Elsevier

Sedimentary Geology

Volume 405, 15 July 2020, 105688
Sedimentary Geology

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

https://doi.org/10.1016/j.sedgeo.2020.105688Get rights and content

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 600 m of clastic deposits. The 60–80 m of Cenomanian unconsolidated sands and clays, deposited in alluvial-deltaic to lacustrine settings, compose the upper part of the post-riftmedium-grained siliciclastic reservoir, the Sainshand Formation which hosts uranium roll-front systems. Dolomite cemented sandstone layers with 10–20 cm 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 δ18O (−10 ± 1‰ V-PDB) and δ13C (−7 ± 1‰ V-PDB). The δ18O values are interpreted as inherited from typical meteoric waters quite close to present day waters. The δ13C 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.

Introduction

Models of primary dolomite cement formation (Last, 1990; Last etal., 2010; Last etal., 2012; Casado etal., 2014) in continental settings without any marine influence (Colson and Cojan, 1996; Alonso-Zarza and Martín-Pérez, 2008) have been proposed in various settings: (i)climate controlled ephemeral continental (playa) lakes with evaporation occurring during semi-arid episodes, precipitating dolomite (Cojan, 2010), (ii) progressive Ca depletion of meteoric water during surface run-off due to evaporation and groundwater infiltration, causing Mg saturation, resulting into dolocrete formation (Worden and Burley, 2003) and (iii) precipitation of pedogenic dolomite in soils in the presence of certain types of soil microbes (Kearsey etal., 2012; Roberts etal., 2004; Sánchez-Román etal., 2008; Wright and Wacey, 2005).

Most commonly siderite, ankerite and dolomite are formed along contrasted redox conditions in estuarine conditions (Humphrey, 2000). In such context, the main process is the formation of dolomite by lateral evolution of surface waters submitted to evaporation (Worden and Burley, 2003; Cojan, 2010; Kearsey etal., 2012). Factors controlling dolomite precipitation in saline wetlands submitted to arid climate are described in detail in Mather et al. (2018).

In more deeply buried series, dolomite cements are frequently related to the mixing of brackish waters and deep hydrothermal fluids, a process responsible for the dolomitization of limestones through dissolution and recrystallization cycles (Warren, 2000). This is the case also of most Mississippi Valley type deposits where extended dolomitization occurs near the boundary with basement rocks, generally in relation with the incoming of primary brines issued from sea water evaporation, typically enriched in Mg (Leach etal., 2005; Muchez etal., 2005; Boiron etal., 2010). Spirakis (1996) made a link between dolomite cemented sandstone layers associated to uranium roll-front type deposits that form at depth between the fresh water/deep water (brine) interface. In that case dolomite formation was attributed to organic matter bacterial fermentation (methanogenesis) during early diagenesis and there were also evidences for dolomite forming during the ore stage. However, the origin of local dolomite cementation in an unconsolidated continental sand sequence despite mineralized, like the one studied here, is a much less documented topic (Morad etal., 1990; Spötl and Wright, 1992).

In the intracontinental basins of Mongolia, recent series of drillings for uranium exploration in the fluvio-lacustrine deposits of the Cenomanian Sainshand Formation at Zoovch Ovoo revealed the presence of sporadic cemented sandstone layers within an otherwise unconsolidated sedimentary succession. The carbonate cement in the whole Sainshand Formation in the studied depocenter is exclusively dolomite and is characterized by several petrographic textures and habits. In the upper unit that is also the main outcrop in East Gobi Basin, namely the Bayanshiree Formation, dolomite is absent, whereas calcite was found mostly as joint cements. The exclusive presence of the dolomite cement at depth was thus investigated in this study as the chronology of cements and processes affecting these sands is crucial for the understanding of the formation of the uranium deposit and also for the management of ore exploitation. The purpose of the present study was thus i)to produce a first assessment of the mineralogy and geochemistry of dolocrete within the Sainshand formation, ii) to understand the process at the origin of the different Sainshand dolomite cements, and therefore to get indications on the paleohydrological conditions, iii) to investigate why interstitial fluids were saturated with respect to dolomite throughout the whole sequence of events from sedimentation to burial and uplift and iv) to identify potential implications on uranium deposition.

Section snippets

Geological setting

The area of study is located in south-easternMongolia in East Gobi Basin and in particular in the Zuunbayan sub-basin and the Zoovch Ovoo district, hosting the homonymous roll-front type uranium deposit. The East Gobi Basin extends for 700 km in E-W trend and 300 km in the N-S(Meyerhoff and Meyer, 1987; Prost, 2004). It is a northeast-southwest trending continental basin of multiphase deformation (plate interior poly-phase basin) formatted by Mesozoic extension events (Johnson and Ritts, 2012).

Description

Sediments, such as clays, silts and sands, most of the times grain supported as well as a few consolidated facies represented by dolomite cemented and by matrix (clay) supported sandstones represent the facies recognized in the Zoovch Ovoo depocenter. With respect to the dolomite cemented facies, the sandstones can have various grain sizes from fine to very coarse. The sandstones are mostly composed of quartz (40 to 80%), feldspars as K-feldspars (microcline, orthoclase, some % to 25%) and

Dolomite burial diagenesis

By considering Prost (2004) and Graham etal. (2001) it can be estimated that the maximum burial depth for the Sainshand formation was 500-600 m. By taking into account basin modelling by Prost (2004) it may be deduced that the temperature of the formation did not exceed 30–35 °C. According to Barker and Pawlewicz (1994), the mean random vitrinite reflectance of kerogen subjected to a maximum temperature of 62 °C during burial could not exceed 0.40%. Vitrinite reflectance measurements in the

Conclusion

Within the unconsolidated succession of the Sainshand Formation in the Zoovch Ovoo area, the scarce occurrence of dolomite cemented facies provided a unique opportunity to reconstruct a relative chronological succession from early to burial diagenesis conditions. They are also among the rare indicators available to documents fluid-rock interactions as most detrital minerals have remained unchanged since sedimentation.

The petrographical and geochemical study of cements in the Sainshand Formation

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was performed and funded in the research framework between ORANO Mining (France) and CREGU (Centre de Recherches sur la Géologie des Matières Premières Minérales et Energétiques), in the Laboratory of GeoRessources in Nancy, France (contract number 40077759 of 15/03/2016). We are grateful to COGEGOBI's geological team for the hospitality and the equipment provided during the field work as well as for taking care of the shipping of the samples from Mongolia to France. People from the

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