广西北部摩天岭地区铀成矿特征浅析

摩天岭地区铀成矿与岩体风化剥蚀、大气降水、储矿构造闭合性及特定的还原环境密切相关;岩体表层风化剥蚀物和变质岩残留体为铀成矿提供铀源,表层浸取作用使铀从铀源体中浸出,并随大气降水沿构造进入地下,硅质热液活动将成矿流体加热并上升到近地表成矿。从铀源、水源、热源、储矿构造及还原环境等方面对其成矿特征进行了简单叙述,并建立了铀成矿模式。     

某产铀岩体锆石的标型特征及其地质意义

某岩体为一产铀的加里东期复式岩体。本文研究了该岩体中副矿物锆石的形态、包体和环带及其延长度、粒度的变化,将其与围岩中副矿物锆石进行对比并结合岩体中锆石的稀土元素地球化学特点,表明该岩体是区域上的沉积变质岩重熔交代的产物。笔者还从岩体中锆石的含铀性,ZrO_z/HfO_2值与岩石中Na_2O的关系探讨了成矿中铀和钠的来源,并从副矿物锆石的标型方面提出了产铀岩体的评价标志。     

Relations entre la structure des massifs granitiques et la distribution de l'uranium dans le Morvan

Résumé L'objet principal de cette note est l'étude des manifestations précoces, mais déjà décisives de la concentration de l'uranium au sein des roches ignées. Le comportement de cet élément s'y montre distinct de ceux du thorium et de l'étain. Le style de variation de l'uranium proche de celui du thorium dans les roches volcaniques et les granites à évolution calme, se trouve modifié dès les premières manifestations tectoniques cassantes des massifs granitiques, avec une mobilité brutalement accure. L'uranium se fixe alors dans les zones précocement fracturées et consolidées de la partie haute des massifs de granite à biotite. Il y forme des enrichissements faits pour l'essentiel d'une cristallisation très fine de minéraux de type uraninite, largement disséminée dans la roche, constituant une véritable préconcentration (âge présumé 280–300 M.A.). A une date plus tardive, soit à partir de la phase tectonique saalienne (250 M. A.), a eu lieu la concentration principale sous forme de pechblende filonienne. Il est alors formulé une opinion génétique selon laquelle l'uranium des filons proviendrait de celui des uraninites, préalablement lessivé, puis redéposé, sous l'influence de processus supergènes et hydrothermaux profonds.

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The main subject of this note is the study of the early but already decisive occurrences of uranium concentration within igneous rocks. The behaviour of this element seems to be different from that of thorium and tin. The mode of uranium variation, similar to that of thorium in volcanic rocks and in quietly evolving granites, is modified by the first fracturing of the granitic systems, with an abruptly increased mobility.Then uranium becomes fixed in early broken and reconsolidated zones in the higher parts of biotite granite systems. It is mainly enriched as a very fine crystallisation of uraninite type minerals, widely scattered in the rock, making up a real preconcentration (presumed age 280–300 M.Y.). At a later date, i. e., since the saalian phase (250 M.Y.), the main concentration of pitchblende veins took place. Thus one is lead to the genetic conclusion that the uranium of the veins could proceed from uraninite previously leached, and then redeposited under the control of both supergene processes and hydrothermal processes of deep seated origin.

     

Occurrence of uranium in metasedimentary enclaves within basement granite, near Peddur and Kottur, Karimnagar District, Andhra Pradesh

Several radioactive anomalies due to uranium and thorium, associated with the mesedimentary enclaves (Archaean) within granite (Archaean to Early-Proterozoic) have been recorded in parts of Karimnagar Granulite Terrain, Karimnagar Dist. At Peddur and Kottur, Uraninite has been identified in the samples of metasediments. The metasediment from these two places have been subjected to granulite facies of metamorphism and host high values of uranium with negligible thorium. In Peddur, samples of metasediments have assayed as high as 1.96% U₃O₈ with negligible thorium, and in Kottur up to 0.059% U₃O₈. Leaching studies on these samples have indicated that most of the U₃O₈ present is leachable. This discovery has opened up the possibility of finding uranium mineralisation in Archaean metasediments and thus provides a thrust for uranium exploration in similar geological environs in India. Further, the basement granite along with the metasedimentary enclaves has the potential to act as a provenance for a possible unconformity type or sandstone type U-deposit in the rocks of overlying Pakhal and Gondwana Supergroup, in Pranhita-Godavari Basin, situated to the east of this area.     

Atmospheric input vs in situ weathering in the genesis of calcretes: an Sr isotope study at Gálvez (Central Spain)

Strontium isotopes have been analyzed in a typical calcrete profile developed on granite in the Toledo mountains, Central Spain. The pedogenic carbonates show clear petrographic evidence of pseudomorphic replacement of the weathered parent granite.Calcretes display ⁸⁷Sr/⁸⁶Sr ratios between 0.70961 and 0.71059 in sharp contrast to the granite whole rock (0.72856) and minerals (0.71359 to 0.91351). This difference shows that the contribution of Sr from the granite to the calcretes is at most 33% and may be as low as 3%. Direct measurements in rains and aerosols show that the allochtonous source of Ca and Sr is clearly related to the atmospheric input, mainly as dry deposit.A slight decrease of Sr concentration is observed from the upper horizon composed of continuous calcrete to the deeper calcrete veins in the saprolite. This may be due to a kinetic control of the Sr/Ca fractionation, and different crystallization rates of the carbonates in the different units of the profile.Finally, local groundwaters have Sr isotopic compositions similar to the calcretes and the atmospheric input, very different from waters running on the granite.     

Quantifying time scales of pedogenic calcrete formation using U-series disequilibria

Time scales of pedogenic calcrete development are quantified by subsampling carbonate from within a mature (stage V) pedogenic calcrete profile from southeast Spain and dating the material by U-series disequilibria. The location of the earliest and latest cements can be estimated by comparing previous studies of calcrete morphological development with micromorphological analysis of the study profile. Carbonate was sampled and dated from three locations within the profile: (1) below the lower surface of clasts within the hardpan (representing the earliest cement present—207±11 ka), (2) from the centre of cement filled pores within the hardpan (reflecting the final plugging of the calcrete hardpan—155±9 ka) and (3) from the laminar calcrete overlying the hardpan (representing the latest cement—112±15 ka). These results show that the hardpan took between 73 and 31 ka to form, whilst the mature stage V profile took between 121 and 69 ka to form. This is the first time that rates of mature calcrete development have been established by direct radiometric dating of the authigenic carbonate. The technique is appropriate for dating mature calcretes in dryland regions worldwide and offers the opportunity of increasing our understanding of the spatial and temporal variability in rates of pedogenic calcrete development.     

桂北平英花岗岩锆石U-Pb年代学、Hf同位素、地球化学特征及其地质意义

通过对桂北平英花岗质岩体详细的锆石U-Pb年代学、Hf同位素组成及岩石地球化学特征的研究,论证了岩体的形成时代、成因类型、源区性质及其与宝坛锡矿的成矿关系。该岩体中心相-粗粒黑云母花岗岩的锆石LA-MC-ICP-MS U-Pb定年表明,其~(206)Pb/~(238)U加权平均年龄为834.2±5.1 Ma,属新元古代构造岩浆活动的产物。平英花岗岩具有高硅、富碱、强过铝质的特征,岩石富集Cs、Rb、U、Ta而亏损Ba、Sr、Ti等元素。球粒陨石标准化稀土配分曲线呈右倾形和强烈的Eu负异常(Eu/Eu*=0.05~0.31)。花岗岩中锆石的εHf(t)值介于-12.6~-1.6之间,峰值在-4.8~-3.0之间;二阶段模式年龄T_(DM)~C(Hf)在1.83~2.51 Ga之间,峰值在1.9~2.0 Ga之间。这些特征表明平英岩体形成于该区古元古代富硼基底的部分熔融作用,并经历了高度的分异演化过程。桂北九万大山—元宝山地区的新元古代黑云母花岗岩具有良好的锡成矿潜力,是华南多时代花岗岩演化及锡多金属成矿系列的重要组成部分。     

U‐Th‐Pb systematics of some granitoids from the northeastern Yilgarn Block,Western Australia and implications for uranium source rock potential

The Mount Boreas‐type granite and spatially associated syenitic granitoid of Western Australia yield Pb‐Pb ages of 2370 ± 100 Ma and 2760 ± 210 Ma, respectively. Th‐Pb ages, although less precise, are concordant with these ages, and therefore the apparent ages are interpreted to be the crystallisation ages for these two units. U‐Pb ages are variable and for the most part anomalously old, which suggests a Cainozoic uranium loss. However, this loss is generally small (< 3μg/g); therefore, neither granitoid in its fresh state provides a good source for nearby calcrete‐hosted uranium deposits. The possibility remains that the Mount Boreas‐type granite that has been completely weathered during the Tertiary could have been a source for the calcrete‐type uranium deposits in W.A. Although the Mount Boreas‐type granite is highly fractionated, it does not bear a strong geochemical imprint of a sedimentary precursor. This feature contrasts it with apparently fresh granitoids from other parts of the world that have lost large amounts of uranium (～ 20μg/g) and are associated with large roll‐type and other low temperature‐type uranium deposits.     

花岗岩演化与铀钍元素富集的关系:以粤北贵东岩体为例

对粤北贵东岩体铀钍丰度变化特征的研究表明,原地重熔过程导致铀钍元素向花岗岩体的上方汇聚富集,并造成铀、钍元素在空间上的分离,即在花岗岩体中,铀的丰度带位处钍丰度带之上。陆壳多次原地熔融(重熔)不但导致复式花岗岩体形成,同时造成铀元素在晚期岩体中的富集。贵东岩体内燕山早期岩体铀钍含量的东西差异被认为与卷入熔融的铀源层(寒武-震旦系)的初始埋深有关,而复式岩体铀含量的南北差异,则被解释为与晚期重熔界面倾斜方向所导致的晚期岩体的剥蚀深度有关。高铀含量和高铀钍比值并存往往是晚期岩体埋深较浅的表现,对于深部隐伏矿床的寻找有重要指示作用。     

云南个旧卡房锡矿田花岗岩黑云母矿物化学特征及其成岩成矿意义

黑云母作为花岗岩中含量最高的暗色矿物,其成分特征对指示岩石成因与成矿起着重要作用。云南个旧卡房锡多 金属矿床的形成与花岗质岩浆活动密切相关,卡房花岗岩包含有大量黑云母,通过电子探针测试方法,对该花岗岩中的 黑云母成分进行了系统的研究。结果显示,黑云母为富铁黑云母,具有富硅、铁、铝、钾、钛,贫锰、镁、钙、钠等特 征,含铁指数为0.67~0.83。黑云母的成分特征暗示其结晶温度为500~708 ℃,结晶压力为202~538 MPa,对应的结晶深度为 7.64~20.35 km,表明卡房花岗岩形成于中低温环境、属于中深成相。综合研究认为,卡房锡矿田花岗岩具有高铁指数以及 氧逸度由高到低变化趋势等特征是锡成矿的有利条件,可以作为在本区寻找锡矿的重要标志。     

Hydrothermal alteration processes in the giant Dahutang tungsten deposit,South China: Implications from litho-geochemistry and mass balance calculation

The giant Dahutang tungsten (W) deposit has a total reserve of more than 1.31 Mt WO₃. Veinlet-disseminated scheelite and vein type wolframite mineralization are developed in this deposit, which are related to Late Mesozoic biotite granite. Four major types of alterations, which include albitization, potassic-alteration, and greisenization, and overprinted silicification developed in contact zone. The mass balance calculate of the four alteration types were used to further understanding of the mineralization process. The fresh porphyritic biotite granite has high Nb, Ta, and W, but low Ca and Sr while the Jiuling granodiorite has high Ca and Sr, but low Nb, Ta, and W concentrations. The altered porphyritic biotite granite indicated that the Nb, Ta, and W were leached out from the fresh porphyritic biotite granite, especially by sodic alteration. The low Ca and Sr contents of the altered Neoproterozoic Jiuling granodiorite indicate that Ca and Sr had been leached out from the fresh granodiorite by the fluid from Mesozoic porphyritic biotite granites. The metal W of the Dahutang deposit was mainly derived from the fluid exsolution from the melt and alteration of W-bearing granites. This study of alteration presents a new hydrothermal circulation model to understand tungsten mineralization in the Dahutang deposit.     

赣东南大富足成铀岩体锆石U-Pb定年和构造背景与含矿性

大富足花岗岩体位于江西和福建的交界处,与铀成矿关系密切。通过激光剥蚀-多接收器电感耦合等离子体质谱(LA-MC-ICPMS)锆石U-Pb定年技术对大富足岩体进行了精确的年龄测定,获得岩体的中粗粒斑状黑云母(二长)花岗岩和中细粒含斑黑云母花岗岩年龄分别为(233.4±1.6)Ma(MSWD=0.30,n=13)和(233.1±1.6)Ma(MSWD=0.48,n=12),表明岩体形成于华南印支期花岗质岩浆活动第一阶段末期。大富足岩体属高钾钙碱性系列,具有相对低含量的SiO2和较高含量的Al2O3及ACNK值(1.12～1.36),并富含黑云母、白云母等富铝矿物,显示大富足岩体为典型的强过铝质S型花岗岩,暗示其形成于同碰撞挤压背景,是富含泥质成分变质沉积岩的部分熔融的结果。岩体具有较高的U、Th含量,独立晶质铀矿物和活动铀浸出率,暗示大富足岩体具有良好的铀成矿潜力。     

锡田含W,Sn花岗岩体的地球化学特征及其形成构造背景

以二长花岗岩类为主体的锡田岩体,分布在南岭花岗岩套北部边缘,形成时代为燕山早期。岩石具有高硅、富碱以及W,Sn,Mo,Bi,Cu和U,Th丰度高的特点。与湘南地区的骑田岭、香花岭等含Sn花岗岩体特征有相似之处,具有找W,Sn矿的较好潜力。岩石地球化学综合研究表明,其具有典型的后造山(post-orogenic。)作用形成的“A型花岗岩”类岩石的地球化学特征,从而显示锡田岩体形成时该区处于陆壳开始拉张裂陷的构造背景。     

Calcrete conglomerate, case-hardened conglomerate and cornstone - a comparative account of pedogenic and non-pedogenic carbonates from the continental Siwalik Group, Punjab, India

The occurrence of authigenic carbonates formed in three different environmental situations, within the continental Siwalik Group, has been used to compare the lithological and petrographic characters of the contrasted lithofacies. The three lithofacies are: (1) calcrete conglomerate, (2) case-hardened conglomerate, (3) cornstone (pedogenic, nodular calcrete). The calcrete conglomerate facies laterally intertongues with the channel conglomerates. It consists of pisolites which are interpreted to have formed from carbonate-rich spring waters emerging on to the gravelly substrate of dry, abandoned channels. The laminae characteristics of these pisolites are distinctly different from those of marine origin and also from comparable biogenic materials. Case-hardened conglomerate occurs in the youngest part of the Siwalik stratigraphic column, in boulder conglomerates having limestone as the principal clast component. This lithofacies has resulted from cementation of the conglomerate through continued dissolution and re-precipitation of calcite, by meteoric water, downwards from the surface. It displays a coarsely crystalline, sparry calcite cement with no evidence for displacive growth or replacement by calcite. Cornstones (nodular calcrete) occur in several sedimentary cycles of the Middle Siwalik Sub-Group. These are immature and commonly associated with thinly-bedded sandstones (levée) and red shales (overbank). This lithofacies is a result of concentration of carbonate through capillary action associated with pedogenic activity. Ooids developed in cornstone are essentially micritic in nature and usually composed of less than five indistinct laminae.     

The central Australian groundwater discharge zone: Evolution of associated calcrete and gypcrete deposits

The groundwater discharge zone of central Australia is marked by a chain of playas extending 500 km from Lake Hopkins in Western Australia through Lake Neale, Lake Amadeus and numerous smaller playas to the Finke River in the Northern Territory. This great valley is devoid of surface drainage, and the playas and their associated landforms and chemical sediments are a result of groundwater transmission, and discharge from a large regional flow system. The valley contains extensive groundwater calcrete deposits, which are commonly silicified. The playas contain gypsum and glauberite deposits resulting from the evaporative concentration of discharging groundwater to 250 g/1total dissolved solids. Thin gypcrete crusts have developed on the playa margins and islands as a result of induration of bordering gypsum dunes. Diagenetic changes in the calcrete and gypcrete are caused by dissolution and reprecipitation through groundwater movement.

Calcrete and gypcrete samples from playas near Curtin Springs, NT, dated by electron spin resonance (ESR) — calibrated by ¹⁴C dates on calcrete and the uranium content of gypcrete — are mostly in the range 8000–16 000 years BP, and are evidence of groundwater discharge conditions similar to those of the present day. Calcrete in the vadose zone, above the present water‐table, gives ESR dates in the range 22 000–27 000 years BP suggesting episodes of high intensity rainfall at that time. Phreatic calcrete, below the water‐table, with ages in the range 34 000–75 000 years BP, provides evidence of older episodes of calcretization.

The ESR and ¹⁴C dates for the chemical sediments suggest that this groundwater flow system has been an important feature in the region for much of the Late Quaternary. Although there are problems with the interpretation of ESR dating for chemical sediments with diagenetic alteration, the technique shows promise as a dating tool.     

Geochemical and C, O, Sr, and U-series isotopic evidence for the meteoric origin of calcrete at Solitario Wash, Crater Flat, Nevada, USA

Calcite-rich soils (calcrete) in alluvium and colluvium at Solitario Wash, Crater Flat, Nevada, USA, contain pedogenic calcite and opaline silica similar to soils present elsewhere in the semi-arid southwestern United States. Nevertheless, a ground-water discharge origin for the Solitario Wash soil deposits was proposed in a series of publications proposing elevation-dependent variations of carbon and oxygen isotopes in calcrete samples. Discharge of ground water in the past would raise the possibility of future flooding in the unsaturated zone at Yucca Mountain, Nevada, site of a proposed high-level nuclear waste repository. New geochemical and carbon, oxygen, strontium, and uranium-series isotopic data disprove the presence of systematic elevation-isotopic composition relations, which are the main justification given for a proposed ground-water discharge origin of the calcrete deposits at Solitario Wash. Values of δ¹³C (−4.1 to −7.8 per mil [‰]), δ¹⁸O (23.8–17.2‰), ⁸⁷Sr/⁸⁶Sr (0.71270–0.71146), and initial ²³⁴U/²³⁸U activity ratios of about 1.6 in the new calcrete samples are within ranges previously observed in pedogenic carbonate deposits at Yucca Mountain and are incompatible with a ground-water origin for the calcrete. Variations in carbon and oxygen isotopes in Solitario Wash calcrete likely are caused by pedogenic deposition from meteoric water under varying Quaternary climatic conditions over hundreds of thousands of years.     

千里山花岗岩体地质地球化学及与成矿关系

千里山花岗岩由似斑状黑云母花岗岩、等粒黑云母花岗岩和花岗斑岩组成。前两期岩体分别与两期钨多金属矿化有关，后一期与铅锌银成矿密切相联。两套花岗岩虽然均来自地壳，但取于不同源地。该岩体既为富Ｆ、Ｌｉ，Ｒｂ，Ｂｅ，Ｇａ的ＢＥＬＩＦ花岗岩，又是富Ｕ，Ｔｈ的高热花岗岩。     

MATERIAL COMPOSITION CHANGES IN THE GREISENIZATION OF KOUNRAD MASSIF GRANITES

Greisenization does not occur apart from ore bodies associated with the Kounrad granite massif. Primary minerals in granite altered and replaced during greisenization are quartz, feldspars, biotite, sphene, ore minerals, and apatite. Zircon, cyrtolite, and monazite are virtually unaffected by greisenization. Greisen-introduced minerals are muscovite, rutile group minerals, pyrite, fluorite, topaz, and molybdenite. Greisenization of Kounrad granite massif resulted in addition of more material than leached in all levels except at the base of vein zones. The excess of addition over leaching in middle and upper levels is mainly associated with upward migration of material. The value of total and individual factors of addition and leaching depends primarily on original rock composition more so than original porosity and rock structure. --R.M. Hutchinson.     

Mechanism of calcrete formation in the Lower Cretaceous (Neocomian) fluvial deposits,northeastern Iran based on petrographic,geochemical data

The Kopet-Dagh basin of northeastern Iran was formed during the Middle Triassic orogeny. From Jurassic through Miocene time, sedimentation was relatively continuous in this basin. The Shurijeh Formation (Neocomian), which consists of red bed siliciclastic sediments that were deposited in fluvial depositional settings, crops out in the southeastern part of the Kopet-Dagh basin. In addition to clastic lithofacies, non-clastic facies in the form of calcrete paleosols, were identified in this formation. The calcrete host rocks are mainly sandstone, pebbly sandstone. The calcrete in middle unit in the Shurijeh Formation consists of, from bottom to top: incipient calcrete, nodular calcrete, massive calcrete horizons. The maturity pattern of these calcrete gradationally increases from bottom to top in this unit. Lack of organo-sedimentary structure (mainly plant roots), diversity of calcite fabric, suggest that the studied calcretes have a multi-phase development: a short vadose phase followed by a long phreatic phase. These calcretes are neither pedogenic nor groundwater calcretes. Petrographic studies show that they are composed of micritic textures with a variety of calcite fabrics, microsparitic/sparitic veins, displacive, replacive fabrics, quartz, hematite grains. Cathodoluminescence images, trace elemental analysis (Fe, Mn increased, Na, Sr decreased) of calcrete samples show the effects of meteoric waters during the calcrete formation when water tables were variable. In this study, we conclude that evaporation, degassing of carbon dioxide are the two main factors in the formation of non-pedogenic or groundwater calcrete. The sources of carbonate were probably parent materials, surface waters, ground waters, eolian dusts, numerous outcrops of limestones that have been exposed in the source area during Neocomian time.     

地浸过程中铀迁移特征及条件的实验研究

通过室内模拟试验，对地浸过程中铀的迁移规律进行了研究。结果表明：①溶浸液中铀浓度值在达到饱和浓度前，随地浸路径延长而逐渐增大；②铀的存在形式及其含量与水文地球化学条件相关，当1．9＜pH＜4．4时，为UO2SO4和UO^2 2；当4．4＜pH＜5．1时，为UO2SO4和(UO2)a(OH)^5 ；②随着水文地球化学条件的变化，铀迁移经历了溶解迁移、水解沉淀、再溶解迁移的多次旋回，即当pH＞4．6、Eh＜420mV时，溶解状态的铀产生沉淀；当2．0＜pH＜4．6，420mV＜Eh＜650mV时，U^6 溶解迁移；当pH＜2．0、Eh＞650mV时，U^4 溶解迁移；④根据溶浸液中铀浓度和pH、Eh临界值，铀地浸路径可划分出充分浸出带、有效浸出带、铀沉降带和未浸出带，不同特征的浸出带随浸出时间的变化而有序移动。