GEMOC ARC National Key Centre

R.M. Davies1, Suzanne Y. O'Reilly1 and W.L. Griffin1,2
1. GEMOC Macquarie, 2. CSIRO Exploration and Mining

Diamonds in eastern Australia occur in alluvial deposits overlying Phanerozoic basement. The primary source of the diamonds is not known. No feeder pipes of kimberlite or lamproite are known, and none of the usual indicator minerals occurs in association with the diamonds.

Alluvial diamonds from Wellington, Bingara, Copeton and Airly Mountain (NSW) form two distinct groups, here termed A and B (Davies et al., 1999a). Group A diamonds are similar to those found in kimberlites and lamproites worldwide and are inferred to have formed in Precambrian lithospheric mantle. Group B diamonds have unusual characteristics that indicate they formed in a subduction environment. Surface features of diamonds of both groups indicate that all were emplaced at the surface by magmas.

Group A diamonds are similar to diamonds found in kimberlitic and lamproitic hosts in Archean and Proterozoic cratons worldwide in their primary crystal form, internal structure, mineral inclusion composition (mainly peridotitic) and carbon isotopes. Re-Os ages (3.4 and 2.1 Ga; Pearson et al., 1998) determined in situ for sulfide inclusions in two Group A diamonds constrain the origin of these diamonds to ancient mantle sources. This age information and the extensive surface abrasion structures and radiation damage suggest that the Group A diamonds represent an older group of diamonds that have been in secondary collectors for a significant time. If this is so, it is feasible that the Group A diamonds may be derived from a variety of sources, including possible (but unidentified) sources in Antarctica.

Group B diamonds are unlike any other diamond suites worldwide in their combination of shape, surface features, strained and irregular internal structures (Davies et al., 1999b), enriched carbon isotopes, and Ca-rich eclogitic mineral inclusions. These features are best explained as a product of diamond growth in a high-P/T dynamic environment such as a subducting slab. The diamonds may have formed during arc-continent collision at the time of the development of the New England Fold Belt. This would explain why the major known concentration of Group B diamonds is within the New England Fold Belt at Copeton and Bingara, and is consistent with the Phanerozoic ages for diamond emplacement determined from mineral inclusions (326±43 Ma; 39Ar-40Ar for multiple clinopyroxene inclusions from a single diamond; D. Phillips, pers. comm., 1998; 218±6 Ma; U-Pb for an in situ sphene inclusion; Davies and Kinny unpubl. data).

All Group B diamonds have resorption textures that indicate emplacement by magma, and all show sorting and some degree of abrasion. Group B diamonds that make up a small proportion of the alluvial diamond population at Wellington, in the Lachlan Fold Belt, are typically more abraded than Group B diamonds from Copeton and Bingara.

Pearson, D. G., Davies, R.M., Shirey, S.B., Carlson, R.W. and Griffin, W.L., 1998, The age and origin of eastern Australian diamonds: Re-Os isotope evidence from sulfide inclusions in two diamonds from Wellington, New South Wales: Extended Abstract 7th International Kimberlite Conference, South Africa, p. 664-666.

Davies, R. M., O'Reilly, S. Y., and Griffin, W. L., 1999a, Diamonds from Wellington, NSW: new insights into the origin of eastern Australian diamonds: Mineralogical Magazine, v. 63, p. 447-471.

Davies, R. M., O'Reilly, S. Y., and Griffin, W. L., 1999b, Growth structures and nitrogen characteristics of Group B diamonds from Wellington and Bingara, eastern Australia: Proceedings of the Seventh International Kimberlite Conference, p. 156-163.