鞍本地区大孤山条带状铁建造含铁矿物和相分带特征及形成环境分析

摘要

鞍山-本溪条带状铁建造(Banded Iron Formation,简称BIF)位于华北克拉通东北缘,是世界上典型BIF之一,也是我国最重要的铁矿资源基地.大孤山位于鞍山地区南部矿带,为新太古代典型的Algoma型BIF,与华北克拉通其它大多数BIF相比,具有较低变质程度(绿片岩相-低角闪岩相)和较完整的沉积相分布特征.因此,通过大孤山BIF的研究有利于追踪Algoma型BIF的原生矿物组成及其后期成岩-变质过程,进而通过分析原生矿物形成的物理化学条件探讨古海洋环境.依据原生矿物共生组合及产出特征,可将大孤山BIF沉积相划分为氧化物相(30％)、硅酸盐相(50％)和碳酸盐相(20％).氧化物相主要分布于主矿体南部,主要矿物组成为磁铁矿和石英;硅酸盐相分布于主矿体中部,主要矿物组成除了石英和磁铁矿之外,还有黑硬绿泥石、绿泥石、镁铁闪石等;碳酸盐相分布于矿体北部,主要矿物组成为菱铁矿、磁铁矿和石英等.本文通过大孤山BIF岩相学观察和含铁矿物化学成分研究,推测原生沉积物的组成为无定形硅胶、三价铁氢氧化物和富铝粘土碎屑,在经历了成岩和低级变质作用后转变为具不同相带的条带状铁建造.通过分析磁铁矿、菱铁矿和黑硬绿泥石等矿物在不同PO2-PCO2和pH-Eh条件下的共生相图可知,这些矿物均是在较低氧逸度、中到弱碱性环境下形成.综合考虑矿物成分、共生组合及受变质作用较弱等信息,本文推测制约原生矿物形成的控制因素主要是古海水氧化还原状态、酸碱度、CO2含量和硫逸度.%Anshan-Benxi area is located in the northeast margin of North China Craton,which is the most important resource base of the banded iron formation (BIF) type iron ores in China.As a typical Algoma-type BIF,Dagushan BIF locates in the south ore belt in Anshan area,and comparing with characteristics of other BIFs in China,the Dagushan BIF is obviously distinct mainly in mild metamorphism (greenschist-to lower amphibolite-facies) and integrated depositional facies.Thus,the Dagushan BIF can be used as a good probe to pursue the precursor materials,to invert the primary minerals and their evolutions as well as to discuss the environment of ancient oceans.Hosted in the metasedimentary sequence of Yingtaoyuan Formation in Anshan Group,the Dagushan BIF strikes integrally north-west and dips steeply north-west at 30° to 60°.Oxide (30％),silicate (50％) and carbonate (20％) facies iron formations are recognized based on predominant iron minerals within the iron-rich layers.The oxide facies is composed of magnetite and quartz;the silicate facies consists mainly of stilpnomelane,chlorite and cummingtonite other than quartz and magnetite;while the most prominent carbonate minerals in the carbonate facies is siderite.Integration of petrographic evidence and mineral chemistry indicates that the most likely precursor materials were probably amorphous silica gels,Al-rich detrital clay,which has converted to the minerals today due to subsequent processes,such as diagenesis and metamorphism.The PO2-PCO2 and pH-Eh fields of these minerals (and/or their precursors) indicate anoxic and near-neutral to slightly alkaline conditions for the original depositional environment,from which we can infer that the significant factors affecting the formation of minerals are probably redox statement of ancient ocean,pH value,the content of CO2 and sulphur fugacity.