辽东裂谷带菱镁矿镁同位素特征及其对矿床成因的制约

As the world famous magnesite mineralization belt,the magnesite deposit in the eastern Liaoning province, which accounts for about 30% of the whole world reserves, has complex sources.The formations and evolutions of magnesite deposits have experienced multi-geo-processes, including evaporation deposite, regional metamorphism and hydrothermal metasomatism etc, therefore the genesis of the magnesite deposit is still in debate. Magnesium isotope, as the new isotopic tracer, could distinguish between the mantle-derived igneous carbonatites and sedimentary carbonate, and present significantly isotopic fractionation during the processes of carbonate deposition and weathering. Therefore, this project will focus on the magnesium isotopic system, and discuss the magnesium source of the magnesite deposit in the eastern Liaoning province with the mineral deposits geology and geochemistry. Based on the compositions of magnesium isotope and its potential frationation, more evidence and understanding will be presented for the genesis and the evolution of magnesite deposit. Furthermore,the mineralization and mechanism of magnesite in the eastern Liaoning province will be discussed by considering about the geological background, and the method would be established on using magnesium isotopic system to trace the process of mineralization. These works could display a standard research example in using isotopic system to trace the source of mineralization.

作为世界著名的菱镁矿矿区，辽东裂谷带菱镁矿（其储量可达世界储量的30%）成矿物质来源复杂，矿床形成演化经历了多期地质过程（蒸发沉积、区域变质、热液交代等），矿床成因至今仍存在不同认识。镁同位素作为一种新兴的同位素示踪剂，能够区分幔源火成碳酸岩和化学沉积碳酸盐岩，在碳酸盐沉积和风化过程中发生显著分馏，因此本项目拟以镁同位素体系为研究重点，结合矿床地质和矿床地球化学研究，探讨辽东裂谷带菱镁矿矿床（海城-大石桥一带）的镁质来源，分析不同区域和类型的矿床及围岩中镁同位素的组成、相互关系和可能的分馏过程，进而为矿床的成因演化研究提供新的证据和认识。在充分考虑地质背景的基础上，结合镁同位素组成特征，反演辽东菱镁矿经历的主要地质过程，探讨其成矿过程与机制，建立应用镁同位素体系进行成矿作用直接示踪的工作方法，为成矿物质来源的同位素直接示踪提供研究范例。

本项目首次在世界上最大的菱镁矿成矿带（赋存于21亿年前的大石桥组；铧子峪矿区）获得了菱镁矿的镁同位素数据。研究结果表明，绝大多数菱镁矿矿石的镁同位素组成介于-0.95‰至-0.49‰之间（-0.75 ± 0.26‰，2SD），比绝大多数的灰岩和白云岩重，也比大石桥组三段的菱镁矿（-1.14 ± 0.15‰，2SD）和大理岩（-1.22 ± 0.35‰，2SD）重。并且，菱镁矿矿石的镁同位素组成（δ26Mg值）总体上呈现由底至顶逐渐变重的趋势：从底部大约-0.90‰逐渐增加到顶部-0.60‰左右。基于镁、铁、碳、氧同位素组成、元素含量以及野外地质特征，推断古元古代层状菱镁矿矿床的形成主要由两个过程控制：（1）大量的富镁碳酸盐沉淀在类似于泻湖的半封闭海相蒸发环境中，同时还形成富镁的卤盐、卤水和少量的菱镁矿。（2）在埋藏成岩的过程中深部的富镁成岩流体交代富镁碳酸盐形成菱镁矿。埋藏成岩阶段形成的菱镁矿在区域变质过程中发生重结晶转换为菱镁矿矿石。.这种巨型的层状菱镁矿矿床主要出现在前寒武的地层中，在现代和显生宙大多数沉积环境中缺失。这一现象可能与富镁的海水（镁质来源于太古宙富镁超基性岩石的风化）密切相关。富镁的海水能够促进产生富镁的碳酸盐或者沉积物，进而在埋藏成岩过程和区域变质过程中转化为菱镁矿矿床。