庐枞盆地泥河铁矿床中硬石膏微量元素特征、沉淀机制及其对铁成矿作用的制约

Precipitation mechanism,REE characteristics of anhydrite in Nihe deposit and its relation to mineralization

泥河铁矿床是长江中下游成矿带庐枞火山岩盆地中典型的磁铁矿-磷灰石型铁矿床。硬石膏是矿床中的主要脉石矿物,在矿床的各个阶段均有发育。本文在详细的野外地质工作和岩相学观察基础上,系统的对矿床各成矿阶段硬石膏开展LA-ICP-MS分析测试工作对硬石膏形成机制进行了分析,初步探讨了硬石膏在铁成矿过程中的作用及意义。矿床中的硬石膏分为三类:与透辉石、磁铁矿、黄铁矿、磷灰石等矿物共生的紫色板状硬石膏(Type Ⅰ);与黄铁矿共生的白色板状硬石膏(Type Ⅱ)以及呈独立脉状产出的白色糖粒状硬石膏(Type Ⅲ)。Type Ⅱ和Type Ⅲ硬石膏REE含量远低于Type Ⅰ硬石膏,Type Ⅱ和Type Ⅲ硬石膏的稀土配分模式相似,二者与Type Ⅰ硬石膏具有很大差别。泥河铁矿床硬石膏中REE含量受结晶学因素的影响较小,主要影响因素为温度、流体的演化过程以及络合物阴离子的类型和数量。早期高温热液中REE主要以Cl络合物形式迁移,矿床中较早沉淀的富稀土矿物捕获了大量REE,使体系中REE含量大幅度减少,致使Type Ⅱ和Type Ⅲ硬石膏稀土含量偏低。晚期流体盐度的降低导致了Cl-/SO42-比值变低,热液中稀土元素的减少以及络合物配位体的改变导致了Type Ⅱ和Type Ⅲ硬石膏稀土配分模式趋于平滑。此外Type Ⅲ硬石膏个别点显示出轻稀土富集的特征,说明热液晚期可能存在硬石膏的溶解再沉淀过程,该过程会改变硬石膏中的稀土配分模式。硬石膏的加入可以提高体系氧逸度,提高热液中铁元素的形成磁铁矿的比率从而利于形成大规模铁矿体。

Nihe is a major magnetite-apatite deposit located in the northwestern the Lu-Zong volcanic basin in the Middle-Lower Yangtze River Valley Metallogenic Belt. Anhydrite is extensively developed in the deposit. Based on the detailed field geological work and petrographic observation, the anhydrite can be divided into three types on the basis of the mineral assemblage and stage：Type I, tabular violet crystals coexisting with diopside, magnetite, pyrite and apatite; Type Ⅱ, tabular white crystals coexisting with pyrite; and Type Ⅲ, equant pink or white crystals in carbonate-anhydrite veins. We present in situ LA-ICP-MS geochemical data for the three different types of anhydrite in Nihe deposit. LA-ICP-MS data indicate that type I anhydrite contains much more REE than the other two types, and the REE pattern and Eu anomaly is also quite different （（La/Yb）N=1.1~466, δEu=0.3~2.4）. Type Ⅱ and Type Ⅲ have similar REE pattern. Crystallography has little influence on anhydrite REE content in the Nihe iron deposit, temperature, fluid evolution and complex type is main factor controlling the REE pattern. In the early high-temperature hydrothermal solution, REE mainly migrates in the form of Cl complex. After precipitating the REE-rich minerals, the REE content greatly decreased in the fluid, resulting in a lack of REE in types Ⅱ and Ⅲ, and their REE patterns are flat. In addition, decreasing fluid salinity caused lower Cl-/SO42- ratios in later stages. Furthermore, some type Ⅲ anhydrite has elevated LREEs, suggesting some late-stage dissolution and reprecipitation of anhydrite due to fluctuating fluid compositions. Addition of abundant anhydrite to the ore forming fluid increases fO2, allowing iron to be transported far away and helpful to the formation of large-scale iron ore bodies.