(U-Th)/He Geochronology of Channel Iron Deposits, Robe River, Hamersley Province, Australia – Implications for Ore Genesis

摘要

Two drill core samples of haematite/goethite from the Robe River (Western Australia) channelrniron ore deposits (CID) were dated using (U-Th)/He methods in order to constrain the timing ofrniron oxide formation and thereby provide a temporal context for CID genesis. (U-Th)/He ages (Hernages) from these samples range from the late Oligocene to Late Miocene and despite a high degreernof scatter, they corroborate relationships expected from the internal ooidal stratigraphy: Forrnindividual ooids, the ages from haematitic core are older than or indistinguishable from the agesrnof the surrounding goethitic cortex. The goethitic cortices are, in turn, older than the ferruginisedrnwood fragments recovered from the cementing goethitic matrix.rnThe data suggest the following succession of ore formation phases:rn1. Haematitic cores in ooids of both samples formed in the Early to Middle Miocene.rn2. Goethitic cortices of both samples formed in the late Middle to early Late Miocene. Thisrnappears to be the age of goethite cortex formation regardless of depth in the core, which doesrnnot support the top-down infi ll model, genetic model proposed by Heim et al (2006).rn3. Wood fragments form a prominent component of the matrix and were ferruginised during thernLate Miocene. Thus the data suggest that the unique environmental conditions for the CIDrnformation existed during the Miocene.rnA methodological implication of this study is that the temperature utilised for He-extractionrnfrom iron oxides has a critical impact on the mobility of parent nuclides. The typical ～1100℃ laserrnheating used for crystalline minerals like apatite or zircon induces loss of U and Th and results inrnerroneously old ages.