Petrography and geochemistry of uranium mineralised Precambrian granitic-pegmatitic rocks of Mawlait, West Khasi Hills district, Meghalaya

During radiometric investigation at Mawlait, significant uranium mineralisation (0.024–0.22%U₃O₈) was located mainly within the small pegmatite (garnet bearing quartzofeldspathic rock), which are locally segregated within migmatite at Umiang River section. Pink granite and granite gneisses are the dominant lithounits of the study area showing fertile character and spotty radioactivity at several places. Radioactivity in these rocks is mainly contributed by discrete uraninite grains along with some zircon and xenotime. Granites are peraluminous, low-Ca in nature and their geochemical signatures suggest derivation from a felsic source. Discriminant diagrams using Rb, Nb and Y indicate ‘within plate’ to ‘volcanic arc’ nature of the rock. The uraniferous pegmatitic veins within migmatite appear to have formed due to localised metamorphic segregation during late stage of anatexis. Petromineralogical and geochemical studies suggest that the uranium mineralisation in granitic-pegmatitic rocks of the area is mainly syn-magmatic type.     

Reconnaissance of quaternary sediments from Khasi Hills,Meghalaya

Khasi Hills area of Meghalaya, one of the highest rainfall zones of the world has been explored to understand the spatial and temporal extent of Quaternary sediments. In general such deposits are restricted to shallow depths in most of the sites except in one site where it is 600 cm thick. AMS C-14 dates of these deposits suggest that sediments deposited are mostly of Holocene or in rare cases belong to later part of Pleistocene. Early Quaternary deposit is either missing or yet to be explored from this region. Natural hazards combined with human activity effecting degradation of vegetation and sediment cover are probably the main cause for poor Quaternary exposure at the region.     

Palaeochannel controlled Cretaceous sandstone-type uranium deposit of Lostoin area,Mahadek basin,Meghalaya

Sandstone–type uranium deposit hosted by reduced feldspathic sandstone sequence of lower part of Cretaceous Mahadek Formation occurs at Lostoin area of Mahadek basin and is confined within a prominent palaeochannel of NNE-SSW trend with flow towards SSW. The channel-fill comprises of about 10 m to 55 m grey to dark grey coloured, reduced, coarse to medium grained feldspathic sandstone along with abundant carbonaceous matter (5 to 15% by visual estimation) and is overlain by 5 m to 80 m of oxidized sub-feldspathic sandstone. Correlatable medium grade (～ 0.1% U₃O₈) uranium mineralization is intercepted in the boreholes aligned along the palaeochannel. Uraniferous solutions generated from the fertile provenance of south Khasi batholith flow through the channel and precipitated uranium at suitable reduction traps. Gentle slope of 2° to 3° of the basement channel facilitated prolonged interaction of uraniferous solution with reductants causing ore grade concentration. It has been envisaged that the control of palaeochannel and palaeobasement topography of the ore body would continue towards the SSW beyond Wahblei river.     

Assessment of the radiological hazards of sand sediments collected from streams and streamlets of the uranium deposit areas in West Khasi Hills District,Meghalaya, India

Primordial radionuclides in sand sediments that are often used as constructing materials are one of the sources of radiation hazard in dwellings. Activity concentrations of the primordial radionuclides of ⁴⁰K, ²²⁶Ra and ²³²Th have been measured in sand sediments collected from streams and streamlets lying within and around the uranium mineralization deposit blocks of Kylleng-Pyndensohiong, Mawthabah Areas of West Khasi Hills District, Meghalaya, India. The technique of gamma-ray spectroscopy using a NaI(Tl) detector with a PC-based multi channel analyser was applied for determination of the activity concentrations. The activity of the sand sediments obtained in this study ranges from 95.3 to 1,088.8 Bq kg⁻¹ for ⁴⁰K; 38.3 to 784.1 Bq kg⁻¹ for ²²⁶Ra and 78.0 to 316.1 Bq kg⁻¹ for ²³²Th. Sand sediments from two sampling locations lying within the mineralization zone show highest concentrations of these radionuclides. The radiological hazards of the sand sediments were calculated using various models given in the literature. The radium equivalent activity was found to be higher than the accepted standard criterion value of 370 Bq kg⁻¹ and the values of external and internal hazard indices were also found to be higher than unity in these two sampling locations. Besides these two sampling locations, a sampling location lying at a nearby distance from the mineralization zone also exhibits hazard indices values greater than unity.     

Simple statistical and mineralogical studies as petrogenetic indicator for neoproterozoic mylliem porphyritic granites of east Khasi Hills,Meghalaya, Northeastern India

The porphyritic granite body (∼ 600 Ma) around Mylliem (25°32′N: 91°52′E), east Khasi hills district, Meghalaya occurs as a distinct intrusive body into the host rock of low- grade meta-sediments belonging to Proterozoic Shillong Group. The porphyritic granite body manifests prominent lath- shaped feldspar phenocrysts giving rise to dominant porphyritic texture. Further, this porphyritic granite body is characterized by primary foliation (defined by parallely arranged feldspar laths), tongues and appophyses and xenoliths of older metamorphics. Petrographically, the Mylliem porphyritic granite is grey leucocratic, coarse grained, phanerocrystalline with modal variants ranging from granite to granodiorite tending to tonalite. Simple statistical studies based on correlation coefficient values involving modal variables of the Mylliem porphyritic granite bodies appear significant in majority of the cases indicating magmatic crystallization. Use of relevant thermometric method indicates temperature of equilibration of the body in the range of 369°C to 507°C. The crystallization of the Mylliem porphyritic granite pluton initiated at an average lithostatic pressure in the tune of ∼11 Kb followed by dominant P_H2O controlled milieu. Spatially projected mineralogical parameters suggest a general trend of the magmatic cooling of the body from margin inward with sudden influx of volatile matters that occasionally offsets mineralogical trends.     

Interpolation study on ambient gamma levels in parts of Khasi Hills,Meghalaya (India): Preliminary findings for U exploration

This paper discusses an experimental approach to examine uranium exploration avenue over the geologically extended parts of Mahadek basin in Meghalaya, amid some of the environmental constraints. Study comprises periodic measurements of prevailing ambient gamma levels across 320 georeference points, in relation to the major litho units of Mahadek basin, covering 673 line km of Khasi Hills. Acquired sample data points were then analysed in geostatistical software (Surfer^TM) to develop analytical model of sample variogram having bearing on the uranium exploration in the area. Study findings have given encouraging surface indicators with mostly elevated gamma levels over the parts of West Khasi Hills. Delineated gamma anomalous zones are lithologically well correlated including to that of existing uranium occurrences in the basin. Identified anomalous zones over the parts of West Khasi Hills by this study work, are mainly associated with the Mahadek sandstone (Upper and Lower Mahadek) and Precambrian basement granites. Lower Mahadek sandstone is host rock for uranium mineralisation in the basin. Initial findings suggest with the closer spatial resolution (～1 km) of sample data points, the approach adopted by the study work holds promising application in locating potential uranium exploration targets especially to the extended and inaccessible parts of the basin.     

Petrology and Geochemistry of Dolerite Dykes, West Gar0 Hills, Meghalaya: A Preliminary Study

The Precambrian gneissic basement of West Gar0 hills, Meghalaya has been traversed by dolerite, metadolerite and basaltic rocks. The dolerite and basaltic rocks are tholeiites and show basic to intermediate composition with Mid- Oceanic Ridge Basalt (MORB) chemical affinity. These dolerite dykes are slightly older than the Sylhet traps and do not indicate genetic relationship with the latter. Relative age relationships suggest that the dykes are Jurassic in age and different from the dykes related to Sylhet volcanism and other ultrabasic and basic dykes in the area. Geochemical data suggest that these dykes have formed from a differentiated product of primary picritic magma of upper mantle source. This dyke activity may be related to the extensional tectonism during the Jurassic period due to the breakup of the Gondwana supercontinent.     

Structure,composition, and depositional environments of the lower cretaceous rocks of the Zhuravlevka terrane,Central Sikhote Alin

The structure, mineralogical-petrographic composition, and formation settings of the Berriasian-Albian terrigenous rocks of the Zhuravlevka terrane (Central Sikhote Alin) are considered. The rocks are interpreted as sediments accumulated in a virtually nonvolcanic basin along the transform plate boundary. Judging from the composition of terrigenous rocks, the main sources of clastic material were represented by the eroded granitic-metamorphic rocks of a mature continental crust, and, possibly, granitoid intrusions of ancient arc roots. Genetic features of the sediments suggest their accumulation on the shelf, underwater slope, foothill, and the adjacent basin plains of an oceanward-open marginal sea.     

约旦南部瓦迪巴希亚地区铀矿化地球物理特征

通过对瓦迪巴希亚（Wadi Bahiya）地区开展地质调查、岩石取样分析以及地面γ能谱测量等综合研究,认为研究区铀矿化主要产于上白垩统—古新统MCM组近地表风化破碎的灰褐色砂土状泥灰岩之中,矿化类型为现代氧化作用成因的钙结岩型,研究区γ能谱测量铀质量分数平面特征表现为异常规模小,晕圈梯度变化大,深部揭露表明,自近地表向深部铀质量分数逐渐降低,岩石取样分析结果与γ能谱测量具有对应性,铀矿化主要位于地表以下0.5～1.5m的范围内。     

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。     

鄂尔多斯盆地东胜地区沉积体系与砂岩型铀成矿

沉积体系分析在可地浸砂岩型铀矿床的研究中起着非常重要的作用.本文以沉积体系分析和层序地层学为依据,对鄂尔多斯盆地东胜地区中侏罗统直罗组沉积体系特征、沉积相的空间展布、沉积环境的演化和层序地层学等方面进行了研究,认为：（1）沉积相的平面分布控制着砂体的空间展布,进而影响着赋铀砂体的空间分布;（2）沉积相和沉积环境的演化创造了良好的岩相及岩性组合条件,有利于层间氧化作用的进行;（3）沉积层序控制了3层结构的岩性空间组合.     

3075地区变质岩的原岩形成环境及其与铀成矿的关系

本文用各种手段和方法恢复3075地区变质岩的原岩及其形成环境,研究了各类岩石中铀的丰度、配分状态及其在变质作用中的活动规律、成矿作用。提出:变质岩的原岩形成环境是影响铀在原岩中初始富集的重要因素;原岩中铀的配分状态影响铀在变质作用中的行为和迁移富集的程度。由此认为,变质作用、混合岩化作用是有利于铀活化、转移、富集成矿的外部条件,而变质岩的原岩形成环境及原岩中铀的配分形式是形成铀成矿物质基础的决定因素。     

舍伯吐地区第三系沉积体系及对可地浸砂岩型铀成矿的作用

舍伯吐地区位于松辽盆地西南部,笔者通过沉积体系分析认为,该区第三系中新统大安组以低水位体系域的辫状河相沉积为主,上新统泰康组以水进和高水位体系域的曲流河相沉积为主,目前已发现的铀矿化主要产于泰康组水进体系域的曲流河边滩亚相中。综合分析认为,有利的成矿部位应为低水位体系域的大安组辫状河河道心滩亚相。     

鄂尔多斯盆地南缘彬县地区铀成矿关键时期的古地貌特征

中侏罗世为直罗组下段沉积期,辫状河道为有利的铀储层砂体,铀矿(化)体主要赋存于辫状河砂体中,受沉积古地貌控制,主要发育在低洼区。晚侏罗世、早白垩世-古新世、始新世-中新世是砂岩型铀成矿期,古地貌控制了地下古水流的流向,控制着氧化还原带的分布和铀矿的富集:晚侏罗世铀预成矿期,古隆起区受到潜水淋滤,含铀物质向斜坡和低洼区迁移;早白垩世-古新世铀主成矿期,古地貌控制了含氧含铀水的水流方向,氧化还原带主要分布在古地貌斜坡区。     

Stratigraphy of the upper cretaceous and lower tertiary strata in the Tethyan Himalayas of Tibet (Tingri area,China)

The 1500-m-thick marine strata of the Tethys Himalaya of the Zhepure Mountain (Tingri, Tibet) comprise the Upper Albian to Eocene and represent the sedimentary development of the passive northern continental margin of the Indian plate. Investigations of foraminifera have led to a detailed biozonation which is compared with the west Tethyan record. Five stratigraphic units can be distinguished: The Gamba group (Upper Albian - Lower Santonian) represents the development from a basin and slope to an outer-shelf environment. In the following Zhepure Shanbei formation (Lower Santonian - Middle Maastrichtian), outer-shelf deposits continue. Pebbles in the top layers point to beginning redeposition on a continental slope. Intensified redeposition continues within the Zhepure Shanpo formation (Middle Maastrichtian - Lower Paleocene). The series is capped by sandstones of the Jidula formation (Danian) deposited from a seaward prograding delta plain. The overall succession of these units represents a sea-level high at the Cenomanian/Turonian boundary followed, from the Turonian to Danian, by an overall shallowing-upward megasequence. This is followed by a final transgression — regression cycle during the Paleocene and Eocene, documented in the Zhepure Shan formation (?Upper Danian - Lutetian) and by Upper Eocene continental deposits. The section represents the narrowing and closure of the Tethys as a result of the convergence between northward-drifting India and Eurasia. The plate collision started in the Lower Maastrichtian and caused rapid changes in sedimentation patterns affected by tectonic subsidence and uplift. Stronger subsidence and deposition took place from the Middle Maastrichtian to the Lower Paleocene. The final closure of remnant Tethys in the Tingri area took place in the Lutetian.     

Coralline algae from the prang formation (middle-late Eocene) of the Lumshnong Area,Jaintia Hills,Meghalaya

The present paper records nine species of coralline algae from the Prang Formation of middle-late Eocene age from the Jaintia Hills, Meghalaya. The algae are associated with the larger foraminifera including Nummulites, Alveolina and Discocyclina throughout the succession. The temporal distribution of algal species indicates that seven species are known from the Eocene. Of the remaining two, one ranges from the Palaeocene to Eocene and the other is Palaeocene in age. The coralline algae and larger foraminifers are differentiated into two associations, the lower one indicating inner-ramp environment of high energy and the upper one showing deposition in relatively calm waters of deeper ramp environment.     

松辽盆地庆安地区白垩纪沉积体系和目的层地球化学特征及与铀矿化的关系

本在分析松辽盆地所处大地构造背景和构造演化特征的前提下，着重研究了松辽盆地庆安地区白垩系的沉积特点、沉积体系特点和泉头组目的层地球化学特征，初步论述该地区砂岩型铀矿成矿特点及与白垩纪地层的关系，并提出了该地区可能产出的铀矿床类型。     

Lithofacies,depositional environments,regional biostratigraphy and age of the Chitarwata Formation in the Bugti Hills,Balochistan, Pakistan

The Oligocene-early Miocene Chitarwata Formation records a critical interval of terrestrial sedimentation that predates the Siwalik deposits on the Potwar Plateau of north-central Pakistan. This Oligocene-early Miocene time interval has long been considered as lacking in the entire Indo-Pakistan region. The Chitarwata Formation is widely exposed in the Sulaiman Range, but has never been described in detail in the Sulaiman Lobe, where the famous fossiliferous strata called ‘Bugti Bone Beds’ have been known for over a century and half. The Chitarwata Formation represents coastal-delta at the base, and plain and fluvial environments at the top. Lithofacies and sedimentary structures of the Chitarwata Formation in the Bugti area are described in detail, and show a clearly distinct lithologic pattern, different from that reported from the Zinda Pir area. The Chitarwata Formation also records an important transition in the evolution of the drainage systems in the area during the late Paleogene and early Neogene. This transition from the west-flowing paleo-Indus fluvial system to the development of the ancestral Indus drainage system may explain the numerous hiatuses that characterize the Chitarwata Formation. The abundance of fossil mammals from the Chitarwata and overlying Vihowa formation in the Bugti Hills provides critical biochronologic information that sheds new light on biostratigraphic correlation with the Zinda Pir area and for the entire Sulaiman Range.