Antimony and Mercury Isotopes Trace the Deep Roots of China’s Giant Antimony Belt

Deep beneath southern China lies one of the most extraordinary concentrations of a single metal anywhere on Earth. The South China antimony belt supplies more antimony than any other metallogenic province on the planet, yet geologists have argued for decades about where all that metal actually came from. Now a team of geochemists has turned to an unusual pair of chemical detectives—antimony and mercury isotopes—and their verdict is striking: the metal was delivered by two very different sources, one rooted in ancient crustal rocks and another reaching all the way down into Earth’s recycled mantle.

The study, published in Communications Earth & Environment, was led by Degao Zhai of the State Key Laboratory of Geological Processes and Mineral Resources at China University of Geosciences in Beijing, together with Gang Zhao, Qingfei Wang and Jiajun Liu from the same institution, and an international team including Anthony E. Williams-Jones of McGill University, Runsheng Yin of the Chinese Academy of Sciences in Guiyang, Ryan Mathur of Juniata College, Manuel Keith of Ludwig-Maximilians-Universität München, Jian-Feng Gao and Di Chen, and Junwei Xu of the Geophysical and Geochemical Survey Institute of Hunan. Their work addresses a stubborn problem in economic geology: for large, low-temperature hydrothermal deposits, direct tracers of metal sources have remained frustratingly limited.

Antimony is far more familiar to most people than they realize. The element is a critical ingredient in flame retardants, lead-acid batteries, semiconductors and a range of military and industrial applications, which is why many governments now classify it as a strategic raw material. China dominates global production, and the South China belt—with world-famous districts such as Xikuangshan—sits at the heart of that dominance. Understanding where its metal comes from is therefore not just an academic puzzle; it directly shapes how exploration geologists decide where to drill next.

The traditional difficulty is that antimony deposits in South China formed from relatively cool hydrothermal fluids, typically far below the temperatures associated with magmatic ore systems. In hotter systems, geochemists can often link ores to specific intrusions or volcanic rocks using a battery of tracers. In these low-temperature systems, however, the fluids traveled long distances, interacted with many different rocks along the way, and left behind few unambiguous fingerprints of their ultimate origin. Competing models have variously invoked the Precambrian basement, sedimentary sequences, or deep crustal and mantle reservoirs, but the evidence has never been decisive.

Zhai and colleagues attacked the problem with a novel strategy: measuring the isotopic compositions of antimony itself, in tandem with mercury, in stibnite—the antimony sulfide mineral that makes up the ores. Isotopes are atoms of the same element with different masses, and their ratios in a mineral act like a chemical passport, recording the reservoir from which the element was derived. Antimony isotope analysis is a young technique, and this study represents one of its most ambitious applications to ore-forming systems. Mercury isotopes, by contrast, are better established, and they bring a special advantage: the mass-independent fractionation signature of mercury, denoted Δ199Hg, is largely unaffected by the ordinary chemical processing that happens as fluids move through the crust, making it an unusually faithful recorder of whether material passed through surface environments or came from the deep Earth.

The results are remarkable for their sheer variability. The measured δ123Sb values of the ores span from −0.64 to +0.69 per mil, the δ202Hg values range from −1.86 to +2.08 per mil, and the Δ199Hg values extend from −0.20 to +0.14 per mil. In isotope geochemistry, such wide spreads are not noise; they are a message. A single, uniform source would produce a narrow cluster of values. Instead, the data point to a mixing relationship between two isotopically distinct reservoirs that contributed metal to the ore-forming fluids in varying proportions across the belt.

The first reservoir is the Precambrian basement, the ancient metamorphic rocks that underlie much of southern China. Metamorphic devolatilization—the process by which heat and pressure drive fluids and volatile elements out of deeply buried rocks—can liberate antimony and mercury from these crustal rocks and release them into circulating hydrothermal systems. The isotopic signatures of the ores that align with this reservoir indicate that the old basement was not a passive bystander but an active supplier of metal to the belt.

The second reservoir is far more dramatic: a deep, recycled mantle-related source. Mercury’s mass-independent signatures in particular carry information about whether the element once cycled through Earth’s surface environment—oceans and atmosphere—before being dragged back into the interior by subduction and later returned through deep fluids. The data support a model in which some of the antimony in the South China belt was ultimately derived from material that had been recycled into the mantle and was released again from depth, a conclusion that connects these shallow, cool ore systems to the planet’s grandest geochemical conveyor belt.

This dual-source picture resolves a long-standing tension in the literature. Earlier debates often treated the question as either-or: either the metal came from the crust or from depth. The coupled isotope approach shows that the answer is both, with the relative contributions varying from deposit to deposit. That insight matters for exploration, because deposits fed predominantly by one reservoir or the other may occur in different structural and geological settings, and the isotopic composition of ore minerals could in principle serve as a vectoring tool, helping geologists assess the character of a system from early-stage samples.

Beyond the immediate implications for antimony, the study demonstrates the broader power of a technique that is only now coming of age. Coupled antimony and mercury isotope analysis offers a potentially powerful tool for identifying metal sources in ore deposits worldwide, the authors argue, and its application may help refine deposit models globally. It also opens a window onto the dynamic interplay between Earth’s interior and crustal metal cycling—the slow, planet-scale circulation that moves elements between the mantle, the crust and the surface over hundreds of millions of years.

For a metal that has become a centerpiece of supply-chain security debates, the new results carry practical weight. If the giant South China belt owes its wealth to a combination of ancient basement rocks and deeply recycled mantle material, then exploration models built on shallow crustal processes alone may be missing part of the story. The isotopic fingerprints locked inside stibnite crystals now suggest that some of the world’s most important antimony ores are, quite literally, gifts from the deep Earth—delivered upward through fluids that connected the planet’s interior to its crust in ways geologists are only beginning to decode.

Subject of Research: Metal sources of the South China antimony belt traced with coupled antimony and mercury isotopes

Article Title: Coupled Sb-Hg isotopes reveal mixed basement and deeply recycled mantle-related metal sources for the South China antimony belt

Article References: Zhai, D., Zhao, G., Williams-Jones, A. E., Yin, R., Mathur, R., Keith, M., Wang, Q., Liu, J., Chen, D., Gao, J.-F., & Xu, J. (2026). Coupled Sb-Hg isotopes reveal mixed basement and deeply recycled mantle-related metal sources for the South China antimony belt. Communications Earth & Environment. https://doi.org/10.1038/s43247-026-04108-4

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04108-4

Keywords: antimony, mercury isotopes, South China, ore deposits, economic geology, geochemistry, Precambrian basement, mantle recycling, stibnite, hydrothermal fluids, metal sources, critical metals