Key Mineral Deposit Geology Research

1. Key characteristics of mineral deposits

      Compared with traditional bulk minerals, key mineral deposits are often characterized by “rarity”, “association” and “fineness”: (1) The crustal abundance of key metal elements is extremely low (generally 10⁻⁶ or less), and mineralization requires an extraordinary enrichment of hundreds or even tens of thousands of times the element, making the mineralization conditions very harsh . This also results in the extreme unevenness of the global distribution of key metal mineral resources; (2) Deposits formed independently by key metal elements are relatively rare. They are mostly produced in other deposits in the form of symbiotic and associated elements, with complex element combinations, such as key metals Ge, Ga, and Ga in coal-bearing systems. Li and U, etc., REE in vanadium-titanium magnetite deposits, sedimentary lithium associated with bauxite and Re, Se and Te associated with porphyry copper deposits, etc., are key metal resources; (3) the carrier minerals of key metal elements are usually very small and have extremely low content. They often exist in the form of adsorption, isomorphism and extremely small mineral inclusions, so they are not easy to be directly observed; (4) the geochemical behavior and mineralization mechanism of key metal elements are complex, the mineralization types are diverse, and the cognition is difficult; (5) the distribution of key metal minerals is extremely uneven globally, resulting in "bottleneck" and being "bottlenecked". The above geological attributes determine that the source tracing of key mineral elements, mineral exploration and evaluation theory and methods, and green and efficient extraction and separation technology of elements all face great challenges.

2. Research on Key Mineral Deposits

      The geochemical behavior and mineralization mechanisms of key metallic elements are complex, and the mineralization types are diverse and complex. In recent years, significant progress has been made in the research on the genesis mechanisms and mineralization conditions of key minerals. The current research status of several typical and important key minerals is summarized below.

2.1 Rare Earth Metal Deposits Associated with Granites

      These deposits are the main mineralization type for metals such as Li, Be, Rb, Cs, Nb, Ta, Zr, Hf, and W, including two subtypes: highly differentiated granitic granitic and alkaline granitic. Representative examples of the former internationally include the Sn-Nb-Ta-Li-Be rare metal mineralization in the Beauvoir intrusion of the northern part of the French Central Block. Examples of the latter are found in the Pitinga region of Brazil, the Khininy region of the Kola Peninsula in Russia, the Ilimaussaq region of Greenland, and the Thor region of northwestern Canada. Partial melting of crustal material, continuous energy supply, and large extensional structures conducive to highly differentiated magma evolution are often considered important conditions for the formation of granitic rare and rare earth metal mineralization. The enrichment of key metallic elements in these deposits is believed to be related to highly differentiated magma crystallization and hydrothermal alteration under complex tectonic metamorphic deep melting processes.

2.2 Rare Earth Metal Deposits Associated with Granite and Alkaline Pegmatites

      Ore minerals are the carriers of useful elements, and minerals and mineralization are symbiotic and co-evolving. Pegmatites rich in rare earth metal minerals such as spodumene, petalite, lepidolite, beryl, beryllium silicate, sodium beryllium silicate, and pyrochlore are rock types where rare earth mineralization is frequently found. Among them, deposits associated with granite pegmatites (peraluminous) mainly occur in the late orogenic period and post-orogenic period, with elemental assemblages of Li, Cs, Ta, as well as Nb, Rb, Be, Sn, B, P, and F, etc., closely related to S-type granites, and the ore-forming materials mainly originate from marine sedimentary materials such as black shale. Deposits associated with alkaline pegmatites mainly occur during non-orogenic periods, characterized by elemental assemblages of Nb, Y, F, and REE, as well as Ta, Ti, Zr, U, Th, and F, with relative depletion of B and they are often closely associated with A-type granites, representing co-originating magmatic evolution and mineralization. Besides granitic pegmatites, alkaline pegmatites are typically also associated with zirconium and niobium mineralization, along with rare earth element (REE) mineralization. The ore-forming melts/fluids of pegmatite-type REE deposits often exhibit low viscosity, high water content, high dispersibility, and high alkalinity. Three enrichment mechanisms—fluid immiscibility, highly crystallized differentiation of granitic magmas rich in fluxing components, and hydrothermal alteration—are commonly used to explain the formation of these deposits.

2.3 Rare-earth metal deposits associated with alkaline and carbonate rocks

      Many large rare earth (REE) deposits discovered worldwide are associated with alkaline carbonate rocks, primarily concentrated in a few countries such as the United States, Brazil, and Australia. These deposits are generally characterized by their large scale, high grade, and complex composition, with light REEs being a prominent feature. Heavy REE deposits also occur in some carbonate rocks, such as the Lofdal carbonate-type REE deposit in Namibia. These deposits are mainly distributed along the margins of ancient cratons. The ore-forming rocks are typical mantle-derived carbonate rocks, originating from enriched mantle altered by subducting slabs. Early subducting slab alteration and the recycling of REE-rich oceanic sediments are likely the main reasons for the REE-rich mantle in the source region. The mineralization process often involves a magmatic stage, a pegmatite stage, and a hydrothermal stage, with REE precipitation and mineral formation primarily occurring during the hydrothermal stage. The ore-forming fluids also undergo a transformation from magma to hydrothermal, exhibiting characteristics of high temperature, high salinity, and high CO2 content. The fluid evolution process in carbonate rocks is very rapid, which is not conducive to the occurrence of long-distance mineralization. Therefore, rare earth mineralization may occur in the carbonate rock body or in the surrounding rocks adjacent to the carbonate rock body.

2.4 Rare and Precious Metal Deposits Associated with Mafic-Ultramafic Rocks

      Key metallic element deposits associated with mafic-ultramafic rocks mainly include platinum group elements (PGEs) and cobalt (Co) deposits in magmatic copper-nickel sulfides, as well as chromite deposits in layered intrusions and ophiolites. Ni, Cu, and PGE possess unique geochemical properties distinct from other trace elements, making them closely associated with mantle-derived magmatic sulfide deposits. PGE has an extremely high sulfide magma/silicate magma partition coefficient, thus it often occurs in layered intrusions and copper-nickel sulfide deposits, such as the Bushveld Complex in South Africa, the Great Dyke Intrusion in Zimbabwe, the Noril’sk Intrusion in Russia, the Stillwater Intrusion in the United States, and the Sudbury and Lacdes Iles Intrusions in Canada. The first three contain over 90% of global PGE resources. Chromite deposits in large, layered rock masses are generally enormous. The Bushveld, Great Dyke, and Stillwater rock masses contain approximately 70% of the world's chromite resources. Pit-shaped chromite deposits in ophiolites are more common, with high grades but usually relatively small sizes. Only a very few form super-large deposits with reserves exceeding 100 million tons, such as the Kempirsai deposit in Kazakhstan and the Ray-Iz rock mass in the Polar Urals region of Russia. The magmatic processes associated with PGE mineralization are highly complex. Studies of many deposit examples have revealed that when iron-rich magma originating from the mantle reaches sulfide saturation, it melts away sulfide melts rich in PGE. Later hydrothermal activity can also reactivate PGE within the sulfides, leading to secondary enrichment and mineralization.

2.5 Hydrothermal Sulfide Deposits with Associated Rare and Dispersed Metals

      Rare and dispersed elements in nature are mainly distributed in a dispersed state within minerals composed of other elements. They are usually considered as associated components of polymetallic deposits, and therefore many scholars believe they cannot form "independent deposits." The extraordinary enrichment of rare and dispersed elements often requires very harsh conditions and special geochemical processes. For example, the extraordinary enrichment of Re is constrained by mantle melting processes and the recycling of oceanic crust sediments, as well as magmatic oxygen fugacity and degassing processes; In is closely related to Sn, and the crystallization process of mafic minerals in magma and volatile components control the amount of In in the ore-forming hydrothermal fluids; the mantle-derived nature, high oxygen fugacity, and high volatility of alkaline rocks promote the migration and enrichment of Te; the enrichment degree of Cd varies significantly among different types of Pb-Zn deposits. Some rare and dispersed metal elements are mostly found in hydrothermal and sedimentary sulfide deposits, such as Ge, In, Se, Cd, Te, and Tl. In copper deposits, Se and Te are mainly associated; in cassiterite sulfide deposits, In is mainly associated; in Pb-Zn deposits, Ge, In, Ga, and Cd are mainly associated; Ti is mainly enriched in pyrite of hydrothermal deposits; and Te (tellurite) can be found as an associated mineral in some epithermal gold and silver deposits. Furthermore, Ga is commonly associated with bauxite, and gibbsite is one of the carrier minerals of Ga. In germanium-bearing coals, Ge mainly exists in humic bodies in an organic-bound state. Over the past 20 years, research on rare and dispersed metals has mainly focused on deposit types and mineralization specificity, resource reserve assessment, and mechanisms of extraordinary enrichment. It has also broken through the misconception that "rare and dispersed metals cannot form independent deposits," discovering some independent rare and dispersed element deposits. Taking Te-rich minerals (such as tellurite) in hydrothermal gold and silver deposits as an example, Te is often produced in association with gold and silver minerals and sulfides. The ore-forming fluids are generally of medium to low temperature and medium to low salinity, and are weakly acidic to neutral. They have a high tellurium fugacity (fTe2). In hydrothermal gold and silver deposits, Te mainly comes from the mantle, magmatic hydrothermal fluids and host rocks.

2.6 Sedimentary Lithium Deposits

      Sedimentary lithium deposits mainly include two subcategories: clastic rock type and carbonate rock type. Currently, the development and utilization of sedimentary lithium deposits globally is not high. Apart from the Jadar super-large lithium-beryllium deposit in Serbia, most are still in the exploration stage and have not yet achieved actual production. Other major reported sedimentary lithium deposits worldwide are mainly distributed in the western United States, western Mexico, southeastern Peru, and Outer Mongolia. Clastic clay-type lithium deposits are mostly associated with bauxite, coal, and sedimentary potash deposits. Li mainly exists within the crystal lattice of montmorillonite group minerals or illite, belonging to structural lithium. Carbonate clay-type lithium deposits are closely related to the weathering and deposition of carbonate rocks. Li mainly exists in the montmorillonite facies via adsorption. Reducing, low-energy, stagnant, and confined paleogeographic environments are conducive to Li enrichment. In addition to Li, sedimentary lithium deposits may also contain enrichments of Ga and REE.

2.7 Key metallic deposits associated with weathering and sedimentation

      These secondary weathering-enriched deposits encompass most key mineral types, including rare earth, rare dispersed, and rare precious metals. They primarily include brine-type lithium deposits, adsorbed rare earth deposits in offshore mud, ion-adsorbed rare earth deposits from granite weathering and leaching, and cobalt and rare earth element resources in oceanic manganese nodules and crusts. Based on mineralization conditions and existing data, it is inferred that Southeast Asia may contain abundant ion-adsorbed rare earth deposits. Weathering crust-type key metal deposits mainly originate from the weathering of felsic magmatic rocks and basic-ultrabasic rocks, with the ore body composition showing a strong inheritance from the parent rock. The activation and migration of these elements are closely related to groundwater activity and their adsorption and desorption in secondary minerals, ultimately leading to the enrichment of these elements into minerals.