Carbonatization and ore-mineralization in listvenite-bearing mantle/slab transition zones: Insights from OmanDP and laboratory experiments – LISZT

LISZT aims at identifying and modelling the thermo-hydro-chemical mechanisms of CO2-metasomatism producing mantle-derived listvenites and at defining criteria for evaluating their potential as sources of rare and critical metals. It brings together 3 French research institutes internationally recognized for their expertise in studying hydrothermal alteration of CO2-fluxed ultramafic rocks. LISZT proposes an approach combining field studies of CO2-mineralization in mantle rocks and listvenites, taking advantage of the ongoing Oman Drilling Project, with laboratory experimental and modelling investigations to assess what controls ore-rich mineralizations in listvenites. LISZT addresses the challenge “Knowledge of mineral resources, ore-deposits and environmental impacts” of Axis B.1.1 in ANR Action Plan 2018.
The LISZT research program is organized into 3 workpackages (WPs):
WP1 investigates the links between the micro- to decameter-scale heterogeneities in the structure (microtomo- to core scale analysis), mineralogy and petro-geochemistry of listvenites and associated ophicarbonates and serpentinites (redox; major and trace elements; halogens; oxygen, carbon and clumped isotopes; Fe-Zn-Cu isotopes) and the basement hydrodynamic properties (GM, IPGP). It focusses on OmanDP Site BT1, where was drilled for the first time the transition from listvenites to the possible source of CO2-rich fluids: the ophiolite metamorphic sole. Compared to ongoing studies of OmanDP Site BA1/2, where serpentinization and CO2-mineralization are still active, LISZT results will allow discriminate between ongoing and earlier (slab-derived) CO2-metasomatism and identify the parameters controlling mineralization in the Oman listvenites.
WP2 experiments will investigate the feedback mechanisms controlling the hydration, oxidation and carbon mineralization and silicification reactions associated to CO2-rich fluid flow into ultramafic rocks with a special focus on the mobility of metals (GET, GM). GET and GM have developed the techniques and approaches to study carbonatization reactions in ultramafic rocks and provided data on the efficiency of CO2-trapping reactions in Mg-bearing carbonates and pointed also to the associated changes in silica activity. The LISZT experimental program builds upon this expertise. GET experiments will aim at assessing the effect of CO2-rich fluid compositions (XCO2, K, Na and Cl concentrations) on the mobility of metals concentrated in mantle rocks over the range of temperatures (up to 300°C) expected during the formation of listvenites. GM experiments will investigate the effects of the structural and chemical heterogeneity of natural rocks on the rates and sustainability of the reactions.
WP3 (GM, GET) aims at modelling the processes driving the mineralization of these economically-valuable metals and to derive conclusions concerning the genesis of listvenites occurring around the world. It builds upon the results of WPs 1-2 that will identify and quantify the thermodynamic (P,T,X) and physico-hydro-geochemical parameters controlling carbona(tiza)tion efficiency (permeability changes, dominant products, cracking) in relation to silicification mechanisms. This information will be used to parametrize numerical reactive-transport models and perform sensitivity analysis and conclude on the potential sources of ore-mineralization: a control by the composition and structure of the host-rock (remobilization hypothesis) or by fluid composition (contamination hypothesis). We will use the modeling platform developed at GM for multi-scale thermo-hydrochemical simulations. Simulations will be performed at the National Academic Computer Center (CINES, Montpellier). Finally, an up-to-date review of the geological (occurrence, environment) and geochemical data available on listvenites will allow testing the model and resulting criteria to assess the ore-potential of listvenite outcrops depending on their geological environment.