Multi-superimposed mineralization process in Chengchao iron deposit,southeastern Hubei Province: Evidence from the study of magnetite
The Chengchao iron deposit,located in the southeastern Hubei Province,is the largest skarn iron deposit in the MiddleLower Yangtze River Valley metallogenic belt( MLYRB) and most orebodies distributed along the contact zones between the Early Cretaceous intrusions and Triassic strata. To further investigate the deposit formation mechanism,in this contribution,detailed field and microscope observation of magnetite have been focused on various locations and occurrence of ores and mineralized skarn as well as magnetite in the intrusions. And four generations magnetite has been identified in different types of ores and mineralized skarn. The earliest generation magnetite( Mt1) is heterogeneous and undergone dissolution-reprecipitation process. The second generation magnetite( Mt2) is homogeneous and develops in oscillatory texture. The third generation magnetite( Mt3) is characterized with homogeneous and absence of oscillatory texture. The last generation magnetite( Mt4) shows dendrite and xenomorphic granular texture,and lack of oscillatory texture. EMPA data reveal that big compositional variations are existed among them,predominantly for the high concentration elements Si,Al,Ca,Mg,while differences for low content elements Ti,Cr,V,Zn,Ni are relatively small. Factors,such as fluid oxygen fugacity,temperature,concentration of elements,water-rock reaction may jointly account for the differences.Compared with multiple generations magnetite,those within intrusions,not only the texture,but also the compositon,especallly for Ti content,show large differences. Based on the above analysis,and considering the difference with other types magnetite,we porposed that the magnetite in Chengchao is of hydrothermal origin rather than magmatic. According to the semi-quantitative simulation calculation results,the former three generations magnetite contribute more than 96% Fe and paly a crucial role for the forming of Chengchao iron deposit. Superimposed mineralization process provide a deeper understanding for the ore genesis of Chengchao iron deposit,and give an inspiration for the enrichment mechanism of skarn-type iron deposits.