宁芜和尚桥铁氧化物-磷灰石矿床(IOA)成矿过程研究:来自磁铁矿LA-ICP-MS原位分析的证据

铁氧化物-磷灰石矿床(IOA)是全球铁矿资源重要的供给矿床类型之一,受到国内外科研和矿产开采工作者的广泛关注。对铁氧化物-磷灰石矿床研究的争议主要集中在矿床成因上,即岩浆成因或者热液成因。作为一类具有多阶段成矿作用的矿床,IOA型矿床很难用热液或者矿浆成因予以简单概括,需要动态地看待成矿作用。和尚桥铁矿床是一个大型的铁氧化物-磷灰石(IOA)矿床,位于中国东部长江中下游多金属成矿带宁芜矿集区中。和尚桥铁矿床成矿作用含有三个清晰的磁铁矿矿化阶段,分别形成浸染状(Mt1)、角砾状(Mt2)和脉状(Mt3)矿石。对各阶段磁铁矿矿石中磁铁矿进行激光剥蚀等离子质谱(LA-ICP-MS)微区成分测试。在成矿过程中,从早到晚,磁铁矿表现出了从具有岩浆成因特征向具有热液成因特征的方向演化。磁铁矿中Mg和Al含量升高,Cr含量先降低后略微升高,Mn、Co、Ni和V含量先降低后升高,Mo和Sn含量先升高后降低的趋势,表明成矿过程中各阶段围岩及大气水对成矿流体的贡献不一。结合前人研究成果,我们认为和尚桥铁矿床中磁铁矿铁质的来源与安山质侵入岩密切相关,可能来源于岩浆不混溶作用形成的铁质富集流体(熔体),磁铁矿在高温热液环境中结晶沉淀。成矿过程具有多阶段性,推测在各成矿阶段间隙,富铁流体得到富集,同时地层物质不断的加入并导致了磁铁矿成分显示出越来越多的热液成因信息,地层物质(特别是膏盐层)对成矿过程起到了重要的控制作用。

Study on the ore-forming process of the Heshangqiao IOA deposit in the Ningwu ore district:Insight from magnetite LA-ICP-MS in-situ analysis data

Iron Oxide-Apatite (IOA) deposits form an important type of iron deposits, and have attracted much attention of geologists for scientific researches and mining explorations. The genesis of IOA deposits has remained controversial worldwide, with the major divergences in interpretation of the magmatic or hydrothermal (metasomatic) nature of the magnetite ore. Actually, in most mineral deposits or deposit systems, the mineral precipitation processes are dynamic, including time gaps in ore-formation between two mineralization stages, which should not be classified into one simply genesis. The Heshangqiao iron deposit is one multi-stage iron oxide-apatite deposit on a large scale, located in the Ningwu ore district in the Middle-Lower Yangtze River metallogenic ore belt of East China. There are three magnetite mineralization stages, forming disseminated, breccia, and vein-type ores respectively. In-situ LA-ICP-MS trace elemental analyses of magnetite have been performed on three types of ores. In the ore-forming process of the Heshangqiao iron deposit, from early to late, magnetite showed evolution from magmatic characteristics to hydrothermal characteristics. In the magnetite, Mg and Al contents increased, Cr content decreased initially and then increased slightly, Mn, Co, Ni, and V contents decreased initially and then increased, whereas Mo and Sn increased initially and then decreased. These indicate that the contribution of surrounding rock (sedimentary rock, especially evaporate) to the ore-forming fluid is different in different stages. In combination with the results of previous studies, we infer that magnetite from the Heshangqiao IOA deposit originates from an iron-enriched fluid (melt) which might be formed by the magma immiscibility, and precipitated in the high-temperature hydrothermal environment. The metallogenic process has many stages; in the interval between two stages, the metal-bearing fluid becomes enriched, and involved different amount of sedimentary material which is a key controller in this ore-forming system.