Garnets, Diamonds, Diatremes and Subduction at Bingara, in the New England Orogen of New South Wales, Australia

B. JaneBarron*, L.M. Barron** and G. Duncan

*Consulting Petrologist, 7 Fairview Ave., St Ives, NSW 2075, Australia.

**New South Wales Department of Mineral Resources PO BOX 536, St Leonards, NSW 2065, Australia.

Rimfire Pacific Mining NL, Level 13,379 Queen Street Brisbane, Queensland 4000, Australia.

Corresponding author: email, barronjl@optusnet.com.au

Abstract

At Bingara-Copeton, NSW Australia, alluvial diamond deposits are Cainozoic headless placers in the Phanerozoic New England Orogen, more than 1500km distant from the nearest craton.Recent studies have focussed on unusual calc-silicate eclogitic, and mafic-eclogitic inclusions in the diamonds such as grossular garnet, clinopyroxene, coesite, sphene, and molybdenite, with rare peridotitic inclusions of olivine, pentlandite and chromite. In addition, their unique morphology and surface features suggest a local origin. Known age dates on some diamond inclusions are 340 ± 28Ma -early Carboniferous; 326 Ma -mid Carboniferous; and 218 ± 6 Ma -late Triassic. Formation of these non-cratonic mainly eclogitic metamorphic/metasomatic diamonds has been linked to subduction, either through direct crystallisation within a downgoing slab, or through more indirect mechanisms related to subduction. Here we link their origin to the well documented Phanerozoic geology of the New England Orogen generated by prolonged westerly subduction (associated with terrane accretion) from Ordovician, but particularly during early Carboniferous to about mid-Triassic. Each subduction episode creates an opportunity for trapping portions of a diamondiferous subduction slab within the lithosphere by terminating subduction. There is also evidence for crustal doubling due to tectonic underplating.

     A variety of post subduction alkaline igneous rocks and many diatremes (several reported as diamondiferous), as well as obducted blocks (mainly along the 1200km long Peel-Yarrol Fault zone) could have delivered diamonds and related indicator minerals to the surface. Abundant angular (proximal) garnet is widespread in the modern drainage south of Bingara, and forms the most important indicator of eclogitic diamond that is independent of ubiquitous diaggregated spinel lherzolite minerals from mantle xenoliths.

     More than half of > 2000 microprobe analyses show that olivine, pyroxenes and chromite (mainly related to spinel lherzolite) and some ilmenite lack indicator chemistry. Major element chemistry of 960 garnets is assessed using the Schulze (1997) chemical screen (based on eclogitic garnet chemistry from kimberlite and diamond exploration programs, and regional indicator mineral surveys worldwide).

In terms of Ca-Mg-Fe the Bingara garnets, form a large population that ranges from Cr-poor pyrope through Mg-rich almandine-pyrope to Fe-rich almandine and even Fe-Mn almandine-spessartine compositions. There is a distinct break at 22 wt% FeO, splitting the garnets into two main chemical groups; those from mantle eclogites  (< 22wt% FeO) and those from crustal sources (> 22wt% FeO + MnO). Na₂O and TiO₂ levels discriminate a small, but well constrained Group I diamond eclogitic garnet population from Cr-poor megacrysts and relatively lower pressure Group II eclogitic garnet. The diamond eclogitic garnets are from three samples on basanite ?capped twin hills called Tom and Jerry. About 2% of the garnets are crustal but have Na₂O > 0.5 wt%. Rare garnets of grossular composition also are present. Euhedral Ca-Fe-rich andradite garnets form a separate but minor group.

Twenty four representative garnets were selected for trace- and HREE-element chemistry. Results indicate both MORB and arc mafic protolith with dominantly unfractionated patterns. The crustal garnets with elevated Na₂O are highly enriched in HREE and have strongly fractionated patterns that compare with garnets from leucocratic orthogneiss within obducted UHP terranes. The REE patterns further suggest that the low Cr-pyrope garnets are Mg-rich eclogite compositions derived from picritic oceanic protolith, while the euhedral andradite garnets are related to hydrothermal alteration of serpentinised dunite along the Peel Fault zone. The latter confirms a sparse ophiolitic contribution to the Bingara samples.

Bingara garnet chemistry indicates a variety of igneous protoliths and metamorphic grades, and proves that a high pressure- (HP) to ultrahigh- pressure (UHP) eclogitic terrane at depth has been sampled by igneous intrusions, independent of the ophiolite at the surface.

A single (0.265 carat) white diamond, recovered from the north-east slope of Tom and Jerry, has the unique morphology and surface features of other stones from Copeton-Bingara deep leads.

Therefore, the complex Phanerozoic tectonic history of subduction-accretion and tectonic underplating in the New England region, together with garnet-based evidence presented here for an extensive eclogitic to locally diamond eclogitic terrane with a variety of protoliths at depth, strongly supports a local origin for the diamonds that belong in this tectonic setting.