Elsevier

Ore Geology Reviews

Volume 135, August 2021, 104173
Ore Geology Reviews

Characteristics and genesis of ion adsorption type REE deposits in the weathering crusts of metamorphic rocks in Ningdu, Ganzhou, China

https://doi.org/10.1016/j.oregeorev.2021.104173Get rights and content

Highlights

  • A new type ion adsorption type REE deposit in the weathering crusts of metamorphic rocks was investigated.

  • Hydrothermal alteration is a prerequisite for the iREE mineralization of metamorphic bedrock.

  • Ore bodies, approximately five times as enriched as the bedrocks, are primarily located in the middle and upper parts of the completely weathered horizon.

  • Permeability is one of the most important factors affecting the weathering of metamorphic rocks, and controlling the migration of REEs and emplacement of ore bodies in the weathering crusts.

Abstract

Over the past few decades, numerous ion adsorption type rare earth element (REE) deposits have been discovered in the weathering crusts of granitic and volcanic rocks, and a great deal of studies have been conducted on their characteristics and genesis. Recently, ion adsorption type REE deposits in the weathering crusts of metamorphic rocks were discovered for the first time in Ningdu County, China. Unlike the extensively studied weathering crusts of granitic rocks, the geochemical and mineralogical characteristics of the weathering crusts of metamorphic rocks and the mobility of REEs during weathering processes remain unclear. Our research shows that the bedrocks in the Ningdu area are primarily metamorphic sandstone that contain ~ 200 ppm of total REEs. REEs in the metamorphic rocks are primarily hosted in hydrothermal REE-epidote and rhabdophane-(Ce), implying that hydrothermal fluids, which might have introduced REEs and provided the materials for REE mineralization, are likely one of the most critical factors for mineralization in the weathering crusts of metamorphic rocks.

The weathered products of fine-grained metamorphic bedrocks are significantly less permeable than those of granitic rocks; consequently, the weathering crusts are rather thin, usually less than 20 m. The degree of weathering is relatively low, with chemical index of alteration (CIA) values of less than 80. Clay minerals consist primarily of illite, kaolinite, and hydrobiotite. The low permeability of the weathered products causes a short migration distance for water medium carrying REE ions, and the ore bodies in the weathering crusts of metamorphic rocks are primarily located in the middle and upper parts of the completely weathered horizon. Average REE concentrations in the ore bodies are ~ 1000 ppm, of which 70%–80% are ion-exchangeable. The REE distribution patterns of the weathering crusts are largely similar to that of the bedrocks, except for the strong negative Ce anomalies in the topsoils that are caused by the pre-existing positive Ce anomaly layer above the ore body that is thin and removed due to weathering and erosion. Our research results deepen the understanding of the genesis of ion adsorption type REE deposits in the weathering crusts of metamorphic rocks and indicate that the hydrothermal process is the key mineralization factor.

Introduction

Rare earth elements (REEs), including the lanthanide group elements (La–Lu) and yttrium (Y), are crucial raw materials used in a variety of high-technology applications. In modern society, the global market demand for REEs, especially heavy REEs (HREEs, including Gd-Lu + Y), is dramatically increasing. However, though global REE resources are rich in light REEs (LREEs, including La-Eu), they lack HREEs. The weathering crust ion-adsorption type REE (iREE) deposits, widely distributed in South China, are almost the exclusive source of global HREEs and supply nearly the total global HREEs demand (Hoshino et al., 2016, Xie et al., 2016, Li et al., 2017, Goodenough et al., 2018).

Since 1969, hundreds of iREE deposits have been discovered in South China, and iREE deposits have recently been discovered outside China, such as in Madagascar (Berger et al., 2014), Malawi (Le Couteur, 2011), Brazil (Rocha et al., 2013), and Southeast Asia (Sanematsu et al., 2009). According to statistics, almost all known iREE deposits are hosted in the weathering crusts of granitic intrusions (Bao and Zhao, 2008, Sanematsu and Watanabe, 2016) and felsic volcanic rocks (Fu et al., 2019b, Qin et al., 2019). Most of the mineralized granites or volcanic rocks are rich in REEs, which are primarily hosted in less weather-resistant minerals (Chi et al., 2012, Li et al., 2017). Compared to that of granites and volcanic rocks, it was believed that the weathering crusts of sedimentary and metamorphic rocks are not prospective for iREE deposits. On the one hand, REE-bearing minerals include monazite, xenotime, and zircon, which are more resistant to chemical weathering, and could not be a source of REE ions in weathered materials (Hu, 1986). On the other hand, sedimentary and metamorphic rocks, such as mudstone, siltstone, and phyllite, are characterized by fine grain size and dense structure, resulting in low permeability, which inhibits the downward transfer of fluid and the rock-water reaction. Therefore, the weathering crusts developed above these rocks are relatively thin and are characterized by a low degree of weathering and REE accumulation (Hu, 1986).

However, new iREE deposits have recently been discovered in the weathering crusts of metamorphic rocks, such as the Getenzui and Wanan iREE deposits (Liu et al., 2014, Zhao et al., 2016, Zhao et al., 2018, Qiu, 2017, Wang et al., 2018, Wang et al., 2019, Chen et al., 2019). Though a few studies address these new deposits, most previous studies primarily focused on the type and mineralogical and geochemical characteristics of the metamorphic rocks to determine the reasons for weathering and mineralization. Previous comparative studies on different metamorphic rocks in this area show that the high REE content in metamorphic sandstone and metamorphic tuff favor mineralization, whereas phyllite and schist are unfavorable for mineralization due to their resistance to weathering (Wang et al., 2018). Studies on REE mineralogy in metamorphic rocks have discovered a large number of weathering resistant minerals such as REE phosphate and zircon, which are not conducive to mineralization. However, REE minerals of hydrothermal origin, such as zircon, have also been found (Zhao et al., 2018). Therefore, REE-rich hydrothermal fluids are speculated to bring REEs into metamorphic rocks and promote supergene REE enrichment in the weathering crusts (Zhao et al., 2018). The occurrence of REEs in bedrocks is generally believed to be one of the most important factors controlling REE mineralization. However, detailed investigations on hydrothermal REE-bearing minerals in metamorphic rocks are still lacking, and the relationships between weathering and REE mineralization in metamorphic rocks are also poorly addressed.

The objective of the present study was to (i) elucidate the weathering characteristics of metamorphic rocks, and the migration and fractionation of REEs in the weathering crusts; (ii) clarify the key factors controlling REE mineralization in the weathering crusts of metamorphic rocks. A weathering profile of metamorphic rocks in the Ningdu County was carefully sampled, followed by detailed petrographic and geochemical studies of the parent rocks and regolith of the profile. We show that hydrothermal alteration provides REEs needed for mineralization and is prerequisite for iREE mineralization in metamorphic bedrocks. Moreover, permeability is one of the most important factors affecting the development of the weathering crusts above metamorphic rocks, which in turn controls the emplacement of ore bodies and the migration and fractionation of REEs in the weathering crust.

Section snippets

Geological backgrounds

Ningdu county is part of the jurisdiction of Ganzhou City. Ganzhou City, in southern Jiangxi province, is located in the Nanling W–Sn–REE–Nb–Ta–U ore belt, which is also the most important iREE ore province in China. Magmatic and metamorphic rocks are widely distributed in this area (Fig. 1). The magmatic rocks are primarily granites forming in Caledonian, Hercynian-Indosinian, and Yanshanian periods (Sun, 2006). Of these, Yanshanian granites are most widely distributed, with a total outcrop

Profile characteristics and sampling strategy

The sampling sites were located on a small hill within the scope of REE exploration area in Ningdu County. The weathering crust is well-developed, with a thickness of ~ 20 m. To ensure the integrity of the weathering crust, two sampling sites were selected at the hilltop and hillside (Fig. 2).

Sampling site 1 is located on the hillside and exposed by recent house construction (Fig. 2a and 2b). The profile consists of thick regolith and a small amount of exposed bedrocks. The height of the slope

Bedrock

The bedrock of the weathering profile is metamorphic siltstone (Fig. 3a) and fine-grained sandstone (Fig. 3c and 3e), with blastopelitic, blastopsammitic, and microscopic scaly crystalloblastic textures (Fig. 3). The original rock of the metamorphic rock may be greywacke formed by weathering and fragmentation of granitoids that are rapidly deposited across a short transportation distance. The three genetic types of minerals in the bedrocks include i) Clastic minerals in the original sedimentary

Occurrence and source of REEs in bedrocks

The occurrence of REEs in the sericite phyllite in northeastern Jiangxi province was suggested to be primarily ion-adsorption on the surface of sericite (Ling and Liu, 2002). However, as verified in our studies on the extraction of iREE from bedrock with ammonium sulfate and acid dissolution of bedrock, REEs are primarily hosted in a small amount of REE-bearing accessory minerals, whereas the contribution of iREEs is negligible. Bedrocks are the primary source of REEs in the weathering crusts,

Conclusions

REEs are approximately five times as enriched as the bedrocks in the weathering profile of metamorphic rocks in Ningdu County, China. REEs in metamorphic rocks host in REE-bearing minerals, such as REE-epidote and rhabdophane-(Ce), which are formed by REE-rich hydrothermal fluids. The hydrothermal alteration may be a key factor for mineralization. Due to the low permeability of the weathering crusts of metamorphic rocks, REE ore bodies are generally located in the upper and middle parts of the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Key R&D Program of China (Grant No. 2017YFC0602306); Major Project of Basic and Applied Basic Research in Guangdong Province (Grant No. 2019B030302013); Supported by the Key Research Program Project of Institute of Geology and Geophysics, Chinese Academy of Sciences (Grant No. IGGCAS-201901); Key Project of Guangzhou Science and Technology Plan (Grant No. 201804020037); the National Natural Science Foundation of China (Grant Nos. 41773113,

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