宁芜陶村磁铁矿矿床成矿流体及成矿作用

陶村磁铁矿矿床位于长江中下游成矿带宁芜盆地中段,矿床地质特征及岩浆构造背景与Kiruna型磷灰石-铁氧化物矿床相似。本文在野外工作基础上,通过流体包裹体测温和氢氧硫同位素研究,探讨该矿床成矿流体性质、来源及成矿作用。陶村主要矿石类型为浸染状和脉状磁铁矿,脉状矿石形成稍晚。通过包裹体显微测温获得:磷灰石中包裹体的均一温度集中在210~390℃,盐度集中在15%~23%NaCl_(eqv);石英中包裹体的均一温度集中在330~390℃,盐度主要集中在9%~13%NaCl_(eqv),此外还存在部分高盐度原生包裹体。石英的δD为-96‰~-54‰,δ~(18)O_(H2O)除了一个为8.3‰,其余为1.9‰~4.0‰,指示原始成矿流体为岩浆来源,后期有地表水加入。黄铁矿δ~(34)S为4.8‰~9.3‰,平均值为7.4‰,综合中段地区硫同位素资料,认为成矿流体中的硫来自三叠纪膏盐层与岩浆硫的混合。结合矿床地质特征,陶村成矿作用过程可概括为:岩浆出溶形成的高温含矿气液同化三叠纪膏盐层,带入SO_4~(2-)、Cl~-、Na~+等矿化剂;这种高温气液在岩体内以钠质交代形式富集Fe后,于岩体上部形成浸染状磁铁矿,岩体顶部和边部断裂部位形成(网)脉状磁铁矿。

Ore-forming Fluid and Mineralization of the Taocun Magnetite Deposit in the Ningwu Basin

The Ningwu Basin, located in the Middle-Lower Yangtze River metallogenic zone, eastern China, is characterized by the widely distributed iron oxide-apatite deposits. The Taocun magnetite deposit is one of these deposits, which is located in the middle part of the basin. On the basis of field observation, fluid inclusion and hydrogen, oxygen, and sulfur isotope analyses are carried out on purpose of delineating the origin of the ore-forming fluids, and the mineralization of the Taocun magnetite deposit. Microthermometric analyses of fluid inclusions in apatite yielded homogenization temperatures of 210 390 ? ℃ and salinities of 15% 23%NaCl? eqv. While the fluid inclusions in quartz are mainly homogenized at 330 390 ? ℃ and with salinities of 9% 13%NaCl? eqv, whereas some of the fluid inclusions in quartz show much extraordinary high salinities, indicating the heterogeneity of the ore-forming fluid. The values of δD and δ18OH2Oof the fluid inclusions and quartz vary from ?96‰ to ?54‰, and 7.7‰ to 9.8‰ (one is 14.1‰), respectively, which suggests that the original ore-forming fluid was magmatic water, and meteoric water was participated in the late stage of hydrothermal mineralization. The δ34S values of pyrite (4.8‰ to 9.3‰), which are similar to those of the ores in entire middle part of the Ningwu Basin, suggest that sulfur in ore-forming fluid has a mixed source of the magma and the Triassic evaporite strata. It is therefore inferred that Fe in the diorite-porphyry was leached by the high temperature ore-bearing magmatic fluid that assimilated mineralizers (e.g., SO42?, Cl?, Na+) from the Middle Triassic evaporate salt layers during albitization, and the ore-bearing fluids precipitated as disseminated magnetite in the upper part of diorite-porphyry, and then formed different types of magnetite veins in the margin and the top of the intrusion.