Study On The Characteristics Of Wall-rock Alteration Of The Nihe Iron Deposit In Lujiang, Anhui Province,China

The Ni-he iron deposit is located in the northwest of Luzong volcanic basin, in the Middle and Lower Yangtze River metallogic belt, it is a newly discovered large scale deposit. Based on the detail field work and laboratory work, this paper studied the deposit geology, the geological characteristics and the mechanism of the alteration, and discussed the genesis of the Nihe iron deposit, achieved some new understanding and results.The formation of the Nihe iron deposit can be divided into three stages, the pyroxene (garnet)-anhydrite-magnetite stage, the chlorite-epidote-carbonate stage, and the argillation-siliceous stage. The argillation-siliceous stage contains four substages, the anhydrite-pyrite stage, the quartz-pyrite stage,the kaolinite-hydromica stage and the barite-galena stage.The wall rocks in the Nihe iron deposit are intensely and pervasively altered. The alteration contains different types, stages and zonations. Based on the alteration minerals assemblage, the alteration in the Nihe iron deposit can be divided into melanocratic alteration zone, hybrid alteration zone and leucocratic alteration zone from top to bottom.The alteration and mineralization is a continuous process. The early stage includes anhydrite-pyroxene alteration or anhydrite-garnet alteration, accompanying with magnetite mineralization. The intermediate propylitization-like stage includes chlorite alteration, epidote alteration and carbonates alteration, accompany with minor pyrrhotite, pyrite, siderite, and hematite mineralization. The late stage includes anhydrite-pyrite alteration, quartz-pyrite alteration and kaolinite-hydromica alteration, accompany with massive pyrite mineralization.The early alteration stage is charavterized by Fe、Ca、Mg and minor Si addition, with a volume of volatile (F、Cl and CO2). There is no obvious component addition or loss in the middle alteration stage. However, the volatile (H2O and CO2) constitutes turned the anhydrous silicate minerals such as pyroxene and garnet into the hydrous silicate minerals such as chlorite and epidote and calcite and siderite. A large amount of Ca、Fe、S and Si are enriched at the late stage and formed the anhydrite ore bodies and pyrite ore bodies.Based on the comprehensive analysis of wall rock alterations, the evolution of the igneous rocks, the diagenetic and metallogenic epochs, and the source of the ore-forming fluids,as well as comparised with the typical iron deposit in the Middle and Lower Yangtze River metallogic belt, we considers that the Nihe iron deposit has close relationships with the subvolcanic magmatic hydrothermal fluids activities, which belongs to the generalized porphyry iron deposit.