Development of fracture-controlled flow regimes and gold mineralisation, Porgera gold deposit, PNG

Angela Halfpenny and Stephen F Cox

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

The internal structure of a composite Stage 1 and Stage 2 vein. Section 1 and 3 represent the Stage 2 mineralisation, which exhibits crustiform quartz interlayered with roscoelite-rich layers.  The gold is associated with the roscoelite-rich bands and a patch of gold is marked by the white arrow.  Section 2 shows the Stage 1 vein.

Within the framework of the rapidly developing understanding of the dynamics of stress and fluid pressure regimes in contemporary, active magmatic systems, this project is exploring how stress states, stress field orientations and fluid pressures evolved during the development of the large, intrusion-related,  hydrothermal gold system at Porgera in the highlands of Papua New Guinea. Fieldwork is being used to document the geometries and styles of vein systems, their overprinting relationships, and relationships to growth of fault networks.  This is allowing us to examine how stress states, fluid pressure regimes, and the orientations of stresses evolved during the multi-stage evolution of the hydrothermal system. We are also evaluating what processes drove the growth of fracture-controlled flow networks and the evolution of fluid pathways. A key goal is to understand how the evolution of fracture-controlled fluid pathways and reactions impacts on the distribution of economic mineralisation in multi-stage, intrusion-related hydrothermal systems.

Work in 2008 has focussed on developing a 4D understanding of the evolution of vein distribution, geometries and internal textures during five distinct stages of vein development. The Porgera gold deposit exhibits at least two gold-bearing vein formation stages. The development of the richest vein-hosted Au mineralisation is associated with the growth of several low-displacement faults, which exhibit a complex kinematic evolution involving both dextral and normal slip histories during mineralisation.

b. Internal texture of a complex Stage 2 vein. Section 1 shows the wall rock. Section 2 is a pyrite-rich layer.Section 3 exhibits quartz-rich, crustiform banding which grades out into section 4 which shows a dilatational breccia containing wall-rock and quartz-rich clasts with a crustiform overgrowth.Section 4 also exhibits a vuggy centre to the vein. Section 5 exhibits crustiform banding and was in contact with the wall rock.

The varied internal structures of Au-bearing veins and fault zones, such as textural and mineralogical zoning, in some cases provide evidence for multiple opening and sealing events (Figure 1). Flow in such fracture-controlled hydrothermal systems is unlikely to have been continuous.  Instead, flow is interpreted to have occurred as numerous, episodic pulses associated with repeated cycles of breaching of the overpressured, magmatic fluid reservoir by failure events. Breaching events are followed by propagation of fracture arrays driven by migration of a fluid pressure pulse, then progressive sealing of fractures as flow rates decay.  Ongoing work is focusing on defining the architecture of major, ore-producing fluid pathways.