Controversy in Genetic Models for Proterozoic High-Grade, Banded Iron Formation (BIF)-Related Iron Deposits - Unifying or Discrete Model(s)?

摘要

A genetic model for the genesis of high-grade (>65 wt per cent Fe) Proterozoic, banded iron formation (BIF)-related iron deposits such as the Tom Price, Mount Whaleback deposits in the Hamersley Province (Australia), the N4E, N4W, N5 deposits at Carajas (Brazil), the Aguas Claras, Conceicao, and Casa de Pedra deposits in the Iron Quadrangle (Brazil), the Donkerpoort West, Kwagas East deposit in the Thabazimbi iron ore district (South Africa), or the Saksagan ore field at Krivoy Rog (Ukraine), needs to take into account the: 1. timing of regional metamorphism with respect to the upgrade of BIF to high-grade iron ore; 2. significance and relationship of the proto-ore to the hydrothermal alteration minerals and zonation and the process of iron enrichment; 3. source and oxidation state of the hydrothermal fluids that caused the iron enrichment; 4. types of geochemical processes that caused the hydrothermal alteration and iron enrichment; and 5. significance of the widespread carbonatisation of the host rocks as a prerequisite for high-grade BIF-related iron mineralisation. We argue that a unifying model for high-grade BIF-related iron deposits cannot, presently, adequately explain the geological and geochemical characteristics observed in these deposits and, therefore, provide two end-member models that take into account the diversity of geological and geochemical ore deposit features. Our discrete model for the genesis of high-grade, BIF-related iron deposits is based on the interpreted tectonic setting, distinct hydrothermal fluid source of, and processes observed in, major BIF-related iron deposits. We divide two end-members: 1. Proterozoic deep fault-magmatic (Carajas) type, and 2. Proterozoic rift-basin (Hamersley) type. Both end-members experienced varying degrees of deep weathering episodes that effected the hypogene enrichment and, ultimately, yielded the high-grade iron ore mined in many places today.