Origin of the Devonian carbonate-hosted Banbianjie Ge-Zn deposit, Guizhou Province, South China: Geological, mineralogical and geochemical constraints

This paper investigates the origin of the newly discovered Devonian carbonate-hosted Banbianjie Ge-Zn deposit in South China. This deposit is rich in Ge (>900 t @ 110 ppm Ge) and contains 7.7 Mt of sulfide ores at a mean grade of 5.1 wt% Zn. The mineralization is stratabound or veined and is structurally controlled by the E-W trending Huangsi fault. Sulfide ores are dominated by sphalerite and dolomite with subsidiary pyrite, marcasite, and calcite, and minor galena. The mineralization forms massive, veined, brecciated, and disseminated structures and the hydrothermal minerals present in the deposit have colloform, fibrous, granular, and acicular textures. The hydrothermal dolomite has positive Ce (delta Ce = 1.13-1.20) and negative Eu (delta Eu = 0.43-0.68) anomalies that are indicative of formation in a weakly acidic, reduced, and low-temperature environment. The delta 13CPDB and delta 18OSMOW values of the dolomite range from -0.97%o to -0.24%o and +13.89%o to +21.84%o, respectively, indicating that the dissolution of carbonate wall rocks was involved in the formation of hydrothermal dolomite. Theoretical C-O isotopic calculations further suggest that the hydrothermal dolomite formed as a result of water/ rock (W/R) interaction and CO2 degassing. The delta 34S values of the sulfides (-11.23%o to -4.95%o) are markedly lower than those of the Devonian seawater sulfate and sulfate from evaporates in the Devonian strata (+21.9%o to +25.9%o), suggesting that the reduced sulfur was derived from evaporates within the ore-hosting sedimentary rocks by bacterial sulfate reduction (BSR). The Banbianjie sulfides have an upper crustal Pb isotopic affinity, as demonstrated by their 206Pb/204Pb (18.335-18.343), 207Pb/204Pb (15.725-15.735), and 208Pb/204Pb (38.288-38.326) ratios, suggesting the metals were derived from metamorphosed basement rocks. These data suggest that: i) the Banbianjie deposit formed as a result of a combination of weakly acidic, reduced, and epigenetic low temperature mineralized fluids, trap structures, and favorable lithologies; ii) W/R interaction and CO2 degassing caused the precipitation of hydrothermal dolomite, whereas mixing of metals and S2- was the main formation mechanism of the sulfides; and iii) the Banbianjie Ge-Zn deposit is a typical Mississippi Valleytype (MVT) deposit.

Automated summary

This paper investigates the origin of the newly discovered Devonian carbonate-hosted Banbianjie Ge-Zn deposit in South China. The deposit is rich in Ge and sulfide ores, and the mineralization formed in a weakly acidic, reduced, and low-temperature environment.