Neoproterozoic banded iron formations
Tóm tắt
Two epochs of the formation of ferruginous quartzites—Archean-Paleoproterozoic (3.2–1.8 Ga) and Neoproterozoic (0.85–0.7 Ga)—are distinguished in the Precambrian. They are incommensurable in scale: the Paleoproterozoic Kursk Group of the Kursk Magnetic Anomaly (KMA) extends over 1500 km, whereas the extension of Neoproterozoic banded iron formations (BIF) beds does not exceed a few tens of kilometers. Their thickness is up to 200 m and not more than 10 m, respectively. The oldest BIFs are located in old platforms, whereas Neoproterozoic BIFs are mainly confined to Phanerozoic orogenic (mobile) zones. Neoproterozoic BIFs universally associate with glacial deposits and their beds include glacial dropstones. In places, they underlie tillites of the Laplandian (Marinoan) glaciation (635 Ma), but they are more often sandwiched between glaciogenic sequences of the Laplandian and preceding Sturtian or Rapitan glaciation (730–750 Ma). Neoproterozoic BIFs are rather diverse in terms of lithology due to variation in the grade of metamorphism from place to place from low grades of the greenschist facies up to the granulite facies. Correspondingly, the ore component is mainly represented by hematite or magnetite. The REE distribution and (Co + Ni + Cu) index suggest an influence of hydrothermal sources of Fe, although it was subordinate to the continental washout. Iron was accumulated in seawater during glaciations, whereas iron mineralization took place at the earliest stages of postglacial transgressions.
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