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Figure 2: The Wairakei geothermal field near
Taupo, New Zealand.

The escape of the super-heated fluids, then
results in a sudden reduction in pressure along
the fault causing boiling and cooling of the fluid
with subsequent precipitation of very fine
grained quartz, calcite and adularia. These
minerals form layers alternating with layers of
native gold or electrum (a mixture of gold and
silver). This layering is called colloform-
crustiform banding and is often associated with
high grade ore, including the highly prized bonanza grade.
In the active geothermal fields of New Zealand, the formation of silica scales on the inside of pipes which
feed steam into power station turbines is a maintenance issue. Geochemical analysis of these scales
indicates the presence of appreciable quantities of gold and silver. An epithermal gold deposit in the
making!

High sulphide deposits

As the name suggests, high sulphide deposits tend to be more sulphide-rich; containing pyrite (iron
sulphide), enargite (copper-arsenic sulphide) as well as gold and silver. They appear to be genetically
and spatially linked to deep seated porphyry copper-gold deposits. High sulphide deposits are formed in
a similar manner to low sulphide deposits, just at a greater depth with a more direct link to the
underlying magma.

Mining Techniques
Historically epithermal gold has been mined by underground techniques as miners followed the narrow,
twisting veins from the surface outcrops far underground. Modern mining techniques, mean that today
most epithermal deposits are mined using open-cut pit techniques.
Gold
Many civilisations have had an eternal love affair with gold. As a consequence, wars have been fought
over it and many lives have been lost. It has become a symbol of wealth and power in many countries of
the world. It is traded as a currency and is viewed as a safe haven in times of economic uncertainty. Gold
is number 79 on the periodic table of the elements. It has the symbol Au,

Which is an abbreviation of aurum; the Latin word for gold. It is in the same chemical group as silver and
copper, and therefore shares some similar properties, such a high electrical conductivity, malleability
and resistance to oxidation.
Gold is prized for its bright yellow color

What are its uses?
and metallic lustre; its resistance to tarnishing in water and air; its softness and ease of working into
intricate designs; and its scarcity. All of which has made gold an ideal metal for jewellery making and
ceremonial items. In fact, gold is too soft in many cases and must be alloyed with copper and silver to
increase its toughness and resistance to wear. Nine carat gold (9ct) contains only 37.5% gold whilst 18
carat gold (18ct) consists of 75% gold. In many parts of Asia, 24 carat or pure gold is the only type of gold
jewellery worn despite its softness.

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