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Keywords:

  • diamond;
  • dyke;
  • mantle;
  • nephelinite;
  • New England Orogen;
  • New South Wales;
  • nitrogen;
  • subduction

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.