DESCRIPTIVE MODEL OF SUPERIOR Fe

MODEL 34a

By William F. Cannon

DESCRIPTION Banded iron-rich sedimentary rock, generally of great lateral extent, typically layered on centimeter scale with siliceous (chert) beds interlayered with iron-rich beds.

GENERAL REFERENCE James (1954).

GEOLOGICAL ENVIRONMENT

Rock Types Commonly interlayered with quartzite, shale, dolomite.

Textures Iron-formations and host rocks commonly contain sedimentary textures typical of shallow-water deposition in tectonically stable regions.

Age Range Mostly Early Proterozoic (2.0± 0.2 b.y.). Less commonly Middle and Late Proterozoic.

Depositional Environment Stable, shallow-water marine environment, commonly on stable continental shelf or intracratonic basin.

Tectonic Setting(s) Now commonly preserved in forelands of Proterozoic orogenic belts.

Associated Deposit Types Sedimentary manganese deposits may occur stratigraphically near or be interbedded with iron-formations.

DEPOSIT DESCRIPTION

Mineralogy Hematite, magnetite, siderite, fine-grained quartz.

Texture/Structure Nearly always banded at centimeter scale; very fine grained where not metamorphosed.

Alteration None related to ore deposition. Commonly metamorphosed to varying degrees or weathered and enriched by supergene processes.

Ore Controls No primary controls of local importance. Supergene ores may be localized by irregularities in present or paleo erosion surface.

Weathering Alteration of original iron mineral to Fe-hydroxides and hematite. Silica partly to totally leached. End product of weathering is high-grade supergene ore.

Geophysical Signature Magnetic anomalies.

EXAMPLES

Mesabi Range, USMN (James, 1983)