Structural controls on fluid movement during deformation and mineralisation at the Porgera gold deposit, Papua New Guinea




Munroe, Stuart McRae

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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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