Abstract

Early Proterozoic strata of the Emily district at the far northern end of the Cuyuna iron range define the southwestern closure of the Animikie basin in east-central Minnesota. As such, the rocks of the Emily district are correlative with strata of the well-known Animikie Group of the Mesabi Range. However, unlike the monoclinal nature of the Mesabi Range, strata in the Emily district are deformed into a series of broad, open, eastward-plunging folds with near-vertical axial planes. Geometric relationships imply that the basal contact of the Animikie Group overlies an unconformity cut onto older folded rocks of the Cuyuna North Range. That unconformity marks a boundary between twice-deformed rocks of the Penokean fold and thrust belt and the once-deformed rocks of the Animikie basin.On the Mesabi Range, the Animikie Group consists of a lower quartz arenitic sequence (Pokegama Quartzite), an intermediate iron-rich sequence (Biwabik Iron-formation), and an upper black shale-graywacke sequence (Virginia Formation). In the Emily district, however, the stratigraphic position of the Biwabik is occupied by several lenticular units of iron-formation intercalated within both Pokegama- and Virginia-like materials. The main iron-formation--termed "Unit A"--can be divided into seven lithotopes that were deposited during two transgressive-regressive cycles in a basin having a strandline to the west and deeper water to the north and east. Well-rounded grains of terrigenous quartz, which persist throughout much of the formation, imply that sedimentation occurred relatively close to the strandline.Although Unit A has many mineralogical, textural, and chemical attributes indicative of ordinary iron-formation, it locally contains manganese oxides in amounts 10 to 100 times greater than the norm of 0.6 to 0.8 percent in the Biwabik Iron-formation. Manganese oxides occur principally in the coarser grained parts of Unit A as disseminated grains, as thin pods or lenses, and as layers as thick as 1.5 m that typically contain about 10 percent Mn; some contain as much as 20 to 30 percent Mn. Manganese oxides are particularly abundant in two laterally persistent zones about 15 to 18 m thick that have manganese tenors enriched to the range of 10 to 50 percent. Both zones more or less coincide with stratigraphic positions occupied by oolitic-pisolitic iron-formation. The zones contain various proportions of psilomelane and cryptomelane, as well as hematite and quartz. Goethite and manganite are locally abundant; where they occur, they are secondary phases that formed during a period of intense chemical weathering that modified these rocks in Late Jurassic or Early Cretaceous time.The manganese oxides are most likely epigenetic. They are confined to the porous and permeable parts of Unit A; rocks with the greatest primary porosity--such as the oolitic-pisolitic lithotope and quartz arenitic rocks in the epiclastic lithotope--have the most manganese. We suggest that the manganese was deposited by a refluxing process involving reducing solutions that leached manganese from older rocks of the Cuyuna North range. The manganese was subsequently precipitated where the reducing solutions came into contact with oxidizing conditions in the depositional basin just below the sediment-water interface. Much of the manganese must have been precipitated early in the diagenetic history of the iron-formation, because it occupies pore spaces normally filled with silica cement.

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