Archean banded iron-formations, Yilgarn Block, Western Australia

Abstract

Banded iron-formation is a widely distributed although relatively minor lithology in the Archean Yilgarn Block, Western Australia, occurring in supracrustal sequences composed mainly of metamorphosed volcanogenic rocks in the northern and eastern parts of the Yilgarn Block and also in highly deformed and metamorphosed predominantly metasedimentary sequences in the gneissic terrain in the southwest. In the central part of the Yilgarn Block structural mapping and stratigraphic reconstruction indicates that individual banded iron-formation units extend over an area of at least 40 X 60 km and could extend over a much larger area. Most banded iron-formation units appear to be 5 to 50 m thick and units of 100 to 150 m are uncommon. Three scales of layering occur in iron-formation units: (a) interlayering of banded iron-formation and Fe shale. Fe shales range in thickness from several millimeters to 5 to 6 m; (b) mesobanding is the most prominent structure in the banded iron-formations; (c) millimeter scale laminations, including microbanding, occur in both banded iron-formation and Fe shales. Other structures in the banded iron-formation include pods, macules, and small spheroidal nodules. Despite modification by metamorphic and deformation processes which, among other effects, have caused thinning of mesobands, all the structures can readily be equated with those in the Hamersley Basin iron-formations. There is a wide range in metamorphic grade. At the lowest grades banded iron-formation contains quartz, magnetite, greenalite, stilpnomelane, minnesotaite, siderite, and ankerite. Fe shales contain chamosite, stilpnomelane, siderite, and pyrite. The highest grades are characterized by quartz-magnetite-clinopyroxene-orthopyroxene in iron-formation and hornblende-almandine + or - biotite assemblages in Fe shale. Mineral assemblages are essentially identical to those in many metamorphosed Proterozoic iron-formations.The bulk chemical compositions of the Yilgarn Block iron-formations are very similar to other Precambrian iron-formations from very different stratigraphic settings. Fe shales have highly variable compositions characterized by a marked positive Al 2 O 3 -TiO 2 correlation and high Cr contents.The similarity in the bulk chemical composition of iron-formations in the Yilgarn Block and other Precambrian sequences implies that deposition of these iron-formations occurred in very similar chemical environments. It is suggested that although Fe shales may be related in some way to volcanic activity, the main constituents of the banded iron-formations are not directly derived from volcanic sources. Banded iron-formations appear to have been precipitated from ocean waters which, relative to younger oceans, contained higher concentrations of Fe and Si in solution.