RI-39 Manganiferous Zones in Early Proterozoic Iron-Formation in the Emily District, Cuyuna Range, East-Central Minnesota

Early Proterozoic strata in the Emily district of the Cuyuna iron range of east-central Minnesota unconformably overlie older folded rocks of the North range (North range group). They are correlative with strata of the Animikie Group on the Mesabi iron range, which consist of a lower quartz arenitic sequence (Pokegama Quartzite), an intermediate iron-rich sequence (Biwabik Iron Formation), and an upper feldspar-rich, graywacke-shale sequence (Virginia Formation). In the Emily district, however, the stratigraphic position of the Biwabik Iron Formation is occupied by three units of iron-formation separated by intervening sequences of black shale. Manganese occurs in the lowest iron-rich unit (informally termed Unit A of the Ruth Lake area). Unit A can be divided into six lithotopes. They are: (1) an epicIastic lithotope of quartz-rich siltstone and shale; (2) a mixed epiclastic jaspery chert litho tope; (3) an oolitic and pisolitic lithotope; (4) a thick-bedded lithotope of cherty or granular iron-formation; (5) a mixed thick- and thin-bedded lithotope characterized by thick intervals of slaty or nongranular iron-formation; and (6) a ferruginous chert lithotope. In general, lithotopes 1, 2, and 3 have shallowwater attributes, whereas lithotope 6 was deposited in quieter, presumably deeper water. Lithotopes 4 and 5 interfinger, and thus were deposited in generally similar sedimentological regimes in water of intermediate depth that was variably affected by currents. Unit A was deposited during two trangressive-regressive cycles in a basin that deepened to the north. Well-rounded grains of terrigenous quartz, which persist throughout lithotopes 1-5, imply that much of the sedimentation occurred relatively close to strandline. Although Unit A in the Ruth Lake area has many mineralogical and chemical attributes typical of "ordinary" iron-formation, it contains manganese at levels that are one or two orders of magnitude larger than the norm. Manganese oxides are distributed throughout lithotopes 1-5 as disseminated grains, as thin pods or lenses, and as layers as thick as 1.5 meters that typically contain about 10 percent Mn, but some contain as much as 20-30 percent. In addition, Unit A contains two major, laterally persistent zones about 15 to 18 meters thick, in which the manganese tenor has been enriched to the 10-50 percent range by secondary processes. Both enriched zones more or less coincide with stratigraphic positions occupied by the ooliticpisolitic lithotope. They contain various proportions of psilomelane, cryptomelane, hematite, and quartz. Goethite and manganite may be locally abundant, and where they occur they are secondary phases that formed during a period of intense chemical weathering in Late Jurassic or Early Cretaceous time. Primary stratigraphic, textural, and mineralogic attributes of the Ruth Lake strata correspond to those used by James (1955) to define the oxide (hematite) facies of ironformation. The hematite and chert are syngenetic, but the manganese oxides are more likely epigenetic. It is inferred that the manganese oxides were deposited in porous and permeable parts of the Ruth Lake sequence by a reflux process involving reducing solutions that leached manganese from older rocks of the North range group. The principal mechanisms for manganese concentration are inferred to have been early diagenetic, and therefore to have operated in Early Proterozoic time. Mesozoic weathering phenomena have been imposed on the rocks and have caused some redistribution of manganese. The abundance of manganese makes the Ruth Lake area in the Emily district a potential target for in situ mining techniques currently being developed by the U.S. Bureau of Mines and the Mineral Resources Research Center of the University of Minnesota