Evolution of iron accumulation in the history of the Earth

Abstract

Introduction: Features of iron accumulation in the history of the Earth were considered in the studies of N. M. Strakhov (1963), B. M. Mikhailova et al. (1984), V. T. Frolova (1999), V. N. Kholodova (2005), A. D. Savko (2008) and others. At the same time, new data that have appeared in recent years on the large-scale concentration of this metal in supergene, sedimentary, and volcanic-sedimentary deposits, its reserves and resources, the genesis and role of organic matter, allow us to expand our understanding of the evolution of iron accumulation over time, which is the purpose of this article. Research methods and factual material: The main volumes of accumulated iron ores are concentrated in weathering crusts (WC), sedimentary, volcanogenic-sedimentary and metamorphogenic-sedimentary formations. Therefore, formational, facies, paleogeographic, comparative lithological, and stage methods of studying them were used. The factual material for this article is based on data from the three-volume “Historical Minerageny”, which provides information about more than 5000 significant deposits of various minerals, including several hundred for iron ores. World iron reserves for 2021 from the US Geological Survey (USGS) were used. Results and discussion: In the Precambrian history of the Earth, there were three distinct stages in the formation of jaspilites: Neoarchean (during 2.8–2.5 billion years), Paleoproterozoic (during 2.5–2.3 billion years) and Neoproterozoic (during 720–635 million years). During the first stage, the formation of iron concentrations occurred in the marine basins of greenstone belts. It came from endogenous sources, and its ores occur in effusive-sedimentary strata. The total reserves of ores of this age amount to the first tens of billions of tons. During the Paleoproterozoic (Early Karelian) stage, the formation of iron ore deposits occurred on the territory of the protoplatforms of the Kenorland supercontinent due to iron coming from the WC in the demolition sources. Under anaerobic conditions on the land’s surface, soluble Fe+2 were transported to sea basins. There, under the influence of oxygen generated by cyanobionts, it was converted into Fe+3 and precipitated to the bottom as iron oxide. This is how the largest metal deposits in the world were formed with reserves of hundreds of billions of tons. After the Great Oxidation Event (2.3 Ga), the formation of banded ferruginous quartzites (BIF formation) ceased as free oxygen appeared in the atmosphere, converting iron into its immobile form in the oxide. It resumed after more than a billion years in a glacial climate with the formation of specific ores in tillites. At the end of the eon, in the Cryogeny, during the alternation of glaciations and interglacials, there were deep anoxic rift basins formed during the collapse of the supercontinent Rodinia. Their waters were contaminated with hydrogen sulphide due to the decomposition of organic matter. In these waters Fe+3 converted into Fe+2. During interglacial periods, the connection between the rift basins and the World Ocean was restored. Its waters brought O2, which oxidized iron, converting it into sediment. It contained inclusions of erratic material that came from melting floating ice. This stage included the huge accumulations of jaspilites of El Mutun and Bolivia with 40 billion tons of ore. The total reserves of iron of this era with the main mineral hematite were the first hundreds of billions of tons. In the Phanerozoic, in contrast to the Precambrian with its accumulation of iron in remote parts of paleoreservoirs, it shifts to coastal-marine zones and onto land. In the seas, and less often in lagoons and lakes, oolitic ores of the Lorraine type are formed, composed of hydrogoethite, chamosite, siderite, and hydrohematite. On land, iron accumulates in WCs rocks in the form of hematite (martite, iron mica), hydrohematite, goethite, hydrogoethite siderite, and magnetite. The analysis of the distribution of iron reserves in the Phanerozoic showed that its accumulation occurred during the era of levelling of territories and a warm humid climate, intensive crust formation on land and a weakened supply of terrigenous matter to sea basins. Such conditions existed in the Early Silurian (North America), Late Devonian-Early Carboniferous (Eastern Europe), Jurassic (Western Europe, Australia), Cenozoic (countries with tropical and subtropical climates). The last era was especially large-scale, when most of the explored HGIO accumulated in the WCs. The reserves of Lorraine type ores are also significant. According to various sources, in the West Siberian iron ore basin alone they amount to about 400–900 billion tons. The fact that in many cases industry prefers Precambrian ores is explained by the possibility of obtaining cheap concentrates from ferruginous quartzites containing more than 60% iron, practically devoid of harmful impurities. Conclusions: Iron ores are mostly typical exogenous minerals, formed under the influence of various tectonic, paleogeographic and biochemical factors. The evolution of iron accumulation has a pulsation-directed trend with pronounced epochs of ore formation. In the Archean, iron ore accumulations gravitated towards greenstone belts, in the Paleoproterozoic they gravitated towards protoplatforms, in the Neoproterozoic they gravitated towards rift depressions, and in the Phanerozoic they gravitated towards platforms. Precambrian ores are represented mainly by rocks of the BIF formation, Phanerozoic res are represented mainly by rocks of WCs and oolitic formations. Over time, the mineral and petrographic types of ores also evolved. The Archean and Paleoproterozoic are characterized by hematite-magnetite ores BIF, the Neoproterozoic – hematite GIF, Phanerozoic is characterized by hematite, hydrohematite, goethite, hydrogoethite for WCs and oolitic hematite-chamosite-siderite of the Lorraine type.