Geological characterization of IMOF

2.1. Stratigraphy and formation history of the sediments and aquifers of the IMOF The region of the IMOF is located at the edge part of the large Turan Plate in the central part of the Chu-Syrdarya Basin. The Basin is filled with friable sediments from the Cretaceous to the Quaternary. Lithified subplatform sediments of the middle-upper Palaeozoic lie in the foundation of the Basin. Friable platform sediments are continental Cretaceous of up to 320 m in thickness and shallow-marine and marine Palaeogene sediments of up to 200 m in thickness. The Cretaceous- Palaeogene series is overlaid by the red-coloured sandy-clay Oligocene- Quaternary rock complex. The formation of these sediments is connected with the young Alpine orogeny and mountain uplift in the East and, mainly, in the South-East in the region of the Tyan-Shan mountain system.

From the analysis of the regional history it is very important to emphasize the following: 1) the formation of the thick permeable Cretaceous series; 2) the universal existence of the overlaying marine Palaeogene clay series is able to play the role of regional upper confinement; 3) the intensive uplift of the Tyan-Shan mountain system in the Southeast of region, which allowed the active penetration of oxygen-bearing waters into the aquifer of the friable platform sediments; 4) the formation of the large infiltration type Chu-Sarysu artesian basin.

Cretaceous sediments, including all profitable uranium ore at the IMOF, play the main role in the sediment series of the region. Cretaceous rocks are sediments of the large alluvial plain and are, mainly, grained sediment from fine-grained sand to gravel. Clay rocks amount to not more than10-20% of series. In this case, clay beds, as a rule, have a small thickness and do not expand very much. This fact creates some difficulties in separating the series on the horizon and subhorizon. The separation of such stratigraphic units is very important for jointing ore intervals at different well profiles. This was especially important during the first stages of the search, when the inexplicit jointing could lead to mistakes in choosing the direction and in density exploration well profiles and leading to lagging exploration tempos and cost increases.

Using the data of the sediment cycle and electro-logging (Fig. 3), Cretaceous sediments were separated on the 3 horizons with an increase in thickness in an East-West direction. There are different opinions about the age of these horizons. After consideration, the European

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stratigraphic scheme was accepted. Nevertheless, the original horizon names will be used as was accepted in practice.

FIG. 3. Lithology – stratigraphical column of IMOF.

The lower horizon is mynkuduksky (mk), dated as Lower Turonian. The thickness of the horizon varies from 30 up to 90 m. All of the lower part is generally made up of more coarse-grained sediments. The upper part, being the end of the alluvial cycle, is made up of more fine-grained sands. Uranium ore is located, mainly, in the lower part of the horizon.

The superstratum inkuduksky horizon (ink) dates to the Upper Turonian-Santonian, is the most thick (130–150 m). The mynkuduksky horizon is separated clearly on three subhorizons (mk1, mk2, and mk3). These subhorizons are not separated by clear confinements, but their alluvial cycle characteristic allows us to separate them as individual units of a series. Uranium ore is generally located in the lower and middle subhorizons.

The upper horizon, called the zhalpaksky and dated as Campanian- Maastrichtian has been researched the least because it includes uranium ore in the East part of IMOF only (Akdala section). The sediments of this horizon are characterized by less granularity and higher organic carbon content in the lower part of the horizon.

Cretaceous sediments are 80–90% grained rocks. Sands in the mynkuduksky horizon are, mainly, presented by medium-grained, and the inkuduksky contains hetero-grained sand with gravel. The mineralogical sediment content presented in Table 1 shows that 80% of consist is practically insolvable debris.

The tectonic elements and structural features of the Palaeozoic basin foundation surface feebly influence the distribution of the orebodies in the Cretaceous sediments within the IMOF area. Nevertheless, such influence at the Mynkuduk deposit is revealed in the locations of Akdala and East sections and in the orientation of the Central section (Fig. 7). At the Inkay deposit, the connection of the orebodies with the structural elements is less noticeable.

The Cretaceous and Palaeogene sediments are hydrogeologically complex, including a huge volume of the underground waters of the artesian Chu-Syrdarya Basin. The area of recharge is watershed of the Tyan-Shan mountain systems. The Tyan-Shan caused the hydrodynamic regime of Basin and the NW direction of the underground water movement. This direction was preserved despite the Karatau Range uplift in the Quaternary. The uplift of the Karatau Range had little influence on the basin hydrodynamic. Change of the mineralization and the direction of the underground waters are noticeable near the Range only, and are practically non-existent in the IMOF region. The discharge of the underground waters occurs in a direction away from IMOF. The natural velocity of the ground waters movement is not more that 2m/year. Mineralization of Cretaceous waters varies from 1 to 6 g/l. The Palaeogene water is fresh. It is a water source for use by the local population. IMOF aquifer characteristics are shown in Table 2.

2.2. BOZ development, formation and morphology of ores

The BOZ is a unique geological element of the environment of Southern Kazakhstan and greatly influenced the development and formation of the uranium ore. It is very important to examine the BOZ peculiarities because their existence is the main factor contributing to the development of such a large-scale ore formation.

The BOZ extends 500 km from the Tyan-Shan mountain range. The infiltration nature of the artesian basin and continued (beginning from Oligocene) period of BOZ development created very favourable conditions for BOZ expansion. In addition, the mostly speckled-coloured character of the basin sediments did not require essential oxygen consumption as the oxygen-bearing waters filtered through the permeable sediments. Therefore the redox front expanded such a significant distance and is located in the gray-coloured sediments of the palaeovalleys of the latitudinal extension at the Mynkuduk deposit and the meridional extension at the Inkay deposit. Karatau uplift did not influence the redox front position and formation of the orebodies. In any case, specific curvatures of orebodies in plan (Figs 6 and 7) show NW Tyan-Shan waters movement vector.

Expanding on the significant distance, the BOZ oxidated the large volume of sediments. The BOZ also mobilized and transported a large uranium quantity of uranium from the oxidated rocks. At the same time, organic material was also oxidated with the generation of hydrocarbon gases on the redox front. The accumulation of hydrocarbon gas has been determined through the analysis of drilling core samples. Grade hydrocarbon gases exceed normal level on the redox front up to 4-8 times. This fact is very important because it explains the large-scale ore formation on the redox front. The matter is that the mynkuduk and inkuduk horizon sediments are characterized by a low organic carbon content (not more than 0.04%).

This quantity is, apparently, not enough for the creation of the essential reducing conditions.

Therefore, the role of gases in assisting the creation of the essential reducing conditions is exceptionally important. The influence of the gas could also help explain the existence of the orebody over such a large area. In this way, BOZ directly fulfilled several functions. These include uranium mobilization from oxidated sediments and transportation in a dissolved condition over significant distances; reduce condition formation on the redox front; uranium

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precipitation on the redox front. In this case, the BOZ is the ore-generating and ore forming agent. Therefore, the uranium deposits formed in the friable sediments at the redox front should be named the BOZ deposit, as we name, for example, the vein deposits.

BOZ does not end simultaneously in the series as a whole. In connection with the different permeability levels of the different horizons and subhorizons, the BOZ is separated on several oxidation tongues penetrating the bed dip at different distance (Fig. 4). The inkuduk horizon has the most permeability, as will be shown below under the deposits descriptions. Therefore, the redox front in the inkuduk horizon extends the 10-18 km further than in the mynkuduk horizon (Figs 6 and 7).

In connection with the alluvial character of sediments, separated horizons and subhorizons are large sediment macrocycles. They are separated on the great number of microcycles with different permeability, in which small tongues from 1-2 to 5-10 m in thickness are developed.

At the ending of such tongues, in favourable conditions the orebodies are formed as rolls with different extension wings, and bed bodies which have a form depending on the lithological composition of sediments. Different conditions caused the variety of the morphology orebodies (Fig. 5). Nevertheless, the main morphology elements are the bag part and wing parts of the rolls. The bag part attains some 10-20 m in thickness, and the wing parts attains several metres as well. Uranium ore extends sometimes along the redox front for 10–20 km, forming the highly profitable orebodies.