黄山东铜镍矿的韧性变形特征和磁黄铁矿形成机制研究

Ductile Deformation Characteristics and Formation Mechanism of Pyrrhotite in the Huangshandong Ni-Cu Sulfide Deposit

黄山东铜镍硫化物矿床产于东天山地区的黄山—镜儿泉韧性剪切带中,大地构造上属中亚造山带东天山觉罗塔格岛弧带。该铜镍矿所在的黄山东镁铁—超镁铁岩体呈纺锤状侵位于晚石炭世火山岩中,其边部发生了与区域剪切带总体走向一致的强烈糜棱岩化作用。矿床中的部分矿体发生强烈韧性变形,其中17号矿体完全产于韧性变形带内,岩石和矿石都发生了强烈的破碎和蚀变而形成矿石糜棱岩。在上述韧性变形带内,还发育一定规模的网脉状和细脉状富铜碳酸盐—硫化物脉。论文在野外地质和构造形迹观察的基础上,对黄山东矿床不同类型的矿石开展了细致的显微岩相和矿相学观察,识别出三种类型矿石:原生矿石、强烈变形矿石和热液叠加矿石。海绵陨铁结构的原生矿石中,脉石矿物几乎不发生蚀变和变形,矿石矿物仅发生脆性破裂;强烈变形矿石中,脉石矿物和矿石矿物均发生强烈变形,主要以纤闪石的波状消光和膝折、金云母的书斜构造、磁黄铁矿的定向拉长为特征;热液叠加矿石中的磁黄铁矿普遍发育颗粒的扁平化、重结晶,局部可见磁黄铁矿的退火平衡结构。黄山东铜镍硫化物矿床的侵位与变形时间与区域黄山-镜儿泉剪切带的韧性剪切作用时间相一致。岩体冷却过程经历的强烈韧性剪切变形作用不但造成矿石矿物的强烈韧性变形而形成矿石糜棱岩,还使伴生脉石矿物发生细粒化和热液蚀变,释放出流体和成矿元素,并叠加于变形的矿石和岩石之上,从而形成了网脉状和细脉状矿体。黄山东铜镍矿的原生硫化物固熔体铁含量较高,因而在硫化物熔体结晶过程只形成六方磁黄铁矿而无伴生单斜磁黄铁矿和黄铁矿。在热液叠加过程中,流体沿边缘和裂隙面交代早期六方磁黄铁矿,形成单斜磁黄铁矿反应边。本次研究还发现六方磁黄铁矿形成的新机制:即在高硫逸度和高氧逸度的条件下,随着体系温度的降低,单斜磁黄铁矿可从热液六方磁黄铁矿中出溶形成呈叶片状单斜—六方磁黄铁矿交生体。

Huangshandong Ni-Cu sulfide deposit,located in the Huangshan-Jing’erquan ductile shear zone,belongs to the Jueluotage island arc belt of the Eastern Tianshan area,Central Asian Orogenic Belt. The Huangshandong mafic-ultramafic pluton,in shape offusiform,intruded in the Late Carboniferous volcanic rocks with intensive foliation near the margin of the pluton which is consistent with the strike of the regional ductile shear zone. Some orebodies in the deposit also show intensive ductile deformation. The No.17 orebody was totally controlled and hosted by the ductile shear zone. The rocks and ores from the No.17 orebody show intensive deformation and alteration and could be called as ore mylonites. Considerable amounts of network and veinlet copper-rich carbonate-sulfide veins can be also observed in this ductile shear zone. In this contribution,detailed microscopy petrography and mineralography observation were carried out on different types of ores from the Huangshandong deposit. Combined with field geological investigation and ore fabric observation,three different types of ores were distinguished,that is,magmatic ores,deformed ores and fluid-overprinted ores. Magmatic ores with sideronitic fabric are barely altered and deformed,and sulfide minerals in it only show some brittle fractures. Deformed ores develop intensive deformation fabric in silicate and sulfide minerals,such as the kink bands,wavy extinction of uralite,domino structure of phlogopite and the elongation of pyrrhotite. Fluid-overprinted ores are characterized by strongly deformation,flattened grains and recrystallization of pyrrhotites,and annealing of pyrrhotites can also be occasionally observed. Intruding of Huangshandong pluton is coeval with ductile deformation and regional ductile shearing. During intruding and cooling of the pluton,intensive regional ductile shearing induced ductile deformation of sulfide ores and then ore mylonites formed. Gangue minerals in the deformed orebodies were crushed to fine-grained minerals and altered. In these processes,fluid and metallogenic elements were released and overprinted into deformed rocks and ores,forming network and veinlet ores. Owing to containing higher iron in original sulfide solid solution,the magmatic Ni-Cu sulfide ores in Huangshandong deposit could only form hexagonal pyrrhotite without associated monoclinic pyrrhotite and pyrite during the crystallization process of sulfide solid solution. During hydrothermal overprinting process,fluid overprinting occurred along margins and fractures of earlier stage magmatic hexagonal pyrrhotite,formed monoclinic pyrrhotite reactional rims. Our studies argued that monoclinic pyrrhotite can also be formed during hydrothermal precipitation processes. In this process,monoclinic pyrrhotites can exsolved from hexagonal grains in high oxygen and sulfur fugacity environments,as the cooling of such hydrothermal fluids.