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The metallogenic system and the redox trap in the roll-front type uranium deposit of Zoovch Ovoo, Mongolia. Role of the C and S phases

Abstract : The objective of the PhD thesis concerns the analysis of the processes governing the carbon-uranium-sulfide system in the aquifers of the roll-front type Zoovch Ovoo uranium-deposit (Mongolia). The Cenomanian Sainshand Formation hosting the ore deposit contains reservoirs made of silicoclastic sediments deposited in fluvio-lacustrine settings mostly characterized by unconsolidated silty clays and sands ranging from arenite to sub-arkose. Occasionally some decimetres thick sandstone beds are dolomite cemented. Four different dolomite fabrics are recognized and classified according to their crystal boundaries and geochemistry. Isotopic analysis indicates that they were formed from meteoric waters with the carbon source being probably the detrital material itself issued from basement rock erosion. Dolomites are genetically related to each other with possible recrystallization episodes that are indicated by gradual depletion in REE and Mn. The first dolomite type was depositionally controlled, while the next three parageneses are diagenetic. The third dolomite type is found only in samples that have experienced roll-front waters, whereas the fourth post-dates uranium formation. As a result the dolomite cements paragenetic evolution is used as reference to document the whole burial history of the deposit. Organic matter particles, pyrite and uranium are very intimately related. Organic matter present in clay, silt and sand is always represented by poorly preserved detrital elements of higher plant origin and is very immature (peat stage; vitrinite reflectance <0.4%). At the scale of the sedimentary particles, organic matter complexes U(VI). However, despite a systematic association to UO2, it cannot be considered as directly involved in the reduction mechanism of uranium because of the low burial temperatures (max 50°C) and hence very limited diagenesis. Nevertheless, the investigation reveals that organic matter is the most important element in the U trapping process. Indeed, it acts as a substrate sustaining and focusing bacterial activity into specific sedimentary layers. In turn the microbial communities catalyze the reduction of sulfur, iron and especially uranium leading to two major consequences: 1) the precipitation of UO2 onto organic matter particles and 2) the formation of pyrite which acts as an additional reducer for U(VI). It appears that the uranium ore deposit at Zoovch Ovoo is primarily controlled by the distribution of the detrital organic matter in the sedimentary system as well as by the permeability network allowing the input of S, U and micro-organisms. The rather shallow aquifers probably underwent a nearly continuous bacterial activity which is responsible of many chemical features and processes controlling the U and sulfide precipitation. However, main uranium deposition events occur since the beginning of exhumation when enhanced fluid movement sustains roll-front activity. The latter displaces and combines the ingredients of the biochemical trap, allowing progressive enrichment in uranium through time.
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Submitted on : Wednesday, November 6, 2019 - 9:06:45 AM
Last modification on : Wednesday, November 3, 2021 - 5:25:12 AM

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  • HAL Id : tel-02350541, version 1


Dimitrios Rallakis. The metallogenic system and the redox trap in the roll-front type uranium deposit of Zoovch Ovoo, Mongolia. Role of the C and S phases. Geochemistry. Université de Lorraine, 2019. English. ⟨NNT : 2019LORR0079⟩. ⟨tel-02350541⟩



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