Mineralogy and microstructures of carbonaceous gold ores

Abstract

This paper describes mineralogical and geological aspects of the Owl Creek and Hoyle Pond gold mines, located in Archean metasediments and metavolcanics in the Abitibi greenstone belt in northern Ontario, Canada. The occurrence of “invisible gold” of Cenozoic age, hosted in structurally-controlled settings in Paleozoic strata of the southwestern U.S.A., has been known and exploited for over thirty years. Petrographic observations are combined with electron microprobe data in order to describe aspects of gold mineralization from these Ontario mines and from selected analogous sites elsewhere. Details of selected exposures of Archean-Proterozoic lithologies, which are carbonaceous but not strongly enriched in gold, are briefly compared with features of the mine geology. Although no mine generates carbonaceous ores alone, the proportion of such ores varies widely from mine to mine, and district to district. Questions remain, particularly with regard to the chemical behaviour of the reduced carbon, but two common factors of prime economic relevance are established. These are the importance of (a) fluid focusing through incompetent carbonaceous units, and (b) localization of high (commonly visible) gold values on vein-wallrock contacts.