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!
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