福建马坑铁矿的成因及其与高硅花岗岩的关系——来自铁同位素的制约

Fe-Skarn deposit is a kind of high-grade iron ore deposits with extremely high economic significance, which is generally relevant to intermediate-mafic plutons. Makeng iron deposit in Southwest Fujian Province is characterized with skran type mineralization, however, significantly differs from those typical Fe-skarn deposits. Makeng iron deposit is a skarn deposit that associated with a high silica (SiO2>75%) granite both in special and temporal. Hence, the genesis of Makeng iron deposit is in hot debate. As the mineralized element, figuring out the sources of iron is the key to solve the genesis of the Makeng deposit. Fe isotope is a powerful tool for tracing the source of Fe and the ore-forming process in magmatic-hydrothermal systems. Detailed studies of the Fe isotope fractionations in Makeng deposit helps to figure out the origin of Fe, and then reveal the connections between the high silica granite and the Fe-skarn deposit. In addition, we try to build a model for the Fe isotopic fractionations in the Fe-skarn deposit based on the study of the Fe sources. The built model is of significance for the study of the other skarn mineralization deposits.

矽卡岩型铁矿是一种高品位的铁矿类型，具有很高的经济价值，它一般与中基性侵入岩有关。闽西南马坑铁矿具有矽卡岩型铁矿特征，但与典型的矽卡岩型铁矿不同的是，该矿矿体在时间上和空间上均与高硅花岗岩(SiO2>75%)密切相关，因此，马坑铁矿的成因一直存在争议。作为成矿元素，厘清铁的物质来源是解决上述争议的重要途径。铁同位素是示踪铁的来源和成矿过程的有效手段，对马坑铁矿进行详细的铁同位素研究有助于厘清铁的来源，从而揭示高硅花岗岩与铁矿的联系。此外，在厘清铁的物质来源的基础上，本基金尝试建立矽卡岩铁矿中铁同位素分馏的模型，该模型的建立有利于其它矽卡岩矿种的矿化过程和矿床成因的研究。

马坑铁矿的成因尚存争议，制约了其找矿方向和思路。本项目研发了多种针对特殊样品的同位素分析手段，综合利用Sr-Nd同位素体系示踪马坑铁矿的物质来源；利用Fe等同位素研究其形成过程；利用U-Pb同位素体系制约成矿年代。两件矿石样品的磷灰石的U-Pb年代为136Ma早于大洋岩体的倾入时间，表明此矿床形成年代为燕山晚期，而非海西期。矿石样品Nd同位素介于辉绿岩和大理岩之间，无论是年代还是物质上均与高硅大洋花岗岩无关。矿体铁同位素与辉绿岩呈现互补的关系，表明辉绿岩流体蚀变出的铁是矿床的主要来源。综上，马坑铁矿为矽卡岩型矿床，其成矿年代为燕山晚期，对应找矿策略应放在辉绿岩和碳酸盐岩的交界处。