Low-pressure (P) high-temperature (T) metamorphism previously attributed to crustal thickening accompanying nappe emplacement in the Paleoproterozoic Willyama Supergroup at Broken Hill is reinterpreted as a response to crustal extension (D1) and bimodal magmatism commencing circa 1700 Ma. D1 deformation occurred under amphibolite-granulite facies conditions, producing layer-parallel fabrics (S1) and metapelitic mineral assemblages in which andalusite dominated over sillimanite. Crustal extension ceased soon after 1640 Ma to be followed by crustal thickening, higher-pressure metamorphism, and northeast vergent recumbent folding (D2) attendant on inversion of the original D1 extensional structures. D2 recumbent folding has previously gone unrecognized but probably peaked around 1600 Ma before further amphibolite facies metamorphism between 1600 and 1590 Ma accompanying upright folding and northwest directed thrust faulting (D3). In the light of this revised tectonothermal history, geodynamic models erected to explain elevated thermal gradients and low-P-high-T granulite facies metamorphism in tectonically thickened crust may not be as relevant to Broken Hill as once thought. Above average crustal temperatures and low-P-high-T metamorphism in the Willyama Supergroup more likely occurred in response to voluminous magmatic intrusion accompanying crustal thinning, mantle upwelling and emplacement and unroofing of midcrustal rocks along a major detachment surface. A second major structural and/or stratigraphic break in the Willyama Supergroup is indicated by the absence of D1 fabrics from the uppermost part of the sequence.

Continuously cored ODP Leg 189 sites document the marine sequences deposited before, during and after the Tasmanian (Australian-Antarctic) Gateway opened (~33.5 Ma) and deepened. The sites are all on continental crust: one west of Tasmania, three on South Tasman Rise (STR) and one on East Tasman Plateau (ETP). The Tasmanian `land bridge? linked Australia and Antarctica and incorporated parts of Tasmania and STR; one site lay in the gradually widening but restricted Australo-Antarctic Gulf (AAG) in the Indian Ocean, and the others in the more open proto-Pacific Ocean. The main four sites vary in sub-seafloor depth from 776 to 959 m, and their oldest sequence from lowest Upper Eocene (AAG) to Maastrichtian (ETP). The sites are broadly similar, with variations depending on tectono-sedimentary setting. Depositional rates seldom exceeded 4 cm/ky. Until the Oligocene, the region was near the Antarctic margin in very high palaeolatitudes, and dinocyst, diatom and magnetostratigraphic data provide most dating and marine environmental information. From the Late Cretaceous through Late Paleocene (95-55 Ma) left-lateral strike-slip motion moved the Tasmanian region northwest past Antarctica, and Tasman Basin rifting and seafloor spreading occurred in the east. Deltaic sequences filled depocentres with dark, restricted, paralic and marine mudstones (drilled on ETP, STR). At the Paleocene/Eocene boundary (55 Ma) Australia-Antarctic motion changed to north-south along the Tasman Fracture Zone west of STR, and Tasman Basin spreading ceased. South of eastern STR an oceanic basin opened. Fast spreading, beginning in the Middle Eocene, carried this region northward (~43 Ma). In the Early and Middle Eocene, deposition continued of dark, largely deltaic, and broadly similar shallow marine mudstones (thinnest on ETP). Proto-Pacific mudstones become more open marine with time, but AAG mudstones remained restricted. In the Late Eocene (37-33.5 Ma) the continental margins sagged, the water deepened, and some currents may have flowed through shallow seaways. Sedimentation rates declined as winnowing increased and hiatuses formed. On the AAG margin restricted shallow marine mudstone and sandstone were deposited. In the proto-Pacific, as the water deepened in the latest Eocene, marine mudstone gave way to winnowed marine glauconitic siltstone and sandstone. Rapid subsidence followed the final separation of STR and Antarctica. In the proto-Pacific, strong currents swept the shelves and opening straits, and an Early Oligocene hiatus was overlain by Oligocene open marine bathyal carbonates. The AAG margin was now less restricted, but calcareous mudstones had only gradual carbonate increases through into the lower Miocene. From the Oligocene on, calcareous nannofossils, planktonic foraminifers and magnetostratigraphic data provide most dating and marine environmental information. The Neogene sequences, which consist of bathyal chalk and oozes, with limited disconformities in parts of the Miocene, have proved ideal for detailed palaeo-oceanographic/climatic isotope studies - rare in the Southern Ocean.

Knowledge of the spatial and temporal relationships between fluid flow, the generation of structures, and crustal architecture is essential to understanding a mineral system. In regions dominated by cover, such knowledge leans heavily on interpretation of potential field data. Forward modelling and inversion of cross-sections, based on solid geology maps, provide better than a first approximation but reliability decreases with extrapolation from the sections. Stereo-models of crustal architecture are possible using closely spaced sections but they are more rigorously produced by 3D inversion. Inversion programs derive a physical property distribution that reproduces potential field observations in a manner consistent with a series of model parameters and geological constraints. The inversion techniques used in this study are based on the smooth-model potential field inversion software, MAG3D and GRAV3D, developed at the University of British Columbia?Geophysical Inversion Facility (UBC?GIF). We tuned some of the parameters and modified the methods for use in regional-scale rather than deposit-scale inversions. The volume of crust chosen for study, centred on the Olympic Dam deposit, is 150 kmx ? 150 kmy ? 10 kmz. Because a buffer is required to minimise edge effects, we model a volume of 198 kmx ? 198 kmy ? 18 kmz, discretised into 1 kmx ? 1 kmy ? 0.5 kmz cells. A series of trial inversions were run on a desktop PC with an Intel? Pentium? 4 2.0 GHz processor and 2 GB of RAM. The initial trials were designed to investigate the feasibility of doing regional-scale inversions and to show where development of methods and software support were needed. For tractable computation, it is necessary to split each volume into a number of overlapping tiles that can be processed independently then rejoined. Even so, runs took up to 40 hours. The time elapsed can be substantially reduced if processing is performed as a distributed application across a network with each PC dedicated to a single tile. The inherent non uniqueness of potential field inversion means that, even after some models have been rejected on `geo-logical? grounds, a number of reasonable models will remain. Tests that prove or disprove the models may be devised but actual physical testing may not be practical. However, we can make, probabilistic determinations of the distribution of Fe oxide alteration, which may be used to map likely fluid pathways and as guides to ore. Such predictions are amenable to testing available in exploration programs.

Structure and Metamorphic Evolution of Key Areas in the Eastern Fold Belt, Mount Isa Inlier Poster

Current metre deployments, suspended sediment measurements and surface sediment samples were collected from three locations within distributary channels of the tidally dominated Fly River delta in southern Papua New Guinea.

Estimates of significant wave height and period, together with tidal current speed over a semi-lunar cycle, were used to predict the area on the Australian continental shelf over which unconsolidated sediment was mobilised (threshold exceedance). These sediment-entraining processes were examined independently to quantify their relative importance on the continental shelf.

As Australia separated from Antarctica and drifted northward the Tasmanian Gateway opened, allowing the Antarctic Circumpolar Current to develop. This current began to isolate Antarctica from the influence of warm surface currents from the north, and an ice cap started to form. Eventually, deepwater conduits led to deepwater circulation between the southern Indian and Pacific Oceans. The existence of these conduits ultimately allowed ocean conveyor circulation. Continuing Antarctic thermal isolation, caused by the continental separation, contributed to the evolution of global climate from relatively warm early Cenozoic ?Greenhouse? to late Cenozoic ?Icehouse? climates. ODP Leg 189 addressed the interrelationships of plate tectonics in the gateway, circum-polar current circulation, climate and sedimentation, and global climatic changes. DSDP drilling had led to a basic framework of paleoenvironmental changes associated with gateway opening, but was not a full test of the various interrelationships. Using the DSDP results, Kennett, Houtz et al. (1975) proposed that climatic cooling and an Antarctic ice sheet (cryosphere) developed from ~33.5 Ma as the ACC progressively isolated Antarctica thermally. They suggested that development of the Antarctic cryosphere led to the formation of the cold deep ocean and intensified thermohaline circulation. Leg 189 gathered data that support this hypothesis. Leg 189 continuously cored sediments in the gateway, which was once part of a Tasmanian land bridge between Australia and Antarctica. The bridge separated the Australo-Antarctic Gulf in the west from the proto-Pacific Ocean to the east. This region is one of the few in the Southern Ocean where almost complete Cenozoic marine sequences could be drilled in paleo-water depths that were shallow enough to allow preservation of calcareous micro-organisms for isotopic studies. The Leg 189 sequences described by Exon, Kennett, Malone et al. (2001) reflect the evolution of a tightly integrated and dynamically evolving system over the past 70 million years, involving the lithosphere, hydrosphere, atmosphere, cryosphere and biosphere. The most conspicuous changes in the region occurred over the Eocene-Oligocene transition (Figure 1) when Australia and Antarctica finally separated. Before the separation, the combination of a warm climate, nearby continental highlands, and considerable rainfall and erosion, flooded the region with siliciclastic debris. Deposition kept up with subsidence. After separation, a cool climate, smaller more distant landmasses, and little rainfall and erosion, cut off the siliciclastic supply. Pelagic carbonate deposition could not keep up with subsidence. Leg 189 confirmed that Cenozoic Antarctic-Australia separation brought many changes. The regional changes included: warm to cool climate, shallow to deep water deposition, poorly ventilated basins to well-ventilated open ocean, dark deltaic mudstone to light pelagic carbonate deposition, microfossil assemblages dominated by dinoflagellates to ones dominated by calcareous pelagic microfossils, and sediments rich in organic carbon to ones poor in organic carbon.

The MECO hydrodynamic model was adapted for the Torres Strait - Gulf of Papua region at 0.051 resolution. Validation of the hydrodynamic model was carried out against observed current meter data and calculated tidal sea levels. Dispersal pathways of sediments derived from the Fly River, and from a resuspension event along the northern Great Barrier Reef were investigated using an Eulerian approach. Sediment input into Torres Strait is found to be greater during the Trade season by approximately 10%. Wave data were also obtained, and together with hydrodynamic model output, sediment mobility due to currents, waves and wave-current interactions was considered for both the Trade and Monsoon seasons. Sediment mobility in the Gulf of Papua is dominated by wave motion, whereas Torres Strait is a mixed environment of waves and tidal currents.

Chemical modeling of gold mineralisation in the Lachlan Fold Belt shows that gold can be precipitated over a wide temperature range (from 320 to 200 ?C in this study) from CO2-bearing, low salinity, aqueous fluid flowing upwards through faults in turbiditic sequences. In agreement with field observations, the veins are predicted to be mostly quartz (> 93 vol.%) with minor amounts of pyrite, arsenopyrite and muscovite (sericite) precipitating above 230 ?C. The predicted alteration assemblage contains pyrite, arsenopyrite, calcite, muscovite (sericite), chlorite and feldspar. Varying some of the chemical characteristics of the initial fluid has resulted in the following changes to the model: Preventing the fluid from boiling stops gold precipitating below 310 ?C but has little effect on the vein mineralogy or the mineralogy of the surrounding alteration assemblage. Removing CO2 from the fluid also prevents gold precipitation in the veins below 300 ?C. The modeling also generates an alteration assemblage with a number of Ca-rich minerals as less calcium carbonate exists in this system. Removing sulfur species from the initial fluid decreases the amount of gold precipitated by more than a factor of ten, which is to be expected if sulfur ligands are the main species for gold transport. However, the vein assemblage and the lack of sulfide minerals in the surrounding alteration assemblage also suggest that sulfur species are important in this mineral system. Increasing the initial oxidation state (?O2) of the fluid inhibits gold precipitation in the veins above 260 ?C and leads to a high proportion of dolomite in the surrounding alteration assemblage. On the other hand, decreasing the initial oxidation state of the fluid lead to gold precipitation over a range of temperatures below 310 ?C but predicts that mainly graphite ? quartz precipitates in the veins and that the surrounding alteration assemblage is dominated by feldspar proximal to the veins. This style of mineralogy is not commonly observed in gold deposits in the Lachlan Fold Belt. Increasing the initial pH of the fluid inhibits the amount of minerals that precipitate in the veins, which are dominated by calcite at high temperatures and graphite at low temperatures and corresponding minor amounts of gold. The proximal alteration assemblage is dominated by K-feldspar with amphibole, biotite and epidote. This mineral assemblage is not commonly observed in these deposits. Decreasing the initial pH of the fluid allows gold to precipitate below 280 ?C but generates a proximal alteration assemblage dominated by pyrophyllite, which again is not commonly observed. The results are in agreement with the widely accepted premise that gold is transported as bisulfide complexes and that the ore-bearing fluid is typically a low-salinity, mixed aqueous-carbonic fluid with low-moderate CO2 contents (Ridley and Diamond, 2000). However, the modeling has shown that the absence of certain physico-chemical processes or fluid constituents, such as boiling or lack of CO2 may inhibit gold precipitation in some environments. Large fluctuations in ?O2 or pH will also significantly change the vein and alteration mineralogy and generally reduce the amount of gold that is precipitated. This suggests that these fluids remain rock buffered during their journey from the source to the trap site.

The warm greenhouse world of the Late Cretaceous created an ocean that was poorly stratified latitudinally and vertically. Periodically these oceans experienced globally significant events where oxygen minimum zones enveloped the continental margins. Evidence of the effect of one of these Ocean Anoxic Events (OAE?s) is preserved in the southern high latitude strata of the Otway Basin in southeast Australia. During the Late Cretaceous, thick sequences of mudstone-dominated deltaic sediments (the Otway Delta) were deposited in an elongate inlet (ca. 500km wide) between Antarctica and Australia located at least 70?S. The initial Turonian strata of this delta (the Waarre Formation) were deposited in marginal marine delta plain to delta front conditions. The overlying Flaxman Formation and basal Belfast Mudstone preserve evidence of transgressive inner to middle shelf upper delta to prodelta conditions. These Turonian units were subject to periodic dysoxia. The conditions that created this dysoxia in the region were similar to those of the high northern latitude Cretaceous Interior Seaway of North America where intermittent freshwater input and deepening seas caused periods of thermohaline stratification and reduced bottom waters. The overlying Coniacian to Santonian Belfast Mudstone was deposited in outer shelf to upper slope prodelta conditions subject to periodic fluctuations in dysoxia with normal marine salinities. After a period when the oxygen minimum zone contracted, upward-increasing dysoxia in the Belfast Mudstone herald the onset of the Coniacian to Santonian OAE 3. This was the last OAE of the Late Cretaceous, prior to the onset of more ?modern? oceanic conditions. The fluctuations in TOC and hydrogen index in these strata reflect variable dysoxic conditions similar to that reported for OAE 3 in the tropical eastern Atlantic by Hofmann et al. (2003). This periodicity implies a very active and dynamic Late Cretaceous hydrosphere. Eventually, hyposaline conditions or higher sedimentation rates due to upper delta progradation and shallowing in the Santonian caused the local extinction and dissolution of many of the calcareous benthic taxa of the Belfast Mudstone.