Sources of the Laoxiongdong carbonate-hosted Pb–Zn deposit in Southwest China:constraints from S–Pb–Zn isotopic compositions

The Sichuan-Yunnan-Guizhou(SYG)Pb-Zn mineral province,which has significant Pb-Zn repositories in China,is situated in the western Yangtze Block.Despite decades of research,the Pb-Zn source of deposits is still disputed between the basement rocks,sedimentary wall rocks,or the Emeishan flood basalts.The newly-discovered Laoxiongdong Pb-Zn deposit is hosted by the Late Ediacaran Dengying Formation in the SYG province.Moreover,the Laoxiongdong orebodies lie between regional deep faults and close to the Proterozoic basement and Emeishan basalts.Hence,this deposit represents a good case study on the ore-forming material source for the regional Pb-Zn mineralization.The Laoxiongdong Pb-Zn ores are massive,vein,or disseminated and have mainly sphalerite,galena,pyrite,quartz,and calcite.In this paper,we present new sulfide S-Pb-Zn isotope data of the deposit.The sulfideδ34SCDT values(+11.8 to+16.5‰)suggest that the reduced sulfur was mainly sourced from evaporites in the Late Ediacaran-Cambrian sequences.Pb isotopic ratios(206Pb/204Pb=18.004-18.107,207Pb/204Pb=15.652-15.667,and 208Pb/204Pb=38.037-38.248)suggest that the lead metal was primarily originated from the basement rocks.The sphaleriteδ66Zn values(+0.16 to+0.37‰)are also highly similar(within error)to those of basement rocks(+0.10 to+0.34‰),suggesting a basement-rocks zinc source with minor contributions from the carbonate host rocks and Emeishan flood basalts.The narrow sphalerite Zn isotopic range(0.21‰)also indicates that the Zn isotopic fractionation between the sphalerite and initial fluid was limited during the sphalerite ore precipitation.Therefore,we propose that both the Late Ediacaran Dengying Formation rocks and Proterozoic basement rocks were important ore-forming material source for the Laoxiongdong deposit,whereas the Emeishan flood basalts represent only a minor ore-material source.

Zinc isotopic behavior of mafic rocks during continental deep subduction

Zinc isotopes may act as a new tool of tracking recycling of crustal materials that causes compositional heterogeneity of the mantle.This application relies on an investigation of Zn isotopic behaviors during slab subduction.In this study,we report Zn isotopic compositions for a suite of metabasalts(greenschists,amphibolites,and coesitebearing eclogites)from the Dabie Orogen(China),which were formed via the subduction of mafic rocks into different depths and up to>200 km.Three out of eight greenschists are characterized by lighterδ^(66)ZnJMC-Lyon(0.10‰-0.16‰)than those of global basalts(0.28‰±0.05‰),which may be caused by crustal assimilation of the protoliths by sedimentary rocks due to their extremely high ^(87)Sr/^(86)Sr(up to 0.7130)and lowεNd values(down to−12.3).The remaining greenschists have relatively low ^(87)Sr/^(86)Sr and theirδ66Zn values(0.21‰-0.38‰)overlap the ranges of amphibolites(0.18‰-0.32‰)and coesite-bearing eclogites(0.18‰-0.36‰).There is no correlation betweenδ^(66)Zn and sensitive indicators of dehydration(Rb/TiO_(2),Ba/Yb,and H_(2)O+),suggesting that no detectable Zn isotope fractionation has occurred during the deep subduction of mafic rocks even into>200 km,which is attributed to the limited loss of Zn during prograde metamorphism and dehydration.Thus,Zn isotopic compositions of the deeply subducted mafic rocks are inherited from their protoliths.Considering that these metamorphosed rocks have higherδ66Zn than that of the mantle value by up to 0.2‰,the recycled/subducted mafic crust can incorporate isotopically heavy Zn into the mantle.The subducted slabs may partially melt and generate a metasomatized mantle,resulting in changes of Zn isotopic composition of the hybridized mantle as have been observed in some mantle xenoliths and basaltic lavas.

Copper and zinc isotope variations in ferromanganese crusts and their isotopic fractionation mechanism

Ferromanganese(Fe-Mn)crusts are potential archives of the Cu and Zn isotope compositions of seawater through time.In this study,the Cu and Zn isotopes of the top surface of 28 Fe-Mn crusts and 2 Fe-Mn nodules were analysed by MC-ICP-MS using combined sample-standard bracketing for mass bias correction.The Zn isotope compositions of the top surface of Fe-Mn crusts are in the range of 0.71‰to 1.08‰,with a mean δ^(66)Zn value of 0.94‰±0.21‰(2 SD,n=28).The δ^(65)Cu values of the top surface of Fe-Mn crusts range from 0.33‰to0.73‰,with a mean value of 0.58‰±0.20‰(2 SD,n=28).The Cu isotope compositions of Fe-Mn crusts are isotopically lighter than that of dissolved Cu in deep seawater(0.58‰vs.0.9‰).In contrast,the δ^(66)Zn values of Fe-Mn crusts appear to be isotopically heavy compared to deep seawater(0.94‰±0.21‰vs.0.51‰±0.14‰).The isotope fractionation between Fe-Mn crusts and seawater is attributed to equilibrium partitioning between the sorption to crusts and the organic-ligand-bound Cu and Zn in seawater.The Cu and Zn isotopes in the top surface of Fe-Mn crusts are not a direct reflection of the Cu and Zn isotopes,but a function of Cu and Zn isotopes in modern seawater.This study proposes that Fe-Mn crusts have the potential to be archives for paleoceanography through Cu and Zn isotope analysis.

Zinc Isotope Characteristics in the Biogeochemical Cycle as Revealed by Analysis of Suspended Particulate Matter(SPM) in Aha Lake and Hongfeng Lake, Guizhou, China

Zn isotope is a useful tool for tracing biogeochemical processes as zinc plays important roles in the biogeochemistry of natural systems. However, the Zn isotope composition in the lake ecosystems has not been well characterized. In order to resolve this problem, we investigate the Zn isotope compositions of suspended particulate matter(SPM) and biological samples collected from the Aha Lake and Hongfeng Lake, and their tributaries in summer and winter, aiming to explore the potential of this novel isotope system as a proxy for biogeochemical processes in aqueous environments. Concentration of dissolved Zn ranges from 0.65 to 5.06 μg/L and 0.74 to 12.04 μg/L for Aha Lake and Hongfeng Lake, respectively, while Zn(SPM) ranges from 0.18 to 0.70 mg/g and 0.24 to 0.75 mg/g for Aha Lake and Hongfeng Lake, respectively. The Zn isotope composition in SPM from Aha Lake and its main tributaries ranges from -0.18‰ to 0.27‰ and -0.17‰ to 0.46‰, respectively, and it varies from -0.29‰ to 0.26‰ and -0.04‰ to 0.48‰, respectively in Hongfeng Lake and its main tributaries, displaying a wider range in tributaries than lakes. These results imply that Zn isotope compositions are mainly affected by tributaries inputting into Aha Lake, while adsorption process by algae is the major factor for the Zn isotope composition in Hongfeng Lake, and ZnS precipitation leads to the light Zn isotope composition of SPM in summer. These data and results provide the basic information of the Zn isotope for the lake ecosystem, and promote the application of Zn isotope in biogeochemistry.