Mineral Assemblages And Nd-Hf-O Isotopes Research Of The Baiyun Pyrophyllite Deposit, Wutai Country, Shanxi Province

The Baiyun pyrophyllite deposit,Wutai of Shanxi Provnice,is located in the Archean granite-greenstone belt of the North Chian Carton?NCC?.The formation age and origin of ore-forming fluids are still in lack of research.Nevertheless,the genetic relationship between the gold mineralization in wall rocks and the pyrophyllite deposit,is still unknown.In this context,based on detailed field observations,mineralogy,geochemistry and H-O-Nd-Hf isotope of pyrophllite ores were carried out,aiming to shed light on the timing of the hydrothermal activity,sources of ores and its relationship with the wall rock gold mineralization.The highly altered zircons from the pyrophyllite ores,yielded a lower intercepted U-Pb age of 136±7 Ma?MSWD=0.99?,which is interpreted to be representative of fluid alteration timing,reflecting the pyrophllite formation temporally consistent with the peak timing of the lithospheric destruction in the NCC.Hydrothermal alteration in the mine was dominated by pyrite-phyllic and pyrophyllite alteration.The gold mineralization was mainly developed in the pyrite-phyllic alteration zone.Log?O₂-pH diagram exhibited that the early pyrite-phyllic alteration fluid was weak acid to partial neutral?pH=5.24⁵.87?with relatively low oxygen fugacity,by contrast,the pyrophyllite alteration was acidic?pH=2.07².20?with relatively high oxygen fugacity.The mass-balance calculation showed that the contents of SiO₂,Na₂O and K₂O from the pyrophyllite ores were dramatically increased by comparison with the wall rocks,whereas,the other major oxides were prominently moved out.The abundances of Nb,Ta,Th and U were relatively moved in and Sr,Ba,Zr,Hf were relatively decreased.The above elements characteristics exhibited that the ore-forming fluids may be riched in volatile components of F,Cl and H₂O.Comparison of the metal elements contents of pyrophyllite ores and wall rocks,we concluded that the fluids for pyrophyllite alteration leading to a superimposed destroy on the early formed gold mineralization,are not conducive for Au precipitation.Considering the H-O isotope,sources of hydrothermal fluids for the Baiyun pyrophyllite deposit were magmatic affinity.Nd-Hf isotopic compositions of pyrophyllite ores exhibite that the ore-forming materials were mainly derived from mantle,with remarkable isotopical materials of asthenosphere.In all,several viewpoints can be derived as followings:?1?The Baiyun pyrophyllite deposit formed in Cretaceous?130¹35 Ma?,highly consistent with the time of regional magmatism and mineralization,as products of lithospheric destruction;?2?Nd-Hf isotopic compositions of the ore-forming materials were mainly from depleted and enriched mantle component,reflecting their genesis from mantle upwelling;?3?No genetic relationships between the pyrophyllite alteration and the gold mineralization.The fluids for pyrophyllite alteration leading to a superimposed destroy on the early formed gold mineralization,are not conducive for Au precipitation...