Müller et al. (2005) have presented important new age data for mafic intrusions in the Hamersley province of Western Australia, which they use to constrain the ages of the Ophthalmian orogeny and giant iron-ore deposits. Their 2208 ± 10 Ma age for dolerite sills intruded into the Turee Creek Group is within analytical error of the 2209 ± 15 Ma maximum age determined by Martin et al. (1998) for the Cheela Springs Basalt near the base of the unconformably overlying Wyloo Group. Interpretation of these statistically indistinguishable and apparently contradictory ages is dependant on critical field relationships that the authors have not presented or do not appear to have considered.
The assumption that ca. 2208 Ma dolerite sills were intruded at approximately the same stratigraphic level throughout the region would imply a facies change from glacigenic diamictites in the Hardey syncline (Martin, 1999) to deltaic and marine quartzites in the Turee Creek syncline (Thorne and Seymour, 1991; Martin et al., 2000). This implied facies change, and resultant differences in lithology, grain size, and paleocurrent directions, is not accounted for. Such a correlation also does not consider that dolerite-hosting quartzites in the Turee Creek syncline are conformably overlain by vesicular basalts of the Cheela Springs Basalt (Thorne et al., 1991), a stratigraphic relationship consistent with their correlation with the Beasley River Quartzite as exposed throughout the Hamersley province. Undated dolerite sills intrude the Beasley River Quartzite at the eastern end of the Hardey syncline and along the northern flank of the Wyloo Dome (Seymour et al., 1988; Thorne and Tyler, 1996), as well as in the Duck Creek syncline. Undated dolerite sills also intrude the overlying Cheela Springs Basalt (Trendall, 1979; Thorne and Seymour, 1991; Martin et al., 2000).
An important field relationship for the interpretation of dolerite sills that have intruded into the Turee Creek Group is the presence of a highly discordant sill at the eastern end of the Wyloo Dome. This sill cuts across stratigraphy from within the Turee Creek Group up to the unconformity at the base of the Beasley River Quartzite (Seymour et al., 1988; Trendall, 1979). My own unpublished mapping shows that the sill locally intrudes ~50 m above the unconformity, separating the basal Three Corner Conglomerate Member from the remainder of the formation. Interestingly, the youngest detrital zircon in the Beasley River Quartzite is 2420 ± 18 Ma (Geological Survey of Western Australia, 2005; sample number 169084), which does not appear to support erosion of ca. 2208 Ma dolerites on a ~180 m.y. unconformity (Müller et al., 2005).
Regional field relationships of both dated and undated dolerite sills in the southern Hamersley province indicate that they are unreliable for stratigraphic correlation. An alternative interpretation of the ca. 2208 Ma dolerites is that they are sub-volcanic mafic intrusions genetically related to the Cheela Springs Basalt (Krapež, 1999). This interpretation provides a source for the ca. 2209 Ma detrital zircons from near the top of the Cheela Springs Basalt and accounts for the absence of detrital zircons of this age in the Beasley River Quartzite. Furthermore, the Beasley River Quartzite and basal Cheela Springs Basalt were folded along the same east-west axes as the Turee Creek Group (Martin et al., 2000). These multiple lines of evidence imply that the Beasley River Quartzite is older than ca. 2208 Ma and was deformed during the Ophthalmian orogeny (Powell et al., 1999; Martin et al., 2000). Also, the Panhandle folding event, assigned an age of ca. 2031–2008 Ma by Müller et al. (2005), cannot be related to the much younger 1830–1780 Ma Capricorn orogeny (Cawood and Tyler, 2004). Clearly further dating, particularly of mafic intrusions and detrital zircons, is required in order to resolve the timing and significance of the Ophthalmian orogeny.
The ca. 2008 Ma age proposed for the giant iron-ore deposits of the Hamersley province is a reliable maximum age for mineralization at the Paraburdoo mine, but may not necessarily apply to all deposits. Small ore clasts are present in the basal Cheela Springs Basalt and Beasley River Quartzite, suggesting that some iron enrichment occurred prior to ca. 2209 Ma (Martin et al., 1998). This observation is supported by recent studies of the timing of regional-scale metamorphic fluid flow (Rasmussen et al., 2005) and the formation of hematite veins associated with the Mount Whaleback deposit (Brown et al., 2004).