基鲁纳式铁矿床与矽卡岩型铁矿床的成因联系：以大冶铁矿床为例

The Daye district is located in the westernmost portion of the Middle-Lower Yangtze River Metallogenic Belt along the northeastern margin of the Yangtze Craton. Numerous iron skarn deposits have been recognized in the district, with Daye, Chengchao and Jinshandian being the most representatives. The Daye iron deposit is characterized by well development of skarn assemblages, associated with intermediate intrusions that are emplaced into late Triassic marine sedimentary rocks. However, our recent work has led to discovery of magnetite-apatite (Kiruna type) iron ores widely distributed in the Daye iron deposit, which have occurrence, mineral association, spatial distribution, and alteration assemblages distinctly different from the well-known skarn iron ores in the same deposit. In this proposal, we propose to undertake a comprehensive study on the newly-discovered Kiruna-type ores based on detailed field investigations. A variety of analytical techniques will be used to characterize the miscrotextures and trace element compositions of magnetite from both types of ores and precisely constrain the age of the Kiruna-type ores using apatite and titanite U-Pb dating. Then, microthermometry of fluid inclusions in apatite, in-situ Sr-O isotope analysis of apatite and sulfide sulfur isotopes of sulfides will be used to provide information on the composition, nature, source, and evolution of ore-forming fluids responsible for the Kiruna-type iron ores. The results, when combined with previous geochemical and geochronological data in the Daye iron deposit, will be used to better understand the genesis of Kiruna type ore and its possible genetic relationships with the skarn iron ores. This study will yield an improved metallogenic model for the well-known Daye-type iron deposits and provide important information useful in mineral exploration of the Kiruna-type iron ore deposit in the Jinniu volcanic basin of the Daye district.

鄂东地区位于长江中下游成矿带最西端，是我国最重要的矽卡岩型铁矿集区之一，已探明大冶、程潮等大型矽卡岩型铁矿床。但申请者最近在大冶铁矿床中发现大量磁铁矿-磷灰石型矿石（与典型的基鲁纳型铁矿床相似），其野外产状、空间分布、蚀变矿化特征等与矽卡岩型铁矿石显著不同。本项目拟综合利用多种先进的分析测试技术方法，首先对大冶铁矿两类铁矿石中磁铁矿及磷灰石的显微结构及微量元素化学组成，在此基础上对热液富U-Th矿物U-Pb定年，准确限定两类铁矿石的成矿时代。通过磷灰石中流体包裹体的研究，结合基于显微结构分析的磷灰石Sr-O同位素原位分析和硫化物的硫同位素分析，揭示磁铁矿-磷灰石型矿石的成矿流体的组成、性质、来源、演化及外部流体在成矿中的作用。通过以上研究深入讨论磁铁矿-磷灰石型矿石的形成机制及与矽卡岩型铁矿床之间可能存在的成因联系，同时可以为鄂东地区特别是金牛火山盆地寻找同类型铁矿床提供重要的理论指导。

磁铁矿-磷灰石（Iron oxide-apatite, IOA）矿床又被为基鲁纳型铁矿床，其成因还存在激烈大争论。我国的Kiruna型矿床以庐枞及宁芜盆地的与次火山岩有关的玢岩型铁矿床最为典型，是我国最为重要的富铁矿矿床类型之一。鄂东地区的金牛盆地与庐枞及宁芜等盆地共同组成了长江中下游成矿带的七大火山盆地，它们之前在地质背景上存在巨大的相似性，但是前人对金牛盆地的火山岩出露区开展过大量的地质找矿工作但一直没有取得突破。本次研究在在大冶铁矽卡岩铁-铜矿床中及金牛火山岩盆地边缘的矽卡岩型铁矿床深部发现了与宁芜和庐枞盆地类似的磁铁矿-磷灰石矿石，这一发现不仅对进一步深入研究和理解本区磁铁矿-磷灰石型富铁矿的成因机制具有重要意义，而且为鄂东南及整个长江中下游未发现火山岩盆地铁矿床的寻找提供了新的方向和目标。.研究表明磁铁矿-磷灰石矿石主要赋存在火山岩及次火山闪长玢岩中呈脉状穿插或者呈不规则状胶结围岩。矿石具有典型的磁铁矿-磷灰石-透辉石 (阳起石) 组合，明显不同鄂东南地区出现矽卡岩型铁矿床。两个含矿闪长岩样品样品的锆石U-Pb年龄分别为131.4 ± 0.2 Ma (n = 42; MSWD = 1.7) 和131.4 ± 0.2 Ma (n = 42; MSWD = 1.7)。这一年龄与赋矿火山岩的年龄（131.4± 0.2 Ma, MSWD = 1.2）（锆石：130 ± 2 Ma, MSWD = 1.7；榍石：131 ± 1 Ma, MSWD = 0.71）的年龄在误差范围内基本一致。磁铁矿-磷灰石矿石中热液榍石的U-Pb年龄为130Ma，这代表了磁铁矿-磷灰石矿石的年龄。矽卡岩型和IOA矿石中的热液榍石的U-Pb年龄也为~130Ma，表明两类矿石时间上具有一致性。结合系统的磁铁矿微量元素及矿石全岩分析，我们认为这两类矿石在时间、空间及成因上具有密切的成因联系，是同一套岩浆热液系统在不同温度及围岩条件下形成的不同产物。

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