Venue: Denver, USA
Travel report by Des Lascelles
Thanks to a grant from Geoconferences, I was able to present in person a poster entitled “The Mt Gibson iron-ore deposit; a non-supergene origin for high-grade hematite ore derived from banded iron-formation” at SEG 2002 in Denver, USA, on 14-16 June 2002.
The Mt Gibson iron-ore deposit is hosted in an Archean banded iron formation (BIF) within the Retaliation greenstone belt in the Yilgarn Craton. Study of a large volume of diamond drill core from completely unweathered BIF revealed a number of features that contradicted current theories on the supergene origin of the high-grade hematite deposits. Completely unweathered magnetite enrichments, reported as up to 85 wt % Fe2O3 in drill intersections up to 90 metres long, could be related to surface hematite enrichments with a strike length of 1km. The large high-grade hematite deposits throughout the world are saprolitic as shown by the preservation of primary textures. Although silica was highly mobile within the surface soil horizons it was found to remain constant from bedrock to the top of the saprolite layer and was not selectively removed by supergene processes of purely chemical weathering.
Low-temperature hydrothermal enrichments are also present at Mt Gibson where they take the form of carbonate replacement of chert and silicates that are upgraded by supergene weathering through leaching of the carbonate minerals. However, the selective removal of quartz from BIF by high-temperature hydrothermal solutions was contra-indicated by the presence of quartz-hematite veins showing that the iron oxides are equally mobile at higher temperatures.
Thus neither supergene nor hypogene processes are capable of forming major high-grade iron ore deposits from cherty BIF and therefore large bodies of chert-free BIF (including magnetite-carbonate BIF) existed prior to weathering of the deposits.
I found the conference to be a very rewarding experience, not only because it was my first visit to the United States, but also for the wide range of topics presented at the conference and the opportunity to visit some large world-class porphyry copper deposits. Unfortunately, none of the papers held much relevance to my own research as the main interest seemed to be in gold and copper with minor emphasis on platinum group metals and diamonds, and very few of the attendees showed interest in my poster. Iron-ore geology appeared to be somewhat out of fashion. However, I did have some interesting and informative discussions with those who were interested.