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

Some of the minerals formed during weathering are amenable to radiometrie dating and may be used to establish chronologies of weathering, which in turn relate to climate and the history of water in the landscape. Here we explore the potential of secondary U phosphates from the weathering zone of the Lake Boga Granite, northern Victoria, for U-Th dating and construction of a robust weathering history over the last ≈500 ky. The ages obtained are in the interval 115-560 ky, with some samples having ²³⁰Th-²³⁴U systematics beyond the range of the method. In principle, U-phosphate deposition is expected to occur under humid conditions, raising the possibility that U-phosphate ages may provide independent time-scales for the pluvial periods represented by Australia’s Quaternary megalakes. The data available now provide some support for a link with pluvials but a number of issues, largely related to the exact significance of the measured U-Th ages, need to be sorted out first before we can be confident about secondary U-phosphate chronologies.

The Harts Range Metamorphic Complex (HRMC) in the eastern Arunta Province is part of the exhumed core of the intracratonic Alice Springs Orogen. SHRIMP U-Pb dating of intrusive rocks and metamorphism in the HRMC has constrained the timing and character of tectonism, showing that mutliphase deformation under high grade metamorphic conditions spanned at least 50 m.y. The age of a syn-tectonic pegmatite dyke in the northern HRMC records ductile shearing on E-W trending shear zones during or just after the ~380 Ma Pertnjara Movement, possibly related to extension after the main contractional phase. The age of a second syn-to late-deformational pegmatite dyke in the eastern Harts Range suggests that a SW-vergent fold and thrust system in the eastern HRMC formed during the ~360 Ma Brewer Movement. This deformation coincided with the intrusion of small granitic bodies in the northern HRMC, and the formation of rare metamorphic zircon in metasedimentary rocks during a period of crustal thickening. Metamorphic monazite and zircon reflect high-grade metamorphism during the ~330 Ma Mount Eclipse Movement, which coincided with the formation of a flat-lying, kyanite grade foliation in basement rocks of the HRMC. This foliation and later large-scale doming might reflect extensional collapse of the Alice Springs Orogen towards the end of the orogenic cycle. ⁴⁰Ar-³⁹Ar cooling ages indicate that much of the HRMC was exhumed at that time. Localisation of ASO tectonism in the eastern Arunta Province appears to be a result of thermal weakening associated with the ~480-460 Ma Larapinta Event, which partitioned plate boundary stresses into central Australia.

Prestack seismic data provides additional information over poststack seismic data for determining fluid content and reservoir quality. Rapid access to prestack seismic data permits the interpreter to understand whether seismic amplitude changes are related to fluid effects or to seismic processing artifacts such as multiples or poor stacking. Workflows that use prestack data have long been the domain of specialists. Extending prestack data access to complete workflows for AVO analysis and fluid substitution present a challenge for the average interpreter. A ‘wizard’ interface has been designed that allows specialists to provide a template for more effective resolution of these specialist processes by the layman within any asset team. Collaborative workflow design and testing with three major oil companies has demonstrated the potential for interpreters to fully utilize prestack seismic data within the conventional poststack seismic interpretation workflow.

The Ogasawara Plateau is part of the Marcus-Neeker Ridge. This plateau impinges upon the trench of the Izu-Bonin-Mariana subduction zone, which is located in the northwest Pacific. The effect of The Ogasawara Plateau on the subducted slab hi this region was investigated using seismic tomography as well as 3D tomographic and numerical models. A region of anomalously low seismic velocity detected within the subducted slab beneath the southern Izu-Bonin arc, to the northwest of the impinging Ogasawara Plateau, coincides with a cluster of earthquake hypocentres. Preliminary 3D unite element models of a plateau of Varying density embedded in a rolling back subducting slab indicate that this anomalous region could represent a density variation within the slab such as a subducted portion of the plateau, or a region of higher than average strain rate induced by resistance of a portion of the plateau to subduct.

Graphitic and iron-rich rocks commonly are considered to be efficient sites for deposition of gold in lode-gold systems, and are given special attention in exploration programs. In this work, thermodynamic modelling methods are used to clarify the mechanisms and efficiency of deposition of native gold in graphite- and iron-bearing wallrocks of lode-gold deposits under greenschist facies conditions. In all of the models, a fully equilibrated “lode-gold-type” mineralising fluid is initially generated by reaction of a simplified CO2-H2S-NaCl-Au-bearing fluid with granodiorite at middle greenschist facies conditions (P=300MPa, T=420°C), followed by reaction of the fluid with a specified rock composition at lower greenschist facies conditions (P=200MPa, T=320°C) reflecting uprise of the mineralising fluid to a higher crostai level. Effects of variable graphite and iron on gold deposition are evaluated by calculating mineral alteration assemblages (including native gold) where graphite and iron are allowed to vary across specified ranges, and effects of increasing amounts of mineralising fluid are evaluated by allowing fluid/rock (F/R) ratio to range widely. The results reveal that no gold is deposited where graphite and iron are absent from the wallrock, only a small amount of graphite is required in a graphite-bearing rock to achieve maximum deposition of native gold, only a moderate amount of Fe is required in an Fe-bearing rock to achieve maximum deposition of native gold, and Fe-bearing rocks deposit gold much more efficiently than graphite-bearing rocks. These results confirm the known importance of redox conditions for deposition of native gold, but elevate the importance of sulphidation reactions over graphite oxidation reactions in efficiency of gold deposition from lode-gold mineralising fluids.

The Manukau Subgroup, New Zealand comprises a Miocene submarine volcaniclastic apron consisting of the products eroded and erupted from a large offshore partially emergent basaltic to andesitic volcanic complex. The submarine volcaniclastic apron is well exposed for over 30 km along the coast. The apron records the depositional history of a marine basin at bathyal water depths and the onset of fan progradation due to volcanism. The exceptional exposure, variation in fan architecture (distal, lobe, upper slope and channel), facies (e.g. sandstone, pumice breccias, conglomerates and pillow lavas) and origin (primary, resedimented or epiclastic), allows analysis of a fades model for submarine volcaniclastic fans. The fades analysis shows that submarine volcaniclastic fans differ from other clastic sedimentary fan models in the origin, supply and types of clasts.

Eucairo Rhyolite is one of three voluminous (>675 km³) units in the upper part of the Gawler Range Volcanics. Eight stratigraphic sections through the Eucarro Rhyolite were sampled for anisotropy of magnetic susceptibility (AMS). The unit has AMS characteristics similar to those of lavas. AMS foliations (Kmax/Kint plane) consistently strike northwest and are very steeply dipping. Seven sections show a change from steep southwesterly dips near the base to vertical or northeasterly dips in the middle and near the top. Furthermore, the AMS pattern implies outflow from a source located to the southwest of present outcrops. In the remaining section, the AMS pattern implies outflow from the northeast. The opposing flow directions may be delineating the boundary between adjacent lava flow lobes. This study suggests that felsic lavas can have dimensions comparable with those of mafic lavas.

Elemental (TOC, C/N), isotopic (δ¹³Corg) and molecular (¹³C-NMR) analysis of sapropels within the Holocene carbonate mudstone successions of three coastal lakes of the Coorong region, South Australia, indicates that they are composed primarily of aquatic organic matter derived from algae and other photoautotrophs that utilised HCO3^- as their dissolved inorganic carbon source. Isotopie analysis of sedimentary carbonate (δ¹³Cinorg, δ¹⁸O) suggests that this lacustrine organic matter was subject to post-depositional methanogenesis, that lake water salinity has increased during the late Holocene and that sapropel deposition in the southernmost two of these lakes was not restricted to times of wetter environmental conditions.

Dredge samples from the Three Kings Ridge between New Caledonia and New Zealand revealed the presence of an old collisional orogen. The samples included 38 Ma high-P metamorphic rocks, mantle peridotite, probably from the forearc of an island arc, and continental-derived rocks with old Gondwana-derived zircons and fossil leaves. The samples suggest that a large island existed to the east of the Three Kings Ridge between 38 Ma and 21 Ma, when extension associated with the opening of the South Fiji basin caused the island to subside below sea level.

A suite of low-Ti Silurian to Early Devonian basaltic rock and plagiogranite in the Tumut area have previously been interpreted as back-arc tholeiitic basalt and ophiolitic rocks formed in an oceanic environment. Re-examination of these rocks and their geological contacts show that they closely resemble low-Ti continental rift tholeiites and their felsic shallow intrusive differentiation products. The mafic rocks have strong similarities to the low-Ti Jurrassic break up dolerites in Tasmania.

The basalts and intrusive rocks were probably derived from the sub-continental mantle during Silurian-Devonian rifting well after the initial emplacement of the ophiolitic rocks in the Tumut area.

⁴⁰Ar/³⁹Ar dating of fine-grained white micas from gold deposits, formed under low-grade metamorphic conditions, is often complicated by multiple mica populations, recoil loss/redistribution of ³⁹ArK and loss of ⁴⁰Ar. Recent studies have suggested that pyrite armours enclosed mica inclusions against argon loss and, therefore, may provide improved age information in low grade terranes.

This premise is tested by applying the ⁴⁰Ar/³⁹Ar pyrite dating method to three gold deposits in Victoria, namely Stawell, Fosterville and Walhalla. These analyses indicate that, although pyrite behaves as a partially closed system, the method is able to ‘see through’ younger hydrothermal overprints. At the same time, ⁴⁰Ar/³⁹Ar analyses of pyrite from the Fosterville gold mine failed to constrain the timing of gold deposition due to the encapsulation of detritai mica grains by the pyrite. These results emphasize the importance of petrological control on sample selection and combining ⁴⁰Ar/³⁹Ar analyses of pyrite and matrix mica.

Public domain total magnetic intensity (TMI) airborne data covering the Australian continent have been collated into a new database of grids. The cell resolution of each grid is optimal with regard to the original survey flight-line spacing. Data for all the grids have been matched in one inverse operation by using the statistics of data differences in the grid boundary overlap regions. Quality control and long wavelength accuracy utilised the independently acquired AWAGS long traverses.

A variety of products useful for interpretation, from continental-scale composites to small selected areas of data, can easily be generated from the database. Example processes include reduction to the pole (RTP), upward continuation, horizontal and vertical derivatives, pscudogravity, the analytic signal and multiscalc edges.

The production of a high-resolution RTP grid of Australia is described, using a super-computer facility, and the calculation of multi-scale edges from the grid.

Approximately 75% of Australia is covered by public-domain, airborne gamma-ray spectrometric surveys. However, all the older surveys are in units of c/s and their data values depend on the survey instrumentation and acquisition parameters. Also, many of the newer surveys were inadequately calibrated with the result that data values on adjacent surveys are not necessarily comparable. This limits the usefulness of these data.

Geoscience Australia and State Geological Surveys are working towards establishing a national baseline database of Australian gamma-ray spectrometric data that is consistent with the global radioelement baseline established by the International Atomic Energy Agency. This will be achieved by: (a) ensuring consistency in the calibration and processing of new gamma-ray spectrometric data through the use of standard processing procedures and calibration facilities that are tied to the global datum, and; (b) adjusting older surveys to the global datum through back-calibration and automatic grid merging.

Surveys that are registered to the same datum are easily merged into regional compilations which facilitate the recognition and interpretation of broad-scale regional features, and allow lessons learnt in one area to be more easily applied to other areas.

A model for the environmental and climatic evolution of the western Murray Basin during the Late Cainozoic can be determined from analysis of the sedimentary record. Stratigraphic measurements of cliff-sections along the Murray River gorge tract suggest a complex interaction between coastal and estuarine environments during the Pliocene. Predominantly estuarine conditions were restricted to the north-western part of the Murray Basin, centred in the vicinity of Morgan. Further east, more open-marine, coastal environments prevailed leading to deposition of the vast Loxton-Parilla Sands strandplain succession. This interaction between active coastal sand/dune systems and more restricted estuarine conditions maybe analogous to the modern setting of Lake Alexandrina and Lake Albert, at the mouth of the Murray River, and the barrier sands/dunes of the Sir Richard and Younghusband Peninsulas, on the Mt. Gambier coastal plain. Analysis of strontium isotope ratios suggest that estuarine conditions in the western Murray Basin originated in the Early Pliocene and continued until well into the Late Pliocene.

Since it was first recognised that all the Pre-Palaeozoic rocks in Queensland lie west of 140°E, there has heen considerable debate about the location of the edge in eastern Australia of what later became known as Rodinia. This paper suggests the Lawloit Fault as the present-day eastern edge in southeastern Australia. The Lawloit Fault was recognised in magnetic interpretations as separating rocks with different structural styles, with the older rocks on the western (Rodinian) side. It is interpreted as the boundary between the extensional structures of the Stansbury Basin and the collisional structures of the early Tasman Fold Belt and where the magnetic rocks of the Dimboola Subzone have indented the Rodinian edge. This boundary also fits well with other, independent, data sets including the presence of rift tholeiites in the west as opposed to oceanic basalt in the east, unusual granite chemistry along its southern continuation and the 3-D shear wave seismic modelling. If the Lawloit Fault is the edge of Rodinia, that edge can be traced over at least 300 km in western Victoria.

IC and b spacing values of white micas in slates from the Tabberabbera Zone have been used to assess the P and T conditions of metamorphism. IC values range from 0.19 to 0.34, with upper anchizone conditions (T = 250–300° C) over much of the zone, and a belt of epizone rocks (T > 300° C) along the eastern edge of the zone. Average b spacing is 9.023 ± 0.006 ¬, with no discernible trends across the zone. This value is typical of medium-P metamorphic belts, with a geothermal gradient of about 30° C/km. For a temperature of 300° C, this equates to a pressure of ~0.3 GPa, and a depth of about 10 km, the same as migmatites of the Omeo Zone, adjacent to the east. The medium-P metamorphism is difficult to fit to an accretionary prism origin for the zone, unless an active spreading ridge was subducted under it.

Zealandia is a mainly submerged continental block, one third the size of Australia. Before 85-55 Ma Tasman Sea spreading, orogenic trends in Queensland, New South Wales, Victoria and Tasmania continued along what is now the Lord Howe Rise towards their counterparts in New Zealand.

Geologically credible comparisons between selected trans-Tasman and SW Pacific geological provinces and units, especially those that contain significant mineralisation in Australia, can help provide a useful exploration framework for New Zealand.

Additional geological events that have affected Zealandia’s regional mineral prospectivity include more widespread 125-85 Ma magmatism and extensional exhumation, superposition of Neogene volcanic arcs in the North Island, and localised Neogene exhumation in the South Island.

The paper describes application of litho geochemical surveys for petroleum and gas prospecting within direct detection of hydrocarbons escaping from subsurface accumulations and source beds – most common technique.

The geochemical data came from shallow probes soil samples and shallow probes of soil gas. The adsorbed soil gases were analyzed for methane, ethane, ethylene, propane, propylene, iso-butane and normal butane by gas chromatography using a Flame Ionization detector. Samples of soil were analyzed for number of elements using spectral analysis.

Direct relationships between distribution of hydrocarbons and certain elements in the vicinity of oil and gas accumulations were discovered. Those elements form halos marking structures perspective for oil and gas (Ni, Co, Cr, Sc, Mo, Li, B).

Presence of positive correlation between hydrocarbon halos and active elements anomalies lends to the deduction that hydrocarbons migrating from oil and gas reservoirs affect on distribution elements in overlapping layers. These elements can be used for detecting chemical changes in the soils and rocks associated with the oil and gas deposits.

Important result of these studies litho geochemical survey pretty accurate showed board of Orenburg Gas Condensate Oilfield, which has very complicated geological structure.

Main advantage of using the complex litho and athmo geochemical methods is make geochemical surveys cheaper. Litho geochemical studies could be used to find out main regularities of the surveyed area. To approve anomalies discovered by litho geochemical surveys athmo geochemical and over methods should be used.