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Abstract: Mining of Cenozoic alluvial deposits at Copeton and Bingara (Eastern Australia) has produced two million macrodiamonds (0.25 ct median size). Raman spectroscopy is used to identify included minerals within uncut Copeton diamonds, with sealed chamber remnant pressures of 31.7 to 35.6 kbar for coesite, 13.6 and 22.7 kbar for clinopyroxene, and 7.6 kbar for grossular garnet. Assuming elastic behaviour, these values generate inclusion entrapment PT loci which intersect, restricting diamond formation conditions: from 250 °C, 43 kbar to 800 °C, 52 kbar. Larger than error (±100 °C and ±4 kbar), this range shows a systematic variation in inclusion composition with diamond zoning and N properties. Published research shows 1) Copeton and Bingara diamonds are unique, and 2) modern alluvium in the Bingara district carries mantle-formed garnet, captured by post-tectonic alkali basalt from an extensive diamondiferous ultrahigh pressure (UHP) terrane that stalled at depth because it is dominated by mafic eclogite. The combined Raman and geological results indicate two sets of subduction UHP diamond formation conditions/protolith are required, firstly cooler oceanic slab and secondly including higher temperature continental crust. The Copeton and Bingara stones are UHP macrodiamonds, and Carboniferous 40Ar/39Ar age dates on clinopyroxene inclusions should be interpreted as ages of crystallisation, representing the termination of subduction. The characteristic features of ruptured inclusions and etched percussion marks on Copeton and Bingara diamond indicate volcanic delivery to the earth''s surface. Alluvial deposits elsewhere in Eastern Australia may carry similar diamond along with diamond of different origin. [Copyright &y& Elsevier]

Raman spectroscopy is used to identify minerals in isolated sealed inclusion chambers in some New South Wales (Australia) diamonds and to determine the remnant internal pressure on each inclusion with values of each: 3.03, 3.31 and 3.54 - 3.62 GPa for coesite, 0.42, 1.36 and 2.3 GPa for diopsidic omphacite, and 0.8 GPa for grossular garnet, each determination for a different uncut stone. The diamond stress due to remnant pressure was found to be proportional to the maximum strain birefringence around each inclusion, as measured by the Berek compensator, with the constant (K = 1000 / 5.8, R2 = 0.93) independent of the inclusion mineral, size, orientation and shape within the limits of error. Thus, simple birefringence measurements of diamond adjacent to inclusions can be used to estimate remnant pressures on inclusions. The calibration shows that one of the stressed diamonds has an anomalously low strain birefringence. [ABSTRACT FROM AUTHOR]

Investigation of coarse (>2 mm) heavy-mineral concentrates from the White Hills Gravel near St Arnaud in western Victoria provides new evidence for the age and provenance of this widespread palaeoplacer formation. A prominent zircon - sapphire - spinel assemblage is characteristic of Cenozoic basaltic-derived gemfields in eastern Australia, while a single diamond shows similar features to others found in alluvial deposits in northeastern Victoria and New South Wales. Dating of two suites of zircons by fission track and U - Pb (SHRIMP) methods gave overlapping ages between 67.4 ± 5.2 and 74.5 ± 6.3 Ma, indicating a maximum age of Late Cretaceous for the formation. Another suite of minerals includes tourmaline (schorl - dravite), andalusite, rutile and anatase, which are probably locally derived from contact metamorphic aureoles in Cambro-Ordovician basement metapelites intruded by Early Devonian granites. U - Pb dating of rutile grains by laser ablation ICPMS gave an age of 393 ± 10 Ma, confirming an Early Devonian age for the regional granites and associated contact metamorphism. Other phases present include pseudorutile, metamorphic corundum of various types, maghemite and hematite, which have more equivocal source rocks. A model to explain the diverse sources of these minerals invokes recycling and mixing of the far-travelled basalt-derived suite with the less mature, locally derived metamorphic suite. Some minerals have probably been recycled from Mesozoic gravels through Early Cenozoic (Paleocene - Eocene) drainage systems during various episodes of weathering, ferruginisation and erosion. Comparison between heavy-mineral assemblages in occurrences of the White Hills Gravel may allow distinction between depositional models advocating either separate drainage networks or coalescing sheets. Such assemblages may also provide evidence for the present-day divide in the Western Uplands being the youngest expression of an old (Late Mesozoic - Early Cenozoic) stable divide separating north- and south-flowing streams or a much younger feature (ca 10 Ma) which disrupted mainly south-flowing drainage. [ABSTRACT FROM AUTHOR]