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Abstract

The 56 Moz Ladolam gold deposit, with an average gold grade of 2.42 g/t, has previously
been recognised as the world’s largest known epithermal gold deposit. It is located on
Lihir Island, in the New Ireland Province of Papua New Guinea, and is part of the Tabar-
Lihir-Tanga-Feni chain of Pleistocene alkalic volcanoes. Alkalic magmatism occurred in
an extensional tectonic regime, in a location where initial arc magmatism was related to
subduction with the Manus-Kilimailau trench, but was later reactivated as a back arc due
to the subduction of the Solomon Sea plate along the New Britain trench. Lihir island is
made up of at least five volcanic blocks, and is surrounded by an uplifted Quaternary
limestone reef. The Ladolam gold deposit is located on the eastern side of the island, in the
Luise volcanic edifice. Mineralisation is localised along north-dipping structural zones.
There are two large ore zones (Minifie and Lienetz) and several smaller (Kapit, Coastal,
Pacific and Borefields) in a 3 km2 surface area.
The Kapit ore body is hosted by diverse volcano-sedimentary and hydrothermal
facies. The coherent facies include basalt, diorite and andesite. The volcaniclastic facies
comprises two types of polymict matrix-supported breccia. Hydrothermal breccia facies
include anhydrite-carbonate-quartz-cemented breccia and pyrite-cemented breccia.
The Kapit ore zone is characterised by five different paragenetic stages. Porphyrystyle
stage 1 features were not observed at Kapit, but are preserved at Lienetz and Minifie.
Stage 2 anhydrite-cemented breccias and veins at Kapit are associated with porphyrystyle
biotite-K-feldspar-magnetite potassic alteration. Stage 3A pyrite-cemented breccia,
stage 3B quartz-chalcedony-pyrite-cemented breccias and veins, and stage 3C carbonateanhydrite-
quartz-cemented breccias and veins are all associated with phyllic alteration. In
contrast, stage 4 disseminated pyrite and pyrite veinlets are associated with intermediate
argillic to advanced argillic alteration assemblages.
Gold at Kapit, and elsewhere at Ladolam, is contained in refractory pyrite and
marcasite. The different types of pyrite and marcasite identified from the Kapit, Lienetz
and Minifie ore zones are acicular, banded, colloform, euhedral, framboidal and anhedral.
LA-ICPMS analyses have revealed that the colloform, framboidal, anhedral ± acicular
marcasite-pyrite grains have the highest contents of gold and other trace elements. Paragenetically, highest gold contents in Fe-sulfides occur in stage 3A (Lienetz), 3B (Kapit)
and 3C (Minifie) quartz-chalcedony-pyrite-cemented breccias and veins, stage 3A (Kapit)
and stage 3B (Minifie) pyrite-cemented breccias, stage 2B (Lienetz) quartz-anhydritebarite-
cemented breccias, and stage 4 (Kapit) disseminated pyrite and pyrite veinlets. At
the deposit scale, Au in pyrite is positively correlated with Ag, As, Sb, Cu, Se, Pb, and Tl.
Analyses of fluid inclusions, coupled with observed mineralisation and alteration
mineral assemblages from stage 2 (Kapit and Lienetz), show that mineralising fluids were
relatively hot (>250°C), saline (4-6 wt% NaCl eq. in average) and had near-neutral pH. In
contrast, the mineralising fluids from stage 3 (Kapit, Lienetz and Minifie) were significantly
cooler (150-250°C), more dilute (1-4 wt% NaCl eq. in average), and acidic. Trends in fluid
inclusion data are interpreted to be the result of mixing between magmatic-hydrothermal
fluids (5-10 wt% NaCl eq., >350°C) and steam–heated seawater (~3.2 wt% NaCl eq.,
150-200°C). Distinct increases of salinity suggests that adiabatic boiling occured during
mineralisation; most likely during multiple phases of hydrothermal brecciation. Gold is
likely to have been transported as a hydrosulfide complex (e.g. Au(HS)₂¯) and boiling and/
or mixing destabilised the Au-hydrosulfide complex and triggered gold precipitation.
Gold contents in the mineralisisng fluids were up to 8 ppm, based on LA-ICPMS analyses,
much higher than detected in the modern geothermal waters (up to 16 ppb). The positive
correlation of Ag, Tl, As and Sb with Au in fluid inclusions imply that Ag, Tl, As and
Sb were transported as hydrosulfide complexes such as H₂As₃S₆¯, HSb₂S₄¯, AgHS(aq) and
TlHS(aq).
Ladolam is concluded to be a hybrid gold deposit that contains early porphyrystyle
alteration features, semi-massive pyritic ores hosted by hydrothermal breccias
that have affinities with shallow submarine VHMS-style gold mineralisation, late-stage
low sulfidation style epithermal veins and breccias and modern subaerial to submarine
geothermal features. The history of the Ladolam deposit, including the Kapit, Lienetz and
Minifie ore bodies is composed of a succession of catastrophic events triggering formation
of different mineralised breccias and veins in an environment that evolved from submarine
to subaerial after a major sector collapse event. Mineralising events can be separated
into two distinct phases, transitional porphyry-VHMS (pre-sector collapse) and VHMSepithermal
(post-sector collapse).

Item Type:	Thesis - PhD
Authors/Creators:	Ageneau, M
Keywords:	gold, Lihir, epithermal, pyrite geochemistry, fluid inclusions
Copyright Holders:	The Author
Copyright Information:	Copyright 2013 the Author
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