Low-temperature alteration of uranium–thorium bearing minerals and its significance in neoformation of radioactive minerals in stream sediments of Wadi El-Reddah,North Eastern Desert,Egypt

The stream sediments of Wadi El Reddah(North Eastern Desert,Egypt)are geochemically and mineralogically investigated.Their content of radioactive and other heavy minerals is mainly represented by thorite,uranothorite,zircon,monazite,xenotime,columbite,fergusonite,and unknown rare earth elements(REEs)bearing minerals as well as cassiterite.Special emphasis on REE content of thorite,uranothorite,zircon and xenotime has been done to correlate them with the increase of uranium contents in these sediments.The key evidence for the presence lowtemperature alteration processes includes the presence of some zircon crystals as remnants after complete dissolution of the overgrowth zircon in severe acidic environment,the sulphur content,biogenic minerals,occurrence of unusual minerals as cassiterite pore filling in zircon,variation in the REEs content from the surrounding granites to the stream sediments and the abundance of monazite in the surrounding granites.Most minerals are partially and/or completely altered,which indicated by the pseudomorphism of zircon by xenotime,thorite,and uranothorite.     

Characteristic mineralogy of the Zhutishi granite:Implication for petrogenesis of the late intrusive granite

Many granitic batholiths occur in the form of com-plexes, presented principally by a temporal-spacial association between two stages of intrusion, in the Nanling region. Compared with main intrusive gran-ites, late intrusive granites are characterized by fine- grained texture, Si- and Al-enriched composition, and small occurrence as stock or apophysis. On the basis of its rock chemistry (e.g., increasing aluminium saturation index) and geochemistry (e.g., Eu depletion, decreasing concentratio…     

Niobium–tantalum oxide minerals in alluvial placer deposits from the Ngoura area,East-Cameroon

This work presents a preliminary mineralogical characterization of Nb-Ta oxide minerals from alluvial placers located in the Ngoura area,eastern Cameroon.The heavy minerals are characteristic of short hydromechanical transport and highlight the stability of Nb-Ta oxides during the weathering and transport.Nb-Ta mineralization in placer deposits from the Ngoura area may derive from local sedimentation.The assemblage of Nb-Ta oxides in the studied placers consists of tapiolite-(Fe) and tantalite-(Mn).Tapiolite-(Fe) exhibits high concentrations in Ta_2O_5(78.07-80.46 wt%) and FeO(12.18-13.66 wt%) and plots within a narrow range of Ta~*(0.900-0.933) and Mn~*(0.037-0.119),which correspond to the ranges typically observed in tapiolite-(Fe) worldwide.Tantalite-(Mn) shows Ta_2O_5 contents ranging from 62.17 to 69.45 wt%,Nb_2O_5 from 12.09 to 17.37 wt%,MnO from 7.63 to 12.49 wt%and FeO from 1.48 to 6.62 wt%.It is also characterized by a wider range of Mn~~*(0.538-0.891) and relatively homogeneous Ta/(Ta+Nb)(0.683-0.779) ratios.Texturally,the studied Nb-Ta minerals exhibit oscillatory zoning characterized by bright Ta-rich zones alternating with dark Nb-rich zones.This oscillatory zoning is progressive in some minerals and the alternating bands may appear regular to wavy with gradual transitions.Oscillatory zoning in Nb-Ta oxides from the Ngoura placers is considered to be a primary magmatic feature and is tentatively explained as a result of magmas mixing,rapid cooling or degassing/decompression of the igneous system.The geomorphology and the tropical humid climate of the eastern region offer suitable conditions for the deposition of alluvial placers.Therefore,niobium-tantalum minerals from the Ngoura placers must have been sourced from the weathering and erosion of alkaline granites and pegmatites widespread in the study area.     

Lewisian gneiss geochemistry and Archaean crustal development models

The geochemistry of Lewisian amphibolite-facies gneisses from northwest Scotland is described with particular reference to the rare earth elements (REE) and compared with the geochemistry of Lewisian granulite-facies gneisses. The results show that there are no significant differences between “Laxfordian” amphibolite-facies and “Scourian” granulite-facies gneisses in terms of REE and other immobile trace elements (at equivalent silica levels), although the mobile radioactive heat-producing elements, K, Rb, Th, U, are significantly lower in the granulites. In both types the basic gneisses have moderately fractionated REE patterns while the intermediate and acid gneisses have strongly fractionated REE patterns with low heavy REE abundances and decreasing levels of total REE with increasing SiO₂. The most silicic gneisses develop large positive europium anomalies.These gross chemical similarities between gneisses from intermediate (amphibolite-facies) and lower (granulite-facies) crustal levels constrain models for the evolution of the Archaean crust. The depletion of K, Rb, Cs, Th and U in granulites, but not other incompatible trace elements cannot be explained by magmatic processes. The positive Eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation. Fractionation of radioactive heat-producing elements from other trace elements is a result of granulite-facies metamorphism with these elements being removed by an active fluid phase. The apparent lack of partial melting in lower crustal granulites suggests a model for Archaean crustal growth largely through underplating by primary tonalitic magmas.     

The Early Paleozoic Tiefosi syn-collisional granite in the northern Dabie Orogen:Geochronological and geochemical constraints

The Tiefosi granitic pluton is located 5 km northwest of Xinyang City,northern Dabie Orogen,and was emplaced in the Proterozoic Qinling Group. SHRIMP zircon U-Pb dating suggests its crystallization at 436 ± 11 Ma. It is composed of monzogranite and syenogranite containing some amounts of muscovite and few mafic minerals. The rocks are characterized by high and restricted SiO2 content,low FeO,Fe2O3 and MgO contents,high K2O/Na2O ratio,and display high-K calc-alkaline and peraluminous (ACNK>1.1) characteristics. They are generally enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE). They can be divided into three groups in light of rare earth elements (REE) and trace elements. Group I is moderate in ΣREE and characterized by the absence of Eu anom-aly,high (La/Yb)N ratio,and moderate Rb/Sr and Rb/Ba ratios. Group Ⅱ has moderately negative Eu anomaly,low (La/Yb)N ratio and high ΣREE contents,Rb/Sr and Rb/Ba ratios. Group Ⅲ displays positive Eu anomaly,moderate (La/Yb)N ratio,and low ΣREE,Rb/Sr and Rb/Ba ratios. The calculated εNd(440Ma) values of the rocks vary from 8.8 to 9.9 and Nd depleted mantle model ages are about 2.0 Ga,which resemble those of the paragneisses from the Qinling Group. The results indicate that the Tiefosi granite is crust-derived,syn-collisional S-type granite. Generation of Group I was related to low degree melting of the Qinling Group,while Group Ⅱ was formed by fractionational crystallization of plagioclase from Group I magmas,and Group Ⅲ resulted possibly from magma mingling with plagioclase cumulates. The Tiefosi granite was formed within crustal level related to the collision between the North China and South China blocks in the Early Paleozoic time.     

A preliminary study of rare-metal mineralization in the Himalayan leucogranite belts,South Tibet

The Himalayan leucogranite occurs as two extensive (>1000 km) E-W trending belts on the Tibetan Plateau with the unique features. The leucogranite comprised biotite granite, two-mica/muscovite granite, tourmaline granite and garnet granite, which have been identified in previous studies, as well as albite granite and granitic pegmatite that were identified in this investigation. Fifteen leucogranite plutons were studied and 12 were found to contain rare-metal bearing minerals such as beryl (the representative of Be mineralization), columbite-group minerals, tapiolite, pyrochlore-microlite, fergusonite, Nb-Ta rutile (the representative of Nb-Ta mineralization), and cassiterite (the representative of Sn mineralization) mainly based on the field trip, microscope observation and microprobe analysis. The preliminary result shows that the Himalayan leucogranite is commonly related to the rare-metal mineralization and warrants future investigation. Further exploration and intensive research work is important in determining the rare-metal resource potential of this area.     

Diversity of Mesozoic tin-bearing granites in the Nanling and adjacent regions,South China: Distinctive mineralogical patterns

The Nanling and adjacent regions of South China host a series of tin deposits related to Mesozoic granites with diverse petrological characteristics. The rocks are amphibole-bearing biotite granites, or(topaz-) albite-lepidolite(zinnwaldite) granites,and geochemically correspond to mealuminous and peraluminous types, respectively. Mineralogical studies demonstrate highly distinctive and critical patterns for each type of granites. In mealuminous tin granites amphibole, biotite and perthite are the typical rock-forming mineral association; titanite and magnetite are typical accessory minerals, indicating high fO2 magmatic conditions;cassiterite, biotite and titanite are the principal Sn-bearing minerals; and pure cassiterite has low trace-element contents. However,in peraluminous tin granites zinnwaldite-lepidolite, K-feldspar and albite are typical rock-forming minerals; topaz is a common accessory phase, indicative of high peraluminity of this type of granites; cassiterite is present as a uniquely important tin mineral,typically rich in Nb and Ta. Mineralogical distinction between the two types of tin granites is largely controlled by redox state,volatile content and differentiation of magmatic melts. In oxidized metaluminous granitic melts, Sn4+ is readily concentrated in Ti-bearing rock-forming and accessory minerals. Such Sn-bearing minerals are typical of oxidized tin granites, and are enriched in granites at the late fractionation stage. In relatively reduced peraluminous granitic melts, Sn2+ is not readily incorporated into rock-forming and accessory minerals, except for cassiterite at fractionation stage of granite magma, which serves as an indicator of tin mineralization associated with this type of granites. The nature of magma and the geochemical behavior of tin in the two types of granites thus result in the formation of different types of tin deposits. Metaluminous granites host disseminated tin mineralization,and are locally related to deposits of the chlorite quartz-vein, greisen, and skarn types. Greisen, skarn, and quartz-vein tin deposits can occur related to peraluminous granites, but disseminated mineralization of cassiterite is more typical.     

Multiple-aged granitoids and related tungsten-tin mineralization in the Nanling Range, South China

The Nanling metallogenic belt in South China is characterized by well-developed tungsten-tin mineralization related to multiple-aged granitoids. This belt is one of the 5 key prospecting and exploration areas among the 19 important metallogenic targets in China. Important progress has been made in recent years in understanding the Nanling granitoids and associated mineralization, and this paper introduces the latest major findings as follows: (1) there exists a series of Caledonian, Indosinian, and Yanshanian W-Sn-bearing granites; (2) the Sn-bearing Yanshanian granites in the Nanling Range form an NE-SW trending aluminous A-type granite belt that stretches over 350 km. The granites typically belong to the magnetite series, and dioritic micro-granular enclaves with mingling features are very common; (3) the Early Yanshanian Sn- and W-bearing granites possess different petrological and geochemical features to each other: most Sn-bearing granites are metaluminous to weakly peraluminous biotite (hornblende) granites, with zircon ?_Hf(t) values of ca. ?2 to ?8, whereas most W-bearing granites are peraluminous two-mica granites or muscovite granites with ?_Hf(t) values of ca. ?8 to ?12; (4) based on the petrology and geochemistry of the W-Sn-bearing granites, mineralogical studies have shown that common minerals such as titanite, magnetite, and biotite may be used as indicators for discriminating the mineralizing potential of the Sn-bearing granites. Similarly, W-bearing minerals such as wolframite may indicate the mineralizing potential of the W-bearing granites. Future studies should be focused on examining the internal relationships between the multiple-aged granites in composite bodies, the metallogenic peculiarities of multiple-aged W-Sn-bearing granites, the links between melt evolution and highly evolved ore-bearing felsic dykes, and the connections between granite domes and mineralization.     

Study of the minerogenetic mechanism and origin of Qinghai nephrite from Golmud,Qinghai, Northwest China

Electronic microprobe analysis showed that all QN samples are mainly composed of tremolite and minor accessory minerals, such as diopside, calcite, serpentinite, and magnetite. According to the cation coefficients, the crystallo-chemistrygenesis illustration demonstrates that all QN deposits are contact metasomatic. Depending on the mole percent of Fe~(2+(3+))/(Mg~(2+)+Fe~(2+(3+)) and the content of Cr, Co, and Ni in all QN samples measured by X-ray fluorescence spectroscopy(XRF) and inductively coupled plasma-mass spectrometry(ICP-MS), green and azure-green QNs are characterized as serpentinite-related contact metasomatic deposit(S-type), whereas white, green-white, brown, blue-violet, yellow, and viridis QNs are dolomite-related contact metasomatic deposit(D-type). The assemblages and chemical composition of accessory minerals of the eight-color QN samples show evident characteristics, which reveal four possible ore-forming processes. We also measured trace and rare earth elements(REEs) in these samples through ICP-MS to deduce the origin of and the changes in metallogenic conditions. The chondrite-normalized REE patterns of D-type QN exhibit moderately negative Eu anomalies with moderate light REE enrichment, flat heavy REE(HREE), and low(50)REE concentrations, similar to dolomitic marble. Green QN samples of S-type show enrichment in HREE and moderately negative Eu anomalies, which is consistent with characteristics of dunite. Whereas azure-green QN samples of S-type exhibit a right-dipping V-type curve with severe depletion in Eu(δEu=0.36–0.47), in accordance with the characteristics of gabbro from Yushigou ophiolite in North Qilian mountains. Furthermore, this finding is consistent with the content of trace elements and the petrographic analysis results. On the basis of several significant differences in the characteristic elements, which may have been affected by the metallogenic environment, we inferred the differences in oxygen fugacity and basicity of mineralization environments in different-colored QNs.     

Geological characteristics and mineralization setting of the Zhuxi tungsten (copper) polymetallic deposit in the Eastern Jiangnan Orogen

The Zhuxi ore deposit is a super-large scheelite(copper) polymetallic deposit discovered in recent years. It grew above copper/tungsten-rich Neoproterozoic argilloarenaceous basement rocks and was formed in the contact zone between Yanshanian granites and Carboniferous-Permian limestone. Granites related to this mineralization mainly include equigranular, middle- to coarse-grained granites and granitic porphyries. There are two mineralization types: skarn scheelite(copper) and granite scheelite mineralization. The former is large scale and has a high content of scheelite, whereas the latter is small scale and has a low content of scheelite. In the Taqian-Fuchun Basin, its NW boundary is a thrust fault, and the SE boundary is an angular unconformity with Proterozoic basement. In Carboniferous-Permian rock assemblages, the tungsten and copper contents in the limestone are both very high. The contents of major elements in granitoids do not differ largely between the periphery and the inside of the Zhuxi ore deposit. In both areas, the values of the aluminum saturation index are A/CNK1.1, and the rocks are classified as potassium-rich strongly peraluminous granites. In terms of trace elements, compared to granites on the periphery of the Zhuxi ore deposit, the granites inside the Zhuxi ore deposit have smaller d Eu values, exhibit a significantly more negative Eu anomaly, are richer in Rb, U, Ta, Pb and Hf, and are more depleted in Ba, Ce, Sr, La and Ti, which indicates that they are highly differentiated S-type granites with a high degree of evolution. Under the influence of fluids, mineralization of sulfides is evident within massive rock formations inside the Zhuxi ore deposit, and the mean SO_3 content is 0.2%. Compared to peripheral rocks, the d Eu and total rare earth element(REE) content of granites inside the Zhuxi ore deposit are both lower, indicating a certain evolutionary inheritance relationship between the granites on the periphery and the granites inside the Zhuxi ore deposit. For peripheral and ore district plutons, U-Pb zircon dating shows an age range of 152–148 Ma. In situ Lu-Hf isotope analysis of zircon in the granites reveals that the calculated e_(Hf)(t) values are all negative, and the majority range from -6 to -9. The T_(DM2) values are concentrated in the range of 1.50–1.88 Ga(peak at 1.75 Ga), suggesting that the granitic magmas are derived from partial melting of ancient crust. This paper also discusses the metallogenic conditions and ore-controlling conditions of the ore district from the perspectives of mineral contents, hydrothermal alteration, and ore-controlling structures in the strata and the ore-bearing rocks. It is proposed that the Zhuxi ore deposit went through a multistage evolution, including oblique intrusion of granitic magmas, skarn mineralization, cooling and alteration, and precipitation of metal sulfides. The mineralization pattern can be summarized as "copper in the east and tungsten in the west, copper at shallow-middle depths and tungsten at deep depths, tungsten in the early stage and copper in the late stage".     

Geochemistry and petrology of garnet‐bearing S‐type Shir‐Kuh Granite,southwest Yazd,Central Iran

The Jurassic Shir‐Kuh granitoid batholith in Central Iran intrudes Lower Jurassic sandstones and shales. The batholith consists of three main facies: (i) a granodioritic facies to the north; (ii) a monzogranitic facies spread throughout the batholith; and (iii) a leucogranitic facies along the northwestern margin. The granodiorites are composed mainly of plagioclase, quartz, K‐feldspar, biotite, and some muscovite, garnet, cordierite, ilmenite, zircon, apatite, and monazite. This facies contains variable amounts of restite minerals which are mainly defined by calcic plagioclase cores and small aggregates of biotite. The monzogranites, with mineral assemblages similar to those in the granodiorites, range from relatively mafic (cordierite‐bearing) to felsic (muscovite‐rich) rocks. The leucogranites, exposed as small stock and dykes, consist mainly of quartz, K‐feldspar, and sodic plagioclase. The batholith is peraluminous, calc‐alkaline, and typical of S‐type, as indicated by Na₂O content (2.74%), molecular Al₂O₃/(CaO + Na₂O + K₂O) (A/CNK) ratio (1.17), K₂O/Na₂O ratio (1.39), and isotopic data ([⁸⁷Sr/⁸⁶Sr]_i = 0.715). The rocks are characterized by enrichment in large ion lithophile elements such as Rb, Th and K and depletion in high field strength elements such as Nb and Ti. Chondrite‐normalized rare earth element (REE) patterns are characterized by light rare earth element (LREE) enrichment, with values of (La/Yb)_N between 4.5 and 19.53, unfractionated heavy rare earth element (HREE) with values of (Gd/Yb)_N between 0.98 and 2.88, and a distinct negative Eu. The parental magma of the Shir‐Kuh Granite was derived from a plagioclase‐rich metasedimentary source (local anatexis of metagreywacke) in the crust, with heat input from mantle melt components. The separation of restite crystals from the primary melt followed by the fractional crystallization appears to have been an effective differentiation process in the batholith.     

Trace element compositions of jadeite (+omphacite) in jadeitites from the Itoigawa-Ohmi district, Japan: Implications for fluid processes in subduction zones

Abstract   Trace-element compositions of jadeite (±omphacite) in jadeitites from the Itoigawa-Ohmi district of Japan, analyzed by a laser-ablation inductively coupled plasma mass spectrometry technique showed chemical zoning within individual grains and variations within each sample and between different samples. Primitive mantle-normalized patterns of jadeite in the samples generally showed high large-ion lithophile element contents, high light rare earth element/heavy rare earth element ratios and positive anomalies of high field strength elements. The studied jadeitites have no signatures of the protolith texture or mineralogy. Shapes and distributions of minerals coupled with chemical zoning within grains suggest that the jadeitites were formed by direct precipitation of minerals from aqueous fluids or complete metasomatic modification of the precursor rocks by fluids. In either case, the geochemical characteristics of jadeite are highly affected by fluids enriched in both large-ion lithophile elements and high field strength elements. The specific fluids responsible for the formation of jadeitites are related to serpentinization by slab-derived fluids in subduction zones. This process is followed by dissolving high field strength elements in the subducting crust as the fluids continue to circulate into the subducting crusts and serpentinized peridotites. The fluids have variations in chemical compositions corresponding to various degrees of water–rock interactions.     

Distribution of rare earth elements of granitic regolith under the influence of climate

The distribution and anomalies of rare earth elements(REEs) of granitic regolith were studied in Inner Mongolia and Hainan Island, China. One profile showed slight REE enrichment of an upper layer and no obvious light REE/heavy REE(LREE/HREE) fractionation(La_N/Yb_N of 0.9). The second profile was significantly enriched in REEs and enriched in LREEs in the upper portion(La_N/Yb_N1.8). Eu, Ce, and Gd anomalies of the two profiles are different. Slightly negative Eu, Ce, and Gd anomalies in NMG-3-1 indicate slow dissolution of primary minerals and little secondary products; in contrast, a positive Eu anomaly in HN-2 suggests the vegetation cycle may contribute to soil. The Ce anomaly of HN-2 reflects oxidation of Ce and coprecipitation by Fe-and Mn-oxides and organic matter. Correlation between Ce and Gd anomalies in HN-2 suggests Ce and Gd are both influenced by redoxreduction.     

Geochronological Significance of the Post-Orogenic Mafic- Ultramafic Rocks in the Hongqiling Area of Jilin Province, Northeast China

INTRODUCTIONThe mafic-ultramafic complexesinthe Hongqilingarea were emplacedintothe metamorphic rocksof the Hulan Group. Age determination of the intrusion and metamorphism of the Hulan Groupmetamorphic rocks is crucial for the study of petrogenesis and evolution, orogenesis and itsdevelopment of the region. However ,so far it has been difficult to determine the geochronology ofmafic-ultramafic rocks inthe area ,thusthe age obtainedfromprevious data hadto be used.Inrecentyears ,withthe …     

Origin of Li-F-rich granite: Evidence from high P-T experiments

In South China and some regions around the world, there is a special type of rocks. These rocks are usually ultra-acidic, peraluminous, rich in Na and volatile components, such as H2O, F, B and P, and with higher concentrations of lithophile rare metal elements, including Li, Rb, Cs, Be, Ta, Nb, Sn, W, etc. Rocks of this type are commonly called Li-F-rich, rare-metal bearing granitic rocks, or Li-F granites for short[1]. The economic importance and distinct forma-tion mechanism of Li-…     

LA-ICP-MS zircon U-Pb dating and phenocryst EPMA of dikes,Guocheng, Jiaodong Peninsula: Implications for North China Craton lithosphere evolution

Widespread dike swarm, including diorite-, monzonite-porphyry and lamprophyre, intruded in the al- tered breccia gold deposits along basin marginal faults, Guocheng, Jiaodong Peninsula. Petrography exhibits biotite enclaves in amphibole phenocrysts and the presence of acicular apatites in these dikes. Electron probe microanalyses (EPMA) show that the amphibole and clinopyroxene phenocryst’s mantle in diorite porphyry and lamprophyre respectively has sharply higher MgO (Mg#) and Cr2O3 contents in contrast to their cores. The plagioclase phenocryst in monzonite porphyry has reverse zoning. These results indicate that the magma mixing between mantle-derived mafic and crust-derived felsic magmas occurred in the original process of the dikes. Zircon cathodoluminescence (CL) images show well-developed magmatic oscillatory zones and the acquired LA-ICP-MS zircon U-Pb weighted mean 206Pb/238U ages are 114±2 Ma (MSDW=1.5) for monzonite porphyry (GS1) and 116±1 Ma (MSDW=0.8) for diorite porphyry (GS2), respectively. Earlier magmatic events in the northwest Jiaodong Peninsula represented by some inherited or captured zircons also occur in these dikes. Magmatic zircons from GS1 and GS2 display consistent chondrite-normalized REE patterns and Nb/Ta values, implying that they may share a similar or same source. HREE enrichment and obvious negative Eu anomalies of these zircons preclude garnet presented in their source. Our results, combined with preciously pub- lished data, indicate that dike intrusion and gold mineralization among quartz vein, altered tectonite and altered breccia gold deposits are broadly contemporaneous throughout the Jiaodong Peninsula. These also imply that the intensive crust-mantle interaction and asthenospheric underplating had oc- curred in the Early Cretaceous in the Peninsula, together with foundering of lower crust in the early Mesozoic, representing the different stages of lithosphere thinning in the North China Craton (NNC).     

广东南山花岗岩形成时代、地球化学特征与成因

广东南山花岗岩体位于陂头复式岩体西端,锆石的SHRIMP U-Pb年龄为158.1±1.8Ma,是燕山早期岩浆活动的产物。岩石化学特征显示岩体以高硅、富碱、贫Ca和Mg以及高TFeO/MgO、低CaO/Na2O为特征。其K2O/Na2O〉1,A/NK=7.8~11.92,A/CNK=1.33~1.68,属过铝质碱性岩石。在稀土和微量元素组成上,岩石富含稀土元素（除明显的负Eu异常,δEu=0.09~0.16）以及Zr、Y、Th、U、Nb等高场强元素,贫Ba、Sr、Ti等,高10000x Ga/Al（比值大于2.6）。在Zr、Nb、Ce、Y对10000×Ga/Al以及TFeO/MgO-SiO2等A型花岗岩多种判别图上,投影点主要落在A型花岗岩区,而与高分异的I、S型花岗岩明显不同。这些特征均指示,南山岩体具有铝质A型花岗岩的特点。通过Y-Nb-3Ga和Y-Nb-Ce构造环境判别图解将其进一步划分为A2型花岗岩,代表其形成于拉张的构造背景之下。本文在此研究基础上,认为南山花岗质岩浆可能形成于相对挤压的中侏罗世。而在晚侏罗世早期相对拉张的作用下,岩石圈减薄,软流圈地幔上涌,地壳的泥质岩和少量砂质岩受到幔源流体富集后发生部分熔融后上侵形成铝质A型花岗岩,且有较强的结晶分异作用。     

Zircon chemistry and new laser ablation U–Pb ages for uraniferous granitoids in SW Cameroon

The Ekomedion two-mica granite,southwestern Cameroon,has potential for uranium and molybdenum mineralization.Here,we present LA-ICP-MS U-Pb ages,Lu-Hf isotope characteristics,trace element concentrations and Ti-geothermometry of zircon from this granite hosting U-Mo mineralization in pegmatitic pods.The majority of zircon are CL-dark though some CL-bright cores were also identified.U-Pb zircon age data range from 121±3 to743±11 Ma with only 5 of 34 ages being near concordant.The concordant mean age of 603±12 Ma is similar to ages of granitic intrusions along the Central African Shear Zone in Cameroon.Apparent ages with mean of261±6 Ma reveal open system behavior with respect to Pb and/or U.Zircon e Hfi values range from-20.3 to-0.3.This implies that U-Mo was remobilized during partial melting of the surrounding gneiss.Zircon Th/U>0.1 as well as an increasing Hf with decreasing Th/U indicates that fractional crystallization was the main factor that controlled U-Mo mineralization in pegmatitic pods.Y and Y/Ho ratios cluster from 29 to 33 close to the chondritic ratio of 28 and indicate fractionation of Y and Ho with low F contents during the earliest stages of crystallization.Late stage accumulation of F-rich magmatic-hydrothermal fluids impacted U-Mo mineralization as a ligand.Zircon contains a prominent negative Eu anomaly pointing to a fractionating system rich in plagioclase.Calculated Ti-in-zircon temperatures span 672℃to1232℃with the temperatures at the high end reflecting interference from mineral inclusions in the zircon grains while the lower temperature values are linked to crystallization.     

Rare earth element mobility during granite alteration: Evidence from southwest England

Geochemical analyses of granitic rocks from southwest England reveal that the rare earth elements (REE) were potentially mobile during hydrothermal and supergene alteration. In particular, trivalent REE were removed from the system during K-silicate alteration, Eu was lost during sericitic alteration; all REE were lost during tourmalinization, and light REE were lost during chloritization and argillic alteration. The fluids themselves had low concentrations of REE; in only one case (chloritization) were heavy REE introduced during alteration. Analysis of separated minerals indicated that the behaviour of the REE could be partly explained in terms of their different affinities for the primary and secondary assemblages. Thermodynamic calculations indicated that REE mobility is enhanced by the presence of fluorine in the alteration fluids partly because REE form more stable complexes with F than with Cl and partly because elements such as Ti, Zr and P that form REE-bearing minor phases are themselves potentially mobile.     

Role of Lithology and Subsurface structures detected by potential field data in controlling the radioactive mineral accumulation at Natash area,Eastern Desert,Egypt

Wadi Natash area is located in the southern part of the Eastern desert of Egypt. It has a great importance for containing accumulations from the radioactive minerals of Uranium, Thorium and Potassium. An integrated potential study was carried out on the study area with the aim of locating depths to causative bodies with sufficient magnetic susceptibility that may represent magmatic intrusions with relation to the radioactivity location and delineate the subsurface structures affecting the area. Both magnetic and Bouguer data as well as radiometric data were interpreted rapidly for source positions and depths using Euler deconvolution, Werner deconvolution and 3D modeling techniques. The results deduced from the trend analyses show that the major fault trend affecting the area have NNW-SSE (Red Sea–Gulf of Suez trend) direction intersected by the less predominant NNE-SSW(The Gulf of Aqaba–Dead Sea trend) and WNW-ESE (Najd Fault System) fault trends. The causative bodies were imaged at depths ranging from 0.3 km to about 1.5 km. The depths along the interpreted profiles display discontinuities in potential field markers due to presence of the NNW-SSE fault trends act as pass channels for the hydrothermal solutions.It can be stated that the radioactive mineral accumulations were caused by the hydrothermal solutions rich with radioactive minerals as a result of intruding Natash volcanic to the granitic rocks. The Qouseir clastics and the Nudian sand stone were affected by these solutions and show a positive response for the radioactive minerals.