Hydrothermal alteration in the Jiaojia gold deposit Jiaodong China

Jiaodong Peninsula,the most important gold province in China,is an area with concentration occurrence of largesuperlarge gold deposits,the proved reserves in Jiaodong Peninsula account for nearly 1 /3 of the country's. Fracture zone altered type gold deposit is the most important deposit type which accounts for more than 90% of the proved gold reserves in Jiaodong Peninsula.Jiaojia gold deposit is named after"Jiaojia-type"fracture zone altered rock type gold deposit,large-scale sericite-quartz alteration zone( with the width of 20 ~200m) and potassic alteration zone( with the width of 50 ~300m) occur in the gold deposit. Altered rock type gold orebody mainly develops in the sericite-quartz alteration zone which is in the footwall of Jiaojia fault zone. Based on the detailed geologic observations in the field,this paper found out the alteration type and mineral assemblage of the Jiaojia gold deposit,collected different types of alteration rock samples scientifically,and conducted rock geochemistry element analysis which uses the method of the mass balance to discuss the regularity of elements migration in the hydrothermal alteration process and mechanism. Thereinto,potassic alteration occurs in the premineralization,potassic granite is usually as lumpy and breccia residual in the pyrite-sericite-quartz and sericite-quartz altered rock which is controlled by the secondary faults in the footwall of the Jiaojia fault,and the scale is controlled by the faults. The scale of sericite-quartz altered rock is the largest in the footwall of the Jiaojia fault with the width of 10 ~ 200 m,sericitequartz altered zone controlled by secondary faults is relatively small,usually presenting as wide 0. 1 ~ 1m veins in the potassic granite,which indicates sericite-quartz alteration is later than potassic alteration. Compared with biotite granite,rocks from various alteration zones show high contents of K2 O and low contents of Al2O3,CaO and Na2 O,but elements like Si,Fe,and Mg have different characteristics. Potassic granite are rich in K2 O,while both sericite-quartz altered rock and pyrite-sericite-quartz altered rock are characterized with increasements of MgO and Fe2O3. During the process of potassic alteration,SiO2 and K2O were added,indicating that the forming fluid is silicon-rich,alkaline and oxidation. In the process of alteration from potassic granite to pyrite-sericite-quartz altered rock,the Fe2O3 increased obviously,which may be caused by the decomposement of biotite and other melanocratic minerals.Furthermore,sulfophilic elements like Au,Ag,As,Pb and Zn also increased significantly. Part of the gold may be derived from Linglong biotite granite. In the process of potassic alteration,the metasomatism between the wall rock and silicon-rich,alkaline and oxidation fluids,extracted gold in the form of high valence ion from wall rocks. To be specific,dispersed reduced gold( Au0) was activated to be oxidized( Au+,Au3 +),and migrated with the fluids in the form of AuH3SiO4. In the process of sericite-quartz alteration,the descent of the content of SiO2 induced the decomposement of AuH3SiO4. Fe2 +and Fe3 +were consumed to form the pyrites,which induced the deposition and enrichment of gold. Gold activate,migrate and deposit run through all these process.