Two-stage enrichment of germanium in the giant Maoping MVT Pb-Zn deposit, southwestern China: Constraints from in situ analysis of multicolor sphalerites

The Maoping Pb-Zn deposit is one of the largest deposits in the Sichuan-Yunnan-Guizhou (SYG) MVT province in southwest China, which is also one of the biggest germanium producers in China. The hydrothermal minerali-zation process in this deposit can be divided into: (I) dolomite-pyrite stage, (II) sphalerite-galena stage, and (III) calcite stage. Five colors of sphalerites are recognized, including (C1) black, (C2) red brown, (C3) deep brown, (C4) light brown, and (C5) light red sphalerite. Pure colored C1, C2, C3 and C4 are dominanting in the entire sphalerite sequence, and there are also few rhythmic zonal sphalerites (e.g., Z1, Z2) composed of above four colored sphalerites. The C5 sphalerite precipitates last, and often cross-cuts other colored sphalerites in fine veins. Galena usually postdates than multicolor sphalerites. Based on mu-XRF and LA-ICP-MS mapping analysis, the Fe and Cu respectively dominate the variations of many other trace elements (e.g., Mn, Ag, Cd, Ga, Ge). The C5 and C1 sphalerites show the highest germanium contents (geometric mean 49 ppm for C5; 22 ppm for C1), followed by C4 (geometric mean 13 ppm). The contents of germanium and copper show a strong linear corre-lation for all colored sphalerites, in which germanium incorporates into sphalerite by the mechanism of 2Cu+ + Ge4+ -> 3Zn2+, while the nano-inclusion of Cu(Ag)-As(Ga)-S sulfosalt may evenly distribute in C5 sphalerite, resulting in its higher (Cu/Ge)mol ratios. According to the temperature and sulfur fugacity deduced from element compositions of sphalerite, the higher sulfur fugacity is more beneficial to the enrichment of germanium, relative to temperature. The sulfur isotope data suggest that the sulfur in C1, C2, C3 and C4 sphalerites originated from the Zaige and Baizuo Formations via bacterial sulfide reduction (BSR), in contrast to the sulfur in C5 sphalerite that mainly derived from the Datang Formation, with significantly higher sulfur fugacity and lower temperature. It is concluded that the fluids with germanium and reduced-sulfur derived from the Zaige and Baizuo Formations, mixed with the metal-bearing (e.g., Pb, Zn, Ag) basin brine, and caused the first-stage germanium enrichment in C1, C2, C3 and C4 sphalerites. Then the second-stage enrichment of germanium in C5 sphalerite was due to the involvement of fluid with germanium and reduced-sulfur derived from the Datang Formation.