Structural controls on fluid movement during deformation and mineralisation at the Porgera gold deposit, Papua New Guinea
Abstract
The Porgera gold mine is a large, high-grade, vein and fault-hosted deposit located in the
highlands of Papua New Guinea. Gold mineralisation at Porgera is spatially associated
with oxidised, hydrous magmas of the Porgera Intrusive Complex. Prior to emplacement
of the intrusive complex, the Porgera area experienced a period of shortening due to
collision of a southward-moving arc with the northern edge of Papua New Guinea. Two
stages of mineralisation which have distinct mineral assemblages and structural
association, formed shortly after emplacement of the intrusive complex at a depth of
approximately 3 km.
Stage I veins are spatially associated with the exposed stocks of the intrusive complex.
The stage I veins are hosted by extension fractures, which formed in response to high
fluid pressures generated during magmatic volatile exsolution (from depth) within the
intrusive complex. The formation of sub-horizontal stage I veins indicates fluid
pressures, at least periodically, exceeded the lithostatic load. The stage I veins
predominantly contain base metal sulphides (pyrite + sphalerite +galena). Native gold
occurs as inclusions within pyrite resulting in gold grad~s up to 20 git within the veins.
Stage II mineralisation is hosted by faults and fractures of the ENE-striking Roamane
Fault Zone. The Roamane Fault Zone is a late structure (post stage I mineralisation)
which hosts the stage n mineralisation as the matrix to multiple generations of breccia and
cataclasite. The largest single concentration of stage II mineralisation occurs within the
immediate footwall of the principal displacement zone, where a breccia and cataclasite
zone up to 5 m wide (footwall breccia) dips steeply SSE. This mineralisation forms the
high-grade core to the Porgera deposit (Zone VII and VllA). Stage II mineralisation is
also hosted by subsidiary, E-striking (steeply dipping) and ENE-striking (sub-vertical)
fault-fractures. Subsidiary fault and fracture-hosted veins are particularly prevalent in the
footwall of the underground mine and hangingwall of the surface mine. The stage II
mineralisation is dominated by quartz+ calcite+ pyrite+ roscoelite (vanadium-rich
sericite). Native gold occurs as inclusions within pyrite and is also closely associated
with roscoelite mineralisation. Gold grades up to thousands of grams per tonne occur in
these veins.
Stage I veins have no spatial or temporal relationship with the Roamane Fault Zone and
are always overprinted by the fault-hosted stage II veins. Stage I veins are single-event
fractures, most of which show no evidence for fault-related episodic fracturing and
brecciation. This suggests that the Roamane Fault Zone was formed and became active only during stage II mineralisation. Had the fault zone been active earlier, then some
control of stage I veining by the active fault zone would be expected in the presence of
high fluid pressures (up to lithostatic) inferred from the stage I veins.
In plan, the geometry of the Roamane Fault Zone is suggestive of dextral strike-slip
shear, however detailed examination of the fault structure suggests that predominantly
dip-slip normal movement occurred on the fault zone during stage II mineralisation. An
early foliated cataclasite in the footwall breccia and E-striking subsidiary splay faults are
interpreted to have been formed during dextral strike-slip movement on the basis of the
geometry of the fault zone, however no kinematic indicators from the early foliation have
been identified due to re-brecciation and stage II mineralisation. Post-stage II veins
(containing calcite+ anhydrite+ gypsum) were deposited in faults and fractures of the
Roamane Fault Zone during waning of the hydrothermal system. Slickensides associated
with late carbonate vein growth indicate dextral strike-slip movement occurred on the
fault zone during this late mineralisation. Therefore, deformation associated with the
Roamane Fault Zone evolved from dextral strike-slip after stage I veining (before stage II
mineralisation), to normal slip during stage II mineralisation, to dextral strike slip after
stage II mineralisation. Normal movement on the fault zone may have been induced by
uplift or magmatic resurgence of the intrusive complex at depth.
The sequence of deformation at Porgera reflects the evolution of the region following
collision of a southward-moving arc terrane with the northern edge of Papua New
Guinea, which blocked southward subduction. Melting of the blocked, subducted slab is
interpreted to have contaminated mantle partial melts to produce the oxidised, hydrous,
alkaline magmas of the Porgera Intrusive Complex. NNE-directed shortening of the
Papuan Fold Belt, which occurred immediately after collision (from 10 Ma) had ended
before the Porgera Intrusive Complex was emplaced (at approximately 6 Ma). ANNE-directed
maximum principal stress ( Ơ₁) which prevailed during shortening continued in
the same orientation during stage I veining although no shortening occurred during
mineralisation. A change in the style of deformation from predominantly extension
fracturing during stage I mineralisation to fault-slip during stage II mineralisation is
marked by the formation of the Roamane Fault Zone. The change in tectonic environment
which resulted in the formation of the ENE-striking Roamane Fault Zone is probably a
result of post-collisional relaxation of the NNE-directed maximum principal stress ( Ơ₁),
such that the NNE-directed stress became the minimum principal stress (Ơ₃ ).
Formation of the Roamane Fault Zone resulted in a change in localisation of fluid flow
within the hydrothermal system. Fluid flow during stage I veining occurred throughout the intrusive complex, such that the stage I veins are spatially associated with the exposed
stocks, although the density of the stage I veins is higher near the current mine. Stage II
mineralisation on the other hand, is hosted by a single fault zone of approximately 1000 m
strike length and 540 m vertical extent. During stage II mineralisation, ENE-striking,
sub-vertical subsidiary fault-fractures in the footwall of the Roamane Fault Zone, allowed
rapid vertical migration of fluids which were then focussed into the footwall breccia
during stage II mineralisation. The highest grades and thickest section of the footwall
breccia corresponds to the steepest section of the principal displacement zone, which
would have been the most dilational during normal fault-slip.
Layered cataclasite and cyclical stage II mineralisation, punctuated by brecciation indicates
multiple episodes of fault slip occurred during stage II mineralisation. Injection cataclasite
and implosion breccia associated with stage II mineralisation indicates sudden fault
failure. Lattice calcite, deposited in the stage II mineralisation, immediately after some
brecciation events indicates a significant pressure decrease and phase separation of the
fluids within the fault zone in response to increased fracture-permeability. Accretionary
pyrite spheres from the footwall breccia indicate this structure had a very high
permeability and experienced rapid fluid flow immediately after fault rupture.
Repeated episodes of rapid fault slip have enhanced permeability, decreased fluid pressure
within the fault and focussed fluids from depth and the surrounding wall-rock.
Subsequent stage II mineralisation and sealing of fault structures has resulted in an
increase in the fluid pressure surrounding the fault, prior to further fault failure. This
fault-valve style mechanism (Sibson 1987) enabled focussing of fluids from higher
pressure regions, at depth and from the surrounding wall-rock during stage II
mineralisation. The fluid focussed by the fault zone was subjected to rapid changes in
pressure and temperature that resulted in stage n calcite + quartz mineralisation which
contains no gold. High-grade gold mineralisation was probably generated by mixing of
fluid focussed by the fault zone from depth with fluids derived from the carbonaceous
shale wall-rock (Cameron et al. 1995). Efficient mechanical mixing of these two fluids in
the Roamane Fault Zone was probably achieved during deformation by a suction-pump
style mechanism (Sibson 1992). It is most likely that immediately after fault slip, a
suction-pump action occurred which caused rupture arrest adjacent to the steepest part of
the principal displacement zone (footwall breccia), which was the most dilational part of
the fault zone during normal fault-slip. Implosion breccia indicates suction-pumping may
also have occurred on a much smaller scale in dilational jogs associated with subsidiary
faults.
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