MINERALOGY AND PETROLOGY OF THE BOURBON PRECAMBRIAN IRON DEPOSIT OF SOUTHEASTERN MISSOURI, AND ITS COMPARISON WITH THE OLYMPIC DAM-TYPE DEPOSITS OF SOUTH AUSTRALIA

The Bourbon deposit represents one of seven 'windows' into Mesoproterozoic iron ore deposition in southeastern Missouri. Similarities exist between these Missouri ores and those of Olympic Dam, Australia including: 1) spatial and genetic association with alkalic-silicic rocks; 2) magnetite/hematite-rich ore; 3) Cu-rich phases; 4) REE mineralization; 5) U and Th mineralization; and 6) Precambrian age of host lithology. The magnetite-rich Bourbon ore zone was emplaced in rhyolite and lies within the Southeast Missouri Magnetic High. Magnetite occurs as disseminated grains in the host rhyolite porphyry and magnetite-rich veins cut the host rock near the margins. Locally, hematite (martite) has replaced magnetite mainly along the octahedral grain boundaries. Some specular hematite has also been detected near the contact of the deposit. Important gangue phases include actinolite, apatite, fluorite, quartz, and alkali feldspar with strong chloritization of biotite present locally. Near the depostional contact, intense silicification of the wall rock results in abundant quartz overgrowths laced with magnetite. In sum, the Bourbon iron deposit shares many mineralogic and petrologic similarities with Olympic Dam, Australia.