Oolitic ironstones are iron-rich and chert-poor sedimentary rocks containing concentrically coated grains composed of iron (oxyhydr)oxides and iron phyllosilicates that offer a unique window into iron cycling in ancient coastal environments. These enigmatic deposits are common in the Phanerozoic stratigraphic record yet lack clear modern analogues, and curiously are thought to be absent from Precambrian strata, suggesting a secular control on their deposition. Here we describe a previously unreported ironstone from the middle Tonian (ca. 850 Ma) Katherine Group in the Wernecke Inlier (Yukon, Canada), and show that similar deposits can be found—albeit rarely—throughout the Proterozoic. We investigate the origin of this unit and evaluate its palaeoenvironmental significance, and in light of an extensive literature review, present a holistic model for Precambrian ironstone deposition. The Katherine ironstone occurs in multiple horizons in the McClure and Abraham Plains formations and contains iron ooids and oncoids composed dominantly of authigenic hematite and berthierine, with detrital quartz grains. Textural relationships demonstrate that these coated grains formed on the seafloor with synsedimentary reworking, and the fine interlamination of these phases in grain coatings suggests redox and pH fluctuation during ironstone genesis. Facies associations indicate that the ironstones accumulated in a range of low-energy, shallow marine environments (tidal mudflats and coastal embayments). Geochemical analyses offer insights into genetic processes, and the radiogenic Nd isotope composition and negative Eu anomalies of the Katherine ironstone suggest a continental iron source. We present a model whereby abundant iron, cations, and silica—requisite for the authigenesis of iron phyllosilicates—were supplied from chemical weathering and preferentially enriched in coastal environments due to gradients in pH, Eh and salinity. This continental input would have led to intense iron cycling coupled to organic matter respiration, iron phyllosilicate authigenesis (i.e., reverse weathering). The enrichment of authigenic Fe(III) (oxyhydr)oxides and Fe(II) phyllosilicates took place on a broad coastal plain influenced by both autogenic and allogenic fluctuations in relative sea level, likely in a humid, tropical climate. The lenticular and episodic nature of ironstones in the Proterozoic stratigraphic record suggests that a unique combination of environmental conditions fostered ironstone accumulation. By reviewing the literature on oolitic ironstones, we re-evaluate the temporal distribution of these deposits compared to Archaean–Palaeoproterozoic iron formations, and show that the Great Oxidation Event may have been a prerequisite for ironstone deposition, which may implicate oxidative chemical weathering or suboxic, marine iron cycling. In general, we suggest that the Precambrian record of oolitic ironstones represents an important deep-time archive of iron and nutrient cycling in coastal settings.

中文翻译：

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元古代的鲕粒铁矿石、大陆铁通量和逆风化：来自育空地区托尼安凯瑟琳群的见解


鲕粒铁矿石是富含铁和贫燧石的沉积岩，含有由铁（羟基）氧化物和铁页硅酸盐组成的同心包覆颗粒，为了解古代沿海环境中的铁循环提供了独特的窗口。这些神秘的沉积物在显生宙地层记录中很常见，但缺乏明确的现代类似物，奇怪的是，人们认为前寒武纪地层中不存在这些沉积物，这表明对其沉积的长期控制。在这里，我们描述了一种以前未报道过的铁矿石，该铁矿石来自韦内克内利尔（加拿大育空地区）托尼阶中期（约 850 Ma）凯瑟琳群，并表明在整个元古代都可以发现类似的矿床（尽管很少见）。我们调查了该单元的起源并评估了其古环境意义，并根据广泛的文献综述，提出了前寒武纪铁矿石沉积的整体模型。凯瑟琳铁矿石产于麦克卢尔和亚伯拉罕平原地层的多个层位中，含有主要由自生赤铁矿和 Berthierine 组成的铁鲕和铁鲕，以及碎屑石英颗粒。结构关系表明，这些包覆颗粒是在海底通过同沉积再加工形成的，并且颗粒包覆中这些相的精细层状结构表明了铁矿石成因过程中的氧化还原和 pH 值波动。相组合表明，铁矿石聚集在一系列低能量、浅海环境（潮汐泥滩和沿海海湾）中。地球化学分析提供了对成因过程的深入了解，凯瑟琳铁矿石的放射性 Nd 同位素组成和负 Eu 异常表明存在大陆铁源。 我们提出了一个模型，其中丰富的铁、阳离子和二氧化硅（页硅酸铁自生所必需的）由化学风化提供，并且由于 pH、Eh 和盐度的梯度而优先富集在沿海环境中。这种大陆输入会导致强烈的铁循环，并伴有有机物呼吸、页硅酸铁自生（即逆风化）。自生 Fe(III)（羟基）氧化物和 Fe(II) 页硅酸盐的富集发生在广阔的沿海平原上，受到相对海平面自生和异体波动的影响，可能是在潮湿的热带气候中。元古代地层记录中铁矿石的透镜状和幕式性质表明，独特的环境条件组合促进了铁矿石的积累。通过回顾有关鲕粒铁矿石的文献，我们与太古宙-古元古代铁矿层相比，重新评估了这些沉积物的时间分布，并表明大氧化事件可能是铁矿石沉积的先决条件，这可能意味着氧化化学风化或缺氧、海洋铁循环。总的来说，我们认为鲕粒铁岩的前寒武纪记录代表了沿海环境中铁和养分循环的重要深层档案。