Abstracts from the 4th Biennial Conference on Asian Current Research on Fluid Inclusions held in Brisbane, Australia from 10 - 12 August 2012

Exploring for the Future (EFTF) is an Australian Government program led by Geoscience Australia (GA), in partnership with state and Northern Territory governments. The EFTF program (2016-2024) aims to drive industry investment in resource exploration in frontier regions of onshore Australia by providing new precompetitive data and information about their energy, mineral and groundwater resource potential. Under the EFTF program, GA’s National Hydrogen Project and in collaboration with Minerals Resources Tasmania (MRT) undertook a study of hydrogen and helium potential of south-east Tasmania with the sampling of cores from Jericho 1 on Bruny Island. This well was selected based on the availability of core and historic reports of hydrogen-rich natural gases from this well and petroleum exploration wells in the region. Sampling of cores was done at MRT’s Core Repository in Hobart. Geoscience Australia commissioned a fluid inclusion stratigraphy (FIS) study on the downhole samples. Here, volatile components ostensibly trapped with fluid inclusions are released and analysed revealing the level of exposure of the well section to migrating fluids. Integration of thin section (TS) preparations reveal the extent of gas and fluid trapping within fluid inclusions while microthemometry (MT) gives an estimation of fluid inclusion trapping temperature. For Jericho 1, FIS analysis was performed on 179 cores between 87 m and 640.6 m base depth, together with 7 samples prepared for TS and 1 sample for MT. To support this study, lithostratigraphic tops were compiled by MRT. The results of the study are found in the accompanying documents.

Raman spectroscopy has been an invaluable method for the non-destructive analysis of fluid inclusions for over 30 years1 and since then it has also been applied to the study of melt inclusions2. While the analysis of gas species in these inclusions is relatively straight forward, the identification of stable and metastable solid phases in inclusions is more challenging due to the limited availability of reference Raman spectra for some minerals. Some examples of inclusions with challenging Raman spectra are discussed below.

The Paleoproterozoic Westmoreland region is located 1250 km southeast of Darwin. The Westmoreland region is flanked on the southeast by the Paleoproterozoic Mt Isa Inlier and the Neoproterozoic South Nicholson Basin and in the northwest it is overlapped by Mesoproterozoic sediments of the McArthur Basin. The northern and southern ends of the McArthur basin share many geologic attributes including correlative stratigraphic rock types, which suggests that there is potential for unconformity-related uranium deposits in the southern McArthur basin and adjacent Westmoreland region. In fact, over fifty occurrences of uranium (some with minor gold) and copper mineralisation have been recorded in the Westmoreland region. Fluid inclusion studies have been carried out on selected uranium and copper prospects on the Northern Territory side of the Westmoreland region. Four types of inclusions have been observed, (Type A) Vapour-rich inclusions containing 30 100 vol.% vapour. Varying amounts of CO2 ± N2 ± CH4 have been detected in these inclusions, (Type B) Liquid-rich inclusions with up to 30 vol.% vapour, (Type C) Liquid-only inclusions, and (Type D) Three-phase (vapour + liquid + solid) liquid-rich inclusions containing a small daughter crystal. Type A, vapour-rich inclusions and some Type B, liquid-rich inclusions homogenised over the range 171 to 385 °C and are thought to be related to early metamorphic events. Other Type B and Type D inclusions typically homogenised between 100 and 240 °C with a mode around 120 °C, while the presence of liquid-only inclusions suggests trapping at temperatures below 50 °C. Eutectic melting temperatures indicate the presence of CaCl2 in the fluids but final melting temperatures show the presence of both high and low salinity brines. This suggests mixing between saline basinal fluids and low salinity meteoric fluids that continued down to temperatures below 50 °C.

This web-enabled system allows researchers to retrieve fluid inclusion data from anywhere in the world. The concept is to build a free and widely available web-based library of fluid properties for a range of geological fluids. The database is being developed as an "open" project, which intends to bring together researchers interested in the properties of geological fluids or fluid inclusions.

This report is a synopsis of research compiled and carried out within the Predictive Mineral Discovery CRC F3 project "Micrometallogeny of hydrothermal fluids". The F3 project's original objectives were: (1) Geology-driven terrain and ore fluids investigations to evaluate the chemistry and fluid processes within mineral systems in order to extend the focus of fluid studies beyond direct ore deposit analysis. (2) LAICPMS and PIXE technique and methodology development were techniques utilized throughout the project and methodologies were developed for their combined application. (3) Diamond-cell autoclave experiments component of the project was a scoping-collaboration in year one of F3 with the Museum of South Australia. (4) Database development and fusion with numerical modelling resulted in development of a web-based database for fluid inclusion research has been successful and is currently accessed at http://www.ga.gov.au/minerals/research/methodology/geofluids/flincs_about.jsp

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.

Fluid inclusion studies have been carried out on quartz veining from Jackson's Pit and Eva uranium mines and the Dianne and St Barb copper prospects in the Westmoreland region. Four types of inclusions have been observed. Type A, vapour-rich inclusions, contain 30 - 100 vol.% vapour with varying amounts of CO2 ± N2 ± CH4. Type B, liquid-rich inclusions, contain up to 30 vol.% vapour. Type C inclusions are liquid-only. Type D, three-phase (vapour + liquid + solid) liquid-rich inclusions, contain a small daughter crystal. Type A, vapour-rich inclusions and some Type B, liquid-rich inclusions homogenised over the range 171 to 385°C. Other Type B and Type D inclusions typically homogenised between 100 and 240°C with a mode around 120°C, while the presence of liquid-only inclusions suggests trapping at temperatures below 50°C. This may indicate three phases of fluid flow in the region with progressively cooling fluids. Eutectic melting temperatures as low as -79.8ºC in Type B and C inclusions suggest the presence of CaCl2 and other salts in the fluids. Final ice meeting temperatures for Type B and C inclusions fall into two groups. The first group has final melting temperatures below -10ºC while the second group shows final meeting above -10ºC and more typically close to 0ºC indicating the presence of low salinity fluids. This suggests mixing between saline basinal fluids and low salinity meteoric fluids that continued down to temperatures below 50°C.

The Maldon gold deposit in central Victoria lies within the contact aureole of the Harcourt Granite and associated granitic dykes. The ore-bearing fluids are characterised by the presence of CH4-rich fluids, which exhibit complex freezing and heating behaviour, as well as mixed CO2-low-salinity aqueous fluids. Raman analysis indicates that the CH4-rich inclusions from Maldon contain 30 to >70 mol.% CH4 (and <40 mol.% N2) and have highly variable ratios of CH4/CO2, ranging from 0.12 to 15.6. The presence of graphite in the CH4-rich inclusions indicates either accidental trapping of the solid phase at high temperature or post-trapping changes (i.e. incomplete reactions). Higher-salinity brine inclusions only occur locally. The Harcourt Granite is a moderately reduced, I-type granite and the reduced fluids are believed to have formed within (or in close proximity to) thermal aureoles of the dykes or granites during contact metamorphism. We conclude that the Maldon deposit is an 'orogenic' gold deposit that was metamorphosed and/or remobilised during the emplacement of post-orogenic intrusions/dykes. The late-stage magmatic fluids and retrograde metamorphic fluids have produced many of the features associated with other well documented reduced intrusion-related gold systems. This suggests that some 'orogenic' gold deposits may have been completely overprinted by later magmatic/metamorphic events and are now only evident as reduced intrusion-related gold systems.

Exploring for the Future (EFTF) is an Australian Government program led by Geoscience Australia (GA), in partnership with state and Northern Territory governments. The EFTF program (2016-2024) aims to drive industry investment in resource exploration in frontier regions of onshore Australia by providing new precompetitive data and information about their energy, mineral and groundwater resource potential. Under the EFTF program, the Onshore Energy Project undertook a study of petroleum prospectivity of the onshore Officer Basin in South Australia and Western Australia. Birksgate 1 well in South Australia was selected based on the occurrence of gas and oil shows reported in the well completion report. Sampling of cuttings and cores was done at Geoscience Australia's Petroleum Data Repository in Canberra. Geoscience Australia commissioned a fluid inclusion stratigraphy (FIS) study on the downhole samples. Here, volatile components ostensibly trapped with fluid inclusions are released and analysed revealing the level of exposure of the well section to migrating fluids. Integration of thin section (TS) preparations reveal to extent of gas and fluid trapping within fluid inclusions while microthemometry (MT) gives an estimation of fluid inclusion trapping temperature. For Birksgate 1, FIS analysis was performed on 414 cuttings and 33 cores between 150 feet and 6161 feet base depth, together with 14 samples prepared for TS and 3 samples for MT. To support this study, lithostratigraphic tops were compiled by Geoscience Australia. The results of the study are found in the accompanying documents.