安徽铜陵冬瓜山矿床中磁黄铁矿矿石结构特征及其成因意义

安徽铜陵冬瓜山矿床是长江中下游地区具有代表性的大型层状硫化物矿床,磁黄铁矿为矿床中的主要硫化物矿物.该矿床主要由层状硫化物矿体组成,伴有矽卡岩型和斑岩型矿体.在层状矿体上部,磁黄铁矿主要为块状构造,而层状矿体下部,磁黄铁矿多为层纹状、条带状构造,具有显著的沉积结构构造特征.野外地质观察及室内矿相学研究表明,层状矿体中磁黄铁矿矿石遭受了强烈的变质作用及热液交代作用.进变质过程中形成的结构主要为胶黄铁矿转变为黄铁矿以及进一步变质转变为磁黄铁矿、磁铁矿时形成的交代残留结构.退变质过程则以磁黄铁矿的退火、黄铁矿变斑晶的生长和单纯六方磁黄铁矿的形成为特征.岩浆热液对单纯六方磁黄铁矿的交代作用形成了单斜和六方磁黄铁矿的交生结构.这些结构特征表明层状矿体中的磁黄铁矿并不是岩浆热液成因,而主要为石炭纪同生沉积胶黄铁矿、黄铁矿在燕山期岩浆侵入所引起的热变质作用下脱硫所形成,并在热变质作用之后又受到岩浆热液的叠加交代.磁黄铁矿的结构特征显示冬瓜山矿床的形成经历了同生沉积、热变质、热液交代等多个阶段,支持其为同生沉积-叠加改造型矿床.

Ore textures and genetic significance of pyrrhotite from Dongguashan ore deposit in Tongling area, Anhui Province

Pyrrhotite is the main sulfide mineral in the Dongguashan ore deposit, one of the most representative stratiform sulfide deposits in the Midd le-Lower Yangtze River metallogenic belt. The Dongguashan ore deposit is mainly composed of a large stratiform sulfide ore body associated with skarn and porphy ry ore bodies. Pyrrhotite in the upper part of the stratiform ore body is mostly of massive structure, whereas that in the lower part is mainly of banded or lay ered structure. Field and microscopic investigation shows that pyrrhotite ores u nderwent intense metamorphism and hydrothermal replacement. Phase conversion from colloform pyrite through pyrite to pyrrhotite and magnetite occurred in the co urse of prograde metamorphism. Retrograde metamorphism is characterized by the p resence of annealing texture in pyrrhotite, the growth of porphyroblastic pyrite and the formation of hexagonal pyrrhotite. The replacement of pure hexagonal py rrhotite by magmatic hydrothermal fluids led to the formation of the intergrowth of hexagonal and monoclinic pyrrhotite. The textures of pyrrhotite in the stratiform ore body indicate that, instead of being magmatic hydrothermal origin, pyrrhotite is the product of desulfurization of the synsedimentary colloform pyrite and pyrite through thermal metamorphism caused by Yanshanian magmatic intrusion . After thermal metamorphism, pyrrhotite was replaced by magmatic hydrothermal f luids. The textures of pyrrhotite also suggest that the ore deposit has experien ced three ore-forming stages, namely, synsedimentation, thermal metamorphism and hydrothermal replacement, which supports the submarine exhalative sedimentation -magmatic hydrothermal reformation model for the Dongguashan ore deposit.