Devonian Evaporites of South Kazakhstan: Occurrences, Evidence for Dissolution from the Balaturlan Breccia Outcrop, and Implications for Lithium Brines and Sedimentary-Hosted Copper

Abstract

Although the occurrences of Famennian (Devonian) salt diapirs in Shu-Sarysu and Syr-Darya basins are documented during petroleum exploration in the 1970s, very little is known about their depositional systems, post-depositional histories, and potential role in copper mineral systems and lithium-enriched brines. In the absence of subsurface cores and direct outcrop exposure, this thesis first examined the age-equivalent strata in the Bolshoi Karatau and Zhambyl Mountains (outcrop study, petrographic analysis, 3D models) and then, applying sound geological principles, integrated them with data from previous regional studies and unpublished well reports. The main study results about this Famennian unit include a range of novelties such as: i) Extremely large (several thousands of square kilometres) coeval paleo-evaporite system comprised of 3 distinct sub-environments: deep-water salts to the South (Syr-darya basin), lagoonal evaporites to the North (Chu-Sarysu basin), and top-reef evaporites in between (Bolshoi Karatau); ii) The facies associations and an abundance of evaporite pseudomorphs confirm that 50-70m thick and basin-scale developed Balaturlan Breccia is a product of salt-dissolution of saltern platform evaporites, by meteoric waters during periods of sub-aerial exposures; iii) The subsurface anhydrites layers and breccia occurrences in both basins are remnants of halokinetic movements of mobile salts during the formation of diapirs coupled with the evaporite dissolution processes, while an abundance of gypsum veins in the outcrop are product of anhydrite transformation during the uplift (Ulanbel); iv) Recently discovered lithium brines in Upper Devonian and Lower Carboniferous reservoirs are directly related to dissolution of the Devonian evaporites. Unlike other basin evaporites (eg, Permian), these were enriched by lithium during the deposition by influx of lithium ions sourced from weathering and leaching of vast paleo-exposures of the Devonian granites, volcanic rocks and ash. Additional enrichment may have occurred through post-depositional leaching of these rocks or from deeper sources; v) The exploration perspectivity for sedimentary-hosted copper deposits should be focused on areas with potential redox interface, which is formed by the interaction of oxidized saline brines with either reduced hydrocarbon accumulations in the Famennian or gray sandstones in the Carboniferous. Chloride ions derived from the dissolution of the Devonian evaporites act as the carrier of the leached copper. Implications of this study are significant as they provide important information for refining the palaeogeography and paleoclimate of the region, point to multiple economic and exploration significances, and demonstrate the workflow for identifying paleo-evaporite systems in other basins worldwide.