Quantifying time scales of pedogenic calcrete formation using U-series disequilibria

Time scales of pedogenic calcrete development are quantified by subsampling carbonate from within a mature (stage V) pedogenic calcrete profile from southeast Spain and dating the material by U-series disequilibria. The location of the earliest and latest cements can be estimated by comparing previous studies of calcrete morphological development with micromorphological analysis of the study profile. Carbonate was sampled and dated from three locations within the profile: (1) below the lower surface of clasts within the hardpan (representing the earliest cement present—207±11 ka), (2) from the centre of cement filled pores within the hardpan (reflecting the final plugging of the calcrete hardpan—155±9 ka) and (3) from the laminar calcrete overlying the hardpan (representing the latest cement—112±15 ka). These results show that the hardpan took between 73 and 31 ka to form, whilst the mature stage V profile took between 121 and 69 ka to form. This is the first time that rates of mature calcrete development have been established by direct radiometric dating of the authigenic carbonate. The technique is appropriate for dating mature calcretes in dryland regions worldwide and offers the opportunity of increasing our understanding of the spatial and temporal variability in rates of pedogenic calcrete development.     

Soil-zone microfabrics in calcrete and in desiccation cracks from the upper Jurassic purbeck formation of dorset

Soil-zone microfabrics, alveolar-septal structure, needle-fibre calcite, and calcans are described from horizontal calcrete layers, stringers, and infillings in vertical desiccation cracks from an Upper Jurassic limestone in the Lower Purbeck Formation of Dorset. These calcrete palaeosols occur in an oolitic limestone (the Hard Cap) which represents former evaporitic lagoonal to carbonate mudflat environments. The calcretes occur 6-10 cm below the Great Dirt Bed, a former rendzina soil with rooted tree remains. Desiccation cracks and vugs formed in the oolitic sediment before Great Dirt Bed times. After formation of the Great Dirt Bed, soil-water rich in dissolved CaCO₃ preferentially flowed through natural conduits in the underlying sediment, namely the desiccation cracks and vugs. Calcrete precipitated in these cracks and vugs around decaying plant roots, and probably, during more arid (evaporative) climatic periods. These palaeosol microfabrics are among the first to be described from the British Jurassic and were probably preserved due to the semiarid Lower Purbeck climate where rapid oxidation of organic matter limited the amount and strength of carbonic acid generation, thereby limiting extensive dissolution of early formed soil-zone carbonate. Early diagenetic cementation of the sediment also aided microfabric preservation by sealing off soil-zone structures from subsequent diagenetic fluids.     

Distribution and speciation of gold in biogenic and abiogenic calcium carbonates - Implications for the formation of gold anomalous calcrete

Calcrete (pedogenic Ca carbonate) is an important sampling medium for geochemical gold (Au) exploration in semi-arid and arid regions of Australia, because it is widespread, easy to sample and calcium (Ca) shows a strong positive correlation with Au, but not with base metals, in calcrete overlying buried Au mineralization. In this study we show that the formation of Au-anomalous calcrete can be biomediated through the activity of resident microorganisms, and may not simply be the result of passive nucleation on inactive cells or evapotransporative processes. Calcified microfossils are highly abundant in calcrete from the Barns Au-prospect in South Australia. These microfossils are morphological analogues of calcified cells and biofilms formed in laboratory experiments conducted with active bacterial cultures enriched from Au-anomalous calcareous sand from the Barns prospect. Calcium carbonates precipitated by these cultures consisted mostly of calcite, which is the main carbonate mineral in calcrete. Synchrotron micro-X-ray fluorescence (S-μXRF) mapping was used to assess the distribution of Au, Zn, Ca and other metals in Ca carbonates precipitated by active bacterial cultures. On a μm-scale the distribution of Au was heterogeneous in these Ca carbonates and differed from base metal distribution, thus mimicking the spatial separation of these metals observed in calcrete. The speciation of Au in Ca carbonates precipitated by active bacteria was measured using micro-X-ray absorption near edge structure spectroscopy (μ-XANES) and resembled that observed in Au-anomalous calcrete closely. While metallic Au was observed in Au ‘hotpots’, ionic Au was detected in the halo surrounding the ‘hotspot’. In contrast, the precipitates produced in the presence of dead bacterial cells or by raising solution pH or pCO₂, i.e., hydroxylapatite, portlandite and vaterite, respectively, did not reflect the mineralogy of calcrete. Gold distribution and speciation in vaterite, formed by raising pCO₂, were homogenous and did not reproduce the variation observed in calcrete and Ca carbonates precipitated by active cells. Increasing the supersaturation with respect to Ca in solution by incremental drying of the medium produced only X-ray amorphous precipitates, or hydroxylapatite in the presence heat-killed cells. In conclusion, this study shows that active microbial processes that combine biogenic Ca carbonatogenesis with Au precipitation are likely to drive the formation of Au-anomalous calcrete.     

The association of gold with calcrete

The discovery that Au accumulates in calcrete (pedogenic carbonate or caliche) was made in 1987 by CSIRO. Calcrete is a general term describing accumulation of alkaline earth metals in soils of arid and semi-arid terrains around the world. The principal constituent of calcrete is calcite while Au is a noble metal. Calcrete has been a significant tool in a number of Au deposit discoveries, so understanding the mechanisms by which these diametrically different components come together is valuable for enhancing future discovery. Numerous laboratory experiments, case histories and exploration models have been published (most from Australia) yet we do not fully understand the mechanisms involved. It is timely, therefore, twenty-five years on since the first publication of this phenomenon, to review this highly unusual but economically important association.Critical to any review on Au in calcrete is to first consider calcretes themselves. The nature of a particular calcrete, where it has formed and mode of formation is relevant to how, where and why Au accumulates within it. This review commences with a background, nomenclature, history, classification and some examples of calcrete types found near Au deposits. How calcretes form, their origins and the role of biota is considered. Their locations in the regolith and landscape, as well as exploration models for Au in calcrete are discussed. A section on the chemistry of Au in calcretes details what we know about possible mechanisms of formation and considers what laboratory experiments on microorganisms and abiotic experiments tell us. Following on is a summary of practical aspects of identifying, collecting and analysing samples for exploration purposes. Selected mineral exploration case histories are described and how they fit into models of exploration and different regolith settings. Concluding sections include a summary and implications of this accumulated knowledge to discovering Au deposits.     

Occurrence of cyclic palustrine and calcrete deposits within the lower Pliocene Hagul formation, East Cairo district, Egypt

A depositional model of the lower Pliocene Hagul formation, which is exposed in the East Cairo district (Egypt), is proposed with more than 10 depositional cycles recognized. Field occurrence, detailed petrographic investigation and geochemical analysis revealed that the sediments within each cycle are the result of three sequential sedimentological processes: (1) alluvial sedimentation, (2) calcretization, and (3) precipitation of palustrine carbonate. It was concluded that Hagul formation has been deposited within the distal part of an alluvial plain during three successive climatic conditions: a humid climate during which alluvial sediments were deposited, a semi-arid climate with episodic precipitation which was favorable for pedogenic calcrete development, and a sub-humid climate during which groundwater level was gradually elevated and groundwater calcrete accumulated. Rising groundwater level continued until shallow wetlands covered the area and palustrine limestone was precipitated. Variations in the thickness and the nature of the host sediment, calcrete and palustrine limestone cycle suggest that each of the sedimentation processes varied from cycle to cycle.     

Zeolitic diagenesis of late Quaternary fluviolacustrine sediments and associated calcrete formation in the Lake Bogoria Basin, Kenya Rift Valley

Late Quaternary fluviolacustrine siltstones, mudstones and claystones (Loboi Silts) on the northern margins of the saline, alkaline Lake Bogoria in the Kenya Rift Valley contain up to c. 40% authigenic analcime and minor natrolite. The zeolitic sediments are reddish brown and up to 1 m thick. The amount of analcime increases upward in the profile, but decreases with distance from the lake. The altered sediments show many pedogenic features including zeolitic root mats, rootmarks, concretions and carbonate rhizoliths. Residual patches of calcrete locally cap the zeolitic rocks. The profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake. The analcime occurs as submicroscopic (0–5–2–5 μ.m) subhedral and euhedral crystals, which have an average Si/A 1 ratio of 2–33 (as determined by X-ray microanalysis) or 2–18 (d-value of 639 analcime peak). The analcime formed in lake marginal sediments (soils) by reaction of silicate detritus with Na₂CO₃ rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration. Poorly ordered clay minerals were probably the main reactants. Authigenic illite may have been a by-product of the reactions. Chemical analyses suggest that pore waters supplied some of Na⁺, and possibly K⁺ and SiO₂. The associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration. The Ca²⁺ may have originated from infiltrating dilute runoff and groundwater. Authigenic smectite was precipitated in open porosity following analcime formation. The zeolitic alteration at Lake Bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline, alkaline lake sediments.     

A Holocene calcrete from North Yorkshire, England: implications for interpreting palaeoclimates using calcretes

Glacial gravels of Late Devensian Dimlington Stadial age (26 000–13 000 years BP) at West Tanfield, North Yorkshire, England, have been cemented by carbonate-rich solutions to produce a strongly indurated calcrete horizon. The low-Mg cements occur as drusy spar, needle fibres, alveolar septal structures, micrite and micropinnacles, indicative of vadose-zone cementation. Some complex pore partition structures attributed to precipitation along meniscus films also occur. These partitions separate air-dominated and water-dominated microenvironments of the vadose zone. The abundance of vadose fabrics shows that the accumulation is not a groundwater calcrete. In addition, much of the carbonate appears to have been precipitated by biological mediation. Carbon and oxygen isotopic data suggest that the carbonate did not form as a result of freezing, as has been suggested for some ‘arctic’soil carbonates. The pollen history of the area since the Devensian suggests that this calcrete precipitated at low temperatures; this contrasts with widely reported occurrences of calcrete in soils of hot arid or semi-arid regions, and suggests that palaeo-calcretes should not be used as absolute palaeoclimatic indicators. The unusual occurrence, albeit localized, of a thick calcrete under a cool and wet climate probably reflects the well-drained nature of the gravels, the abundance of CaCO₃ as limestone clasts in the gravel and a high degree of biological activity beneath a forest cover, which created a local environment favouring carbonate precipitation.     

Mechanism of calcrete formation in the Lower Cretaceous (Neocomian) fluvial deposits,northeastern Iran based on petrographic,geochemical data

The Kopet-Dagh basin of northeastern Iran was formed during the Middle Triassic orogeny. From Jurassic through Miocene time, sedimentation was relatively continuous in this basin. The Shurijeh Formation (Neocomian), which consists of red bed siliciclastic sediments that were deposited in fluvial depositional settings, crops out in the southeastern part of the Kopet-Dagh basin. In addition to clastic lithofacies, non-clastic facies in the form of calcrete paleosols, were identified in this formation. The calcrete host rocks are mainly sandstone, pebbly sandstone. The calcrete in middle unit in the Shurijeh Formation consists of, from bottom to top: incipient calcrete, nodular calcrete, massive calcrete horizons. The maturity pattern of these calcrete gradationally increases from bottom to top in this unit. Lack of organo-sedimentary structure (mainly plant roots), diversity of calcite fabric, suggest that the studied calcretes have a multi-phase development: a short vadose phase followed by a long phreatic phase. These calcretes are neither pedogenic nor groundwater calcretes. Petrographic studies show that they are composed of micritic textures with a variety of calcite fabrics, microsparitic/sparitic veins, displacive, replacive fabrics, quartz, hematite grains. Cathodoluminescence images, trace elemental analysis (Fe, Mn increased, Na, Sr decreased) of calcrete samples show the effects of meteoric waters during the calcrete formation when water tables were variable. In this study, we conclude that evaporation, degassing of carbon dioxide are the two main factors in the formation of non-pedogenic or groundwater calcrete. The sources of carbonate were probably parent materials, surface waters, ground waters, eolian dusts, numerous outcrops of limestones that have been exposed in the source area during Neocomian time.     

Dolomite dissolution: An alternative diagenetic pathway for the formation of palygorskite clay

Palygorskite is a fibrous, magnesium‐bearing clay mineral commonly associated with Late Mesozoic and Early Cenozoic dolomites. The presence of palygorskite is thought to be indicative of warm, alkaline fluids rich in Si, Al and Mg. Palygorskite has been interpreted to form in peritidal diagenetic environments, either as a replacement of detrital smectite clay during a dissolution–precipitation reaction or solid‐state transformation, or as a direct precipitate from solution. Despite a lack of evidence, most diagenetic studies involving these two minerals posit that dolomite and palygorskite form concurrently. Here, petrological evidence is presented from the Umm er Radhuma Formation (Palaeocene–Eocene) in the subsurface of central Qatar for an alternative pathway for palygorskite formation. The Umm er Radhuma is comprised of dolomitized subtidal to peritidal carbonate cycles that are commonly capped by centimetre‐scale beds rich in palygorskite. Thin section, scanning electron microscopy and elemental analyses demonstrate that palygorskite fibres formed on both the outermost surfaces of dissolved euhedral dolomite crystals and within partially to completely dissolved dolomite crystal cores. These observations suggest that dolomite and palygorskite formed sequentially, and support a model by which the release of Mg²⁺ ions and the buffering of solution pH during dolomite dissolution promote the formation of palygorskite. This new diagenetic model explains the co‐occurrence of palygorskite and dolomite in the rock record, and provides valuable insight into the specific diagenetic conditions under which these minerals may form.     

The Cherty Rock,Elgin: A petrographic and isotopic study of a Permo-Triassic calcrete

The Cherty Rock is a laterally persistent horizon that extends across the Inner Moray Firth Basin, northeastern Scotland. The southern exposures of the Cherty Rock near Elgin reveal an indurated carbonate and silica-rich horizon (0·75-10m thick) developed in the uppermost part of the largely aeolian Lossiemouth Sandstone Formation. Laminated, pisolitic, and brecciated textures within the Cherty Rock confirm that the horizon represents an analogue to Recent calcretes of semiarid areas. Textures characteristic of silcretes are widespread in the Cherty Rock but these originate from silica replacement of calcite. The carbon and oxygen stable isotopic composition of micrite in the Cherty Rock (mean σ ¹³C = -6·9%°, mean σ ¹⁸O = -5·7%°, both relative to PDB) is comparable to that of other Permo-Triassic calcretes and modern soil carbonate, suggesting that similar processes were responsible for their formation. The σ ¹³C and σ ¹⁸O values for calcite in the Cherty Rock may indicate that evaporative processes contributed to its development. σ ¹⁸O data from spar (mean σ ¹⁸O = -9·8%° PDB) and macroquartz (mean σ ¹⁸O = 19·7% SMOW) indicate that these are later (burial) cements. The Cherty Rock is a useful stratigraphical and palaeoclimatic indicator, and its presence in the Inner Moray Firth basin suggests a period of tectonic stability in the basin during late Triassic times.     

The application of cathodoluminescence to interpreting the diagenesis of an ancient calcrete profile

A laterally extensive calcrete profile has been identified in the Late Asbian (Lower Carboniferous) shallow marine shelf limestones of the Llangollen area, North Wales. The upper surface of the profile is defined by a laterally discontinuous palaeokarstic surface and by laminated calcareous crusts which developed within the underlying limestone. The profile contains a unique series of early pore-filling vadose cements which only occur down to 1 m below the palaeokarstic surface. Cathodoluminescence reveals that these cements pre-date the late pore-filling meteoric phreatic cements which occur throughout local Asbian lithologies. A spar cement stratigraphy has been established for the calcrete profile. Subaerial vadose cements comprise two generations of non-luminescent cement, followed by a brightly luminescent generation which occasionally shows an acicular habit. This needle-fibre calcite represents the final stage of vadose cementation. Precipitation of vadose cements was contemporary with subaerial alteration and micritization of the limestone. Textures, visible only with cathodoluminescence, provide evidence of recurrent periods of fabric dissolution. The most extensive phase of dissolution occurred immediately after the precipitation of the non-luminescent subaerial vadose cements. Several different textures have been recorded, each reflecting the morphology of a partially dissolved substrate. Dissolution textures are generally confined to the walls of the larger pores and to early brecciation fractures. These probably acted as fluid pathways in the calcrete during early subaerial diagenesis. Much of the non-marine micrite in the calcrete profile appears as needle-fibre calcite under cathodoluminescence. This acicular calcite was probably formed in response to localized supersaturation of meteoric pore fluids caused by periods of near-surface evaporation. Since needle-fibre luminescence is strongly variable, these ambient conditions are not believed to have directly controlled the activator ion concentrations of cementing pore waters. Needle-fibre calcite is considered to be a cement precipitate which has almost completely recrystallized to micrite, probably during the late stages of subaerial diagenesis. Two generations of subaerial micrite which define a ‘micrite stratigraphy’, have been distinguished under cathodoluminescence. Reconstructing the diagenetic history of this ancient calcrete profile has revealed that subaerial alteration was multistaged, with many diagenetic processes acting simultaneously during a single phase of emergence.     

Chemical mass balance of calcrete genesis on the Toledo granite (Spain)

The chemical mass balance of calcrete genesis is studied on a typical sequence developed in granite, in the Toledo mountains, Central Spain.

Field evidence and petrographic observations indicate that the texture and the bulk volume of the parent rock are strictly preserved all along the studied calcrete profile.

Microscopic observations indicate that the calcitization process starts within the saprolite, superimposed on the usual mechanisms of granite weathering: the fresh rock is first weathered to secondary clays, mainly smectites, which are then pseudomorphically replaced by calcite. Based on this evidence, chemical mass transfers are calculated, assuming iso-volume transformation from the parent rock to the calcrete.

The mass balance results show the increasing loss of matter due to weathering of the primary phases, from the saprolite towards the calcrete layers higher in the sequence. Zr, Ti or Th, which are classically considered as immobile during weathering, are also depleted along the profile, especially in the calcrete layer. This results from the prevailing highly alkaline conditions, which could account for the simultaneous precipitation of CaCO₃ and silicate dissolution.

The calculated budget suggests that the elements exported from the weathering profile are provided dominantly by the weathering of plagioclase and biotite. We calculate that 8–42% of the original Ca remains in granitic relics, while only 15% of the authigenic Ca released by weathering is reincorporated in the calcite. This suggests that 373 kg/m² of calcium (i.e., three times the original amount) is imported into the calcrete from allochtonous sources, probably due to aeolian transport from distant limestone formations.     

Potential of microbial methane formation in a high-temperature hydrocarbon seep

Hydrocarbon seepage is a surface expression where fluids mixed with sediments and hydrocarbons are expelled through fracture systems that potentially tap into gas–petroleum reservoirs. Hydrocarbons released from most seeps appear to be thermogenic on the basis of their relative abundance and isotopic composition. The potential for subsurface microbial processes modifying these geochemical fingerprints remains poorly constrained. In this study, microcosm incubations were conducted on mud slurries supplied with/without various methanogenic precursors at temperatures ranging from ambient conditions to 90 °C, in order to assess microbial CH₄ formation in the subsurface beneath hydrocarbon seeps. The analyses indicated that CH₄ production was positive at ?80 °C, regardless of whether or not or which precursors were added. However, the pattern of CH₄ production rates varied with the precursor and temperature. In general, the optimum CH₄ production from H₂/CO₂ and formate occurred over a wide range of temperatures (?40 °C), whereas that from acetate, methanol and methylamine was restricted to relatively lower temperatures (40–50 °C). The CH₄ recoveries, together with the C isotopic compositions of CH₄, further indicated that the quantities of CH₄ produced could not completely account for the quantities of precursor consumed, suggesting that a complex metabolic network was involved in the transformation of the added precursor and organic C inherited from inoculated sediments. Microbial CH₄ was estimated to constitute 7–61% of the CH₄ observed using experimentally-derived apparent isotope fractionations as the end member compositions. This illustrates the possibility that microbial CH₄ produced at shallower depths could quantitatively and isotopically alter deeply-sourced thermogenic CH₄ in hydrocarbon seep environments.     

Brecciation textures and tepee structures in Quaternary calcrete (caliche) profiles from eastern Spain: The plant factor in their formation

Field studies of brecciation textures and tepee structures in Quaternary calcretes from eastern Spain reveal that vegetation plays a fundamental role in their formation, alteration and destruction. Biophysical and biochemical activities of higher plant systems produce brecciation textures and some tepee structures. Brecciation of indurated calcrete profiles form rhizo-breccias which may be used as indicators of terrestrial conditions in ancient successions.     

Occurrence of calcrete in the Phu Kradung Formation of the Mesozoic Khorat Group, NE Thailand

The Phu Kradung Formation of the Mesozoic Khorat Group is deposited by meandering river system. Floodplain deposits in the Nong Bua Lamphu section, northeastern Thailand contain paleosols with abundant calcretes. Calcretes occur within about 60 horizons in the studied section. Occurrences of calcretes are related with traces of life, such as roots and burrows. Microstructures of calcretes are mixture of biogenic and non-biogenic origin. It is suggested that the calcrete formation in the Phu Kradung Formation was affected by abundant biological activity.     

Dependence of microbial magnetite formation on humic substance and ferrihydrite concentrations

Iron mineral (trans)formation during microbial Fe(III) reduction is of environmental relevance as it can influence the fate of pollutants such as toxic metal ions or hydrocarbons. Magnetite is an important biomineralization product of microbial iron reduction and influences soil magnetic properties that are used for paleoclimate reconstruction and were suggested to assist in the localization of organic and inorganic pollutants. However, it is not well understood how different concentrations of Fe(III) minerals and humic substances (HS) affect magnetite formation during microbial Fe(III) reduction. We therefore used wet-chemical extractions, magnetic susceptibility measurements and X-ray diffraction analyses to determine systematically how (i) different initial ferrihydrite (FH) concentrations and (ii) different concentrations of HS (i.e. the presence of either only adsorbed HS or adsorbed and dissolved HS) affect magnetite formation during FH reduction by Shewanella oneidensis MR-1. In our experiments magnetite formation did not occur at FH concentrations lower than 5 mM, even though rapid iron reduction took place. At higher FH concentrations a minimum fraction of Fe(II) of 25-30% of the total iron present was necessary to initiate magnetite formation. The Fe(II) fraction at which magnetite formation started decreased with increasing FH concentration, which might be due to aggregation of the FH particles reducing the FH surface area at higher FH concentrations. HS concentrations of 215-393 mg HS/g FH slowed down (at partial FH surface coverage with sorbed HS) or even completely inhibited (at complete FH surface coverage with sorbed HS) magnetite formation due to blocking of surface sites by adsorbed HS. These results indicate the requirement of Fe(II) adsorption to, and subsequent interaction with, the FH surface for the transformation of FH into magnetite. Additionally, we found that the microbially formed magnetite was further reduced by strain MR-1 leading to the formation of either dissolved Fe(II), i.e. Fe²⁺, in HEPES buffered medium or Fe(II) carbonate (siderite) in bicarbonate buffered medium. Besides the different identity of the Fe(II) compound formed at the end of Fe(III) reduction, there was no difference in the maximum rate and extent of microbial iron reduction and magnetite formation during FH reduction in the two buffer systems used. Our findings indicate that microbial magnetite formation during iron reduction depends on the geochemical conditions and can be of minor importance at low FH concentrations or be inhibited by adsorption of HS to the FH surface. Such scenarios could occur in soils with low iron mineral or high organic matter content.     

Volatile organic acids and microbial processes in the Yegua formation,east-central Texas

Geochemical and microbiological evidence indicates that viable microorganisms produce and consume volatile organic acids (VOA) in the Yegua formation. Acetic and propionic acid concentrations in mudstones range from 200 to 1270 and 20 to 38 nmol·gdw⁻¹ respectively, whereas concentrations in sands are 50–200 and less than 20 nmol·gdw⁻¹. VOA concentrations in sediments and in laboratory incubations suggest net production of VOAs by microorganisms in mudstones, and net consumption of VOAs by SO₄ reducing bacteria (SRB) in sands. Notably, SRB activity is mostly confined to aquifer sands.Vertical diffusion and advection were modeled to estimate acetic acid transport from aquitard to aquifer. Assuming that SRB completely respire the acetic acid transported into the aquifer (3.2 μmol·l⁻¹·m·a⁻¹), the CO₂ production rate in the aquifer sands is 5.3 μmol·l⁻¹·a⁻¹. This slow mineralization rate of in situ organic matter is within the range for deep aquifers, and probably accounts for the long-term survival of microorganisms in oligotrophic environments. Finally, the microbial communities in Yegua sediments appear to exhibit a loose commensalism, with microorganisms in aquitards providing VOAs for respiratory processes (i.e., SO₄ reduction) in aquifers.     

A new model for the formation of the Somma Caldera

Summary The evolution of the Mt. Somma caldera is reconstructed by means of volcanological, geological and geomorphologic data. The present caldera shape results from two caldera forming events: (a) break-up of the upper part of the older structure, which occurred between 17 and 8Kyr B.P. through a series of external volcaniclastic debris flow depositions and internal collapses into a cored-out vent. (b) W–SW directed sector collapse of the enlarged Mt. Somma crater, caused by excess vapour pressure generated during the Avellino eruption (3.5Kyr B.P.). At this time, the morphology of the Mt. Somma crater probably resembled the demolished crater formed in 1980 at Mt. St. Helens. Flank failure of Mt. Somma during the Avellino eruption was probably caused both by a migration of the vent to the west and a drastic change of the hydrogeological conditions at depth. Flank failure processes were extended to the S–SE sector of the Mt. Somma edifice during the 79 A.D. and 472 A.D. eruptions. After the 472 A.D. eruption interplinian activity during the Middle Ages resulted in the formation of the Vesuvius cone inside the Mt. Somma caldera.     

A unified theory for the formation of galactic structures

A new theory for the formation of the main structures of galaxies is proposed: these structures are viewed as low-frequency normal modes in disks consisting of precessing stellar orbits. Mathematically, the theory is based on an integral equation in the form of a classical eigenvalue problem, with the eigenvalues being equal to the angular velocities Ω_p of the modes. Analysis of the general properties of the master integral equation (without finding concrete solutions) shows that it admits two types of solutions: barlike and spiral. The numerical algorithms are discussed and particular solutions of the integral equation are presented. If resonance interaction can be neglected, the bar mode represents a neutral perturbation of the disk. This mode can be amplified by the effect of the long-range gravitational field of the mode on stars located in the vicinity of the corotation and outer-Lindblad resonances. Spiral perturbations are waves with zero total angular momentum, and spiral modes are excited at the inner-Lindblad resonance. The approach proposed is compared to currently accepted mechanisms for the formation of galactic structures. In particular, Toomre's application of the swing amplification mechanism to explain the formation of global modes is critically discussed. In addition, we show that it is not correct to simulate the real stellar velocity dispersion in a galaxy using softened gravity.     

Biomediation of calcrete at the gold anomaly of the Barns prospect,Gawler Craton,South Australia

Anomalously high Au concentrations (2.5 to 50 ppb) in regolith carbonate accumulations, such as calcrete and calcareous sands in aeolian sand dunes overlying Au mineralisation of the Gawler Craton, South Australia, show a marked covariance of Au with K, Mg and most notably Ca. This relationship appears to be linked to the authigenic formation of smectites and carbonates within the aeolian dunes in the region. However, little is known about the processes involved in the formation of carbonates under semi-arid and arid conditions. In this study the geochemical properties of aeolian dunes along several depth profiles of 2 to 4 m are investigated in order to assess the relationship between Au mobility and calcrete formation. In the profiles a strongly systematic relationship between Au and the increasing Ca–Mg contents at depth highlights the close association between the enrichment of Au in the calcrete and the underlying hydrothermal mineralisation. Intense calcrete formation and concurrent Au enrichment also occurs in the vicinity of roots penetrating the dune. Thin section petrography and cathodoluminescence show that most of the calcrete in the regolith profiles is micritic; some sparic crystallites have also been identified. To demonstrate the presence of microbial processes that may mediate the formation of calcrete, samples from a depth profile in the dune were taken under sterile conditions. After amendment with urea and incubation of up to 24 h, up to 18 mg/l of NH₄⁺ were detected in near surface samples. At depth of 2.3 m 1 mg/l NH₄⁺ were detected compared to a control that contained below 0.05 mg/l NH₄⁺. These results suggest that the genesis of calcrete and pedogenic carbonate in the area may be partly biologically mediated via processes such as the metabolic breakdown of urea by resident microbiota which generates a pH and pCO₂ environment conducive to the precipitation of carbonate. In the process of urea breakdown organic Au complexes such as Au-amino acid complexes may become destabilised in solution and Au may be co-precipitated, resulting in the fine, non-particulate distribution of Au throughout the micritic calcrete carbonate. In conclusion, this study suggests a coupled mechanism of biologically mediated and inorganic mechanisms that lead to the formation of Au-in-calcrete anomalies.