Metallogenic model of the Jinchang Au-Ni deposit in the Ailaoshan belt, SW China, determined on the basis of pyrite trace element contents, in-situ sulfur isotope composition and PGE geochemistry

The Ailaoshan metallogenic belt in southwestern China contains numerous shear zone-related Au deposits with Au reserves greater than 200 tonnes. However, the source of ore-forming fluids and metals is controversial, hampering the construction of genetic models. In this study, we selected the Jinchang Au-Ni deposit in the northern segment of the Ailaoshan belt for detailed investigation of pyrite chemistry and in-situ sulfur isotope compositions using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The petrographic and pyrite chemistry analyses revealed four types of hydrothermal pyrite. Py1 pyrite is euhedral to subhedral and has in-situ sulfur isotope (δ³⁴S) values of -5.30 to 6.26‰ (median: -2.93‰) and low concentrations of Au, Ni, As, Sb, Cu, Zn, Pb, and Ag. Py1 is interpreted to have formed during the early hydrothermal stage. Py2 pyrite is subhedral to anhedral and porous with variable size. Although Py2 and Py3 show different textural relationships, we conclude that both of them formed during the intermediate hydrothermal stage as evidenced by similar sulfur isotope compositions and trace element contents, and common intergrowth textures. Py2 and Py3 pyrite have in-situ δ³⁴S values ranging from -11.6 to 1.29‰ (median: -4.45‰) and relatively elevated concentrations of Au, Ni, As, Sb, Cu, Zn, Pb, and Ag. In Py2 and Py3 pyrite, Ni concentrations correlate with Co concentration, whereas Au exhibits moderate positive correlations with Ni, Co, and As, indicating that Au, Ni and other metals were predominantly sourced from the Jinchang ultramafic rocks. Py4 pyrite is euhedral in shape and has relatively low in-situ δ³⁴S values of -23.5 to -4.34‰ (median: -8.33‰). The metal concentrations in Py4 pyrite are lower than those in Py2 and Py3 pyrite. In general, the metal concentrations in the different pyrite populations display systematic changes, with low levels in Py1, higher levels in Py2 andPy3, and low levels again in Py4.

The average total platinum-group element (PGE) concentration of the Jinchang Ni-bearing ultramafic rocks is 26.3 ppb, similar to the average value of the primitive mantle of 22.0 ppb. The Au-Ni ores from the Jinchang deposit have an average Pd/Pt ratio of 5.98, similar to that of hydrothermal Ni ores but higher than that of typical magmatic Ni ores. In contrast, the Jinchang ultramafic rocks have low Pd/Pt ratios between 0.57 and 1.13, suggesting that the PGEs in the Jinchang intrusion are of magmatic origin. Similarly, the Rh/Ir ratios of Au-Ni ores are consistent with those of hydrothermal Ni ores but differ from those of magmatic Ni ores.

The pyrite chemistry, in-situ sulfur isotope compositions, and PGE geochemistry suggest that the ore-forming fluids and metals associated with the Jinchang Au-Ni deposit were dominantly derived from the Jinchang ultramafic intrusion. Mesozoic felsic magmatism or Cenozoic regional strike-slip activity may have triggered migration of the ore-forming fluids, leaching of metals from the Jinchang intrusion and precipitation of these metals at the boundary between the intrusions and carbonaceous rocks.