Mineral Resources Potential - Geothermal Resources

Metallic Minerals 39
to 40O0c). The fluids use the fault/fracture zones as permeable
channels along which to tow from their region of origin untii they
reach a point where in any of a number of factors -chemical
reactions with country rock andlor changes. in the temperature
andlor pressure -causes the fluids to precipitate. The gold
precipitates out of solution along with the quartz vein material.
These regional fault systems develop during the waning stages of
continental collision and hence can form at significantly later
periods than the host rocks; as such, they are termed "epigenetic."
The actual host rocks of the orogenic mesotl~ern~al lode
deposits are affected by these fnultlfracture origins and can range
fmn mylonites to fault gouge. Mylonites indicate deformation
under confining pressures sufficiently high that the rock
recrystallizes to a fine grain size. This is plastic or ductile
behavior, and indicates that the vein formed deep in the earth's
am@, Alternatively, if the faultlfrachlre cuts a rock at a level cIose
, to the earth's surface, then it does not have the same confining
pressure and hence will break into fault gouge.
Typical orogenic mesothermal lode gold deposits consist of
quark veins with gold, pyrite andlor arsenopyrite, The gold is
usually pure gold and can be present in textures ranging from
solitary grains to grains intimately ii~tergrown with sulphide
minerals. In some deposits, gold is present as "invisible"
intergrowths with suIphj& minerals such as arsenopycite (that is,
the gold is in the crystal lattice of the sulphide mineral). In other
deposits, the gold is not pure but electnun -a mineral made up of
gold, with 20% to 80% silver. Orogenic mesothermal vein gold
systems are characterized by abundant, typically iron-rich,
hydrothermal carbonate alteration assemblages which spread into
the host rock from the vein. They represent pulses of fluid which
flowed along the hctudfault plane into the surrounding country
lock with which they am not in chemical equilibrium, producing
chemical reactions and the resultant alteration halo.