Diamonds from Brickclay Creek, near Walcha, are associated with a Cenozoic nephelinitic intrusion and expand evidence for multiple origins for diamonds from New South Wales. Although the diamonds show similar morphology and C-isotope characteristics to the unusual subduction-related diamonds from the Bingara–Copeton fields, the greater topographic elevation of the associated rocks, and particularly the N aggregation relationships of the diamonds indicate a separate local source. Both yellow and white diamonds from Walcha show limited 1aB nitrogen aggregation (0–7%), similar to that of Ellendale 4 diamonds from Cenozoic pipes in Western Australia. A model is proposed for their local origin from a relatively young (Mesozoic?) subducted mantle slab below the eastern New England Orogen.
Keywords:diamond;dyke;mantle;nephelinite;New England Orogen;New South Wales;nitrogen;subduction