Supergene modification of magnetite and hematite shear zones in banded iron-formation at Mt Richardson, Yilgarn Craton, Western Australia

Abstract

The Mt Richardson deposit contains high-grade (>57 wt% Fetotal) iron ore zones hosted by several banded iron-formations (BIFs) in the c. 3504 Ma Illaara greenstone belt. The BIFs and intervening mafic igneous rocks are exposed along a 700 m-wide, 140 m-high, NNW-trending ridge that demarks the eastern limb of a greenstone belt-scale tight, upright, moderately SSE-plunging syncline. Local expressions of this fold generation include parasitic metre-scale fold hinges, an axial planar fabric, and intersection lineation in BIFs. Margins of tightly folded BIFs are commonly sheared and altered by hypogene magnetite and quartz. Although primary bands are strongly overprinted by disseminated magnetite, the concomitant growth of quartz translates to a negligible increase in the iron content of hypogene-altered BIFs. Reactivation of magnetite–quartz-altered shear zones and interaction with more oxidized fluids led to the replacement of magnetite by martite, precipitation of crystalline hematite, and recrystallization of quartz. Mafic igneous rocks are altered to fine-grained chlorite via replacement of igneous amphibole, biotite, and plagioclase within 30 m of sheared and hypogene-altered BIFs. Hypogene-altered mafic igneous rocks are enriched in Fe2O3total and W, and depleted in MnO, K2O, Ca, Na2O, and Sr compared to least altered rocks. Subsequent reactivation of shear zones and the circulation of supergene fluids led to dissolution and replacement of primary and hypogene quartz by goethite, and further oxidation of magnetite to martite. Resultant goethite–hematite ore zones are silica-poor, iron-rich, porous, and friable. Alteration indices for supergene-modified, hypogene ore zones in BIFs include hypogene magnetite and hematite in BIFs, chlorite-replacement of primary minerals in mafic igneous rocks, intense supergene goethite–hematite–kaolinite alteration, thinning of primary quartz bands in BIFs, and the presence of broad high-strain zones. Prime exploration targets in the Illaara greenstone belt are areas where these alteration indices coincide with fold-thickened BIFs. Given the friable nature of high-grade iron ore zones, mineralized occurrences are often at least partly hidden and require remote detection methods, followed by drilling, for testing. Mt Richardson differs from most iron deposits in the Yilgarn Craton by the absence of early hypogene carbonate mineral replacement of primary quartz bands in BIFs. Instead, Mt Richardson demonstrates the greater role of supergene fluids that exploit existing structures and permeability via reactivation of hypogene magnetite- and hematite-rich shear zones. In this regard, the Mt Richardson deposit has greater affinity with iron deposits in the Pilbara Craton.