Gallium (Ga), germanium (Ge), thallium (Tl) and cadmium (Cd) resources in China

Dispersed metals are global strategic resources for the development of advanced technologies and future energy. In recent years, due to their limited resources, uneven geographical distribution, and high supply risk, dispersed metals are critical resources that most industrial countries compete for. The signatures of dispersed metals are of “dispersed”, “companion”, and “fine”, suggesting that these metals are difficult to form independent deposits. Thus, deposits enriched in dispersed metals were formed with special geological processes. In comparison with other metals (e.g., Zn and Cu), studies on the mineralization of dispersed metals are limited, restricting theoretical understanding and breakthroughs in prospecting of these metals. Based on the global reserves and geographical distribution of dispersed metals, we systematically summarized the distribution, major deposit type, occurrence, predominant enrichment process, geological, and geochemical signatures of gallium (Ga), germanium (Ge), thallium (Tl), and cadmium (Cd) in China to investigate the distribution characteristics and forming conditions of these elements, and to promote their prediction accuracy and evaluation level. Studies have shown that Ga is widely distributed in all kinds of rocks and has reached to industrial grade in some ore deposits. According to mineralization, Ga-rich deposits in the world can be roughly divided into weathered-sedimentary, hydrothermal, pegmatitic, and magmatic deposits. The Ga-rich deposits of industrial significance in China mainly include three types: Bauxite-associated, lead-zinc-associated, and coal-associated Ga deposits. In China, Ge deposits can be divided into “coal-type” Ge deposits (e.g., the Lincang and the Wulantuga Ge deposits) and “lead-zinc-type” Ge deposits (mainly located at the Yunnan-Sichuan-Guizhou region). China is rich in Tl resources, i.e., approximately 9000 t, mainly distributed in Yunnan, Guizhou, Anhui, Guangdong, Hubei, Guangxi, and Liaoning provinces. Owing to the relatively sparse research on Tl mineralization, there is no recognized classification of Tl deposit types at present. According to the differences in element combination, occurrence state, and metallogenic conditions, two types of deposits with industrial significance are roughly divided, namely low-temperature hydrothermal and massive sulfide Tl deposits. The global distribution of Cd resources is highly consistent with that of zinc resources, which are dominantly distributed in Australia, China, Peru, Mexico, and the United States. In terms of tectonic background, Cd resources in China are mainly distributed in lead-zinc metallogenic belts, such as the three rivers Tethys metallogenic belt, the low-temperature metallogenic zone at Sichuan-Yunnan-Guizhou tringle area, the Nanling metallogenic belt, and the greater Khingan metallogenic belt. In this review, the following prospects should be strengthened in future: (1) Typical dispersed metal deposits and clarification of the major resource types with industrial significance; (2) the occurrence of dispersed metals in minerals by high-precision and high-spatial-resolution microanalysis techniques; (3) the mechanism of super-normal enrichment of dispersed metals; and (4) new types of dispersed metal deposits. This review will be helpful to better understand the reserves of dispersed metals in China and will enhance the support capabilities of strategic resources.