Acta Petrologica Sinica  2022, Vol. 38 Issue (1): 1-8, doi: 10.18654/1000-0569/2022.01.01   PDF    
Spodumene: The key lithium mineral in giant lithium-cesium-tantalum pegmatites
GROVES DI1,2, Zhang Liang1, GROVES IM3, SENER AK2     
1. China University of Geosciences, Beijing 100083, China;
2. Asgard Metals Pty Ltd, WA 6018, Australia;
3. Insight Geology Pty Ltd, VIC 3178, Australia
Abstract: As alternate energy sources have become a global issue, the demand for lithium has increased and there has been greater exploration for lithium pegmatite deposits. Although the geology of many of these deposits is poorly documented in the modern literature, it is evident that most major to giant pegmatites have spodumene as the economic lithium mineral rather than other lithium minerals. These largely Archean economic pegmatites throughout the world are normally sub-horizontal to gently dipping, in contrast to many near-vertical pegmatite-swarms, commonly have complex 3D geometries, are strongly mineralogically and geochemically zoned, and are hosted in amphibolite-facies wallrocks. The complex geometries and flat dips are consistent with syn-metamorphic emplacement in dilation zones within a compressional to transpressional regime with a near-vertical minimum principal stress, sigma 3. This would have allowed injection of multiple pulses of lithium-enriched melt and greater time for vertical pegmatite differentiation of volatile-rich pegmatitic melts during prolonged cooling, and importantly, for spodumene to crystallize as the most common medium to high P-T lithium phase in flat-lying lithium-rich zones.
Key words: Critical elements    Lithium minerals    Spodumene    Syn-metamorphic pegmatites    Lithium-cesium-tantalum deposits    
锂辉石:巨型花岗伟晶岩锂-铯-钽矿床中关键含锂矿物
GROVES DI1,2, 张良1, GROVES IM3, SENER AK2     
1. 中国地质大学(北京),北京 100083;
2. 阿斯加德金属有限公司,珀斯 6018;
3. 洞见地质有限公司,墨尔本 3178
摘要: 寻找传统化石燃料的替代能源已成为全球性议题。受动力电池消费的拉动,锂资源需求急剧上升,伟晶岩型锂矿勘查热度持续攀升。虽然众多伟晶岩型锂矿地质特征尚不清晰,已有证据表明锂辉石是大多数大型-巨型伟晶岩型锂矿床的主要含锂矿物。与许多近直立的伟晶岩脉群不同,世界范围内大多数太古代伟晶岩矿脉往往呈近水平或缓倾斜在角闪岩相围岩中产出,它们往往具有复杂的三维形态并发育明显的矿物和地球化学分带。这些太古代伟晶岩脉通常形成于挤压或压剪构造体制下同变质环境中,成岩期最小主应力(σ3)近竖直。因此,伟晶岩常常侵位于近水平的构造局部引张区而形成复杂的几何学形态。压性的构造环境为富锂熔体多次脉动式注入和富含挥发分熔体垂向结晶分异提供了充足的时间;锂辉石在中高温压条件下结晶成为缓倾富锂带中最为常见的含锂矿物。
关键词: 关键金属    含锂矿物    锂辉石    伟晶岩    锂-铯-钽矿床    
1 Introduction

As long-lived production of lithium from historic mines and production of other critical elements from Lithium-Cesium-Tantalum (LCT) pegmatites has slowed or ceased, exploration programs have been accelerated to meet increasing demand in this expanding world of technical innovation (Dessemond et al., 2019; Xiong et al., 2020). Many of the world's largest lithium pegmatite deposits have only been discovered in the past few years and there has been limited published research in scientific journals. Instead, the information is commonly restricted to data on company websites which emphasize exploration success and resource estimates at the expense of geological information on the pegmatites, and particularly on the nature of the host rocks. However, it has become clear that, except in exceptional cases, spodumene (LiAlSi2O6, Fig. 1) is the dominant economic lithium mineral, despite the abundance of other lithium minerals (Table 1).

Figure 1 Spodumene crystals (a) triphane from Nuristan, Laghman Province, Afghanistan; (b) kunzite from Mawi pegmatite, Nuristan, Afghanistan. Photos from Arkenstone: www.irocks.com. Permission sought

Table 1 Variety of lithium-bearing minerals commonly found in natural rocks, particularly pegmatites and sediment-hosted deposits

In this short preliminary paper, the reason for the predominance of spodumene in major to giant lithium pegmatites is sought and a robust model developed to explain repetitive critical parameters of the pegmatites and their host environment while realizing that not all critical data are available for the newly discovered deposits and total resources are difficult to assess for historical deposits with significant past production. Many of the references used are quite old because this was when fundamental research on the nature and zoning of the pegmatites was carried out. Apart from the references listed in Table 2, a literature search reveals that most modern references concentrate on specific aspects of lithium pegmatites and do not provide a holistic interpretation. For example, the mineralogy and/or mineral chemistry of associated minerals in spodumene pegmatites are described by Badanina et al. (2015), Feng et al. (2019), and Maneta and Baker (2019). Knoll et al. (2018) and Fei et al. (2020) discuss the petrogenesis and geochronology of spodumene pegmatites, and Barros and Menuge (2016) and Liu et al. (2020) discuss the genetic relationships between spodumene pegmatites and adjacent granite intrusions. Wilde et al. (2021) describe a new potentially giant lithium pegmatite province in Mali, and Steiner (2019) reviews the pegmatite literature and sets out pragmatic lithium exploration guides. However, none of these papers with the exception of a description of cristobalite from the Jiajika pegmatite, deal with known giant lithium-cesium-tantalum pegmatites, the subject of this paper.

Table 2 Overview of global major to giant lithium-bearing pegmatites for which there is adequate information on tonnage and grade, and on both pegmatites and host rocks
2 Characteristics of large lithium pegmatites with economic ore grades

As noted above, it is not possible to prepare a list of lithium-bearing pegmatite deposits in terms of a consistent set of characteristics for the multitude of global LCT pegmatite deposits compiled by Bradley et al. (2017). Instead, a more restricted list of many of the largest deposits is provided in Table 2, using the proviso that the deposit is economically significant and has a known resource and, where possible, there is confirmation of the principal lithium phase, and information on the attitude of the pegmatites and nature of the host rocks, particularly their metamorphic grade. To achieve this, mining company websites have been used as a source of information because, as discussed above, many of the newly discovered deposits have no scientific data of research quality. However, as the parameters sought are relatively basic, it is considered that Table 2 is reliable. The global occurrence of these pegmatites is shown in Fig. 2. There is some emphasis on Australia because of a 2017 publication that summarizes deposit data.

Figure 2 Map of the world showing location of the well-documented major lithium-bearing pegmatites listed in Table 2 (adapted after Bradley et al. 2017)
3 Syn-tectonic and syn-metamorphic timing of emplacement

Although Černý (1991, 1992) pointed out that many LCT and other pegmatites appeared to have been emplaced syn-tectonically, this appears to have been downplayed in many subsequent papers with most diagrammatic figures showing planar, near-vertical zoned pegmatites in the roof zones of granite plutons (Bradley et al., 2017).

Several independent lines of evidence suggest that the large to world-class spodumene pegmatites are syn-tectonic and syn-metamorphic bodies. Importantly, they are very rarely planar, more commonly having complex shapes in plan (Fig. 3a). They may also have extensive syn-metamorphic metasomatic alteration zones around them (Morgan and London, 1987). In rare cases, a syn-deformational model has been erected to explain both the complex shape and complex zoning of the pegmatites such as the giant Greenbushes pegmatite from the Yilgarn Block of Western Australia (Partington, 1990, 2017; Fig. 3b). Another good example of a giant pegmatite with a complex syn-deformational geometry is the Pilgangoora pegmatite from the Pilbara Block of Western Australia (Sweetapple et al., 2017; Fig. 4). Most of the economically important lithium pegmatites (Table 2) also have gentle dips or are sub-horizontal, as shown for the Pilgangoora pegmatite (Fig. 5) and the giant Kenticha pegmatite of Ethiopia (Küster et al., 2009; Fig. 6). This is entirely consistent with their emplacement in flat-lying dilation zones in a compressional to transpressional stress field in which the minimum principal stress (sigma 3) was subvertical. An analogy can be made to flat-lying extensional or 'ladder' veins in late-tectonic orogenic gold deposits (Robert and Paulsen, 2001). The gentle dip of the pegmatites is also critical for most efficient open-cut mining.

Figure 3 Geological map and structural model of Greenbushes lithium-bearing pegmatite (adapted after Partington, 2017) (a) map of giant Greenbushes lithium-bearing pegmatite, Western Australia, showing complex shape in amphibolite host rocks; (b) schematic structural model to explain complex zonation of the Greenbushes pegmatite during syn-metamorphic shearing

Figure 4 Simplified geological map of the complex, syn-deformational geometries of the giant Pilgangoora lithium-tin-niobium-tantalum pegmatite swarm (after Sweetapple et al. 2017) Dashed line shows cross section of Fig. 5 in this map

Figure 5 Schematic cross section based on diamond drill holes of the Pilgangoora pegmatites showing their gentle dip and consistent lithium grades (adapted after Sweetapple et al., 2017)

Figure 6 Geological and mineralogical characteristics of Kenticha pegmatite, Ethiopia (after Tadesse and Zerihun, 1996; Küster et al., 2009) (a) cross section through the Kenticha pegmatite deposit, Ethiopia, highlighting, the flat dip, non-planar shape (complex in plain view), and strong vertical mineralogical zonation; (b) field photograph showing the flat-lying Kenticha pegmatite; (c) subvertical coarse grained spodumene crystals in the Kenticha pegmatite
4 Significance to formation of world-class high-grade lithium deposits

Pegmatites are typically emplaced at 500~600℃ and may cool rapidly if intruded into cold wall rocks, although Simmons and Webber (2008) demonstrated that the presence of fluxes and volatiles in pegmatite melts may suppress crystallization temperatures as low as 350℃. However, if emplaced broadly syn-tectonically and syn-metamorphically at upper greenschist to amphibolite facies (Table 2), at >500℃, the wall rocks would be at a similar temperature to the pegmatite fluid-rich melt for a significant time. This would slow cooling of even flux- and volatile-rich melts, and thus enhance differentiation and local enrichment of incompatible elements, and promote the formation of giant crystals, although these may also crystallize in post-metamorphic pegmatites (Simmons and Webber, 2008). The sub-horizontal form of the pegmatite bodies would also promote increased efficiency of vertical differentiation of pegmatite melts, with the formation of sub-vertical large to giant crystals (Fig. 6c). The inferred sub-horizontal dilation zones into which pegmatite melts were emplaced would also enhance the opportunity for multiple volatile-rich melt influxes with consequent both increase in thickness and critical element enrichment in the resultant pegmatite body (Fig. 7). Some authors have inferred such development of multiple melt pulses to explain complexities in pegmatite formation (Nabelek et al., 2010; Anderson et al., 2013). They would also help explain the ubiquitous occurrence of lithium grades of 0.54% to 3.40 % Li2O related to spodumene throughout the entire thickness of some flat-lying pegmatites at Pilgangoora in Western Australia (Sweetapple et al., 2017; Fig. 8).

Figure 7 Schematic representation of sub-horizontal pegmatite melt emplaced into flat dilation zone in amphibolite host rocks due to vertical sigma 3 (a) initial melt pulse with differentiation of upper lithium zone; (b) second melt emplaced during further dilation with differentiation of second lithium zone; (c) third pulse of melt to produce the final geometry of the pegmatite with a thick upper lithium zone

Figure 8 Detailed graphic log of intercept at 106~121m in diamond drill hole PLS197M showing remarkable concentrations of Li, Sn, Nb, and Ta over the entire width of the pegmatite (after Sweetapple et al., 2017)
5 Spodumene as the key economic lithium mineral

There are few modern thermodynamic data relating to the stability of lithium silicate minerals. However, based on available experimental data (Stewart, 1978; London, 1984; Chakoumakos and Lumpkin, 1990), the higher pressures of the upper-greenschist to amphibolite facies environments (>2.5kbar) would favor spodumene formation at likely crystallization temperatures of 500℃ to 300℃, thus prolonging crystallization of flux- and volatile-rich melts and favoring the formation of giant crystals (Fig. 9). At lower pressures (< 2.0kbar) in the greenschist facies, petalite would form at high temperatures, making giant crystals less common. Other lithium silicates form at lower temperatures, explaining their absence in most economic lithium pegmatites.

Figure 9 Experimental P-T phase diagram for the bulk system 20 eucryptite 80 quartz (mol/mol) in the system LiAlSiO4-SiO2-H2O, showing stability field of spodumene (adapted after London, 1984: figure 1, with experimental conditions shown) Abbreviations: Bsp-β-spodumene; Ecr-eucryptite; Pet-petalite; Qtz-quartz; Spd-spodumene; Vrg-virgilite

The anomalous abundance of the economic spodumene-bearing pegmatites in Archean greenstone belts is probably related to a combination of higher heat flow due to mantle plume activity and preservation of environments at the required crustal level due to the long-term stability of Archean cratons with anomalously thick lithosphere (Groves et al., 2005, and references therein).

6 Conclusions

Largely Archean, economic LCT pegmatites throughout the world are normally sub-horizontal to gently dipping, in contrast to many near-vertical pegmatite-swarms. They were emplaced during regional compression or transpression and regional metamorphism and, as a result, are hosted in amphibolite-facies wallrocks, have complex geometries with flat dips, are strongly mineralogically and geochemically zoned, and have metasomatic reaction zones against their wallrocks. Their flat dips suggest emplacement in dilation zones with minimum principal stress, sigma 3, sub-vertical, which would favor multiple pulses of lithium-rich melt. These syn-metamorphic conditions would have allowed greater time for vertical differentiation of flux- and volatile-rich pegmatitic melts during prolonged cooling, and importantly, expand the opportunity for large crystals of spodumene to crystallize within the critical P-T window of regional metamorphism. These critical parameters can be used to assess the potential of lithium-cesium-tantalum pegmatites to become giant economic deposits through exploration.

Acknowledgements      We sincerely thank Deng Jun, Yang Liqiang, Wang Qingfei, Zu Bo and Xiong Yiqu for the opportunity to interact with them, and Shen Guanwen, Yu Tianwei, Li Jia and Wang Haoshuai for their help with the first draft of the figures.

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