安徽铜陵矿集区铜官山矿田胶状黄铁矿矿物学特征及其对成矿作用的制约

Mineralogical characteristics of colloform pyrite from Tongguanshan ore field and its implications for mineralization of the stratabound sulfide deposit in Tongling mineralization cluster,Anhui Province

铜官山矿田是铜陵矿集区内五大矿田之一,矿田内顺层产出的层状硫化物矿体是铜金矿床的主矿体,矿体内含有较多的胶状黄铁矿,其成因的争议制约了对铜金矿床成矿作用的解析。本文主要利用场发射扫描电镜(FE-SEM)等纳米矿物学手段,并结合光学显微镜、粉晶X射线衍射(XRD)、微区激光拉曼光谱分析等方法,对矿田内铜官山矿床及天马山矿床内层状硫化物矿体中胶状黄铁矿矿石的矿物组成、微形貌、微结构等特征进行系统研究,结果表明胶状黄铁矿矿石多呈胶状、鲕状结构,具有同心环状构造,同心环被赤铁矿、菱铁矿与黄铜矿脉穿切。同心环主要由白铁矿+有机质与胶状黄铁矿交替组成。胶状黄铁矿的黄铁矿颗粒粒径从纳米至亚微米均有分布,以自形-半自形立方体为主,少数呈他形,脉体边部胶状黄铁矿颗粒较大,自形程度较高,重结晶显著。矿石中含有少量白云石、伊利石微晶体,与胶状黄铁矿紧密共存,显示典型沉积特征。共存石英磨圆度较高,存在次生加大现象,表面存在胶状黄铁矿印模,显示为碎屑成因。这些综合信息表明胶状黄铁矿非岩浆热液成因,而是与石炭系地层同沉积成岩成因,并可能有微生物作用参与。高孔隙率、高化学活性及较高有机质含量的胶状黄铁矿可能为燕山期岩浆热液演化的含铜成矿流体提供了沉淀剂,对矿田内铜金硫化物矿体的层控性发挥了重要的控制作用。

Tongguanshan ore field is the representative of the five major ore fields in Tongling mineralization cluster,and its metallogenesis remains a topic of debate. The stratabound sulfide orebodies dominate the main copper-bearing sulfide orebodies in Tongguanshan ore field. The colloform pyrite is common in this type of stratabound sulfide orebody,and its formation and genesis are still controversial which on the contrary limits identification of the stratabound sulfide orebody. In this paper,the mineral composition,morphology and micro-structure of colloform pyrite ores in Tongguanshan deposit and Tianmashan deposit were systematically studied through field emission scanning electron microscopy( FE-SEM),optical microscopy,powder X-ray diffraction( XRD) and micro-area laser Raman spectroscopy. The results show that colloform pyrite ores display colloform,ooliticand concentric structure,and the concentric structure was cut by chalcopyrite and hematite,siderite veins. The concentric structure is mainly composed of marcasite +organic matter and colloform pyrite. The grain sizes of colloform pyrite range from several nanometers to hundreds of nanometers and the granular pyrites are dominated by euhedral-subhedral cubic shape with minor irregular microcrystals. The colloform pyrite crystals at the edge of veins have large particle size,and have much high degree of crystallinity or recrystallization. Dolomite and illite in ores show small particle size and often coexist with colloform pyrite,showing typical sedimentary characteristics. The coexisting quartz has high degree of roundness,secondary enlargement texture and colloform pyrite mold impression on its surface,which shows that it is clastic origin. These systemic information indicate that colloform pyrite in Tongguanshan ore field was direct precipitation from the sub-closed marine basin environment instead of magmatic hydrothermal system,and may deposit through biogeochemistry processes. Colloform pyrite with high porosity,high chemical activity and high content of organic matter content maybe act as ore-forming metal element precipitant for copper( gold)-bearing hydrothermal fluid being derived from Yanshanian magma and limits formation of stratabound occurences of stratabound sulfide orebodies in the ore field and Tongling mineralization cluster.