Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes

Abstract

Corundum xenocrysts from alkaline basalt fields differ in characteristics and hence lithospheric origins. Trace element, fluid/melt inclusion and oxygen isotope studies on two eastern Australian corundum deposits are compared to consider their origins. Sapphires from Weldborough, NE Tasmania, are magmatic (high-Ga, av. 200 ppm) and dominated by Fe (av. 3300 ppm) and variable Ti (av. 400 ppm) as chromophores. They contain Cl, Fe, Ga, Ti and CO 2 -rich fluid inclusions and give d 18 O values (5.1À6.2%) of mantle range. Geochronology on companion zircons suggests several sources (from 290 Ma to 47 Ma) were disrupted by basaltic melts (47 Ô 0.6 Ma). Gem corundums from Barrington, New South Wales, also include magmatic sapphires (Ga av. 170 ppm; d 18 O 4.6À5.8%), but with more Fe (av. 9000 ppm) and less Ti (av. 300 ppm) as chromophores. Zircon dating suggests that gem formation preceded and was overlapped by Cenozoic basaltic melt generation (59À4 Ma). In contrast, a metamorphic sapphire-ruby suite (low-Ga, av. 30 ppm) here incorporates greater Cr into the chromophores (up to 2250 ppm). Fluid inclusions are CO 2 -poor, but melt inclusions suggest some alkaline melt interaction. The d 18 O values (5.1À6.2%) overlap magmatic sapphire values. Interactions at contact zones (T = 780À940ºC) between earlier Permian ultramafic bodies and later alkaline fluid activity may explain the formation of rubies.

Sapphires related to alkali basalts from the Cerová Highlands, Western Carpathians (southern Slovakia): composition and origin

“…Sutherland et al 2002Sutherland et al , 2009Garnier et al 2005;Zaw et al 2006;Peucat et al 2007). Generally, Fe and Ga reveal higher concentrations in magmatic sapphires (~1800 to 13,000 ppm Fe and ~70 to 570 ppm Ga), than in metamorphic ones which have Fe and Ga contents less than 3000 and 75 ppm, respectively.…”

“…The 18 O value in gem corundum represents a useful tool for deciphering its geological origin (Yui et al 2003Giuliani et al 2005Giuliani et al , 2007Giuliani et al , 2009Garnier et al 2005;Zaw et al 2006;Sutherland et al 2009). As the mantle and crustal rocks show distinct oxygen isotope compositions, the 18 O value enables us to investigate the origin and source of corundum.…”

“…Several principal genetic groups can be subdivided based on sapphire 18 O values (Giuliani et al 2007): pink sapphires hosted in cordieritite ( 18 O = 1.7 to 2.9 ‰), coloured and blue sapphires in lamprophyre ( 18 O = 5.4 to 6.8 ‰), in syenitic rocks (4.4 to 8.3 ‰), in desilicated pegmatites associated with amphibolites and pyroxenites (4.2 to 7.5, locally to 11.2 ‰), in biotitite in gneiss (4.6 to 9.0 ‰), in calc-silicates and skarns (7.7 to 10.7 ‰), in desilicated pegmatites in marble (15.5 to 15.9 ‰), and blue-reddish sapphire with ruby in marble (16.3 to 22.3 ‰). Giuliani et al 2005Giuliani et al , 2007Zaw et al 2006). Additional O-isotopic composition of sapphires from biotitites in gneiss from Madagascar are presented in order to complete an earlier database (Giuliani et al , 2007).…”

Petrogenesis of alkaline basalt-hosted sapphire megacrysts. Petrological and geochemical investigations of in situ sapphire occurrences from the Siebengebirge Volcanic Field, Germany

“…Most authors (e.g., Sutherland et al 1998b) relate the HFSE enrichment of the sapphires and the presence of Columbite Group, Pyrochlore Group and of Nb-rich rutile mineral inclusions to highly evolved SiO 2 undersaturated silicate melts such as syenites, or nepheline syenites. A further observation leading these authors to favor syenites as parental melts is that repeatedly melt inclusions of syenitic composition were reported from magmatic sapphires (e.g., Pakhomova et al 2006;Zaw et al 2006;Izokh et al 2010). Up until now, carbonatites have not been considered as potential parental melts, although many authors (e.g., Coenraads et al 1990;Van Long et al 2004;McGee 2005;Nechaev et al 2009;Izokh et al 2010;Palke et al 2017) reported that CO 2 must have played a significant role in the formation of these sapphires, as CO 2 -rich fluid inclusions are commonly observed in many magmatic sapphires.…”

Morphology and mineral chemistry of monazite–zircon-bearing stream sediments of continental placer deposits (SE Germany): Ore guide and provenance marker

“…The zircon found in the stream sediments are weakly translucent and multicolored, showing yellow to brownish tints and, in places, achieve jeweler's quality. Detrital zircon of this type were also observed in alkaline basalts in NSW, Australia, Thailand, Vietnam, Cambodia and Saxony, Germany, where zircon is also associated with sapphire of gem quality (Garnier et al, 2005;Tröger and Seifert, 1963;Zaw et al, 2006). In the chessboard classification scheme sensu Dill (2010) this type of deposit has been categorized as 39B, zircon-sapphire deposits within alkaline basalts.…”