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
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
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
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
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
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
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
genuine sapphire -
made by nature
Resource on the Australian Sapphire Industry