Genesis of Granite Pegmatite-type Lithium Deposits

1. Geological Characteristics of the Deposit

As the lightest metal in nature, lithium is highly reactive, leading to tectonic constraints on the formation of endogenous lithium deposits. Globally, pegmatite-type lithium deposits mainly occur in relatively stable, closed tectonic environments, such as ancient crystalline shields and blocks; however, some occur in active orogenic tectonic environments (such as the Kings Hill spodumene deposit in the Appalachian Fold Belt of the United States). Dome structures, anticlines, and synclines are the main tectonic environments for fold-controlled granite pegmatite-type lithium deposits.

Granite pegmatite-type lithium deposits are often hosted in metasomatic pegmatite veins on the upper (or edge) parts of large granite intrusions, occurring in stratiform or vein-like patterns. Previous studies have shown that the occurrence of granite pegmatite-type lithium deposits is also influenced by tectonic fractures. Rich in volatiles and rare elements such as lithium, pegmatite magma separates from the parent granite magma, rises and fills structural fissures in the surrounding strata, and slowly crystallizes and fractionates in a relatively closed and stable environment to form pegmatite veins of various types, including microcline, microcline-albite, albite, albite-spodumene, and lepidolite (silica). In pegmatite deposit zoning, industrially valuable lithium ore bodies are mostly hosted in albite-spodumene pegmatite zones and quartz-spodumene pegmatite zones.

The mineralization of pegmatite veins is related to the spatial distribution distance between pegmatite bodies and follows certain patterns. From the edge of the pegmatite body outwards, the degree of pegmatite evolution changes from simple to complex, and the degree of rare metal mineralization changes from weak to strong. The outermost edge of a complete vein system is a quartz vein zone. Granite pegmatites far from the parent magma chamber have good mineralization, while those close to the parent magma chamber have poor mineralization. Generally, early-formed rare metal deposits are located close to the parent rock mass and are relatively small in scale. Those located far from the parent rock mass are mostly large-scale late-stage deposits, with magma migrations over longer distances and a longer mineralization period, resulting in more complex mineral zoning and a higher degree of geochemical differentiation. In addition to lithium, other rare metals, including beryllium, niobium, tantalum, and tin, also show good mineralization.

Figure 1. Zoning of granite and pegmatite as differentiation increases.

Pegmatite-type lithium deposits can be divided into two main categories based on the presence or absence of banded structures:

1) Banded pegmatite-type lithium deposits, such as the Greenbushes lithium mine in Australia, are typical examples. They have a complex composition, with spodumene content around 20%. Besides lithium-bearing minerals, they often contain other rare metal minerals that can be comprehensively utilized, such as beryl and niobite.

2) Non-banded pegmatite-type lithium deposits, such as the Kings Hill deposit in the United States. Spodumene content can reach about one-quarter of the total rock mass. These are mostly independent lithium deposits, and may also contain small amounts of beryllium and tantalum. Spodumene is the main industrial mineral in pegmatite lithium resources; about 30% of the lithium extracted from the ore each year comes from spodumene. As an abundant ore lithium resource, spodumene is a very stable chain-structure aluminosilicate mineral, often occurring in platy or columnar forms, and can also be found in rod-shaped or dense cryptocrystalline massive aggregates. Its Li₂O content can reach 3%–8%. In addition, lithium is also found in minerals such as lepidolite, petalite, lepidolite, and nepheline.

Globally, granite pegmatite-type lithium deposits occur in stable shields, blocks, and active orogenic belts. Granite pegmatite-type lithium deposits are among the most highly evolved products of granitic melts; for example, they can develop large spodumene crystals. The granite and pegmatite exhibit spatial zoning, and the host rocks are mainly granite pegmatite and altered granite. They are often found at the edges of granite intrusions and are frequently associated with various rare metals such as beryllium, niobium, and tantalum.

2. Diagenetic and Mineralization Ages

Previous studies have extensively investigated the diagenetic and mineralization ages of granite pegmatite-type lithium deposits. Granite pegmatite-type lithium deposits occur from the Archean to the Cenozoic. Among them, the Pilgangoora deposit in Australia, with a mineralization age of Archean, is a world-class lithium-tantalum deposit, intruding within the Pilbara Craton greenstone belt.

The Bikita deposit in Zimbabwe contains mineralized pegmatites with a mineralization age of 2617 Ma. This deposit is the world's largest lepidolite mine, located within the Archean Shield of South Africa and Zimbabwe, an area containing some of Africa's oldest rocks. The Tanco lithium deposit at Lake Bernick, with a mineralization age of 2600 Ma, is located in the Canadian Shield of North America. This deposit is associated with Archean microcline granites. The Greenbushes pegmatite-type lithium deposit in Australia underwent three stages of mineralization: pegmatite crystallization at 2527 Ma, alteration of the pegmatite into minerals under hydrothermal activity at 2430 Ma, and reactivation into minerals around 1100 Ma through deformation and metamorphism. The pegmatites in the King Mountain lithium deposit in the Appalachian Orogen in the United States formed between 373 and 265 Ma, dating to the Hercynian period of the Paleozoic. The youngest large granitic pegmatite-type lithium deposit discovered to date is located in the Hindu Kush Mountains of northeastern Afghanistan, where the pegmatites are mostly genetically related to Oligocene two-mica granites.

Figure 2. Metallogenic epoch map of typical granite pegmatite-type lithium deposits worldwide.

3. Properties and Evolution of Ore-forming Fluids

Based on previous analyses of the properties of ore-forming fluids in granite pegmatite-type lithium deposits, it is believed that the ore-forming fluids in these deposits are predominantly low-salinity fluids generated under medium-high pressure and medium-temperature conditions. The formation of the ore-forming melts and fluids in the pegmatite system is thought to have two main stages: 1) a magmatic stage at 600–850°C, where the pegmatite melt differentiates from the granite; and 2) a magmatic-fluid transition stage at 300–700°C. At 500–700°C, the fluid primarily forms abundant beryl crystals; at 500–600°C, it primarily forms abundant spodumene crystals; and at 300–450°C, a water-sodium chloride-carbon dioxide system forms within the fluid. These two stages are not independent; crystallization differentiation is the dominant process, permeating the entire process. As crystallization differentiation progresses, hydrothermal metasomatism also intensifies. Early crystallization differentiation has little effect on the accumulation of rare metal elements, resulting in low mineralization. As the magma evolves, crystallization differentiation continues, and ore-forming elements such as Li enter the exsolution fluids of the residual magma, leading to Li-rich characteristics in later-stage fluids. The mainstream view holds that pegmatites are the result of highly differentiated magma, and that the rare metal content is positively correlated with the degree of differentiation and evolution of the pegmatite magma; generally, as the differentiation and evolution of the pegmatite melt deepens, rare metals such as Li continuously accumulate.

During the migration of ore-forming fluids, under reducing conditions, gases such as CO2 and CH4 readily combine with elements such as Li in the fluid to form relatively stable complexes, enhancing the migration and enrichment capacity of Li. When the ore-forming fluid migrates to open systems such as fractures, the pressure drops rapidly, resulting in decompression boiling. CO2 gas escapes from the water-sodium chloride-carbon dioxide system, leading to fluid phase separation and fluid immiscibility; the water-sodium chloride-carbon dioxide system separates into two phases: water-sodium chloride and carbon dioxide. Simultaneously, volatile gases such as CO2, CH4, and N2 separate from the fluid, significantly altering the properties of the ore-forming fluid and leading to the aggregation and mineralization of lithium-bearing minerals under suitable conditions. Spodumene is the ore-bearing mineral in most granite pegmatite-type lithium deposits. This is because spodumene is the earliest lithium-rich mineral to crystallize in magma, and when the temperature and pressure conditions are 320℃ and 162.12–405.30 MPa, petalite can also decompose into spodumene and quartz. Boiling of the ore-forming fluid or liquid immiscibility leading to phase separation is an important mineralization mechanism in pegmatite-type lithium deposits. Furthermore, volatile components (F, P, CO2) also play a significant role in the migration, enrichment, and precipitation of granite pegmatite-type lithium deposits. Different magmatic and hydrothermal processes can lead to the mineralization diversity of granite pegmatite-type lithium deposits and their associated rare metals.

4. Sources of Mineral Formations

Favorable geodynamic settings for lithium enrichment in endogenic magma occur during the late orogenic period, typically back-arc accretion, contemporaneous with continent-continent collisions, and related to local crustal thickening. The post-orogenic extensional environment favors lithium enrichment in exogenic processes. Due to its moderately incompatible nature, Li tends to accumulate in silicate melts and the crust during partial melting and crust-mantle differentiation, respectively. In the magmatic evolution sequence (ultrabasic to acidic rocks), Li content gradually increases, reaching its highest levels in granites and pegmatites at the very end. Currently, there is some debate regarding the genesis of rare-metal granite pegmatites. Some scholars believe that granite pegmatites are formed by low-level partial melting of lower crustal material. However, most granite pegmatites are genetically and spatially related to granites, and lithium granites are often accompanied by lithium granite pegmatites. Therefore, the magmatic crystallization differentiation genesis of pegmatites is widely accepted. The Neoarchean pegmatite-type lithium deposits of Pilgangrooa and Greenbushes in the Pilbara region of Australia are associated with highly differentiated two-mica granites and monzogranites. The Beauvoir albite granite deposit in France is a super-large lithium-beryllium deposit with granite-type lithium deposits developed at its edges. Lithium-beryllium pegmatites are LCT-type pegmatites, mostly exhibiting compositional characteristics of S-type granites.

Granite-type lithium deposits are generally considered endogenous, but in reality, the source of lithium in these deposits remains unresolved. Recent studies indicate that lithium mineralization has a "cyclical" characteristic, capable of being both "endogenous and exogenous," or "exogenous and endogenous." Lithium in brine can be obtained from deep hydrothermal brines, or it can be enriched by clay matter in sedimentary basins after weathering and erosion of ore-bearing granites (see Figure 3). Conversely, lithium-rich claystones, through deep burial, remelting, granitization, and crystallization differentiation, can form endogenous lithium deposits. By comparing the concentration of magmatic lithium in different tectonic environments and measuring in-situ trace elements within quartz melt inclusions, BENSON et al. concluded that magmas with moderate to extremely high lithium enrichment are genetically related to the involvement of felsic continental crust. GODFREY et al., through analyzing lithium isotope fractionation during salt lake formation, suggested that lithium-rich water from adjacent geothermal and volcanic sedimentary areas is the main source of lithium in the Hombre Muerto salt lake in the central Andes. ORBERGER et al. analyzed the lithium isotope composition of different aquifers in the Puna region of Argentina, showing that lithium mainly originated from andesites, pegmatites, and igneous clastic sediments. These studies indicate that lithium in granite pegmatites can be inherited from sedimentary rocks, while lithium enriched in sedimentary basins can also be provided by volcanic activity.