The Cenozoic detrital iron deposits of the Hamersley Province, Western Australia

Abstract

Three major types of economic secondary iron ores occur in Western Australia, mainly in the banded iron-formation (BIF)-rich Hamersley Province of the Pilbara:the dominant BIF-hosted bedded iron deposits (BID; ∼40 billion tonnes (Bt); 58–65 wt% Fe); and the detrital ores, mainly in the three province-wide Cenozoic sequences that include coeval non-ore sediments:Miocene channel iron deposits (CID; ∼>15 Bt; 54–58 wt% Fe) of the Cenozoic Detritals 2 (CzD2); andEocene CzD1 and Pliocene CzD3 detrital iron deposits (DID; ∼3.5 Bt; 40–60 wt% Fe).Striking differences exist between the massive CID resources and the much smaller underlying and overlying DID. CID are essentially riverine alluvial ooidal rocks with abundant small fossil wood fragments and variable peloids, but with only extremely rare, recognisable lithic remnants. The original matrix is typically ramifying layered goethite. Eocene DID are mainly alluvial with only minor pisoids, whereas Pliocene–Quaternary DID are dominantly coarse colluvial gravels, with minor pisoids, both derived from and largely retaining the original textures of BID, hardcap or variably ferruginised surface BIF. The coluvial DID matrix is typically ferroan-aluminous soil, resulting in canga where replaced by goethite, which may be dehydrated to hematite in part by exposure. The Cenozoic deposits described in detail in this paper occur in two dominant geomorphological environments: the southern Marra Mamba to Brockman Iron Formation strike valleys (MBSV), containing all three Cenozoic sequences; and the much later northern Brockman IF plateau valleys (BPV) that include only the Miocene and Pliocene sequences. Minor basinal/deltaic alluvials occur in the Proterozoic. The Cenozoic detritals formed in different climatic regimes, with an extended dry period forming a prominent province-wide dehydrated carapace on the Eocene DID. The Miocene ‘optimum’ followed with its thick scrub-covered deep regolith that produced the fossil wood-rich CID, succeeded by the arid cool period of the Oakover limestone/calcrete. A major renewal of exposure and erosion in the Pliocene resulted in the extensive iron gravels of the Pliocene–Quaternary.