SMEDG – PREVIOUS MEETING

DATE: Thursday 9 June 1994

VENUE: Rugby Club, Crane Place, Circular Quay

SPEAKERS: S. R. (Steve) Lismund, Principal Geologist, NSW Geological Survey.

TOPIC: “A New Model for the Origin of some Diamonds in Eastern Australia.” The above region is characterised by thin hot Phanerozoic crust, but has past production of perhaps 500,000 carats of alluvial diamonds, mostly from NSW. Diamonds have been found in several NSW intrusive bodies of alkali basaltic parentage, but this has not encouraged exploration because the traditional models for diamond formation require thick lithosphere more than 1000 Ma old and a kimberlite or lamproite carrier. A new model, termed the Subduction or S-diamond Model, shows how a range of diamond types can form in a dynamic window within a subducting slab at relatively shallow depths. This makes them accessible to subsequent carrier magmas such as leucitite, nephelinite and basinite, all characteristic rock types for diatremes in eastern Australia. The model suggests new guidelines and indicator minerals to identify prospective areas, and indicates likely ages for the crystallisation of the diamonds. Alluvial diamond is the best indicator mineral in Phanerozoic terranes, but sodium levels in garnets can test for the presence of xenoliths of diamond eclogite facies in a diatreme. Eastern NSW is prospective for diamonds!