Mineralization processes for sphalerite in MVT deposits: insights from the Niujiaotang–Qingshuitang ore field, Guizhou, SW China

Although Mississippi Valley-type (MVT) deposit mineralization processes are well characterized, the associated ore-forming fluids, which are crucial to understanding Pb and Zn mineralization, should be investigated further. With more than 0.7 million tonnes (Mt) of Zn reserves at average grades ranging from 4 to 8%, the Niujiaotang–Qingshuitang Zn ore field comprises representative deposits such as those of Caiyuanhe, Heli and Qingshuitang. The ore bodies, which exhibit stratiform, vein-like and lenticular forms, are hosted within the Cambrian Qingxudong Formation dolomite and are spatially associated with the NE-trending Mandong fault. This Zn ore field in SW China is a classic example of Zn-rich mineralization. However, the genetic mechanisms and processes responsible for the Zn-rich nature of these MVT Pb‒Zn deposits remain poorly understood. This study aims to characterize Zn mineralization in the Niujiaotang–Qingshuitang Zn ore field, SW China. The REE concentrations in hydrothermal dolomite (Dol2) from the syn-ore stage vary between 5.63 and 32.50 ppm (with an average of 14.07 ppm), whereas in hydrothermal dolomite (Dol3) from the post-ore stage, the values range from 45.63 to 95.69 ppm (with an average of 72.03 ppm). The fluid inclusions of Dol2 and Dol3 exhibit homogenization temperatures ranging from 98 to 204°C (with an average of 143°C) and from 98 to 195°C (with an average of 132°C), with corresponding salinity ranges of 13.0–29.5 wt% (with an average of 20.9 wt%) and 16.0–25.7 wt% (with an average of 18.9 wt%) NaCl equivalent, respectively. Sphalerite inclusions exhibit homogenization temperatures ranging from 95 to 230°C (with a mean of 143°C) and salinities ranging from 13.8 to 28.9 wt% (with a mean of 23.7 wt%) NaCl. The δ 13 C fluid , δ D fluid and δ 18 O fluid values of the ore-forming fluids range from −4.4 to −3.8‰ (with a mean of −4.1‰), −36.4 to −27.8‰ (with a mean of −31.9‰) and 4.9 to 5.7‰ (with a mean of 5.3‰), respectively. In this study, an Fe- and Zn-enriched NaCl–H 2 O basement brine is identified as the ore-forming fluid responsible for the Zn-rich MVT deposits. During its low- to medium-temperature migration, the fluid was channelled by the Mandong fault and Wangsi anticline structural framework, which provided ideal pathways and traps. During migration, the fluid leached sulfur (reduced from evaporites to H 2 S) from wall rocks and ultimately precipitated within dolomite faults and fractures. The distinct Zn-rich character of the deposits stems from the following two key factors: a primary derivation from the Zn-rich basement of the Yangtze Block and a chemical preference for Zn enrichment, which is driven by the differing fractionation coefficients of Zn- and Pb-chloride complexes in the NaCl–H 2 O system. This study advances the understanding of the genesis of MVT deposits worldwide.