Abstract:
The Karaotkel deposit of fossil Ti-Zr placers is situated in the north-western edge of the Zaysan intermontane depression in Eastern Kazakhstan. The geological structure of the mineral sands is characterized by complex morphology and a two-layered assembly, formed as the result of long-term regional geodynamic processes during the Meso- and Cenozoic. Our study is aimed to establish a genetic model for the formation of the continental Karaotkel Ti-Zr placers by utilising novel data on geology, mineralogy, geochemistry and geochronology. Sedimentological facies analyses indicate an environment of coastal wave-built formation of the Late Cretaceous kaolin-dominated weathering crust and Paleogene sediments of the North Zaysan formation. A set of geochemical indices is applied on the base of key geochemical parameters of placer composition to allow quantitative and qualitative assessment and to establish genetic trends. Mineralogical composition of the heavy fraction of the productive sediments of the Ti-Zr placer (horizon “A” of the North Zaysan formation) was determined by XRD and microprobe-probe analyses. Microprobe analyses determined the elemental composition of mineral grains of zircon, ilmenite, monazite, staurolite, titanite, epidote and gold. Gold and monazite were found also as micro-inclusions in minerals. The sediments of the horizon “A” are characterized by elevated grades of light rare-earth elements (LREE) over heavy rare-earth elements (HREE). The weathering processes reveal a gradual transition from a moderately warm and humid climate to a cold and dry climate as the placer deposit was formed. The intercalations between horizontal- and crossbedding characterize the conditions of placers’ sedimentation by eluvial, eluvial-alluvial, alluvial-lacustrine and lacustrine environments. U-Pb dating of zircons from the productive horizon “A” of the Karaotkel placer deposit showed a crystallization age of 287 ± 1 Ma. This is identical with the age of the Karaotkel-Preobrazhenka multi-phase igneous intrusion suggesting that it can be considered as the source of the Karaotkel Ti-Zr placer. In addition, comparative analyses of elemental composition of zircons confirmed that monzonites and syenites of earlier phases of the Karaotkel-Preobrazhenka intrusion are the likely source rocks for the Karaotkel mineral sands. The formation of these Ti-Zr placers comprise the following stages: 1) establishment of the bedrock of the Karaotkel-Preobrazhenka multiphase igneous intrusion of the Permian Cisuralian epoch; 2) development of weathering crust during Upper Cretaceous period; 3) washing-out of the weathering crusts during the North Zaysan formation in the Paleogene, and 4) differential redeposition by temporary streams that resulted in the accumulation of Ti and Zr minerals.
