GEMOC ARC National Key Centre

CHARACTERISTICS AND POSSIBLE ORIGINS OF DIAMONDS FROM WELLINGTON AND BINGARA NSW

R. M. Davies¹, S. Y. O'Reilly¹ and W. L. Griffin^1,2
1. GEMOC Macquarie  2. CSIRO Exploration and Mining

The eastern margin of Australia hosts diamond deposits that occur in paleodrainages beneath Tertiary basalt, and in present day alluvial systems. Neither identified source rocks, nor the traditional suite of indicator minerals is recognised in association with the diamonds. Their occurrence overlying Phanerozoic basement terranes of the Tasmanide Orogen, raises the possibility that they may have a younger eastern Australian lithospheric origin, thus potentially identifying new environments for diamond formation and preservation. This study investigates the nature and origin/s of alluvial diamonds from Wellington, NSW and 350 km north at Bingara, NSW, by characterising the diamonds on the basis of their morphological features, mineral inclusions, carbon isotopes, internal structures, nitrogen contents and nitrogen aggregation states.

Diamonds at Wellington and Bingara are of two types, here termed group A and B. At Wellington, group A diamonds dominate in the ratio of 4:1, while at Bingara, the population is essentially of the B type. Surface features of all diamonds are characterised by polished forms that have been strongly rounded by resorption. Etch features are similar to those of diamonds from kimberlite and lamproites, ie indicative of diamond transport to the surface in a magma. Unique to group B diamonds are irregular forms with frosted pits and strong deformation features. Mild abrasion is evident on most stones, and radiation damage is more common in the group A diamonds, suggesting different alluvial histories for the two groups.

The compositions of syngenetic mineral inclusions indicate that the group A diamonds formed in a dominantly peridotitic mantle volume; a small number of stones contain eclogitic inclusions. Olivine (Fo 93) is the dominant inclusion. Rare pentlandite and chromite also occur. The group B diamonds have only eclogitic inclusions with the exception of one diamond containing olivine (Fo 89); the inclusion suite includes a wide range of diopside - omphacite clinopyroxenes (3 to 40% Jd), Ca-rich garnet (61 - 83% Gr), coesite, sphene and molybdenite. Sphene and molybdenite have not previously been recognised as syngenetic inclusions in diamond. Furthermore the extremely calcium-rich compositions of the garnets and many of the clinopyroxenes are unique.

Carbon isotope measurements for group A diamonds range between d13C -10‰ and -3‰, within the world range for peridotite diamonds, and suggest a derivation from a mantle carbon source. Group B diamonds are 13C-enriched (d13C = -5‰ to +3‰), a signature that may suggest a crustal origin for the carbon.

Internal growth features of the group A diamonds are characterised by planar octahedral layers that suggests growth in mostly stable conditions. In 25% of these diamonds, intermediate zones are truncated by one or more resorption episodes with overgrowths of octahedral layers. All diamonds show a late resorption episode that probably occurred in the emplacing magma. Nitrogen contents of group A diamonds are generally high (250 to 2500 atomic ppm). Corresponding nitrogen aggregation states indicate two sub-groups: the main group show a range between 6 to 42% IaB; and a small group of stones has low nitrogen contents (< 900 at. ppm) but high IaB aggregation states (44-95%), indicative of longer mantle thermal maturation histories, and/or greater plastic deformation.

Low-nitrogen group B diamonds show evidence of rapid and unstable growth contemperaneous with deformation in the form of non-planar growth facets and displacement and brecciation of structures that have been rehealed, with episodes of diamond precipitation, and show subsequent overgrowths. In these diamonds, central structures are nitrogen rich (ca 1000 ppm), and rim zones are nitrogen poor (<100 ppm). Group B diamonds with high nitrogen contents have homogeneous structures. The high nitrogen B diamonds appear to be an early growth stage, as do the low nitrogen diamond cores. B diamonds show high degrees of nitrogen aggregation, relative to nitrogen contents with mixes of IaA to IaB types.

The characteristics of the group B diamonds are consistent with formation in a subducted oceanic plate. Rodingitisation of basaltic dikes in peridotite prior to subduction, or mixing with Mg-carbonates, followed by subduction could account for the calcium rich inclusion suite, as well as the heavy carbon isotopic signatures. A subduction origin may link the formation of these diamonds to the New England Orogen. The group A diamonds may be derived from a more conventional source such as cratonic Proterozoic lithosphere of central Australia, or underlying eastern Australia at some time before 200 Ma.