The Ladolam gold mine lies within the Luise Caldera, located on the eastern side of the north-south elongated, 20 x 12 km, Lihir Island within the Tabar-Lihir-Feni-Tanga chain of islands northeast of New Ireland.
Lihir is a major bulk gold deposit with mineralisation represented by an earlier un-economic porphyry stage, and a later, overprinting low sulphidation advanced argillic phase that accompanied the introduction of the bulk of the gold mineralisation. The three main orebodies, Minifie, Lienitz and Kapit (the latter two connected by the Link zone) are located within a generally north-south elongated area inside the breached 5.5 x 3.5 km Luise Caldera on the east coast of Lihir Island. All fall within a 1.5 to 2 km radius of the centre of the caldera. Minifie has plan dimensions of the order of 700 x 400 m with mineralisation extending from 50 m above to 150 m below sea level. Lienitz is 600 x 300 m in plan and is mineralised from 140 m above to 250 m below sea level, although the bulk of the ore is from sea level to 200 m below.
Lihir Island is one of a series of four volcanic island groups which rise from a submarine platform and form a chain roughly parallel to and 50 km to the north-east of New Ireland in Papua New Guinea. This chain is also parallel to and 100 km south-west of the Kilinailau Trench where the Pacific plate subducted below the Melanesian Arc which is peripheral to the Australian Plate until the Mid Miocene. The magmatic arc of the Tabar-Lihir-Feni-Tanga chain of islands however, is apparently related to subduction below the New Britain Trench to the south, exploiting a pre-existing crustal weakness from the earlier subduction, and/or a transform fault associated with extension on the Manus Spreading Centre.
The 3 to 1 Ma Luise Caldera occupies the youngest of several Miocene to Holocene alkaline volcanoes developed on the island. The oldest rocks on the island comprise 350 m of Pliocene to Miocene mafic lavas and volcaniclastics of the Londolovit Block on the northern tip, while a NE-SW elongated, unconformably overlying and partly fault bounded block of Pliocene to Pleistocene mafic lavas, agglomerates and lahars around 500 m thick, the Wurtol Wedge adjoins, the Londolovit Block and stretches across the island to the west coast. The Londolovit Block and Wurtol Wedge probably represent portions of a deeply eroded Late Miocene to Pliocene volcano. Sub-economic gold with associated potassic, phyllic and argillic alteration is hosted by altered intermediate volcanics and intrusives. The southern part of the island is occupied by the Pliocene to Pleistocene Kinami Volcano lavas, pyroclastics, breccias and derivative epiclastics. This volcano has associated phyllic and argillic alteration, which grades into potassic zones at depth, which in turn pass out into propylitic zones. Low grade gold accompanies the phyllic alteration. The Pleistocene Huniho Volcano, which has a number of satellite cones and craters makes up the north-western segment of the island with tephras and lahars overlying mafic lavas. The late 5.5 x 3.5 km Luise Caldera is the remnants of a volcanic edifice on the central east coast, north of the Kinamo volcano and east of the Wurtol Wedge. The associated volcanics include trachy-basaltic lavas, pyroclastics and breccias. All three of the remnant volcanic centres have undergone some form of seaward collapse, particularly the most recent, the Luise volcano. Raised coral reefs fringe most of the island, but are absent in Luise Harbour, suggesting they predate the caldera flooding event.
Volcanic rocks predominate in the upper parts of the ore zones of the Luise Caldera, and the margins of the system, and are underlain by intrusives. The host volcanic sequence occupies most of the floor of the Luise Caldera and comprises intermediate (latitic, andesitic and trachytic) lavas, tuffs and volcanic breccias. These extrusives are intruded by a series of fine to medium grained, quartz poor but silica saturated monzonitic to monzodioritic porphyries ranging from pyroxene microdiorite to biotite syenite and some andesite porphyry.
Intrusion-related potassic alteration occurred in the period 0.917 to 0.342 Ma, while the epithermal gold mineralisation is dated at 0.336 Ma, possibly continuing to 0.1 Ma, although the geothermal system is presently active. While the Luise Caldera trends elongate NNE, it is cut by north-south structures associated with the deep fractures that are interpreted to localise the magmatism of Lihir Island. NW-trending fractures are also evident and host the NE-dipping Minifie mineralisation. The Luise volcano is interpreted to have collapsed sideways at about 0.34 Ma. NE-dipping listric-style faults are interpreted to have developed within the remaining underlying part of the edifice. This failure removed about 1 km from above an active porphyry Cu-Au deposit. In doing so, it is believed to have removed the confining pressure and allowed epithermal fluids to escape up the listric faults produced during the collapse, and as such initiated development of the epithermal mineralisation.
Mineralisation is coincident with a NNE trending zone of hydrothermally altered volcanics and breccias intruded at depth by porphyries. Three stages of alteration have been identified,
i). an early porphyry style accompanied by low grade Cu-Au-Mo mineralisation at depth, with a potassic core (mostly phlogopitic biotite-anhydrite with lesser orthoclase, magnetite with pyrite, chalcopyrite and molybdenite) and a peripheral propylitic phase (chlorite ±amphibole ±albite ±epidote ±calcite ±magnetite);
ii). A transitional zone, which forms the bulk of the ore at Minifie, characterised by pervasive adularia ±illite and fine grained, refractory auriferous pyrite within extensive shallow hydrothermal breccias located above biotite altered porphyritic stocks, and grading abruptly downward into sub-economic anhydrite-K feldspar ±pyrite alteration of the porphyry system; and
iii). a younger, overprinting and shallower low sulphidation epithermal style as in the Lienetz area, comprising advanced argillic (alunite ±opaline silica ±kaolinite ±sulphur), argillic (kaolinite ±smectite ±illite) and phyllic (illite ±K-feldspar ±silica). Alteration generally forms a horizontal layering, with porphyry style potassic and propylitic assemblages at depth, grading up through phyllic to epithermal argillic and advanced argillic phases nearer the surface.
Alunite and opal fill veins and stockworks near the surface, grade to quartz and adularia at intermediate depths, underlain by anhydrite and carbonate. Sulphides and gold mineralisation generally mimic the horizontal alteration pattern, although some follows steep 'feeder' fractures. The horizontal zonation represents a surface oxidation regime, related to mixing with ground water, passing down progressively into argillaceous altered and silica clay, through to a boiling layer and then the anhydrite sealed zone. Grades are best from the silica-clay to the boiling zones. Gold is predominantly fine grained and contained within pyrite and marcasite. The overall sulphide or reactive sulphur content averages 6%. Geothermal activity is still taking place.
The total Identified Mineral Resource at January 2004 (based on a 1.5 g/t Au cut-off) was 442.5 Mt @ 3.14 g/t Au for 1390 tonnes (44.7 Moz) of contained gold. Proved + probable reserves totalled 163.5 Mt @ 3.88 g/t Au for 635 tonnes (20.7 Moz) of contained gold.
The total Measured + Indicated + Inferred Mineral Resource at August 2011 (Newcrest Mining website, 2012) was 830 Mt @ 2.1 g/t Au for 1745 tonnes (56 Moz) of contained gold. Proved + Probable Reserves (a subset of the resources) totalled 400 Mt @ 2.4 g/t Au for 960 tonnes (31 Moz) of contained gold.
