LAMINATED IRON FORMATIONS IN THE MESOPROTEROZOIC ST. FRANCOIS MOUNTAINS IGNEOUS TERRANE, SE MISSOURI. DO THEY REPRESENT ALGOMA-TYPE BIF?

Some of the earliest preserved rocks in the Earth’s geologic record are represented by a unique group of sedimentary rocks known as banded iron-formations (BIF), which are comprised of alternating iron- and silica-rich laminae most commonly in the form of iron oxides (hematite and/or magnetite) and microcrystalline quartz (jasper). Considerable debate still persists over the nature of these rocks and one of the notable points is their apparent exclusivity to the Archean and Proterozoic rock record. This unusual characteristic is further deepened by an apparent hiatus of BIF deposition within the Proterozoic extending for nearly one billion years duration from 1.8 to 0.8 Ga. Recent work within the St. Francois Mountains Iron district of SE Missouri has uncovered a number of previously undescribed deposits that appear to represent BIF’s, yet occur in a terrane that is 1.5-1.4 Ga in age, during this apparent haitus. These Missouri deposits share many mineralogical and sedimentalogical similarities to the extensive and well-studied BIF occurrences found in the Labrador Trough (Canada), Nabbeau Basin (Australia) and Mesabi Range (Minnesota) that characterized the end of BIF deposition worldwide around 1.9-1.8 Ga. They as well exhibit strikingly similar REE patterns suggesting that the same conditions, mechanisms or processes responsible for the creation of BIF in the Archean and early Proterozoic were still active in the middle Proterozoic.