Al-rich diopside in alluvial ruby and corundum-bearing xenoliths, Australian and SE Asian basalt fields

Abstract

Alluvial rubies and sapphires are found in palaeodrainage deposits along the Cudgegong-Macquarie River system, central eastern New South Wales, Australia. A pink to red suite contains Cr (up to 0.6 wt.% Cr2O3) as the main chromophore, exceeding Fe (up to 0.5 wt.%Fe2O3). Corrosive etching suggests a prior xenocrystic Mesozoic-Cenozoic basaltic transport, while Cr2O3/Ga2O3to Fe2O3/TiO2ratios indicate an original metamorphic source. Syngenetic mineral inclusions include Al-rich diopside, meionite and anatase. The Al-rich diopside (‘fassaite’) contains extremely high Al2O3(20–21 wt.%). A blue-green suite contains Fe (up to 0.8 wt.% Fe2O3) as a dominant chromophore, while a rare nepheline-anorthoclase composite inclusion supports a magmatic phonolitic origin. The Cudgegong- Macquarie ruby formation is compared with a garnet granulite origin proposed for Thailand rubies and a xenolith of corundum-bearing garnet granulite from Ruby Hill, Bingara, Australia. Clinopyroxenecorundum thermometry suggests the Cudgegong-Macquarie rubies formed atT>1000–1300ºC, a high equilibrationTfor proposed lithospheric granulites. These rubies form a distinctive suite compared to other rubies from Australian and SE Asian basalt fields, but have some similarities with eastern Thailand rubies.

Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China

“…Nevertheless, the associations of corundum together with clinopyroxene and garnet (Sobolev et al, 1998) or Alrich clinopyroxene have been identified in diamond (Hutchinson et al, 2001(Hutchinson et al, , 2004 as well as in corundum (Saminpanya and Sutherland, 2011;Sutherland et al, 2003;Sutthirat et al, 2001). At the Juina kimberlite deposit, high Cr 2 O 3 ruby inclusions in diamond are associated with Al-rich pyroxene (up to 10 wt% Al 2 O 3 ), ferropericlase and Mg-perovskite (Hutchinson et al, 2001).…”

Diversity in Ruby Geochemistry and Its Inclusions: Intra- and Inter- Continental Comparisons from Myanmar and Eastern Australia

“…The Macquarie River ruby grades into pink sapphire and its source region is poorly constrained, as stones were recovered as accessories in gold dredging operations in the alluvial river bed. It accompanies diamonds and represents a distinctive highly Mg-rich, high temperature genetic type [47]. The New England ruby is a rare accessory in gem field workings within the dissected Maybole shield volcano sequences [48].…”

“…Photos of ruby features from Macquarie-Cudgegong River system alluvial deposits, NSW (a) ruby fragment, a few mm across with pyrope garnet inclusion (center). (b) Facetted stones of ruby and pink sapphire, up to 3 mm across[47]. Photographer G. Webb, Australian Museum.…”

Trace-element compositions of sapphire and ruby from the eastern Australian gemstone belt

“…In the case of Australian basaltrelated corundum, the conditions required for formation are enigmatic: corundum crystals have mineralogical associations suggesting a wide range of pressure, temperature and geochemical conditions during their generation (e.g. Sutherland and Coenraads, 1996;Sutherland et al, 2003;Zaw et al, 2006). In eastern Australia, sapphires and rubies seem to have been transported to the surface as xenocrysts in alkali basalts (see Table 1), but they are also found in xenoliths in rare instances (Sutherland et al, 2003(Sutherland et al, , 2015.…”