Controlling factors of sphalerite and galena deposition in Baiyinnuo er skarn deposit, Inner Mongolia, China

The temperature,pH,f O 2 and Cl-activity have been considered to be significant physicochemical parameters which controlled sulphide mineralization.Especially,the change of pH is considered to play one of the key roles in skarn deposit because of the involvement of marble.But few scholars have evaluated the impact of each parameter on mineralization.In this study,the authors have constructed thermodynamic model,calculated the important aqueous species activities in Baiyinnuo er deposit,and evaluated the influence of these parameters on the solubility of Pb and Zn.The results show that the log f O 2 decreased from-33.834 to-39.256,pH decreased from 4.0 to 3.7 and the Cl-activity increased from 0.25 to 0.30 in the main mineralization stage.The effect of f O 2 was readily ruled out because the reaction of Pb and Zn precipitation did not involve changes in the valence of the element.The decreased pH was impossible to be a mineralization factor,which was confirmed by metal solubility evaluation.The Cl-activity was also considered to be a negative factor.In fact,the dominant factor of mineralization was temperature,not the combination of various parameters as previously thought.Even in the skarn stage,the main factor to make the metal solubility>10%was also the high temperature.A significant increase in pH only occurred in the post-ore stage in response to the production of large amount of calcite.

Timing of the formation of the Baiyinnuo'er skarn Zn–Pb deposit,NE China:evidence from sulfide Rb–Sr dating

The Baiyinnuo’er deposit in northern China is located in the south section of the Great Xing’an Range,and it is the largest skarn Zn–Pb deposit in the region.Skarn and Zn–Pb orebodies mainly occur between the different units of the Permian Huanggangliang Formation,or within the contact zone between the intrusive rocks and the marble.Although Baiyinnuo’er has been well investigated previously,the timing of the Zn–Pb mineralization is still controversial,largely due to the lack of appropriate ore or alteration minerals that could be directly used for isotopic dating.In this study,we report the results of Rb–Sr isotopic analysis for sphalerite and pyrite samples from the Baiyinnuo’er orebodies,which yielded two isochron ages of 137.4±3.4 and 140.0±7.8 Ma,respectively,constraining the Zn–Pb mineralization time of the deposit as the Early Cretaceous.The data are also consistent with the age of the granitoids in the mining area,indicating a potential genetic relationship between the Early Cretaceous magmatism and mineralization.Many other intrusion-related hydrothermal deposits(including the two typical skarns,Huanggang and Haobugao)in the southern Great Xing’an Range also share similar mineralization ages(i.e.,140–130 Ma).Together,these data suggest an Early Cretaceous mineralization event in this region,and this largescale mineralization could be related to the regional tectonic regime transition from compression to extension as a result of the rollback of the subducted Paleo-Pacific plate.The initial87 Sr/86 Sr ratios of the sphalerite and pyrite samples are 0.70569 and 0.70616,respectively,implying that the ore-forming material could have a significant contribution from the mantle components.The current study shows that sulfide Rb–Sr dating could be used in deciphering the timing of skarn deposit formation.