Oxidative fluids facilitate Cd enrichment in a magmatic-hydrothermal system: An example from the Kangjiawan Pb-Zn polymetallic deposit, South China

The Kangjiawan, as one of the representative magmatic-hydrothermal Pb + Zn polymetallic deposits in the central Qin-Hang Metallogenic Belt (QHMB), South China, contains 16.59 million tons of ore reserves with 0.5 Mt Pb metal reserves and 0.56 Mt Zn metal reserves. Episodic magmatic and associated hydrothermal ore fluids produced a variety of sphalerite generations with distinct mineral assemblages and compositions, making the Kangjiawan an excellent target for studying the controls on the enrichment of critical metals such as Cd, In, and Sn in Pb-Zn deposits. In this contribution, we systematically collected the samples from the different horizontal and vertical levels. Then, a series of microscopic identification, electron probe micro-analyzer (EPMA), and laser ablation inductively coupled plasm mass spectrometer (LA-ICP-MS) analyses were carried out. We identified four generations of sphalerite, that is, Sp1, Sp2, Sp3, and Sp4, with largely different mineral assemblages. A group of minor and trace elements such as Mn, Fe, Cd, Cu, Ag, and Pb with great variations in sphalerite are detected. It is notable that Cd in sphalerite displays an obvious elevation from Sp1 (mean = 2583 ppm), Sp2 (2489 ppm), and Sp3 (3249 ppm) to Sp4 (6324 ppm, a maximum up to 9356 ppm). Further temperature estimation via empirical GGIMFis geothermometer shows there is a clear drop from higher-temperature Sp1, Sp2, and Sp3 (mean = 320 degrees C) to low-temperature Sp4 (160 degrees C). Moreover, the concentrations of Mn sensitive to redox state also have a significant drop from early Sp1 (mean 3461 ppm), Sp2 (6373 ppm), and Sp3 (6763 ppm) to late Sp4 (5.28 ppm). Combined with a massive of pyrrhotite inclusions in Sp1, Sp2, and Sp3, against Sp4, we deduce that Sp1, Sp2, and Sp3 are formed in a relatively reductive condition, but Sp4 is strongly oxidative. Collectively, we conclude that the redox state, besides temperature, acts as an important factor for Cd enrichment, and the oxidized, lowtemperature hydrothermal fluids facilitate Cd enrichment in the Kangjiawan magmatic-hydrothermal system.