Geological Structure of the Severniy Aurtas Field
According to geological and geophysical studies the geological structure of the Aurtas area is typical of young Epihercynian platform the folded basement of which is composed of dislocated rocks of Permian-Triassic complex. The platform cover is represented by sediments of the Jurassic, Cretaceous, Paleogene and Neogene systems. Widely developed are the Quaternary sediments of different genetic type.
The Aurtas field is inclined to the area of convergence of the western pericline of Karasyaz-Taspask anticline and eastern pericline of Kuyulusk structure. From the south these structures border with Zhetybai-Uzen tectonic stage.
The bedding angle of the sediments is within 3-5 degrees. Given the existence of sharply reduced (in terms of the platform cover) regional breaks, angular unconformities and nature of occurrence of the layers four structural stages: Retleyasov, middle Jurassic – lower Miocene, middle-upper Miocene and upper Paleocene-Quaternary. The Aurtas field is timed to the third (middle Miocene) structural stage.
The Aurtas field is located within the northern part of the plateau-like upland, the surface of which is enclosed by layer of ore-bearing limestone-coquina of middle Sarmatian age. The escarpment of the plateau exposes underlying sediments of Miocene and Paleogene. In the fore-escarpment valleys on the surface there exposed rocks of Cretaceous.
The upper Albian substage (Kal) is developed at the eastern border of the field, and represented by sands, siltstones with horizons of spherical nodules and subordinate interbedded clays. The section is crowned with nodular phosphorite horizons.
- Learned thickness amounts to 35 m.
The Cenoman stage (Ksm) is represented by clays (at the bottom), siltstone and sandstone with horizons of spherical nodules. At the bottom of the stage the horizons of nodular phosphorite are registered.
The Senonian superstage (Ksn) begins with a layer of marl which is gradually replaced up the section by writing chalk and less by sandy limestones.
- The total thickness of the Senonian sediments is of 80-120 m.
The Danian stage (Kd-P) is normally represented by bryozoan limestone, sometimes slightly sandy.
- The thickness of the Danian ranges within 25-65 m.
The Paleogene system (P) has a broad development in the field area and the surrounding areas. The Paleogene sediments occur with angular and stratigraphic unconformity at different horizons of Cretaceous. In the Paleogene amount the following stages are allocated: Simferopol, Bodrak and Almin, as well as undifferentiated Oligocene sediments, occurring concordantly in the section.
The Simferopol stage (Ps) is represented by two batches: lower-sand-carbonate and upper-carbonate.
- The thickness of the stage reaches 42 m.
Bodrak stage (Pb) includes the layer of greenish-gray, cinnamon-brown and chocolate clays interbedded with marl and inclusions of faunal remains.
- The thickness of the stage is 35 meters.
The Almin stage (Ra) is complicated with monotonous layer of white, loose marcasite nodules. In the south-western part of the field in the section there are registered veinlets of colorless celestine-calcite.
- The thickness of the sediments reaches 20-25 m.
The Oligocene (P) is represented by a monotonic layer of greenish-gray clays interbedded with occasional thin siltstone and marl nodules.
The Neogene system is universally developed within the field and lies with erosion and angular unconformity at various horizons of older sediments.
In the section of the Neogene there are distinguished (bottom-up) Konksk horizon, lower and middle Sarmatian substages.
The Konksk horizon is represented by clays with lenticular interbeds of organogenic-clastic limestone. At the bottom of the horizon there lies a basal conglomerate.
- The thickness of the horizon is 5.5-9.5 m.
The Sarmatian stage is represented by the lower and middle substages.
The lower substage is composed of clays interbedded (0.1-0.5 m) with marl and limestone. The clays are usually plastered with abundance of fauna remains.
- The thickness of the sediments reaches 17 m.
The middle Sarmatian substage is represented by two batches: lower one – sand and clay interbedded with detrital limestones and upper one – represented by limestone-shell rock.
The lower batch is ore-underlying horizon, and the upper batch is ore-bearing.
The ore-bearing limestone horizons usually have the following structure: organogenic, organo-detrital, oolitic and oolite-organo-detrital. The limestone is usually composed of pelecypod fragments of different size and shape, cemented by cryptocrystalline carbonate material and celestine-barite.
- The thickness of the sediment reaches 30 m.
The Quaternary sediments are represented by loam and sand clay of different genetic types.
- Their thickness does not exceed 2-3 m and is occasionally up to 5 m.
Mineralization – Three Types
Within the Aurtas field three types of mineralization are distinguished according to structural textural features.
The first type – nested and vein-disseminated mineralization in limestones of upper Sarmatian age and is universally developed in the northern and north-eastern wing of the ore-bearing area. Here ore minerals are recorded in the form of disseminations, veinlets and nests scattered.
In the areas of fracture zones there being formed quite rich, densely disseminated and massive ores. This type of mineralization has unbalanced development both in the plan and at the section of the ore thickness, forming localized lens 0,1-3,3 m thick and characterized by the content of strontium oxide from 0.1 to 35.34% (12.99% on average) and barium oxide from 0.39 to 14.12% (or 5.09% on average).
Structural and Technological Ore Types
Within the Aurtas field there are distinguished disseminated, medium-thick disseminated and massive ores.
The disseminated ores contain:
- up to 15% of barium and strontium oxides and are defined as poor in quality
- medium-disseminated ores contain from 15 to 20% of barium and strontium (of medium quality)
- thick disseminated ores contain from 20 to 30% (rich)
- and massive ores containing more than 30% of barium and strontium oxides are characterized as highly rich in ore quality.
According to technological properties of the three types of ores are distinguished:
The criterion for allocation of these ore types was the concept of barium module estimated by the findings from analyses of the laboratory and technological studies.
Strontium Oxide, Barium Oxide Resource
The celestine ores are characterized by the ratio of strontium oxide and barium oxide (barium module) equal to or greater than 4, barite-celestine ores have barium module equal to 1 to 4, and barite ore equal to less than 1. The last two types of ore are primarily developed in the contours of the North site.
Morphological types of ores
The ore horizon composes a single interstratial ore deposit of stratiform type with horizontal bedding.
From the north-west and south-east the ore deposit is limited by escarpment (steep cliffs) of the Sarmatian plateau; the north and north-east border is controlled by the line of contact of Sarmatian limestones and underlying Neogene and Cretaceous rocks. From the south the ore deposit tapers out; the border loses its continuity and is registered by individual isolated ore blocks.
The thickness of the ore horizon varies from a few centimeters to 11.5 meters, with an average of about 3 m. The total area of the Aurtas field is about 110 square meters. km., of which about 40% is barren area.