Depositional Environment of the Paleoproterozoic Yuanjiacun Banded Iron Formation in Shanxi Province, China
Depositional Environment of the Paleoproterozoic Yuanjiacun Banded Iron Formation in Shanxi Province, China
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DOI：
10.2113/econgeo.110.6.1515
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发表时间：
2015-09
期刊：
影响因子：
5.8
通讯作者：
Changle Wang;K. Konhauser;Lianchang Zhang
中科院分区：
文献类型：
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作者：
Changle Wang;K. Konhauser;Lianchang Zhang
山西省古元古代袁家村条带状含铁建造（BIF）是华北克拉通中的一个水道型BIF。该BIF位于吕梁群下部袁家村组的变质沉积岩序列中，经历了下部绿片岩相变质作用。氧化铁（磁铁矿和赤铁矿）、碳酸盐和硅酸盐相都存在于富铁层中。从碳酸盐岩向氧化物相铁地层的东向过渡伴随着矿物组成的变化，从西部的菱铁矿到过渡带的磁铁矿-铁白云石和磁铁矿-硬黑锰矿组合，再到东部的磁铁矿和赤铁矿。在BIF内也可以垂直观察到这些不同的侧向相，即，铁矿物组合由菱铁矿经磁铁矿向富赤铁矿铁建造的上切变化。氧化物相BIF形成于海岸附近，而碳酸盐（菱铁矿）和硅酸盐相组合形成于较深的沃茨。基于对盆地尺度上这些变化的详细分析，BIF在海侵事件期间在从风暴波底以下的深沃茨到相对浅的沃茨的环境中沉淀。BIF样品显示出明显的海水状稀土+ Y的配置文件，其特点是积极的La和Y异常和重稀土富集相对于轻稀土后太古代澳大利亚页岩归一化图。一致的正Eu异常也被观察到，这是典型的还原，高温热液流体。此外，轻微的负到正的Ce异常，和一个大范围的轻，重稀土元素的比例，存在于氧化物相BIF。这些特征与δ 56 Fe值一致为正值相结合，表明BIF的沉积发生在沿着化学跃层，在该化学跃层中，深部缺氧、富铁和富硅热液流体的上涌与较浅且轻度含氧的海水混合。铁白云石显示出高度亏损的δ 13 C值，富含碳酸盐的BIF具有高含量的有机碳，表明在从水柱沉积的生物质的埋藏过程中，羟基氧化铁前体的异化Fe（III）还原;相同的生物质可能与溶解的Fe（II）的原始氧化有关。在较浅的沃茨中形成的含铁量较高的BIF相的事实表明，河流来源的营养物质将是最小的，从而限制了浅沃茨中的初级生产力，并最大限度地减少了通过异化Fe（III）还原还原赤铁矿所需的有机碳源。相比之下，在更靠近热液喷口的较深沃茨，营养物质丰富，生物量生产力高，加上碳埋藏增加，导致富铁碳酸盐沉积。袁家村BIF的沉积在大氧化事件（GOE;约。2.4-2.2 Ga）证实，深海沃茨在这段时间内仍然是偶尔含铁的，但浅水沃茨充分氧化，Fe（II）氧化不再需要直接联系到近端蓝藻活动。
The Paleoproterozoic (~2.38–2.21 Ga) Yuanjiacun banded iron formation (BIF), located in Shanxi Province, is a Superior-type BIF in the North China craton. This BIF is within a metasedimentary rock succession of the Yuanjiacun Formation, in the lower Luliang Group, which has undergone lower greenschist-facies metamor phism. Iron oxide (magnetite and hematite), carbonate, and silicate facies are all present within the iron-rich layers. The eastward transition from carbonate- into oxide-facies iron formations is accompanied by a change in mineralogical composition from siderite in the west through magnetite-ankerite and magnetite-stilpnomelane assemblages in the transition zone to magnetite and then hematite in the east. These distinct lateral facies are also observed vertically within the BIF, i.e., the iron mineral assemblage changes upsection from sider ite through magnetite into hematite-rich iron formation. The oxide-facies BIF formed near shore, whereas carbonate (siderite)- and silicate-facies assemblages formed in deeper waters. Based on detailed analyses of these variations on a basinal scale, the BIF precipitated during a transgressive event within an environment that ranged from deep waters below storm wave base to relatively shallow waters. The BIF samples display distinctively seawater-like REEs + Y profiles that are characterized by positive La and Y anomalies and HREEs enrichment relative to LREEs in Post-Archean Australian shale-normalized diagrams. Consistently positive Eu anomalies are also observed, which are typical of reduced, high-temperature hydrothermal fluids. In addition, slightly negative to positive Ce anomalies, and a large range in ratios of light to heavy REEs, are present in the oxide-facies BIF. These characteristics, in combination with consistently positive δ 56Fe values, suggest that deposition of the BIF took place along the chemocline where upwelling of deep, anoxic, iron- and silicarich hydrothermal fluids mixed with shallower and slightly oxygenated seawater. The ankerite displays highly depleted δ13C values and the carbonate-rich BIF has a high content of organic carbon, suggesting dissimilatory Fe(III) reduction of a ferric oxyhydroxide precursor during burial of biomass deposited from the water column; that same biomass was likely tied to the original oxidation of dissolved Fe(II). The fact that the more ferric BIF facies formed in shallower waters suggests that river-sourced nutrients would have been minimal, thus limiting primary productivity in the shallow waters and minimizing the organic carbon source necessary for reducing the hematite via dissimilatory Fe(III) reduction. By contrast, in deeper waters more proximal to the hydrothermal vents, nutrients were abundant, and high biomass productivity was coupled to increased carbon burial, leading to the deposition of iron-rich carbonates. The deposition of the Yuanjiacun BIF during the onset of the Great Oxidation Event (GOE; ca. 2.4–2.2 Ga) confirms that deep marine waters during this time period were still episodically ferruginous, but that shallow waters were sufficiently oxygenated that Fe(II) oxidation no longer needed to be tied directly to proximal cyanobacterial activity.