Geological Significance of Magnetic Patterns Related to Magnetite in Sediments and Metasediments – A Review

Complete magnetic interpretation involves understanding the geological significance of magnetic mineral bearing (generally magnetite bearing) rock units traced out by magnetic surveying — especially as boundaries of magnetite bearing rock units sometimes do not coincide with boundaries of geologically mapped rock units.Magnetic patterns related to magnetite in sediments and metasediments are most often stratiform in character but may also be related to shearing or to local granitization.Magnetite in sediments is unusual but may occur in the following forms: (i) Precambrian chemical precipitate from oxygen poor marine basins, forming extensive banded iron formations; (ii) volcanogenic chemical precipitate possibly associated with base metals; (iii) detrital magnetite in chemically "fresh" sediments; (iv) residual (lateritic) maghemite.Magnetite is more common in metasediments, and its formation during metamorphism depends primarily on the oxidation state (Fe3 + /(Fe3 + + Fe2 +) ratio) inherited from the sediment. During metamorphism low oxidation ratios lead to iron silicate formation; intermediate ratios favour magnetite formation; high ratios favour hematite formation. Magnetic surveying thus frequently delineates pre-metamorphic sedimentary environments in metasediments.Other factors which favour magnetite formation during metamorphism are (i) low silica; (ii) low titanium; (iii) dehydration; (iv) excess aluminium; (v) equilibration to lower temperatures in titaniferous rocks; (vi) absence of carbon.Distinctive magnetic patterns may form in shear zones because of their high permeability to fluids which may be either weakly oxidizing or weakly reducing.