Origin of Oligocene channel ironstones of Lisakovsk deposit (Turgay depression, northern Kazakhstan)

Abstract

Channel ironstone deposits (CID) represent a unique variety of iron-rich sedimentary rocks, which are mostly found in Northern Kazakhstan and in Western Australia. The origin and the depositional conditions of this rare variety of ironstone are poorly understood. This study investigates the nature of the Lisakovsk CID and assesses the iron source(s) and the paleo-depositional conditions. Four lithofacies representing fluvial and deltaic channel environments have been identified. Ooids, peloids and micro-oncoids are allogenic, while proto-ooids formed authigenically. (Hydro-)goethite and botryoidal cements formed during the last stage of diagenesis. Upper Cretaceous marine ooidal ironstones are the main iron source for the Lisakovsk CID, sourcing most of the metal content. Similarly, major oxides and trace metals of the Lisakovsk CID are derived primarily from the underlying marine ironstone. However, the REE patterns reveal a complex origin of the CID, including hydrogenic, diagenetic and hydrothermal. The Lisakovsk deposit formed in three stages. The first stage involved the uplift and erosion of Cretaceous ironstone and the establishment of paleo-drainage pattern along major faults. During the second stage, iron-rich detritus got redeposited along fluvial channels. The cement of the original ironstones was dissolved, while ooids, peloids, and micro-oncoids remained unaltered in the fluvial and deltaic channel environments. During the final stage, ironstone cementation with proto-ooids and rare metals took place during the diagenesis.