pp. 541-567 ◽  
Author(s):  
Benjamin Lee ◽  
Martyn Unsworth ◽  
Knútur Árnason ◽  
Darcy Cordell
Keyword(s):  
Electrical Resistivity ◽  
Volcanic Field ◽  
Geothermal Field ◽  
Fault System ◽  
Impedance Tensor ◽  
Magnetotelluric Data ◽  
Near Surface ◽  
Low Resistivity ◽  
Rhyolite Magma ◽  
Resistivity Model

SUMMARY Krafla is an active volcanic field and a high-temperature geothermal system in northeast Iceland. As part of a program to produce more energy from higher temperature wells, the IDDP-1 well was drilled in 2009 to reach supercritical fluid conditions below the Krafla geothermal field. However, drilling ended prematurely when the well unexpectedly encountered rhyolite magma at a depth of 2.1 km. In this paper we re-examine the magnetotelluric (MT) data that were used to model the electrical resistivity structure at Krafla. We present a new 3-D resistivity model that differs from previous inversions due to (1) using the full impedance tensor data and (2) a finely discretized mesh with horizontal cell dimensions of 100 m by 100 m. We obtained similar resistivity models from using two different prior models: a uniform half-space, and a previously published 1-D resistivity model. Our model contains a near-surface resistive layer of unaltered basalt and a low resistivity layer of hydrothermal alteration (C1). A resistive region (R1) at 1 to 2 km depth corresponds to chlorite-epidote alteration minerals that are stable at temperatures of about 220 to 500 °C. A low resistivity feature (C2) coincides with the Hveragil fault system, a zone of increased permeability allowing interaction of aquifer fluids with magmatic fluids and gases. Our model contains a large, low resistivity zone (C3) below the northern half of the Krafla volcanic field that domes upward to a depth of about 1.6 km b.s.l. C3 is partially coincident with reported low S-wave velocity zones which could be due to partial melt or aqueous fluids. The low resistivity could also be attributed to dehydration and decomposition of chlorite and epidote that occurs above 500 °C. As opposed to previously published resistivity models, our resistivity model shows that IDDP-1 encountered rhyolite magma near the upper edge of C3, where it intersects C2. In order to assess the sensitivity of the MT data to melt at the bottom of IDDP-1, we added hypothetical magma bodies with resistivities of 0.1 to 30 Ωm to our resistivity model and compared the synthetic MT data to the original inversion response. We used two methods to compare the MT data fit: (1) the change in r.m.s. misfit and (2) an asymptotic p-value obtained from the Kolmogorov–Smirnov (K–S) statistical test on the two sets of data residuals. We determined that the MT data can only detect sills that are unrealistically large (2.25 km3) with very low resistivities (0.1 or 0.3 Ωm). Smaller magma bodies (0.125 and 1 km3) were not detected; thus the MT data are not sensitive to small rhyolite magma bodies near the bottom of IDDP-1. Our tests gave similar results when evaluating the changes in r.m.s. misfit and the K–S test p-values, but the K–S test is a more objective method than appraising a relative change in r.m.s. misfit. Our resistivity model and resolution tests are consistent with the idea of rhyolite melt forming by re-melting of hydrothermally altered basalt on the edges of a deeper magma body.

A seismic-reflection-based regional cross section of the southern Abitibi greenstone belt

1995 ◽  
Vol 32 (2) ◽  
pp. 135-148 ◽  
Author(s):  
S. L. Jackson ◽  
A. R. Cruden ◽  
D. White ◽  
B. Milkereit
Keyword(s):  
Greenstone Belt ◽  
Seismic Reflection ◽  
Lower Layer ◽  
Middle Layer ◽  
Shear Zones ◽  
Low Pressure ◽  
Crustal Thickening ◽  
Upper Crust ◽  
Abitibi Greenstone Belt ◽  
Seismic Reflection Profiles

Seismic reflection profiles from the southern Abitibi greenstone belt reveal four first-order subdivisions: (1) Between 0 and ~4.5 s, the upper crust is weakly reflective, with prominent local to laterally extensive reflections. (2) Between ~4 and ~9 s, the crust is strongly and heterogeneously reflective with laterally continuous reflections. (3) From ~9 to ~13 s, the crust is more homogeneously reflective and displays downward decreasing reflectivity. (4) Below ~13 s (Moho?) the upper mantle is weakly reflective. The upper layer may correspond to subgreenschist–greenschist-facies supracrustal rocks cut by low-angle shear zones and intruded by regional tabular batholiths; the middle layer, to ductiley deformed amphibolite-facies gneisses, granitoids, and (or) metasediments; and the lower layer, to more homogeneously deformed granulite-facies rocks. North-dipping, low-angle reflections extending beneath both diverse supracrustal assemblages and regional batholiths may represent structural detachments upon which both the supracrustal assemblages and batholiths were imbricated and translated southward. However, the preservation of regional low-pressure metamorphic rocks and the common para-autochthonous relationships between assemblages suggest that thrust-related vertical separations and the magnitude of crustal thickening were not large. Steeply dipping regional shear zones within the greenstone belt appear to disrupt subhorizontal reflections down to ~15 km and may represent late-tectonic strains, which were progressively concentrated into linear zones during continued north–south shortening. The crustal-scale structure determined from the seismic reflection profiles, combined with surface geology, is compatible with post-2.70 Ga north–south shortening accommodated by south-directed(?) thrusting in a thermally softened mid crust and by upright folding in the upper crust. This scenario is comparable to recently proposed models for the Paleozoic, high-temperature, low-pressure Lachlan fold belt of Australia.

Geophysical characterization of the Menengai volcano, Central Kenya Rift from the analysis of magnetotelluric and gravity data

Geophysics ◽  
2013 ◽  
Vol 78 (4) ◽  
pp. B187-B199 ◽  
Author(s):  
Antony Munika Wamalwa ◽  
Kevin L. Mickus ◽  
Laura F. Serpa
Keyword(s):  
Gravity Data ◽  
Gravity Models ◽  
High Resistivity ◽  
Geothermal System ◽  
Low Density ◽  
Fracture Density ◽  
Density Region ◽  
Magnetotelluric Data ◽  
Low Resistivity ◽  
2D Resistivity

In this study, we qualitatively analyze detailed gravity and broadband magnetotelluric data in and surrounding the Menengai volcano of the East African rift in Kenya to assess geothermal potential of the region. Three-dimensional gravity models obtained by inverting residual gravity anomalies and 2D resistivity models obtained by inverting the transverse electric and transverse magnetic magnetotelluric modes show several common features. Our models show that a low-resistivity zone above a higher resistivity zone correlates with a low-density region located 1–4 km beneath the volcano. These zones may be related to a high temperature gradient or hydrothermally altered, fractured rocks. Additionally, a low-resistivity ([Formula: see text]) and a low-density region located approximately 4–6 km below the volcano may be related to molten material that is the source of heat for the geothermal system. The low-resistivity ([Formula: see text]) regions that correlated with a denser ([Formula: see text]) region within the caldera are bounded by high-resistivity ([Formula: see text]), high-density ([Formula: see text]) volcanic units implying that the dense and electrically resistive volcanic material is relatively cool and lacks significant fluid content that can lower resistivity. At shallow depths, 0.5–1.5 km below the caldera, a low-resistivity and low-to-moderate density region is interpreted as a zone with high fracture density that consists of clay minerals resulting from hydrothermal alteration. These results agree well with the results from previous seismic studies on the depth of the suggested molten rocks.

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