G51/B1-77

80% coverage, north east & north west corner missing D52/B1-155 Contour interval: 25

H52/B1-58 Contour interval: 10

D52/B1-173 Vertical scale: 10

I50/B1-51 Vertical scale: 10

The Carlo Sandstone is part of the Toko Group of the Georgina Basin. This basin, which straddles the Northern Territory-Queensland border, comprises carbonate and clastic rocks of Cambrian and Early Ordovician age; the Toko Group forms the uppermost and dominantly clastic part of the sequence. The Carlo Sandstone, which is of possible Early Ordovician age (? latest Arenig), consists of very fine to fine quartzose sandstone containing siltstone and mudstone pellets. Minor siltstone and mudstone interbeds are present. The unit is conformable with the underlying Nora and overlying Mithaka Formations. Sedimentary structures present include: flute marks, current crescents, various other current lineations, ripple-marks, cross stratification, lamination and various biogenic sedimentary structures. The palaeocurrent measurements indicate uni-directional currents. Fossils are rare except in the uppermost part of the unit where gastropods, pelecypods, nautiloids, trilobites, brachiopods and fish are present. A study of geometry, lithology, sedimentary structures, palaeocurrents and fossils indicate subtidal to intertidal depositional conditions. Of various depositional models, a barrier model is favoured. The Nora Formation represents the offshore, below wave base sediments; the Carlo Sandstone the barrier, either dunes or bars; and the Mithaka Formation the lagoon-bay sediments. The sediments were deposited in an elongate epicontinental sea that also covered the Amadeus, Wiso and Canning Basins.

Scale model studies are often used to determine the distribution of secondary magnetic fields when the numerical approach is unreasonably difficult. Reduction in size (L) is compensated by an increase in conductivity (o) or frequency (f), or both. In most cases the results are expressed in a way whereby they are independent of the absolute dimensions of the model and the general modelling relation oufL^2 = constant, is valid. However in some applications such as transient electromagnetic (TEM) modelling it is necessary to determine the power level of the full-scale system. An expression relating the voltage levels of the model and the full-scale system is: Vm/V = LmT/LTm where the subscript m represents the model system. This expression must he used in conjunction with the general modelling relation in order to compensate for changes in the modelling parameters. It is possible to scale time, as well as dimensions and conductivity and, using the above relationships, many geological situations may be simulated with a single model. By varying either one of the parameters conductivity or dimensions while keeping the other constant, the same transient decay curve can be obtained by transposing the individual curves (plotted on a log-log scale) along the time and response axes. An example illustrates that by modelling field cases absolute quantities such as conductivity can be estimated. Modelling of the Woodlawn orebody gave bulk conductivities of 5 S/m and 20 S/m. The analysis of TEM data using early and late time responses must be done with care. The response typical of a late time may be similar to an early time response if the electrical or dimensional properties of the conductor are varied. More correctly one should analyse responses in terms of large and small values of the parameter oufL^2/T.

The late Palaeozoic acid igneous petrographic province, covering some 120 000 km^2 In the Cairns-Townsville hinterland, has associated uranium mineralization characterized by various combinations of uranium, fluorine, and molybdenum. Mineralization of this type has been described from other parts of the world, but is best known in the USSR. Information about the Australian deposits and occurrences is very limited, but it is apparent that the mineralization is mainly of hydrothermal origin and genetically related to extensive late Palaeozoic magmatism. Zones of high porosity and permeability within 300 to 1500 m or so of the surface, which were accessible to mineralized hydrothermal fluids at temperatures of about 150°C to 300°C, appear to have been the most favourable sites for the deposition of uraniferous minerals. Suitable porosity and permeability may be of sedimentary, diagenetic, volcanic, hydrothermal, or tectonic origin, but flat, or gently dipping coarse sedimentary rocks at the base of acid volcanic sequences are possibly the most suitable hosts for layer-like and stockwork deposits. Intensely jointed rocks, hydrothermal alteration zones, breccia pipes, fault zones, unconformities, flow-top breccias, and faulted or fractured fold hinges, may also have sufficient porosity and permeability to host mineralization. Permeable basement fractures, dyke swarms, and intrusive contacts, as well as some of the potential hosts themselves may have provided channelways from the mineralizing source. Suitable channelways and deposition sites are common throughout the province, and offer the prospect of additional discoveries of uranium deposits of similar and related types.

A geochemical study of the Cullen Granite in the Northern Territory, involving major elements and 25 trace elements (Li, Be, F, S, V, Co, Ni, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, Mo, Sn, Ba, La, Ce, W, Pb, Th, U) has been carried out. The results support field observations in that they indicate that one of the five phases of granite identified is younger and more highly fractionated than the rest. The existence of a relationship between the geochemistry of the Cullen Granite and mineralisation contained within the granite and surrounding sediments is suggested by only a few of the trace elements determined. High U concentrations within the granite near Edith River can be correlated with known mineralisation in the granite, although no U mineralisation has been reported from a region southwest of Frances Creek where the U content of the granite is also high. Anomalously high concentrations of Cu and W in the granite may be related to mineralisation in nearby sediments. The well documented tendency for the Sn content of intrusive rocks associated with Sn deposits to be higher than in those rocks without deposits is not supported by the Cullen Granite data. Although numerous Sn deposits occur in and around the granite, its Sn content is near or below the limit of detection.

A program of field tests of a new electromagnetic sounding technique was conducted in the Northern Territory. The tests were conducted to evaluate the operational features of the technique, and to determine its ability to provide useful depth information over localised conductive structures. The results are compared with earlier work which used an alternative electromagnetic sounding technique, and with drilling results. The tests demonstrated the operational and interpretive flexibility of the new technique. In particular they showed that the technique can be applied with normal, unmodified, electromagnetic prospecting equipment. The depth information which the new technique provided was in good agreement with control information.