Phosphorite deposits: A promising unconventional resource for rare earth elements

• A comprehensive review of phosphorite deposits of different types of the world. • Their formation mechanisms through geological time, geochemistry, mineralogy, and REE-enrichment mechanisms. • The use of machine learning models to predict REE distribution in phosphate ore deposits. • Recovery techniques, elemental, isotopic and mineralogical characterization, and environmental issues. The green energy transition relies heavily on critical metals, such as rare earth elements (REEs). However, their reserves are primarily focused in a few countries, such as China, which accounts for approximately 70% of global production. Hence, several countries are currently looking for alternative resources for REEs. Alternative REE resources in the supply chain include recycling of e-waste, industrial waste like red mud and phosphogypsum, coal ash, mine tailings, ocean floor sediments, and even certain types of sedimentary deposits like phosphorites where REEs are present in lower concentrations but at larger volumes compared to primary ore deposits which are becoming targets by REEs industry. Currently, several studies are going on the development of eco-friendly REEs extraction technologies from phosphorite deposits. Consequently, advanced data analysis tools, such as Machine Learning (ML), are becoming increasingly important in mineral prospectivity and are rapidly gaining traction in the earth sciences. Phosphorite deposits are mainly used to manufacture fertilizers as these rocks are known for their significant phosphorus content. Moreover, these formations are considered a prospective resource of REEs. The different types of phosphorite deposits such as continental, seamount, and ore deposits worldwide reported concentrations of ∑REE upto 18,000 µg/g. Due to the augmented claim of REEs for various ultra-modern, and green technology applications that are required to switch over to a carbon–neutral environment, these phosphorite deposits have become an important target mostly because of their relatively higher content of REEs especially heavy rare earth elements (HREE). For example, Mississippian phosphorites reported ∑HREE 7,000 µg/g. To have a comprehensive understanding of the REEs potential of these phosphorite deposits which also include several Chinese phosphorite deposits, this study is undertaken to review the phosphorite deposits in the world and their REEs potential, in addition to some of the associated aspects such as applications and formation mechanisms for different types of phosphorite deposits such as igneous phosphate deposits, sedimentary phosphorite deposits, marine phosphorite deposits, cave phosphate deposits, and insular guano deposits. Other important aspects include their occurrences, types, geochemical characteristics, the REEs enrichment mechanisms, and various recovery methods adopted to recover REEs from different phosphorite deposits. The present review paper concludes that the recent studies highlight the global potential of phosphorite deposits to satisfy the increasing demand for REEs. Extracting REEs from phosphorite presents no significant technological or environmental difficulties, as long as radioactive elements are eliminated. In India, more comprehensive geological surveys, along with the advancement of new methods and evaluations, are required to locate phosphorite deposits with high concentrations of REEs.