Geochemistry of granites and metasediments of the urban area of Vila Real (northern Portugal) and correlative radon risk

Radon concentration was evaluated in dwellings of the urban area of Vila Real (Northern Portugal). The area is mainly composed of Hercynian granites and Cambrian metasediments, and CR-39 passive detectors (n = 112) were used for the purpose. The results obtained in winter conditions suggest that the most productive geological unit is the Hercynian granite G1 (geometric mean of 364 Bq/m³), while Cambrian metasediments of the Douro Group show the lowest average indoor radon concentration (236 Bq/m³). The geological, geochemical and radiological data obtained suggest that the most effective control on the radon concentrations of the area is related with the uranium content of the rocks; indeed, the highest contents were observed in granite G1 (21 ppm) and the lowest in the metasediments (3 ppm). This is also confirmed by the results obtained for groundwater, where granites present the highest concentrations of dissolved radon (up to 938 Bq/l), uranium (5–18 ppb) and gross α activities (0.47–0.92 Bq/l). No important radiometric anomalies were found in relation with geological structures such as faults, veins and contacts, but a moderate increase of the uranium content can occur locally in such structures. Petrographic observations and SEM studies show that uranium is mainly contained within the rock in heavy accessory minerals (apatite, zircon, monazite, xenotime), which reduces radon emanation. Notwithstanding, due to the high U contents granites show a significant potential to induce indoor radon concentrations in dwellings in excess of the recommended value of 400 Bq/m³. Overall, we can conclude that the region of Vila Real presents a moderate to high radon risk in dwellings and groundwater.     

Radon distributions in the Hail Region of Northern Saudi Arabia

Indoor radon measurements were carried out in a total of 420 dwellings and 17 schools in Hail region of Saudi Arabia, using NTDs based radon dosimeters. The duration of the measurements was one year, from April 2008 to April 2009. The indoor radon concentrations varied from 4 to 513 Bq/m³ with an overall average of 45 Bq/m³ for all surveyed dwellings. These passive measurements were confirmed by the active measurements. The anomalous concentrations above 200 Bq/m³ were observed in 13 dwellings, representing 3.1 % of the total surveyed dwellings. In Inbowan village alone, it was found that 7.6 % of the dwellings have indoor radon concentration above 200 Bq/m³. The highest average indoor radon concentration of 64 Bq/m³ was found in Inbowan village while the lowest average of 24 Bq/m³ was found in Majasah village. The city of Hail showed an average indoor radon concentration of 49 Bq/m³. The average indoor radon concentration in one area located at the edge of the Aja Mountain in Hail city was 111 Bq/m³. The elevated indoor radon concentrations in many dwellings in the Hail region, prompted us to measure outdoor ground radon in such locations using gas monitor. It was found that radon concentrations at a depth of 0.5 m varied significantly from place to place ranging from 1.2 to 177 kBq/m³. The outdoor radon concentrations are generally correlated with the indoor radon measurements. Radon exhalations from construction materials and soil samples from the Hail region were also measured. It was found that radon exhalations from soil samples are higher than that of construction materials by a factor of at least 3 and reaching up to 11. These results indicate that soil is the main source of indoor radon. Geological interpretations of the results are also given.     

粤北产铀与不产铀花岗岩中铀矿物特征的电子探针研究及其找矿意义

晶质铀矿的含量、形貌、成分、铀矿物类型、与铀矿物共存的矿物组合等特征可以作为产铀与不产铀岩体的判别标志,为花岗岩型铀矿找矿工作提供了一种新的技术手段。长江岩体和九峰岩体是粤北地区典型的产铀与不产铀花岗岩体,本文利用电子探针测试了九峰岩体的铀矿物并与长江岩体进行对比研究。结果表明九峰岩体的铀矿物主要为晶质铀矿,其化学年龄可分为两组,分别为~160 Ma、~105 Ma,与长江岩体的两组晶质铀矿年龄基本一致;其中第一组年龄代表岩体的成岩年龄,第二组年龄与粤北地区~105 Ma的基性岩脉侵入时代相对应;但九峰岩体缺少长江岩体中~74 Ma的成矿年龄。相比于长江岩体,九峰岩体的铀矿物受到后期热液事件的影响较小,U没有发生明显的活化、转移,因而未能富集成矿,没有形成具有工业价值的铀矿床。     

U–Pb zircon geochronology of Silurian–Devonian granites in southeastern Australia: implications for the timing of the Benambran Orogeny and the I–S dichotomy

Despite extensive geochemical study and their importance to granite studies, the geochronology of Silurian to early-Devonian granitic rocks of southeastern Australia is poorly understood. In order to provide an improved temporal framework, new ion microprobe U–Pb zircon ages are presented from these rocks, and previous work is critically reviewed. Geochronological control is best in the Berridale Batholith, where S- and I-type granites have a close spatial relationship. In this region, there is a small volume of I-type granite that crystallised at 436 Ma, followed closely by a large volume of S-type granite at 432 Ma. I-type granite is abundant in a second peak at ca 417 Ma, although the Jindabyne pluton from the Kosciuszko Batholith is slightly older, at 424 Ma. A broader survey of S-type granite throughout the eastern Lachlan Orogen shows that the 432 Ma event is ubiquitous. There is no temporal overlap between S- and I-type granites in the Kosciuszko and Berridale Batholiths, which suggests that factors other than variations in degree of crustal contamination (which may include variation in tectonic setting, heat-flow, mass transfer across the crust–mantle boundary and/or availability in source materials) contribute to the diversity in granite types. The S-type granitic rocks occupy an aerial extent of greater than 28 000 km², and geochronological constraints suggest that the crystallisation of these granites took place over a relatively small interval, probably less than 10 m.y. This implies a magmatic flux of over 64 km³/Ma per km strike length, comparable to other high-flux granitic belts. Previous work has linked the Benambran Orogeny to the generation of the S-type granites, and so the age of these granites constrains the age of Benambran Orogenesis     

Late Triassic U-bearing and barren granites in the Miao'ershan batholith,South China: Petrogenetic discrimination and exploration significance

The Miao'ershan uranium ore district is one of the most important granite-hosted uranium producers in South China. There are several Triassic granite plutons in the Miao'ershan batholith, but uranium ore deposits mainly occur within the Douzhashan granitic body. Precise zircon U–Pb dating indicated that these Triassic granite plutons were emplaced during 204 to 215 Ma. The Douzhashan U-bearing granite lies in the central part of the Miao'ershan batholith, and has higher U contents (8.0 to 26.1 ppm, average 17.0 ppm) than the nearby Xiangcaoping granite (5.0 to 9.3 ppm, average 7.0 ppm) and the Yangqiaoling granite (6.4 to 18.3 ppm, average 11.5 ppm) in the south part of the batholith. The Douzhashan granite is composed of medium-grained two-mica granite, whereas the Xiangcaoping and Yangqiaoling granites are composed of porphyritic biotite granite. Both the Xiangcaoping and Douzhashan granites have high A/CNK ratios (> 1.10), high (⁸⁷Sr/⁸⁶Sr)_i ratios (> 0.720) and low ε_Nd(t) values (− 11.3 to − 10.4), suggesting that they belong to strongly peraluminous S-type granites. The Douzhashan granite has low CaO/Na₂O ratios, high Rb/Sr and Rb/Ba ratios, indicating a partial melting origin of clay-rich pelitic rocks. In contrast, the Xiangcaoping granite formed from clay-poor psammite-derived melt. The Yangqiaoling granite shows different geochemical characteristics with the Douzhashan and Xiangcaoping granites, indicating a different magma source. The Yangqiaoling granite has higher ε_Nd(t) of − 9.4 to − 8.3 and variable A/CNK values from 0.98 to 1.19, suggesting a mixture source of meta-sedimentary rocks and meta-igneous rocks. Crystallization fractionation is not the main mechanism for U enrichment in the Douzhashan granite. We suggest that U-rich pelitic rock sources may be the key factor to generate peraluminous U-bearing granites in South China. Searching for those granites which are reduced, strongly peraluminous and were derived from U-rich pelitic rocks, is the most effective way for exploring granite-hosted U deposits.     

Isotope geochronology,geochemistry, and mineral chemistry of the U-bearing and barren granites from the Zhuguangshan complex,South China: Implications for petrogenesis and uranium mineralization

The Zhuguangshan complex carries some of the most important granite-hosted uranium deposits in South China. Here we investigate the Changjiang and Jiufeng granites which represent typical U-bearing and barren granites in the complex, using zircon U-Pb ages, whole-rock geochemistry, Sr-Nd isotopic and zircon Hf isotopic data, and mineral chemistry, to constrain the petrogenesis and uranium mineralization. LA-ICP-MS zircon U-Pb dating shows that both the Changjiang and Jiufeng granites were emplaced ca. 160 Ma. These rocks show high silica, weakly to strongly peraluminous compositions, enrichment in Rb, Th, and U, and depletion in Ba, Nb, Sr, P, and Ti. These features coupled with the high initial ⁸⁷Sr/⁸⁶Sr ratios, negative εNd(t) values and εHf(t) values, and the Paleoproterozoic two stage model ages of these two granites suggest that the two granites belong to S-type granites, and the parental magmas of the two granites were derived from the Paleoproterozoic metasedimentary rocks. However, the granitoids show different mineralogical characteristics. The biotite in the Changjiang granite belongs to siderophyllite, marking higher degree of chloritization, whereas the biotite in the Jiufeng granite is ferribiotite, characterized by only slight chloritization. Compared with the Jiufeng granite, the biotite in the Changjiang granite has lower crystallization temperature and oxygen fugacity, but higher F content, and the uraninite has higher UO₂ content but lower ThO₂ content, and stronger corrosion. The chemical ages of uraninites from both granites are (within error) consistent with the zircon U-Pb ages and are considered to represent the emplacement ages of granites. Chemical ages of pitchblende in the Changjiang granite yield 118 ± 8 Ma, 87 ± 4 Ma, and 68 ± 6 Ma, representing multiple episodes of hydrothermal events that are responsible for the precipitation of U ores in the Changjiang uranium ore field. Our study suggests that the degree of magma differentiation and physicochemical conditions of the magmatic-hydrothermal system are the key factors that control the different U contents of these two granites. The mineralogical characteristics of uraninite and biotite can be used to distinguish between U-bearing and barren granites, and serve as a potential tool for prospecting granite-hosted uranium deposits.     

Age of dispersed uranium mineralization in rocks of the framework of the Strel’tsovka uranium ore field and the Yamsky site,Eastern Transbaikal region

An isotopic geochronological study of dispersed uranium mineralization was performed in the granitic rocks of the Urtui pluton in the framework of the Strel’tsovka uranium ore field and in the Yamsky site of the Urov-Uryumkan granite-gneiss arch. Two stages of such mineralization—783 ± 26 Ma in the Urtui granitic pluton and 138.6 ± 2.3 Ma in the Yamsky site—have been established. The emplacement of granite pertaining to the Unda Complex disturbed the U-Pb isotopic system in uraninite from the Urtui granitic pluton and resulted in redeposition of uranium phase dated at 262 ± 34 Ma. The young, probably, recent process gave rise to the redistribution of radiogenic lead in the U-bearing phases developing after uraninite.     

粤北青嶂山岩体江头矿区铀矿微区矿物学、年代学特征及其成矿动力背景制约

粤北诸广和贵东是华南最重要的两个花岗型铀矿密集区,青嶂山(龙源坝)岩体位于两者之间,是华南花岗岩型铀矿研究薄弱地区。江头铀矿区地处青嶂山岩体北部与南雄断陷盆地的结合部位,该矿区的铀成矿年代学研究几为空白。本文通过电子探针方法研究了青嶂山岩体、及与该岩体密切相关的江头矿区中的铀矿物微区矿物学特征,获得岩浆成因的晶质铀矿与热液成因的沥青铀矿的U-Th-Pb化学年龄,探讨了华南铀成矿作用动力学背景及成矿地质体。研究表明:青嶂山岩体粗粒斑状黑云母花岗岩和中粒斑状黑云母花岗岩中的铀矿物主要有晶质铀矿、铀石,部分晶质铀矿存在明显铀释放的特征,其晶质铀矿化学年龄分别为246.8±8.8Ma、161.5±8.0Ma,与前人获得的锆石U-Pb年龄结果在误差范围内一致,分别代表了区内印支期与燕山期花岗岩体的成岩年龄,表明在南雄断陷盆地形成之前,青嶂山岩体与诸广岩体可能为一有机整体,有着相同的成岩、成矿环境。江头矿区矿石中铀矿物主要为沥青铀矿,伴有少量钛铀矿、铀石等,沥青铀矿化学年龄分别为121.3±9.8Ma、98.8±8.0Ma、73.2±8.8Ma,分别代表区内3期铀成矿作用的时代,结合华南中生代以来...     

Geochemical characteristics of the niobium-rich arfvedsonite granites, Younger Granites province of Nigeria

Arfvedsonite granites are most prevalent in the northern sector of the Nigerian anorogenic ring-complex province wherein they form the main granitic rocks at Kudaru and Fagam and are important components of Kila-Warji, Ririwai and Dutsen-Wai ring-complexes. The albitized variety of these rocks hosts pyrochlore to varying extents depending on the degree of albitization and are, therefore, important targets for niobium investigation. Geochemical data of the granites reveal that niobium has a mean concentration of 111 ppm in the arfvedsonite granite, increasing to 168 ppm in the aegirine arfvedsonite granite and reaching 1568 ppm in the albite arfvedsonite granite. Niobium is thus enriched in the albite arfvedsonite granite by a factor of 8-11 relative to its mean value in the aegirine arfvedsonite and arfvedsonite granites, respectively. Uranium contents show a sympathetic trend with niobium, being also enriched in the albite arfvedsonite granite relative to its abundance in both the aegirine arfvedsonite granite and arfvedsonite granite by a factor of 15. The uranium abundance in the albite arfvedsonite granite is more than 48 times higher than the mean background values in low-calcium granite.The REE fractionation patterns in all three arfvedsonite granite varieties are characterized by enrichment of both the light (La-Sm) and heavy (Gd-Lu) rare earth elements and a significant negative Eu anomaly. The albite arfvedsonite granite is, however, preferentially more enriched in the heavy REE relative to the aegirine arfvedsonite and the arfvedsonite granites. A plot of the ∑REE against Na₂O and niobium reveals positive correlation in the arfvedsonite granites. There is also a linear relationship and strongly positive correlation between Nb and Na₂O because the pyrochlore is most abundant in the most extensively albitized variety of the arfvedsonite granites.     

Protomylonite evolution potentially revealed by the 3D depiction and fractal analysis of chemical data from a feldspar

Tourmalinization associated with peraluminous granitic intrusions in metapelitic host-rocks has been widely recorded in the Iberian Peninsula, given the importance of tourmaline as a tracer of granite magma evolution and potential indicator of Sn-W mineralizations. In the Penamacor-Monsanto granite pluton (Central Eastern Portugal, Central Iberian Zone), tourmaline occurs: (1) as accessory phase in two-mica granitic rocks, muscovite-granites and aplites, (2) in quartz (±mica)-tourmaline rocks (tourmalinites) in several exocontact locations, and (3) as a rare detrital phase in contact zone hornfels and metapelitic host-rocks. Electron microprobe and stable isotope (δ¹⁸O, δD, δ¹¹B) data provide clear distinctions between tourmaline populations from these different settings: (a) schorl–oxyschorl tourmalines from granitic rocks have variable foititic component (^X□ = 17–57 %) and Mg/(Mg + Fe) ratios (0.19–0.50 in two-mica granitic rocks, and 0.05–0.19 in the more differentiated muscovite-granite and aplites); granitic tourmalines have constant δ¹⁸O values (12.1 ± 0.1 ‰), with wider-ranging δD (?78.2 ± 4.7 ‰) and δ¹¹B (?10.7 to ?9.0 ‰) values; (b) vein/breccia oxyschorl [Mg/(Mg + Fe) = 0.31–0.44] results from late, B- and Fe-enriched magma-derived fluids and is characterized by δ¹⁸O = 12.4 ‰, δD = ?29.5 ‰, and δ¹¹B = ?9.3 ‰, while replacement tourmalines have more dravitic compositions [Mg/(Mg + Fe) = 0.26–0.64], close to that of detrital tourmaline in the surrounding metapelitic rocks, and yield relatively constant δ¹⁸O values (13.1–13.3 ‰), though wider-ranging δD (?58.5 to ?36.5 ‰) and δ¹¹B (?10.2 to ?8.8 ‰) values; and (c) detrital tourmaline in contact rocks and regional host metasediments is mainly dravite [Mg/(Mg + Fe) = 0.35–0.78] and oxydravite [Mg/(Mg + Fe) = 0.51–0.58], respectively. Boron contents of the granitic rocks are low (<650 ppm) compared to the minimum B contents normally required for tourmaline saturation in granitic melts, implying loss of B and other volatiles to the surrounding host-rocks during the late-magmatic stages. This process was responsible for tourmalinization at the exocontact of the Penamacor-Monsanto pluton, either as direct tourmaline precipitation in cavities and fractures crossing the pluton margin (vein/breccia tourmalinites), or as replacement of mafic minerals (chlorite or biotite) in the host-rocks (replacement tourmalinites) along the exocontact of the granite. Thermometry based on ¹⁸O equilibrium fractionation between tourmaline and fluid indicates that a late, B-enriched magmatic aqueous fluid (av. δ¹⁸O ~12.1 ‰, at ~600 °C) precipitated the vein/breccia tourmaline (δ¹⁸O ~12.4 ‰) at ~500–550 °C, and later interacted with the cooler surrounding host-rocks to produce tourmaline at lower temperatures (400–450 °C), and an average δ¹⁸O ~13.2 ‰, closer to the values for the host-rock. Although B-metasomatism associated with some granitic plutons in the Iberian Peninsula seems to be relatively confined in space, extending integrated studies such as this to a larger number of granitic plutons may afford us a better understanding of Variscan magmatism and related mineralizations.     

Geological and tectonic influence on water–soil–radon relationship in Mandi–Manali area,Himachal Himalaya

Radon measurements in soil and groundwater (springs, thermal springs and handpumps) were made in a variety of lithological units including major thrusts between Mandi and Manali in Himachal Himalaya. Analysis of radon data in light of lithological controls and influence of deep-seated thrusts has been made to elucidate the causative factors for anomalous emanation of radon. The lithological types include banded gneisses, schists, quartzite, granite, phyllites, volcanics and mylonites. The low-grade metasedimentries of Shali and Dharamsala generally show low and narrow range of radon concentration in water (5.6–13.4 Bq/l) as well as in soil (1.8–3.2 kBq/m³) except for the samples related to thrusts. On the other hand, sheared and deformed rocks of Chail and Jutogh show moderate radon content (average 5.03 kBq/m³, range 2.9–11.1 kBq/m³) in soil. However, the groundwater radon concentration shows wide variation in different types of sources (2.1–80.8 Bq/l). The quartzite and volcanic rocks of Rampur formation in this area present as a window separated by Chail thrust. Radon emanations on these rock types are relatively high (6.3–68.1 Bq/l in water and 5.5–15.9 kBq/m³ in soil) and are exceptionally high in samples that are related to uranium mineralization, deep-seated thrusts and hot springs (13.5–653.5 Bq/l). It is generally observed that anomalous high radon content is associated with mineralization, deeper source and tectonic discontinuities. Whereas it is obvious that subsurface radioactive mineralization would facilitate enhanced radon production, however, thrust plains provide easy pathways for escape of gases from the deeper sources. Shallow and deep sources of the groundwater have contrasting radon content particularly in the deformed and metamorphosed rocks of Jutogh and Chail. Shallow groundwater sources, mainly handpumps, have lower radon concentration due to limited superficial water circulation, whereas deeper sources, mainly perennial springs, show higher radon content because of larger opportunity for water–rock interaction.     

Occurrence of uraniferous iron and manganese oxides in biotite granite North East Gabal El Sela area, South Eastern Desert, Egypt

Optical microscopy, X-ray diffraction (XRD), and back-scattered electron imaging (BSE) have been used to determine the mineralogical composition of the uraniferous iron and manganese oxides and the associated U-minerals hosted in biotite granite that occurred north east Gabal El Sela area south Eastern Desert, Egypt. These mineralizations were found as veinlets fractures filling associated with strongly kaolinitic alteration of the coarse-grained biotite granite. XRD determined that the geothite mineral form the main constituent of uraniferous iron oxide in addition to tapiolite, and kaolinite minerals, where as uraniferous manganese oxide composed of pyrolusite, ramsdellite, and cryptomelane. BSE confirmed that the associated uranium minerals represented by uranothorite, kazolite, and zentime in addition to columbite-bearing minerals. Uranothorite and columbite-bearing minerals are the most abundant minerals in this mineralization. Petrographically, biotite granite is composed mainly of quartz, in addition to K-feldspars, biotite and muscovite with minor zircon, garnet, apatite, uranium-rich thorite and iron oxide. Petrochemical studies and tectonic discrimination diagrams for this granite reveal that they are classified as granite to alkali feldspar granite, originated from calc-alkaline magma having peraluminous nature and developed in within-plate tectonic environment. Field radiometric measurements revealed the localization of two high radiometric anomalies associated with iron and manganese oxides, within this anomaly uranium content range from 65 to 85 ppm. Alpha Track-etch Detectors of radon gas registrations revealed high track density reach up to 15,448.7 Bq/m³.     

Uranium-bearing and barren granites from the Taoshan Complex,Jiangxi Province,South China: Geochemical and petrogenetic discrimination and exploration significance

The Taoshan uranium ore district is one of the most important granite-hosted uranium producers in South China. The Taoshan granitic complex can be petrographically classified into several units of Caijiang, Huangpi, Daguzhai, and Luobuli, but the uranium deposits only occur within the Daguzhai granite unit. LA-ICP-MS zircon U–Pb dating indicates that both the Daguzhai granite and the Huangpi granite were emplaced at 154 ± 2 Ma. U contents (average 19.5 ppm) of the Daguzhai granite are higher than those of the Huangpi granite (average 7.3 ppm). The Daguzhai granite is composed of medium-grained two-mica granite, and the Huangpi granite is composed of medium- to coarse-grained biotite granite. These two granites show obvious differences in major element, trace element and isotopic geochemical characteristics. Compared to the Huangpi granite, the Daguzhai granite has higher A/CNK ratios, higher P₂O₅ contents and lower CaO contents, and is more enriched in Rb, Ba, U, and more depleted in Sr, Eu and Ti. The ε_Nd(t) values of the Daguzhai granite vary from − 12.2 to − 11.0 with two-stage model ages of 1.84 to 1.93 Ga. The ε_Nd(t) values of the Huangpi granite are slightly higher (− 9.7 to − 8.6) and the Nd model ages are younger (1.64 to 1.73 Ga). Comparative studies imply that the Daguzhai granite belongs to typical S-type and might be derived from the partial melting of parametamorphic rocks from metamorphic basement of the Zhoutan Group. In contrast, the Huangpi granite belongs to fractioned I-type, which might be derived from the partial melting of a mixture of ortho- and para-metamorphic rocks of the Zhoutan Group. These different magma sources might explain the different U contents of the two granites. In general, the source factor is an important controlling factor for the genesis of U-bearing granites in South China. U-bearing granites in South China show some common mineralogical and geochemical characteristics, which can be used to guide further exploration of granite-hosted U deposits.     

A uranium and thorium enriched province of the Fennoscandian shield in southern Norway

The Precambrian Flå, Iddefjord, and Bohus granites lie along a line striking roughly northwest which crosses the Permian Oslo Province to the southwest of Oslo. Radioelement investigations in the three bodies show they all contain abnormally high thorium and uranium concentrations relative to the published literature on average radioelement contents of granitic rocks. Trend surface analysis of the radioelement distribution in the Iddefjord granite suggests there was relative movement of uranium to the east with respect to thorium, possibly as the result of Permian activity in the adjacent rocks. Geological considerations, radiometric evidence and published gravimetric data suggest that the 3 granites represent a continuous belt enriched in thorium and uranium during the Sveconorwegian orogeny. A portion of the belt was later involved in the Permian igneous activity which produced the igneous Oslo Province. There is some evidence that the Permian Drammen and Finnemarka granites represent that part of the belt which was modified in Permian time.     

花岗岩晶质铀矿的产出概况及其富集因素的初步研究

本文采用人工重砂法调查了同内花岗岩体中晶质铀矿的含量。初步证明,花岗岩内主要的脉型铀矿床、矿田都位于晶质铀矿含量高的(大于5g/t)花岗告体内。晶质铀矿在花岗岩岩浆中的富集,需要四方面因素的配合:重熔母岩相对富铀;重熔规模大,充分分异演化;铀含量高,钍含量低,以及REE、Nb、Ta、zr、P等含量相对较低;成岩时氧逸度较低。确定花岗岩中是否含晶质铀矿(并结合岩体出露面积大小)是判断该岩体是否具产铀潜力的最关键的依据。但由于品质铀矿有时在岩体中分布不很均匀,因而最好能有3个以上样品的测定值,以便对该岩体能否作为铀源体作出确凿的判断。     

赣中紫云山花岗岩晶质铀矿的电子探针U-Th-Pb化学定年

紫云山岩体是赣中地区与钨铀成矿关系极为密切的过铝质花岗岩体,但目前该岩体的成岩时代尚不明确.通过偏光显微镜、扫描电镜、电子探针等手段,首次开展了紫云山花岗岩中赋存晶质铀矿的精细矿物学研究.结果表明:晶质铀矿主要赋存于黑云母之中,少数被黄铁矿包裹,部分晶质铀矿被不同程度溶蚀和交代,表明晶质铀矿是本区花岗岩型铀矿的主要铀源矿物之一.利用电子探针U-Th-Pb化学定年法测得蕉坑单元 (J₃J)5颗晶质铀矿年龄为154.5~168.9 Ma,加权平均年龄为161.8±2.4 Ma (MSWD=0.26,n=26),庙前单元 (J₃M) 三颗晶质铀矿年龄为152.8~164.7 Ma,加权平均年龄为159.7±3.2 Ma (MSWD=0.2,n=15).获得的年龄与南岭地区主要含钨花岗岩的侵入时间高度一致,对应华南中生代大规模岩浆活动的第二阶段.晶质铀矿年龄与华南含钨花岗岩锆石U-Pb年龄非常一致,验证了过铝质富铀花岗岩中晶质铀矿电子探针定年方法的可行性.      

Ammonium in granites and its petrogenetic significance

Ammonium is present as a trace constituent in all granites, with an average concentration of 45 ppm (NH₄⁺), equivalent to 35 ppm of elemental N. It shows wide variations related to petrography and region, but the only significant correlation between ammonium and other geochemical parameters is that it is most abundant in peraluminous granites and least abundant in peralkaline granites. These variations can be related to (a) the amount of sedimentary material in the magmatic source region, and (b) redox conditions in the source region. The ammonium content of granitic magmas can also be modified by fractionation or contamination. Hydrothermal alteration has a major effect on the ammonium content of granitic rocks, and variation due to this cause may exceed the magmatic variation. Most hydrothermally altered granites are enriched in ammonium as a result of the transfer of ammonium from sedimentary country rocks by the hydrothermal fluids.     

Radon in Himalayan springs: a geohydrological control

 This paper presents the results of radon measurements in springs of the Himalayan region by using radon emanometry technique. The radon was measured in different springs, draining from different geohydrological setups, and from stream water in order to find the geohydrological control over radon concentration in groundwater emanating in the form of spring. The radon values were found to vary from 0.4 Bq/l to 887 Bq/l, being observed lowest for a turbulent stream and highest for the spring. The radon values were recorded highest in the springs draining through gneiss, granite, mylonite, etc. Radon concentrations have been related with four spring types viz. fracture-joint related spring, fault-lineament related spring, fluvial related spring and colluvial related spring, showing geohydrological characteristics of the rocks through which they are emanating. The high radon concentration in fracture-joint and fault-lineament spring is related to increased ratio of rock surface area to water volume and uranium mineralisation in the shear zones present in the close vicinity of fault and thrust. The low concentration of radon in fluvial and colluvial springs is possibly because of high transmissivity and turbulent flow within such deposits leading to natural de-emanation of gases. Received: 6 January 1998 · Accepted 11 May 1999     

Assessment of soil radon potential in Hong Kong, China, using a 10-point evaluation system

Radon and its progenies have been ranked second of being responsible for lung cancer in humans. Hong Kong has four major groups of uranium-rich plutonic and volcanic rocks and is suffering from radon emanated therefrom. However, there is a lack of radon potential maps in Hong Kong to resolve the spatial distribution of radon-prone areas. A ten-point radon potential system was developed in Germany (2005) to predict radon potential using both the in situ geogenic and geographic parameters under hierarchical ranking. Primarily, the ten-point system requires the desk study of the geological environment of sampling sites, which has an advantage of saving resources and manpower in extensive radon potential mapping over the traditional soil radon concentration sampling method. This paper presents a trial of radon potential mapping in Hong Kong to further verify the system. Despite some slight departures, the system demonstrates an acceptable correlation with soil radon concentrations (R ² = 0.62–0.66) from 768 samples of mainly intermediate radon potential. Hong Kong has a mean soil radon concentrations of 58.9 kBqm^?3, while the radon potential from the ten-point system achieves an average of 4.93 out of 10 over the territory. The vicinity of fault zone showed high soil radon concentrations and potentials, which were conducive to uranium enrichment and rapid soil-gas diffusion near faults. High uranium-238 content in soil was found to cause high soil radon concentration with a large R ², 0.84. The Jurassic granite and volcanic crystal tuff cover more than 85 % of the whole Hong Kong area, and they show relatively high radon concentrations (Geometric mean 83 and 49 kBqm^?3, respectively) which are associated with their high uranium contents (Geometric mean 234 and 197 Bqkg^?1, respectively). While indoor radon concentration is an important factor for radon risk assessment, this study has not considered the correlation between indoor radon concentration and radon potential. The reason is that almost all buildings in Hong Kong are high-rise buildings where indoor radon concentrations are governed only by the radium content in the building materials and the ventilation conditions.