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.

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

2019 ◽  
Author(s):  
Joel B. H. Andersson ◽  
Tobias E. Bauer ◽  
Edward P. Lynch
Keyword(s):  
Iron Oxide ◽  
Hydrothermal Alteration ◽  
Structural Model ◽  
Spatial Relationship ◽  
Structural Evolution ◽  
Shear Zones ◽  
Geological Mapping ◽  
Northwestern Part ◽  
Mineral Associations ◽  
Specific Alteration

Abstract. In this field-based study, a ~ 90 km long Palaeoproterozoic metasupracrustal belt in the northwestern part of the Norrbotten ore province (northernmost Sweden) has been investigated in order to characterize its various structural components and thus constrain its structural evolution. In addition, hydrothermal mineral associations are described and linked to identified deformation phases. New geological mapping of five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka, Kaitum West and Fjällåsen-Allavaara) indicates two major compressional events (D1, D2) that affected the belt whereas each deformation event can be related to specific alteration styles typical for iron oxide-apatite and iron oxide Cu-Au systems. D1 generated a regionally distributed penetrative S1 foliation and oblique reverse shear zones with southwest block up sense-of-shears in response to NE–SW crustal shortening. D1 is associated with regional scapolite ± albite alteration formed coeval with regional magnetite ± amphibole alteration and calcite under epidote-amphibolite metamorphism. During D2, folding of S1 generated steeply south-plunging F2-folds in low strain areas whereas most strain was partitioned into pre-existing shear zones resulting in reverse dip-slip reactivation of steep NNW-oriented D1 shear zones and strike-slip dominated movements along steep E–W-trending shear zones under brittle-ductile conditions. The hydrothermal alteration linked to the D2 deformation phase is more potassic in character and dominated by K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulphides, and calcite. Our results underline the importance of paired structural-alteration approaches at the regional- to belt-scale 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. Our new structural model harmonizes with earlier petrological/geochemical tectonic models of the northern Norrbotten area and emphasizes the importance of reactivation of early formed structures.

scholarly journals Precise Location and Mapping of the Main Central Thrust Zone in Reference to Micro-Structures and Deformation along Khudi-Tal Area of Marsyangdi Valley

2020 ◽  
Vol 22 ◽  
pp. 33-40
Author(s):  
Lokendra Pandeya ◽  
Kabi Raj Paudyal
Keyword(s):  
Regional Metamorphism ◽  
Static Condition ◽  
Shear Zones ◽  
Geological Mapping ◽  
Lesser Himalaya ◽  
Main Central Thrust ◽  
Thin Sections ◽  
Ductile Shearing ◽  
Shear Sense ◽  
Micro Structures

Geological mapping was carried out along Marsyangdi valley in the Khudi - Dahare -Tal area on a scale of 1: 50,000 covering about 142 square kilometers. Recent study aims to locate the Main Central Thrust (MCT) precisely based on lithostratigraphy, micro-structures, deformation, and metamorphism. Several thin sections were observed to study the metamorphism, deformation, and micro-structures developed in the rocks. The rocks sequences in both the Higher Himalaya and the Lesser Himalaya have undergone polyphase metamorphism and deformation. The Lesser Himalaya experienced first burial metamorphism (M1) followed by garnet grade inverted metamorphism related to the MCT activity (M2) followed by retrograde metamorphism (M3) whereas the Higher Himalaya has undergone regional high-pressure/ high-temperature kyanite/ sillimanite- grade prograde regional metamorphism (M1) followed by the (M2) related to ductile sharing which in turn is overprinted by the later post-tectonic retrograde garnet to chlorite grade metamorphism during exhumation. The polyphase deformation is indicated by the cross-cutting foliation and many other features. The deformation phase D1 is associated with the development of the bedding parallel foliation due to burial in both the Higher Himalaya and the Lesser Himalaya. Isoclinal folds and crenulation cleavage were developed before the collision is categorized as D2. Development of nearly N- S trending mineral and stretching lineation, south vergent drag folds, folded S2 cleavage and microscopic shear sense indicators, rotated syn- tectonic garnet grains, etc. were developed during the deformation D3 related to the ductile shearing through the MCT. Various brittle faults and shear zones cross-cutting all earlier features were developed during D4 during the upheaval. The rocks in the MCT zone are affected by intense sharing and mylonitization as indicated by the presence of many mylonitic structures in the thin sections throughout the Lesser Himalaya in the area. Features like polygonization and ribbon quartz with evidence of sub-grain rotation, mica fish, syn-tectonic rotated garnet grains indicate the ductile shearing in the MCT area suggesting the dynamic recrystallization in the MCT zone whereas rocks of the Higher Himalaya show the evidence of recrystallization under static condition. The MCT zone was mapped precisely based on the microstructures and deformation.

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.

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

1997 ◽  
Vol 269 (1-2) ◽  
pp. 51-63 ◽  
Author(s):  
O. Tavchandjian ◽  
A. Rouleau ◽  
G. Archambault ◽  
R. Daigneault ◽  
D. Marcotte
Keyword(s):  
Structural Geology ◽  
Shear Zones ◽  
Geostatistical Analysis

Evidence for hydrothermal alteration and source regions for the Kiruna iron oxide–apatite ore (northern Sweden) from zircon Hf and O isotopes

Geology ◽  
2017 ◽  
Vol 45 (6) ◽  
pp. 571-574 ◽  
Author(s):  
Anne Westhues ◽  
John M. Hanchar ◽  
Mark J. LeMessurier ◽  
Martin J. Whitehouse
Keyword(s):  
Iron Oxide ◽  
Hydrothermal Alteration ◽  
Northern Sweden ◽  
Source Regions ◽  
O Isotopes

scholarly journals Engineering Geological and Geotechnical Approaches for the Construction of Powerhouse Cavern of Tehri Pumped Storage Plant (1000 MW) - A Case Study

2019 ◽  
Vol 24 ◽  
pp. 35-44
Author(s):  
Rajeev Prasad ◽  
Nishith Sharma
Keyword(s):  
Underground Structure ◽  
Shear Zones ◽  
Structural Features ◽  
Geological Mapping ◽  
Left Bank ◽  
Rock Composition ◽  
Northern India ◽  
Set Up ◽  
Pumped Storage

Construction of underground Cavern in the Himalayan region is full of challenges and uncertainties. Experience has shown that construction in Himalayan regions requires good understanding of geology, adequate site investigations, proper design and selection of suitable construction methodology and technology. The most commonly encountered geological problems during excavation of underground structure in Hydroelectric Projects are, Fault/Thrust/Shear Zones squeezing and swelling, wedge block failure etc. Tehri Pumped Storage Plant (PSP) is located at the left bank of river Bhagirathi in the state of Uttarakhand in Northern India. This case study indicates about the geological challenges faced and their remedial measures during the construction of Tehri PSP Powerhouse Cavern having dimension of 203m x 24m x 58m.3D-geological mapping with 1:100 scales was carried out in excavated central drift of powerhouse to evaluate the rock composition, behavior of rock mass, structural features and further investigation to finalize the layout and orientation. During the investigation Sheared Phyllite with bands of thinly Phyllite Quartzite rock were encountered in the end portion of central drift of powerhouse which had posed a mammoth challenge in designing the powerhouse cavern. Keeping in view the recommendations of geotechnical experts and the design consultants, decision were made to shift the cavern further by 50 m to avoid Sheared Phyllite bands. The shifting of cavern led to the reorientation of structures like control room, service bay and location of units etc. This paper briefly describes the Engineering Geological and Geotechnical set up of powerhouse with proper investigation approaches and excavation sequences highlighting the importance of orientation and Sheared Phyllite Zone.

Iron Oxide (U–Th)/He Thermochronology: New Perspectives on Faults, Fluids, and Heat

Elements ◽  
2020 ◽  
Vol 16 (5) ◽  
pp. 319-324
Author(s):  
Emily H. G. Cooperdock ◽  
Alexis K. Ault
Keyword(s):  
Fluid Flow ◽  
Iron Oxide ◽  
Heat Exchange ◽  
Iron Oxides ◽  
Fault Zone ◽  
Frictional Heating ◽  
Fault Zones ◽  
Chemical Behavior ◽  
Rock Interaction ◽  
Dynamic Motion

Fault zones record the dynamic motion of Earth’s crust and are sites of heat exchange, fluid–rock interaction, and mineralization. Episodic or long-lived fluid flow, frictional heating, and/or deformation can induce open-system chemical behavior and make dating fault zone processes challenging. Iron oxides are common in a variety of geologic settings, including faults and fractures, and can grow at surface-to magmatic temperatures. Recently, iron oxide (U–Th)/He thermochronology, coupled with microtextural and trace element analyses, has enabled new avenues of research into the timing and nature of fluid–rock interactions and deformation. These constraints are important for understanding fault zone evolution in space and time.

Paleotopography controls weathering of Cambrian-age profiles beneath the Great Unconformity, St. Francois Mountains, SE Missouri, U.S.A.

2020 ◽  
Vol 90 (6) ◽  
pp. 629-650
Author(s):  
Katie J. Pevehouse ◽  
Dustin E. Sweet ◽  
Branimir Šegvić ◽  
Charles C. Monson ◽  
Giovanni Zanoni ◽  
...  
Keyword(s):  
Fluid Flow ◽  
Spatial Relationship ◽  
Soil Formation ◽  
Sedimentary Basins ◽  
X Ray Diffraction ◽  
Soil Thickness ◽  
Subaerial Exposure ◽  
Soil Creep ◽  
Paleozoic Sedimentary Rock ◽  
Fluid Alteration

ABSTRACT Precambrian (1.4 to 1.5 Ga) granite and rhyolite in the St. Francois Mountains at the northeast corner of the Ozark Plateau in Missouri has been altered down to a depth up to 8 meters below the Great Unconformity (the contact between Paleozoic sedimentary rock and underlying Precambrian). Petrographic, geochemical, and mineralogic data indicate that at least two events generated this alteration: 1) surficial weathering due to subaerial exposure of the granite before Cambrian burial—this material is preserved as a paleosol; and 2) alteration due to reaction with basinal fluids channeled along the unconformity from nearby sedimentary basins long after burial by Paleozoic strata. To assess the variation between surficial weathering and basinal fluid alteration, we measured and sampled for petrologic, geochemical, and mineralogic data in the rock at and just below the Great Unconformity at three paleoelevations. Whole-rock geochemical oxide and X-ray diffraction data indicate that K-metasomatism and highly crystalline illite occurred in each profile. The K increase reflects crystallization of authigenic feldspar and illite from basinal fluids that were channeled along the Great Unconformity during younger Paleozoic fluid-flow events. Each profile also exhibits an upward increase in altered feldspar crystals and highly crystalline kaolinite, and an upward decrease in Ca and Na. Such changes reflect soil formation due to reaction with meteoric water before Cambrian burial, indicating that the altered granite was a paleosol before Paleozoic basinal fluid-flow events. Notably, the paleosol at the highest paleoelevation displays the least amount of paleoweathering and the paleosol at the lowest displays the greatest amount of paleoweathering. These results demonstrate that not only can characteristics of the paleosol just below the Great Unconformity be recognized in the St. Francois Mountains, despite subsequent alteration, but also it is possible to detect variations in soil thickness that were controlled by slope steepness and, therefore, water availability and/or soil creep or failure. This spatial relationship is compatible with studies of modern soils which indicate that soil character varies with position on a slope.

scholarly journals Kinematics, deformation partitioning and late Variscan magmatism in the Agly massif, Eastern Pyrenees, France

2020 ◽  
Vol 191 ◽  
pp. 15 ◽  
Author(s):  
Jonas Vanardois ◽  
Pierre Trap ◽  
Philippe Goncalves ◽  
Didier Marquer ◽  
Josselin Gremmel ◽  
...  
Keyword(s):  
Cross Sections ◽  
Shear Zones ◽  
Geological Mapping ◽  
Deformation Pattern ◽  
Detailed Structural Analysis ◽  
Eastern Pyrenees ◽  
Structural Work ◽  
Tectonic Forces ◽  
Orogenic Plateau ◽  
Extensional Collapse

In order to constrain the finite deformation pattern of the Variscan basement of the Agly massif, a detailed structural analysis over the whole Agly massif was performed. Our investigation combined geological mapping, reappraisal of published and unpublished data completed with our own structural work. Results are provided in the form of new tectonic maps and series of regional cross-sections through the Agly massif. At variance from previous studies, we identified three deformation fabrics named D1, D2 and D3. The D1 deformation is only relictual and characterized by a broadly northwest-southeast striking and eastward dipping foliation without any clear mineral and stretching lineation direction. D1 might be attributed to thickening of the Variscan crust in a possible orogenic plateau edge position. The D2 deformation is a heterogeneous non-coaxial deformation, affecting the whole massif, that produced a shallowly dipping S2 foliation, and an anastomosed network of C2 shear zones that accommodated vertical thinning and N20 directed extension. D2 is coeval with LP-HT metamorphism and plutonism at ca. 315–295 Ma. D2 corresponds to the extensional collapse of the partially molten orogenic crust in a global dextral strike-slip at the scale of the whole Variscan belt. The D2 fabrics are folded and steepened along a D3 east-west trending corridor, called Tournefort Deformation Zone (TDZ), where the Saint-Arnac and Tournefort intrusives and surrounding rocks share the same NE-SW to E-W subvertical S3 foliation. Along the D3 corridor, the asymmetrical schistosity pattern and kinematic criteria suggest a D3 dextral kinematics. The D3 deformation is a record of E-W striking dextral shearing that facilitated and localized the ascent and emplacement of the diorite and granitic sheet-shaped plutons. D3 outlasted D2 and turned compressional-dominated in response to the closure of the Ibero-Armorican arc in a transpressional regime. The progressive switch from D2 thinning to D3 transpression is attributed to the lessening of gravitational forces at an advanced stage of extensional collapse that became overcome by ongoing compressional tectonic forces at the southern edge of the Variscan orogenic plateau.

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