山西新太古界柏芝岩组条带状铁建造中的菱铁矿:成因机制与古环境意义

菱铁矿是前寒武纪条带状铁建造(BIF)中重要的矿物组分和古海洋信息载体,但它可能具有原生、早期成岩和晚期成岩多种成因,这在一定程度上限制了其在古海洋条件分析中的应用。虽然前人对菱铁矿开展了广泛的地球化学分析,但在岩相学研究方面相对薄弱。为进一步揭示BIF中菱铁矿的成因机制,以山西代县羊角沟矿区新太古界柏芝岩组的BIF为研究对象,开展了系统的岩相学工作。研究表明,该BIF主要由厘米级交互的富铁与富硅条带构成,其中普遍缺少水体扰动沉积构造,偶见交错层理和风暴碎屑,表明主要沉积于风暴浪基面之下。菱铁矿的主要产出形式有3种:(1)亚毫米级条带,其内"悬浮"有风暴成因碎屑颗粒,具有水柱或沉积物/水界面的原生成因特征;(2)在富铁条带中的晶体间隙充填,可能为早期成岩成因;(3)富硅条带中存在绿泥石层间脉状充填或截切石英和铁白云石的脉体,具有晚期成岩成因。原生菱铁矿的产出,表明新太古代在风暴浪基面之下的海水强烈缺氧、富铁并具有低硫酸盐浓度的特征。尽管原生菱铁矿条带的产出表明菱铁矿具有反映海洋化学条件的潜力,但多种成因菱铁矿的同时产出,也要求在应用菱铁矿分析古海洋条件时应当分组构进行。

Siderite in banded iron formation of the Neoarchean Baizhiyan Formation,Shanxi Province:genesis and palaeoenvironmental implications

Siderite is one of the major mineral components in Precambrian banded iron formation( BIF) and an archive for palaeoenvironmental reconstruction. However,it can exist as primary,early diagenetic,and late diagenetic origins,thus limiting its application in palaeoenvironmental analysis to a certain extent. Previous studies mainly focused on geochemical features of siderite, whereas its petrographic features were paid less attention. In order to further reveal the origin of siderite in BIF,this study took BIF from the Neoarchean Baizhiyan Formation in Yangjiaogou Mining Area,Dai County,Shanxi Province as a target,and carried out a systematic petrographic analysis. The results show that the BIF in the Baizhiyan Formation is mainly composed of alternating centimeter-scale, iron-rich and silicon-rich bands. Wave-agitated structure is rare,but occasionally cross lamination and storm debris are present,suggesting that the BIF was mainly deposited below the storm-wave base. There are three main occurrences for the siderite,including:( 1) sub-millimeter layers with detrital particles "suspended"in them,indicating that they may have the primary origin formed in water column or sediment-water interface;( 2) dispersed anhedral grains in iron-rich layers,which may be of an early diagenetic origin;and( 3) veins penetrating into layers of chlorite,or truncating quartz and ankerite,which is of a late diagenetic origin. The occurrence of primary siderite in the BIF from the Baizhiyan Formation suggests that the Neoarchean seawater below the storm-wave base was strongly anoxic, iron-rich, and of low sulfate concentration. Siderite has the potential to reflect the information of marine chemistry;however,the occurrence of siderite with different origins in one sample requires fabric-specific analysis.