Comparison of supergene mimetic and supergene lateritic iron ore deposits

Abstract

Supergene lateritic iron ores are uncommon compared to the vast tonnage of banded ironformation (BIF) hosted microplaty hematite ores or the dominant supergene mimetic martitegoethite ores of the Hamersley area. In Australia and to a minor extent in India and South America, mimetic replacement of gangue minerals by iron hydroxyoxides is a major factor in forming supergene iron ore from BIF and results in the superb preservation of the original BIF features. This includes primary banding down to the finest inclusions, as is a result of pseudomorphing of the BIF gangue at depth by goethite. The enrichment as a whole is an absolute accumulation of Fe at depth, dissolved from the surface BIF, with a removal of the gangue elements without loss of the original BIF mineral textures. Thus the ores grow upward to meet the downward destruction of the surface BIF by erosion. In contrast the supergene lateritic ores including the ‘blue dust ores’ are residual ores resulting from the relative accumulation of Fe. In the supergene lateritic ores derived from BIF, removal of the gangue is accompanied by repetitive solution and precipitation by secondary hematite and goethite. The final product of lateritisation of BIF and BIF-derived ores is typically complex due to successive overprinting of earlier stages of alteration during the process and because of the different precursors. This progressive lateritic modification tends to produce a varied surface complex including massive, vermiform, and botryoidal, or nodular aluminous/siliceous goethite and hematite, variously known as ferricrete, duricrust, canga, or hydrated zone hardcap. The significant difference between these ores and the supergene mimetic martite-goethite ores is that they grow downward, with the total destruction of any of the parent BIF or BIF-ore textures.