川西会理–会东地区火山岩型富铁矿床与中元古代岩浆活动的成因联系

Recently, more and more Mesoproterozoic igneous rocks have been found in the southwestern margin of the Yangtze craton. In this region several volcanic-hosted Fe deposits are known to be spatially closely associated with Mesoproterozoic mafic volcanic rocks. The genetic relationship between them, however, is poorly known due to lack of critical geochemical data for the deposits and associated mafic rocks. Fund is thus requested to carry out a systematic geochemical study to assess the role of Mesoproterozoic magmatism in the formation of associated volcanic-hosted Fe deposits in the region. The results from this study will improve our understanding of the poorly-known Mesoproterozoic tectonic-magmatic-metallogenic event in the southwestern margin of the Yangtze craton, which in turn will improve the success rate of exploration targeting for the volcanic-hosted Fe deposits in the region..The systematic geochemical study we have proposed will focuss on several selected Mesoproterozoic volcanic-hosted Fe deposits and associated meta-volcanic and intrusive rocks in the Huili-Huidong district, Sichuan province. We will acquire mineralogical, petrological and isotopic data including precise zircon U-Pb age determination for the mafic rocks as well as the Fe deposits. The age data will be used to establish a temporal relationship between mafic magmatism and the spatially asociated Fe metallogeny. Mineral compositions, together with whole-rock major and trace element compositions, will be used to determine the compositions of the parental magmas. Trace elements, Sm–Nd and zircon Hf–O isotopes will be used to determine the style of magmatism and possible tectonic setting for the meta-volcanic rocks and coeval mafic intrusions. The style of mineralization, the sources of Fe and oxygen, and the genetic controls on the Fe mineralization will be examined using field observations and laboratory studies such as ore textures, mineral assemblages and paragenesis, trace element cencentrations and O–Fe isotopes of magnetite and coexisting minerals. We are confident that the results from this study will shed lights on the currently under-appreciated, Mesoproterozoic tectonic-magmatic-metallogenic event in the southwestern margin of the Yangtze craton.

近年来的研究工作显示扬子地块西南缘存在较多的中元古代岩浆活动，且与该区广泛产出的火山–次火山岩型富铁矿床有密切的联系，但已有的研究仍然较为薄弱。对这些富铁矿床及其相关基性火山–次火山岩的研究，有助于深入认识扬子地块西南缘该时期的构造–岩浆演化及其与Fe成矿作用的成因联系。本项目以四川会理和会东地区火山–次火山岩型富铁矿床及其相关的中元古代火山岩和侵入岩为研究对象。通过矿床学、同位素年代学和典型矿物的元素–同位素地球化学的系统研究，确定该区富铁矿床的成矿时代和成因类型，探讨成矿物质来源和成矿机制；通过岩石学、矿物学、高精度年代学、元素和同位素地球化学等系统研究，揭示火山岩和侵入岩的形成时代、岩浆性质、起源及演化特征，探讨火山岩与同时代侵入岩的成因联系，进而更加深入地阐明该区该时期岩石圈演化过程；并通过岩石和矿床的同位素年代学和元素–同位素地球化学的对比研究，揭示岩浆活动与成矿作用的成因联系。

扬子地块西南缘广泛产出火山–次火山岩型富铁矿床。本项目通过四川会东菜园子地区富铁矿床与云南省大红山铁铜矿床进行研究，探讨这些富铁矿床与玄武岩、辉绿岩脉的成因联系。四川会东菜园子地区玄武岩的磷灰石Pb–Pb和U–Pb年龄均为约1.8Ga，菜园子辉绿岩的锆石Pb–Pb年龄为1.8Ga。因此，菜园子基性岩浆活动形成于~ 1.8 Ga。菜园子玄武岩和辉绿岩分别具有拉斑质和碱性特征，有高的Zr、Ti、V和Zr/Y值，指示其形成于大陆裂谷背景。~1.8 Ga板内岩浆活动是扬子地块西南缘从 Columbia 超大陆初始裂解的时间。菜园子铁矿的磁铁矿具有高Si，低Ti、Cr、V特征，表明其具有热液成因。菜园子磁铁矿与岩浆热液矿床和IOCG矿床中磁铁矿配分模式类似，指示成矿热液可能与岩浆热液有关。菜园子磁铁矿具有显著高含量的W–Mo元素，说明玄武岩和花岗岩均为菜园子磁铁矿的形成提供了成矿物质。而云南省大红山铁铜矿区地层定年约为1.75Ga，辉绿岩的年龄为1.70Ga。该矿床内两种矿石中磁铁矿在各种矿石中均有发育。磁铁矿元素特征表明铁矿石和铁铜矿石中铁矿化流体具有相似的化学组成，来源可能相同。铁矿化阶段的晚期出现少量硫化物（如辉钼矿），指示铁矿化流体随着磁铁矿的沉淀逐渐向低氧逸度演化。此外，本研究发现大红山矿床具有极其高钒含量的热液磁铁矿颗粒（V:~10000 ppm）。现有的资料表明高V磁铁矿倾向于在高还原的海相环境中产生。因此，本研究为支持中国西南前寒武纪层状铁铜矿床形成于海相环境中提供了独立的证据。以上研究表明，出露于不同地区的火山岩型富铁矿床主要产于古元古代晚期（1.8–1.7 Ga）地层，这些玄武岩可能形成于海相环境中，矿体中铁的物质来源可能来自火山岩。这些富铁矿床及其相关基性火山–次火山岩的成因研究，有助于深入认识扬子地块西南缘该时期的构造–岩浆演化及其Fe成矿作用的成因联系。