2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 12
Presentation Time: 4:20 PM


GUTZMER, Jens1, BEUKES, Nicolas J.1, MUKHOPADHYAY, Joydip1, PACK, Andreas2, HAYASHI, Ken-ichiro3 and SHARP, Zachary4, (1)Department of Geology, Rand Afrikaans Univ, P.O. Box 524, Auckland Park, 2006, South Africa, (2)Institut fuer Mineralogie und Geochemie, Universitaet zu Koeln, Zuelpicher Strasse 49b, Cologne, 50674, Germany, (3)Department of Mineralogy, Petrology, and Economic Geology, Tohoku Univ, Aoba-ku, Sendai, 980-8578, Japan, (4)Department of Earth and Planetary Sciences, Univ of New Mexico, Northrop Hall, 200 Yale Blvd. NE, Albuquerque, NM 87131, jg@na.rau.ac.za

Most world-class high-grade iron ore deposits are the product of enrichment of Precambrian iron formations but processes responsible for enrichment remain poorly understood. At present it is thought that some of the deposits are of hydrothermal and some of supergene origin. The supergene deposits are closely related to major unconformities whereas the hydrothermal deposits appear to be structurally controlled. However, there are no established geochemical or petrographic tools available to differentiate between the two types of deposits. This is because the iron ores are essentially composed of hematite, a mineral with wide stability field and simple mineral chemistry. However, the oxygen isotopic composition of the ore-forming hematite appears to hold relevant metallogenetic information. Deposits that are though to be of hydrothermal origin based on their geological setting are marked by oxygen isotopic compositions as low as –7‰ (relative to SMOW). This signature is quite in contrast to ancient and modern unconformity bounded supergene deposits that yield isotopic signatures between +2‰ and –3‰. The results obtained for ancient supergene deposits are especially noteworthy, as they may find application to constrain the oxygen isotopic composition of ancient meteoric water. In geologically recent times, the formation of high-grade iron ore deposits is known to require lateritic weathering conditions and the oxygen isotope signature of iron oxides contained in lateritic weathering profiles is in apparent equilibrium with co-existing meteoric water. A lateritic origin has also been predicted for ancient supergene iron ore deposits in South Africa of Paleoproterozoic age. From the ore-forming hematites in these deposits, an oxygen isotope signature of Paleoproterozoic meteoric water can be derived that is well within the range of modern meteoric water in tropical and subtropical environments. This finding leads to the conclusion that processes controlling the global water cycle may have been very similar for at least the last 2 billion years of Earth history.