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Summary Airborne electromagnetics (AEM) has successfully mapped and characterised groundwater systems in a range of landscapes and geological settings in the East Kimberley Region of north-western Australia. The AEM data enabled rapid imaging of key elements of hydrogeological systems in near-surface Cenozoic paleovalley, alluvial fan and colluvial sediments, and in underlying tectonically-inverted sedimentary basins. Rapid mapping and assessment of groundwater systems, MAR targets and salinity hazards involved the integration of AEM data with Ground Magnetic Resonance (GMR), seismic reflection, drilling and pump tests, borehole geophysics, soils, regolith, geological and structural mapping, and hydrogeological and hydrochemical investigations. AEM survey design was aided by the use of spatio-temporal analysis of Landsat data to identify areas of potential surface-groundwater interaction. A suite of equivalent 1D AEM inversion models produced comparable images of the sub-surface hydrostratigraphy and faults. However, 2.5D inversions produced different solutions in key locations. 3D inversions were subsequently performed, and drilling and tectonic analysis was used to assess all AEM inversion models. Recognising zones of structural complexity was important in the successful development of appropriate AEM inversion strategies and models. Overall, the success of groundwater system mapping has been due to the use of AEM within a broader, inter-disciplinary, multi-physics project framework.

Hydrothermal zircons occur as an integral component of the alteration and mineralization assemblage in the Gidginbung high sulphidation Au-Ag-(Cu) deposit in the Lachlan Fold Belt, Australia. Their hydrothermal origin is demonstrated by (1) their paragenetic relationships to ore and mineral assemblages within the orebodies, (2) the higher concentration of zircon within the orebody relative to that of the host rocks, (3) the presence of ore and alteration minerals as solid mineral inclusions within the zircons, (4) similar fluid inclusion compositions in these zircons and the associated ore and gangue minerals, and (5) their trace element composition and zonation as measured by the CSIRO-GEMOC nuclear microprobe. The trace element chemistry and mineral inclusion assemblage of these hydrothermal zircons differ markedly from those in coeval intrusive, volcanic and sedimentary lithologies. This chemical ‘fingerprinting’ permits the hydrothermal character of the zircons and their association with mineralization to be demonstrated unequivocally, thereby enabling the mineralizing event to be dated with confidence and precision at 436.4±3.1 Ma. Isotopic dating of hydrothermal zircons may have particular application where related ore deposits have been overprinted by later magmatic, hydrothermal, metamorphic, deformation and/or crustal weathering processes that disturb isotopic systems in other less stable minerals. This account shows that uncritical acceptance of U-Pb zircon ages as dating primary igneous events is unwise.

Experience over the past 15 years has demonstrated that the use of airborne electromagnetics (AEM) for near-surface hydrogeological investigations often requires high resolution data to map key functional elements of the hydrogeological system. In the Broken Hill Managed Aquifer Recharge (BHMAR) project, the AEM acquisition strategy was governed by the need to rapidly identify and assess a number of potential managed aquifer recharge (MAR) and groundwater resource targets over a large area (>7,500 km2) of the Lower Darling Valley, with a high degree of confidence, and in very short timeframes. A flight line spacing of 200-300m successfully mapped the key elements of the hydrostratigraphy, neotectonic features, and 14 potential MAR and groundwater targets. Subsequently, AEM data were re-inverted, enabling comparison of line spacing of 200 m, 600 m, 1 km, 2 km, 5 km and 10 km. Utilising the information gained from this exercise, a novel survey design was developed using a systems analysis approach incorporating a range of conceptual hydrogeological and geological models, morphotectonic and structural mapping, and temporal remote sensing and hydrogeological data. Trialling this approach, a few widely-spaced AEM transects identified potential new groundwater resources over a large area of the Middle- and Upper- Darling River Valley.

In this paper, we present novel developments of the Lateral Parameter Correlation (LPC) method for invoking lateral smoothness in model sections of one-dimensional (1D) inversion models. The LPC method is a three-step procedure consisting of (1) individual inversion of all soundings, (2) a lateral correlation procedure, and (3) a final individual inversion. The lateral correlation involves solving a simple constrained inversion problem with a model covariance matrix that ensures lateral smoothness. The method separates inversion from correlation and is much faster than methods where inversion correlation are solved simultaneously. In the new developments, a strictly horizontal correlation can be performed, thereby avoiding the model artifact sometimes seen when correlating along layers. Furthermore, a solution to the intractable computation times arising with large data sets is formulated employing a tessellation of the plane and an averaging scheme within subareas that reduces the size of the numerical LPC inversion problem while maintaining correct correlation within a very large area. A field example shows the improvements obtained with the strictly horizontal correlation. The LPC method is very flexible and is capable of correlating models from inversion of different data types including information from boreholes, and it lends itself easily to 'embarassingly parallel' computation.

Estimating groundwater storage volumes is important for assessing the potential capacity and recovery efficiencies of Managed Aquifer Recharge (MAR) strategies. However, such storage estimates are challenging, particularly in areas of poor data density. This challenge faced the Broken Hill Managed Aquifer Recharge (BHMAR) project tasked with assessing groundwater options in the Darling River floodplain, NSW, Australia. Within the ~7500 km2 area, 14 fresh groundwater zones were identified with AEM and a workflow was developed to estimate the groundwater volumes in these targets. Pore fluid sampling from sonic core provided the opportunity to develop simple AEM conductivity thresholds to produce surrogate groundwater salinity maps for each AEM depth slice. Combining the AEM data with other project datasets such as downhole geophysics, detailed sonic-core logging and laboratory analyses facilitated the mapping of hydrostratigraphy, textural classes and saturation within AEM depth slices, which were then used to calculate bulk aquifer volumes. Nuclear magnetic resonance (NMR) free-water statistics were used in effective porosity estimates, to calculate groundwater volumes. Although preliminary, the volume estimates were essential in the identifying, prioritising, interpreting and reporting of groundwater resource targets during the project and to make recommendations for future phases of investigation.

The Virtasalmi metavolcanic belt forms part of the Early Proterozoic Svecofennide Terrane in central Finland. The preserved supracrustal lithologies (40% of the assemblage) have undergone extensive synvolcanic hydrothermal alteration, polyphase regional ductile deformation, low pressure regional polyphase metamorphism to granulite facies with subsequent amphibolitisation, and extensive emplacement of a synkinematic I-Type plutonic suite. Despite these overprinting events, three chemically distinct suites of metatholeiites and one suite of coeval coarser-grained intrusive lithologies are recognized, and their petrogenesis resolved. Primary igneous geochemistry is preserved mainly by the HFSEs and Cr and Ni. Variable major and LIL element mobility is demonstrated. Significant differences in geochemistry are reflected in different metamorphic assemblages between suites. Preservation of vesicular pillow lavas indicate subaqueous extrusion of the three metatholeiitic suites. Trace element geochemistry indicates that Suite I compositions are controlled by low pressure fractional crystallization. In contrast, Suite III amphibolite compositions are controlled by batch partial melting, with limited modification by open system fractional crystallization. These differences in fractionation history point to the evolution of these two main suites within distinct upper crustal magmatic plumbing systems. Also, differences in more-HYG/less-HYG element and other incompatible element ratios, point to derivation of Suites I and III by different degrees of partial melting from different, heterogeneous mantle source regions. Coeval eruption of these suites indicates that variations in source geochemistry and melting processes reflect spatial rather than temporal differences. The volumetrically smaller Suite II comprises lava flows and pillowed units that are interlayered with, but have a more depleted chemistry than the other suites. Marked variation in Cr and Ni contents within a narrow range of Zr contents in individual flows in Subgroups IIa/1 and IIa/2, and marked TiO 2 enrichment in Subgroup IIb, suggests that the more depleted character of this suite is due to fractional crystallisation and crystal settling in high level magma chambers. Suite IV amphibolites comprise a number of small intrusive bodies that are emplaced within the volcanic pile. They are coarser grained than the enveloping amphibolites, and are interpreted as tectonised relics of high level magma chambers and/or sills emplaced into the (coeval) metavolcanic assemblage. They have a depleted geochemistry and there is textural and geochemical evidence to suggest a partial cumulate origin. Similarities in fractionation histories suggest that Suite IV intrusions are cogenetic with Suites I and II. Enrichment in incompatible elements, particularly Nb, demonstrates that the Virtasalmi metatholeiites are not related to supra-subduction zone magmatism. All the amphibolites are Within Plate Basalts or E-Type MORBs, with similar compositions known today from a range of tectonic settings. However, the interlayering of E-MORB metavolcanic rocks with marbles and voluminous fine grained clastic metasediments is more likely to indicate an intracratonic rift (passive margin?) geotectonic setting. Subsequent deformation and metamorphism of the volcanics and syn-kinematic calc-alkaline intrusives are interpreted as accretion at a destructive margin. The latter was most probably a continental calc-alkaline arc, various crustal levels of which are now exposed in the study area.