The
evolution of economic precious metal and copper-gold deposits in high-level
igneous terranes is dependent on very efficient fluid focussing. This is most
readily achieved by contemporaneous deformation, igneous intrusion and
hydrothermal activity. Mineralisation in these environments generally occurs in
fault and fault-vein arrays developed in the cover sedimentary and volcanics
sequences above older regional scale fault systems. Such reactivated fault
systems are particularly prone to developing tile complex fracture arrays,
extreme local dilation and dramatic increases in permeability that are necessary
to efficiently focus hydrothermal fluid and produce large mineral deposits.
The arc
systems of tilePacific
rim are typically structurally and
tectonically complex. At any given period in the evolution of an arc system, it
may be dominated by extensional, compressional or strike-slip deformation.
Indeed, it is common for the tectonic environment to change repeatedly through
the development of a single arc segment. As a result, structures formed in one
tectonic environment are commonly complexly reactivated in quite different
environments later in arc evolution. These complexly reactivated structures have
greater potential to localise extreme fracturing, dilation and fluid flux than
primary structures. Application of this knowledge to practical exploration
requires an understanding of the primary geometry of the controlling fault
systems, the fault movements during reactivation, intrusion and mineralisation,
and the consequent location and geometry of dilational sites. Simple 2D
lineament analysis and targeting on lineament intersections does not
suffice.
Chemical
processes that results from structurally focussed fluid flow may also contribute
to permeability enhancement and further enhance fluid flux. Two main types of
chemical feedback are important. The first involves selective dissolution of
minerals during alteration and tile consequent production of increased porosity
and permeability. The second occurs where alteration changes the mechanical
properties of tile rock mass, especially where it leads to a greater propensity
for fracture (eg by silification). In near-surface environments (< ~1 km),
geomorphic processes complement fault systems in focussing fluid flow to develop
sub-horizontal bodies of mineralisation, such as the giant Ladolam gold deposit
(>42 million ounces of gold).

Along with classification
schemes which stress crustal level, host rocks and alteration assemblages, the
styles of high level 'epithermal' and reated deposits must also be considered in
respect of their controlling co-active fault structures. Such structural
understanding, combined with understanding of tile chemical processes involved
(eg host rock reactivity), provides a major tool for exploration particularly
in terrains obscured by deep weathering. Geological interpretation based on
integration of field mapping with magnetic, radiometric and other remotely
sensed imagery provides a powerful complementary tool to the traditional
geochemical and geophysical targeting techniques. Timely and effective
geological input is capable of reducing exploration risk and providing a more
cost-effective basis for drilling and development. Examples of deposits
associated with fault reactivation in extensional, compressional and strike-slip
settings within arc systems are discussed in this
context.