Targeting REE deposits associated with carbonatite and alkaline complexes in northeast India

Rare earth elements (REEs) are considered critical metals globally. About 62% of the global resources of REEs occur associated with carbonatites and alkaline complexes. However, the entire production of REEs in India currently comes from monazite-bearing beach sands, although a variety of REE enriched source rocks, particularly carbonatites and alkaline complexes occur in different parts of the country. There is, therefore, a significant potential in the county for new REE deposit discoveries associated with carbonatites and alkaline complexes. This paper describes a generalised carbonatite-alkaline complex related REE mineral systems model and applies a knowledge-driven model to demarcate REE exploration targets in the Karbi-Meghalaya plateau, NE India. The main components of the mineral systems are (1) pockets of metasomatised subcontinental lithospheric mantle (SCLM) which form fertile source regions for REE-bearing fluids; (2) extensional geodynamics; (3) permeable lithosphere architecture for tapping REE-enriched fluids from SCLM and focusing them to near-surface levels, and (4) a post-emplacement tectonic regime that preserves the deposits. Spatial proxies representing each of these components are mapped and integrated using fuzzy inference system (FIS) to identify prospective targets. Systemic and stochastic uncertainties associated with the model were quantified to aid target selection and further work. Main recommendations of this exercise are the following: (1) project-scale ground exploration for the Sung valley and Jasra complexes; (2) further regional-scale data collection for the Mikir Hills in the areas surrounding Samchampi and Barpung Complexes and around the swarms of dykes in the Garo Hills around Swangkre; (3) follow-up exploration in the areas north of Silchar and south of Nongstoin and (4) detailed geochemical sampling and surface or air-borne radiometric surveys for the Mawpyut Ultramafic Complex. The REE-mineral-systems model and the workflow demonstrated in this article could be used for targeting REE deposits in geologically similar terrains worldwide.