The structure and geological development of the Erickson gold mine, Cassiar District, British Columbia, with implications for the origin of mother-lode-type gold deposits

1989 ◽  
Vol 26 (12) ◽  
pp. 2645-2660 ◽  
Author(s):  
P. G. Anderson ◽  
C. Jay Hodgson

The Erickson gold mine is a typical gold quartz vein deposit. The veins are hosted by a thrust-imbricated, gently dipping, synformal allochthon of low-grade metamorphic, Devonian to Upper Triassic basalts, argillites, and peridotites of oceancrustal origin belonging to the Sylvester Group, part of the Slide Mountain assemblage. The Sylvester allochthon lies concordantly on Devonian miogeoclinal sedimentary rocks of the North American continental margin and was emplaced in the Middle Jurassic as a result of the collision of the Quesnel arc with North America. The veins in the mine are hosted mainly by a moderately dipping system of shear zones with approximately orthorhombic symmetry, indicating a triaxial bulk, inhomogeneous strain pattern superimposed on the earlier formed, gently dipping thrusts. Steeply dipping extension veinlets, rotation of schistosity, and downdip slickenlines indicate the maximum shortening axis was subvertical. The veins display complex superimposed ribbon and breccia textures, indicating incremental growth. Most of the gold occurs in association with tetrahedrite, sphalerite, and chalcopyrite in steeply dipping, late, grey quartz veinlets localized within and striking perpendicular to the main veins. The vein-forming event, dated at 130 Ma, appears to have been related to extension and high heat flow associated with the rise of the Omenica geanticline, in turn the result of crustal thickening caused by the collision of the amalgamated Quesnel arc – North America plate with Stikinia in the Middle Jurassic.

2021 ◽  
Vol 2 (4) ◽  
Author(s):  
Xin Wang

Angiosperms are the single most important plant group in the current ecosystem. However, little is known about the origin and early evolution of angiosperms. Jurassic and earlier traces of angiosperms have been claimed multiple times from Europe and Asia, but reluctance to accept these records remains. To test the truthfulness of these claims, palaeobotanical records from continents other than Europe and Asia constitute a crucial test. Here I document a new angiosperm fruit, Dilcherifructus mexicana gen. et sp. nov, from the Middle Jurassic of Mexico. Its Jurassic age suggests that origin of angiosperms is much earlier than widely accepted, while its occurrence in the North America indicates that angiosperms were already widespread in the Jurassic, although they were still far away from their ecological radiation, which started in the Early Cretaceous.


1990 ◽  
Vol 127 (2) ◽  
pp. 101-116 ◽  
Author(s):  
U. Pognante ◽  
D. Castelli ◽  
P. Benna ◽  
G. Genovese ◽  
F. Oberli ◽  
...  

AbstractIn the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallines sensu stricto (HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as M1 and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb–Sr and U–Th–Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time.Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the ‘cooler’ Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the late- metamorphic Zanskar Fault.


1992 ◽  
Vol 29 (3) ◽  
pp. 388-417 ◽  
Author(s):  
Andreas G. Mueller

The Norseman mining district in the Archean Yilgarn Block, Western Australia, has produced 140 t of gold and about 90 t of silver from 11.24 × 106 t of ore. The district is located within a metamorphic terrane of mafic and minor ultramafic greenstones, intruded by granite cupolas and swarms of porphyry dykes. The orebodies consist of laminated quartz veins, controlled by narrow (0.5–5 m) reverse shear zones that, in general, follow the contacts of metapyroxenite or porphyry dykes. Petrological studies of four shear zones, exposed on the Regent shaft 14 level, Ajax shaft 10 level, and in the stope above the North Royal shaft 5 level, show that the host rocks were metamorphosed to hornblende–plagioclase amphibolites and actinolite–chlorite rocks at temperatures of 500–550 °C prior to mineralization.At the localities studied, intense wall-rock replacement and low-grade (0.5 g/t) gold mineralization are confined to ductile or brittle–ductile shear structures. Alteration is similar in both ultramafic and mafic greenstones, and consists of an inner zone of biotite–quartz–calcite–plagioclase rock with minor actinolitic hornblende and quartz–calcite–actinolite veinlets, and an outer zone, locally developed, of chlorite–calcite–quartz rock. At an estimated pressure of 3 kbar (300 MPa), fluid temperatures during wall-rock alteration are constrained by the hydrothermal mineral assemblages to 480 ± 30 °C in two shear zones on the Regent shaft 14 level, and to 450 ± 20 °C in one shear zone in the North Royal shaft 5 level stope. The mole fraction of CO2 of the fluids is estimated at [Formula: see text], and the sulphur fugacity at 10−6 bar (10−1 kPa) (at 450 °C), based on the assemblage pyrrhotite + pyrite ± arsenopyrite. The development of an outer chloritic alteration zone at North Royal is related to the lower fluid temperature at this locality.High-grade (up to 75 g/t Au, 283 g/t Ag) veins formed within three of the shear zones studied at fluid temperatures of 400 °C and less, by the successive accretion of quartz laminae, separated by films of retrograde chlorite and sericite. The assemblage of ore minerals in the veins differs from that in the altered wall rocks, and includes disseminated galena, Pb–Bi–Ag tellurides, and native gold, which coprecipitated with the quartz. The orebodies at Norseman show affinities to Phanerozoic and Archean gold skarn deposits.


2017 ◽  
Vol 188 (1-2) ◽  
pp. 9 ◽  
Author(s):  
Marc Jolivet ◽  
Anastasia Arzhannikova ◽  
Andrei Frolov ◽  
Sergei Arzhannikov ◽  
Natalia Kulagina ◽  
...  

The Late Jurassic - Early Cretaceous tectonic evolution of SE Siberia was marked by the closure of the Mongol-Okhotsk ocean. While this geodynamic event led to compressive deformation and denudation in a wide area encompassing the North-Altay, Sayan and Baikal Patom ranges, it was contemporaneous to widespread extension from the Transbaikal region situated immediately north of the suture zone to the Pacific plate, affecting eastern Mongolia and northeastern China. In this study we review the paleontological and sedimentological data available in the Russian literature and provide new macro-floral and palynological data from the Mesozoic sediments of three Transbaikal basins. These data are used to describe the paleoenvironmental and paleoclimatic evolution of the Transbaikal area in order to assess the topographic evolution of the region in relation with the closure of the Mongol-Okhotsk ocean. We establish that the Transbaikal basins evolved in a continuously extensional tectonic setting from at least the Early-Middle Jurassic to the Early Cretaceous. The associated sedimentary environments are characterized by retrogradation from alluvial fan–braided river dominated systems prevailing during the Early to Middle Jurassic initial opening of the basins to meandering river– lacustrine systems that developed during the Late Jurassic - Early Cretaceous interval. No evidence of high relief topography was found and we conclude that, while compression and denudation occurred in the North Altai, Sayan and Patom ranges, in the Transbaikal region, the docking of the Mongolia-North China continent to Siberia was a “soft collision” event, possibly involving a major strike-slip displacement that did not lead to an orogenic event implying strong compressive deformation, crustal thickening and topography building.


2020 ◽  
Vol 50 (1) ◽  
pp. 83-130 ◽  
Author(s):  
Pietari Skyttä ◽  
Pär Weihed ◽  
Karin Högdahl ◽  
Stefan Bergman ◽  
Michael B. Stephens

AbstractThe Bothnia–Skellefteå lithotectonic unit is dominated by turbiditic wacke and argillite (Bothnian basin), deposited at 1.96 (or older)–1.86 Ga, metamorphosed generally under high-grade conditions and intruded by successive plutonic suites at 1.95–1.93, 1.90–1.88, 1.87–1.85 and 1.81–1.76 Ga. In the northern part, low-grade and low-strain, 1.90–1.86 Ga predominantly magmatic rocks (the Skellefte–Arvidsjaur magmatic province) are enclosed by the basinal components. Subduction-related processes in intra-arc basin and magmatic arc settings, respectively, are inferred. Changes in the metamorphic grade and the relative timing of deformation and structural style across the magmatic province are linked to major shear zones trending roughly north–south and, close to the southern margin, WNW–ESE. Zones trending WNW–ESE and ENE–WSW dominate southwards. Slip along the north–south zones in an extensional setting initiated synchronously with magmatic activity at 1.90–1.88 Ga. Tectonic inversion steered by accretion to a craton to the east, involving crustal shortening, ductile strain and crustal melting, occurred at 1.88–1.85 Ga. Deformation along shear zones under lower-grade conditions continued at c. 1.8 Ga. Felsic volcanic rocks (1.90–1.88 Ga) host exhalative and replacement-type volcanogenic massive sulphide deposits (the metallogenic Skellefte district). Other deposits include orogenic Au, particularly along the ‘gold line’ SW of this district, porphyry Cu–Au–Mo, and magmatic Ni–Cu along the ‘nickel line’ SE of the ‘gold line’.


2007 ◽  
Vol 44 (7) ◽  
pp. 925-946 ◽  
Author(s):  
Jerry C DeWolfe ◽  
Bruno Lafrance ◽  
Greg M Stott

The Beardmore–Geraldton belt consists of steeply dipping, intercalated panels of metavolcanic and metasedimentary rocks along the southern margin of the granite–greenstone Wabigoon subprovince in the Archean Superior Province, Ontario. It is an important past-producing gold belt that includes classic epigenetic iron-formation-hosted deposits near Geraldton and turbidite-hosted deposits, north of Beardmore. The Brookbank gold prospect belongs to a third group of related gold deposits that formed along dextral shear zones localized at contacts between panels of metasedimentary and metavolcanic rocks. The Brookbank prospect occurs along a steeply dipping shear zone at the contact between footwall polymictic conglomerate and hanging-wall calc-alkaline arc basalt. Early during shearing the basalt acted as a structural and chemical trap that localized brittle deformation, veining, and gold deposition, ankerite–sericite–chlorite–epidote–pyrite alteration, and the replacement of metamorphic magnetite and ilmenite by gold-bearing pyrite. This produced a low grade (≤5 g/t Au) ankerite-rich alteration zone that extends up to 20 m into the hanging-wall basalt. Later during shearing, gold was deposited within higher grade (≤20 g/t Au) quartz–orthoclase–pyrite alteration zones superimposed on the wider ankerite-rich alteration zone. Auriferous quartz–carbonate veins oriented clockwise and counter-clockwise to the shear zone walls are folded and boudinaged, respectively, consistent with dextral slip along the shear zone. A key finding of the study is that different groups of gold deposits in the belt, including epigenetic iron formation gold deposits near Geraldton, formed during post-2690 Ma regional dextral transpression across the belt.


2021 ◽  
pp. 73-130
Author(s):  
Alan D. Chapman* ◽  
Doug Yule ◽  
William Schmidt ◽  
Todd LaMaskin

ABSTRACT The Klamath Mountains province and adjacent Franciscan subduction complex (northern California–southern Oregon) together contain a world-class archive of subduction-related growth and stabilization of continental lithosphere. These key elements of the North American Cordillera expanded significantly from Middle Jurassic to Early Cretaceous time, apparently by a combination of tectonic accretion and continental arc– plus rift-related magmatic additions. The purpose of this field trip is twofold: to showcase the rock record of continental growth in this region and to discuss unresolved regional geologic problems. The latter include: (1) the extent to which Mesozoic orogenesis (e.g., Siskiyou and Nevadan events plus the onset of Franciscan accretion) was driven by collision of continental or oceanic fragments versus changes in plate motion, (2) whether growth involved “accordion tectonics” whereby marginal basins (and associated fringing arcs) repeatedly opened and closed or was driven by the accretion of significant volumes of material exotic to North America, and (3) the origin of the Condrey Mountain schist, a composite low-grade unit occupying an enigmatic structural window in the central Klamaths—at odds with the east-dipping thrust sheet regional structural “rule.” Respectively, we assert that (1) if collision drove orogenesis, the requisite exotic materials are missing (we cannot rule out the possibility that such materials were removed via subduction and/or strike slip faulting); (2) opening and closure of the Josephine ophiolite-floored and Galice Formation–filled basin demonstrably occurred adjacent to North America; and (3) the inner Condrey Mountain schist domain is equivalent to the oldest clastic Franciscan subunit (the South Fork Mountain schist) and therefore represents trench assemblages underplated >100 km inboard of the subduction margin, presumably during a previously unrecognized phase of shallow-angle subduction. In aggregate, these relations suggest that the Klamath Mountains and adjacent Franciscan complex represent telescoped arc and forearc upper plate domains of a dynamic Mesozoic subduction zone, wherein the downgoing oceanic plate took a variety of trajectories into the mantle. We speculate that the downgoing plate contained alternating tracts of smooth and dense versus rough and buoyant lithosphere—the former gliding into the mantle (facilitating slab rollback and upper plate extension) and the latter enhancing basal traction (driving upper plate compression and slab-shallowing). Modern snapshots of similarly complex convergent settings are abundant in the western Pacific Ocean, with subduction of the Australian plate beneath New Guinea and adjacent island groups providing perhaps the best analog.


2021 ◽  
Author(s):  
Rodrigo Prudente de Melo ◽  
Marcos Aurélio Farias de Oliveira ◽  
Richard J. Goldfarb ◽  
Craig A. Johnson ◽  
Erin E. Marsh ◽  
...  

Abstract The Alto Guaporé gold province, southwestern Amazon craton, contains gold deposits that have been mined since the beginning of the 18th century and these deposits, together, have modern-day, pre-mining gold resources of at least 1.8 Moz. The ore is associated with quartz vein systems along the southeastern part of the Aguapei belt, a ~35 km wide and ~500 km long, NNW-trending shear zone formed due to reactivation of a terrane-bounding suture. The Aguapei belt evolved by ca. 1150–1100 Ma rifting and deposition of siliciclastic sediments in an aulacogen basin, followed by deformation and low-grade metamorphism of the sedimentary sequences during 1100–900 Ma terrane collision along the craton margin. The deformation was characterized by a compressional regime until ca. 950 Ma and transition to a transpressional setting during the final 50 m.y. The gold deposits are hosted in a variety of structures that are second-order to the main Aguapei shear zone. The Ernesto and Pau-a-Pique deposits are located ~40 km apart and at jogs along the Aguapei belt. They are marginal to pre-ore igneous rocks, with Ernesto hosted in the basal part of the metasedimentary Fortuna Formation that overlies tonalite and Pau-a-Pique at the contact between metasedimentary rocks and diorite. Three deformational phases comprise the compressional (D1 to D2) to transpressional (D3) tectonic events. In the Pau-a-Pique deposit and the deeper level of the Ernesto deposit, the ore-bearing veins are bedding parallel and follow D2 strike-slip and reverse fault zones, respectively. However, the veins formed during D3 reactivation of the older structures by an array of oblique accommodation faults. In contrast, ores at shallower levels of Ernesto, both in discordant and bedding-parallel veins, are hosted within a ~20-m-thick rigid metaconglomerate with associated dilation due to the structural complexity as sedimentary rocks of the Aguapei Group were folded around the dome-shaped roof of the pre-ore tonalite. The ores in both deposits, as well as in many other deposits of the province, are characterized by disseminated and vein-hosted pyrite. Gold occurs mainly as inclusions in the pyrite, with other hydrothermal phases comprising muscovite, Fe-Ti oxides, and minor apatite, chalcopyrite, and galena. Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO2-rich fluid at ~300°C and ~2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of δ18O values of quartz (12 ± 1‰) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of δD for hydrous minerals (–116 to –55‰) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40Ar/39Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores.


2018 ◽  
Vol 46 (1) ◽  
pp. 102 ◽  
Author(s):  
Samanta Serra-Varela ◽  
Pablo D. González ◽  
Raúl E. Giacosa ◽  
Nemesio Heredia ◽  
David Pedreira ◽  
...  

In San Martín de los Andes area (Argentinian Patagonia) the Palaeozoic basement consists of metamorphic and plutonic rocks. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemical and petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes. Detrital zircon analysis (U-Pb Q-ICP-LA-MS) yielded a maximum depositional age of 501±14 Ma (Series 3 Cambrian) for this sedimentary protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of the metamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regional foliation is locally affected by open folds that develop an incipient crenulation cleavage (S3). The high-grade metamorphism includes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut it in presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a U-Pb Concordia age of 434.1±4.5 Ma (Llandovery-Wenlock, Silurian). The age of maximum sedimentation and the anatectic age constrain the metamorphic evolution of the basement into the lower Palaeozoic (between upper Cambrian and lower Silurian). The igneous rocks of the basement are granodiorites, tonalities, and some gabbros that cut the metamorphic basement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grade metamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age of these igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. 370 and 400 Ma (Devonian). The maximum sedimentation age for the protolith and its metamorphic evolution seems to be related to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. In contrast, the Devonian igneous rocks of San Martín de los Andes could represent a Devonian magmatic arc, related to subduction processes developed at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.


2019 ◽  
Vol 118 (11) ◽  
pp. 365-371
Author(s):  
J Dorasamy ◽  
Mr Jirushlan Dorasamy

Studies, especially in the North America, have shown a relationship between political orientation and moralfoundation. This study investigated whether moral judgements differ from the political orientation of participantsin South Africa moral judgment and the extent to which moral foundations are influenced by politicalorientation.Further, the study investigated the possibility of similar patterns with the North AmericanConservative-Liberal spectrum and the moral foundation. There were 300participants, 78 males and 222 females,who completed an online questionnaire relating to moral foundation and political orientation. The results partiallysupported the hypothesis relating to Liberal and Conservative orientation in South Africa. Further, this studypartially predicted the Liberal-Conservative orientation with patterns in the moral foundation, whilst showingsimilar findings to the North American studies. A growing rate of a neutral/moderate society is evidenced in SouthAfrica and abroad, thereby showing the emergence of a more open approach to both a political and generalstance.”””


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