Diamonds from Wellington, NSW: insights into the origin of eastern Australian diamonds

Diamonds from alluvial deposits near Wellington, New South Wales, have been characterized on the basis of morphological features, mineral inclusions, C isotope signatures, N content and aggregation state and internal structure. The diamonds are of two types. The larger group (Group A) is indistinguishable from diamonds found worldwide from kimberlitic and lamproitic host rocks. This group is inferred to have formed in a peridotitic mantle source in Pre-Cambrian subcratonic lithosphere. The second group (Group B) is unique in its internal structures (which show evidence of growth in a stress field and non-planar facets), has unusually heavy C isotopic compositions and contains Ca-rich eclogitic inclusions. This group is inferred to have formed in a subducting slab. Diamonds of both groups have external features (corrosion structures and polish) indicating transport to the surface by lamproitic-like magmas. The diamonds show evidence of long residence at the earth's surface and significant alluvial reworking: they are not accompanied by typical diamond indicator minerals.