Dolomite cements in Cenomanian continental sand deposits: Time evolution and significance (Zoovch Ovoo U-deposit, East Gobi Basin, Mongolia)

Abstract

Shallow buried unconsolidated sands generally provide very little information about diagenesis as most detrital minerals remain unchanged. However, in rare cases carbonate cemented nodules or sandstone layers may occur inside unconsolidated series. These cements could help to reconstruct the chronology of events from early to late phase stages of diagenesis. The Late Cretaceous sequence of the Zoovch Ovoo depocenter in East Gobi Basin is represented by 600m of clastic deposits. The 60-80m of Cenomanian unconsolidated sands and clays, deposited in alluvial-deltaic to lacustrine settings, compose the upper part of the post-rift medium-grained siliciclastic reservoir, the Sainshand Formation which hosts uranium roll-front systems. Dolomite cemented sandstone layers with 10-20cm thickness occur among the unconsolidated rock facies. Calcite is absent from this formation, but is present only in the overlying, also outcropping, Bayanshiree Formation. Samples from the dolomite cemented sandstone layers were investigated in detail to uncover their origin and diagenetic history. Four dolomite cement types were recognized that indicate recrystallization episodes and were classified based on the size and shape of the crystals, namely: (i) microcrystalline dolomite frequently associated with siderite, (ii) euhedral dolomite also associated with siderite, (iii) subhedral dolomite and (iv) finally anhedral dolomite. Their REE content varies significantly from dolomite I to IV, in particular by a strong depletion in LREE about 30 times. The contrasted precipitation of calcite in Bayanshiree and dolomite cements in Sainshand formations could be attributed to different Mg/Ca ratio of the circulation fluids in the two aquifers. Both carbonates display however rather homogenous oxygen and carbon isotopic compositions for δ 18 O (-10±1‰ V-PDB) and δ 13 C (-7±1‰ V-PDB). The δ 18 O values are interpreted as inherited from typical meteoric waters quite close to present day waters. The δ 13 C values indicate a mixed source of both organic and mineral carbon. All data taken into account, a full paragenetic succession was constructed. It includes the evolution of dolomite during burial diagenesis and the effects of the oxidizing roll-front uranium rich waters in the system. The latter induces partial dissolution followed by precipitation of a dolomite phase typical of roll-front zones. Carbonate cements can be thus considered as the best and rather unique geochemical indicators for the recognition of a burial history framework to paleofluid circulations and fluid-rock interactions in these intracontinental series of unconsolidated sands.

Conditions for uranium biomineralization during the formation of the Zoovch Ovoo roll-front-type uranium deposit in East Gobi Basin, Mongolia

“…Clay intraclasts up to pebble size are often present in sand layers and detrital rounded dolomite grains. Dolomite is microcrystalline and often associated with siderite (Rallakis et al, 2020).…”

“…The consolidated lithologies are only a few dm thick and correspond to dolomite-and pyrite-cemented sandstones, the cement accounting for 30% of sandstone volume on average. Four successive generations of dolomite are distinguished (Rallakis et al, 2020): type I (microcrystalline, pre-roll-front), type II (euhedral, pre-roll-front), type III (active roll-front) and type IV (post-roll-front).…”

“…Detrital sediments are neither affected by diagenesis nor by cementation. The dolomite cemented sandstone layers are an exception and their formation is linked to proto-dolomite deposition (Rallakis et al, 2020). Dolomite types I (microcrystalline) and II (euhedral) were formed during early diagenesis and are associated with siderite.…”

“…The partial dissolution of the dolomite I and II due to the reactivity with the roll-front waters is responsible for forming secondary porosity and is followed by the formation of subhedral (type III) dolomite (Rallakis et al, 2020). The roll-front also affects the first euhedral pyrite and sphalerite generations, causing their epigenetic replacement by UO 2 .…”

The Role of Organic Matter on Uranium Precipitation in Zoovch Ovoo, Mongolia

“…Uranium mineral phases were studied by means of electron probe micro-analysis (Table 1) in samples of variable lithologies (e.g., matrix supported sandstones and sandstones cemented by dolomite), also discussed in [36]. Two phases were identified-uraninite and coffinite, sometimes occurring together.…”