A textural and mineralogical study of the relationship of iron ore to banded iron-formation in the Hamersley iron province of Western Australia

Abstract

Microscope studies of a wide range of ores and fresh, oxidized, or partly iron-enriched, banded iron-formation (BIF) have shown a consistent sequence of alteration of the primary constituents, chert, magnetite with or without hematite, carbonates, and silicates, leading to the formation of martite-(hematite)-goethite ores which form the bulk of the reserves of the Hamersley Iron Province. Primary hematite survives with little change; magnetite oxidizes to hematite (martite) or in part to lacunar phases between magnetite and maghemite (kenomagnetite), which in turn commonly is altered by hydration to goethite. Subsequent leaching of this goethite in preference to martite leads to a variety of skeletal textures which may help identify the original magnetite, despite destruction of martite lamellae by recrystallization. Chert, carbonates, and silicates are either replaced by goethite or leached from the system, resulting in thinning of the orebodies with respect to their parent banded iron-formations. Preservation of macroscopic features, such as mesobanding, and microscopic features, such as pseudomorphs, is compelling evidence of supergene enrichment which, together with residual upgrading by leaching of these iron-formations, produces ores which typically assay 62 percent Fe, ignition loss of 4 to 6 percent, and >0.07 percent P. With increasing maturity, part of the goethite has converted to fine-grained hematite in some of these martite-(hematite)-goethite ores. Though overall the secondary hematite is quantitatively unimportant, some deposits may be significantly upgraded by this process, and it is this variability that is implied by the use of brackets in the classification.A second ore type, characteristic of the Tom Price and Whaleback mines, consists essentially of residual primary components, martite and hematite, with abundant secondary hematite, most of it platy but some anhedral. These martite-hematite ores appear to have formed by a low-temperature process in which some of the goethite or its precursor, in ores of the martite-(hematite)-goethite type, has converted to platy hematite in contrast to the fine-grained and often cryptocrystalline material of the normal ores. This partial transformation has resulted in increased permeability of the deposits which in turn has allowed leaching of the remnant goethite. The Paraburdoo 4E mine represents an arrested stage of development in which a large proportion of this unconverted goethite is still present, whereas the Tom Price and Whaleback deposits represent the most mature ores in the province. In these, leaching has removed the bulk of the goethite, leading to typical assay values of >64 percent Fe, 1 percent loss on ignition, and <0.05 percent P.The supergene ores of the province can be classified in terms of a maturity scale based on the type and amount of secondary hematite replacing goethite. Thus martite-goethite ores without significant secondary hematite are considered to be the least mature. Examples of these have been found in a limited survey of the Marra Mamba Iron Formation ores. The Brockman Iron Formation deposits at Koodaideri, the Brockman syncline, and Metawandy provide examples of more mature ores with increased secondary hematite and are classified as martite-(hematite)-goethite ores. Ancient deposits (early to middle Proterozoic) with abundant secondary platy hematite but with significant residual goethite are classified as the Paraburdoo type. Substantial leaching of this goethite leads to the most mature ores of the province, the martite-hematite deposits of the Tom Price-Whaleback type.