Subduction model for the origin of some diamonds in the Phanerozoic of eastern New South Wales

Abstract

Eastern New South Wales has produced over 500 000 carats of diamonds, mostly from the Copeton‐Bingara area in the north. A local derivation is indicated by their distinct tribal character and lack of surface damage, while carbon isotopic values and composition of mineral inclusions are unlike those in diamonds from conventional diamond‐producing areas, for example Yakutia (Siberia), Kapvaal (South Africa), or Argyle (Western Australia). The eastern New South Wales tectonic setting is also unlike that for ‘conventional’ diamonds, involving a Phanerozoic sequence of accreted subduction terranes, with relatively thin hot crust. A subduction diamond model is developed to explain the origin and geology of eastern New South Wales diamonds. This model involves prolonged subduction of mature oceanic crust, resulting in the development of a low‐temperature metamorphic window into the diamond stability field within the downgoing slab at half the depth required by conventional models. The diamonds are preserved at depth by termination of subduction, and brought to the surface by obduction or by entrainment in suitable magmas. The type of diamond formed depends on the original source rock (diamond affinity is shown in brackets): carbonaceous‐rich and carbonaceous‐poor trench sediments (CaFeMgMn‐silicate, eclogitic), graphite‐poor basalt and gabbro (eclogitic) and lherzolite (peridotitic). The model predicts that the New South Wales diamonds are young (Phanerozoic) and that their features, age, associated minerals and geographic distribution relate to New South Wales tectonic provinces. The subduction diamond model extends the range of indicator minerals to include macro‐diamond, corundum, Na‐garnet, with a new series of carrier magmas (basanite, nephelinite, leucitite). Unsuccessful previous exploration in New South Wales, based on conventional models, has produced some positive results when reinterpreted in terms of the subduction diamond model. A re‐evaluation of diamond potential of eastern New South Wales is warranted.