Diamonds of multiple origins from New South Wales: further data and discussion

Abstract

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.

Refining accretionary orogen models for the Tasmanides of eastern Australia

“…Instead, it would indicate that ascent occurred in the late Paleozoic, 'partially' in the widespread late Carboniferous 344, 340, 325, 300 Ma exhumation events referred to above and/or by magmas perhaps generated during early Permian rollback (see above) intersecting stalled or hanging subducted slabs. In the same vein, Sutherland & Barron (2003) suggested that the younger, late Triassic 218 AE 6 Ma 206 Pb-238 U concordia intercept age of a titanite inclusion in a group B diamond from Bingara (Davies et al 2002) implied derivation from a Mesozoic slab.…”

Eclogitic and Ultrahigh-Pressure Crustal Garnets and Their Relationship to Phanerozoic Subduction Diamonds, Bingara Area, New England Fold Belt, Eastern Australia

“…At Bingara-Copeton in northern New South Wales, the deposits have an unknown primary source, but about 500,000 carats of diamonds were produced, mainly from 19th century mining (MacNevin, 1977;Brown, 1995). Compared with diamonds from ancient cratons, delivered to the surface in deep sourced kimberlitic and lamproitic intrusions, recent studies demonstrated unusual features of eastern Australian paleoplacer diamonds; octahedra are rare, and most diamonds have rounded, modified octahedral (mainly dodecahedral) shapes with mechanical twin defects (naats), abundant surface pits and microdisks (minute raised and indented circles and ovals), unique strained and brittledeformed internal structures, 13 C-enrichment and variable states of nitrogen aggregation (MacNevin, 1977;Sobolev 1984;Taylor et al, 1990;Meyer et al, 1997;Davies, 1998;Davies et al, 1998Davies et al, , 2002Barron et al, 2000;Griffin et al, 2000;Sutherland and Barron, 2003). Compared with diamonds from ancient cratons, delivered to the surface in deep sourced kimberlitic and lamproitic intrusions, recent studies demonstrated unusual features of eastern Australian paleoplacer diamonds; octahedra are rare, and most diamonds have rounded, modified octahedral (mainly dodecahedral) shapes with mechanical twin defects (naats), abundant surface pits and microdisks (minute raised and indented circles and ovals), unique strained and brittledeformed internal structures, 13 C-enrichment and variable states of nitrogen aggregation (MacNevin, 1977;Sobolev 1984;Taylor et al, 1990;Meyer et al, 1997;Davies, 1998;Davies et al, 1998Davies et al, , 2002Barron et al, 2000;Griffin et al, 2000;Sutherland and Barron, 2003).…”