Chapter 5: The Low-Grade, Neoproterozoic, Vein-Style, Carbonaceous Phyllite-Hosted Paracatu Gold Deposit, Minas Gerais, Brazil

2020 ◽  
pp. 101-120
Author(s):  
Nicholas H.S. Oliver ◽  
Brian Thomson ◽  
Flavio H. Freitas-Silva ◽  
Rodney J. Holcombe
Keyword(s):  
Hanging Wall ◽  
Black Shales ◽  
Passive Margin ◽  
Ductile Deformation ◽  
Low Grade ◽  
Physico Chemical ◽  
Mineralized Zone ◽  
Brasiliano Orogeny ◽  
Carbonaceous Phyllite ◽  
Zinc Deposits

Abstract The Paracatu deposit in Brazil is a shallowly dipping, bulk-tonnage, low-grade, vein-style orogenic Au orebody hosted in very strongly deformed Neoproterozoic carbonaceous phyllite of the southern Brasília fold belt. At regional to district scales, the gold orebody lies along the eastern, hanging-wall edge of a major thrust of the ~630 Ma Brasiliano orogeny. This thrust cuts through a facies transition between clastic-dominated rocks of the Canastra Group and carbonate-dominant rocks of the Vazante Group, deposited at ~1000 Ma in a rift to passive-margin environment on the flank of the São Francisco craton. At the same scales, the footwall of this major thrust system hosts numerous structurally controlled zinc deposits including Vazante and Morro Agudo. At Paracatu, ore genesis occurred primarily by the formation of early tectonic quartz sulfide-carbonate veins, prior to substantial ductile deformation (boudinage), local physico-chemical reworking of these veins, and redistribution of some gold. Structural, geochemical, and isotopic data indicate a strong influence of the local rocks (cm to 100-m scales) on many ore ingredients, and the quartz and carbonate in ore veins were most likely derived locally (cm to m scales). However, the coassociation of gold and arsenic with the boudinaged veins and a major thrust, and the absence of metal enrichments normally associated with syngenetic metalliferous black shales, supports a model of far-field derivation of gold within this metasedimentary package (km to 10-km scales). Transport of metal-bearing fluids toward a favorable structural and chemical site during thrusting and orogenesis was possibly focused, during precipitation to ore grades, by the position of transverse structures in the basement, which also influenced deposition of the adjacent zinc deposits. Successful mining of the low-grade resource was initially favored by the subhorizontal orebody geometry and weathering characteristics, and subsequently by high production rates from the 100-m-thick mineralized zone.

scholarly journals The thrust contact between the Canastra and Vazante groups in the Southern Brasília Belt: structural evolution, white mica crystallinity and implications for the Brasiliano orogeny

2016 ◽  
Vol 46 (4) ◽  
pp. 567-583 ◽  
Author(s):  
Manuela de Oliveira Carvalho ◽  
◽  
Claudio de Morisson Valeriano ◽  
Pamela Alejandra Aparicio González ◽  
Gustavo Diniz Oliveira ◽  
...  
Keyword(s):  
Structural Evolution ◽  
White Mica ◽  
Low Grade ◽  
Kink Bands ◽  
Metasedimentary Rocks ◽  
Kübler Index ◽  
Crenulation Cleavage ◽  
Thrust Sheets ◽  
Brasiliano Orogeny ◽  
Carbonaceous Phyllite

ABSTRACT: Two regional thrust-sheets of Neoproterozoic metasedimentary rocks occur in the Southern Brasília Belt, northwest Minas Gerais. The lower one comprises the Vazante Group, that is formed in the studied area, from base to top, by the Serra do Garrote (metapelites interlayered with carbonaceous phyllite), Serra do Poço Verde (beige to pink stromatolitic metadolomite with interlayered greenish slates), Morro do Calcário (gray stromatolitic metadolomite interlayered with gray slates) and Serra da Lapa (phyllite with dolarenitic lenses interlayered with slates) formations. The upper thrust sheet consists of the Canastra Group (Paracatu formation): laminated sericite phyllites and carbonaceous phyllites interlayered with quartzite. The Braziliano orogeny resulted in four phases of contractional deformation, associated with low-grade metamorphism. The first two (D1 and D2) are ductile, while the third and fourth ones (D3 and D4) are brittle-ductile. D1 developed a slaty S1 cleavage subparallel to the primary layering, with shallow to steep dips to NW. D2 developed a crenulation cleavage (S2) that dips moderately to NW and is associated with tight to isoclinal folds. D3 and D4 phases developed crenulations and open folds and kink bands. S3 dips steeply to NW, while S4 has moderate to steep dips to NE and SW. White mica crystallinity (Kübler index) measurements in metapelites indicate that both the Canastra and Vazante groups reached anchizone/epizone conditions, and metamorphic discontinuities along thrusts indicate that the peak of metamorphism is pre or syn-thrusting.

40Ar/ 39Ar laser dating of Zongwulong ductile shear zone in northeastern Tibetan Plateau ：Constrains on the closure time of the northmost Paleo-Tethys ocean

2021 ◽  
Author(s):  
Wanli Gao ◽  
Zongxiu Wang
Keyword(s):  
Shear Deformation ◽  
Shear Zone ◽  
Ductile Deformation ◽  
Low Grade ◽  
Ductile Shear Zone ◽  
The South ◽  
Oblique Collision ◽  
Ductile Shear ◽  
Tethys Ocean ◽  
Tectonic Belt

<p><strong><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.67d6c7216eff55356050161/sdaolpUECMynit/12UGE&app=m&a=0&c=5572aca4b392eef83f52919e1be673e9&ct=x&pn=gepj.elif&d=1" alt="">Abstract</strong>：The Zongwulong tectonic belt (ZTB) is located between the northern Qaidam tectonic belt and the south Qilian orogenic belt and contains Late Paleozoic and Early- Middle Triassic strata. Structural features and geochronology of Zongwulong ductile shear zone have key implications for the tectonic property of the ZTB. This study integrated field structure, microscopic structure and <sup>40</sup>Ar/<sup>39</sup>Ar laser probe analysis. The shear zone strikes ~NEE-SWW and dips at a high angle, with a NWW-SEE trending and WE stretching lineation, indicating the shear zone as a thrust- slip shear ductile shear. The asymmetric folds, rotating porphyroclast,structural lens and crenulation cleavage can be seen in the field. Mica fish, S − C fabrics, σ type quartz porphyroclastic and quartz wire drawing structure can also be observed under microscope, indicating that the strike- slip- related ductile deformation and mylonitization occurred under low- grade greenschist facies conditions at temperatures of <em>300° C − 400° C</em>.  The highly deformed<br>mylonite schist yielded <sup>40</sup>Ar/<sup>39</sup>Ar ages <em>(245.8±1.7)Ma </em>and <em>(238.5±2.6)</em>Ma for muscovite and biotite, respectively, indicating that the shear deformation occurred during the Early- Mid Triassic. Combined with comprehensive analysis of regional geology and petrology, the authors hold that the age of ductile shear deformation represents the time of Triassic orogeny in the ZTB. The oroginic activity was probably related to the oblique collision between the South Qilian block and the Oulongbuluke block after the closure of the northermost Paleo-Tethys ocean.</p>

scholarly journals Metallogenic Evolution of the Mackenzie and Eastern Selwyn Mountains of Canada’s Northern Cordillera, Northwest Territories: A Compilation and Review

2013 ◽  
Vol 40 (1) ◽  
Author(s):  
Luke Ootes ◽  
Sarah A. Gleeson ◽  
Elizabeth Turner ◽  
Kirsten Rasmussen ◽  
Steve Gordey ◽  
...  
Keyword(s):  
Northwest Territories ◽  
Hanging Wall ◽  
Gold Deposits ◽  
Mineral Deposit ◽  
Iron Formation ◽  
Low Grade ◽  
Base Metals ◽  
Geologic History ◽  
Coal Deposits ◽  
Mackenzie Mountains

The Mackenzie and eastern Selwyn Mountains, Northwest Territories, Canada, are the northeast expression of the Cordilleran orogen and have a geologic history that spans the last one billion years. The region has undergone a diverse tectonic evolution, which is reflected in an equally diverse collection of mineral deposits and prospects. More than 300 of these deposits and prospects have been documented in this area of the Northwest Territories and here they are categorized into mineral deposit types and their mode of formation evaluated and highlighted. Stratiform/stratabound Cu-Ag occurrences are hosted in the Neoproterozoic Coates Lake Group, generally preserved in the hanging wall of the Cretaceous Plateau fault, and define a belt through the central part of the Mackenzie Mountains. Low-grade phosphatic stratiform iron (47.5% Fe) occurs as iron formation in the Neoproterozoic Rapitan Group in the very northwest of the Mackenzie Mountains. Sedimentary exhalative Zn-Pb (± Ba) deposits are preserved in Cambrian through Devonian strata of the Selwyn Basin in the eastern Selwyn Mountains. Numerous carbonate-hosted Zn-Pb (± base-metals) occurrences are located in the Paleozoic strata of the Mackenzie Platform in the Mackenzie Mountains. Cretaceous felsic-intermediate plutons, which occur throughout the eastern Selwyn Mountains, are associated with tungsten skarn (proximal to intrusions), base-metal skarn (distal from intrusions), rare metals, semi-precious tourmaline related to pegmatites, and vein-hosted emeralds. Other resources of potential interest include coal deposits, placer gold, and possible Carlin-type gold deposits that have recently been identified farther west in the Yukon.SOMMAIRELes monts Mackenzie et ceux de la chaîne orientale de Selwyn, dans les Territoires du Nord-Ouest, au Canada, sont l'expression au nord-est de l'orogène de la Cordillère, et leur histoire géologique s’étale sur le dernier milliard d’années. La région a été l’hôte d’une évolution tectonique diversifiée, et cela se reflète par une suite tout aussi diversifiée de gisements minéraux et d’indices prometteurs. Plus de 300 de ces dépôts et indices prometteurs ont été documentées dans cette région des Territoires du Nord-Ouest, et le présent article ils sont classés en types de gîtes minéraux, et l’attention est portée sur leur mode de formation. Les gisements de Cu-Ag stratiformes ou stratoïdes sont encaissés dans le Groupe néoprotérozoïque de Coates Lake, et ils sont généralement préservés dans l'éponte supérieure de la faille du plateau crétacé, et ils forment une bande qui traverse la partie centrale des monts Mackenzie. Le fer se retrouve dans des gisements phosphatées stratiformes à faible teneur (47,5% Fe) qui provient de formations de fer dans le Groupe néoprotérozoïque de Rapitan situé dans la pointe nord-ouest des monts Mackenzie. Des gisements sédimentaires exhalatifs de Zn-Pb (± Ba) sont préservés dans des strates cambriennes à dévoniennes du bassin de Selwyn dans la portion est des monts Selwyn. De nombreux indices de Zn-Pb (± métaux communs) dans des roches carbonatées des strates paléozoïques de la plate-forme de Mackenzie, des monts Mackenzie. Des plutons felsiques intermédiaires crétacés, qui pointent tout au long de la chaîne est de Selwyn, sont associées à des skarns de tungstène (proximaux), à des skarns de métaux communs (distaux), à des concentrations de métaux rares, de tourmaline semi-précieuses liés aux pegmatites, et à des émeraudes filoniennes. Parmi d’autres ressources d'intérêt, on retrouve des gisements de charbon, d'or alluvionnaire, et d’éventuels gisements d'or de type Carlin qui ont été découverts récemment plus à l'ouest au Yukon.

Coexistence of pyrophyllite, I-S, R1 and NH4+-rich illite in Silurian black shales (Sierra de Albarracín, NE Spain): metamorphic vs. hydrothermal origin

Clay Minerals ◽  
2010 ◽  
Vol 45 (3) ◽  
pp. 383-392 ◽  
Author(s):  
B. Bauluz ◽  
I. Subías
Keyword(s):  
Organic Matter ◽  
Clay Mineral ◽  
Hydrothermal Alteration ◽  
Black Shales ◽  
Low Grade ◽  
Lower Silurian ◽  
Igneous Activity ◽  
Upper Silurian ◽  
Ne Spain ◽  
Regional Tectonics

AbstractA set of Silurian black shales from Sierra de Albarracín (NE Spain) corresponding to two different sections was studied to determine the relative influence of diagenesis, igneous activity, and regional tectonics on the clay-mineral genesis. The coexistence of pyrophyllite, I-S interstratifications (R1), ammonium-rich illite, potassium illite, kaolin, and chlorite is not the result of prograde evolution during diagenesis – very low-grade metamorphism. Three different stages may be inferred: (1) sedimentation of black shales (Aeronian, Lower Silurian, to basal Ludfordian, Upper Silurian) and the subsequent diagenetic process producing the coexistence of quartz, illite, kaolinite, organic matter, etc.; (2) intrusion of andesitic sills producing hydrothermal alteration and crystallization of pyrophyllite, ammonium-rich illites, smectite, I-S R1 phases and jarosite; and (3) and folding of shales and sills and development of penetrative schistosity during the late Variscan leading to illite and paragonite recrystallization reaching the anchizone grade.

Accretion tectonics in northern Eritrea revealed by remotely sensed imagery

1993 ◽  
Vol 130 (2) ◽  
pp. 177-190 ◽  
Author(s):  
S. A. Drury ◽  
S. M. Berhe
Keyword(s):  
Remotely Sensed ◽  
Ductile Deformation ◽  
Low Grade ◽  
Arc Volcanism ◽  
Remotely Sensed Imagery ◽  
Pan African ◽  
Major Fault ◽  
High Level ◽  
Back Arc ◽  
Layered Sequence

AbstractNew details from remotely sensed images of the structure and disposition of broad lithological variations in the Pan-African of northern Eritrea are discussed in the context of accretionary tectonics. The recognition of major north-south structural discontinuities allows the area to be divided into three discrete terranes with apparently different histories of deformation and metamorphism, magmagenesis and sedimentation. The central Hagar Terrane is dominated by large ultramafic masses with a volcano-sedimentary layered sequence, and shows the effects of major sinistral transpression and lateral expulsion. It is bounded to the west by a major fault, the Barka suture, and abuts the older Barka Terrane that comprises metasediments with evidence for polyphase ductile deformation and pre-kinematic dyke emplacement. The Hagar Terrane is thrust against the eastern Nacfa Terrane, which is dominated by low-grade calc-alkaline metavolcanics and immature volcanoclastic sediments intruded by syn-kinematic plutons. These units are pre-dated by an earlier high-grade basement and post-dated by high-level unmetamorphosed silicic volcanics and redbed sediments. The complex is suggested to have been assembled by oblique accretion from the southeast after arc volcanism in the Nacfa Terrane and back-arc extension in the Hagar Terrane ended with the cease of subduction.

U–Pb constraints on the thermotectonic evolution of the Vernon antiform and the age of the Aberdeen gneiss complex, southeastern Canadian Cordillera

2006 ◽  
Vol 43 (2) ◽  
pp. 213-244 ◽  
Author(s):  
P Glombick ◽  
R I Thompson ◽  
P Erdmer ◽  
L Heaman ◽  
R M Friedman ◽  
...  
Keyword(s):  
Shear Zone ◽  
Hanging Wall ◽  
Ductile Deformation ◽  
Granitic Rocks ◽  
Tectonic Activity ◽  
Canadian Cordillera ◽  
Metamorphic Complex ◽  
Seismic Reflection Data ◽  
Gneiss Complex ◽  
Shuswap Metamorphic Complex

The Aberdeen gneiss complex is composed of complexly deformed migmatitic orthogneiss and paragneiss situated within the core of the Vernon antiform, a structure defined by a series of subparallel reflectors visible at upper to middle crustal depths (6–18 km) in seismic reflection data from the Vernon area of the Shuswap metamorphic complex. The Vernon antiform and the Aberdeen gneiss complex lie within the footwall of the gently west dipping (top to the west) Kalamalka Lake shear zone. Migmatitic gneiss exposed within the antiform records evidence (recorded as age domains in complexly zoned zircon grains) of three metamorphic events, occurring at 155–150, 90, and 66–51 Ma. The timing of magmatic events within the antiform includes emplacement of diorite at ~232 Ma, tonalite at ~151 Ma, granodiorite at 102 Ma, and monzonite at 52 Ma. Middle to Late Jurassic metamorphism resulted in widespread migmatization. Early Tertiary metamorphism (66–51 Ma) was coeval with the emplacement of granitic rocks and exhumation typical of other areas of the Shuswap metamorphic complex. Highly deformed orthogneiss situated within the hanging wall of the Kalamalka Lake shear zone, comprising the superstructure, was emplaced at ~171 Ma. Ductile deformation had ceased by 162 Ma. The complex metamorphic and magmatic evolution of the Vernon antiform, which is similar to other areas of the southern Canadian Cordillera including the Nicola horst, Mount Lytton – Eagle plutonic complex, Cariboo Mountains, and Mica Creek area, may reflect episodic tectonic activity at the plate margin.

Mesoproterozoic dyke swarms in foreland and nappes of the central Scandinavian Caledonides: structure, magnetic fabric, and geochemistry

2007 ◽  
Vol 144 (3) ◽  
pp. 525-546 ◽  
Author(s):  
R. O. GREILING ◽  
J. C. GRIMMER ◽  
H. DE WALL ◽  
L. BJÖRK
Keyword(s):  
Thin Layers ◽  
Passive Margin ◽  
Magnetic Fabric ◽  
Low Grade ◽  
Silicate Mineral ◽  
Geophysical Field ◽  
Mafic Dykes ◽  
Magnetic Fabrics ◽  
Basement Rocks ◽  
Plate Reconstructions

As an example of microstructural and magnetic fabric evolution, and geochemistry of mafic dykes during a subsequent orogenic overprint, a major Mesoproterozoic dyke complex in Scandinavia, the Västerbotten complex of the Central Scandinavian Dolerite Group, is traced westwards into the crystalline nappes of the early Phanerozoic Caledonian orogen. Using geophysical, field, microscopic, magnetic and geochemical information, dykes and sills are characterized, and their overprint during Caledonian orogeny documented. The Västerbotten complex is composed of sets of dykes, trending NE–SW, NW–SE and WNW–ESE, respectively. Similar dykes are exposed in allochthonous positions (Lower and Middle allochthons) in the Caledonian fold-and-thrust belt. The autochthonous dykes are generally undeformed and retain both their primary texture and mineralogy. Chilled margins are well preserved. In the Caledonian Lower and Middle allochthons, similar dykes in crystalline basement rocks are progressively faulted and sheared when proceeding from the marginal to the interior parts of the orogen. Dyke margins are more likely to be sheared than the interior parts of dykes. In the Lower Allochthon, under very low- and low-grade metamorphic conditions, dykes are distinctly less competent than granitic host rocks. Thick dykes are more competent than gneisses; thin dykes do not show such competence contrasts. In the Middle Allochthon, metre-scale dykes with patches of altered plagioclase phenocrysts can still be discerned in low-strain domains. Highly sheared dykes are drawn out to thin layers of centimetre thickness. Dykes are deformed together with the crystalline country rocks under greenschist-grade metamorphic conditions without major competence contrasts. Magnetic fabrics show an evolution similar to the silicate mineral fabrics. The magnetic fabrics in the dykes are transformed successively from ferromagnetic–magmatic in the Autochthon to ferromagnetic deformational in the Lower Allochthon and, finally, paramagnetic deformational in the Middle Allochthon. As a consequence, the magnetic susceptibility decreases for several orders of magnitude. Geochemically, the dykes are dominantly sub-alkaline basalts typical for continental tholeiites and can be distinguished from the Neoproterozoic dykes in the Särv-Nappe equivalents (highest part of the Middle Allochthon), which show a more MORB-like (E-MORB) magmatic signature. Preliminary age information from a dyke in the Lower Allochthon of the Børgefjell area and the Middle Allochthon is consistent with the assumption that these dykes are time equivalent with the Central Scandinavian Dolerite Group. Therefore, the studied dykes may represent an extension of the Västerbotten complex or a new complex of the Central Scandinavian Dolerite Group. According to section restorations, the Caledonian allochthons were situated further WNW relative to their present position, and, originally, the mafic dykes cut across all of the Fennoscandian lithosphere, at least to the present Atlantic margin and the earlier passive margin of the Baltica terrane. As a consequence, these dykes may provide a link for pre-Caledonian and pre-Grenvillian plate reconstructions.

Deformational geometry and tectonic significance of a portion of the Chilliwack Group, northwestern Cascades, Washington

1988 ◽  
Vol 25 (3) ◽  
pp. 433-441 ◽  
Author(s):  
Moira Smith
Keyword(s):  
High Pressure ◽  
Volcanic Rocks ◽  
Ductile Deformation ◽  
Metamorphic Rocks ◽  
Low Grade ◽  
North Cascades ◽  
Clastic Rocks ◽  
Tectonic Significance ◽  
Group A ◽  
Event Based

The northwestern Cascades structural province can be interpreted as an accretionary complex comprising fault-bounded blocks of pre-Tertiary metamorphic rocks of diverse age and lithologic type. This paper documents the deformation in a portion of the Chilliwack Group, a unit in this complex. The Chilliwack Group is a thick sequence of volcaniclastic sedimentary rocks, calc-alkaline volcanic rocks, and limestone that is metamorphosed to low-grade blueschist facies. The rocks underwent ductile deformation during a Late Cretaceous orogenic event, producing a subhorizontal foliation and, in appropriate lithologies, subhorizontal stretching lineations that trend N20°W. Finite strain sustained by coarse clastic rocks produced RXZ values averaging 3.5. The deformation at least partially postdates the high pressure metamorphic event, based on the presence of bent and broken high-pressure mineral grains. Although early studies postulated west-vergent thrust imbrication of units in the northwest Cascades, the N20°W direction of apparent elongation in the Chilliwack Group, consistent with the direction of motion along segments of the Shuksan fault elucidated in other more recent studies, may reflect significant, highly oblique components of convergence during formation of the western North Cascades collisional orogen.

scholarly journals The Erzgebirge, Germany, a subducted part of northern Gondwana: geochemical evidence for repetition of early Palaeozoic metasedimentary sequences in metamorphic thrust units

1998 ◽  
Vol 135 (6) ◽  
pp. 785-801 ◽  
Author(s):  
BIRGIT MINGRAM
Keyword(s):  
Granulite Facies ◽  
Passive Margin ◽  
Radiometric Dating ◽  
Chemical Compositions ◽  
Low Grade ◽  
Granulite Facies Metamorphism ◽  
Margin Setting ◽  
New Finding ◽  
Early Palaeozoic ◽  
Metamorphic Terranes

One of the major metamorphic terranes of the Bohemian Massif, the Erzgebirge, is interpreted to record a subducted part of a Palaeozoic margin of Gondwana. A geochemical study on non-calcareous metasediments from the various metamorphic units from lower greenschist to granulite facies metamorphism supports a recently established thrust model. Geochemical discrimination and correlation from the metasediments of the Erzgebirge suggest repetition of an early Palaeozoic metasedimentary sequence in metamorphic thrust units. This new finding is in line with recent radiometric dating of intercalated metarhyolitic rocks, which yielded ages of around 480 Ma. It is furthermore supported by correlation with a low-grade standard section in Thuringia, which represents the transition from an orogenic belt to a passive margin setting, with highly mature sediments. Significant geochemical signatures have been identified in three different lithotypes, which reappear in at least three metamorphic units of the Erzgebirge. Geochemical correlation of these units was established using simple comparison of averages and with statistical techniques. The identification of significant geochemical signatures from different lithotypes in metamorphic suites has important implications for terrane analysis and reconstruction of ancient tectonic settings.The repetition of lithologies and their distinct chemical compositions in progressively metamorphosed units is useful for examining element mobility during Barrovian metamorphism. Statistical comparison implies that Li is progressively depleted from the greenschist to amphibolite facies, whereas Ca exhibits some enrichment. All the other elements studied are considered to be immobile.

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