In-Cd-Ga enrichment in sphalerite from the Gaocheng skarn Pb-Zn-Ag deposit, Yunkai Domain, South China

Abstract

Indium (In), Gallium (Ga), and Cadmium (Cd) are strategic critical metals with widespread applications in high-tech industries. The growing demand and global supply limitations have led to widespread attention on the genesis and exploration of deposits rich in In, Ga, and Cd. The Yunkai Domain (Cathaysia Block, South China) contains numerous Zn-Pb polymetallic deposits, but the potential of In, Ga, and Cd in these deposits has been insufficiently studied. To fill this gap, we selected the Gaocheng deposit, a representative skarn-type Zn-Pb-Ag deposit, to investigate the content, spatial distribution, and incorporation behavior of In, Ga, and Cd in sphalerite, which serves as their major mineral host. Field surveys and microscopic observations revealed that the Gaocheng is featured with a variety of skarn minerals such as diopside, epidote, and garnet. Three generations of sphalerite have been identified with distinct mineral assemblages and colors. Electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses revealed that each sphalerite has distinct compositions, in particular In, Ga, and Cd concentrations. In Sp1 (n = 56), In (mean = 414 ppm), Ga (mean = 177 ppm), and Cd (mean = 11517 ppm) are the most enriched. The concentrations in the subsequent two generations of sphalerite show a decreasing trend, with Sp2 (n = 52) having In (mean = 271 ppm), Ga (mean = 119 ppm), and Cd (mean = 6196 ppm), and Sp3 (n = 49) having In (mean = 48 ppm), Ga (mean = 73 ppm), and Cd (mean = 5732 ppm).

Binary diagrams indicate that the substitution mechanism of In into the sphalerite lattice is: Cu⁺ (or Ag⁺) + In³⁺ ↔ 2Zn²⁺, and Ga follows a similar mechanism: Cu⁺ (or Ag⁺) + Ga³⁺ ↔ 2Zn²⁺, while the substitution mechanism for Cd is: Cd²⁺ ↔ Zn²⁺. It is noticeable that the effect of the indium window, the highly enriched In, correlating with the specific Cd concentrations (4000–8000 ppm, 10000–14000 ppm), is observed in the Gaocheng sphalerite. Then, we use the GGIMFis geothermometer to calculate the crystallization temperatures of three sphalerite generations that exhibit a clear decreasing trend from 331 °C (mean) for Sp1 through 320°C for Sp2, to 309 °C for Sp3. Based on temperature and thermodynamic calculations, the sulfur fugacity (fS₂) shows a gradual decreasing trend, with Sp1 (−8 to −5), Sp2 (−10 to −6), and Sp3 (−13 to −9). Meanwhile, the oxygen fugacity (fO₂) of Sp1 and Sp2 in the earlier stages ranges from −34 to −28.5, with pH values between 4.9 and 6.7, while in the later stages, the oxygen fugacity (fO₂) of Sp3 ranges from −36 to −31.5, with pH values between 4.6 and 6.6. Collectively, we conclude that high temperature, elevated sulfur fugacity, higher oxygen fugacity, and weakly acidic physicochemical conditions are favorable for the incorporation of In, Ga, and Cd into sphalerite, with temperature and sulfur fugacity playing a more dominant role. This novel finding helps establish key exploration indicators for skarn Zn-Pb deposits in the future, enhancing our understanding of the enrichment of In, Ga, and Cd elements.