Origin and Timing of Banded Iron Formation-Hosted High-Grade Hard Hematite Deposits—A Paleomagnetic Approach
M. O. de Kock, D. A. D. Evans, J. Gutzmer, N. J. Beukes, H. C. Dorland, 2008. "Origin and Timing of Banded Iron Formation-Hosted High-Grade Hard Hematite Deposits—A Paleomagnetic Approach", Banded Iron Formation-Related High-Grade Iron Ore, Steffen Hagemann, Carlos Alberto Rosière, Jens Gutzmer, Nicolas J. Beukes
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The processes responsible for the transformation of banded iron formations to hard high-grade hematite ore, and their timing, remain poorly understood despite many recent advances. The paleomagnetic method allows for the estimation of ore genesis timing as a complement to other techniques. The effectiveness of the paleomagnetic method at dating, and testing proposed models for, the genesis of hard high-grade hematite ore deposits is illustrated by two South African examples. A new dataset is reported for the Thabazimbi deposit that independently constrains the age of ore formation between 2054 and 1930 Ma, while previously published data from the Sishen-Beeshoek deposits highlight the association of those deposits with weathering preceding the development of a marked Paleoproterozoic-aged unconformity (older than 2060 Ma). Paleomagnetic results are in both cases consistent with proposed models of ore genesis (i.e., extensive carbonate metasomatism and meteoric fluid interaction at Thabazimbi and ancient supergene processes at Sishen-Beeshoek). The antiquity of these South African examples appears to reflect a common theme among other hard high-grade hematite deposits from around the world, as revealed by a review and reevaluation of existing paleomagnetic literature. This review represents a first attempt at providing a synopsis of hard high-grade hematite deposits within a temporal framework. The apparent Paleoproterozoic to Mesozoic age distribution of deposits as discussed in this review, which must be tested and verified by both the expansion of the database and improvement of current available data, has important implications for proposed models of ore genesis, as well as for exploration.
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The spark to put together this volume on banded iron formation (BIF)-related high-grade iron ore was born in 2005 during a steamy night in Carajás where the iron research group from the Universidade Federal Minas Gerais, Vale geologists, Carlos Rosière and Steffen Hagemann, were hotly debating the hypogene alteration genesis for the high-grade, jaspilite-hosted Serra Norte iron ore deposits. A couple of caipirinhas later we decided that the time was opportune to put together a volume that captured the new and innovative research that was being conducted on BIF-related high-grade iron ores throughout the world. We had little problem convincing our South African colleagues Jens Gutzmer and Nic Beukes to join the effort and decided that the 2008 biannual Society of Economic Geologists' (SEG) meeting in South Africa would be the perfect place to present this project through a combined field trip and workshop near Sishen.
The enthusiastic support that we received from the research community, SEG, and industry to put this volume together was generated by the significant increase in exploration activity, and with it the need for more detailed information on what exactly controls the location of high-grade iron orebodies, and renewed research interest around the world in models for the genesis of BIF-related high-grade iron ore, and particularly the relative importance of hypogene and supergene processes in formation of high-grade ore.
This volume concentrates on new research on the characteristics and metallogenesis of BIF-related high-grade iron ores. It contains a state of the art series of papers on established and new iron ore districts and deposits, the different components of the BIF iron mineral system, and how to best explore for this ore type. Although the emphasis of many of the contributions to this volume is on the hypogene aspect of high-grade iron ore formation, it is important to note that most BIF-related iron ore districts have a very pronounced supergene overprint due to deep lateritic weathering. The transformation of many hypogene iron orebodies of reasonable grade and size to the giant deposits exploited today can be related to this geologically recent supergene overprint; most of the past and still much of the present mining of high-grade iron ore relates to soft ore interpreted in most cases to be the direct result of supergene processes. Also mentioned here should be the recent resurgence of a syngenetic model that advocates the formation of chert-free BIF