The Hamerseley Basin is located in the remote northwest of Western Australia. It is an ovoid basin with an area of approximately 150,000 sq km and contains one of the world’s largest deposits of iron ore. The known deposits of iron ore exceed 33,000 million tons at >55% iron. The basin is flanked by Precambrian rocks on both sides and is situated upon Archean basement which originally connected the Pilbara Block to the Yilgarn Block.
In few areas in Western Australia is the close correlation between geology and topography more strikingly demonstrated. The clear topographic expression of the major stratigraphic units has permitted accurate photo-geological interpretation and the recognition of potential ore bearing zones in the jaspillite of the basin.
Maitland made the first extensive geologic traverses through the Hamerseley Basin area in 1903, followed by more in 1904 and 1905. Regional mapping of the basin was carried out during the interval 1959-1970. MacLeod made a major contribution to the geology of the basin in 1962, 1963, 1964 and 1966. Trendall and Blockley (1970) and Trendall (1975) carried out more detailed examinations, mainly orientated towards the iron ore deposits of the region.
Physiography
The Hamerseley and Opthalmia Ranges are the major topographic features. They both owe their existence to the presence of three thick, resistant jaspillite iron formations of wide lateral extent. These units are separated by dolomite, shale and volcanic rocks of much lower resistance to erosion. As a result, the stratigraphic and structural features of the region are boldly outlined by erosion. The clear topographic expression of the major stratigraphic units has permitted accurate photo-geological interpretation and the delineation of major ore bodies. The majority of hill summits in the region, particularly those formed of the Brockman Iron Formation, have a gently domed form. These domes and capping represent remnants of an older land surface which has been strongly incised by erosion. Most of the major iron ore bodies are to be found immediately below this surface. Outcrop within the Hamerseley Basin consists predominantly of Proterozoic sediments of the Fortescue, Hamerseley and Wyloo Groups. The prominent jaspillite ridges belong to the Hamerseley Group.
Stratigraphy
The Proterozoic rocks of the Hamerseley Basin have been divided into 3 groups, the Fortescue, Hamerseley and Wyloo Groups. Collectively, they are known as the Mt. Bruce Supergroup.
Fortescue Group
The Fortescue Group includes the lower formations of the Proterozoic succession and uncomformably overlies the Archean basement. The upper limit of the group is arbitrarily placed at the base of the Marra Mamba Iron Formation. The appearance of this unit marks the change from the initial volcanic activity and clastic sedimentation to the phase of chemically precipitated sediments of the Hamerseley Group.
The Hardey Sandstone is the basal formation and is white to reddish brown and green quartz sandstone. Basalt and volcanic agglomerate are interbedded with the sandstone which is conformably overlain by the pillow lava’s of the Mt Jape Basalt.
The Mt Jape Basalt is of wide extent and consists of dark green, fine grained, often vesicular or amygdaloidal basalt with pillow forms. Intervening pyroclastics consisting of grey ash beds with agglomerate and volcanic bombs occur. Minor amounts of chert, shale and jaspillite are present.
The Jeerinah Formation conformably overlies the Mt Jape Basalt. The formation consists of shale, chert, jaspillite, mudstone, quartzite and dolerite. Shale is the dominant rock type; however, thick dolerite sills occupy over half of the formation thickness in some areas. The Marra Mamba Iron Formation conformably overlies this unit and marks the beginning of the Hamerseley Group.
Hamerseley Group
Sediments of the Hamerseley Group are predominantly of chemical origin. Chert, jaspillite and dolomite are the dominant rocktypes. The Hamerseley Group is subdivided into eight formations.
The Marra Mamba Iron Formation is of wide extent, outcropping over a distance of 480km. Chert and jaspillite are the principle rock types. The jaspillite consists of thin, alternating bands of chert and iron minerals. Magnetite and hematite are the principle iron minerals. At Marra Mamba, the formation contains seams of crocidolite (asbestos). Kneeshaw (1982 has divided this formation into four members: (1) Lower BIF Member; (2) Middle BIF and Shale Member; (3) Upper BIF Member; and (4) Upper Shale Member.
The Wittenoom Dolomite conformably overlies the Marra Mamba Iron Formation and comprises a finely crystalline grey-white dolomite with some chert in the lower section and a series of dolomitic shales and cherts in the upper section. It is conformably overlain by the Mt Sylvia Formation.
The Mt Sylvia Formation includes three thin, but persistent, beds of jaspillite interbedded with dolomitic shales. The top 7m thick jaspillite band is a valuable stratigraphic marker. The jaspillite beds are separated by blocky calcareous shale and thinly bedded fissile shale with dolomitic intercalations.
The Mt MacRae Shale conformably overlies the Mt Sylvia Formation and comprises shale, siltstone and dolomitic shale with thin beds of jaspillite and chert. It is conformably overlain by the Brockman Iron Formation.
The Brockman Iron Formation is the thickest and most widely exposed formation in the Hamerseley Group. Most of the hills in the Opthalmia and Hamerseley Ranges consist of this formation. Rock types are predominantly jaspillite and chert and it has been divided into four members: (1) Dales Gorge Member; (2) Whaleback Shale Member; (3) Joffre Member; and (4) Yandicoogina Shale Member. The Dales Gorge Member comprises the main iron ore horizons and has been split into alternating ore/banded iron formation and shale macro-bands. The Whaleback Shale Member consists of an upper shale zone, a central chert band, and a basal shale zone. The Joffre Member comprises of banded iron formation with minor thin shale horizons and contains small orebodies. The Yandicoogina Shale Member consists of banded iron formation and shales.
The Weeli Wolli Formation comprises interbedded dolerite and jaspillite, with minor shale, and conformably overlies the Brockman Iron Formation. The dolerite represents intrusive sills.
The Woongara Volcanics consists of dacites, tuffs, rhyolites, banded iron formation and siltstones, conformably overlying the Weeli Wolli Formation.
The Boolgeda Iron Formation is divided into three members: (1) a lower member comprising BIF, shale and chert; (2) a middle member of mainly BIF; and (3) an upper member of shale and BIF. Minor iron ore bodies occur.
Wyloo Group
The Wyloo Group rocks are the youngest Proterozoic sediments. They conformably overlie the Hamerseley Group and its subdivision is not clearly defined. The basal Turee Creek Formation overlies the Boolgeeda Iron Formation and comprises mainly greywacke and shale. Local intercalations of quartzite, conglomerate, dolomite and basalt occur.
The Beasley River Quartzite conformably overlies the Turee Creek Formation and is typically cream or white coarse grained quartz sandstone, which is often massive, silicified and glassy with indistinct bedding. Cross bedding and ripple marks have been noted. Local intercalations of conglomerate, dolomite and basalt occur.
The Mt McGrath Beds conformably overly the Beasley River Quartzite and has been clearly divided into four members: (1) the Karlathundra Member; (2) Coolbye Shale Member; (3) Cheeta Springs Member; and (4) Nummana Member. They consist of conglomerate, shale, basalt and sandstone.
The Duck Creek Dolomite conformably overlies the McGrath Beds and is the most consistent member of the Wyloo Group, comprising a tough, light grey to buff, crystalline dolomitic limestone with medium to thick bedding.
The Ashburton Formation has been added onto MacLeod’s (1962) original classification by Daniel’s (1968 and 1970) and occupies much of the outcrop area of the Wyloo Group. It consists of brown, weathered, shale and greywacke with localized sandstone, conglomerate and BIF.
Structure
MacLeod (1966) defined three successive structural zones from north to south. Although this division is valid, Trendall (1975) states the transition between zones is gradational, so there is a gradual increase in structural complexity from north to south across the Mt Bruce Supergroup.. Over most of its length, the northern basal unconformity of the Fortescue Group dips southwards, at no more than a few degrees, the rocks show no signs of folding and exhibit an exceptional lack of disturbance since their formation.
In the east, the basal unconformity of the Fortescue Group, tends to follow the sinuous boundary between granite and metasediments in the underlying Archean basement.. Dips of up to 45 degrees occur, suggesting that granitic domes continued their diapiric ascent after deposition of the Fortescue Group.
The gentle southerly dip of the Proterozoic sediments in the northern half of the basin persists to the main axis of the Hamerseley Range, which forms the axial line of a complex synclinorium. To the south of the Hamerseley Range Synclinorium, the intensity of folding increases steadily. The Mt. Bruce Supergroup becomes folded on a variety of scales along an east-west axis. Folds commonly plunge, so that anticlines pass axially into synclines. Further south, folding is intense with dips close to vertical common.
At both the eastern and western ends, fold axes curve northwards. To the east, gently folded Fortescue Group rocks are separated by a narrow line of faulting from the mainly granitic rocks of the Gregory Range. To the west, strongly folded Wyloo Group rocks are separated by a zone of complex faulting from Hamerseley Group rocks further east.
Sedimentation
Sedimentation began with deposition of the Hardey Sandstone (Fortescue Group) in a shallow depression on a stable continental platform. With deepening of the depression, marine transgression occurred. The Mt Jope Basalts were deposited on the floor of the basin during a subsequent period of volcanic activity . As the basin stabilized, jaspillite and shale were deposited in a deep water basinal environment. Deposition of the overlying Weeli Wolli Formation marked the onset of chemically and biologically(?) precipitated sediments of the Hamerseley Group. A prolonged period of alternating phases of precipitation of iron, silica, lime and magnesia under deep, still water conditions took place. The acid lava’s of the upper section of the group were probably extruded under subaereal conditions.
During deposition of the iron formations, water depth probably reached a maximum of about 250m. The climate was hot and dry with little rainfall and high evaporation (MacLeod, 1966) . The mechanism of iron precipitation was an annual increase of concentration through evaporation (MacLeod, 1966). This resulted in micro-bands with macro bands being formed due to compression and diagenesis. Iron in the basin was probably replenished by volcanic activity in the west.
Deposition of the overlying Wyloo Group sediments marked a return to coarse clastic dominated sedimentation. A period of volcanism is marked by the Cheeta Springs Basalt.
Geological History
The Hamerseley Basin developed between about 2300 and 1800 million years ago and formed by the steady depression of an erosional surface cut across Archean rocks, which are presumed to have extended between the Yilgarn and Pilbara Blocks.. Vigorous volcanic activity was a feature of the early development of the basin. Shortly after, the basin was open to the sea and chemical sedimentation took place. The climate was hot and dry, the basin reached a maximum depth of 250m with a shallow marginal shelf developed in the east. During sedimentation, compaction and subsidence of sediments occurred. After deposition of the Wyloo Group, intense faulting took place in the southern half of the basin, mainly along a east-west line. At the same time, folding occurred, producing a series of plunging anticlines and syncline which became overturned and block faulted in the extreme south.
Much later, the rocks were uplifted to form a plateau, which erosion turned into a series of flat topped mesas and cone shaped peaks.
Conclusion
The Hamerseley Basin developed between about 2300 and 1800 million years ago and formed by steady depression of an erosional surface cut across the Archean basement. The Hamerseley Basin contains one of the world’s largest deposits of iron ore, comprising more than 33000 million tons at >55% iron. The distribution of iron ore deposits is controlled by the Proterozoic stratigraphy, with most deposits contained within the Hamerseley Group. The Hamerseley Group represents a period of predominantly chemical sedimentation, with a high iron concentration. Deposition ended in the Hamerseley Basin approximately 1800 million years ago, with subsequent folding, faulting and erosion producing the current surface expression of flat topped mesas and dome shaped hills.
References
Kneeshaw, M, 1982, ” Mt Newman Iron Ore Deposits”. Mt Newman Mining Co. Notes, pp.16-24.
MacLeod, W.N., 1966, ” The Geology and Iron Deposits of the Hamerseley Range Area, Western Australia”. WA Geol Surv., Bull. 117
Morris, R.C., 1980, ” A textural and Mineralogical Study of the relationship of Iron Ore to Banded Iron Formation in the Hamerseley Iron Province of Western Australia”. Econ. Geol., V.75, pp.184-209
Trendall, A.F., 1975, “Hamerseley Basin”. WA Geol. Surv., Mem. 2, pp.119-143.
