ASEG Extended Abstracts - ASEG2006 - 18th Geophysical Conference, 2006

The CGI data model working group have established an initial geology data model and XML based exchange language to accommodate geological map data, referred to as GeoSciML. The language is based on prior work carried out at North American, European and Australian geological survey and research organisations. Unified Modelling Language (UML) has been used as a design aid for capturing the geological concepts and their properties. The UML model has then been converted to the GML-conformant GeoSciML.

The design of GeoSCiML meets the short-term goal of accommodating the geoscience information presented on geological maps, as well as being fully extensible to include the full range of geological concepts covered by the geosciences. To demonstrate the ability of GeoSciML to deliver data via web feature services, a small subset has been selected as a testbed. This testbed will deliver lithostratigraphic units, boreholes, faults, contacts and compound materials from different national geological surveys.

The spectral-element method provides an accurate alternative to the finite element method for modelling elastic waves in anisotropie media. However its high computational overheads make it impractical for inversion. With the aim of reducing the overheads, we have implemented 2.5D spectral-element modelling of elastic waves, initially for vertically transversely isotropie (VTI) media. We have incorporated perfectly matched layers to minimise artificial boundary reflections, using the split gradient operator for the time domain and complex coordinate stretching for the frequency domain. Traditional 2D modelling does not reflect the true coupling between wave modes. Our preliminary results, presented as images of seismic waveforms, illustrate the expected coupling. We also present an intuitive explanation of our unique implementation of the spectral-element method.

Rock masses comprise densely packed blocks bounded by joints and other discontinuous geological structures. The strength of a rock mass is controlled by the geometry of the blocks and by the frictional properties of the block surfaces. Empirical engineering methods combine structural geology features into design parameters. There are numerous aspects where geological information and methodology can contribute to advancing engineering investigations of rock mass. Three of these aspects are 1) explicit analysis of the anisotropy of fracture systems, 2) structural analysis of anastomose fracture patterns and 3) application of hydrothermal alteration studies to engineering assessment of joint coatings. Investigations of rock mass anisotropy can be enhanced by recording the orientation of fractures relative to a system of reference axes. Anastomose patterns of fractures can be detected in borehole data by recording the relationship between spacing and dihedral angle of adjacent fractures. The systematic definition of mineral assemblages in hydrothermal alteration studies forms a useful framework for characterising the mechanical properties of joint surfaces.

The massive sulphide deposits of the Devonian-Carhoniferous Iberian pyrite belt (IPB) and the Ordovician Bathurst mining camp (BMC) have much in common: relative to others of this group like those of the Hokuroku basin they are large; have high aspect ratios; low Zn, Pb and Cu grades; high Fe/Cu, Zn/Cu and Pb/Zn ratios; high Sn; low fO2 mineral assemblages; evidence of Ba but scarcity of barite; ill-defined or no zone refining; and abundance of framboidal pyrite, mineral banding (bedding) and syn-depositional breccias. Differences include the vent complexes above stockwork pipes in the BMC, and biogenic S (negative dMS) in the IPB. There is evidence of sea-floor deposition, and fluid inclusion data from stockwork quartz in eight IPB deposits predict that the ore-forming fluids reversed buoyancy on mixing with seawater and ponded, sulphides precipitating immediately in the quenched ore-forming fluids. This manner of deposition explains many of the above ore characteristics while the biogenic S may be due to biostatic evolution. There is no reliable fluid inclusion evidence for the BMC but we suggest that the high degree of commonality between the camps points to brine-pool deposition for the BMC ore deposits. A previously proposed model involving buoyant fluids emerging into anoxic ocean water is vitiated by the evidence that host shales were deposited in oxic ocean water during mineralisation, and the lack of evidence for chimneys and rubble mounds that would be predicted by this hypothesis.

Airborne magnetics, ground gravity, seismic and wireline log geophysical datasets are integrated with outcrop and drill core stratigraphic, structural, geochronology and palaeomagnetic information to derive three forward model profiles for the Palaeo-Mesoproterozoic rocks of the central and northern Lawn Hill Platform of northern Australia. Each profile depicts current basin geometries and sedimentary architecture. Two of the profiles are subsequently reconstructed to depict sediment architecture at 1640 Ma. and 1575Ma., the times of fluid migration at the Grevillea and Walford Creek Prospects and the Century zinc deposit.

Surfaces of chronostratigraphic significance are used to constrain the depositional architecture and bound time-equivalent facies packages. This enables basin scale aquitard and aquifer lithologies to be represented, relationships between these shallow- to deep-burial diagenetic packages and basin scale faults to be depicted and fluid migration scenarios to be tested through numerical mechanical and convective modelling simulations.

The two north-south profiles depict an early (1800-1740 Ma.) Leichhardt Superbasin phase of basin-geometries related to E-W extension. This template controlled subsequent extension during the 1740-1680 Ma. Calvert Superbasin. Rocks of the overlying Isa Superbasin can be split into an upper (1640-1575Ma.) and a lower (1680-1640Ma.) basin phase. Rocks of the lower basin phase include the Gun and Loretta Supersequences characterised by facies relationships that show deeper water facies to the south east and east. Rocks of the overlying River-Doom Supersequences define a southward thickening megawedge whose geometry is controlled by growth on the E-W oriented Little Range Fault.

All komatiitic rocks in the 2.7 Ga Abitibi greenstone belt, Canada contain similar PGE abundances, regardless of age, depth of melting, and/or residue composition. Furthermore, all komatiitic rocks in the Abitibi greenstone belt appear to have been initially sulfide undersaturated and show only local evidence of sulfide segregation, consistent with sulfide saturation during lava emplacement and/or crystallization. Ni-Cu-(PGE) mineralization in the Abitibi greenstone belt has been found only in the two oldest assemblages, the Kidd-Munro and Tisdale assemblages. Only these assemblages contain lava channels or channelized sheet flows and sulfide-bearing sediments. Thus, physical volcanology is more important than magma composition in determining the prospectivity of komatiitic rocks in the Abitibi greenstone belt and elsewhere.

Experimental work on the strength of single phases has been used to infer the strengths of the upper and lower crust and mantle. Dislocation creep in plagioclase is commonly inferred to approximate the rheology of the lower crust. Recent work suggests that diffusion-accommodated grain-boundary sliding in plagioclase may be an important deformation mechanism in naturally deformed lower crust. One way to constrain rheology is through field and petrographic studies of naturally deformed systems, such as the Mt. Hay block in central Australia. These rocks demonstrate strain partitioning from the micro- to macroscale and provide an opportunity to study the relationships between structures of different scales. Field work shows map-scale fabric contrasts (lineation dominant versus equal foliation and lineation) between the geographic domains in the Mt. Hay block. At the outcrop scale, plagioclase-rich domains shape defines both foliation and lineation. Microstructural studies also show the partitioning of strain between lithologic domains. These show that plagioclase has different microstructures in different lithologic layers. This suggests different deformation mechanisms are active depending on plagioclase content, which may control variations in meso- and macroscale fabric. EBSD analyses of plagioclase in these different domains will evaluate the importance of dislocation creep in this context.

In 1995 the SALTMAP airborne electromagnetic (AEM) system was surveyed over an area of around 50,000 hectares west of Broomehill in the southwest of Australia. It was the first practical application of a new AEM system developed specifically for mapping salinity. The project was a cooperative study between farmers in the area, World Geoscience Corporation, Department of Agriculture, Water and Rivers Corporation and the CRC-AMET. Previous AEM surveys in the southwest were surveyed using INPUT and QUESTEM but the results had not been incorporated into land management decisions. The aim of the Broomehill study was to develop the information products to enable farm plans to be created using information from the SALTMAP data. Inherent in the approach was the collection of a large range of ancillary geospatial data using geophysics, remote sensing and other techniques and incorporation of these data into a geographic database. Various techniques were explored to ensure that as much information as possible from the geophysics flowed through to the eventual land management plans. These techniques included manual interpretation of magnetic and radiometrie data and the development of new smart data interpretation in GIS. Intermediate information sets such as ‘Salt Hazard’ and regolith maps guided the development of new farm plans to address land degradation in the area. Ten years later the Broomehill study remains as one of the few cases where farm plans were created based on a full range of geospatial data. This paper will report on the methods used and how they could be improved with new interpretation techniques to define a new set of information that could be used to guide land management decisions today.

Zircon megacrysts feature in basalt fields along the Australian-Asian continental margins and form an Indo-Pacific petrological puzzle. These xenocrysts show magmatic growth features under cathodoluminescence imaging. Representative zircons were studied from several sites along this zone (3 Australian, 2 South East Asian, 1 far eastern Russian).

Zircon geochemistry was characterised by electron microprobe and laser ablation inductively coupled plasma mass spectrometry analysis. Hf/Zr ratios (0.013 –0.022) suggest undersaturated to intermediate parental melts, while U/Th (0.2 – 1.7) suggests both fluids and silicate melts participated. Variations in rare earth element contents (av 74 – 2800ppm) and in chondrite normalised parameters Ce^*, Eu^* and Yb/Sm suggest different chemical fractionations took place both within and between zircon sites.

Geochemical signatures for these zircons, compared with those from known lithologies (statistical trees) suggest lamproitic, granitoid and volatile-rich mixed parents provided the zircons.

Although basaltic products dominate post-5 Ma western Victorian volcanism, a revised picture suggests basaltic activity extended back to 8 Ma and was punctuated by felsic episodes around 8, 6 and 2 Ma. The 8 Ma episode produced rhyolite (8.3 ± 0.1 Ma), zircon-bearing tuff (8.1 – 8.3 ± 0.5 Ma) and derived zircon-bearing sediments (8.1 ± 0.6 Ma). The 6 Ma episode formed the prominent trachytes of the Macedon-Trentham province (5.9 – 6.1 Ma) and provided zircon at depth for subsequent basaltic megacrysts. The 2 Ma episode is the first felsic outcrop identified in the main basaltic sequence and a trachyte plug near Creswick gives congruent K-Ar, Ar-Ar and zircon fission-track ages at 2.4 ± 0.4 Ma. The Creswick plug is a porphyritic, metaluminous potassic-sodic trachyte and each felsic episode shows petrological differences. The youngest felsic event may explain distal bentonite deposits in south-western New South Wales.

Triple episodes of felsic fractionation during western Victorian basaltic activity requires a more complex genetic model for its formation. It raises the question of multiple plume-related mantle melting events.

The Multi-channel Analysis of Surface Wave (MASW) method is a shallow seismic method which may complement the traditional seismic refraction method used in engineering applications. Introducing this newly-developed method, this paper presents a case history of an MASW survey in a tailings dam, where the original topography has been covered with tailings (red mud) from an aluminium refinery over the years.

An eight-line 3.6 kilometre MASW survey was carried out in the tailings dam of Queensland Alumina Limited in Boyne Island near Gladstone, Queensland to estimate the depth of the competent layer for construction of a new tailings dam wall.

The field procedure used was similar to the off-end shooting method used for seismic reflection surveys. This was further simplified by the use of a purpose-built 24-channel land streamer. The seismic source was a weight dropping system mounted on a vehicle, which drops a 50kg steel weight from a height of 0.8 metres. The recording system used was a Seistronix RAS-24 seismograph. The data were analysed and inverted using the SurfSeis software by Kansas Geological Survey.

The depths of the original surface and underlying clay are profiled in terms of their S-wave velocities. The result shows the effectiveness of the MASW method even where velocity reversals are present.

A record of refraction survey data is used by Multichannel Analysis of Surface Wave (MASW) method. An analysis of a seismic record gives a one-dimensional S-wave velocity solution at the centre point of the array. A number of sub-arrays for analysis can be selected from a single record, which will increase the spatial samples along the survey line.

Using sub-arrays immediately raises issues of varying offset and lower number of traces. This study examines these issues and proposes a method to generate a detailed S-wave velocity section using a larger number of analysis points without increasing field effort.

Three time-domain airborne EM surveys were recently flown over a known mineral deposit at Barrow Creek, NT. The Ni/Cu/Ag deposit is located in an area characterised by variable conductive overburden and is considered a difficult exploration target.

Data from the three airborne systems and a moving loop ground EM survey were processed and analysed to compare the relative abilities of the systems to locate the target.

It was found that the fixed-wing systems (TEMPEST and GEOTEM) produced clear responses to the known mineralisation that were consistent with the model developed from the ground EM responses. The EM responses measured with the helicopter system (HoistEM) did not indicate the target.

Using a marine seismic reflection survey as a source for onshore and offshore seismic refraction recorders, high quality refraction data were recorded to a maximum offset of 400 km. A two-dimensional velocity model for Line 19 of Geoscience Australia survey 280 in the Bremer Sub-basin and its onshore extension was derived by forward modelling using iterative ray tracing. Our velocity model indicates the presence of a deep crustal root located under the southern margin of the Yilgarn Craton and the Albany-Fraser Orogen. Comparison of the gravity response derived from this model with the gravity profile extracted from regional grids shows reasonable correlation. However, fine tuning of the seismic model is required to better define the complex geometry of the crustal root. Deep crustal roots are common under Proterozoic mobile belts that surround Archaean cratons. However, these results also indicate the presence of a crustal root under the southern margin of the Yilgarn Craton. This is consistent with the view that Proterozoic taphrogenesis penetrated tens of kilometres into the Yilgarn Craton.

The environmental effects upon the lateral distribution of Early Cretaceous microplankton assemblages and the differentiation of shallow and deeper-water assemblages have not been studied extensively in the Australian region (Helby et al, 1987). For this study, Valanginian to Early Aptian assemblages from the Carnarvon Basin and adjacent oceanic areas were investigated to clarify the nature of environmental controls – particularly bathymetry – on species distribution.

A completely new electro-mechanical marine vibroseis concept is introduced that uses a flextensional shell with a uniquely developed form. Two controllable output resonances achieve a very high efficiency and repeatability, with a typical frequency bandwidth of 6 -100 Hz. The marine vibrator can be used as a standard towed acoustic source in shallow water or as a stationary source in transition zone environments. Of particular significance, the vibrator requires only an electrical power supply, cannot leak hydraulic fluids, is easily transported, and is suitable for applications requiring very low environmental impact.

The present work is dedicated to the study of dehydration and ion pair formation in hydrothermal ion-rich solutions. The objectives are to describe at the molecular scale these processes, and relate them to the macroscopic properties of the solvent water.

Thanks to the combination of x-ray synchrotron techniques (x-ray absorption spectroscopy and small angle x-ray scattering), and to the design of a high-pressure and high-temperature set-up, local order in ZnBr2 aqueous solutions is studied. The evolution of ion pair formation (between Zn²⁺ and Br^- ions) with temperature, pressure and ion concentration is shown. Furthermore, the influence of density fluctuations (characteristic of supercritical conditions) on this ion interaction is explained.

This work is a good candidate to show the efficiency of synchrotron measurements as a complement to other spectroscopies, especially in the case of hydrothermal solutions where the experimental conditions are challenging.

Phase relations between mineral assemblages in common sulfide ore deposit types are examined to place constraints on the amount of polymetallic melt produced within the limits of crustal metamorphism. Most sulfide deposit types have the potential to undergo minor melting at moderate temperatures, provided that sulfosalts or tellurides are present. Deposits that contain these phases may start to melt at conditions ranging from lowest greenschist facies to amphibolite facies, whereas those lacking sulfosalts and/or tellurides may only melt once P-T conditions are well into the granulite facies, particularly if galena is also absent. Epithermal gold deposits would melt at the lowest temperatures of all sulfide deposit types. Massive Pb-Zn(-Cu) deposits may start to melt in the lower to middle amphibolite facies, but only if pyrite and arsenopyrite coexist at these conditions. Whereas massive Ni-Cu-PGE deposits can typically generate only trace quantities of melt. Similarly, disseminated Cu deposits, excepting sulfosalt-bearing occurrences, are incapable of generating melt until the granulite facies when melting commences in bornite-rich regions, in the system Cu-Fe-S. Even at granulite facies conditions, the amount of polymetallic melt that can be generated in most deposit types is <1 vol.%. The exception is massive Pb-Zn(-Cu) deposits, where melt volumes significantly exceeding 1 vol.% may be segregated into sulfide magma dykes, allowing mobilization over large distances.