鄂东南程潮铁矿多世代叠加成矿作用:磁铁矿证据

Multi-superimposed mineralization process in Chengchao iron deposit,southeastern Hubei Province: Evidence from the study of magnetite

湖北程潮铁矿是长江中下游成矿带最大的矽卡岩型铁矿床,主要产于早白垩世中酸性侵入岩与三叠系地层的接触带上。为了进一步探讨其富铁矿的形成机制,本文对该矿床中不同产状的磁铁矿和不同岩性侵入岩中的副矿物磁铁矿进行了详细的野外地质观察和显微结构分析,发现在多数磁铁矿矿石和矿化矽卡岩中均存在多世代磁铁矿矿化叠加现象。根据显微观察和BSE图像特征,程潮铁矿中热液期磁铁矿可以划分为四个世代,即Mt1、Mt2、Mt3、Mt4,其中Mt1颗粒表面不均匀,溶解-再沉淀现象明显;Mt2沿Mt1边缘生长,颗粒表面均匀,环带发育;Mt3沿Mt2边缘生长,颗粒表面均匀,环带不发育;Mt4多呈板条状或他形粒状,环带不发育。电子探针分析结果表明:同一世代不同产状或同一产状不同世代磁铁矿之间具有明显的成分差异,其中以Si、Al、Ca、Mg等含量较高的元素差异最为明显,而Ti、Cr、V、Zn、Ni等含量较低的元素差异则相对较小。这些差异性可能与磁铁矿结晶时成矿流体氧逸度、温度、元素浓度和水岩反应比例密切相关。不同世代热液磁铁矿与矿区岩体副矿物磁铁矿对比发现,二者在矿物结构和微量元素组成上存在明显差异,特别以微量元素Ti含量差异最大。程潮铁矿与不同成因类型矿床中的磁铁矿成分对比分析结果,进一步暗示出程潮铁矿中的磁铁矿为接触交代成因,并非矿浆成因。半定量模拟计算结果表明,Mt1、Mt2、Mt3在整个成矿过程中贡献了至少96%的铁质,对成矿起到了决定性作用。多世代磁铁矿矿化叠加过程不仅为揭示程潮大型铁矿的富集过程提供了重要依据,同时也为进一步理解矽卡岩型富铁矿的成矿机制提供了重要启示。

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.