山西吕梁袁家村条带状铁建造沉积相与沉积环境分析

山西吕梁作为华北克拉通上条带状铁建造(BIF)的重要产区之一,位于华北中央构造带中。袁家村BIF分布于吕梁岚县袁家村一带,极有可能是华北克拉通内最为典型的Superior型BIF。与华北克拉通其他大多数BIF相比,袁家村BIF具有明显的差异性,其中包括它的形成时代(2.3~2.1Ga)、铁建造类型和低级变质程度(低绿片岩相)等。因此,研究袁家村BIF具有特殊的研究意义,可为探讨大氧化事件之后古海洋氧化还原状态以及国内Superior型BIF的成因提供研究基础。袁家村BIF产于吕梁群袁家村组变沉积岩系的下部,前人根据上覆和下伏含火山岩地层的时代,推测袁家村组的形成时代为2.3~2.1Ga。BIF整体产状陡倾,沿北北东-北东东向呈L形带状分布。依据原生矿物的共生组合及产出特征,可将BIF沉积相划分为氧化物相(60%)、硅酸盐相(30%)和碳酸盐相(10%)。氧化物相是本区BIF最主要的沉积相,主要矿物为赤铁矿、磁铁矿和石英,从而可进一步划分为赤铁矿(24%)和磁铁矿(36%)亚相;硅酸盐相BIF以大量硅酸盐矿物出现为特征,散布于研究区,主要矿物组成除了石英和磁铁矿之外,还有铁黑硬绿泥石、绿泥石、铁滑石、镁铁闪石和阳起石等。在与碳酸盐相BIF构成过渡相的BIF中,还可发现大量的铁白云石。而碳酸盐相主要矿物为菱铁矿、铁白云石和石英等,主要发育于研究区的南部。依据含铁岩系构造格局特点复原获得了原始沉积相分布略图,沉积相主要呈南北向延展,自东向西显示出相变规律,西边为碳酸盐相,东边为氧化物相,其间是过渡的硅酸盐相。通过袁家村BIF的岩相学和含铁矿物化学成分的研究,可大致推测原始沉积的矿物组成为无定形硅胶、水铁矿、与铁蛇纹石和黑硬绿泥石组成类似的铁硅酸盐凝胶、富Al的粘土碎屑和含铁、镁、钙的碳酸盐软泥。这些沉积物在随后的成岩期和绿片岩相的区域变质作用下发生矿物之间的相互转变。BIF中主要含铁矿物的PO-P-Eh 2CO2和pH相关图解说明除了赤铁矿之外,其他矿物均是在较低氧逸度环境中形成的,且所有矿物共存的水体系为中性到弱碱性。袁家村BIF氧化物相中发育豆粒、内碎屑结构和板状交错层理等原始沉积构造,指示氧化相部分是在相对高能的浅水环境下沉积的。但BIF大部分应该形成于浪基面以下(200m)较为深水的环境中,沉淀可能同时发生于上部氧化和下部还原的水体之中,由于还原弱酸性的深部富铁海水在海侵的过程中上升到浅部相对氧化和弱碱性的浅水环境中,因为Eh、pH及氧逸度等物化条件的骤然变化,最终导致铁质的沉淀和沉积相自上而下的变化。

Analysis of sedimentary facies and depositional environment of the Yuanjiacun banded iron formation in the Lüliang area, Shanxi Province

The Yuanjiacun banded iron formation (BIF), located in the Lüliang area, is likely to be the most representative Superior-type BIF in the North China Craton (NCC). Comparing with characteristics of other BIFs in China, the Yuanjiacun BIF is obviously distinct from them mainly involving three aspects: the formation age (2.3~2.1Ga), the genetic type (Superior) and mild metamorphism (lower greenschist facies). Thus, the Yuanjiacun BIF can be used as a good probe to understand atmospheric evolution and the chemical composition and redox states of the ancient oceans after the Great Oxidation Event (GOE) and is of great significance for relevant research on the Superior BIFs in China. The Yuanjiacun BIF is coherent with the sedimentary rock succession of the Yuanjiacun Formation in the Lower Lüliang Group, and was interpreted to be deposited at 2.3~2.1Ga, based on ages of overlying and underlying volcanic strata. The BIF is distributed in a NNE-NEE direction with a steep occurrence. Oxide (magnetite and hematite), carbonate, and silicate facies iron formations are recognized based on predominant iron minerals within the iron-rich layers. The widespread oxide facies is composed of magnetite, hematite and quartz; the silicate facies is characterized by presence of iron silicate minerals and their metamorphic equivalens, consisting mainly of stilpnomelane, chlorite, minnesotaite, cummingtonite and actinolite other than quartz and magnetite; whereas the carbonate facies is rare in which the most prominent carbonate minerals are siderite and ankerite. Ankerite is commonly found in the silicate-carbonate facies iron formation. The distribution of primary sedimentary facies has been recovered on the basis of structural framework for iron-bearing rock sequences. Sedimentary facies extend along the N-S direction. The eastward transition from carbonate facies into oxide facies iron formation is accompanied by a change in mineralogical composition from siderite-facies iron formation in the west through magnetite-ankerite- and magnetite-stilpnomelane-facies iron formation in the transition zone to hematite-magnetite iron formation in the east. Integration of petrographic evidence and mineral chemistry indicates that the most likely precursor materials were comprised of probably hydrous, Fe-silicate gels of greenalite- and stilpnomelane-type compositions, amorphous silica gels, ferrihydrite, very fine-grained carbonate oozes of variable composition, and Al-rich detrital clay, which have been modified due to subsequent processes, such as diagenesis and metamorphism. The P_O2-P_CO2 and pH-Eh fields of the above minerals (and/or their precursors) indicate anoxic and near-neutral to slightly alkaline conditions for the original depositional environment except for the hematite precursor field (that of Fe(OH)₃), which is very small and exists only at relatively high P_O2 values. Primary depositional features, such as tabular cross-bedding, interclastic and granular texture, are locally found in the oxide-facies iron formation. All these suggest a shallow-water, relatively high energy sedimentary environment for minor parts of iron formation. Nevertheless, the prevalence of alternating magnetite- and silica-rich bands in the Yuanjiacun BIF reflect that most of these occurrences were likely to be deposited in basin deeper than at least 200m, which is the minimum depth for modern storm wave base. Therefore, the Yuanjiacun BIF appears to have formed both in the upper oxic and lower-level reduced marine water due to upwelling of reducing, slightly acidic iron-rich bottom water into an already more oxidizing, and slightly alkaline shallow-water environment during the peak of transgression, ultimately resulting in the vertical facies variation.