Ore‑forming process revealed by sphalerite texture and geochemistry: A case study at the Kangjiawan Pb–Zn deposit in Qin-Hang Metallogenic Belt, South China

Abstract

The origin of the abundant hydrothermal vein-type Pb–Zn deposits in the Qin-Hang Metallogenic Belt (QHMB) in South China remains disputed due to the poor constraints on the ore-forming process. The Kangjiawan (1.96 Mt @ 4.70% Zn and 6.02% Pb), is a representative Pb–Zn deposit in the central QHMB, and its mineralization can be divided into three stages: (I) pre-ore quartz-pyrite, (II) main-ore quartz-pyrite-galena-sphalerite, and (III) Late ore calcite-pyrite-galena-sphalerite. Stage II sphalerite (Sp2) commonly coexists with pyrite and chalcopyrite, and shows a homogeneous internal texture. Stage III sphalerite (Sp3) comprises two generations of sphalerite, namely the earlier, texturally homogeneous Sp3a (coexists with “chalcopyrite disease”), and the younger Sp3b that replaced Sp3a and coexists with galena.Sp2 is Fe-Mn-Co-In-rich, and characterized by the high In/Ga (avg. 2.6) and Zn/Cd (avg. 242.0), but low Zn/Fe (avg. 12.2) ratios, suggesting high ore-forming temperature, consistent with the results of the sphalerite GGIMFis geothermometer (307–314 ℃ with an average of 310 ℃). Sp3a has higher Se, Co, and Ni contents than Sp3b, implying the possible decline of temperature from Sp3a to Sp3b. Meanwhile, the Kangjiawan sphalerite geochemistry suggests a decreasing trend of fS2 from Sp2 (lg fS2 = − 10.5 to − 12.4) to Sp3 (lg fS2 = − 17.2 to − 20.3). These results indicate a dramatic change in the temperature and fS2 from the main-ore to the late-ore stage, probably caused by the mixing between meteoric water and magmatic-hydrothermal fluids in Stage III, as supported by published HO isotope data. The ubiquitous hydrothermal breccias with fine-grained pyrite cement suggest fluid boiling in Stage II. The Kangjiawan sphalerite geochemistry is similar to that of skarn-type sphalerite, consistent with the close spatial–temporal link between the Kangjiawan mineralization and the Shuikoushan granodiorite stock. Therefore, we inferred that the Kangjiawan is a Pb–Zn skarn deposit genetically related to the Shuikoushan granodiorite, with the ore precipitation caused by fluid boiling and meteoric water incursion.