Diamonds from Wellington, NSW: insights into the origin of eastern Australian diamonds

Abstract

A B S T R A C TDiamonds from alluvial deposits near Wellington, New South Wales, have been characterized on the basis of morphological features, mineral inclusions, C isotope signatures, N content and aggregation state and internal structure. The diamonds are of two types. The larger group (Group A) is indistinguishable from diamonds found worldwide from kimberlitic and lamproitic host rocks. This group is inferred to have formed in a peridotitic mantle source in Pre-Cambrian subcratonic lithosphere. The second group (Group B) is unique in its internal structures (which show evidence of growth in a stress field and non-planar facets), has unusually heavy C isotopic compositions and contains Ca-rich eclogitic inclusions. This group is inferred to have formed in a subducting slab. Diamonds of both groups have external features (corrosion structures and polish) indicating transport to the surface by lamproitic-like magmas. The diamonds show evidence of long residence at the earth's surface and significant alluvial reworking: they are not accompanied by typical diamond indicator minerals.

The roles of primary kimberlitic and secondary Dwyka glacial sources in the development of alluvial and marine diamond deposits in Southern Africa

“…It is speculated that primary sources occur in north-western Australia. Alluvial Group A diamonds along the eastern margin of Australia, are thought to have been derived from a source in eastern Antarctica, on the basis of lower Permian fluvio-glacial patterns (Davies et al, 1999).…”

“…In the central Karoo Basin, Dwyka Group sedimentary rocks Veevers et al (1994b), showing highlands, icetransport vectors and terrestrial-marine margin. Localities of alluvial diamonds derived from Permo-Carboniferous glacial sediments were taken from Tompkins and Gonzaga (1989) for South America, Griffin et al (2001) for Myanmar/Sumatra, and Davies et al (1999) for Australia. Fig.…”

Dissolution of diamond crystals in a heterogeneous (metal–sulfide–silicate) medium at 4 GPa and 1400 °C

“…Note that many type-II diamonds (Bowen et al, 2009;Moore, 2009Moore, , 2014Smith et al, 2016) are irregular crystals, and morphologically complex diamonds with disc sculptures and cavities are widespread in placers derived from unknown or debatable primary sources (Patel and Agarval, 1965;Davies et al, 1999;McCandless et al, 1999;Evdokimov et al, 2001;Kaminsky et al, 2001;Tappert et al, 2009).…”

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

“…Nitrogen aggregation in diamonds is subject to kinetic, deformational and crystal‐growth factors (Evans 1992; Evans et al . 1995; Davies et al . 1999a, b; Khachatryan & Kaminsky 2003).…”

“… New South Wales diamond types plotted as %1aB N against N (atomic ppm) fields. Walcha diamond plots (filled circles) define the Walcha field (W), with other diamond types (A1 + A2 and B1 + B2) modified after Davies et al . (1999a, b, 2002).…”