Page 19 - js expolaratıon

mined. Unfortunately, the tonnage of ore in epithermal veins is typically small compared with other
types of lower grade gold deposits.
Gold mineralization within epithermal veins can occur over vertical intervals of a few hundred metres,
vein thicknesses of 1-2m and lengths of up to several hundred metres. In association with epithermal
veins are large alteration haloes where minerals within the country rock have been converted into clay
minerals by the circulating hydrothermal fluids.
Epithermal deposits are typically high grade, small
tonnage and relatively shallow deposits. They are
best mined by underground narrow vein methods.
Rarely, closely spaced veins or vein swarms can
occur, allowing open cut mining methods to
become economically feasible.
The term epithermal was coined to describe a
relatively shallow deposit, hence the Greek word
“epi” meaning shallow and the “thermal” part of the
word referring to the heated fluid which is
necessary for the deposit’s formation.
There are two main types of epithermal gold
deposits; low and high sulphidation (Figure 1).

Figure 1: Diagram showing low and high
sulphidation epithermal deposits

Low sulphide deposits
When ground water comes into contact with hot molten rock deep under volcanoes, silicate minerals
within the rock are dissolved along with metals including gold, silver, arsenic, cadmium, lead, zinc,
antimony and mercury. Large kilometre-scale circulation cells are set up by the movement of these
heated ground waters, which are also known as hydrothermal fluids. These fluids move through cracks,
joints and faults in the rock. As the fluids rise towards the surface, they cool with the precipitation of
mainly quartz, some calcite and adularia, which is a type of potassium feldspar. Within the quartz veins
are found concentrations of the metals being carried in the super-heated ground water.
New Zealand has several commercial epithermal gold deposits- but is also a country where you can
observe the creation of epithermal gold deposits.

Epithermal Gold Deposits In The Making
As the cooling hydrothermal liquids seal up cracks, they create plugs which trap pressure. Earthquakes
can result in an abrupt injection of hot and highly pressurised hydrothermal fluids into newly created
spaces along the fault. Geysers and hot pools are created when these hydrothermal fluids erupt at the
earth’s surface. Around Wairakei in the North Island of New Zealand, steam from these fluids is
harnessed for electricity generation (Figure 2).

SAYFA 18