Geology
Lihir and the other islands of the Tabar-to-Feni chain are volcanic islands of largely Pliocene to recent age rising from a submarine platform which extends between New Ireland and the West Melanesian Trench. The bathymetric trend of the chain is co-linear with Bougainville to the south-east, and with Mussau to the north-west. However, Lihir lies on a seismically active north-north-east submarine spur which suggests structural control at a high angle to the regional arc trend.
The islands in the chain have a varied but predominantly shoshonitic composition which differs strongly from the calc-alkaline arc volcanism of similar age on Bougainville and New Britain. The islands do not overlie the active Benioff zone which dips northerly from the New Britain Trench below Bougainville and New Britain, and the volcanism is too young to have been emplaced above an older, south dipping subduction system linked to the West Melanesian Trench. Their origin is therefore enigmatic, but the magmas may have been generated by partial melting of mantle already modified by earlier subduction at the West Melanesian Trench.
Isotopic age determinations on unaltered rocks from the Tabar-to-Feni islands range from 3.7
(Tabar) to 0.2 million years and a phreato-magmatic eruption in the Feni group has been dated at 2300 years. These data plus the morphology of the islands suggest that the oldest volcanism occurred at the north-western, or Tabar end of the chain, and the youngest volcanics were erupted in the Feni group. Determinations on alteration minerals give similar but slightly younger ages: 0.92 ± 0.15 million years on Lihir and 2.8 ± 1.2 million years on the Tabar islands.
Thermal activity is present within all four island groups of the chain, and epithermal gold mineralisation has been discovered in the Tabar and Feni groups in addition to Lihir.
The northerly trending ridge on which Lihir is built is in a region of moderate seismic activity. Recent earthquakes may reflect northerly and north-easterly faulting, such as the Ramat Fault on New Ireland and other faults mapped on Lihir, which control the location of the
Luise, Kinami and Huniho Volcanoes. There are five Plio-Pleistocene volcanic units on Lihir.
Londolovit Block is a Pliocene or Upper Miocene unit which consists of a 350 m thickness of mafic lava and volcaniclastics resting on a basement of ankaramitic lava. The upper parts of the sequence are transitional to Lower Pliocene foraminiferal limestone.
The Wurtol Wedge overlies the southern margin of the Londolovit Block and consists of a westerly dipping sequence of mafic lava, tephra and lahars about 500 m thick. The unit is considered to be the remnant of the western flank of a stratovolcano whose basal diameter was about 15 km, the eastern half having been destroyed by faulting. It is thought to be of Pliocene age.
Kinami Volcano consists of lava, tuff breccia, agglomerate, and clastic derivatives. An east-facing partly destroyed crater marks the original vent. On its western side, lavas appear to have ponded against a fault scarp which forms the eastern boundary of the Wurtol Wedge. The volcano may be of Pleistocene age.
Huniho Volcano has a westerly facing remnant crater, and consists largely of poorly consolidated tephra and lahars overlying a core of mafic lava. The volcanics overlie both the Londolovit Block and the Wurtol Wedge, and are considered to be Pleistocene.
The Pleistocene Luise Volcano consists of trachybasalt lava, volcanic
breccia, and tuff. It rests on part of an older eroded volcano (possibly the Wurtol Wedge) and lavas on the western flank have ponded against the Wurtol Wedge fault scarp in a manner similar to those of the Kinami Volcano. An elliptical, breached
caldera, measuring approximately 5.5 km x 3.5 km forms Luise Harbour on its north-eastern side. The well-preserved rim rises to just over 600 m and the original,
pre-caldera height of the volcano is thought to have been about 1200 m. The
caldera, which contains the Ladolam deposit, is probably a consequence of explosion or internal collapse, in contrast to similar seaward-facing arcuate features in the Huniho and Kinami Volcanoes which are considered to be the result of debris avalanches.
Reef limestone fringes most of the island and reappears 3 km offshore as the small islands of Mali and
Sanambiet. It forms a platform 100 m high and may range in age from Pliocene, where it is interbedded with the oldest
volcanics, to Recent where it is built on the flanks of the Luise Volcano. It is not present in Luise
Harbour, and therefore even the youngest limestone except for modern, active reefs, pre-dates the Luise
Caldera.
The structural pattern on Lihir includes early N-S faulting and younger E-W and SW faults, some of which may still be active.
Ladolam deposit occurs within an area of about 2.0 km by 1.5 km, entirely within the Luise
Caldera. Most of the Proven and Probable resources come from two areas, Lienetz and
Minifie, which are situated respectively in the central part of the caldera and towards its southern margin. The Kapit area, north of
Lienetz, has substantial potential for additional resources, which may not prove feasible because of its proximity to the ocean and its active geothermal system.
Lienetz deposit has dimensions of approximately 800 m by 400 m and occurs at elevations between 140 m above and 250 m below sea level. Minifie deposit measures approximately 1000 m by 600 m, occurring mostly between the surface (about 50 m above sea level) and 150 m below sea level. The subsurface dimensions of the Kapit mineralised zone are incompletely tested at this stage, but appear to be at least 800 m by 500 m, with most of the known mineralisation occurring between sea level and 200 m below.
The Camp area contains what appears to be a small outlier of mineralisation (approximately 250 m by 100 m) possibly related structurally to the Coastal area.
Zones of low-grade gold mineralisation connect the
Lienetz, Coastal and Kapit areas, but the Minifie deposit appears to be structurally and mineralogically distinct.
Oxide material extends to a maximum depth of approximately 70 m. The oxide zone is best developed in topographically higher parts of the Lienetz and Coastal area, and is of negligible thickness in the Minifie area.
Rocks forming the caldera wall are mostly unaltered mafic lava and
pyroclastics. Weathering, intense alteration, and brecciation make the surface definition of primary rock types within the caldera difficult, but most outcrops are thought to be volcanic. However, drill holes have also intersected abundant monzonitic and syenitic
intrusives, particularly below the Lienetz area but also below the Minifie area.
Only two lithological units, intrusive and
volcanics, are distinguished but their boundaries are highly interpretative. Gold grades show no obvious relationship to
lithologies.
Volcanic rocks dominate in the upper parts of the deposit and on its margins. They comprise lava, tuff, and volcanic
breccia.
Lavas are porphyritic, with phenocrysts of feldspar, pyroxene, and rarely hornblende and
biotite, in a sub-trachytic feldspar-rich ground mass. Most are latites,
andesites, and trachytes, but some are distinctly mafic and contain abundant pyroxenes pseudomorphed by chlorite or biotite and
sulphides. Many of these rocks may have solidified in a sub-volcanic environment, rather than as sub-aerial flows.
Fragment textured rocks, consisting of lava clasts up to several centimetres in size in a matrix of similar composition, are quite common and may have been formed by autobrecciation as a result of degassing of the lava as it solidified.
Most intrusives are quartz-deficient, (but silica-saturated) monzonitic types ranging from
pyroxene-biotite microdiorite to biotite syenite. The monzonite typically consist of 40% to 60% plagioclase laths, 30% to 40% subhedral to euhedral pyroxene, and 10% to 25%
biotite, in a potassium feldspar rich ground mass.
The main intrusive body below Lienetz Hill is thought to extend locally to the surface and the deepest hole drilled in the area of the deposit bottomed in monzonite at 670 m below sea level. Volcanics close to the intrusive margins are metasomatised to
biotite-chlorite-potassium feldspar which grades outward to a chlorite-amphibole-magnetite assemblage. Intrusive lithologies are less common in the Minifie area, and only occur as narrow dykes in the Coastal area. An intrusion has been tentatively identified in the Kapit area.
Pre-caldera magmatic degassing, and
post-caldera hydrothermal processes, probably have both contributed to the extensive brecciation which affects most of the rocks of the deposit. Crackle breccias, consisting of angular, unrotated clasts cemented by pyrite and rock flour, alunite or silica, dominate in the upper and better mineralised parts of the deposit. They are frequently associated with fluidised breccia dykes, millimetres to metres in width, in which the clasts are rotated and milled. Breccia pipes, tens of metres in diameter, composed of
light-coloured, altered clasts in a dark pyrite-rock flour matrix, occur at a number of localities generally along major fractures. They are associated with
fluidisation, crackle brecciation and advanced argillic alteration.
Most alteration types are identified easily in diamond drill core and visual identification is supported by extensive petrologic data. Consequently alteration boundaries are well constrained and can be identified accurately.
Two distinct episodes of alteration are
recognised: an early, porphyry-style, deep-level event characterised by potassic alteration which grades laterally to
propylitisation; and a later, higher-level, epithermal event represented by
argillic, advanced argillic and phyllic assemblages.
Potassic and propylitic alteration, thought to pre-date most of the gold deposition, occurs at depths generally greater than 100 m below sea level. Potassically altered rocks contain abundant secondary potassium feldspar which grades downward to
biotite-potassium feldspar assemblages in which biotite replaces primary mafic minerals, potassium feldspar replaces plagioclase and both minerals also replace the ground mass of porphyritic rocks.
Propylitic alteration, which is marginal to potassic alteration in the
Lienetz, Coastal and Minifie areas, is characterised by the development of chlorite, secondary amphibole,
albite, calcite, magnetite and minor epidote, which pseudomorph primary mafic minerals and plagioclase.
There is good correlation between propylitic alteration and uneconomic gold grades in the zone between the Minifie and Lienetz areas. In contrast, low-grade mineralisation may continue in potassically altered rocks between the
Lienetz, Coastal and Kapit areas.
Advanced argillic, argillic and phyllic alteration both overlie and overprint the porphyry-style alteration in the upper parts of the deposit. It is widespread on the surface and correlates well with high gold grades. It is extensively developed between the surface and about 100 m depth in the Coastal, Lienetz and Kapit areas, but is poorly developed in the Minifie area. It may also extend in probable fracture controlled zones to deeper levels, and has been recorded as a thin layer overlying 'boiling zone' breccia at the potassic-argillic boundary. The typical surface expression is an extremely leached, white, porous, light-weight rock, known locally as 'white rock', which consists of skeletal silica plus kaolin and
alunite. White rock is thought to have resulted from supergene leaching of
pyrite-alunite-potassium feldspar rock, into which it generally grades with depth. Advanced argillically altered rocks also outcrop as dense, hard,
alunite-opal-pyrite rocks on Lienetz Hill and on bluffs in the Coastal area.
Argillic alteration is identified at the surface as
grey-white pyritic or orange-brown limonitic clays. Argillised rocks form an extensive blanket below the advanced argillic zone to 100 m below sea level in the Coastal,
Lienetz, and Kapit areas, but are mostly restricted to the near-surface part of the Minifie area. Gold grades are generally lower than in advanced argillic and potassically altered rocks. In drill holes, argillised rocks are soft,
grey, pyritic clays mostly devoid of original textures. Towards the base of the argillic zone, relict potassic or propylitic alteration is generally preserved.
Phyllic alteration is extensively developed in the intermediate to upper levels of the Minifie and Kapit areas and is also recorded in some drill holes in the Lienetz area. There are few surface exposures of phyllically altered rocks; in drill holes they are medium soft to medium hard, creamy
grey, and often show a fine scale pitted texture. The hardness increases with depth probably due to increased potassium feldspar and silica and reduced
illite; near the surface, the assemblage may grade into argillic alteration through the appearance of
kaolinite. Phyllic altered rocks contain high gold grades in the Minifie area, especially in the more silicified sections.
Analyses show the Coastal area being dominated by the alunite-kaolinite assemblage and the Minifie area by the
adularia-sericite assemblage. The Lienetz area appears to have roughly equal amounts of both assemblages. The distribution of these assemblages is a clear indication of varying fluid compositions over space or time or both.
Veins and stockworks are filled with anhydrite, carbonate, and minor quartz at deeper levels, adularia and localised quartz at intermediate depths, and alunite and opal near the surface. Pyritic veining may occur at all levels.
Anhydrite stockwork veining is developed throughout the deposit in potassically altered rocks at depths between the 850 m and 800 m elevations, and approaches the surface in propylitised rocks between the Minifie and Lienetz ore bodies. It extends in depth to the limit of drilling. As mentioned above, the top of the anhydrite zone is frequently marked by the transition to an open-space breccia (boiling zone). Anhydrite is accompanied by minor carbonate in the Lienetz and Coastal area; in the Minifie area it surrounds and underlies a zone of carbonate veining which in turn grades to a central and upper core of quartz veining. Many of the veins in the Minifie area are composite and may consist of anhydrite-carbonate or carbonate-quartz, but usually not all three. Gold grades in these vein zones are highly variable, but the transition to anhydrite veining is generally accompanied by decreased gold grades.
The dominant opaque minerals are
pyrite-marcasite. Accessory opaque minerals include base metal
sulphides, sulphosalts, magnetite, rutile, leucoxene and gold-silver
tellurides. Microscopically visible particles of free gold are rare.
Free, visible gold in the oxide zone has been identified locally as spongy, irregular particles from 50 µm to 1.5 mm in size, with a fineness ranging between 965 and 992. However, most of the gold in the oxide zone is sub-microscopic in size. In the sulphide zone, free gold is rare but does occur in significant quantity in some areas eg SE
Minifie. Visible gold is associated with quartz.
Most of the gold probably occurs in pyrite in particles less than 0.01 µm in size, and to a lesser extent in chalcopyrite and
marcasite.
Pyrite is the dominant sulphide throughout the mineralised zone in all alteration types with marcasite as an accessory
sulphide. Both appear to have replaced pyrrhotite in part. In the 'boiling zone', pyrite occurs as coarse, coalescent masses coating breccia voids, single
euhedra, and fine dissemination to 200 µm in size; marcasite forms bladed crystals, either separate or on pyrite margins. In phyllic altered rocks, pyrite has been observed as coalesced strings of single crystals replacing breccia matrix, as veinlets up to 5 mm wide, and as spheroidal particles in quartz veins. Pyrite may contain inclusions of quartz, rock flour, magnetite, bornite and
chalcopyrite. Disseminated pyrite within breccia clasts appears to have replaced primary mafic minerals or iron-titanium oxides.
Base metal sulphides chalcopyrite, pyrrhotite,
sphalerite, galena and covellite occur as accessories in all alteration types, but become more abundant in the deeper parts of the potassic zone. Molybdenite occurs locally in potassically altered ore, but is absent from higher level alteration types. Arsenopyrite was only noted in one drill hole.
Sulphosalts tetrahedrite, tennantite, luzonite and enargite are common accessories in advanced argillically altered rocks. They are less common in argillised and phyllically altered types, and have not been identified in the potassic or propylitic alteration zones.
Tellurides, including sylvanite, have been identified in advance argillic alteration, possibly inter-grown with
rutile. A telluride mineral equivalent to Au2AgTe3 has also been identified as discrete grains and inclusions in pyrite from potassically altered 'boiling zone' breccia.
Ruby silver has been identified in one instance in anhydrite sealed, potassically altered
breccia.
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