G Taylor

Regolith profile, mineralogy and geochemistry of the Weipa Bauxite, northern Australia

The regolith profile at Weipa is of two types. At Andoom, to the north of Weipa township, a 30 m profile overprints and overlies sediments of the Rolling Downs Group. At a depth of 23 m saprolite gives way to a Plasmic (pallid) Zone of kaolinite + quartz, which becomes mottled above 12 m. The degree of induration of the Mottled Zone increases upward to form a discontinuous and irregular Transition Zone about 1 m thick at the top of the wet season water-table and at a depth of about 4 m. In many places the Transition Zone is sufficiently indurated by goethite and hematite to be termed a Ferricrete. Above this is pisolithic bauxite and soil. On the Weipa Peninsula, Rolling Downs Group saprolite and Plasmic Zone terminate in a weak paleosol at 18 m, and this is unconformably overlain by sands and clays of the Paleogene Bulimba Formation. A quartz–kaolin Plasmic Zone developed in the clay and sandy clay units of the Bulimba Formation becomes mottled at about 5 m, with a Ferricrete or Transition Zone at around 4 m. Above this is pisolithic bauxite and soil. At both East Weipa and Andoom the Ferricrete is highly irregular, having an undulating surface at a wavelength of the order of 50 m or more and an amplitude of 2–3 m. Locally the surface is interrupted by depressions and holes ranging from 50 cm to 2 m in diameter and penetrating through the Ferricrete, and in places metre high pinnacles extend into the bauxite. The chemical and mineralogical changes that ultimately produce the bauxite pisoliths occur in the Transition Zone. At the base of the Transition Zone, kaolinite comprises 80% of the regolith. At the top of the Transition Zone, the composition is more than 75% Al(OH)3 in the form of gibbsite and ‘amorphous’ gibbsite. In the Transition Zone, goethite and hematite increase from 1–2% below to 15–25%. Over this 1 m interval, the fabric changes from 30–40 mm peds and nodules of generally massive kaolinite to 15 mm nodules of oolithic and sporadically pisolithic bauxite. The bauxites of Weipa and Andoom can be distinguished on the basis of higher Fe at Andoom, and higher quartz + kaolinite at East Weipa. Titanium, Zr, As, Nb and Sc also provide a basis for discrimination. Geochemical profiling of samples from two 30 m drillholes through each deposit shows strong coherence from the fresh Rolling Downs Group rocks to the bauxite at Andoom, and up to the unconformity at East Weipa, indicating these are in situ weathering profiles. The 30 m Andoom profile formed from an initial thickness of about 50 m of sediment. The variable substrate of the Bulimba Formation does not allow such conclusions to be reached for the East Weipa profile. These profiles have been developing, probably, over the last 50 Ma, and they are still forming, and/or changing, today.

Effects of some macrobiota on the Weipa Bauxite, northern Australia

Bioturbation of the Weipa bauxite is effected primarily by termites and tree roots. Termite galleries extend into the Plasmic Zone to a depth of 20 m. Termite nests are constructed from the surrounding soil (averaging 30% kaolinite) and also from kaolinite derived from below the bauxite, with some parts of the nest containing 75% kaolinite. Measurement of termite nest volume from two 1 ha areas indicates nest materials amount to as much as 5 t/ha and that termites bring kaolinite to the surface to the extent of perhaps 10 kg/ha annually. Roots of Eucalyptus tetrodonta, the main forest tree of the region, penetrate the 3–6 m thickness of the bauxite at least as far as the top of the Mottled Zone and commonly some distance further. Penetrating roots displace the loose pisolithic bauxite, and eventually provide channels through the bauxite down which material may fall, thus tending to homogenise the deposit. Evidence of their effect on local redox conditions is revealed in mine faces by zones of variable ferruginous colouration. Tree-fall, mainly the result of cyclones, disturbs the soil and upper bauxite: about 0.5% of a 1 ha area is estimated to have been affected by tree-fall. Small nodules scattered throughout the bauxite, prolate ellipsoidal in shape, averaging 28 × 21 mm, are interpreted as the fossilised pupal cases of ground-dwelling beetle larvae, probably scarabs or weevils.

Genesis of pisoliths and of the Weipa Bauxite deposit, northern Australia

Consideration of the Late Cretaceous and Cenozoic geological, landscape and climatic history of western Cape York suggests that an environment suitable for the development of bauxite has prevailed for the past 100 Ma. Uplift of the western Cape exposed flat-lying Cretaceous Rolling Downs Group glauconitic and feldspathic sandstones to weathering under a climate that was seasonally wet, establishing a deep lateritic profile. The rising highlands along the eastern Cape allowed Lower Cretaceous sediments and Paleozoic rocks to be eroded by west-flowing rivers, moving weathered Rolling Downs Group materials westwards. Some coarser components may have been deposited on the lower-gradient coastal plains, while finer components would have moved farther seawards. By the Paleogene quartzose rocks were exposed along the highland spine. Their weathering and erosion products, together with eroded material from the already deeply weathered Rolling Downs Group rocks, were deposited as sands and clays in broad fans forming the Neogene Bulimba Formation over established lateritic profiles on the Cretaceous rocks. This continued until the Weipa Plateau was isolated by river capture and incision sometime during the Neogene. Continued weathering since the mid-Neogene saw the formation of lateritic profiles on the fan sediments inset into the weathered Cretaceous rocks. The Weipa bauxite deposit has similarities to other northern Australian bauxites, such as that at Gove, in having clear evidence for transport of the pisolithic bauxite. Formation, erosion, fragmentation and cortication of pisoliths have been ongoing over this time. In places, ironstone fragments, pisoliths and ooliths have accumulated in the lower parts of the landscape as unconformable transported bauxite over the weathering profiles. Elsewhere the lower parts of the pisolithic bauxite remain in situ over the weathering profile of either the Rolling Downs Group or the Bulimba Formation, though much bioturbated. The boundary between the upper parts of the lateritic profile (the Mottled Zone) and the pisolithic Bauxite may be erosional (physically unconformable), karstic (chemically unconformable) or conformable through a Transition Zone lying at the level of the wet season water-table demonstrating that processes that produced the lateritic profiles, the pisoliths and their redistribution are, in places, ongoing. The bauxite is at the same time and the same place some millions of years old and a few months old. Redistribution of pisoliths continues on the Weipa Plateau as they are eroded from its margins and redeposited in lower parts of the landscape.

Neotectonic disruption of silicified palaeovalley systems in an intraplate, cratonic landscape: regolith and landscape evolution of the Mulculca range-front, Broken Hill Domain, New South Wales

The landscape expression of a wide range of ancient and contemporary regolith materials in the vicinity of the Mulculca Fault demonstrates two important points: (i) tectonism can be a significant factor in the evolution of landscapes in some parts of the Australian craton; and (ii) ancient and young regolith‐landform features can coexist within a tectonically active landscape. Tectonic activity along the Mulculca Fault has created a range‐front near the Broken Hill Domain ‐ Murray Basin margins. This tectonism defeated a now silicified palaeodrainage system that flowed from the area now occupied by the Barrier Ranges towards the present area of the Murray Basin. Stream defeat led to the development of lacustrine‐overbank conditions within the area of the fault‐angle depression, which was later breached by stream incision across the range‐front. The area of lacustrine‐overbank deposition is now dominated by alluvial (channel and overbank) deposits with minor colluvial and aeolian deposition. Silicification of palaeovalley sediments and adjacent saprolite has been occurring over a broad range of times during landscape development, including several stages of palaeovalley evolution and as minor red‐brown hardpan development in the contemporary landscape. Ferruginised regolith has been developing at many different times during the evolution of the landscape, including: (i) prior to the defeat of the palaeodrainage system; (ii) during the sedimentary infilling of the fault‐angle depression; and (iii) within the contemporary landscape. The variable preservation and, therefore, landscape expression of a wide range of regolith materials that formed over a long period of landscape evolution has persisted even though there has been tectonic activity along the Mulculca Fault. For example, topographically inverted, and therefore ancient, silicified alluvial deposits occur alongside contemporary colluvial and alluvial deposits along the tectonically active range‐front. This is in contrast to simple models invoking long‐term tectonic stability to account for the expression of ancient regolith and landforms in Australian cratonic landscapes.

A core through the Monaro basalt: Bega (BMR) no. 7

A vertical borehole has been drilled through 215.8 m of the Monaro volcanic sequence at a location 20 km south of Cooma. The site is on basalt outcrops near the crest of the Main Divide. The core comprises 198.2 m of mafic volcanic and weathered volcanic rocks overlying 8.5 m of lacustrine sedimentary and hyaloclastite deposits and 9.0 m of weathered basement composed of schist and meta‐sandstone. About 55% of the volcanic sequence is weathered; and some of the weathered material may have been originally volcaniclastic. Seven weathering profiles, up to 12.5 m thick have bauxite zones up to 3.5 m thick at the top. Bauxite comprises ∼9% of the sequence. The fresher rocks are mainly alkali basalt flows, up to 27 m thick, but also include a basanite flow at the top of the sequence and two intervals of alkali dolerite between 65.0 and 75.0 m. The deeply weathered nature of many of the flows indicates prolonged periods of intense chemical weathering under wet climatic conditions between lava eruptions.

Landscapes and regolith of Weipa, northern Australia

Since its discovery during the 1950s the Weipa bauxite has been considered to be the product of in situ weathering of the rocks or sediments below the deposit. It is the worlds largest proven bauxite resource covering some 11 000 km2 with an average thickness of about 2.5 m. It consists of a pisolithic bauxite sitting atop a typical lateritic profile about 10–15 m thick. Few, if any researchers have considered genetic scenarios other than the in situ evolution of the deposit. This paper summarises the history of research and outlines the geological setting of the Weipa Bauxite, and provides some evidence for alternative genetic scenarios.

Impact of fire on the Weipa Bauxite, northern Australia

More than half of western Cape York Peninsula experiences fire every dry season, and the effects of this on the bauxite are twofold: gibbsite is dehydrated to boehmite or alumina and Fe-oxyhydroxides are converted to maghemite. Effects are most significant on the earthen materials of termite nests, particularly those coating the trunks of the common Eucalyptus tetrodonta (Darwin stringybark), and where dead trunks, branches and roots have burnt. Fire-induced dehydration of ooliths in termite nests is suggested as the source of the high-boehmite redsoil in the Weipa Bauxite deposit.

Weipa Bauxite, northern Australia

This thematic issue on the Weipa Bauxite has resulted from our work and that of our students in the region over a 20 year period. In the first paper Landscapes and regolith of Weipa, northern Australia, Taylor, Eggleton, Foster & Morgan describe the geological and landscape setting and the overall history and mining history of the deposit. This sets the scene for four detailed papers that follow on various aspects of the work. Regolith profile, mineralogy and geochemistry of the Weipa Bauxite, northern Australia by Eggleton, Taylor, Le Gleuher, Foster, Tilley & Morgan discusses the nature and origin of the deep lateritic weathering profiles. In Nature of the Weipa Bauxite, northern Australia Taylor, Eggleton, Foster, Tilley, Le Gleuher & Morgan deal with the nature and origin of pisoliths and ooliths which comprise the bulk of the bauxite. Then, Eggleton & Taylor examine the biological influences on the weathering and bauxite in Effects of some macrobiota on the Weipa Bauxite, northern Australia and the influence of fire on the bauxite in Impact of fire on the Weipa Bauxite, northern Australia. The final paper by Taylor & Eggleton, Genesis of pisoliths and of the Weipa Bauxite deposit, northern Australia discusses the origin of the bauxite and its component parts over the last 100 Ma or so. This work would not have been possible without many contributions from others. Here at the outset we wish to acknowledge that help and our gratitude. Rio Tinto Aluminium, formerly COMALCO Ltd, provided ready access to the mines, generous financial support for students, field and accommodation support while at Weipa, advice, information, and samples. The Australian Research Council through Grant A39232594 supported postdoctoral research and research assistance as well as equipment, laboratory and travel expenses. The Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRCLEME), through the Australian Cooperative Research Centres Program, also provided field support as well as financing this publication. Graham Taylor wishes to also thank CRCLEME for conferring Honorary Fellow status on him allowing him to continue this work after the University of Canberra withdrew from the CRC. We thank Dave Andrews, Zoe Kyriazis, Tracey Kostecki, Barry Murray, Evan Parsell, Flo New, John Bower, Sinead Kaufman, and many other Rio Tinto staff for help and guidance at Weipa. We acknowledge also the contributions of former students of ours: Iain Campbell, Ma Chi and Shawn Laffan. Other students associated with this work have co-authorship of papers in this volume. We additionally thank Sue Border, for access to the Skardon River Kaolin Deposit, and Theo Evans and Rolf Oberprieler, CSIRO, for discussions about termite and weevil species, respectively. Lisa Worrall and Jonathan Clarke kindly reviewed the manuscripts and made many positive suggestions for their improvement. We hope this work will stimulate interest in Australia’s deep regolith, its resources and its mysteries. The relationship between all the components of regolith—earth, water, fire, air and biota—is essential to the understanding of process, and so to the development of new thoughts on regolith geology and its application.

Titania in Australian massive silcretes

ABSTRACT Selected silcretes formed by cementation of stream sediments and having >90 wt% SiO2, have been examined optically and by scanning electron microscopy. Such silcretes have quartz framework grains cemented by a plasma of quartz and anatase. Both the plasma quartz and the anatase are euhedral where they line cavities in the silcrete. Such quartz is typically up to 5 µm in diameter; the anatase crystals are platy on (001) and 50–100 nm in diameter. Comparison of the SiO2, TiO2 and Zr content of 138 silcretes with that of 2345 Australian stream sediments suggests the source of titania in silcretes is endogenous. The morphology of the quartz and anatase leads to the conclusion that both precipitated in situ.

Silcrete: an Australian perspective

ABSTRACT We know how most rocks are formed. Silcrete is something of an enigma, for although there are many interpretations of the origins of individual silcrete bodies, such as those in the Paris Basin, England, Botswana and central Australia, an overarching hypothesis capable of explaining all occurrences is still to be found. This paper reviews the literature of research on predominantly Australian silcretes as well as reviewing their occurrence, mineralogy, geochemistry and petrology. Silcrete ages and paleoclimatic significance are also reviewed. Most silcretes are formed low in landscapes along fluvial tracts or lakes but, some may form at breakaway margins as a result of lateral groundwater movement. Following silicification and landscape inversion, many silcretes are left high in the landscape. Most silcretes must form in climates where there is an abundance of water, perhaps seasonally, and of organic acids. The age of a silcrete can be constrained by the fossils it may contain; ages of Australian silcretes so established range through most of the Cenozoic. Lacking fossil evidence, sediments of known stratigraphic age that have been silcreted can only provide a maximum age for the silcrete. Many silcretes in eastern Australia are overlain by basalt, but the age of the basalt can only give the minimum age of the silcreted host below it, not of the silcrete. Silcretes commonly exhibit a number of fabrics; externally glerp structures (also called cockade, ropy or botryoidal), and internally pedogenic and geopetal titaniferous grain-cap fabrics. We conclude that silcretes are formed by the precipitation of silica in various forms, almost always along with titania as anatase, at the time of cementation. Anatase occurs either where it is precipitated or by illuviation, commonly becoming concentrated as geopetal caps or coatings on larger detrital framework grains. This implies that the fluids moving the cementing components largely move downward through the silcreted host. Alternating Ti-rich and Ti-poor laminae in the caps show this process can be repetitious.