LI RESIDENCY IN ALKALINE PALEO-LAKE SYSTEMS: THE PECULIAR CASE OF THE JADAR (SERBIA) DEPOSIT

The Jadar deposit, Serbia, represents a unicum of the Li mineralized volcano-sedimentary systems. Discovered in 2004, Jadar’s Indicated and Inferred Resources of 143.5 Mt contain Li at 0.838%, ranking it as one of the world’s largest hard rock lithium deposits. The main ore mineral is the unique Li-B-silicate Jadarite LiNaSiB₃O₇(OH), which provides significant B resources in addition to Li. This study deconvolves the mineralogical and geochemical Li and B residency during the evolution of the Jadar sedimentary basin. The deposit is hosted in a Miocene lacustrine basin occurring within the Adria-derived Jadar-Kopaonik terrane. Mineralization is largely hosted by dolomicrite and siltstone lithofacies interbedded with tuffs. The formation of the Li-B mineralization follows a proposed two-stage diagenetic model. Firstly, the lake system was marked by a low-standing of the water table and by high evaporation rates. This triggered a set of mineral formation processes that enriched alkalis and increased the pH of the system. Under these conditions, breakdown of volcanic glass in presence of Li-B-Na brines led to the early formation of Li-rich B-silicates. Geochemical data support Li fixation during high salinity conditions in the basin, and points to a sedimentary environment dominated by reducing conditions. Secondly, late-stage basin evolution characterised by subsidence and burial triggered the decay of the organic matter, which released organic and carbonic acids to the system. The pH shift prompted the partial dissolution of B-silicates and other B-phases. This reduction in pH led to a localized redistribution and geochemical decoupling of Li and B ending with the formation of late diagenetic Li-phosphates and secondary borates. Forthcoming analyses will use stable isotopes of the ore minerals and associated dolomite to help to constrain ore-forming conditions that have led to the development of this unique paragenesis.