Geoscience Australia houses a dedicated mineral separation laboratory, which includes specialised down draft fume cabinets to accommodate use of heavy liquids.
Mineral types most frequently separated are zircon, monazite and potassium-rich rock forming minerals including potassium-feldspar and micas. Other mineral types that are separated include titanite, rutiles, apatite, sulphides, tungstates and gold.
Rock samples collected for geochronological analysis from outcrop or drill core can range in size from 100 grams to 30 kilograms in weight. Once they are submitted to the Mineral Separation Laboratory, the following steps are undertaken to separate the minerals required for analysis.
- Sample crushing
Splitters, jaw crushers, ring mills and disc mills are used to reduce the sample to a rock flour in two dedicated crushing rooms.
- Wilfley table method
Wilfley tables are used as a quick, reliable and consistent method for de-sliming bulk rock flours and reducing sample composition to minerals of interest.
- Heavy liquids
Using the known variations in specific gravity between different minerals, heavy liquids are used to further concentrate the minerals required for analysis. In the Geoscience Australia laboratories, two types of heavy liquid are used, tetrabromoethane (specific gravity 2.96) and diidomethane (specific gravity 3.3), which require specialised operating environments.
- Barrier magnetic separation
Arguably the most essential method for bulk heavy mineral concentration, variable current induced magnetic fields interact with mineral grains passing through at set slope and tilt to further separate the sample. Grains with surface slicks, smears coatings, cracks, inclusions and internal fractures may be filtered out in order to select the highest quality grains for analysis.
- Mineral concentrates and determination
Final pure mineral concentrates are achieved through hand-picking individual grains using a binocular microscope. The sample is then ready to be prepared for mass spectrometer analysis.
Sample preparation for mass spectrometer analysis
- Sensitive High Resolution Ion MicroProbe (SHRIMP) mounts
Mineral grains are placed in rows, cast into epoxy resin and polished. The mount is then photographed in plane and transmitted light and for cathode luminescence in order to identify compositional and textural variations within grains. The mount is then gold-coated and ready to be loaded into the SHRIMP for analysis.
- Ar/Ar methods
Mineral concentrates (commonly micas or potassium-feldspar grains) are packed into aluminium foil parcels and placed into irradiation canister. Irradiated samples are then repackaged into tin foil parcels which are dropped into a furnace ready for step heat mass spectrometer analysis.
Geoscience Australia installed a new Sensitive High Resolution Ion MicroProbe IIe (SHRIMP IIe) at the end of 2007 to enable in-house analysis of mineral phases such as zircon and monazite. Along with existing resources and experience, this facility will allow the group to have detailed management of the 'outcrop-to-publication' analytical cycle to ensure that the highest quality data is provided to research projects. The in-house facility will also enable the development of new analytical methods, expanding Geoscience Australia's capability to address increasingly complex geoscience issues.
The Geoscience Australia Geochronology Laboratory also has a strong network of relationships with other geochronology laboratories world-wide that provide access to a range of analytical methods such as Ar-Ar, TIMS U-Pb, etc.