印度三个不同时代地洼区的铀成矿作用(英文)

在印度三个时代不同的地洼区中,出现重要的赋存于沉积岩中的铀矿化作用。这些地洼区是:a)印度南部的元古代库达帕洼地;b)印度东北部梅加拉亚的白垩纪Madadek盆地;c)印度北部的晚第三纪喜马拉雅前渊。库达帕洼地呈新月形,沿南印度克拉通东缘沉积,紧邻东加茨活动带,面积达44,500km~2。该洼地由数个上升、下落断块组成。在这些断块内,浅水砂质(库达帕亚组)和钙质(库尔努尔组)沉积物厚度超过12km,同时从2,000—600Ma,至少有四期粒玄岩墙侵入。铀矿化作用主要呈沥青铀矿(含硫化物)和次生铀矿物形式,沿库达帕洼地面南缘出现于磷质碳酸盐岩和共生物的帕帕格尼组砂质岩、砾岩中。如Tummalapalli、Ammasripall,在梅加拉亚,约200米厚的河流相、海相砂质、泥质沉积物分布于1.5km高的西隆高原南缘的Mahadek盆地。该盆地的铀矿化限于河流相、边缘海相早白垩世“石英粗砂碎屑岩/亚长石砂岩”型Mehadek砾岩中,如Comasahat,Pdensashakap、Domiasiat,并呈沥青铀矿、水硅铀矿、钛钠矿形式。这些矿物与还原剂(如碳、生物成因黄铁矿)紧密共生。在喜马拉雅前渊,铀矿化赋存于河流相锡瓦利克砂岩中,并主要产于锡瓦利克砂岩的下-中或中-上接触面上,如Thein、Morni、Hamirpur,Naugajiarao等地。矿化主要呈沥青铀矿、水硅铀矿(与硫化物共生)和大量次生铀矿物形式——这主要是由于持续至现在的反复的活化作用和沉淀作用。这三个地洼区铀成矿作用的共同特点是河流相沉积物围岩(主要为砂岩,并来源于丰富的酸性源区)、远成热液成矿作用和原始成矿作用时的强烈还原环境。这特征可作为在地洼区寻找赋存于沉积岩中的铀矿化的标志。

URANIUM MINERALIZATION lN THREE TEMPORALLY DIFFERENT DIWA (GEODEPRESSION) REGIONS OF INDIA

Sediment-hosted significant uranium mineralization occurs in three temporally dif ferent diwa(geodepression) regions of India. These. with yoanging of age, are a) Proterozoie Cuddapah depression in South India. b) Cretaceous Mahadek basin of Meghalaya in North -eastern India. and c) Neogene Himalayan foredeep in Northern India. The crescent-shaped, intra-cratonic Cuddapah depression, with its disposition along the eastern margin of the South Indian Craton and bordering the Eastern Ghats mobile belt. covers an area of 44,500 km~2. and is made up of a number of downtbrown and upthrown blocks containing over 12 km thick shallow water arenaceous, (Cuddapah Supergroup) and calcareous (Kurnool Group) sediments, together with mostly dolerite dykes that were emplaced in at least 4 periods from 2000 to 600 Ma. Uranium mineralization, mainly in the form of pitchblende(with sulphides) and secondary uranium minerals occurs along the SW margin of the Cuddapah depression in phosphatic carbonate rocks and associated arenaceous rocks and conglomerate of the Papaghni Group. as at Tummalapalli and Ammagripalli. In Meghalaya, about 200m thick sequence of fluviatile to marine arenaceous and argillaceous sediments occur in the Mahadek basin bordering the southern margin of the 1.5 km high Shillong Plateau. Uranium mineralization in this basin is confined to fluviatile to marginal marine Lower Cretaceous 'quartz arenite/subarkose' type Mahadek sandstone as at Gomaghat. Pdengshakap and Domiasiat. and is in the form of pitchblende, coffinitc and brannerite, intimately associated with reduetants like carbon and biogenie pyrite. In the Himalayan foredeep, flaviatile Siwalik sandstones host uranium minera lization, mainly at the contact of Lower-Middle or Middle-Upper Siwaliks, at a number of places like Thein. Morni. Hamirpur. Naugajiarao etc. The mineralization is mainly in the form of pitchblende, coffinite(associated with sulphides) and a host of secondary uranium minerals due to repeated remobilization and precipitation, which continues even to present day. The common characteristics of uranium mineralization in these three diwa regions are fluviatile host sediments, essentially sandstone, havinga fertile acidic provenance, telethermal mineralization. and a strong redacing environment for primary uraniam mineralization. and such aspects act as guides in prospecting for sediment-hosted uranium mineralization in the Diwa regions.