^(40)Ar/^(39)Ar and (U–Th)/He – ^4He/^3He geochronology of landscape evolution and channel iron deposit genesis at Lynn Peak, Western Australia

Abstract

(U–Th)/He geochronology of authigenic goethite cements from the Lynn Peak channel iron deposit (CID), Hamersley Province, Western Australia, reveals a history of mineral precipitation ranging from ca. 33 to 14 Ma. Massive goethites from nearby weathering profiles at Roy Hill North, a possible source of detrital material during the aggradation of the Lynn Peak channels, yield (U–Th)/He results as old as ca. 64 Ma. The combination of (U–Th)/He geochronology with incremental outgassing ^4He/^3He studies on proton-irradiated samples reveals that Lynn Peak goethites host radiogenic ^4He in low retentivity (LRD) and high retentivity (HRD) domains and that the HRDs account for most of the sample mass and have lost very little of their original ^4He over geologic time. Such high retentivity is especially notable given the goethites were collected from the surface, where they were subject to significant heating by solar irradiation. Minor contamination by detrital fragments of potentially ^4He-rich primary phases (e.g., rutile, ilmenite, zircon) occurs in some samples. Fortunately, the ^4He/^3He method permits characterization of this extraneous ^4He component, which is small (<10 wt.% of the total ^4He in the goethite) and can be corrected out in estimating the goethite formation age. These results indicate that the Lynn Peak channel was already aggraded and undergoing goethite cementation by ca. 33 Ma. The history of aggradation and channel cementation independently measured through ^(40)Ar/^(39)Ar geochronology is consistent with that obtained from the (U–Th)/He and ^4He/^3He record. Laser incremental-heating ^(40)Ar/^(39)Ar geochronology of detrital and authigenic Mn oxides, primarily cryptomelane (KMn_8O_(16)·xH_2O), from the same locality in the Lynn Peak channel reveals that detrital oxides are older than ca. 44 Ma (and as old as ca. 65 Ma) and authigenic oxides are younger than ca. 35 Ma and as young as ca. 16 Ma. Authigenic cryptomelane precipitation and channel cementation occurred throughout the Miocene, with a particularly strong period at around 20 Ma. The ^(40)Ar/^(39)Ar geochronological results suggest that regional weathering profiles, developed before 44 Ma and possibly as early as ca. 65 Ma, were incised and partially eroded in the 44–35 Ma interval. Semi-arid conditions promoted the transport of large volumes of sediments, aggrading the regional drainage system. Metasomatic reactions in the aggraded channels caused the ferruginization and, in some places, manganese replacement of the CIDs; goethite and K–Mn oxide cementation continued throughout the Oligocene and Miocene. Post-Miocene aridification contributed to the preservation of the cemented channel sediments, forming some of the largest readily mineable iron ore deposits on earth.

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