ABSTRACT

Evolving atmospheric oxygen levels during the early Earth history had a profound impact on Earth’s surface processes, especially on the mineralization of redox-sensitive elements such as iron (Fe) and Uranium (U). Here we present a review of the effect of the Great Oxidation Event (GOE) that started at ~2.44 Ga on the style of Uranium mineralization with reference to Indian uranium occurrences. GOE was responsible for the initiation of intense chemical oxidative weathering of the Neoarchean U-rich K-granites mobilizing U from the source rocks in the U⁶⁺ soluble state. By 2.32 Ga, anoxic/euxinic conditions in the oceans developed on a global scale mainly due to the bacterial sulphate reduction that was triggered by enhanced sulphate concentrations in the ocean caused by the GOE. Such a development of anoxic/euxinic conditions acted as a sink for the hexavalent U by reducing it to tetravalent state and its deposition on a large scale in the strata of post-2.0 Ga age. Majority of Uranium deposits of India are hosted in the Proterozoic rock formations unlike the global pattern, where the Phanerozoic strata are more prominent. Among the uranium reserves that occur in the Precambrian sequences of India, the mineralization during the 1.9 to 1.6 Ga time window constitute ~75% of the total. In India, the Archaean U occurrence is exemplified by the Bababudan quartz pebble conglomerates (~2.72 Ga) hosting detrital uraninite, while the Proterozoic period witnessed a marked change with most of the U deposits being of redox-controlled-type in sedimentary formations. A model is proposed to explain such an enrichment in Uranium reserves among the Proterozoic strata of India. It appears that the emplacement of large ion lithophile element (LILE)-enriched granites, the onset of GOE and ensuing oceanic anoxic conditions played a major role in controlling the Uranium mineralization of the late Paleoproterozoic in the Indian scenario.

抽象的

在地球早期的历史中，不断变化的大气氧含量对地球的表面过程产生了深远的影响，特别是对氧化还原敏感元素（如铁（Fe）和铀（U））的矿化。在这里，我们参考印度铀矿的发生情况，回顾了从约2.44 Ga开始的大氧化事件（GOE）对铀矿化方式的影响。GOE负责引发新旧宙斯富铀的K花岗岩的强烈化学氧化风化，后者从U ^6+中的烃源岩中^迁移出U可溶状态。到2.32 Ga时，海洋中的缺氧/厌氧条件已在全球范围内发展，这主要归因于GOE引起的海洋中硫酸盐浓度的升高触发了细菌硫酸盐的减少。缺氧/富氧条件的这种发展通过将六价U还原为四价态并在2.0 Ga时代后的地层中大规模沉积而充当了六价U的汇。与全球格局不同，印度的大多数铀矿床都存在于元古代的地层中，在该地区的生代地层更为突出。在印度前寒武纪序列中发生的铀储量中，在1.9至1.6 Ga时间窗内的矿化量约占总数的75％。在印度，古巴铀矿的发生以巴巴布丹石英卵石砾岩为例（〜2。72 Ga）含有碎屑铀质岩，而元古代时期则发生了显着变化，大部分U沉积物是沉积岩层中的氧化还原控制型。提出了一个模型来解释印度元古代地层中铀储量的这种富集。看来，在印度洋情景中，富含大离子的亲石元素（LILE）的花岗岩的沉积，GOE的爆发和随之而来的海洋缺氧条件在控制晚古元古代铀矿化中起着重要作用。