scholarly journals Origin of Palaeoproterozoic, sub-seafloor Zn-Pb-Ag skarn deposits, Sala area, Bergslagen, Sweden

2021 ◽  
Author(s):  
Nils F. Jansson ◽  
Rodney L. Allen ◽  
Göran Skogsmo ◽  
Thomas Turner
Keyword(s):  
Regional Metamorphism ◽  
Geological Mapping ◽  
Sulfur Source ◽  
Calc Alkaline ◽  
Alkaline Granite ◽  
Metamorphic Overprint ◽  
Sulfide Deposits ◽  
Fluid Infiltration ◽  
Skarn Deposits ◽  
Grossular Garnet

AbstractUnravelling the genesis of metamorphosed mineral deposits can be complicated due to difficulties in separating between primary features and features that formed during the metamorphic overprint. Such uncertainty exists for stratabound and dolomite- and skarn-hosted Zn-Pb-Ag sulfide deposits in 1.89 Ga rocks in the Bergslagen lithotectonic unit (BLU) of Sweden, where a metasomatic vs. regional metamorphic origin for skarns has long been discussed. By integrating geological mapping with new lithogeochemical, mineralogical, and stable isotope data (C, O, S), we show that complexly zoned garnet and clinopyroxene skarns in the Sala area of the central BLU predate mineralization. Sphalerite-galena mineralization formed after the deposition of a younger, more Mn-rich ferroan diopside and andradite-grossular garnet, and is associated with phlogopite, tremolite-actinolite, chlorite, serpentine, and calcite. Mineralization in conjunction with a transition from high-T metasomatism to hydrolytic alteration is inferred. An average δ34SV-CDT of 1.6 ± 1.9‰ in sulfides is consistent with a primordial sulfur source. Trends defined by negative shifts in δ18OV-SMOW and δ13CV-PDB in dolomite and calcite are consistent with fluid infiltration at 300–500 °C. The alteration system is sharply truncated by unaltered, c. 1.89 Ga calc-alkaline granite and porphyritic intrusions, which along with F1 folding of the alteration zones and mineralization suggest that mineralization predate regional metamorphism. The Sala deposits are interpreted as Zn skarn deposits formed in conjunction with the emplacement of intrusions into penecontemporaneous marine volcanic and dolomitized limestone strata. The unusually Mg-rich mineralogy in relation to Zn skarns worldwide most likely reflects the dolomitic precursor.

scholarly journals Petrogenesis of a late-stage calc-alkaline granite in a giant S-type batholith: geochronology and Sr–Nd–Pb isotopes from the Nomatsaus granite (Donkerhoek batholith), Namibia

2021 ◽  
Author(s):  
S. Aspiotis ◽  
S. Jung ◽  
F. Hauff ◽  
R. L. Romer
Keyword(s):  
Partial Melting ◽  
Mineral Assemblage ◽  
Central Zone ◽  
Calc Alkaline ◽  
Pb Isotope ◽  
Alkaline Granite ◽  
Ti Oxides ◽  
Eu Anomaly ◽  
Environment Characteristic ◽  
Southern Central

AbstractThe late-tectonic 511.4 ± 0.6 Ma-old Nomatsaus intrusion (Donkerhoek batholith, Damara orogen, Namibia) consists of moderately peraluminous, magnesian, calc-alkalic to calcic granites similar to I-type granites worldwide. Major and trace-element variations and LREE and HREE concentrations in evolved rocks imply that the fractionated mineral assemblage includes biotite, Fe–Ti oxides, zircon, plagioclase and monazite. Increasing K2O abundance with increasing SiO2 suggests accumulation of K-feldspar; compatible with a small positive Eu anomaly in the most evolved rocks. In comparison with experimental data, the Nomatsaus granite was likely generated from meta-igneous sources of possibly dacitic composition that melted under water-undersaturated conditions (X H2O: 0.25–0.50) and at temperatures between 800 and 850 °C, compatible with the zircon and monazite saturation temperatures of 812 and 852 °C, respectively. The Nomatsaus granite has moderately radiogenic initial 87Sr/86Sr ratios (0.7067–0.7082), relatively radiogenic initial εNd values (− 2.9 to − 4.8) and moderately evolved Pb isotope ratios. Although initial Sr and Nd isotopic compositions of the granite do not vary with SiO2 or MgO contents, fSm/Nd and initial εNd values are negatively correlated indicating limited assimilation of crustal components during monazite-dominated fractional crystallization. The preferred petrogenetic model for the generation of the Nomatsaus granite involves a continent–continent collisional setting with stacking of crustal slices that in combination with high radioactive heat production rates heated the thickened crust, leading to the medium-P/high-T environment characteristic of the southern Central Zone of the Damara orogen. Such a setting promoted partial melting of metasedimentary sources during the initial stages of crustal heating, followed by the partial melting of meta-igneous rocks at mid-crustal levels at higher P–T conditions and relatively late in the orogenic evolution.

Release of Uranium from Uraninite in Granites Through Alteration: Implications for the Source of Granite-Related Uranium Ores

2021 ◽  
Author(s):  
Long Zhang ◽  
Zhenyu Chen ◽  
Fangyue Wang ◽  
Noel C. White ◽  
Taofa Zhou
Keyword(s):  
Uranium Concentration ◽  
Geochemical Data ◽  
Uranium Deposits ◽  
Bulk Rock ◽  
Calc Alkaline ◽  
Alkaline Granite ◽  
Compositional Evolution ◽  
High K ◽  
Backscattered Electron ◽  
Uranium Sources

Abstract Uraninite is the main contributor to the bulk-rock uranium concentration in many U-rich granites and is the most important uranium source for granite-related uranium deposits. However, detailed textural and compositional evolution of magmatic uraninite in granites during alteration and associated uranium mobilization have not been well documented. In this study, textures and geochemistry of uraninites from the Zhuguangshan batholith (South China) were investigated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The geochemical data indicate that the Longhuashan and Youdong plutons are peraluminous leucogranite, the Changjiang pluton is highly fractionated high-K calc-alkaline granite, and the Jiufeng pluton belongs to a high-K calc-alkaline association. Uraninites from the Longhuashan and Youdong granites have lower concentrations of ThO2 (0.9–4.0 wt %) and rare earth elements (REE)2O3 (0.1–1.0 wt %) than those from the Changjiang and Jiufeng granites (ThO2 = 4.4–7.6 wt %, REE2O3 = 0.7–5.1 wt %). Uraninites observed in the Longhuashan, Youdong, Changjiang, and Jiufeng granites yielded chemical ages of 223 ± 3, 222 ± 2, 157 ± 1, and 161 ± 2 Ma, respectively. The samples (including altered and unaltered) collected from the Longhuashan, Youdong, and Changjiang granites are characterized by highly variable whole-rock U concentrations of 6.9 to 44.7 ppm and Th/U ratios of 0.9 to 7.0, consistent with crystallization of uraninite in these granites being followed by uranium leaching during alteration. Alteration of uraninite, indicated by altered domains developing microcracks and appearing darker in backscattered electron (BSE) images compared to unaltered domains, results in the incorporation of Si and Ca and mobilization of U. In contrast, the least altered samples of the unmineralized Jiufeng granite have low U concentrations (5.3–16.4 ppm) and high ΣREE/U (13.6–49.4) and Th/U ratios (2.1–5.6), which inhibit crystallization of uraninite, as its crystallization occurs when the U concentration is high enough to exceed the substitution capacity of other U-bearing minerals. These results indicate that the Longhuashan, Youdong, and Changjiang granites were favorable uranium sources for the formation of uranium deposits in this area. This study highlights the potential of uraninite alteration and geochemistry to assist in deciphering uranium sources and enrichment processes of granite-related uranium deposits.

Rapid fluid-driven transformation of lower continental crust associated with thrust-induced shear heating

2020 ◽  
Author(s):  
Bjørn Jamtveit ◽  
Kristina G. Dunkel ◽  
Arianne Petley-Ragan ◽  
Fernando Corfu ◽  
Dani W. Schmid
Keyword(s):  
Regional Metamorphism ◽  
Shear Zones ◽  
Amphibolite Facies ◽  
Source Rocks ◽  
Granitic Rocks ◽  
Time Interval ◽  
Fluid Infiltration ◽  
Facies Metamorphism ◽  
Shear Heating ◽  
Host Rocks

<p>Caledonian eclogite- and amphibolite-facies metamorphism of initially dry Proterozoic granulites in the Lindås Nappe of the Bergen Arcs, Western Norway, is driven by fluid infiltration along faults and shear zones. The granulites are also cut by numerous dykes and pegmatites that are spatially associated with metamorphosed host rocks. U-Pb geochronology was performed to constrain the age of fluid infiltration and metamorphism. The ages obtained demonstrate that eclogite- and amphibolite-facies metamorphism were synchronous within the uncertainties of our results and occurred within a maximum time interval of 5 Myr, with a mean age of ca. 426 Ma.  Caledonian dykes and pegmatites are granitic rocks characterised by a high Na/K-ration, low REE-abundance and positive anomalies of Eu, Ba, Pb, and Sr. The most REE-poor compositions show HREE-enrichment. Melt compositions are consistent with wet melting of plagioclase- and garnet-bearing source rocks. The most likely fluid source is dehydration of Paleozoic metapelites, located immediately below the Lindås part of the Jotun-Lindås microcontinent, during eastward thrusting over the extended margin of Baltica. Melt compositions and thermal modelling suggest that short-lived fluid-driven metamorphism of the Lindås Nappe granulites was related to shear heating at lithostatic pressures in the range 1.0-1.5 GPa. High-P (≈2 GPa) metamorphism within the Nappe was related to weakening-induced pressure perturbations, not to deep burial. Our results emphasize that both prograde and retrograde metamorphism may proceed rapidly during regional metamorphism and that their time-scales may be coupled through local production and consumption of fluids.</p>

Outline of the geochronology and tectonic units of the basement complex of northeast Africa

1976 ◽  
Vol 350 (1660) ◽  
pp. 127-141 ◽  
Keyword(s):  
Regional Metamorphism ◽  
Central Africa ◽  
Geological Mapping ◽  
Fold Belt ◽  
Basement Complex ◽  
Southern Sudan ◽  
Pan African ◽  
Mozambique Belt ◽  
Unknown Area ◽  
Eastern Uganda

Results of new geological mapping with the help of air and satellite photo­graphy in Sudan together with information from adjacent territories has enabled a map to be drawn showing the dominant basement tectonic trends in a previously geologically unknown area. Over 100 age deter­minations, including 25 unpublished analyses, allow the recognition of Eburnian age events in Central Africa Republic and southeast Libya similar to the 1950 million year (Ma) old Ruwenzori Belt in Uganda and similar events in Zaire. A northeast trending fold belt is recognized in Central Africa, western Sudan and southeast Egypt in which 1000 Ma ages are found. The Pan African age Mozambique belt truncates older structures in eastern Uganda and southern Sudan but is covered by a greenschist volcanic assemblage along the Red Sea coast in which 550 ± 150 Ma old granites and regional metamorphism occur.

Suite subdivision and petrological evolution of granitoids from the Taylor Valley and Ferrar Glacier region, south Victoria Land

1992 ◽  
Vol 4 (1) ◽  
pp. 71-87 ◽  
Author(s):  
Robert W. Smillie
Keyword(s):  
Upper Mantle ◽  
Structural Control ◽  
Geological Mapping ◽  
Dry Valleys ◽  
Crustal Extension ◽  
Calc Alkaline ◽  
Taylor Valley ◽  
Victoria Land ◽  
Early Palaeozoic ◽  
Dyke Swarms

Detailed geological mapping and geochemical analysis of early Palaeozoic granitoid plutons and dykes from the Taylor Valley and Ferrar Glacier region in south Victoria Land reveal two distinct suites. This suite subdivision-approach is a departure from previous lithology-based schemes and can be applied elsewhere in south Victoria Land. The older calc-alkaline Dry Valleys 1 suite is dominated by the compositionally variable Bonney Pluton, a flow-foliated concordant pluton with an inferred length of over 100 km. Plutons of this suite are elongate in a NW-SE direction and appear to have been subjected to major structural control during their emplacement. The younger alkali-calcic Dry Valleys 2 suite comprises discordant plutons and numerous dyke swarms with complex age relationships. Field characteristics of this suite indicate that it was passively emplaced into fractures at higher levels in the crust than the Dry Valleys 1 suite. Whole-rock geochemistry confirms this suite subdivision based on field relationships and indicates that the two suites were derived from different parent magmas by fractional crystallization. The Dry Valleys 1 suite resembles Cordilleran I-type granitoids and is inferred to be derived from partial melting of the upper mantle and/or lower crust above an ancient subduction zone. The Dry Valleys 2 suite resembles Caledonian I-type granitoids and may have resulted from a later episode of crustal extension.

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

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

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

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