Banded Ironstones Research Articles

Four new Western Australian species related to Hibbertia axillibarba (Dilleniaceae)

Hibbertia axillibarba was described in 2000 as a short-range endemic species known only from South Ironcap, a semi-arid, banded ironstone hill in the Western Australian southern Goldfields. Subsequently, several specimens collected from sandplain habitats in the Western Australian wheatbelt, varying from close to, to distant from, South Ironcap, were determined as H. aff. axillibarba or H. cf. axillibarba at the Western Australian Herbarium. Close study of these specimens has shown that none matches H. axillibarba, which on current knowledge remains endemic on South Ironcap; instead, they comprise three distinct species, which are described here as Hibbertia arenicola K.R.Thiele, H. erioclada K.R.Thiele and H. elachophylla K.R.Thiele & T.Hammer. A fourth new species, H. lanulipes K.R.Thiele, is included in this paper as it is likely to be related to the others, despite being morphologically quite different from them. None of these species is widespread and some are likely to be rare and restricted.

Spatiotemporal variation in ecophysiological traits align with high resolution niche modelling in the short-range banded ironstone endemic Aluta quadrata.

Defining plant ecophysiological responses across natural distributions enables a greater understanding of the niche that plants occupy. Much of the foundational knowledge of species' ecology and responses to environmental change across their distribution is often lacking, particularly for rare and threatened species, exacerbating management and conservation challenges. Combining high-resolution species distribution models (SDMs) with ecophysiological monitoring characterized the spatiotemporal variation in both plant traits and their interactions with their surrounding environment for the range-restricted Aluta quadrata Rye & Trudgen, and a common, co-occurring generalist, Eremophila latrobei subsp. glabra (L.S.Sm.) Chinnock., from the semi-arid Pilbara and Gascoyne region in northwest Western Australia. The plants reflected differences in gas exchange, plant health and plant water relations at sites with contrasting suitability from the SDM, with higher performance measured in the SDM-predicted high-suitability site. Seasonal differences demonstrated the highest variation across ecophysiological traits in both species, with higher performance in the austral wet season across all levels of habitat suitability. The results of this study allow us to effectively describe how plant performance in A. quadrata is distributed across the landscape in contrast to a common, widespread co-occurring species and demonstrate a level of confidence in the habitat suitability modelling derived from the SDM in predicting plant function determined through intensive ecophysiology monitoring programmes. In addition, the findings also provide a baseline approach for future conservation actions, as well as to explore the mechanisms underpinning the short-range endemism arid zone systems.

The response to the rebuttal on the revised stratigraphy of the Bida Basin, Nigeria: New observations.

The late Cretaceous Bida Basin lithostratigraphy was carried out in order to establish a correlation between the northern (Bida area) and the southern part (Lokoja area) of the basin. The detailed fieldwork carried out over four years involved field description and mapping of outcrops in the entire basin. The study showed that the basin is underlain by mainly continental sedimentary rocks with minor occurrence of marginal/shallow marine to freshwater flood-plain deposits. The basin fill is characterised mostly by several cycles of amalgamated and/or stacked alluvial fans of the Bida/Lokoja Formation directly overlying the Basement Complex. The study revealed that the Lokoja/Bida Sandstone is overlain directly by the oolitic/pisolitic ironstone of the Agbaja/Sakpe Formations, which, in turn, is succeeded by fine to medium grained, grey and white sandstones, siltstones, grey clays, carbonaceous silts and shales, and ubiquitous concretionary ironstone bands of the Patti/Enagi Formation. Given the current detailed investigation, this study suggests that contrary to the rebuttal published, the Agbaja Ironstone is not the youngest formation in the basin. This study further revealed that the sequence of rocks and fossils used to define the Patti Formation and its probable age do not exist on Mount Patti in Lokoja, rather they are found along excavated road sections, quarries and boreholes around the Ahoko Village. Based on the detailed field work and sedimentological investigations, this study hereby proposes that the name ‘Patti Formation’ be changed to ‘Ahoko Formation’.

A nardoo processing grinding stone from a rockshelter in the Pilbara, Western Australia

Nardoo (scientific name Marsilea spp.) is a small freshwater fern, common across semi-arid and arid Australia. While reported as an important starchy food, albeit requiring complex processing in some regions, it is not well known as a food source in the Pilbara of Western Australia. Here we describe plant residues including megaspores, starch grains, cellulose, and fibres on a portable grinding stone from the surface of a rockshelter in the West Angelas region of the Central Pilbara. These residues display physical characteristics that strongly suggest they derive from nardoo. Given these findings, we encourage researchers to consider nardoo processing and consumption in future ethnobotanical and archaeological studies in the Pilbara. Abbreviations LW: Lynley Wallis; BS: Birgitta Stephenson; PSR: Picro Sirius Red; BIF: banded ironstone formation

Plant endemism in Griqualand West, South Africa

Griqualand West, a region in the semi-arid Northern Cape and North-West provinces of South Africa, has been variously suggested to contain a number of range-restricted plant species, and was proposed to be a local centre of plant endemism. The Griqualand West Centre (GWC), hitherto demarcated by geological features and limited floristic data, is hereby investigated to determine the true levels of endemism and its extent of occurrence. Findings suggest that at least 23 plant species have their natural distribution ranges restricted to the Griqualand West region. These endemics represent 1.4% of the region's flora. Although this is a lower than the predicted level of endemism, it matches the trends of endemicity found in other centres in semi-arid savanna of southern Africa. Many of the GWC endemics show indications of holo-endemism owing to their apparent preference for the Ca- and Mg-rich substrates of the Ghaap Plateau. When the total distributions of all GWC endemic species are considered, then the resulting boundary of the GWC is more extensive than the substantial area already proposed previously (>40,000 km2). This study therefore proposes the concept of ‘core area’ in which distant outlier populations of endemic species (>100 km outside the main distribution range with no suitable habitat in between) are discarded during the demarcation of the centre's boundary. It is proposed that this concept is best applied when assessing extensive areas with few endemic species. A more refined GWC core area will allow for more effective conservation and future research efforts by focussing attention on those areas where high numbers of endemic plant species co-occur. Within the GWC core area, specific regions, such as the increasingly densely populated Kimberley region, the banded ironstone hill ranges, and the unique environment that is the Ghaap Plateau, are highlighted as areas of conservation importance.

Palaeotopography of a Palaeolithic landscape at Bestwood 1, South Africa, from ground-penetrating radar and magnetometry

In order to investigate the buried landscape at the Fauresmith locality of Bestwood 1, outside the town of Kathu in the Northern Cape Province, we performed ground-penetrating radar and magnetometry surveys across the sand-filled central portion of the valley. The radar images a strong continuous reflector which we can assign to the boundary between the Kalahari sands and underlying Banded Ironstone Formation gravels. Moreover, the thickness of the sand delineates a buried depression in the centre of the valley with flat plateaus at the sides. Subtracting the sand thickness from the current topography produces a map of a small stream channel in the northern part of the valley. Analysis of the magnetic gradient data allows us to extend this buried channel further to the south. Our geophysical survey provides a valuable contribution towards understanding the context of hominin occupation along the banks of a small stream in the Kathu Complex.
 Significance:
 
 We provide an example of combining two geophysical methods to map overburden thickness, useful for archaeological landscape interpretation.
 
 .

РУДНО-МАГМАТИЧЕСКАЯ СИСТЕМА SUKUMALAND (ТАНЗАНИЯ)

The overwhelming part of the gold ore potential of the United Republic of Tanzania is confined to the systems of the Neoarchaean greenstone belts (GSB) of the metallogenic province of Lake Victoria in the northwestern part of the country. Based on the interpretation of space imagery, in the western part of the province the Sukumaland ore-magmatic system of concentric-zonal structure has been distinguished. It is formed by a group of arched greenstone belts and associated gold deposits. They form two arcuate branches: the internal and external branch, differing in their composition and structure. The internal arc is represented by the Rwamagaza and Kahama GSB with gold deposits of Tulawaka, Buckreef, Mawe-Meru, Tembo, and Bulyanhulu. The external arc is represented by Geita and Mabale-Buhungurica GSB with gold deposits of Ridge-8, Nyankanga, Geita, Matandani, Kukuluma, Nyanzaga, Kitongo, Golden Ridge, Nyakafuru, and Miabu. Their huge gold potential (over 50 million ounces of gold) stipulates for the urgency of the study of this gigantic structure. GSB of the inner arc (Rwamagaza and Kahama) are characterized by a predominance of basic volcanic rocks, and GSB of the external arc (Geita and Mabale-Buhungurica) – by sedimentary and volcanic-sedimentary formations, in particular, banded ironstone formations (BIF). Accordingly, gold deposits of the inner arc are mainly associated with basic volcanic rocks (Tulawaka, Buckreef, Mawe-Meru, Bulyanhulu, Tembo) while gold deposits of the external arc – with BIF (Geita, Nyankanga, Kukuluma, Matandani). The nuclear part of the ore-magmatic system is of special interest, although manifestations of volcanism are absent there and gold or other mineral deposits are unknown yet, but a tectonic pattern of its structural elements indicates its significant ore potential. In particular, the presence of numerous spatially close systems of radial-circular faults attests to the presence of weakened zones in the space above the core of a hypothetical magma chamber. Gold deposits of different geological and industrial types, yet undiscovered, as well as manifestations of diatreme magmatism may be associated with these weakened zones. In particular, one cannot exclude the possible presence of diamond-bearing volcanic pipes. It is assumed that the ore-magmatic system has originated and evolved in the course of a complex multi-stage ore-magmatic process under the influence of ascending flows of depth fluids due to the long-term functioning of a zone with anomalously heated mantle in that area. During further geological exploration it is recommended to take into account the predictable availability of a single integrated ore-magmatic system of longdevelopment and its structural features.

A revised stratigraphy of the Bida Basin, Nigeria

The Bida Basin is a NW-SE trending inland basin, stretching from Shegwa (NW) to Dekina (SE). It is approximately 350 km long and varies in width from 75 to 150 km. It is roughly elliptical in ground plan and runs perpendicularly to the western margin of the NE-SW trending Benue Trough Complex. Unlike other sedimentary basins in Nigeria, it is characterized by the absence of volcanics, carbonates and rocks of Tertiary age. Dedicated fieldwork carried out over the past four years revealed that the basin is underlain by mainly continental sediments with minor occurrence of marginal/shallow marine to freshwater flood-plain deposits. The initial basin fill is characterized largely by several cycles of simple, but amalgamated and/or stacked fanglomerates (alluvial fan) systems developed from several single stream breaches onto the proximal setting of the basin. These fanglomerates directly overly the Basement Complex and display a fining upward sequence. This unit is generally accepted as the Lokoja/Bida Sandstone. Our studies revealed that the Lokoja/Bida Sandstone is overlain directly by the oolitic/pisolitic ironstone of the Agbaja/Sakpe Ironstone which is in turn succeeded by a sequence of rocks generally referred to as the Patti/Enagi Formation, which is comprised of predominantly fine to medium-grained, grey and white sandstones, grey clays, carbonaceous silts and shales, and ubiquitous concretionary ironstone bands. This observation is opposed to the widely accepted view of previous workers that considered the Agbaja Ironstone as the youngest formation in the basin. This study further revealed that the sequence of rocks used to define the Patti Formation does not exist at the location of Mount Patti in Lokoja, rather, it is found in road cut sections, quarries and boreholes around the Ahoko Village. Following the rules for stratigraphic nomenclature, it is hereby proposed that the name ‘Patti Formation’ should be changed to ‘Ahoko Formation.’ A new geological map was made for the basin which clearly demonstrates the relationship of the formations.

The potential impact of mining on population genetic variation in the Banded Ironstone Formation endemic Tetratheca erubescens (Elaeocarpaceae)

Tetratheca erubescens is a narrowly endemic species including ~6300 plants restricted to a 2-km2 distribution on the south Koolyanobbing Range Banded Ironstone Formation (BIF) in Western Australia. A key objective of the present study was to characterise population genetic variation, and its spatial structuring across the entire distribution of T. erubescens, to enable a quantification of genetic variation that may be affected by proposed mining of the BIF. In total, 436 plants (~30 at each of 14 sites) from across the entire distribution were sampled, genotyped and scored for allelic variation at 11 polymorphic microsatellite loci. Fifty-nine alleles were detected (mean alleles per locus=5.36, range 2–10), and observed heterozygosity was low to moderate and typically lower than expected heterozygosity across all loci (mean observed heterozygosity (Ho)=0.41, mean expected heterozygosity (He)=0.48). Given the restricted distribution of T. erubescens, overall genetic structuring was surprisingly strong (overall FST=0.098). A range-wide spatial autocorrelation analysis indicated a significant positive genetic correlation at distances up to 450m, largely corresponding to the scale of more-or-less continuous distribution within each of two geographic clusters. In support, a STRUCTURE analysis identified an optimal number of genetic clusters as K=2, with assignment of individuals to one of two genetic clusters corresponding with the main geographic clusters. The genetic impact of proposed mining on T. erubescens was assessed on the basis of identifying plants within the proposed mine footprint (all plants from 4 of 14 sites). Repeating analyses of genetic variation after removal of these samples, and comparing to the complete dataset adjusted for sample size, resulted in the loss of one (very rare: overall frequency=0.001) allele (i.e. 58 of 59 alleles (98.3%) were recovered). All other parameters of genetic variation (mean Na, Ne, I, Ho, He, F) were unaffected. Consequently, although up to 22% of all plants fall within the mine footprint and, therefore, may be lost, <2% of alleles detected will be lost, and other genetic parameters remained unaffected. Although these results suggest that the proposed mining will result in a negligible impact on the assessed genetic variation and its spatial structuring in T. erubescens, further research on impacts to, and management of, quantitative genetic variation and key population genetic processes is required.

Middle Pleistocene lithic raw material foraging strategies at Kathu Pan 1, Northern Cape, South Africa

There is a general trend for increased selectivity and longer transport distances of lithic raw materials for flintknapping in the Middle Stone Age compared to the Earlier Stone Age. The Stratum 4a assemblage at Kathu Pan 1, Northern Cape, South Africa, which dates to ~500ka, presents a unique opportunity for assessing Pleistocene raw material foraging strategies at the onset of the Middle Stone Age. In this paper, I identify potential primary and secondary sources for the raw materials used for lithic reduction at Kathu Pan 1 (KP1), and quantify raw material variability at nearby secondary sources. Patterns of raw material selection are identified within the KP1 Stratum 4a assemblage and compared to a sample from the underlying Stratum 4b, an Acheulean assemblage, to test for a temporal pattern. Raw material use during the Stratum 4a occupation of Kathu Pan, which is focused on abundant locally available resources for flintknapping, is consistent with the general pattern for Earlier Stone Age foraging strategies, and differs from evidence at some more recent Middle Stone Age sites that show longer transport distances and increased selectivity for certain high-value materials. While many aspects of technology change at KP1 ~500ka, these technological shifts are not associated with a significant change in the kinds of raw material that were exploited. There are, however, differences in how the raw material was exploited. In Stratum 4a, there is evidence that hominins intentionally exploited the natural banding in banded ironstone formation in order to produce elongated products, and this evidence is lacking in the underlying Acheulean assemblage.

The structural history and mineralization controls of the world-class Geita Hill gold deposit, Geita Greenstone Belt, Tanzania

The Geita Hill gold deposit is located in the Archean Geita Greenstone Belt and is one of the largest gold deposits in East Africa. The Geita Greenstone Belt experienced a complex deformation and intrusive history that is well illustrated and preserved in and around the Geita Hill gold deposit. Deformation involved early stages of ductile shearing and folding (D1 to D5), during which episodic emplacement of large diorite intrusive complexes, sills, and dykes occurred. These ductile deformation phases were followed by the development of brittle-ductile shear zones and faults (D6 to D8). The last stages of deformation were accompanied by voluminous felsic magmatism involving the intrusion of felsic porphyry dykes, within the greenstone belt, and the emplacement of large granitic bodies now forming the margins of the greenstone belt. Early, folded lamprophyre dykes, and later lamprophyre dykes, crosscutting the folded sequence are common, although volumetrically insignificant. The gold deposit formed late during the tectonic history of the greenstone belt, post-dating ductile deformation and synchronous with the development of brittle-ductile shear zones that overprinted earlier structural elements. The main mineralizing process involved sulfide replacement of magnetite-rich layers in ironstone and locally the replacement of ferromagnesian phases and magnetite in the diorite intrusions. The intersection between the brittle-ductile (D6) Geita Hill Shear Zone and different structural elements of ductile origin (e.g., fold hinges), and the contact between banded ironstone and folded diorite dykes and sills provided the optimal sites for gold mineralization.

A Giant Gold System, Geita Greenstone Belt, Tanzania

[Extract] Archean greenstone belts are a well-known source of gold deposits. If we ignore the giant Witwatersrand gold system we can confidently say that the majority of Archean gold comes from greenstone belts. Even though as a general rule greenstone belts are highly prospective for gold some greenstone belts have proved to be much more productive than others. For example most of the gold in Canada's Superior Province comes from Abitibi Greenstone Belt, particularly from Timmins area (e.g. Wyman, 2003). Similarly, most of the gold mined from Australia's Yilgarn Craton comes from the Kalgoorlie camp which accounts for more than half of the gold mined so far (Philips et al., 1996) in the entire craton. Interestingly, although far apart, the geology and mineralization styles of gold deposits in both areas are very much alike suggesting that the processes leading to the formation of giant ore systems must be similar starting from the ground preparation to the mineralization process. In the Tanzania Craton the giant Bulyanhulu gold deposit is also similar to the giant deposits found in the Timmins and Kalgoorlie areas, respectively. That is, lode gold mineralization style, mafic host rocks, ages, map scale patterns, deformation styles and the short premineralization crustal history are all largely similar (e.g. Bateman and Bierlein, 2007). Bulyanhulu is much more similar to the Kalgoorlie camp in the sense that it consists of one major gold deposit, while the Timmins camp is made of a large number of smaller gold deposits. These similarities between three giant gold camps situated on three different continents may indicate that in order to form giant gold deposits certain conditions need to be satisfied. However, that is not always the case and the gold mineralization found in Geita Greenstone Belt does not follow the typical pattern of Archean lode gold deposits. Geita Greenstone belt contains a minimum of fifteen gold deposits which may resemble the Timmins camp, but the mineralization style, alteration, structures and host rocks are different. The mineralization in Geita Greenstone belt is preferentially hosted within deformation zones developed along the contact of banded ironstones and porphyries of various compositions rather than along some major shear systems. The structures associated with the mineralised system are minor, the alteration zone is restricted to the mineralised zone, quartz veins are rare or missing although silicification is common.

Pityrodia iphthima (Lamiaceae), a new species endemic to banded ironstone in Western Australia, with notes on two informally recognised Pityrodia

<i>Pityrodia iphthima</i> (Lamiaceae), a new species endemic to banded ironstone in Western Australia, with notes on two informally recognised <i>Pityrodia</i>

Search for traces of the late heavy bombardment on Earth—Results from high precision chromium isotopes

High precision mass spectrometric analyses of the chromium isotopic composition of metamorphosed turbiditic and pelagic sedimentary rocks and banded iron stones from the ∼3.7 Gyr Isua Supracrustal Belt (ISB) in West Greenland cannot be distinguished from the standard terrestrial 53Cr / 52Cr ratio at our present level of resolution. As a consequence, our search for chemical traces of possible impact-derived meteoritic components (asteroidal and/or cometary material, or accreted cosmic dust) in the Earth's oldest chemical and detrital sediments was negative. Our results, based on the 53Mn– 53Cr short-lived radionuclide system (half-life of 3.7 Myr), cannot confirm the recent findings by [1] [R. Schoenberg, B.S. Kamber, K.D. Collerson, S. Moorbath. Tungsten isotope evidence from approximately 3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth. Nature 418 (2002) 403–405.] of tungsten isotope anomalies (based on the 182Hf– 182W short-lived radionuclide system; half-life of 9 Myr) in these sediments, which were interpreted as indicating a component derived from meteorites. Possible reasons for the failure to trace cosmic material in the ISB metasediments are various: 1. The samples studied are not representative; 2: The sedimentation period did not overlap with the period of late heavy bombardment of the Moon; and 3. The potential chromium anomalies, if present, are too small to be traceable by our present levels of detection. Unequivocal evidence of a late heavy bombardment on the early Earth therefore remains elusive and uncertain.

Expanding the Geological Model for Finsch Mine

Research Article| December 01, 2002 Expanding the Geological Model for Finsch Mine Wayne Barnett; Wayne Barnett De Beers Consolidated Mines Ltd, Venetia Mine, Messina, 0900 Republic of South Africa, e-mail: wayne.barnett@debeersgroup.com Search for other works by this author on: GSW Google Scholar Christopher Preece Christopher Preece De Beers Consolidated Mines Ltd, Finsch Mine, Lime Acres, 8410 Republic of South Africa, e-mail: christopher.preece@debeersgroup.com Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2002) 105 (4): 381–400. https://doi.org/10.2113/1050381 Article history first online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Wayne Barnett, Christopher Preece; Expanding the Geological Model for Finsch Mine. South African Journal of Geology 2002;; 105 (4): 381–400. doi: https://doi.org/10.2113/1050381 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The geological model (lithology and structure) developed for use in geotechnical engineering projects at Finsch Mine, Northern Cape Province, is described in this paper. The lithology model is presented as a detailed stratigraphic column. This study provides new lithofacies descriptions and suggests formation boundary positions for the upper Campbellrand Subgroup stratigraphy exposed by the open pit and underground tunnel network at the mine. The Klippan Formation lagoonal facies, the Kogelbeen Formation subtidal facies and the Gamohaan Formation microbialite facies are interpreted as a transgression sequence deepening into the basinal banded ironstone formation of the Asbestos Hill Subgroup.The structural model is presented as a three-dimensional computer model, and as a summarized model of the structural domains. The structural model is of a northwest to north-northeast varying strike-slip system with “negative flower structure” geometry. Two phases of open folding have developed one trending east-southeast and the other parallel to the faults. Four sub-vertical joint sets are described. The structures are identified as an attenuated expression of the late-stage events of the Namaqua-Natal orogeny at circa 1Ga. The authors suggest that the latest phase of deformation was caused by oblique collision tectonics expressed initially as compression from the south-southwest producing transtensional north-northwest strike-slips faults with a dextral shear sense. Compression may have then swung to the west-southwest producing fault-parallel folds and a conjugate joint set.The described structures were utilized during kimberlite emplacement, including the early intrusion of northeast striking kimberlite dykes. The mechanics of kimberlite emplacement are not discussed, but this work provides important data for such future research. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Amphibole asbestos in Africa and Australia: geology, health hazard and mining legacy

Although asbestiform amphiboles only contributed less than 6% of industrial asbestos worldwide, they have proved more toxic as mineral pathogens than the more commonly mined asbestiform serpentine (chrysotile or white asbestos). Most amphibole asbestos was mined as crocidolite (blue) and amosite (brown) from Precambrian banded ironstones in South Africa and, to a lesser extent, Western Australia. Blue and brown asbestos not only can induce asbestosis at lower concentrations than white asbestos, but they are highly carcinogenic. Early awareness of asbestos-related diseases in producer countries did not translate into preventative action before hundreds of thousands of individuals had been exposed to high fibre levels. The situation was most acute in South Africa where the occupational health status of black Africans in particular was badly neglected. In at least one mine, children were used to trample shipping bags filled with amosite, and in doing so received what is thought to have been the highest ever inhalation dose of amphibole asbestos. South Africa now has the highest rate of asbestos-induced mesothelioma in the world, with this invariably fatal cancer responsible for over 9% of deaths in a study conducted in a former Cape crocidolite mining town.

Silicic Layer-Parallel Shear Zones in a Zimbabwean Greenstone Sequence: Horizontal Accretion Preceding Doming

Detailed structural metamorphic data from the Trojan nickel mine area in the Shamva-Bindura Greenstone Belt (SBGB) show that two groups of ductile structures associated with separate amphibolite facies metamorphic assemblages can be distinguished. D 1/M 1 structures are related to a network of anastomosing shear zones that accommodated W-directed imbricate stacking of the stratigraphy, while metamorphic conditions in the mine area reached about 500°C at pressures of 3-4 kbar. D 1 shears are locally strongly silicified and preserve fine-grained mylonitic textures. They have been wrongly identified in the past as banded iron stones or stratigraphic chert horizons. Truncations of primary layering associated with such fine-grained mylonitic quartzites are tectonic and not stratigraphic in origin, and the units can not be used as tectonic marker horizons, but instead represent glide planes across which tectonic imbrication of the stratigraphy has occurred. An important silicified shear zone of this nature occurs along the boundary of the Iron Mask and Arcturus Formations. D 2/M 2 structures are related to doming of the Chinamora Batholith, and a contact metamorphic overprint and recrystallisation of M 1 assemblages. M 2 temperatures in the mine area reached 565°C. Such high contact metamorphic temperatures at 3 km from the contact of the batholith can only be explained if the entire Chinamore Batholith was emplaced as a relatively hot (>750°C) intrusive body in an already anomalously hot greenstone sequence. Metamorphic fluids during M 1 and M 2 where CO 2-rich and carbonate alteration was pervasive. D 1/M 1 and D 2/M 2 events may have been separated by as much as 70 Ma. D 1 structures in the Trojan area can be related to a large mantled-gneiss, nappe structure represented by the Madziwa Batholith and a mantle of mafic greenstones to the N of the SBGB. The footwall thrust of this nappe occurs along the N boundary of the Shamvaian sediments in the centre of the SBGB. The imbricate stacking of the stratigraphy in the Trojan area can be interpreted as secondary structures in the footwall of the nappe. Horizontal accretion and stacking of crustal fragments during D 1 was followed by thermal perturbations that resulted in the diapiric rise of domes like the Chinamora Batholith during D 2. After doming the terrain appears to have cooled and stabilized, with further deformation partitioned into narrow strike-slip shear zones.

The nature and origin of sideritic ironstone bands in the Tertiary Lowmead and Duaringa Basins, Queensland

Sideritic ironstones in Tertiary lacustrine oil shale from the Lowmead and Duaringa Basins in Queensland, contain two distinctive types of siderite in the ironstone bands: sphaerosiderite in the mudstone and coal, and finely crystalline siderite in the lamosite. The petrological evidence indicates that the siderite in the ironstone bands formed eogenetically by growing displacively within the soft sediment. Chemically the siderite is very pure though the sphaerosiderite sometimes shows compositional zoning. Stable oxygen and carbon isotope analyses of the siderite show a wide range of values from ‐12.8‰ to ‐2.4 %0 δ18O (PDB) and ‐5.5‰ to +12.9‰ δ13C (PDB) for the Lowmead Basin; and ‐9.6‰ to ‐1.2‰ δ18O (PDB) and ‐18.6‰ to +16.4‰ δ13C (PDB) for the Duaringa Basin. The oxygen isotope data indicate that the siderite formed in freshwater environments but not in isotopic equilibrium with the formation waters. Kinetic factors offer the most plausible explanation for the anomalously light δ18O values of many of t...

3-D Magnetic Model Inversion for Structure Mapping

A series of three-dimensional magnetic models has been designed to represent a wide range of geological structures. The models have been incorporated into automatic inversion routines, so that their parameters may be derived quickly from magnetic anomalies. The objective in developing this modelling scheme was to devise tools for applying to aeromagnetic data that would yield quantitative structural information, complementing the qualitative interpretations usually carried out. The models are based on simple slab components, and include single and double slabs, horizontal and plunging fold structures. The number of critical parameters for each model has been minimised to facilitate stable and rapid inversion. The inversion modelling scheme has been tested in the area of the Kurrawang Syncline near Kalgoorlie, WA. Here a magnetic conglomerate marker horizon has been folded and subsequently faulted. The results from the inversions of aeromagnetic data show excellent agreement with the known geology. It has been found that orientations tend to be determined more reliably than do thicknesses and depths. Other structures that have been modelled are the Mount Edwards folded ultramafic belt, near Widgiemooltha, WA, which is complicated by cross-cutting dykes; and a series of banded ironstones that host copper-gold mineralisation at Osborne, Queensland. The modelling technique is useful for testing an initial qualitative interpretation by trying the appropriate model and finding out if it can be made to fit. If a model fits well, then valuable structural information (strikes, dips, plunge etc) can be obtained. Although the modelling procedures were developed for use in reconnaissance mapping with airborne data, they are equally applicable to prospect exploration and ground survey measurements.

Eogenetic siderites in lacustrine oil shales from Queensland, Australia, a stable isotope study

Tertiary lacustrine oil shales from the Lowmead and Duaringa Basins in Queensland, Australia, contain siderite as a ubiquitous minor phase and in ironstone bands within the sequence. There are two distinctive types of siderite in the ironstone bands: sphaerosiderites and finely crystalline siderites. Petrological evidence indicates that the sphaerosiderites in the ironstone bands formed eogenetically by growing displacively within the soft sediment. The geological setting of the basin and the sedimentological evidence indicate a lacustrine environment. The deposition of siderite is therefore considered to be controlled by processes in the lacustrine water body and sediments. The maximal burial depth is estimated to be 1400 m and consequently the oil shales have not undergone any advanced thermal alteration. Stable oxygen and carbon isotope analysis of the siderites show a wide compositional range: δ 18O = −12.8 to − 2.4% vs. PDBG and δ 13C = −5.5 to + 12.9% vs. PDB for the Lowmead Basin: δ 18O = −9.6 to 1.2% vs. PDB and δ 13C = −18.6 to + 16.4% vs. PDB for the Duaringa Basin. The oxygen isotope compositions indicate that the siderite formed in a to + 16.4% vs. PDB for the Duaringa Basin. The oxygen isotope compositions indicate that the siderite formed in a freshwater environment but generally show a range more negative than the expected equilibrium composition with coeval meteoric waters. The carbon isotope composition may reflect deposition within the bacterial oxidation and fermentation zones. The shallow-lake setting with high productivity results in siderite precipitation under high pH either at the water-sediment interface or within the sediment. The δ 13C of the dissolved carbonate will be elevated by intense photosynthesis. Under these conditions of CO 2-depletion/high-pH isotopic disequilibrium during siderite formation will result in δ 13C and δ 18C lower than the equilibrium composition, in agreement with the recorded isotopic data.