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Articles de revues sur le sujet "Aureole tectonics"
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Faure, Michel, Xavier Charonnat, Alain Chauvet, Yan Chen, Jean-Yves Talbot, Guillaume Martelet, Gabriel Courrioux, Patrick Monie et Jean-Pierre Milesi. « Tectonic evolution of the Cevennes para-autochthonous domain of the Hercynian French Massif Central and its bearing on ore deposits formation ». Bulletin de la Société Géologique de France 172, no 6 (1 novembre 2001) : 687–96. http://dx.doi.org/10.2113/172.6.687.
Texte intégralRésumé :
Abstract The Cevennes area belongs to the para-autochthonous domain of the Hercynian Belt of the French Massif Central. Three lithological series, namely: sandstone-pelite, black micaschist and gneiss-micaschist, are identified. They form an imbrication of five tectonic units which overthrust the unmetamorphosed Viganais Paleozoic units to the south and the gneissic Mamejean Unit to the north. The structural, metamorphic and magmatic evolution of the Cevennes area is characterized by three events, namely: (1) southward shearing coeval to a MP/MT metamorphism dated around 340 Ma; (2) post nappe anatexis (T<750 degrees C, P>5 kb); (3) Namurian (ca 315 Ma) E-W extensional tectonics and plutonism. The structure of the Mt-Lozere-Borne granitic complex is constrained by new AMS and gravimetric data. The plutons are the driving power of the hydrothermal convective circulations responsible for an early deposition of diffuse arsenopyrite in the thermal aureole. Gold bearing sulfides are afterwards concentrated in quartz veins along brittle normal and wrench faults around the granite. Lastly, ore bearing quartz pebbles are sedimented in the Stephanian Ales coal basin.
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Brogi, Andrea, Alfredo Caggianelli, Domenico Liotta, Martina Zucchi, Amalia Spina, Enrico Capezzuoli, Alessandra Casini et Elena Buracchi. « The Gavorrano Monzogranite (Northern Apennines) : An Updated Review of Host Rock Protoliths, Thermal Metamorphism and Tectonic Setting ». Geosciences 11, no 3 (8 mars 2021) : 124. http://dx.doi.org/10.3390/geosciences11030124.
Texte intégralRésumé :
We review and refine the geological setting of an area located nearby the Tyrrhenian seacoast, in the inner zone of the Northern Apennines (southern Tuscany), where a Neogene monzogranite body (estimated in about 3 km long, 1.5 km wide, and 0.7 km thick) emplaced during early Pliocene. This magmatic intrusion, known as the Gavorrano pluton, is partially exposed in a ridge bounded by regional faults delimiting broad structural depressions. A widespread circulation of geothermal fluids accompanied the cooling of the magmatic body and gave rise to an extensive Fe-ore deposit (mainly pyrite) exploited during the past century. The tectonic setting which favoured the emplacement and exhumation of the Gavorrano pluton is strongly debated with fallouts on the comprehension of the Neogene evolution of this sector of the inner Northern Apennines. Data from a new fieldwork dataset, integrated with information from the mining activity, have been integrated to refine the geological setting of the whole crustal sector where the Gavorrano monzogranite was emplaced and exhumed. Our review, implemented by new palynological, petrological and structural data pointed out that: (i) the age of the Palaeozoic phyllite (hosting rocks) is middle-late Permian, thus resulting younger than previously described (i.e., pre-Carboniferous); (ii) the conditions at which the metamorphic aureole developed are estimated at a temperature of c. 660 °C and at a depth lower than c. 6 km; (iii) the tectonic evolution which determined the emplacement and exhumation of the monzogranite is constrained in a transfer zone, in the frame of the extensional tectonics affecting the area continuously since Miocene.
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PIPER, J. D. A., N. J. McARDLE et Y. ALMASKERI. « Palaeomagnetic study of the Cairnsmoor of Fleet Granite and Criffel-Dalbeattie granodiorite contact aureoles : Caledonian tectonics of the Southern Uplands of Scotland and Devonian palaeogeography ». Geological Magazine 144, no 5 (19 juin 2007) : 811–35. http://dx.doi.org/10.1017/s0016756807003536.
Texte intégralRésumé :
The plutons of Cairnsmoor of Fleet (392±2 Ma) and Criffel-Dalbeattie (397±2 Ma, both mineral isochron ages) comprise two of four major post-tectonic granitic complexes emplaced into the Southern Uplands, an Ordovician–Silurian back-arc and foreland basin complex formed at the northern margin of the Iapetus Suture. To expand the palaeomagnetic record of the Southern Uplands we have studied palaeomagnetism and magnetic fabrics in traverses spanning contacts of these intrusions with host mudrocks. A uniform anisotropy of magnetic susceptibility (AMS) fabric across the Cairnsmoor of Fleet contact has been enhanced by recrystallization into hornfels near the contact and records a late Acadian regional stress operative during, or soon after, emplacement of the pluton in Middle Devonian times. Magnetization during slow cooling recorded a dual polarity (‘A’) remanence in granite and hornfels with mean direction D/I = 92/−2° (α95 = 6.5°) yielding a palaeopole (Q = 6) at 2°N, 265°E linked to cooling at c. 392 Ma. Subsidiary magnetizations are overprints imparted during Variscan tectonism (‘B’, D/I = 194/6°) and Jurassic rifting within the adjoining Irish Sea Basin (‘C’, c. 160–140 Ma, D/I = 172/−52°). The Criffel-Dalbeattie pluton has more complex AMS fabrics recording both deformation and emplacement effects. Hematite of secondary hydrothermal origin is a significant feature of the rock magnetic record in the aureole, which is otherwise dominated by paramagnetism. The granodiorite is more strongly magnetized than the country rocks, accounting for a positive aeromagnetic anomaly. A fairly dispersed dual polarity remanence (mean D/I = 115/55°, α95 = 18°) in granodiorite and late tectonic porphyrite dykes is probably the oldest magnetization preserved in this pluton because it correlates with an excursion of Britain into southerly palaeolatitudes at c. 410 Ma and indicates an Early Devonian emplacement age. The palaeofield at c. 397 Ma, the currently accepted isotopic age, is recorded by a minority overprinted remanence (mean D/I = 272/2°, α95 = 12°) similar to the record in the Cairnsmoor of Fleet pluton and granites from the adjoining Lake District terrane. Granite complexes of the Southern Uplands Block collectively record regional rotation and excursion of Britain into southerly latitudes between c. 410 and 390 Ma. Comparable Silurian–Devonian palaeomagnetic poles identify common apparent polar wander (APW) in paratectonic and orthotectonic terranes from the Variscan Front in the south to the Laurentian foreland in the north following climactic Acadian deformation. APW between 430 and 390 Ma embracing the (post-closure) history of the Caledonian orogen is a loop executed at rates much higher than typical rates of plate motion and appears to record a component of true polar wander. The ∼110° arc length is identical to polar shift identified between mid-Silurian and Lower–Middle Devonian poles from Gondwana. The two paths superimpose to show that the western margin of Gondwana was in proximity to the SE margin of Laurentia during Acadian deformation in Early–Middle Devonian times and remote from the Caledonides; the residual Rheic Ocean subsequently closed by a combination of pivotal and left lateral strike-slip motions.
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Razanatseheno, M. O. M., A. Nédélec, M. Rakotondrazafy, J. G. Meert et B. Ralison. « Four-stage building of the Cambrian Carion pluton (Madagascar) ». Earth and Environmental Science Transactions of the Royal Society of Edinburgh 100, no 1-2 (mars 2009) : 133–45. http://dx.doi.org/10.1017/s1755691009016156.
Texte intégralRésumé :
ABSTRACTThe 532±5 Ma old Carion pluton is a dark, porphyritic ferro-potassic granitoid emplaced near the late Pan-African Angavo mega-shear zone. A rough normal zoning from tonalitic to granitic compositions can be recognised in the field. Steep magmatic foliations are evidenced by K-feldspar megacryst preferred orientations. Microstructures are either magmatic or typical of incipient solid-state deformation in near solidus conditions. Magnetic susceptibility magnitudes (K) range from 11 to 111×10−3 SI in the pluton and can be correlated to the petrography (highest K values in the tonalites; lowest K in the granites; granodiorites in between). The susceptibility magnitudes display a complex zoning pattern. Combined with the arrangement of magnetic foliation trajectories, it is possible to delineate four nested sub-units, regarded as four magmatic pulses successively emplaced from the west to the east of the pluton. The four pulses are characterised by very similar magma geochemistry, but variable magmatic differentiation. The highest degrees of magnetic susceptibility anisotropies (up to 1·6) are observed along internal contacts between sub-units and along the borders of the pluton. The magnetic lineations are also steeply plunging in some places in each sub-unit, possibly imaging the different feeder zones. Magma emplacement occurred at the end of the activity of the Angavo shear zone, hence avoiding re-orientation of the magmatic structures by the late Pan-African transcurrent tectonics. The diachronicity of the four magmatic pulses is consistent with previously determined palaeomagnetic data, because only the two older sub-units display a magnetic reversal sequence, whereas the two youngest sub-units lack any reversion. Emplacement of these four magmatic batches was responsible for a strain aureole and suggests a diapiric mode of ascent.
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Lux, Daniel R. « 40Ar/39Ar ages for minerals from the amphibolite dynamothermal aureole, Mont Albert, Gaspe, Quebec ». Canadian Journal of Earth Sciences 23, no 1 (1 janvier 1986) : 21–26. http://dx.doi.org/10.1139/e86-003.
Texte intégralRésumé :
New 40Ar/39Ar ages are presented for two hornblendes and one biotite from the dynamothermal aureole that is situated at the base of the Mont Albeit ophiolite. Analysis by the incremental release technique reveals a small component of excess 40Ar in one hornblende, though both exhibit plateau-type spectra. The biotite was analyzed by the total fusion technique. All apparent ages are concordant and average 456 ± 3 Ma.Dynamothermal aureoles form during the obduction process, which juxtaposes ophiolites and amphibolite protoliths during a compressional tectonic event at a plate margin: in this instance, thrusting of the Mont Albert ophiolite over rocks of the Shickshock Group. The concordance of biotite and hornblende ages indicates rapid postmetamorphic cooling and firmly dates obduction at 456 ± 3 Ma ago, an age younger than any of the other northern Appalachian ophiolites. The high-pressure metamorphism of the amphibolite is inconsistent with the present high structural level of the composite Mont Albert – Shickshock allochthon. This implies that obduction and assembly of the allochthon took place in a different environment prior to emplacement in its present high-level position. The age presented here is therefore a maximum limit for the late thrusting event. A north–south diachronism of Humber Zone ophiolite obduction is supported by the new data.
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FESTA, VINCENZO, ALFREDO CAGGIANELLI, ANTONIO LANGONE et GIACOMO PROSSER. « Time–space relationships among structural and metamorphic aureoles related to granite emplacement : a case study from the Serre Massif (southern Italy) ». Geological Magazine 150, no 3 (16 novembre 2012) : 441–54. http://dx.doi.org/10.1017/s0016756812000714.
Texte intégralRésumé :
AbstractTectonic and thermal perturbations, related to emplacement of granodiorite in the upper continental crust, have been investigated in the late-Hercynian basement exposed in southern Calabria (Italy). Here, the structural aureole is marked by the presence of a major rim fold adjacent to the intrusive contact for a length of at least 20 km. Geometrical analysis of the structural aureole and related foliations, lineations and crenulations reveals that the perturbed zone is at least 3000 m wide and characterized by an open synform trending nearly parallel to the intrusive contact. This pattern is compatible with a laccolith-like mode of magma emplacement, related to the accretion of the pluton that shouldered weak phyllitic and slaty wall rocks. The metamorphic aureole, about 1800 m wide, is characterized by biotite, cordierite and andalusite that appear sequentially in spotted schists and hornfelses approaching the intrusive contact. The peak assemblage equilibrated between 535 and 590°C at pressures between 175 and 200 MPa, confirmed by Al-in-hornblende barometry on granodiorite. Microstructural analysis allowed the inference of a time lag between the thermal and tectonic perturbations. With the aid of thermal modelling it was possible to quantify the time required to reach the peak temperature at a distance from the intrusive contact where cordierite spots and andalusite porphyroblasts clearly overprint crenulations. This estimate represents the time limit to accomplish deformation in the inner portion of the aureole and thus indicates a minimum strain rate of 4 × 10−14 s−1 within the country rocks during granodiorite intrusion.
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Zi, Jian-Wei, Birger Rasmussen, Janet R. Muhling, Wolfgang D. Maier et Ian R. Fletcher. « U-Pb monazite ages of the Kabanga mafic-ultramafic intrusions and contact aureoles, central Africa : Geochronological and tectonic implications ». GSA Bulletin 131, no 11-12 (15 avril 2019) : 1857–70. http://dx.doi.org/10.1130/b35142.1.
Texte intégralRésumé :
AbstractMafic-ultramafic rocks of the Kabanga-Musongati alignment in the East African nickel belt occur as Bushveld-type layered intrusions emplaced in metasedimentary sequences. The age of the mafic-ultramafic intrusions remains poorly constrained, though they are regarded to be part of ca. 1375 Ma bimodal magmatism dominated by voluminous S-type granites. In this study, we investigated igneous monazite and zircon from a differentiated layered intrusion and metamorphic monazite from the contact aureole. The monazite shows contrasting crystal morphology, chemical composition, and U-Pb ages. Monazite that formed by contact metamorphism in response to emplacement of mafic-ultramafic melts is characterized by extremely high Th and U and yielded a weighted mean 207Pb/206Pb age of 1402 ± 9 Ma, which is in agreement with dates from the igneous monazite and zircon. The ages indicate that the intrusion of ultramafic melts was substantially earlier (by ∼25 m.y., 95% confidence) than the prevailing S-type granites, calling for a reappraisal of the previously suggested model of coeval, bimodal magmatism. Monazite in the metapelitic rocks also records two younger growth events at ca. 1375 Ma and ca. 990 Ma, coeval with metamorphism during emplacement of S-type granites and tin-bearing granites, respectively. In conjunction with available geologic evidence, we propose that the Kabanga-Musongati mafic-ultramafic intrusions likely heralded a structurally controlled thermal anomaly related to Nuna breakup, which culminated during the ca. 1375 Ma Kibaran event, manifested as extensive intracrustal melting in the adjoining Karagwe-Ankole belt, producing voluminous S-type granites. The Grenvillian-aged (ca. 990 Ma) tin-bearing granite and related Sn mineralization appear to be the far-field record of tectonothermal events associated with collision along the Irumide belt during Rodinia assembly. Since monazite is a ubiquitous trace phase in pelitic sedimentary rocks, in contact aureoles of mafic-ultramafic intrusions, and in regional metamorphic belts, our study highlights the potential of using metamorphic monazite to determine ages of mafic-ultramafic intrusions, and to reconstruct postemplacement metamorphic history of the host terranes.
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Suarez, M., M. Herve et A. Puig. « Cretaceous diapiric plutonism in the southern cordillera, Chile ». Geological Magazine 124, no 6 (novembre 1987) : 569–75. http://dx.doi.org/10.1017/s0016756800017398.
Texte intégralRésumé :
AbstractThe Castores and probably the Santa Rosa plutons of north-west Isla Navarino, southern Chile, have been emplaced by in situ diapirism into metasedimentary rocks of the Upper Jurassic (?)–Lower Cretaceous Yaghan Formation. For the former, this model is consistent with the concentric foliation paralleling the margin of the pluton and the foliation and stratification planes in the metamorphic aureole. Only the southern part of the Santa Rosa Pluton is preserved, and it has some structures similar to those of the Castores Pluton, which can also be interpreted as produced by an inflating diapir. The main intrusive rocks of these plutons are quartz-monzodiorites and quartz-diorites with synmagmatic foliation. They were preceded by minor bodies of hornblende gabbros, and followed by dykes and small bodies of non-foliated granodiorites. Non-foliated to weakly foliated granodiorites, forming the centre of the Castores Pluton, probably represent a younger intrusive pulse.Twelve K–Ar mineral dates from 10 specimens of plutonic rocks, interpreted as near crystallization ages, span the period 80–90 Ma. These dates do not show the sequence of intrusion of the different rock-types, which may suggest that all of them were intruded and cooled in a short period of time. The timing of emplacement of these plutons in relation to tectonism is difficult to determine; however, a post-tectonic emplacement for at least the Castores Pluton, is proposed.
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Smith, J. M., et P. Erdmer. « The Anvil aureole, an atypical mid-Cretaceous culmination in the northern Canadian Cordillera ». Canadian Journal of Earth Sciences 27, no 3 (1 mars 1990) : 344–56. http://dx.doi.org/10.1139/e90-032.
Texte intégralRésumé :
The mid-Cretaceous Anvil batholith, in south-central Yukon near Faro, intrudes Upper Proterozoic to upper Paleozoic strata of the Cordilleran outer miogeocline. From previous work, it was unclear whether biotite, andalusite–staurolite, and garnet isograds near the pluton resulted from pre-Devonian regional metamorphism and subsequent arching in a structural culmination or from mid-Cretaceous instrusion. The present study has documented biotite, andalusite, staurolite, garnet, and sillimanite isograds concentric to the pluton. Prophyroblast–matrix relationships indicate that peak metamorphism occurred during intrusion, which took place under approximately 3 kbar (300 MPa) pressure and heated country rock to temperatures of 600°–620 °C. The metamorphism is thus compatible with a deep, mid-Cretaceous event. Regional uplift of 10 km is implied by the metamorphic minerals. From cogenetic relationships between some phases of the Anvil batholith and the nearby South Fork volcanic rocks, regional uplift appears to have been completed in a few million years in the mid-Cretaceous. The uncharacteristic aureole suggests that mid-Cretaceous events in this region are atypical of the Cordillera and may reflect a unique tectonic history or position in the orogen.
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Brown, E. H. « Magma Loading in the Southern Coast Plutonic Complex, British Columbia and Washington ». Lithosphere 2020, no 1 (10 novembre 2020) : 1–17. http://dx.doi.org/10.2113/2020/8856566.
Texte intégralRésumé :
Abstract The southen end of the 1800 km long Coast Plutonic Complex (CPC), exposed in the Harrison Lake area of British Columbia and in the North Cascades of Washington, bears a record of great crustal thickening -20 to 40 km in localized zones during Late Cretaceous times. During this period, the CPC was positioned at the continental margin during collision/subduction of the Farallon plate. Arc magmatism and regional orogenic contraction were both active as potential crustal thickening processes. Magmatism is favored in this report as the dominant factor based on the delineation of four spatially and temporally separate loading events, the close association of the loaded areas with emplacement of large plutons, and a paucity of evidence of deep regional tectonic contraction. The timing and spatial location of crustal loading events are documented by the following: zircon ages in plutons; an early event of low pressure in pluton aureoles evidenced by andalusite, now pseudomorphed by high-pressure minerals; high pressures in country rock in pluton aureoles measured by mineral compositions in the assemblages garnet-biotite-muscovite-plagioclase and garnet-aluminum silicate-plagioclase; high pressures recorded in plutons by Al-in-hornblende barometry; and uplift ages of plutons derived from K-Ar and Ar-Ar ages of micas and hornblende in plutons.
Thèses sur le sujet "Aureole tectonics"
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PAPESCHI, SAMUELE. « The brittle/ductile transition at upper crustal level : geometry, strain partitioning and fluid circulation. The case study of the Calamita Unit (Elba Island, Northern Apennines, Italy) ». Doctoral thesis, 2019. http://hdl.handle.net/2158/1156134.
Texte intégralRésumé :
This study focuses on the detailed investigation of the brittle/plastic transition in quartz-feldspathic rocks at upper-middle crustal conditions and aims to better understand the role of fluids and temperature during deformation. The Calamita Unit (Elba Island, Italy) is a high metamorphic grade unit (T ~ 650 °C) that has been intruded by a monzogranite body at shallow crustal level (P < 0.2 GPa) and coevally deformed during regional shortening for a limited time span (< 1 Ma). These conditions outline the Calamita Unit as an ideal case study to investigate the brittle/plastic transition at approximately constant pressure (i.e. depth) during temperature decrease, reproducing upper-middle crustal conditions.
The Calamita Schists are a metapsammitic complex exposed in the lower part of the Calamita Unit. Pseudosection modelling and Ti-in-biotite thermometry constrain the peak metamorphic assemblage, marked by andalusite + cordierite + biotite + K-feldspar, at upper amphibolite facies conditions (T ~ 600 – 700 °C; P ~ 0.2 GPa), with microstructures suggesting partial melting. The retrograde path is constrained by chlorite geothermometry in the 300 – 500 °C temperature range.
Detailed field mapping and structural analysis have revealed at map-scale a pattern of heterogeneous deformation characterized by west-dipping high-strain domains localizing eastward-directed deformation interleaved with relatively low-strain domains. In high-strain domains, mylonitic fabrics are, in turn, overprinted in the brittle regime by non-Andersonian subhorizontal faults associated with Riedel shears networks, formed subparallel to C’ shear bands.
Microstructural analysis highlights that temperature decrease and fluid influx controlled the mechanical evolution of the investigated rocks, which are marked by the transition from a high-metamorphic grade foliation to shear bands and mylonites with widespread S-C and S-C’ fabrics, characterized by retrograde, synkinematic white mica and chlorite.
Quartz microfabric displays an evolution from fast grain boundary migration, developed close to peak metamorphic conditions, to subgrain rotation and bulging recrystallization, tracking decreasing temperature during deformation. During decreasing temperature, deformation localized in mylonitic quartz ribbons at amphibolite facies conditions (450 °C < T < 600 °C), where recrystallization was accommodated by dislocation creep of quartz under dominant prism slip, causing the development of strong Y-maximum crystallographic preferred orientations (CPO). Secondary rhomb and acute rhomb slip assisted the recrystallization of grains unfavorably oriented for prism slip, with the activation of slip systems whose misorientation axis lies close to the vorticity axis. At greenschist facies conditions (300 °C < T < 450 °C), deformation localized in phyllonitic domains, producing phase mixing of phyllosilicates and tiny quartz grains. Relic, large quartz grains hardened and fractured along synthetic and conjugate shear bands. The propagation of shear bands occurred under fluid-rich conditions and was controlled by cyclic fracturing and precipitation of new quartz and phyllosilicate grains, deposited by circulating fluids. Precipitated new quartz grains developed a CPO parallel to shear bands controlled by the opening of dilatant sites. The nucleation of fine-grained quartz and ‘soft’ phyllosilicates enhanced strain softening and assisted strain partitioning into localized C’ shear bands at the brittle/plastic transition.
In the brittle regime (T ~ 300 °C), deformation localized on previously formed C’ shear bands, favorably oriented for reactivation, that acted as ductile precursors for misoriented non-Andersonian faults. Brittle deformation in fault zones was controlled by the cyclical interaction between fracturing, taking advantage of weak crystallographic planes in quartz such as the rhombs, and fluid infiltration, assisting the precipitation of new quartz and phyllosilicate grains, lacking a clear preferred orientation.
The data presented in this thesis highlight the role played by fluids during deformation of quartz-feldspathic rocks at the brittle/plastic transition, that efficiently control (1) strain softening of ‘stiff’ domains and (2) strain localization into shear bands that have the potential to act as precursors for non-Andersonian fault zones. The proposed model predicts the development of brittle structures discordantly overprinting ductile fabrics developed in the same kinematic regime, which bears implications for the tectonic evolution of rock volumes (i.e. tectonic units and/or metamorphic complexes) exhumed though the brittle/plastic transition.
Chapitres de livres sur le sujet "Aureole tectonics"
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Paterson, S. R., R. H. Vernon et T. K. Fowler. « Chapter 13. AUREOLE TECTONICS ». Dans Contact Metamorphism, sous la direction de Derrill M. Kerrick, 673–722. Berlin, Boston : De Gruyter, 1991. http://dx.doi.org/10.1515/9781501509612-016.
Texte intégral
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Seltmann, Reimar, Richard J. Goldfarb, Bo Zu, Robert A. Creaser, Alla Dolgopolova et Vitaly V. Shatov. « Chapter 24 : Muruntau, Uzbekistan : The World’s Largest Epigenetic Gold Deposit ». Dans Geology of the World’s Major Gold Deposits and Provinces, 497–521. Society of Economic Geologists, 2020. http://dx.doi.org/10.5382/sp.23.24.
Texte intégralRésumé :
Abstract Muruntau in the Central Kyzylkum desert of the South Tien Shan, western Uzbekistan, with past production of ~3,000 metric tons (t) Au since 1967, present annual production of ~60 t Au, and large remaining resources, is the world’s largest epigenetic Au deposit. The host rocks are the mainly Cambrian-Ordovician siliciclastic flysch of the Besapan sequence. The rocks were deformed into a broadly east-west fold-and-thrust belt prior to ca. 300 Ma during ocean closure along the South Tien Shan suture. A subsequent tectonic transition was characterized by left-lateral motion on regional splays from the suture and by a massive thermal event documented by widespread 300 to 275 Ma magmatism. The Besapan rocks were subjected to middle to upper greenschist-facies regional metamorphism, an overprinting more local thermal metamorphism to produce a large hornfels aureole, and then Au-related hydrothermal activity all during early parts of the thermal event. The giant Muruntau Au deposit formed in the low-strain hornfels rocks at ca. 288 Ma at the intersection of one of the east-west splays, the Sangruntau-Tamdytau shear zone, with a NE-trending regional fault zone, the Muruntau-Daugyztau fault, which likely formed as a cross fault during the onset of left-lateral translation on the regional splays. Interaction between the two faults opened a large dilational zone along a plunging anticlinorium fold nose that served as a major site for hydrothermal fluid focusing. The Au ores are dominantly present as a series of moderately to steeply dipping quartz ± K-feldspar stockwork systems surrounding uncommon central veins and with widespread lower Au-grade metasomatites (i.e., disseminated ores). Pervasive alteration is biotite-K-feldspar, although locally albitization is dominant. Sulfides are mainly arsenopyrite, pyrite, and lesser pyrrhotite, and scheelite may be present both in preore ductile veins and in the more brittle auriferous stockwork systems. The low-salinity, aqueous-carbonic ore-forming fluids probably deposited the bulk of the ore at 400° ± 50°C and 6-to 10-km paleodepth. The genesis of the deposit remains controversial with metamorphic, thermal aureole gold (TAG), and models related to mantle upwelling all having been suggested in recent years. More importantly, the question as to why there was such a focusing of so much Au and fluid into this one location, forming an ore system an order of magnitude larger than other giant Au deposits in metamorphic terranes, remains unresolved.
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Russo, Angela G., Wanda J. Taylor et Patricia H. Cashman. « Late Paleozoic Shortening in South-Central Nevada and Regional Correlations of Major Pre-Sevier Structures ». Dans Late Paleozoic and Early Mesozoic Tectonostratigraphy and Biostratigraphy of Western Pangea, 114–26. SEPM (Society for Sedimentary Geology), 2022. http://dx.doi.org/10.2110/sepmsp.113.05.
Texte intégralRésumé :
Recent tectonic reinterpretations of the Late Paleozoic Southwest Laurentian margins recognize widespread Late Paleozoic deformation as a critical component in the boundary region development. Overprinted late Paleozoic structures record repeated shortening events in both northern and southern Nevada, but spatial and temporal data are currently lacking to resolve the evolution of this margin. The Timpahute Range, south-central Nevada, bridges part of the spatial gap between previous detailed studies of Late Paleozoic deformation. The purpose here is to (1) evaluate structures in the area that do not appear to fit with recognized Sevier hinterland structures (the Central Nevada thrust belt [CNTB]) and (2) consider whether these contractional structures may be Late Paleozoic and possibly link, or not, structures to the north and south. New mapping in the Timpahute Range documents four geometrically or kinematically distinct sets of structures: Tempiute Ridge folds, Schofield Pass fault zone (SPFZ), structures of the CNTB, and Cenozoic extensional faults. The first three are interpreted to represent separate shortening events based on cross-cutting relations and differences in orientations of the Tempiute Ridge folds and SPFZ (north [N]), and structures of the CNTB (northwest [NW]). The Tempiute Ridge folds represent the oldest event, D1. These folds are large, trend N and verge east (E). The SPFZ is west (W)-vergent, cuts across the limb of a D1 fold and represents D2. The SPFZ is interpreted to be older than the CNTB structures, D3, based on positions of fault cut offs, and differences in footwall and hangingwall facies. All of the shortening events predate the newly dated 102.9 ± 3.2 Ma Lincoln stock and its contact metamorphic aureole. New and previous correlations suggest that a belt of Permian deformation extends from southeast (SE) California northward at least to the Timpahute Range. The Tempiute Ridge folds and SPFZ have the same distinctive geometries, styles, and kinematics as structures in the Nevada National Security Site. The mountain-size, E-vergent Tempiute Ridge folds and the W-vergent SPFZ correlate to structures associated with the Belted Range thrust and the W-vergent CP thrust, respectively. The Belted Range thrust previously has been correlated southward into the Death Valley region. Thus, convergence created large-amplitude folds and thrusts for ~200 km along strike. Structures of this age are exposed in northern Nevada but are smaller. These new relations fill a data gap and suggest differences in the size and structural style of Permian structures along strike and corresponding variations in the plate boundary configuration.
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Muntean, John L. « Chapter 36 : Carlin-Type Gold Deposits in Nevada : Geologic Characteristics, Critical Processes, and Exploration ». Dans Geology of the World’s Major Gold Deposits and Provinces, 775–95. Society of Economic Geologists, 2020. http://dx.doi.org/10.5382/sp.23.36.
Texte intégralRésumé :
Abstract Carlin-type gold deposits in Nevada account for ~5% of worldwide annual gold production, typically about ~135 metric tons (t) (~4.5 Moz) per year. They are hydrothermal epigenetic replacement bodies hosted predominantly in carbonate-bearing sedimentary rocks. They are known for their “invisible” gold that occurs in the crystal structure of pyrite. Over 95% of the production from these deposits is from four clusters of deposits, which include the Carlin trend and the Cortez, Getchell, and Jerritt Canyon camps. Despite differences in the local geologic settings, the characteristics of the deposits are very similar in the four clusters. Shared characteristics include: (1) alteration characterized by carbonate dissolution, silicate argillization, and silicification; (2) ore formation characterized by auriferous arsensian pyrite, typically as rims on preore pyrite, followed by late open-space deposition of orpiment, realgar, stibnite, and other minerals; (3) Ag/Au ratios of <1 in ore; (4) an As-Hg-Sb-Tl geochemical signature; (5) low temperatures (~160°–240°C) and salinities of ore fluids (~1–6 wt % NaCl equiv) and fairly shallow depths of formation (<~2–3 km); and (6) lack of mineral and elemental zoning around ore. The four clusters share regional geologic controls related to formation as follows: (1) along the rifted margin of a craton, (2) within the slope facies of a passive margin sequence dominated by carbonates, (3) in the lower plate of a regional thrust fault, and (4) during a narrow time interval in the late Eocene (~42–34 Ma). The geometries and ore controls of the deposits in the four clusters are also very similar. At the deposit scale, ore and hydrothermal alteration are commonly associated with high-angle faults and preore low-angle contractional structures, including thrust faults and folds. The high-angle faults acted as fluid pathways for upwelling ore fluids, which were then diverted into lower angle favorable strata and contractional structures, where fluid-rock interaction led to replacement of carbonate and formation of ore. Rheologic contrasts between lithologies were also critical in diverting fluids into wall rocks. Common rheologic contrasts include contacts between thin- and thick-bedded lithologic units and the margins of contact metamorphic aureoles associated with Mesozoic intrusions. The similarities suggest common processes. Four critical processes are apparent: (1) development of source(s) for gold and other critical components of the ore fluids, (2) formation of fluid pathways, (3) water-rock interaction and gold deposition, and (4) a tectonic trigger, which was renewal of magmatism and a change from contraction to extension in the late Eocene. Consensus exists on these processes, except for the source of gold and other components of the ore fluid, with most models calling upon either a magmatic-hydrothermal source or a crustal source, where metals were scavenged by either meteoric or metamorphic fluids. Future research should focus on Carlin-style deposits in Nevada that exhibit epithermal characteristics and deposits that appear to have a clear genetic association with magmatic-hydrothermal systems associated with upper crustal intrusions. Rather than discrete types of ore deposits, there may be continua between Carlin-type gold deposits, epithermal deposits, and distal disseminated deposits, with the four large camps representing an end member.