Page 21 - Handbook of Gold Exploration and Evaluation

4 Handbook of gold exploration and evaluation

· Coefficient of linear thermal expansion
± 0.0000142 cm/cm/ëC (0 ëC)
· Conductivity
6
Electrical: ± 0.452 10 /cm
Thermal: ± 3.17 W/cmK
Electronic processes create heat, and gold is essential to transfer heat easily from
delicate instruments. A 35% gold alloy is used in the main engine nozzle of the
space shuttle, where temperatures can reach 3300 ëC. It is the most tenacious and
long-performing material available for protection at these temperatures.
· Reflectivity ± high-purity gold reflects up to 99% of infra-red rays. This
makes it ideal for heat and radiation reflection, as in life-saving face shields
for astronauts and fire fighters.
· Density ± 19.32 g/cc @ 300 K
· Melting point ± 1064.58 ëC
3
· Molar volume ± 10.2 cm /mole
· Specific heat ± 0.128 j/gK
· Malleability ± can be hammered into a sheet so thin that light can pass through
it; one troy ounce of gold can be hammered into a one square metre sheet.
· Ductility ± can be drawn into long thin wires; a single troy ounce of gold can
be drawn into a thread approximately 5 miles long.
· Hardness ± 2 to 3 (scale of comparative hardness of minerals from talc (1) to
diamond (10) (Dana, 1890))
· Crystal System - isometric.
As a relatively rare element in the Earth's crust, gold is widely although not
evenly distributed. It evolves as a siderophile element from the Fe-Ni core at
crustal spreading centres and is present in Fe-Ni sulphides in the upper mantle.
During partial melting of these materials the sulphides are partly consumed
while the gold and other metals rise with basaltic fluids into the crust along mid-
ocean ridges and at subduction zones. It is then associated with complex
processes involving convection, subduction, partial melting, hydrothermal pro-
cessing, weathering, erosion, and deposition before being returned to the mantle
for recycling at subduction centres. Significantly large mineral deposits require
the coincidence of particularly favourable processes and source parameters.
Figure 1.1 depicts the basic requirement for the formation of any ore deposit
with gold as an example. The degree of element concentration demonstrates the
importance of an enriched source in minimising the concentration factor and
hence the required efficiency and probability of the gold ore forming process.
Fluids that can carry more than 10 ppb Au are excellent ore-forming solutions.
Gold occurs mainly in its native state, often alloyed with silver, copper,
bismuth, mercury and platinum-group elements, and in tellurides and selenides.
Native gold is distinguished from other minerals having similar yellowish
coloration, often referred to as `fools gold' (e.g. pyrite, chalcopyrite and