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

Many low-lying areas of the Mount William Creek valley are affected by dryland salinity due to shallow watertables across the floodplain. The valley contains a sequence of alluvium consisting of a basal gravel and sand unit of Tertiary age, overlain by Quaternary silts and clays. This sequence lies unconformably on a variety of Palaeozoic basement rocks, which outcrop as rolling hills and mountains towards the catchment boundaries. Potentiometric surface configurations and bore hydrograph data show that the basal Tertiary sequence and the overlying Quaternary sequence do not act as a single unconfined aquifer. Rather, they behave as two aquifers, with the Quaternary silts and clays having some confining capacity over the Tertiary sequence. Hydraulic head differences demonstrate that there is potential for downward leakage between these two units at the floodplain margins and upper catchment throughout the year, and that potential for upward leakage exists in the lower catchment following the winter peak in recharge. The area where upward leakage from the Tertiary sequence can be invoked as a salinity-causing mechanism is spatially and temporally restricted, but it does correspond to some large areas of land salinisation. The high groundwater pressures causing potential upward leakage can easily be attributed to head gained in the adjacent Grampians ranges. However, geological restrictions to lateral groundwater flow through the system may also contribute to the high groundwater pressures in the Tertiary sequence.

Cessation of magmatism in the Banda Arc north of Timor about 3 million years ago has been followed by a period rapid continuing uplift, most dramatically evidenced by the presence of Quaternary coral at ~700 m above sea level on the island of Atauro. In order to understand how these processes constrain slab dynamics during the early stage of continental–arc collision (Timor – Banda Arc) we have mapped the island of Atauro in detail. The basic features of Atauro geology are described, and it is demonstrated how they may relate to slab processes deep beneath.

New seismic reflection data and dredged rock samples confirm that the submarine Kenn Plateau is a thinned continental fragment. It is twice the size of Tasmania and lies 400 km east of central Queensland beyond Cato Trough. East and northeast trending faults separate thinly sedimented basement highs from sedimentary basins with >2 km of Cretaceous and younger strata. In the Late Cretaceous the future plateau fitted against Australia south of Marion Plateau, and consisted of Late Triassic and younger basins unconformably overlying the New England Fold Belt. After Lower Cretaceous rift volcanism, the Kenn Plateau itself formed by Late Cretaceous extension and breakup. Paleocene drifting moved it northeast and rotated it 30° anticlockwise, leaving behind Tasman Basin oceanic basalts. Siliciclastic sediments poured into the basins from the Australian mainland and local highs. After a regional Paleocene-Eocene boundary unconformity, siliciclastic sedimentation resumed nearshore. In deep water, deposition of radiolarian chalks ended at the regional Eocene-Oligocene boundary unconformity, and pure biogenic carbonates accumulated in warming surface waters. From the Middle Eocene, calcarenite formed on the shelves of the subsiding plateau. Some seismic profiles show Middle to Late Eocene compression, probably related to New Caledonian obduction to the east. Hotspots formed parts of two volcanic chains as the plateau moved northward: in the west, Upper Eocene and younger volcanics of the Tasmantid chain; and in the east Upper Oligocene and younger volcanics of the Lord Howe chain. The subsiding volcanoes were fringed by reefs, some of which have persisted until now; others have subsided to form guyots. The plateau has subsided 2000 m or more since its breakup and is now subject solely to pelagic carbonate sedimentation.

The ultramafic horizons of the Wiluna domain (Agnew-Wihina Greenstone Belt, Western Australia) form two main tabular and sheet-like fractionated units, which generally comprise a 5-10m-thick basal pyroxenite layer, a 50-100m-thick meso-to-orthocumulate-textured peridotite horizon, which is overlain by a core of wehrlite, ranging 10-50m in thickness, grading upwards into oikocrystic pyroxenites (30-50m-thick) and upper gabbroic margins (30-50m-thick). Primary textures are exceptionally well preserved: pegmatoidal and dendritic (harrisitic) olivine domains are common in all lithologies, whereas spinifex-textured horizons and flowtop breccias are absent. The fractionation sequence and the geochemical profile of the two ultramafic units are almost indistinguishable, suggesting that the belts may have been structurally duplicated. The ultramafic units display a steady trend towards increasing Fe content up stratigraphy, above a narrow basal zone showing a reversed trend. Calculated Ni contents of cumulus olivines indicate that the magma remained sulfur-undersaturated up to about the middle of the unit, where fractionation of a small proportion of sulfide (R>1000) occurred and depleted the upper fractionated levels in Ni. The presence of various anomalous PGE-enriched horizons (∑Pt-Pd= 200-300 ppb) in the fractionated pyroxenite layer directly overlying the basal ortho-mesocumulate-textured unit indicates the occurrence of localised weak reef-style PGE mineralisation. Reef-style PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich disseminated Fe-Cu sulfide mineralisation zones within thick differentiated intrusions (e.g. Platinova reef in the Skaergaard intrusion) rather than to other zones in komatiite-hosted systems.

The ~ 1333 Ma Voisey’s Bay Intrusion represents one of the earliest magmatic events recorded in the development of the Nain Plutonic Suite. The intrusion was likely emplaced at a depth of ~ 14 km.

Sulphide saturation occurred due to the assimilation of bulk Churchill gneiss in a series of sub-chambers below the current level of erosion. The current economic ore zones formed by the accumulation of magmatic sulphides in physical traps from a pulsed series of sulphide-laden magmas derived from the sub-chambers.

The initial melt had an MgO content of > 8 wt % and a geochemical and isotopic composition similar to some Proterozoic flood basalts.

An array of northeast-trending shear zones in the northwest Gawler Craton of South Australia has heen implicated in widely disparate Proterozoic continental reconstructions. Quantitative constraints on timing of movement along these shear zones are sparse, but potentially provide tests for contrasting reconstructions. New ⁴⁰Ar/³⁹Ar data provide improved age constraints for movement along these shear zones and are interpreted to indicate that the Karari, Tallacootra and Coorabie shear zones were active at ~ 1440 Ma. This timing is ~ 100 Ma younger than suggested in several published reconstructions and is also significantly older than Grenvillian-age tectonism in adjacent provinces to the west and north. The shear zones of the northwest Gawler Craton are interpreted to have formed in response to sinistral transpression, and dissect the region into several geological domains of contrasting metamorphic grade. Despite distinct differences in metamorphic grade the various domains exhibit evidence of a shared event-history prior to deformation at ~ 1440 Ma, albeit at different crustal levels. Movement along the shear zones of the northwest Gawler Craton at ~ 1440 Ma therefore appears to have dissected and reorganised a previously coherent crustal block rather than representing a suture zone between distinct terranes.

This paper will present a synopsis of the results from a three year AMIRA project (P588) entitled ”Epithermal Gold-silver Deposits: Geological, Geochemical and Isotopic Vectors to Target Major Deposits”. Research was undertaken at the Ladolam, (Papua New Guinea), Gosowong, and Mt Muro (Indonesia), Cerro Vanguardia (Argentina) and Twin Hills and Bimurra (Australia) deposits. The sizes and styles of the deposits varied greatly from a low-grade to barren small vein-stockwork system at Bimurra and Twin Hills to high grade veins systems of different scales such as Gosowong, Mt Murro and Cerro Vanguardia to a large breccia-hosted low grade, disseminated system at Ladolam.

At each of the deposits alteration mineralogy and zonation were determined using a combination of surface mapping, drill hole logging, petrology, SWIR (PIMA), XRD and potassium feldspar staining. Electron microprobe analyses of specific minerals, whole-rock and trace element whole rock analyses and stable isotope geochemistry were utilised to characterise the geochemical signature of the hydrothermal alteration.

Results of this research have 1) substantially increase our knowledge of the mineralogical and whole-rock geochemical characteristics of hydrothermal alteration associated with low- and intermediate sulfidation epithermal deposits, 2) developed a range of vectors that point towards ore, both on district and deposit scales, which can be used when exploring in epithermal environments and 3) proposed a set of criteria for distinguishing potential ore grade systems from barren or low grade epithermal systems.

The Western Australian Wheatbelt has been essentially geologically stable since the late Permian although the Archaean basement sustained some movement during the break-up of Gondwanaland and the northward drift of Australia from Antarctica. During the Early Cretaceous, Eocene and more recently, the Wheatbelt region’s weathered mantle has been eroded by rivers. The palaeo-valleys have been infilled with terrestrial and marine sediments, and been subject to ongoing deep weathering. During the Pliocene and Quaternary the region experienced alternating arid and pluvial climates. These cyclic episodes shaped the regolith and impacted on vegetation species and catchment water balances and also promoted the accumulation of massive volumes of salt. In more recent times, this stored salt has interacted with vegetation, soils, surface waterbodies and groundwater systems and left a distinctive and pervasive legacy in the landscape.

Salinisation was manifest in the Wheatbelt of WA from as long ago as 2.8 Ma, concentrating in valley floors as arid and pluvial cycles prevailed and whilst the continent migrated northwards. Today, as agricultural development has altered the water balance on 20 M ha of cleared farmland, salinity is again on the move, further degrading the 300,000 ha of variably saline land that existed before the arrival of Europeans, and spreading across an additional 1.1 M ha of formerly non-saline land. Unchecked by reduced rainfall or human induced changes to the water balance, salinity may expand even further, potentially affecting 1.7-3.4 M ha of the Wheatbelt’s agricultural land and its unique and indigenous resources.

This paper reviews the palaeogeography and palaeoclimates of the region and its hydrogeology and examines the nature of its susceptibility to salinisation. It poses questions about the relationship between palaeo-salinity and contemporary salinity, seeking geomorphic evidence to determine whether salinity is likely to expand beyond extant palaeo-salinity markers. Finally, it considers the likely timeframes involved in the recovery of systems from primary salinity and whether clearing-induced salinity will follow patterns similar to those observed from past saline episodes in the region.

Low-temperature thermochronology on apatites from the Gulf Extensional Province in northern Baja California reveals fission-track ages >27 Ma in the Sierra San Pedro Martir and the Sierra Juarez. This indicates that exposed crustal depths are less than about 2km below the pre-extension land surface. Younger ages are found in the strongly rotated hanging wall blocks in the Sierra San Felipe towards the Gulf coast indicating deeper crustal levels. The more sensitive apatite (U-Th)/He system, however, reveals much younger ages along the foot of the main escarpment and in the San Felipe blocks constraining the time of onset of rapid extension to between 7 and 14 Ma.

The Shoalhaven region of NSW experiences environmental acidification due to acid sulphate soils (ASS). In order to trial an environmental engineering solution to groundwater remediation involving a permeable reactive barrier (PRB), comprehensive site characterisation and laboratory-based batch and column tests of reactive materials were conducted. The PRB is designed to perform in-situ remediation of the acidic groundwater (pH 3) that is generated in ASS. Twenty-five alkaline reactive materials have been tested for suitability for the barrier, with an emphasis on waste materials, including recycled concrete, limestone, calcite-bearing zeolitic breccia, blast furnace slag, and oyster shells. Following three phases of batch tests, two waste materials (recycled concrete and oyster shells) were chosen for column tests that simulate flow conditions through the barrier and using acidic water from the field site (pH = 3). After 105 days in the column, the oyster shells still neutralised the water (pH = 7).

Isa Superbasin strata in the Lawn Hill Platform host major base metal sulfide mineralization including the giant Century Zn-Pb deposit. Mineral paragenesis, organic maturation, K-Ar dating and stable isotope studies demonstrate that long-lived structures such as the Termite Range Fault acted as hot fluid conduits several times during the Paleoproterozoic and Mesoproterozoic. K-Ar dating of illites, in combination with other datasets, may identify three thermal events in the Lawn Hill Platform at 1500, 1440-1400 and 1250-1150 Ma. 1500 Ma is a Late fcan Orogeny age recorded only in the south that may reflect exhumation of a provenance region. The 1440-1300 Ma ages are related to fault reactivation and a thermal/?fluid pulse at ~1440-1400 Ma, with subsequent enhanced cooling. The youngest thermal/fluid flow event at 1250-1150 Ma is recorded mainly in the northeast and is probably related to the assembly of Rodinia.

An extensive carbonate alteration halo in the footwall of the circa 1575 Ma Century Zn-Pb deposit extends some 15 km along strike. Calculations of fluid oxygen and carbon isotope composition based on model temperatures of 120°C for the ore zone siderites and 180°C for siderites and ankerites in the regional carbonate alteration halo indicate similar ore and alteration fluid compositions (°¹⁸O = 3 to 10 per mil; °¹³C = -7 to -3 per mil). This fluid isotope composition is consistent with highly evolved basinal brines and mixed inorganic and organic carbon sources. The good agreement between maximum temperature estimates from illite crystallinity and organic reflectance and inverse correlation with carbonate oxygen isotopes in northeast Lawn Hill platform boreholes indicate thermal maturation and carbonate formation resulted from interaction with the 1250-1150 Ma fluids. The calculated fluid isotope compositions are consistent with an evolved basinal brine (°¹⁸O = 5.1 to 9.4 per mil V-SMOW; °¹³C = -13.2 to -3.7 per mil V-PDB) that contained a significant organic carbon component. Differences in carbon and oxygen isotope systematics of alteration carbonate are significant to their use with lithogeochemical vectors in mineral exploration in the northern Australian basins.

ANSIR, the National Facility for Earth Sounding, has been responsible for the acquisition of 30 regional deep seismic reflection surveys across many different Australian geological regions and structural environments. The seismic data has provided exciting new images of the Australian Crust at depth. Each survey has produced the unexpected and has usually dispelled some ‘sacred-cow’ in geological understanding of the depth-dimension of that region. ANSIR’s deep seismic reflection equipment provides both high quality whole-of-crust images of the Australia continent as well as detailed district and mine scale seismic images.

These seismic surveys are providing a clear picture of the architecture of a region and providing information to develop tectonic models a region. In particular, the seismic data is successfully imaging the major basement geological units and structures and therefore providing the necessary depth constraints to construct 3D geological models of a region. The seismic data is suggesting possible mineral systems that have operated and allowing explorers to assess the potential of an area for mineral deposits.

Results from the Tanami Seismic reflection survey are extremely encouraging for researchers and explorers working in the Proterozoic Tanami region of Western Australia and the Northern Territory. The Tanami seismic reflection project, a joint project between Geoscience Australia (GA), the Northern Territory Geological Survey (NTGS), the Geological Survey of Western Australia (GSWA), Newmont Exploration Pty. Ltd. and Tanami Gold N.L., has defined the crostai architecture of the Tanami region and Proterozoic Arunta region, identified structural anomalies, which appear to control gold mineralisation and generated exploration targets. Main crustal-scale features include a major SE dipping suture that extends from the surface to the Moho and is interpreted as the boundary between the Tanami region and the Arunta Region; a partitioning of the crust into a less reflective upper crust and a more reflective middle to lower crust; a series of SE, NE and NW dipping shallow reflectors within the Tanami region that are interpreted as thrusts; shallow and relatively thin granitic bodies that are less than 10 km thick and a strong association between ore deposits and prospects and the location of crostai structural anomalies, including shallowly-dipping thrusts, pop-up structures and ramp anticlines.

Zircon megacrysts are associated with gem ruby-sapphire deposits along a 12 000 km long zone, from eastern Australia in the south, through South East Asia, to far eastern Russia in the north. Representative zircons were studied from some 6 sites along this zone. This is just a small part of a much broader ongoing study of this zone whose occurrence must be due to some fundamental plate tectonic process.

The time relationship between zircon crystallisation at depth, and transport to the surface via basaltic magmas was studied using zircon FT, U-Pb zircon SHRIMP techniques, and published basalt ages. The results show that for some fields, zircon crystallisation and transport to the surface were almost coeval events, whilst for others, zircons crystallised as much as 200 Ma before later transport.

The Lome Basin on the mid north coast region of New South Wales is a small circular-shaped basin within the southern margin of the New England Fold Belt. It comprises an Early Triassic terrestrial sedimentary sequence (the Camden Haven Group) overlain by rhyolitic volcanics which appear to be co-genetic with the Late Triassic granitoids (the Brothers) which intrude the Lome Basin sedimentary sequence.

Over the last few years, we have conducted fieldwork in the Lome Basin, mainly investigating the magmatic rocks, as part of a larger project on the volcanic sequences of eastern Australia. Initial work resulted in the identification of a number of rhyolitic dykes intruding the Brothers granitoids, previously unrecorded. As a result of this, we then undertook detailed geochronological and geochemical investigations of the magmatic sequences throughout the basin to determine their relationship to each other, and other peripheral sequences, outside of the basin.

Our results show that the Lome Basin had a more complex and protracted magmatic history than previously thought, beginning with Late Triassic rhyolitic volcanics, followed by co-genetic rhyolitic volcanics and granitoids, then Late Jurassic and Early Cretaceous rhyolitic volcanics, terminating in Mid Miocene mafic volcanism.

Normal faults within sedimentary basins are commonly associated with subtle linear features in high-resolution aeromagnetic data. Many of these anomalies arise from the tectonic juxtaposition of sedimentary units of differing magnetic properties. In detail, the anomalies are quite variable in character, even along the strike of individual faults. To understand this variability, we examine the well-exposed San Ysidro fault in the central Rio Grande rift, USA, using magnetic-property measurements and geophysical models. We find that the sources of along-strike anomaly variability arise mainly from (1) differences in levels of erosion that bring different juxtaposed units to the surface and (2) magnetic properties that vary along strike within individual units. Uneven levels of erosion have the most significant effect, owing to the presence of several discrete magnetic sources distributed at different levels of the fault. Although variable fault throw affects the extent to which contrasting units are in contact across the fault, the effect appears to be overwhelmed by the other factors. The results at the San Ysidro fault, especially the possibility of multiple magnetic sources at a single fault, should aid in the geologic interpretation of linear magnetic anomalies related to intrasedimentary faults in general.

In the Tibooburra area, within the northeastern Koonenberry Belt, geological mapping has delineated a sequence of shallow marine sediments, interbedded with distal airfall tuffs that have been dated at 503-496 Ma. These rocks were strongly deformed at approximately 441-424 Ma, producing tight east verging folds and reverse faults (D1), and a penetrative west-dipping cleavage (S1). The sequence was intruded by post-Di monzodioritic sills and dykes (~423-416 Ma) and the geochemically-related I-type Tibooburra Granodiorite (412±10 Ma).

Gold mineralization in the Tibooburra Goldfields is associated with syn-D1 replacement quartz veins which were injected parallel to S1 cleavage and formed long narrow stringer zones. Auriferous fluids were probably oxidized, low-sulphide, CCh-bearing fluids produced during peak M1 metamorphism. Two types of gold-associated alteration are present: narrow phengite-chlorite-pyrite-carbonate halos around quartz veins, and carbonate-sericite ‘bleached’ zones extending kilometres along strike.

The style and timing of gold mineralization and the structural history of the Tibooburra Goldfields has strong similarities to the Victorian Goldfields within the Western Lachlan Orogen and Delamerian Orogen of Victoria. However, there are significant differences in the interpreted tectonic setting between the two areas which have implications for the development of the Gondwana margin and mineral exploration.