Summary:
Golpu and Nambonga are porphyry copper- gold deposits, whilst Wafi is an epithermal gold deposit.
Wafi–Golpu is a complex, multiphase mineralised system, comprising:
- Golpu porphyry copper–gold deposit;
- Wafi epithermal gold deposit;
- Nambonga porphyry gold–copper deposit.
The upper portions of the porphyry systems were overprinted by the high-sulphidation epithermal mineralisation at Wafi.
Golpu
The Golpu deposit extends over about 800 m north–south by 500 m west–east, and was drill tested to more than 2,000 m depth.
Porphyry-style veins are preserved within the clay alteration at the top of the Golpu deposit. These veins have had the copper–iron sulphides removed, leaving only a skeleton of quartz. Irregular molybdenite veins are preserved stable remnants of the porphyry mineralising event.
The dominant copper–gold-bearing sulphide species vary laterally and vertically within the deposit from an inner bornite (plus chalcopyrite) core, to chalcopyrite as the dominant copper sulphide, and grading out to a pyrite-only shell on the mineralisation margin.
The proportion, by volume, of disseminated copper–iron sulphides varies from trace to as much as 10%. Pyrite increases from near absent in the core to 10% on the margin in a reverse relationship to chalcopyrite. Disseminated sulphides are typically located at the site of relict iron-bearing phases including primary phenocrystic hornblende and hydrothermal alteration-derived biotite– magnetite.
The Hornblende Porphyry (Livana Porphyry) is the main mineralised porphyry. Other porphyries act either as weak mineralisers (e.g., Golpu Porphyry) or as benign hosts (wall rock) from adjacent mineralising porphyries. The mineralisation style in benign (wall rock) porphyries can change due to rheological contrast and chemical variations. Quartz vein stockworks occur on the intrusive margin in sedimentary rocks. Significant zones of quartz flooding may be present. The porphyry system is mineralised with gold, copper, silver and molybdenum:
- Gold, copper and silver trend from highest grade in the Hornblende Porphyry (Livana) core to background levels at the mineralised edge;
- Molybdenum content is low in the gold and copper maxima, but increases outwards to a maximum at the copper margin and declines to background beyond the copper mineralisation limits.
Wafi
The Wafi deposit has a surface area of approximately 1,100 m x 800 m and was drill tested to about 600 m below surface. A number of zones, including the A, B, NRG and Link Zones, were defined. The NRG Zone is the nonrefractory portion of the A Zone, and the Link Zone is a more discrete, higher-grade zone characterised by both high sulphidation and low sulphidation mineralisation. It is unclear if the high sulphidation and intermediate sulphidation events at Wafi are independent, or are a continuum of a single event.
The high sulphidation gold event alteration overprints the Golpu porphyry-style alteration and mineralisation, with the diatreme carrying fragments of the earlier porphyry alteration. The high sulphidation event was interpreted to have remobilised pre-existing porphyry-related copper from the phyllic–argillic-altered upper porphyry and deposited this as zoned enargite–tennantite– covellite–chalcopyrite mineralisation. Most of the gold in the high sulphidation overprint was introduced in association with pyrite.
The low sulphidation Link Zone, which occurs on the diatreme margin, between and below the Zones A and B high sulphidation gold mineralisation, is characterised by pyrite with lesser quartz (quartz–sulphide–gold style) veins, which are overprinted by more than one generation of pyrite– sphalerite–galena–carbonate veins (carbonate–base metal–gold style). Selective sampling shows the gold in this zone is related to the arsenian pyrite of the quartz–sulphide-style veins, while the multi-stage lowarsenic carbonate–base metal veins are not well gold-mineralised.
Advanced argillic alteration contains primary copper mineralisation as chalcocite, but gold occurs within pyrite or as electrum associated with pyrite–enargite–tetrahedrite. Mineralisation appears to broadly follow the metasedimentary and volcanic host rocks stratigraphy (i.e., 40–50° to the east, and northeast), and is often sub-parallel to bedding. The copper–gold-bearing lenses occur in kaolinite–chalcocite–pyrite and vuggy quartz ± covellite ± enargite bands that may reach 20 m in thickness.
Nambonga
The stock is a low-grade copper and gold mineralised system, and extends over an area of approximately 200 m x 200 m and to a vertical extent of at least 800 m.
Much of the mineralisation is associated with silicification, either pervasive or as veins. Quartz stockwork veins that may be as wide as 10 mm and stockworks overprint the porphyry, especially in the upper levels.
Mineralisation consists of disseminated and vein-style copper–gold mineralisation and structurally-controlled base metal mineralisation in steeply-dipping lodes. Chalcopyrite is the dominant copper sulphide mineral. Chalcopyrite and pyrite form anhedral grains ranging that can reach 0.2 mm in width, and tend to occur as centrelines to quartz veins. Magnetite forms anhedral grains that can be 0.4 mm wide, and is generally present in the margins of the quartz veins or in the wall rock adjacent to quartz veins. Minor magnetite, pyrite and chalcopyrite are disseminated through the host rock.