Geophysics

New England Sapphire Gemfields

The sapphire bearing region of the New England district comprises Permian granites, porphyries, and acid volcanics and Permo-Carboniferous metasediments overlain by large areas of Tertiary basalt. Most of the high ground is capped by basalt which is up to 300 m thick in places. These basalt areas display a terraced physiography which is controlled by the multiple flows.

The sapphires occur, for the most part, in the alluvial gravels of the present-day stream system but, are also found on the surface (usually in basaltic soil) well above th6 present stream deposits. Many of the streams in the area have cut through the basalt, and the gravels being worked for sapphire have been deposited on Palaeozoic basement rocks. There also appears to be some potential for locating sapphires in different environments beneath the present day alluvial gravels

The sapphire bearing wash has an average thickness of between and 2 m, and usually occurs beneath a clayey, black soil overburden which is about 0.3 m thick but can be much thicker. The wash occurs as narrow lenticular bodies in the present stream and as relatively large areas of alluvium marking the position of older stream beds (some river flats up to 8 ha in area are s bearing.)

The sapphire is concentrated in irregular pockets and horizons in the wash, and usually the very bottom of the wash gives the highest yield. Hence most sapphire plants treat all the wash down to bedrock, where large boulders up to 1 m in diameter occur surrounded by fine, sapphire bearing gravel embedded in a clayey matrix.

The coarse gravel is usually composed of rounded fragments of porphyry, granite, and basalt, and angular fragments of indurated mudstone ("trap") and quartzite.

The finer gravel associated with sapphire is generally composed of black pleonaste (one of the spine] group of minerals); basalt pebbles; ironstone (including bauxite oolites); quartz (both rounded and subangular and as small bipyramidal crystals); fragments of porphyry, granite, and metasediment; orange-brown zircon (usually well rounded); ilmenite; tourmaline; and, more rarely, enstatite. Sapphire~ are usually present as subangular crystal fragments, commonly talk ing the form of tapering hexagonal prisms, though a few grains are well rounded

Until the early 1980s, most of the sapphires won from the alluvial deposits of the New England region of New South Wales were thought to have been derived from the weathering and erosion of basaltic lava.

The occasional recovery of specimens showing a sapphire crystal embedded in basalt was thought to confirm this theory However, careful inspection of the top of the sapphire crystal embedded in basalt shows that the sapphire crystal has ragged edges. This means that the crystal was being chemically attacked by the molten basalt lava and was thus 'foreign` to it. In other words, it was a non-compatible inclusion picked up by the lava.

It is now recognised that the primary source of the sapphires is not molten lava but early explosive phases of volcanic activity which produced ash falls (of the same type as buried Pompei). The term 'volcaniclastic' covers both direct ash-fall deposits and material which has been reworked by alluvial processes. In contrast to the volcaniclastic rocks, the sapphire and corundum content of the basalts is generally low. The volcaniclastic rocks brought their sapphire component up from deep in the crust. The rising molten magma carrying the sapphires was explosively expelled from volcanoes when it came into contact with groundwater. When such volcaniclastic detritus was deposited in pre-existing drainage channels, separation of crystals and rock fragments from fine grained material occurred in places by the process of mass flow. Such deposits have commonly been further reworked by stream action leaving some rich deposits of sapphire, grading even up to a kilogram or more of corundum in a cubic metre of wash.

* Information on this page derived from the New South Wales Dept of Mineral Resources

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