Origin and Timing of Banded Iron Formation-Hosted High-Grade Hard Hematite Deposits—A Paleomagnetic Approach

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 re-evaluation 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. † Corresponding author: e-mail, [email protected] *Current address: Paleoproterozoic Mineralization Research Group, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa. ©2008 Society of Economic Geologists SEG Reviews vol. 15, p. 49–71 deKock et al 6/11/08 7:41 AM Page 49