中国南方典型宁乡式铁矿沉积特征与成矿机制分析

Sedimentary characteristics and metallogenic mechanism of Ningxiang-type iron deposits in South China

关键词:  中国南方  宁乡式铁矿  沉积特征  成矿机制

Key words:  South China  Ningxiang-type iron deposits  sedimentary characteristics  metallogenic mechanism

摘要:

宁乡式铁矿是我国南方最具代表性的沉积型铁矿，为了探讨其沉积特征与成矿机制，本文借助偏光显微镜、扫描电子显微镜和TESCAN综合矿物分析仪，选取采自桂东北、鄂西成矿区的若干矿石样品进行岩石学和沉积学研究，深入剖析铁质鲕粒的显微组构和地球化学特征。结果显示宁乡式铁矿为混积岩，具有典型的鲕状结构，可划分为砂岩型、灰岩型和混合型等三种类型，赋存于中泥盆统信都组、上泥盆统黄家磴组和写经寺组地层中，含矿岩系总体为一套砂岩、泥页岩和泥灰岩组合，形成于区域海侵背景下的滨海、滨–浅海转换带；矿石中铁质鲕粒形态多样，粒径多集中于0.2~0.5 mm，少数铁质鲕粒的矿物相和主量元素呈圈层状分布，核心可为石英或生物碎屑充填，外部为赤铁矿、鲕绿泥石及胶磷矿环带互层。研究表明矿石的形成可划分为成矿物质准备期、铁质鲕粒形成期和铁矿沉积期三个阶段，强烈的古陆风化作用提供了成矿物质来源，成矿物质在机械沉积作用、胶体化学沉积作用和生物沉积作用下富集并沉淀，期间经历了复杂的氧化还原过程，最终压实固结为宁乡式铁矿。

Abstract:

Ningxiang-type iron deposit is the most representative sedimentary iron deposit in South China. In order to explore sedimentary characteristics and metallogenic mechanism of Ningxiang-type iron deposit, the ore samples from northeastern Guangxi and western Hubei are selected for petrological and sedimentological research with the help of polarizing optical microscope, scanning electron microscope and TESCAN integrated mineral analyzer. The microfabric and geochemical characteristics of iron ooids are deeply studied. The results show that Ningxiang-type iron deposit is a kind of hybrid sedimentary with typical oolitic texture, which can be divided into three types:Sandstone-type, limestone-type and mixed-type. It occurs in the Middle Devonian Xindu Formation, the Upper Devonian Huangjiadeng Formation and Xiejingsi Formation. The ore-bearing rock series are generally composed of sandstone, mud, shale and marl, which are formed in the coastal and coastal-shallow transitional zone on the background of regional transgression. The iron oolites in ore samples have various morphologies, and the particle size is mostly concentrated between 0.2 mm and 0.5 mm. The distribution of the mineral phases and major elements in some iron oolites is of annular layers. The core of these iron oolites can be filled with quartz or bioclastics, and the outer annular layers are composed of hematite, chamosite and collophane zones. The study suggests that the forming process of iron ores can be divided into three stages:The preparation period of ore-forming materials, the formation period of iron ooids, and the deposition period of iron ores. Strong paleoterrestrial weathering provides the source of ore-forming materials. The ore-forming materials are enriched and precipitated by mechanical sedimentation, colloidal chemical sedimentation and biological sedimentation, which have experienced a complex redox process. Finally, they are compacted and consolidated into Ningxiang-type iron deposits.