The Bake quartz vein-type gold deposit is one of the most well-known gold deposits in SE Guizhou. The mineralization age of auriferous arsenopyrite determined using Re-Os chronology is 412 ± 21 Ma (MSWD = 2.6), which is in good agreement with the regional-scale geologic evidence and confirms that the Caledonian was an important gold metallogenetic period in the Xuefeng region. Contrastingly, the high initial 187Os/188Os ratio (2.7 ± 1.2) suggests a crustal source for the ore-forming material of the deposit. Furthermore, the REE patterns, Eu/Eu*, Ce/Ce* and Y/Ho ratios of the arsenopyrites are all similar to those of the wall rocks, indicating that the ore was perhaps derived from the wall rocks belonging to the Xiajiang Group.
The Sichuan–Yunnan–Guizhou (SYG) Zn–Pb metallogenic zone in SW China contains >400 carbonate-hosted hydrothermal Zn–Pb deposits. Some of these, such as the Huize, Tianbaoshan, and Daliangzi deposits, are super-large deposits with significant reserves of Cd, Ge, and Ag. However, the sources of these metals remain controversial. This study investigated the Cd isotopic geochemistry of the Huize deposit, the largest Zn–Pb deposit in the SYG area. Sphalerites formed at three stages in the deposit have different colors: black or dark brown (Stage I), red (Stage II), and light-yellow (Stage III). The δ114/110Cd values of the sphalerites are in the order Stage III < Stage I < Stage II. Kinetic isotopic fractionation is likely the key factor causing the lower δ114/110Cd values in the early formed Stage I sphalerites than in later-formed Stage II sphalerites, with cooling of ore-forming fluids being responsible for the still lower values of the Stage III sphalerites. In galena, the δ114/110Cd values are inversely correlated with Cd contents and tend to be higher in high-Zn galena. We speculate that Cd isotopic fractionation was significant during the precipitation of sphalerite and galena, with light Cd isotopes being enriched in galena rather than sphalerite. Comparison of the Cd isotopic signatures and Zn/Cd ratios of different endmembers suggests that the δ114/110Cd values and Zn/Cd ratios of sphalerite from the Huize deposit, as well as other large-scale deposits from the SYG area, are lie in those range of Emeishan basalts and sedimentary rocks and the mean δ114/110Cd values of these deposits show good negative correlation with 1/Cd, suggesting that the ore-forming materials of these deposits were derived from the mixing of Emeishan basalts and sedimentary rocks. This study demonstrates that Cd isotopes can be useful proxies in elucidating ore genesis in large Zn–Pb deposits.
<p>Table S1: EPMA trace-element contents of jamesonite (Jam) and sphalerite (Sp) from the Jianzhupo deposit. Figure S1: General paragenetic sequence of minerals in the Jianzhupo deposit. Figure S2: Backscattered electron images (A to J) and photomicrographs (K to L; under reflected light) of sulphides from the Jianzhupo deposit. Table S2: LA-ICP-MS data (in ppm) obtained for sulfides from the Jianzhupo deposit. Table S3: End-members compositions and mixing modeling on Zn, Cd, and δ114/110Cd values.</p>
Cadmium (Cd) isotopes have great potential for understanding Cd geochemical cycling in soil and aquatic systems. Iron (oxyhydr)oxides can sequester Cd via adsorption and isomorphous substitution, but how these interactions affect Cd isotope fractionation remains unknown. Here, we show that adsorption preferentially enriches lighter Cd isotopes on iron (oxyhydr)oxide surfaces through equilibrium fractionation, with a similar fractionation magnitude (Δ114/110Cdsolid-solution) for goethite (Goe) (-0.51 ± 0.04‰), hematite (Hem) (-0.54 ± 0.10‰), and ferrihydrite (Fh) (-0.55 ± 0.03‰). Neither the initial Cd2+ concentration or ionic strength nor the pH influence the fractionation magnitude. The enrichment of the light isotope is attributed to the adsorption of highly distorted [CdO6] on solids, as indicated by Cd K-edge extended X-ray absorption fine-structure analysis. In contrast, Cd incorporation into Goe by substitution for lattice Fe at a Cd/Fe molar ratio of 0.05 preferentially sequesters heavy Cd isotopes, with a Δ114/110Cdsolid-solution of 0.22 ± 0.01‰. The fractionation probably occurs during the transformation of Fh into Goe via dissolution and reprecipitation. These results improve the understanding of the Cd isotope fractionation behavior being affected by iron (oxyhydr)oxides in Earth's critical zone and demonstrate that interactions with minerals can obscure anthropogenic and natural Cd isotope characteristics, which should be carefully considered when applying Cd isotopes as environmental tracers.
Deep-sea mining magnifies the release of heavy metals into seawater through oxidative dissolution of seafloor massive sulfide (SMS). At present, there is little information about how the metals released into seawater might be affected by the mineral assemblages, seawater conditions, and solid percentages. Here, leaching experiments were carried out to examine the behavior of three sulfides from the Southwest Indian Ridge, under conditions that replicated deep and shallow seawater environments at three solid-liquid ratios. The results demonstrated that sphalerite dissolved rapidly, and the metals released in both experimental conditions were comparable, potentially reflecting galvanic interactions between the sulfide minerals. Large quantities of the released metals were removed from the solutions when hydrous ferric oxides formed, especially for shallow seawater conditions. A comparison of metal concentrations in the leachates with the baseline metal concentrations in natural seawater indicated that most of the released metals, when diluted with seawater, would not have widespread impacts on ecosystems. Based on the obtained unique oxidative dissolution properties of each SMS at variable solid-liquid ratios, targeted wastewater discharge treatments are proposed to minimize impacts from the dissolved metals. This study will support the development of robust guidelines for deep-sea mining activities.
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