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

Imaging of groundwater that interacts with surface watercourses is essential for providing detail needed to accurately manage both resources. It is particularly important where one resource is saline or otherwise polluted, where spatial quantification of the interacting resources is critical to water use planning and where losses from surface waterways need to be minimized in order to transport water long distances. Geo-electric arrays or transient electromagnetic devices can be towed along watercourses to image electrical conductivity (EC) at multiple depths within and beneath those watercourses. It has been found that in such environments, EC is typically related primarily to groundwater salinity and secondarily to clay content. Submerged geo-electric arrays can detect detailed canal-bottom variations if correctly designed. Floating arrays pass obstacles easily and are good for surveying constricted rivers and canals. Transient electromagnetic devices detect saline features clearly but have inferior ability to detect fine changes just below beds of watercourses. All require that water depth be measured by sonar or pressure sensors for successful elimination of effects of the water layer on the data. Presentation of the data using a 3D presentation technique where EC is imaged along vertical ribbons drawn along the watercourses is almost essential for handling the data produced because the meandering paths of rivers and canals combined with the shear volume of data typically acquired results in a geo-referencing dilemma that cannot be accommodated using traditional presentation techniques.

An extensive set of EC Imaging case studies, distributed across canals and rivers of the Australian Murray-Darling Basin, has been collected. They reveal the interaction of various rivers and canals with the underlying groundwater resources. At some sites, watercourses cross prior river channel sands that are being recharged and are suitable for use as water storages with low evaporation losses. Geophysical imagery can be used to optimize recharge and design of such underground storages. At some other sites, little connection between aquifers and surface watercourses is evident. Finally, at downstream ends of geological basins, sites where saline groundwater flows into, or is on the verge of flowing into rivers are evident.

Recent research voyages using the Australian Marine National Facility, have discovered one of the world’s most youthful, magmatically-active backarc basins: the Coriolis Troughs. In detail, the localised structural consequences of the general clockwise rotation and extension inferred for the basement block of the southern New Hebrides Arc has been clearly revealed by 30 kHz multi-beam, sonar swath mapping of the Coriolis Troughs, and major submerged volcanoes in the southern part of the Arc. A left-lateral, transtensional faulting regime predominates both within the Troughs and the submarine Gemini-Oscostar (20° 59’S, 170° 17’E) and Volsmar (21° 30’S, 170° 11 ’E) volcanic complexes.

Gravity and GPS surveys were conducted on the Windmill Islands, East Antarctica, during the 2004/05 summer season. The aims of the study were to investigate the subsurface geology of the area and to deduce the changes in gravity and rock surface heights since the first surveys of the area were conducted almost five decades earlier, caused by glacial isostatic readjustment. The area became ice free following the end of the Last Glacial Maximum (LGM) 5,500 to 8,000 years ago and is thought to contain signals as to the state of nearby Law Dome.

G-model Lacoste and Romberg gravity meters were used to make precise gravity measurements, std +/- 0.04 milligal. Two calibrated hand held single frequency Garmin GPS receivers were used to determine position including elevation, the standard deviation of the elevation being 2.7 m. Ninety three gravity stations were established. Additionally, forty three observations from a survey in 1993-94 were re-reduced and merged with the 2004-05 data to allow a complex three-dimensional subsurface model to be constructed.

The Windmill Islands are at the rock/ice interface and thus this work has given an insight into the likely underlying bedrock structures and variability below Law Dome.

This survey has set the foundation for future time-series gravity work in the Windmill Islands.

Mafic igneous rocks in the Georgetown Inlier of north Queensland are found within the Palaeoproterozoic Etheridge Group, and comprise the extrusive Dead Horse Metabasalt and intrusive Cobbold Metadolerite. This study presents new major, trace and rare earth element data for the mafic rocks, and compares them to existing data from similarly aged mafic rocks of other Palaeoproterozoic terranes in eastern Australia. Based on the geochemical and limited geochronological evidence, both suites of mafic rocks are relatively evolved low-K tholeiites with strong Fe-enrichment trends (Fe203(t) 6.81-21.11 wt%), and were emplaced between ca. 1700 and 1650 Ma. Despite pervasive metamorphism and recrystallization, the rocks appear geochemically similar to basic gneisses of the Broken Hill Block and mafic rocks of the Mt Isa Eastern Succession, particularly the Toole Creek Volcanics. This relationship is of regional significance, as it may point to a tectonic relationship between the terranes, and hence help place the Georgetown Inlier within the Proterozoic framework of northern Australia.

Two 2D anisotropie prestack depth migration algorithms that use wavefield extrapolation are evaluated for tilted transversely isotropie media (TTI).

The first method is anisotropie phase-shift-plus-interpolation (A-PSPI) that is based on an analytical solution of frequency-dispersion enabling the use of an arbitrary distribution of velocities and anisotropie parameters. The second method is anisotropie reverse-time (A-RT) that uses the full wave equation.

We evaluate the accuracy and efficiency of there algorithms. Examples are presented that use numerical and physically modelled data along with a structurally complex real data example. Both A-PSPI and A-RT have excellent performance with A-RT maintaining the advantage of using the full wave equation.

Traveltime computations are an integral part of modelling and imaging seismic data by providing efficient kinematic information on the location of propagated energy. The traveltimes may be computed analytically using simplifying assumptions, or may be estimated on a complex geological structure using raytracing or gridded traveltimes.

A basic requirement for the propagation of gridded traveltimes is the estimation of one point on a corner of a square, given the traveltimes on the other three corners. A number of solutions are available to solve for the unknown time and are based on either a plane-wave assumption, a finite difference solution to the Eikonal equation, or an assumption that the wavefront at the square is curved. A solution for a curved wavefront assumption requires estimating the center of curvature, and requires solving a quartic equation. An alternate method is presented to estimate the center of curvature for a curved wavefront that uses an iterative procedure and does not require solving the quartic equation.

This paper uses large sets of whole-rock geochemical data to make comparisons between the komatiitic rocks of Abitibi (AGB) and Norseman-Wiluna (NWGB) greenstone terranes.

The NWGB komatiite suite has a much higher proportion of highly olivine-enriched cumulates than the AGB suite, as indicated by data-density distributions on plots of MgO vs FeO and MgO vs Cr, although average compositions of the komatiite magmas in the two belts were not significantly different. NWGB komatiites appear generally more contaminated, on the basis of various ratios of strongly to moderately incompatible low-mobility trace elements.

Both factors which are likely contributors to the much higher Ni sulfide resource endowment of the NWGB. The combination of high degrees of contamination and presence of olivine adcumulates in the NWGB attests to the presence of exceptionally high-intensity, prolonged eruptions, capable of forming long-lived entrenched magma pathways, represented by highly olivine-enriched cumulates, and capable of melting substrates to form orebodies. This is in contrast with more episodic, lower volume eruptions in the AGB. The contrast is interpreted as the result of crustal structure and tectonic setting, rather then the size and intensity of mantle plume sources.

The dramatic changes in the landscape since European settlement have radically altered regional water balances in semi-arid areas, which have not only resulted in land salinisation, but have and will cause salinisation of aquifers over the next 100 or so years in semi-arid areas of the Murray Basin. This process is triggered by two main processes - land clearing and extractions for irrigation.

The small amount of salt in rainfall has been concentrated by native mallee vegetation over thousands of years and is stored in the root zone, with salinities approaching that of sea water. Clearance of this native vegetation for dryland cropping has not only increased recharge, but has initiated the flushing of this salt down to underlying aquifers which often contain good quality groundwater.

Irrigation extractions from shallow permeable aquifers have led to the recirculation and concentration of salt.

These salinisation processes require a new interpretation of what sustainability means for groundwater development.

In the last 100 years about two million (mostly eclogitic) diamonds were mined from Tertiary alluvial deposits at Bingara-Copeton, in the low metamorphic grade Phanerozoic New England Fold Belt, NSW. Even today their hard rock source remains unknown, so we targeted several basanitic igneous bodies to the south west of Bingara, and recovered more than 3000 garnets and five diamonds from samples of modern stream sediment and loam.

Chemical classification of the garnets identified some from diamond eclogite and the Cr-poor megacryst suite indicating a mantle source, and formation near or within the diamond stability field. We defined an additional subgroup of UHP-crustal garnets with inherited continental crustal chemistry and unusual exsolution microstructures. These are similar to garnets from schist/orthogneiss, formed from deeply (>200km) subducted leucocratic continental crust, exhumed in the UHP terrane of Dabie Shan, China. They also compare with rare garnet inclusions in diamonds.

Exsolution microstructures in garnets (world wide) are rare. They indicate a chemical change in UHP garnet due to exhumation of slab material and re-equilibration under lower pressure conditions. Such decompressed garnets from Bingara provide evidence for partial exhumation of deeply subducted diamond eclogite-grade oceanic crust and UHP-metamorphic continental crust to much shallower levels.

This study predicts a buried diamond-bearing UHP terrane in the northern New England district, and a two-stage delivery of subduction diamonds and garnets to the surface; firstly by slab exhumation to relatively high levels, then by entrainment in shallow-sourced basanitic magma. Deep-sourced kimberlites and lamproites therefore are not necessary for diamond delivery here, and are unknown from eastern Australia.

Laser Raman spectroscopy is used to identify mineral inclusions in situ within six Copeton diamonds and to determine the remnant internal pressure on them, namely 31-34 kbar for coesite, 13.6 and 23 kbar for diopsidic omphacite, and 8 kbar for grossular garnet. Using a published linear model of the host-inclusion volume system, each remnant pressure value generates a PT locus of diamond formation. The intersections of these define a range of diamond formation conditions from 250 ΰC, 43 kbar to 950 ΰC, 58 kbar with estimated intersection errors of α 70 ΰC and α 4 kbar. This range encompasses at least two sets of ultrahigh pressure (UHP) formation conditions derived from different protoliths involved with the termination of subduction, namely subducted oceanic slab (lower temperatures: eclogitic diamonds) and subducted continental rocks (higher temperatures: calcsilicate diamonds). Coesite has been found as a compound inclusion with omphacite (lower temperature set) and with grossular (higher temperature set). The measurements indicate that the Copeton stones are UHP diamonds (not cratonic), and their Carboniferous argon age dates on pyroxene inclusions should be interpreted as ages of crystallisation. The results and implications are consistent with a local source for Copeton diamonds, and imply there is a buried Carboniferous UHP terrane within eastern Australia.

Pb-Pb step-leaching (PbSL) is a digestion technique based on sequential acid treatment of a mineral resulting in the selective recovery of radiogenic and common Pb components from the crystal lattice, making single-phase Pb-Pb dating possible. Developed originally by R. Frei and colleagues at Bern, the technique has had mixed success. Initial studies produced reliable dates for silicates and mixed sulphides, however there is some conflict surrounding the validity of the PbSL isochrons, with some authors claiming that isochrons might be the product of initial Pb heterogeneity and/or Pb mixing due to post crystallisation U or Pb introduction.

This study builds on previous observations and tests the reliability of PbSL in directly dating ore minerals such as sulphides (chalcopyrite, pyrite, pyrrhotite/pentlandite) and oxides (magnetite), in an attempt to obtain the timing of mineralisation from a phase that is unambiguously linked to the ore-forming process. This is based on the often surprisingly high U contents (relative to what might be anticipated), yielding rather higher than expected initial U/Pb ratios in several types of low-Pb sulphides. The technique is applied to well-constrained samples from a number of predominately Australian mineral deposits covering a range of mineralization types.

Recent drilling in Voisey’s Bay has uncovered significant wallrock mineralization in what was before thought to be a sub-economic occurrence of sulphides within the Reid Brook Dyke. Although technically disseminated, the sulphides within the dyke are electrically well connected and can act as a single conductor of kilometres in size and with a conductance of thousands of Siemens. Using the standard transmitting frequency of the time, we were unable to discriminate the conductance of the disseminated from the massive sulphides. In 2003 we revisited the area with BHEM transmitting at 1Hz with more success. From the low frequency data, we saw evidence for the first time of conductive features that did not coincide with the dyke orientation.

In 2004 we began to utilize the superposition of two data sets from two separate transmitting loops to adjust the angle of the inducing field and null couple to these in-dyke disseminated sulphides. In doing so, we obtained greater precision on the orientations of the conductors, and were able to discriminate between in-dyke conductivity, and wall rock related conductivity. We are now integrating this method with other structural techniques to further develop the deposit model.

This paper attempts to highlight the fundamental role played by tectonics and deformation in the formation and subsequent modification of many komatiite associated nickel sulphide deposits in the Eastern Goldfields province of the Archaean Yilgarn Craton of Western Australia. The influence of tectonics and deformation is manifest at a variety of scales, ranging from hand specimen to craton scale, and has a real impact on how these deposits are targeted, explored, delineated and mined. The influence of tectonics and deformation is as fundamental to the genesis of these deposits as are the well documented magmatic and volcanic processes generally considered to be crucial to their formation.

Post emplacement tectonism has led to the deformation, and in many cases, substantial modification of these essentially magmatic deposits. This ranges from centimetric scale remobilisation as a consequence of metamorphic recrystallisation, through the localised predominantly mechanical remobilisation of sulphides during brittle-ductile deformation of the rock mass, to partial or complete decoupling of sulphides from their parent ultramafics over distances of up to 800 metres in brittle-ductile shear zones and even nappes. Recognition of such modifying processes and consequent three dimensional architecture is essential for successful exploration, orebody modelling and mining of these deposits.

Furthermore, regional tectonic processes may also exert a considerable fundamental influence on the primary emplacement and localisation of komatiites and associated nickel sulphide mineralisation. Many of the thick komatiite sequences and major nickel sulphide accumulations of the Eastern Goldfields province are proximal to cratonic scale NNW trending shear zones which are generally accepted to represent D2 reverse and strike slip fault systems formed during the closure of ensialic greenstone basins. It is proposed that some of these structures represent inverted early extensional faults and that the ultramafics were extruded and emplaced as local volcanic centres adjacent to these basin bounding faults in rift basins and half grabens. If valid, this has important implications for the targeting of, and exploration for komatiite associated nickel sulphide deposits in the Archaean Yilgarn Craton.

This paper explores these ideas by specifically highlighting key geological features of the Emily Ann, Maggie Hays, Waterloo and Honeymoon Well nickel sulphide deposits together with general observations from other deposits within the large scale regional geological context and four dimensional structural framework of the Eastern Goldfields province as a whole.

GeoScience Victoria (GSV) is introducing a new service delivery model to improve capture, storage and delivery of geoscience information. This major project has prompted significant effort to establish a data model with accompanying vocabularies to govern what data to store in the model, and how to arrange and group that data.

The principles behind the vocabulary are that it must be robust, flexible, expandable and hierarchical. The vocabulary, like the data model itself, must satisfy both internal business requirements and international efforts with respect to information exchange in a global environment. There are three components to the vocabulary: the terms themselves with their definitions/synonyms; arranging the terms into schemes; and the grouping of those schemes to provide meaningful information (handled by the data model). Our solution manages the vocabulary in an open source technology, with terms (and synonyms) arranged in hierarchies.

The process demonstrated that while computers may require rigid terminology from us, in some cases this is a real challenge. As an evolving science, many areas of geology have terms that mean different things to different people, with no single correct meaning.

Common vocabularies are essential to the exchange of geoscience information at a national or international level, and GSV strongly encourages their development.

The White Hills Gravel is a widespread gold-bearing palaeoplacer in western and north-central Victoria. Its age is unconstrained by palaeontology but has been considered as Late Cretaceous to early Tertiary. Using coarse (> 1mm) concentrates of heavy minerals from an occurrence of the WHG near St Arnaud, we have obtained evidence for the age and provenance of the formation. U-Pb SHRIMP dating of two populations of zircons gave ages of 74.5 and 68.2 Ma, indicating a maximum age of latest Cretaceous for the WHG. The zircons are believed to have been sourced from basaltic volcaniclastic deposits since removed by erosion. These also provided coloured sapphires, spinel and possibly the sole diamond recovered from the concentrates. Such an assemblage is characteristic of eastern Australian gemfields associated with Cainozoic basaltic lavas. Other minerals in the concentrates include schorl, rutile, andalusite, anatase and metamorphic corundum. U-Pb LA-ICPMS dating of rutile gave an age of 393 Ma, within the age range of Early Devonian granites in the region, which have intruded Cambro-Ordovician metasediments (St Arnaud Group). Sources within contact metamorphosed pelitic metasediments and associated pegmatites are indicated for the rutile, and probably for schorl, andalusite and anatase. Corundum-bearing rocks are unknown in the region, so these may have a more distal and older source. Investigation of heavy mineral concentrates from other occurrences of the White Hills Gravel may enable regional correlation of sources and provide more evidence for particular depositional models

New insights into the crustal structure and evolution of the Mt Isa Eastern Succession have come from constructing a crustal scale 3D model. A revised chronostratigraphy, detailed aeromagnetic data, multiscale wavelet edges of potential field data, and reprocessing and reinterpretation of seismic data suggest hitherto unsuspected links between upper and lower crustal structures.

Three major approximately NS structures are interpreted to underlie the Eastern Succession, respectively along the edge of the Wonga belt, under the Mitakoodi anticline and at the western margin of the contiguous Soldiers Cap outcrop. Major contrasts in potential field data occur along these structures; they also correspond to major changes in stratigraphy. They are interpreted to have originated as extensional faults during basin formation, which were positively inverted during the Isan orogeny from 1.6 to 1.5 Ga.

Inversion apparently involved both the cover sequences and basement rocks, although net extension is preserved on some parts of the major structures. Reactivation of these structures in the ban orogeny may explain some of the complexity of structure within the Eastern Succession, which has hitherto been interpreted as evidence for multiple overprinting deformation. Furthermore, the localisation of contractional features over extensional structures suggests that the Eastern Succession is parautochthonous. The two largest mineral deposits in the Eastern Succession (Cannington and Ernest Henry) are proximal to the easternmost major crustal structure, suggesting the importance of these structures to mineralization.

In seismic reflection tomography, the velocity model of the subsurface is updated by back-projecting travel-time residuals along ray-paths. The travel-time residuals are picked from the seismic data itself and the methodology used to gather these picks is a fundamental part of any velocity inversion workflow. In particular, the density at which the residuals are represented in the four-dimensional data space (inline, crossline, offset and depth/time) appears to have a significant effect on the precision of the velocity updates that are output from the tomographic inversion. Utilising dense, hyperbolic- (or parabolic-) fitting means that the residuals are finely sampled in the data space but does not necessarily represent their true values with great accuracy. Dense, non-hyperbolic fitting offers a similarly fine sampling but with greater adherence to the true residual value. These two methodologies have been compared and contrasted on a complex synthetic dataset. It can be seen from this comparison that the dense, non-hyperbolic tomography offers greater potential for resolving small-scale velocity heterogeneities in the Earth.

A “holistic” method for simultaneously estimating conductivity and calibration models from 1-D inversion of time-domain airborne electromagnetic (AEM) data is proposed. The work extends the concept of holistic inversion that been successfully applied to frequency-domain AEM data. The entire multi-component airborne dataset and available independent conductivity and interface-depth data are simultaneously inverted. A spline-based conductivity model covering the complete survey area is estimated. Unmonitored elements of the system geometry are included as unknown parameters of the calibration model and are solved for in the inversion.

Conventional 1-D inversion methods invert each airborne sample in isolation from other samples. However, by simultaneously considering all of the available information together in a holistic inversion formulation, we are able to exploit the inter-component and spatial coherency characteristics of the airborne data. The formulation ensures that the conductivity and calibration models are optimal with respect to the airborne data and prior information.

There is a well documented relationship between Au mineralized quartz and CO2 rich fluid inclusions within that quartz, indicating the CO2 rich nature of the parent hydrothermal fluids. This relationship has been suggested to be a function of CO2 buffering the Ph of the geothermal fluids in a range which enhances gold solubility. It is also known that the acoustic decrepitation method can identify quartz formed from fluids which had high CO2 contents, as CO2 rich fluid inclusions decrepitate upon heating at temperatures well below that of aqueous inclusions trapped under the same pressure and temperature conditions. CO2 contents at least as low as 5 mole % are readily detectable using the acoustic decrepitation method.

The decrepitation method has been applied to a number of gold deposits in Victoria and shows that CO2 is a common but not ubiquitous constituent of the fluid systems which deposited the quartz and gold. Although the precise relationship between CO2 and gold is not clear, the method provides a means to “fingerprint” quartz veins as an exploration guide. Samples from the Meguma terrane in Nova Scotia also show widespread but variable levels of CO2 rich fluids in the gold deposits.

Knowledge of the CO2 contents of quartz systems is a valuable exploration tool and the acoustic decrepitation method is the easiest way to acquire this information for exploration purposes as it is an automated and quick instrumental procedure.