A fossil hydrothermal system or a source-bed in the Madiyi Formation near the Xiangxi Au–Sb–W deposit, NW Hunan, PR China?

In China, a great number of Au–(Sb–W) deposits or mineralizations are located in NW Hunan. The Xiangxi deposit is the largest and, as 80% of such metal accumulations in this region, occurs in the late Proterozoic—low grade metamorphic red slate Madiyi Formation. Ore bodies of the Xiangxi Mining District are strictly structurally controlled. Gold, antimony, tungsten, and arsenic are strongly concentrated relative to bed rock lithologies. Sulfur, Hg, Mo, and locally Cu, Pb, and Zn are weakly enriched only. The Madiyi Formation was deposited under oxidizing conditions in a shallow marine environment, and is characterized by a high Fe2O3 content, with local accumulations of CaO. The stratigraphic horizon is probably not a source-bed for the Au, Sb, and W mineralizations in this region. High contents of these elements and As were likely introduced by hydrothermal fluids. Background values for Au, Sb, W, and As in the Madiyi Formation are 1.4 ppb, 0.42 ppm, 1.9 ppm, and 1.4 ppm, respectively. The stratigraphic sequence acts as a geochemical barrier controlling the distribution of Au (or/and Sb–W).