(U-Th)/He geochronology of channel iron deposits, Robe River, Hamersley Province, Australia - Implications for ore genesis

Abstract

Two drill core samples of haematite/goethite from the Robe River (Western Australia) channel ironore deposits (CID) were dated using (U-Th)/He methods in order to constrain the timing of iron oxide formation and thereby provide a temporal context for CID genesis. (U-Th)/He ages (He ages) from these samples range from the late Oligocene to Late Miocene and despite a high degree of scatter, they corroborate relationships expected from the internal ooidal stratigraphy: For individualooids, the ages from haematitic core are older than or indistinguishable from the ages of the surrounding goethitic cortex. The goethitic cortices are, in turn, older than the ferruginised wood fragments recovered from the cementing goethitic matrix. The data suggest the following succession of ore formation phases: • Haematitic cores in ooids of both samples formed in the Early to Middle Miocene. • Goethitic cortices of both samples formed in the late Middle to early Late Miocene. This appears to be the age of goethite cortex formation regardless of depth in the core, which does not support the top-down infill model, genetic model proposed by Heim et al (2006). • Wood fragments form a prominent component of the matrix and were ferruginised during the Late Miocene. Thus the data suggest that the unique environmental conditions for the CID formation existed during the Miocene. A methodological implication of this study is that the temperature utilised for He-extraction from iron oxides has a critical impact on the mobility of parent nuclides. The typical ~1100°C laser heating used for crystalline minerals like apatite or zircon induces loss of U and Th and results in erroneously old ages.