Corundum Conundrum


1Unsw, Kensington, Australia

Volcaniclastics and basalts are widely spread across the Anakie Gemfields in Central Queensland, Australia. A principal elevator of sapphires to the surface is widely believed to have been eruptions from Hoy Province volcanoes, followed by disintegration and release of sapphire from lava flows. However, it is noteworthy that volcaniclastic rocks are usually associated with areas where intensive sapphire mining has taken place. Although corundum has been reported from some Hoy plugs, it is extremely rare but is more commonly found by miners in volcaniclastics (called clinker by local miners). Crucially, although numerous basalt clasts of various textural types occur within the mined gravels, despite a hundred years of mining miners have not found any corundum within these clasts. The volcaniclastics from this field vary widely in mineralogy, texture, and chemistry. A surprising result is that some samples comprise over 50% dolomite, with some containing much higher amounts of around 70%. The volcanic origin of these is supported by anomalously high levels of zirconium and titanium. In addition, these alkaline rocks sometimes contain abundant small equant-shaped flakes of biotite, now altered to clays. This raises an important question: were the dolomite-rich volcaniclastics erupted from one or more carbonatite-type volcanos? And were these also the source for the corundums? Also, what were the sources for the diverse suites of zircons and spinels found in the gem gravels? The timing of events is complicated by the range in ages of Hoy plugs from 67 Ma to 14 Ma. The most likely scenario is that lavas flows initially protected earlier volcaniclastics from erosion. After removal of the flows, the volcaniclastics were free to release corundum to the waterways. Trace element geochemistry for seven plugs and four volcaniclastic samples suggests that they are genetically related. Matters are complicated by an undocumented basalt field high in the Drummond Range, west of the sapphire fields where clinker was also observed, along with possible volcaniclastics immediately below the basalt flows.


Forty six years experience within the resources sector including working as a  petrologist, exploration geologist, equity analyst and portfolio manager, commodity analyst, gold fund manager and minerals economist.

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