Iron And Copper Isotope Geochemistry Of Tonqlinq Ore District Of The Middle-Lower Yangtze River Valley

The stable isotopes of transition metals have attracted great interests as new tracers for monitoring metallogenic processes. However, it still needs a lot of investigations on how to use these tracers properly to solve significant scientific problem. The Middle-Lower Yangtze River valley is one of the most important metallogenic belts of igneous rocks-related Cu-Fe-Au-Mo ore deposits in East China, and Tongling is one of the most typical ore districts amongst this belt in terms of metallogenic scale and complexity. Xinqiao manto-type deposit, Dongguashan porphyry-skarn type deposit and Fenghuangshan skarn-type deposit in Tongling district were selected for this study. Through combination of detailed field works with systematic Fe and Cu isotopes investigation, some significant advances have been made:1) Fe isotope compositions of mineral separates and bulk samples from Xinqiao, Dongguashan and Fenghuangshan deposits were investigated for the first time, which displays distinct differences both between and within mineral phases. However, overall variations in δ57Fe values of the three deposits have some overlap. For co-existing sulfides, bornite samples show a tendency towards enrichment in light isotopes, whereas pyrite samples are enriched in heavy isotopes, relative to chalcopyrite.2) Cu isotope compositions of mineral separates and bulk samples from the Xinqiao, Dongguashan and Fenghuangshan deposits were also systematically measured in this study. The overall variations in 865Cu of three deposits overlap with each other and show a tendency towards enrichment in heavy isotopes relative to NBS976. For co-existing sulfides, bornite samples show a tendency towards enrichment in light isotopes relative to chalcopyrite.3) Geochemical behavior of Fe isotopes during fluid exsolution was investigated in this study and regularity of Fe isotopes variations in this process was discovered. Fe isotopes fractionation occurs during fluid exsolution which results in fluid enriched in light Fe isotopes relative to the parental magma. This regularity provides important foundation for tracing metal source using Fe isotopes.4) Geochemical behavior of Fe isotopes during fluid evolution process was investigated in this study and regularity of Fe isotopes variations in this process was discovered. Precipitation of Fe-bearing minerals results in the temporal evolution of Fe isotope compositions of fluid. Precipitation of Fe (III) minerals leaves the remaining fluid enriched in light isotopes, while precipitation of Fe (II) minerals makes the Fe isotopic composition of the fluid progressively heavier. This recognition provides significant foundation for tracing minerlization process using Fe isotopes.5) Spatial zonation of Cu isotopes was observed in Dongguashan deposit and its plausible mechanism was discussed in this study. Away from stock, Cu isotopes compositions become heavier progressively. This spatial zonation maybe caused by Cu isotopes fractionation during fluid evolution, accompied with limited contribution of incorporation of Cu from host rocks during hydrothermal metasomatic processes6) This study provides direct constraints for the first time that Fe in the skarn-type ore deposit is sourced from igneous rocks. Fe isotopes fractionate during fluid exsolution from the stock, and the feature of light Fe isotope enrichment in the exsolved fluid cannot be explained by Fe incorporation from country-rocks. This implies that Fe in the initial fluid must come from the igneous stock. Furthermore, actual evolution of Fe isotope composition of the fluid during subsequent precipitation matches well with prediction of mass fractionation made on the basis of both theoretical calculation and experimental results. This strongly suggests that all Fe precipitated during the mineralization is predominantly from a single source.7) A theoretical model for tracing metellogenic processes of skarn-type deposit by using Fe isotopes was bulid on the basis of the above mentioned investigations, which provides a significant foundation for metallogeny study of skarn-type deposit.