Economic geologists and remote sensing specialists have long been interested in hydrothermal systems for their valuable ore deposits. Alteration mineral assemblages caused by hydrothermal activity typically display certain spectral characteristics due to vibration of the hydroxyl (OH-) anion in the near infrared. This feature can be exploited by satellites using imaging infrared spectrometers. Hyperspectral equipment such as the Australian built HyMap airborne system and the PIMA II field spectrometer collect detailed spectral information, often in contiguous wavelengths, which can be analysed and interpreted to make highly detailed mineralogical and alteration pattern maps . Two Precambrian hydrothermal systems, well suited to testing remote sensing of hydrothermal systems are described here, the first in the northern Flinders Ranges in South Australia, comprising one of Earth?s largest hydrothermal deposits and, the second in the North Pole Dome in the Pilbara region of north western Australia, host to many famous stromatolitic horizons.

The Kangaroo Caves zinc-copper deposit in the Archaean Panorama District in the northern Pilbara Craton, Western Australia contains an Indicated and Inferred Mineral Resource of 6.3 million tonnes at 3.3% zinc and 0.5% copper. The Kangaroo Caves area is characterised by predominantly tholeiitic volcanic rocks of the Kangaroo Caves Formation, which is overlain by turbiditic sedimentary and volcanic rocks of the Soanesville Group. Zinc-copper mineralisation is hosted mainly by the regionally extensive Marker Chert, the uppermost unit of the Kangaroo Caves Formation, and structurally controlled by D1 synvolcanic faults. The upper area of the deposit is characterised by quartz-sphalerite ± pyrite ± baryte ± chalcopyrite, whereas the lower area contains mainly chlorite-pyrite-quartz-carbonate-sericite ± chalcopyrite ± sphalerite. Laser ablation inductively coupled plasma mass spectrometry analyses show that cobalt-nickel ratios in pyrite are significantly greater in the upper, zinc-rich area (median copper/nickel = 0.4) of the deposit than the lower, zinc-poor area (median copper/nickel = 5). Structural analysis of the Kangaroo Caves area combined with Leapfrog modelling of ore and trace element distribution shows that the deposit is predominantly an elongate sheet of zinc mineralisation (-1%), which plunges ~30° to the northeast and is approximately 1000 metres in length. The morphology of the Kangaroo Caves deposit was retained from its original formation, despite rotation during the D2 event. Variations in hydrothermal alteration assemblages, including the copper and nickel contents of pyrite within the deposit and underlying dacite, are interpreted to be the result of variations in the influx and mixing of seawater with upwelling volcanogenic fluids during zinc-copper mineralization. At the Kangaroo Caves area the cobalt-nickel ratio of pyrite can be used as an exploration vector towards high-grade zinc-copper mineralization.

The physical properties of non-porous basement rocks are directly related to the mineralogy of those rocks. The MineralMapper3D software package originally developed by Nick Williams at the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC), Geoscience Australia, uses the physical properties of minerals to provide bounds on estimates of the abundance of specified minerals in non-porous basement rocks. This approach is applicable to both estimates of density and magnetic susceptibility derived from 3D inversions of gravity and magnetic data as well as physical measurements on specimens or down-hole derived physical properties. This users guide descibes the history, installation and operation of the software package.

The physical properties of non-porous basement rocks are directly related to the mineralogy of those rocks. The MineralMapper3D software package originally developed by Nick Williams at the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC), Geoscience Australia, uses the physical properties of minerals to provide bounds on estimates of the abundance of specified minerals in non-porous basement rocks. This approach is applicable to both estimates of density and magnetic susceptibility derived from 3D inversions of gravity and magnetic data as well as physical measurements on specimens or down-hole derived physical properties.

A metamorphic database covering the entire eastern Yilgarn Craton has been compiled from pre-existing mapping, 14,500 sites with qualitative metamorphic information, and 470 new key sites with detailed quantitative metamorphic data including P, T, temperature/depth ratio and P-T paths. The derived temporal and spatial patterns contrast with previous tectonic models and invariant crustal depth with the single prograde metamorphic event of the long-standing metamorphic paradigm. In particular, there are large variations in peak metamorphic crustal depths (12 to 31 km), and five metamorphic periods can now be recognised. &#149; Ma: Very localised, low-P granulite of high temperature/depth ratio (>50ºC/km). &#149; M1: High-P (8.7kb), low temperature/depth ratio (<20ºC/km) assemblages localised to major shear zones with clockwise isothermal decompression P-T paths. &#149; M2: Regional matrix parageneses with T ranging 300-550ºC across greenstone belts and elevated temperature/depth ratio of 30-40ºC/km throughout. Tight clockwise paths evolved through maximum prograde pressures of 6 kb and peak metamorphic pressures of 3.5-5.0 kb. &#149; M3a: An extension related thermal pulse localised on the Ockerburry Fault and post-volcanic late basins. Anticlockwise paths to peak conditions of 500-580ºC and 4.0 kb, define moderately high temperature/depth ratio of 40-50ºC/km. &#149; M3b: Multiple localised hydrothermal alteration events during a period of exhumation from 4 kb to 1 kb. Metamorphic patterns during each event have been temporally and spatially integrated with the new deformation framework (Blewett & Czarnota, 2007c) by a process of metamorphic domain analysis and using metamorphic field gradients. The continual evolution with time of fundamental metamorphic parameters throughout the entire history have been constructed as evolution curves and integrated with the deformation, magmatic, stratigraphic and mineralization history. <p>Related material<a href="https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&catno=69771">East Yilgarn Craton Metamorphism and Strain</a> - Map.</p>

Hyperspectral airborne images from the Eastern Fold Belt of the Mount Isa Inlier, were validated as new tool for the detection of Iron oxide Cu-Au (IOCG) related alteration. High resolution mineral maps derived from hyperspectral imaging (4.5m/pixel) enables the recognition of various types of hydrothermal alteration patterns and the localisation of fluid pathways. Four different types of hydrothermal alteration patterns were identified with the hyperspectral mineral maps: (1) Metasomatic 1: White mica mineral maps were applied to map the spatial distribution of regional sodic-calcic alteration in metasedimentary successions of the Soldiers Cap Group in the Snake Creek Anticline. (2) Metasomatic 2: Alteration zoning is evident from albitised granites, assigned to the Williams-Naraku Suite, along the Cloncurry Fault show characteristic absorption features in the shortwave infrared range (SWIR) and can be detected with white mica mineral maps (white mica composition, white mica content, white mica crystallinity index).

Globally, iron-oxide Cu-Au deposits are a prime exploration target given the super-giant status of Olympic Dam, the type example. The most commonly identified primary ingredients for these deposit types are granites, mafic igneous rocks and oxidised host sequences (e.g., Hitzman 2000). The deposits are also characterised by sodic-potassic and potassic alteration which can be significant on a regional scale. Deposits in the Tennant Creek, Cloncurry and Olympic Dam regions are the three main examples of iron-oxide Cu-Au deposits in Australia: all are Proterozoic. A continent scale-spatial analysis of their regional setting reveals some common elements and suggests that these deposit types are unlikely to be found in the Archaean or the Phanerozoic of Australia. In all three cases, intrusive rocks occur in proximity, but whether the metals come direct from these or are leached from the country rock is unimportant to this analysis. The nearby granitic rocks are all fractionated, and show evidence of release of late stage magmatic fluids. Chemically these granites are a very specific type: all strongly oxidised, metaluminous, high temperature, high Ca granites that formed as a result of well above average geothermal gradients from crustal sources. These granites contain anomalously high concentrations of K, Th and U. Such granites are not found in the Archaean and rare equivalent Phanerozoic granitic types are of an order of magnitude smaller in size. This analysis shows that the known Australian deposits occur in regions of present-day anomalously high heat flow. Heat production is enhanced by high concentrations of K, Th and U in the nearby granites, particularly in the Olympic Dam and Cloncurry areas. Such elevated heat production may have resulted in anomalously long-lived hydrothermal cells and enhanced the metal leaching process. The large areal extent of the alteration zones in these two areas may also be a result of this fundamental crustal anomaly. Mafic igneous rocks are common in the vicinity of known Australia deposits, and might be another essential ingredient of an efficient Fe-Ox Cu-Au mineral system. They are regarded as an additional source of Cu for leaching. The regions with known deposits are dominated by oxidised rock packages containing hematite?magnetite. These oxidised packages are regarded as essential to maintaining the high redox state of the metal-bearing fluids. Elsewhere where similar granites intrude reduced packages, particularly those bearing graphite, Cu deposits are rare. Where oxidised host packages are not intruded by granites these deposit types do not occur. Equivalent oxidised host packages intruded by granites are not common in the Australian Archaean or Phanerozoic.

In addition to typical seafloor VHMS deposits, the ~3240 Ma Panorama district contains contemporaneous greisen- and vein-hosted Mo-Cu-Zn-Sn occurrences that hosted by the Strelley granite complex, which drove VHMS circulation. High-temperature alteration zones in volcanic rocks underlying the VHMS deposits are dominated by quartz-chlorite±albite assemblages, with lesser low-temperature quartz-sericite±K-feldspar assemblages, typical of VHMS hydrothermal systems. Alteration assemblages associated with granite-hosted greisens and veins, which do not extend into the overlying volcanc pile, include quartz-topaz-muscovite-fluorite and quartz-muscovite(sericite)-chlorite-ankerite. Fluid inclusion and stable isotope data suggest that the greisens formed from high temperature (~590C), high salinity (38-56 wt % NaCl equiv) fluids with high densities (>1.3 g/cm3) and high -18O (9.3±0.6-), which are compatible with magmatic fluids evolved from the Strelley granite complex. Fluids in the volcanic pile (including the VHMS ore-forming fluids) were of lower temperature (90-270C), lower salinity (5.0-11.2 wt % NaCl equiv), with lower densities (0.88-1.01 g/cm3) and lower -18O (-0.8±2.6), compatible with evolved Paleoarchean seawater. Fluids that formed the quartz-chalcopyrite-sphalerite-cassiterite veins, which are present within the upper granite complex, were intermediate in temperature and isotopic composition (T = 240-315C; -18O = 4.3±1.5-) and are interpreted to indicate mixing between the two end-member fluids. Evidence of mixing between evolved seawater and magmatic-hydrothermal fluid in the granite complex, along with a lack of evidence for a magmatic component in fluids from the volcanic pile, suggest partitioning of magmatic-hydrothermal from evolved seawater hydrothermal systems in the Panorama VHMS system, interpreted as a consequence swamping of the system by evolved seawater or density contrasts.

No abstract available

Gold deposits in the Archaean Eastern Goldfields Province in Western Australia were deposited in greenstone supracrustal rocks by fluids migrating up crustal scale fault zones. Regional ENE-WSW D2 shortening of the supracrustal rocks was detached from lower crustal shortening at a regional sub-horizontal detachment surface which transects stratigraphy below the base of the greenstones. Major gold deposits lie close to D3 strike slip faults that extend through the detachment surface and into the middle to lower crust. The detachment originally formed at a depth near the plastic-viscous transition. In orogenic systems the plastic-viscous transition correlates with a low permeability pressure seal separating essentially lithostatic fluid pressures in the upper crust from supralithostatic fluid pressures in the lower crust. This situation arises from collapse in permeability below the plastic-viscous transition because fluid pressures cannot match the mean stress in the rock. If the low permeability pressure seal is subsequently broken by a through-going fault, fluids below the seal would flow into the upper crust. Large, deeply penetrating faults are therefore ideal for focussing fluid flow into the upper crust. Dilatant deformation associated with sliding on faults or the development of shear zones above the seal will lead to tensile failure and fluid-filled extension fractures. In compressional orogens, the extensional fractures would be sub-horizontal, have poor vertical connectivity for fluid movement and could behave as fluids reservoirs. Seismic bright spots at 15-25 km depth in Tibet, Japan and the western United States have been described as examples of present day water or magma concentrations within orogens. The likely drop in rock strength associated with overpressured fluid-rich zones would make this region just above the plastic-viscous transition an ideal depth range to nucleate a regional detachment surface in a deforming crust.