The Nena Cu-Au high sulphidation
deposit has formed within the Frieda
River Intrusive Complex in association
with a major dilatant structure.
Multiple intermediate intrusives of
Miocene age are localised by a splay
(Frieda Fault) of the Fiak-Leonard-
Schultz Thrust, a several hundred
kilometre long structure, formed
parallel to the accretionary prism of
northern Papua New Guinea.
The high sulphidation systems
including Nena, crop out as narrow
silica-alunite ridges extending for up
to 5-6km in length within a dilational
structural environment. Alteration at
Mena exhibits two phases of hydro-
thermal activity: an initial stage,
inferred to be volatile-rich, which has
produced zoned alteration; and a later
phase, postulated to have been a
liquid-dominated fluid, which has
brecciated earlier alteration zones and
deposited gangue phases as well as
Cu/Au-bearinq sulphides in open spaces.