The average chemical composition of the Biwabik Iron-Formation and of its principal members is reasonably well known from several previous studies. In this paper the mineral compositions of the formation and its subdivisions are calculated from these chemical data. Because certain components of the formation, such as ferrous iron, occur in oxide, carbonate, and silicate minerals, there is no unique way of assigning such elements to normative minerals. Moreover, because the mineralogy varies with metamorphic grade, the degree of metamorphism must be arbitrarily set. The present calculations were designed to include only the iron minerals magnetite, siderite, minnesotaite, stilpnomelane, and hematite, which are known from field studies to be the chief constituents of the main part of the formation that is essentially unmetamorphosed. According to the plan adopted here, the calculated mineral composition of the Biwabik Formation by weight percent is: quartz = 31.9, iron silicates = 28.0, magnetite = 18.4, combined carbonates = 16.3, hematite = 4.6, and other minerals = 0.8. These values cannot be compared with the over-all modal composition of the unmetamorphosed formation, because that has not been determined. Several lines of evidence, however, suggest that the calculated mineral abundances do not deviate far from the actual abundances.

ABSTRACT The 100-mile-long Mesabi Iron Range contains the Biwabik Iron Formation, the largest of the Lake Superior–type iron-formations in the United States, deposited on the northern edge of the Paleoproterozoic Animikie Basin. This basin has been interpreted as a foreland basin that developed north of the Penokean Fold-and-Thrust Belt (ca. 1850 Ma), or alternatively, as a backarc basin north of the Wisconsin magmatic terrane. The basal unit in the basin, the siliciclastic Pokegama Formation, was deposited upon the ca. 2700 Ma granitic-volcanic basement. It is conformably overlain by the Biwabik Iron Formation, 200–750 ft thick, which consists of four members: lower cherty, lower slaty, upper cherty, and upper slaty. There are two prominent stromatolite zones. Both of the above formations contain attributes of deposition in a tidally influenced environment. The Biwabik is conformably overlain by the Virginia Formation, a thick turbiditic sequence of interbedded black shale, graywacke, and ash beds. All three formations dip southeastward at 10°–20°. The iron-formation (taconite) generally consists of 20%–30% Fe present in carbonates, silicates, and oxides, and 70%–80% SiO 2 . Direct shipping ores, also called natural ores, were originally mined on the Mesabi Iron Range and were instrumental in making the United States an industrial giant and in the winning of WWI and WWII. These ores originated along fault zones in the iron-formation where silica was removed leaving high-grade oxidized hematite-goethite natural ore bodies of 50%–55% Fe. Processing of low-grade magnetic taconite began in 1952, passed the natural ores in tonnage in 1967, and is now totally dominant. Field trip stops will include all three formations, with emphasis on the iron-formation. The final stop is in the folded Thomson Formation, the southerly equivalent of the Virginia Formation.

A drill core (Drill Hole 17,700) of the middle Precambrian Biwabik Iron-Formation from the contact aureole of the middle Keweenawan Duluth Complex in the East Mesabi district, Minnesota, has been studied by petrographic, x-ray diffraction, and electron microprobe techniques. The core provides 400 ft of iron-formation which is underlain by Pokegama Quartzite and is separated from the overlying Duluth Complex by 500 ft of metagraywacke and argillite (Virginia Formation). The iron-formation now consists of metamorphic assemblages containing grunerite, hornblende, actinolite, hedenbergite, olivine, almandine, andradite, magnetite, and quartz. Relict sedimentary textures that are characterized by specific metamorphic mineral assemblages permit correlations with pre-metamorphic mineral assemblages. The following reactions are suggested (all reactions may involve quartz and/or magnetite): grunerite from minesotaite; hornblende + grunerite from stilpnomelane; actinolite and hedenbergite from ankerite; olivine from siderite; almandine from chamosite; and andradite from a calcite-bearing assemblage. Perry and Bonnichsen (1966) suggested a temperature range of 400°C to 650°C in the core during the time of contact metamorphism. This study reveals that the core lies outside the stability fields of iron-carbonates and hypersthene. No mineral assemblage variations indicative of facies changes were observed. Compositions of the iron-silicates do not vary systematically with the inferred thermal gradient; they can be correlated, however, with stratigraphy and specific textures. Estimates of oxygen fugacity based on the inferred temperature of the quartz-magnetite-fayalite buffer assemblage which occurs throughout the core indicate a bulk compositional control on this parameter.

Abstract Analysis of oriented specimens of stromatolites from the Lower Huronian (Animikean of some authors) of the Upper Peninsula of Michigan, the Middle Huronian of the Eastern Mcsabi Range of Minnesota, and the Lpper Keweenawan of Michigan shows that these structures have a preferred direction of growth. Study of the literature on calcareous algae and of numerous occurrences besides those men-tioned has led the writer to certain conclusions concerning the factors by which the colonial growth forms are controlled. The most important of these is believed to be sunlight. The ac-tivity of the algae would be greatest on those portions of the colony receiving the greatest amount of sunlight and therefore colonial growth should be at a maximum in the direction from which the maximum amount of sunlight is received. Thus, the vertical axes of the majority of colonies in a bioherm or biostrome should point toward the equator at an angle determined by the Latitude. Measurements made on stromatolites from the Lower Huronian Kona Dolomite and its equivalent, the Kandville Dolomite, give an average position of one of the poles in Lower Huronian time of 40° West Longitude and 4° North Latitude. Measurements on stromatolites from the Middle Huronian Biwabik Formation give a position of one of the poles of 48° West Longitude and 5° South Latitude. Measurements on stromatolites from the Lpper Keweenawan Copper Harbor Conglomerate give a position of one of the poles of 12° North Latitude and 144° East Longitude.