Cratonic tectonic controls of Late Precambrian iron deposits and Mississippi Valley-type deposits in the Central United States
Heyl, A. V.
published: Nov 27, 1995
ArtNo. ESP136000501005, Price: 19.00 €
Middle Proterozoic age iron, light rare-earth apatite, copper deposits are exposed in the St. Francois Mountains in a larger alkalic granite and rhyolite terrane that is favorable for the occurrence of world-class deposits of the Olympic Dam type. The Precambrian age structural network of lineaments cut the Proterozoic rocks above where movement has been revived in post-Precambrian time including both Paleozoic and Mesozoic age rocks. The Mississippi Valley-type of deposits in Paleozoic-age rocks are located at the crests of domes (Central Kentucky and Central Tennessee districts), or faulted collapsed domes (Illinois-Kentucky) on half-domes (Upper Mississippi Valley and Tri-State districts), or they surround Precambrian islands in an open oval in fault zones and facies changes where shales, carbonate rocks and sandstones pinch out (Southeast Missouri). Some ores were deposited as veins in faults, as replacements in adjacent permeable beds, in cryptovolcanic breccia (part of Illinois-Kentucky), or in fault zones adjacent to and above Precambrian rift valleys (eastern northern Arkansas and parts of the Illinois-Kentucky districts). In the Central States major structures of repeated uplift and subsidence are crossed by a complex system of intersecting structural lineaments that form a cratonic network across the large domes, basins and rift valleys. The lineaments trend northwest, northeast, east and north. Some are curved and many have a strike-slip component of movement. In addition there is the Proterozoic age Midcontinent rift system of the northern Midwest of the U. S. A. This system, where exposed near Lake Superior, has major deposits of native copper and silver deposits, also north of Duluth, Minn. copper, nickel, cobalt deposits, to the south of which in northern Wisconsin are massive sulfide deposits of zinc, copper, lead, silver and gold. They are in a northeast-trending belt of metavolcanic rocks of Early Proterozoic rocks. The Mississippi Valley-type deposits occur in all strata younger than the Precambrian. In each zinc, lead, fluorite, barite district every Paleozoic sedimentary rock (carbonate rocks, sandstones and shales) contain some deposits, but the preferred hosts are hydrothermally (by heated brines) sponged and thinned carbonate rocks beneath shale caps, and in the southern Illinois cryptovolcanic breccias of the center of Hicks Dome. The mineralized breccia is centered in a much larger structural high of about 1200 meters of vertical uplift, and about 15 kilometers across. In southern Illinois are about 10 known intrusive lamphroite, lamphroite breccia bodies that contain among others, fragments of felsic syenites blown up from great depths. Mafic alkalic dikes are also present. Most are of northwest trend but a few have northerly and northeast trends like the fluorite veins in faults. All of the isotopic age data average at 267 million years (Early Permian). The central breccia of Hicks Dome contains rare-earth bearing fluorite, barite, sphalerite, galena, bertrandite, niobiumbearing minerals, brockite, florencite, calcite, pyrite, biotite, rutile, apatite, and zirconium minerals. Also some mafic alkalic dikes, volcanic necks, ash flows and explosion breccias of Cambrian and Devonian age occur in Southeast Missouri. Some explosion breccias near Avon, MO., contain fluorite, barite biotite and ashanite (a niobium, tantalum, uranium oxide). They are of Devonian age, and intrude the Cambrian-age Bonne Terre Dolomite.