The characteristics of the extraordinary germanium enrichment in the Wusihe large-scale Ge-Pb-Zn deposit, Sichuan Province, China and its geological significance.

Germanium (Ge) is an advantageous critical metal in China, and it is mainly mined in sphalerite as a by-product. Although much progress has been made on the Ge enrichment in sphalerite, the substitution of Ge and mechanisms of abnormal Ge enrichment remain poorly understood. The Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic district, located on the southwest margin of the Yangtze Block, is one of the most important production bases of Ge in China. Wherein the Wusihe large-scale deposit (5. 7Mt @ 8. 6% Zn, 2. 0% Pb) is characterized by abnormal Ge enrichment with a mean Ge ore grade of 72. 3 x 10(-6) and Ge concentrations up to 1934 x 10(-6) in sphalerite. The factors controlling such abnormal enrichment warrant further investigation. The Wusihe sphalerite is mainly formed in two hydrothermal stages: the early-stage (Spl) is white-brown-black under single polarized transmitted light, subhedral fine-grained and disseminated; whereas the late-stage (Sp(2)) is white-red or gray-brown-black, euhedralsubhedral fine- to coarse-grained, and massive or veined. The analytical results by the laser laser-ablation inductively coupled plasma mass spectrometry show that Ge concentrations in sphalerite are highly variable, with a range of 3. 17 x 10(-6) to 1934 x 10(-6)(averaged at 563 x 10(-6)), which is up to 1000 times of the Ge abundance of the earth's crust (1. 5 x 10(-6)). Germanium has a positive correlation with Fe, Cu, Ag and Pb, while it is negatively correlated with Cd. Chemical maps show that Ge is incorporated into sphalerite with isomorphic substitution. Structural similarities (SSIM) of the chemical maps show that Ge has the best correlations with Ag in Spl, and with Cu in Sp2. This implies the SSIM method has great implication potential in comparing element correlation. Nevertheless, the concentrations of Cu and Ag are often lower than those of Ge, and the mol ratios of (Ge/Cu) and (Ge/Ag) varies widely, hence a single element correlation analysis cannot directly be used to reveal the substitution. Comprehensive studies have found that the Ge substitution may have little to do with elements such as Cu and Ag, but directly replaced Zn in sphalerite at Wusihe (Ge4+ <>2Zn(2+) and/or Ge2+<>-Zn2+). Furthermore, concentrations within a single sphalerite particle (sector and oscillating zones) have a significant variation, suggesting that the Ge enrichment is crystallographically controlled and related to the precipitate rate of sphalerite. The difference in Ge contents between Spl and Sp2 is related to the fluid composition and changes in the physicochemical conditions the mineralization. Therefore, the abnormal enrichment of Ge in the Wusihe large-scale Ge-Pb-Zn deposit should be a coupling product of fluid composition, physicochemical conditions, precipitation rate and crystallization, and affected by special metallogenic conditions on the mineralogical and deposit scales.