Geophysics

Central Queensland Sapphire Gemfields

The Geology  ( from Qld Dept of Mineral Resources )

Sapphires are of volcanic origin and occur in alluvial deposits in either present day or fossil drainage systems that drain volcanic terrains composed of alkali-volcanic rocks, mainly basaltic lavas and pyroclastics of Tertiary and Quaternary age.

The occurrence of sapphire in Queensland is associated with Tertiary and Quaternary alluvial deposits derived from the weathering and erosion of alkali-volcanic rocks, mainly basaltic lavas, pyroclastics and volcaniclastics of Tertiary age and underlying Palaeozoic basement rocks.

Similar rocks and volcanic terrains are found throughout the highlands of eastern Australia, and extend from Cooktown to Tasmania.

In Queensland, the most important of these volcanic terrains containing significant deposits of sapphire is the Hoy Basalt Province at Anakie and the McBride Basalt Province at Lava Plains.

On the Anakie field basaltic lavas, pyroclastics and underlying granitic and metamorphic rocks were weathered and eroded to form alluvial deposits in a Tertiary palaeodrainage system.

Consequent erosion reworked some of this alluvium, redepositing the materials as more recent deposits in present day drainage systems.

Remnants of the older alluvium occur in many areas as primary, high-level gravels (known as wash by the miners) on elevated ridges between or adjacent to present day drainages.

Deposits may be at the surface or may be covered by up to 20m of overburden.

Sheets of secondary (low-level) wash resulted from erosion of primary wash (eluvium/colluvium) in some areas.

Some younger reworked wash deposits occur in present day drainages as well.

The sapphires and other heavy minerals were transported and deposited in layers of wash, tending to be concentrated in 'runs' along particular channels.

The character and size of sapphire grains and the composition of associated heavy minerals and detritus comprised in the wash varies considerably.

Typically the wash consists of 'billy' (quartzite) boulders, quartz and rock fragments of basaltic lavas, pyroclastic and volcanicalstic rocks and older basement rocks set in a silty or clayey matrix.

The gravel layers are often interlayered with coarse and fine sediments and vary from a few centimetres to over a metre thick and may rest on basement rocks.

Associated minerals found in the sapphire concentrate include zircon, quartz, ilmenite, pleonaste (black spinel), spinel (red), garnet, topaz, tourmaline, diamond (rare), magnesite, hematite, magnetite and limonite.

Qld Dept of Mineral Resources

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genuine sapphire - made by nature

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