Discovery of Deep Sulfide Cumulates Beneath the Jinchuan Ni-Cu-Platinum Group Element Deposit and Its Significance

Background Introduction

Nickel (Ni), copper (Cu), and platinum group elements (PGE) are critical metal resources on Earth, widely used in industries, electronics, and new energy sectors. Approximately 56% of the world’s nickel, 3% of copper, and over 96% of PGE production come from magmatic Ni-Cu-PGE sulfide deposits. These deposits are typically associated with komatiitic, basaltic, and mafic-ultramafic magmatic activities. However, despite their global distribution, the mechanisms behind the formation of these deposits remain poorly understood, particularly how metals are concentrated in magmatic systems to form high-grade deposits.

The Jinchuan deposit, located in northwestern China, is one of the world’s largest single magmatic Ni-Cu-PGE sulfide deposits. Despite its relatively small volume (approximately 1 cubic kilometer), it contains about 6.25 million tons of nickel and 4.0 million tons of copper. The formation mechanism of the Jinchuan deposit has long been a subject of academic debate, especially how such a high concentration of metals was achieved in such a small magmatic conduit. To address this issue, researchers employed nickel and copper isotopes as novel geochemical tools to uncover the formation process of the Jinchuan deposit and the potential existence of hidden deep ore bodies.

Source of the Paper

This paper was co-authored by Yun Zhao, Shui-Jiong Wang, Chunji Xue, and Matthew J. Brzozowski, affiliated with institutions such as the China University of Geosciences (Beijing) and the China University of Geosciences (Xi’an). The paper was published online on October 15, 2024, in the journal Geology, titled Hidden deep sulfide cumulates beneath the Jinchuan Ni–Cu–platinum-group element deposit (China) inferred from Ni–Cu isotopes. The research was supported by multiple funding sources, including the National Natural Science Foundation of China.

Research Process and Results

1. Sample Collection and Classification

Researchers collected various types of sulfide ore samples from the Jinchuan deposit and classified them into three categories based on their structural characteristics: - Disseminated sulfide ores: Sulfides are distributed as grains among silicate minerals, with sulfide content below 30%. - Net-textured sulfide ores: Sulfides form an interconnected network among silicates, with sulfide content ranging from 30% to 50%. - Massive sulfide ores: Sulfides occur as irregular veins, with sulfide content exceeding 50%.

These samples showed minimal weathering or hydrothermal alteration, ensuring the reliability of the isotopic data.

2. Nickel and Copper Isotope Analysis

The researchers conducted nickel and copper isotope analyses on the collected samples. Nickel isotope analysis was performed using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), while copper isotope analysis was based on previously reported chalcopyrite data. The results showed: - The δ⁶⁰/⁵⁸Ni values of disseminated and net-textured sulfide ores were +0.27‰ and +0.22‰, respectively, significantly higher than those of massive sulfide ores (-0.27‰). - The δ⁶⁵Cu values of disseminated and net-textured sulfide ores were +0.36‰ and +0.22‰, respectively, also significantly higher than those of massive sulfide ores (-0.44‰).

3. Correlation Between Isotopes and Elemental Content

The study found that nickel and copper isotope values were negatively correlated with whole-rock PGE, sulfur, nickel, and copper contents but positively correlated with Pd/Ir ratios. This indicates that the enrichment of nickel and copper isotopes is closely related to the fractionation process of sulfide melts.

4. Inference of Deep Sulfide Segregation Event

Using Rayleigh fractionation modeling, the researchers simulated the process of sulfide melt segregation from silicate magma. The results suggest that the nickel and copper isotope characteristics of the Jinchuan deposit can be explained by an early deep sulfide segregation event. Specifically, sulfide melt segregation occurred in the deep magma chamber, leading to the enrichment of heavy nickel and copper isotopes in the residual magma. Subsequently, these magmas underwent further sulfide melt segregation and fractionation in shallow magmatic conduits, forming the currently explored sulfide ores.

Research Conclusions and Significance

The study reached the following main conclusions: 1. Deep Sulfide Segregation Event: The nickel and copper isotope characteristics of the Jinchuan deposit indicate that early sulfide melt segregation occurred in the deep magma chamber, resulting in the enrichment of heavy nickel and copper isotopes in the residual magma. 2. Shallow Sulfide Fractionation: Further sulfide melt segregation and fractionation in shallow magmatic conduits formed the currently explored sulfide ores. 3. Exploration Potential for Hidden Ore Bodies: The study suggests that hidden ore bodies with light nickel and copper isotope signatures may exist in the deep parts of the Jinchuan deposit, representing promising targets for future exploration.

This research not only reveals the formation mechanism of the Jinchuan deposit but also provides new insights for the exploration of other magmatic Ni-Cu-PGE sulfide deposits worldwide. By analyzing nickel and copper isotopes, researchers can more effectively trace the behavior of sulfide melts, offering scientific guidance for deposit exploration.

Research Highlights

1. Application of Isotope Geochemical Tools: This study is the first to systematically apply nickel and copper isotopes to the Jinchuan deposit, revealing its unique isotopic characteristics and geological significance.
2. Discovery of Deep Sulfide Segregation Event: The research proposes for the first time that the Jinchuan deposit may have experienced a deep sulfide segregation event, providing a new perspective for understanding the formation mechanisms of magmatic sulfide deposits.
3. Exploration Potential: The study highlights the potential existence of hidden ore bodies in the deep parts of the Jinchuan deposit, offering important directions for future exploration.

Additional Valuable Information

The study also mentions that the formation of the Jinchuan deposit may be related to magmatic activities at the intersection of mantle plumes and rifts. Such environments are typically favorable for large-scale metal enrichment, as they generate substantial metal-bearing magmas and provide well-defined magmatic conduits. This finding not only applies to the Jinchuan deposit but may also serve as a reference for the study of similar deposits worldwide.

Through innovative isotope geochemical methods, this research elucidates the formation mechanism of the Jinchuan deposit and provides scientific foundations for future exploration efforts. Its findings hold significant academic value and broad application prospects.