257 research outputs found
Detrital Mineral U/Pb Age Dating and Geochemistry of Magmatic Products in Basin Sequences
Dating and geochemical analyses of detrital minerals (mainly zircons) combined with traditional methods, such as heavy minerals and sandstone modes, are a powerful tool in paleogeographic and paleotectonic research and industrial applications
Characterization and (U-Th-Sm)/He dating of Moroccan supergene ore deposits :New insights to unravel the periods and potential causes of weathering processes.
The Construction of a Plutonic Complex in a Continental Arc Setting: The Skookum Butte Stock, Western Montana
This study addresses the petrogenesis of a plutonic complex in a continental magmatic arc setting by examining the Skookum Butte stock (SBS), a satellite pluton of the Idaho-Bitterroot batholith (IBB). The SBS is part of widespread middle to Cretaceous to Eocene magmatism that occurred in the Cordilleran Interior and is dominated by biotite-muscovite granite/granodiorite and biotite-hornblende granite/granodiorite, to subordinate quartz monzodiorite, and monzogabbro. Both field evidence and geochemical data indicate that SBS magmas were generated from at least two distinct sources: a primitive source of mafic to intermediate composition and a more evolved felsic source. The mafic primitive members of the SBS have a significantly lower silica content averaging 59 wt % and are metaluminous. The granitic main-phase melt of the SBS is felsic (63.99-73.82 wt % SiO2) and slightly peraluminous (alumina saturation index = 1.04). There are two distinct felsic members represented by biotite-muscovite and biotite-hornblende granitic rocks that possibly reflect multiple magmatic episodes or differences in magma fractionation. The SBS as well as other Cordilleran Interior granitoids are distinct in mineralogy and geochemistry from granitoids generated in typical subduction-zone settings and these differences indicate a significant amount of crustal contamination. The SBS is lithologically and geochemically similar to other intrusive members of the IBB and these similarities have been determined to reflect melting of common crustal source materials, namely Proterozoic metasedimentary and basement rocks. These results suggest that first stage mafic melts of the SBS were most likely derived in a subduction-zone environment while late stage felsic and largely contaminated melts occurred in response to crustal thickening. Distinct from main-phase subduction related melts of the IBB, the SBS is assumed to have formed as an early stage magma that underwent lesser degrees of partial melting and crustal contaminatio
Geology, Geochemistry and Some Genetic Discussion of The IIC Anomaly, Bafq Ditrict, Central Iran
Zaqia IIC anomaly is located in the east of Bafq city in Yazd province. Alteration associated with mineralization has appeared within volcanic, intrusive and sedimentary rocks. The set of alterations of this anomaly consist potassium, sericitic and silicate alterations, with less sodic-calcic alterations. Iron minerals are observed in the form of veins and masses with various compositions in this deposit. Iron ore in Zaqia IIC anomaly is related to magma and hydrothermal fluids. A collection of accumulated zones is composed of high-temperature minerals at depth to subvolcanic assemblages on the surface. REE patterns in iron ores in IIC anomalies indicate LREE enrichment and Eu negative anomaly. The negative Eu anomaly shows the reducing conditions of the mineralizing fluid. Isotopic studies have been conducted to examine the source of the fluid. The mineralizing fluid source in this anomaly is sedimentary-metamorphism. Field observations, mineralogy, alterations along with ore geochemical data show that a magmatic fluid is turned to an iron-rich brine fluid; moreover, an IIC anomaly is formed
Leveraging Detrital Zircon Geochemistry to Study Deep Arc Processes: REE-Rich Magmas Mobilized by Jurassic Rifting of the Sierra Nevada Arc
Anomalous trace element compositions of Middle to Late Jurassic detrital zircon separated from Sierra Nevada forearc and intra-arc strata reveal processes of differentiation occurring within the deep arc lithosphere. REE-Sc-Nb-Ti-Hf-U-Th covariations define three populations of atypically REE-rich grains that we interpret as crystallizing from (1) differentiates produced by olivine+clinopyroxene+plagioclase+garnet±ilmenite fractionation; (2) mixing between mafic arc magmas and partial melts of Proterozoic Mojave province crust; and (3) compositionally transient, low Gd/Yb magmas generated by hornblende resorption during decompression. We interpret a fourth population of Middle Jurassic to Early Cretaceous zircons having REE contents similar to âtypicalâ arc zircon but with atypically high Gd/Yb ratios as having crystallized from partial melts of recycled arc crust and from deep-arc differentiates that evolved down-temperature through hornblende saturation. We hypothesize that latest Jurassic extension ripped open the arc, facilitating upward migration and eruption of geochemically anomalous zircon-bearing magmas. The anomalous compositions relative to âtypicalâ arc zircon imply that these zircons and their host magmas rarely reach the upper arc crust, where eruption and/or erosion would release their zircon cargo to the clastic system. Focusing on the trace element compositions of zircons of syn-extensional age represents a productive new strategy for learning about deep magmatic reservoirs and early differentiation pathways within the thick lithosphere of continental margin arcs
Apatiidi petrograafia ja haruldaste muldmetallide koostis 2 miljardi aasta vanustes Onega ja PetĆĄenga basseinide setendites: fosfogeneesi paleokeskkonna rekonstrueerimine
VĂ€itekirja elektrooniline versioon ei sisalda publikatsioone.Fosfor on bioevolutsiooniliselt vĂ”tmetĂ€htsusega element, millel on oluline roll nii elu geneetilises koodis, kui ka organismide ainevahetuses. TĂ€napĂ€eval toimub fosfogenees (fosfori eemaldamine veekogudest) ja kaasaegsete fosforiitide moodustumine peamiselt mandrilavade ÀÀrealadel kus ookeanisĂŒvikutest tĂ”usvad toiteaineterikkad hoovused toidavad pinnakihtide kĂ”rget primaarproduktsiooni. TĂŒĂŒpiliselt kujunevad fosfaatsete mineraalide (apatiidi) kristalliseerumiseks piisavad fosfaadi kontsentratsioonid settesisestel anoksilise (sulfiidse) â aeroobse keskkonna redokspiiridel, kus fosfori kuhjumist kontrollivad settes elavad ja primaarset orgaanilist ainet lagundavad mikroorganismid. Esimesed fosforirikkad settekivimid moodustusid Paleoproterosoikumis, ligikaudu kaks miljardit aastat tagasi. Seejuures on tĂ€helepanuvÀÀrne, et sarnase vanusega fosforiite on leitud paljudest kohtadest ĂŒle maakera ning seetĂ”ttu vĂ”ib arvata, et nende vĂ€ljasettimise pĂ”hjustas mingi globaalne sĂŒndmus. Doktoritöös uuriti ĂŒhtesid maailma vanimaid fosforiite, mis pĂ€rinevad kahest erinevast settebasseinist Loode Venemaal. Töö eesmĂ€rgiks oli selgitada keskkonnatingimused nende fosforiitide moodustuse ajal ning vĂ”rrelda neid tĂ€napĂ€evaste fosfogeneesi keskkondadega. PĂ”hitĂ€helepanu keskendus settelises apatiidis leiduvate haruldaste muldmetallide (lantanoidide) jaotumise ning sisalduste selgitamisele, mis vĂ”imaldavad interpreteerida apatiidi tekkekeskkonna redokstingimusi. Uuringute tulemusena selgus, et mĂ”lemas settebasseinis toimus fosfogenees merepĂ”hja settekolonni ĂŒlemises osas, kus valitsesid redokstingimused, mis on sarnased kaasaegse fosfogeneesi keskkondadele. See nĂ€itab, et tĂ”enĂ€oliselt kontrollisid apatiidi vĂ€ljasettimist Paleoproterosoikumis ja kontrollivad ka kaasaegsetes setendites sarnased bioloogilised-fĂŒĂŒsikalis/keemilised protsessid. Arvatavasti mĂ€rgib apatiidi vĂ€ljasettimise algust geoloogilises ajas spetsiifilise anoksilise (sulfiidse) â aeroobse redokspiiri moodustumine sette ĂŒlemises mĂ”ne cm paksuses osas, mis sai vĂ”imalikuks alles peale vaba hapniku ilmumist Maa atmosfÀÀris ligikaudu 2,3 miljardit aastat tagasi.Phosphorus is an essential and non-substitutable bioelement that limits the ecosystem primary productivity. In the modern P-cycle the major sink of phosphorous and formation of P-rich deposits - phosphorites - occurs at continental margins influenced by ocean upwelling currents. The main sites of modern phosphogenesis are the shallow levels of the sediment column close to the sediment water interface within the anoxic to (sub)oxic diagenetic zones. Recent studies of organic-rich sediments on modern continental margins have revealed the utmost significance of sulphur-oxidizing bacteria mediating a redox-dependent phosphorous-cycling by creating sinks for marine phosphorous, and eventually formation of phosphorites. Phosphorite formation is mainly a Phanerozoic (last 0.5 Ga) phenomenon, but the first significant P-rich sedimentary rocks appear seemingly synchronously worldwide in the Paleoproterozoic rock record at around 2 Ga. Appearance of these deposits have been linked to the oxidation of the atmosphere and establishment of the aerobic, modern-type Earth though little is known about the underlining causes and mechanisms of their formation. The aim of this PhD thesis was to study the environmental conditions of phosphogenesis in two ca 2 Ga old sedimentary basins using the Rare Earth Element analysis of sedimentary phosphates. The environmental conditions of the phosphogenesis recorded in apatite in the Paleoproterozoic PilgujĂ€rvi Sedimentary Formation of the Pechenga basin and in Zaonega Formation, Onega basin are similar, suggesting that the phosphogenic events in these sedimentary basins occurred in a similar way. Initiation of the phosphogenesis in these basins possibly marks the development of specific anoxic(sulfidic)-suboxic redoxclines at shallow sediment depth during the Paleoproterozoic that are very similar to the environmental settings found in modern phosphogenic areas. It is important that phosphogenic events archived in 1.97 Ga Zaonega Formation and in 1.92 Ga PilgujĂ€rvi Sedimentary Formation occurred ca 300-400 Ma after the Great Oxygenation Event at 2.3 Ga, suggesting that time was needed to establish diagenetic conditions and redoxcline conditions suitable for concentration of interstitial phosphate in shallow sediments at sea bottom
Workshop on a Cross Section of Archean Crust
Various topics relevant to crustal genesis, especially the relationship between Archean low - and high-grade terrains, were discussed. The central Superior Province of the Canadian Shield was studied. Here a 120 km-wide transition from subgreenschist facies rocks of the Michipicoten greenstone belt to granulite facies rocks of the Kapuskasing structural zone represents an oblique cross section through some 20 km of crust, uplifted along a northwest-dipping thrust fault
The Geology and Geochemistry of Base Metal Sulfide Mineralization in the Foster River area, Northern Saskatchewan, Canada
The Foster River area, northern Saskatchewan, is one of several Pb-Zn and Zn deposits (e.g. George Lake: 7.8 Mt @ 3.9% Zn and 0.5% Pb) along the SE margin of the highly deformed and metamorphosed Wollaston domain, Hearne Craton. Zinc-lead sulfide mineralization in the Foster River area is hosted by feldspathic quartzites within a unit of
psammites and pelites near the middle of the Paleoproterzoic (2.0-1.86 Ga) Wollaston Group. These sediments were metamorphosed to the upper amphibolite facies and subjected to at least four episodes of deformation during the 1860-1750 Ma Trans-Hudson Orogeny. Drilling indicates that the Sito Lake East prospect contains 50,000 t of 4.5% Zn, with one intercept containing 11m of 4.2 % Zn and 0.6% Pb. Boulders from a boulder train near the Fable Lake prospect contain up to 13.2% Zn, 4.0% Pb, and 11 g/t Ag. The sulfides are spatially associated with a package of rocks similar to that found with Broken Hill-type (BHT) deposits (quartzite, gahnite-rich rocks, iron formation, and quartz garnetite). A nodular sillimanite rock that occurs in the vicinity of the Sito East, Sito West, and George prospects is likely to be part of a stratabound hydrothermal alteration zone. Hydrothermal sulfide mineralization consists of pyrite, sphalerite, pyrrhotite, galena, and chalcopyrite, locally spatially associated with graphite. Gangue minerals include garnet, gahnite, magnetite, tourmaline, calcite, and augite. Chondrite-normalized REE patterns of silicate-facies iron formation (garnet-pyroxene-amphibole-magnetite rock) and quartz garnetite show light REE (LREE) enrichment and heavy REE (HREE) depletion, with moderate to large negative Eu anomalies and no or slightly positive Ce anomalies. Such patterns are consistent with meta-exhalites that have a high detrital component (\u3e30%) in the source rock. The compositions of garnet (spessartine-almandine) and gahnite at Foster River are similar to those spatially associated with BHT deposits. Sulfur isotope compositions of pyrite, pyrrhotite, and sphalerite from the Foster River area range from 26.2-38.1 / (n=20) and are consistent with sulfur being sourced from Paleoproterozoic seawater that was modified by microbial sulfate reduction in a restricted basin. The low Pb/(Pb+Zn) and Ag/(Ag+Pb+Zn) ratios of sulfide mineralization, common graphite-sulfide intergrowths, the high sulfur isotope values, and the lack of bimodal volcanics in the stratigraphic sequence suggest that the mineralization in the Foster River area has a sedimentary-exhalative origin with BHT affinities
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