The
structure and tectonic setting has had a strong influence on gold mineralisation
at the Porgera gold deposit. Here, the structural controls on the mineralisation
at Porgera are discussed and placed in a regional tectonic context. The Porgera
gold deposit is related to the emplacement of the Porgera Intrusive Complex.
Prior to intrusive emplacement at Porgera,Papua
New Guinea
was affected by a major fold and thrust episode which occurred in response to
arc-continent collision in the Late Miocene. After crustal shortening the region
experienced a relaxation of compression which is expressed in the deformation
accompanying mineralisation at Porgera. The first stage of mineralisation (stage
I) is hosted by extension fractures, and is spatially associated with exposed
intrusive stocks. A later episode of mineralisation (stage II) is hosted by the
faults and fractures of the Roamane Fault Zone, which was active as a dip-slip
normal fault during stage II mineralisation. Therefore a change in the tectonic
environment from shortening to extensional tectonics can be seen at Porgera
before and during mineralisation. Rapid fault-slip during stage II
mineralisation may have aided high-grade gold mineralisation by subjecting
hydrothermal fluids to changes in pressure, temperature, fluid-fluid and
fluid-rock interaction. Uplift to form the highlands of Papua New Guinea
occurred after mineralisation and was largely complete by 4.0 Ma.