Source of Ore-Forming Fluid and Material in the Baiyun Gold Deposit, Liaoning Province, NE China: Constraints from H-O-S-Pb Isotopes and in-situ Analyses of Au-Bearing Pyrites

The Baiyun deposit is a large gold deposit at the western end of the Liaoji rift zone in Liaoning Province, which has produced both auriferous quartz-vein type and altered-rock type mineralization. The ore bodies are mainly hosted in schist from the Gaixian Formation of the Liaohe Group. A detailed field geological survey showed that the quartz-vein type gold ore bodies are distributed in the near EW-trending and occur in the extensional tectonic space of schist in the Gaixian Formation, and the altered-rock type gold ore bodies are distributed in the near EW-trending structural belt and occur near in the Gaixian Formation of biotite schist, biotite granulite, marble and the upper footwall of dike. To further elucidate the source of ore-forming fluid and material in the Baiyun gold deposit, the H-O isotopes for quartz, S and Pb isotopes, in-situ trace elements for sulfides from quartz-vein and altered-rock type mineralization were studied. The H-O isotopic δD_(V-SMOW) and δ^(18)O_(H2O) values of the auriferous quartz range were from-88.8‰ to-82.2‰ and-1.95‰ to 4.85‰, respectively, suggests that the ore-forming fluids were mainly magmatic water with minor meteoric water. The distribution ranges of in-situ S isotopic compositions of Au-bearing pyrite in the quartz-vein type and altered-rock type ores were-8.38‰–-10.47‰(with average values of-7.89‰) and 11.38‰– 17.52‰(with average values of 11.55‰), respectively, indicating that the S isotopic compositions of the two ore types were clearly different. The in-situ Pb isotopic ratios changed almost uniformly, which showed that they had the same lead isotopic source. Based on the analysis of S and Pb isotopic compositions, the metallogenic materials in the Baiyun gold deposit were primarily from deep magma, and some wall rock materials may have been mixed in the metallogenic process. Co/Ni diagram shows that most Au-bearing pyrites have magmatic-hydrothermal or sedimentary alteration properties, and Au/As ratios were between 0.001 and 0.828(the average value was 0.07), indicating that the ore-forming fluid in the Baiyun gold deposit may have been deep magma. Combining the geological, trace element, and isotopic data, as well as data from previous studies, we propose that the Baiyun gold deposit is a magmatic-hydrothermal ore deposit.

Petrology,Geochemistry,and Sr-Nd-S Isotopic Compositions of the Ore-Hosting Biotite Monzodiorite in the Luanjiahe Gold Deposit,Jiaodong Peninsula,China

The Jiaodong Peninsula is one of the most important Au ore provinces in China.There is an ongoing debate on the correlation between ore formation and magmatism in this province,because few intrusive rocks exhibit a clear association with ore deposits.A mineralized biotite monzodiorite(BM)stock,with disseminated ore,pervasive phyllic alteration,and no deformation,was found in a borehole in the footwall of the Zhaoping fault within the Luanjiahe Au deposit,which may shed light on this debate.The biotite monzodiorite contains explosion breccias,miarolitic cavities,skeletal and den-dritic quartz,and late-stage evolved aplite dikes,and the in-situδ34S values of the disseminated pyrite which is associated with Au mineralization are-1.7‰ to 7.3‰(mean=3.5‰),indicative of a magmatic-hydrothermal system.These findings,combined with the reported age of 123 Ma,show that the intru-sion has close spatial,temporal,and geochemical relationships with Au mineralization in the area.The biotite monzodiorite is metaluminous,high-K calc-alkaline and shoshonitic,with enrichment in light rare earth elements(REEs)and large-ion lithophile elements(LILEs),depletion in high-field-strength elements(HFSEs),and enriched Sr-Nd isotopic compositions.The intrusion may be the product of par-tial melting of enriched lithospheric mantle with a small lower crustal component.The hydrous,Au-bearing,enriched mantle source,and the strongly oxidized magma that was generated,created fa-vorable conditions for Au mineralization.