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Characteristics and origins of breccias in a volcanichosted alkalic epithermal gold deposit, Ladolam, Lihir Island, Papua New Guinea

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Blackwell, JL (2010) Characteristics and origins of breccias in a volcanichosted alkalic epithermal gold deposit, Ladolam, Lihir Island, Papua New Guinea. PhD thesis, University of Tasmania.

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Abstract

The 46 Moz. Ladolam gold deposit is located on Lihir Island, Papua New Guinea and is the
world’s largest low sulfi dation, alkalic epithermal gold deposit in terms of contained gold.
Lihir Island is part of the 150-km-long, alkalic Tabar-Lihir-Tanga-Feni (TLTF) volcanic
island chain that is located in the New Ireland basin. The Ladolam deposit occurs in the
central portion of the dissected Plio-Pleistocene Luise volcano and is composed of four
main ore zones: Minifi e, Lienetz, Coastal and Kapit. This thesis focuses on the volcanic
and subvolcanic facies that host the Minifi e (predominantly mined out) and the Lienetz (the
current focus of mining) ore zones. The Kapit and Coastal ore zones remain unmined at the
time of this study.
The Luise volcano consists of volcano-sedimentary stratigraphy that has been cut
by an intrusive complex and overprinted by various hydrothermal facies. The volcanosedimentary
stratigraphy is dominated by polymictic, matrix-supported breccias interbedded
with lavas and shallow intrusions. The monzonitic intrusive complex is centred on the
northern margin of a prominent 3.5 by 4 km elliptical depression or amphitheatre that is
interpreted to be a relatively large volcanic-sector collapse scar. The sector collapse event is
less than 190 ka based on 230Th – 234U age dating on the uplifted fringing limestone reef that
was partly destroyed during the sector collapse event.
The Minifi e ore zone occurs in a variably altered, southward-dipping volcanosedimentary
succession that was overprinted by at least three major hydrothermal brecciation
and veining events, one in the porphyry environment and two in the epithermal environment.
The volcano-sedimentary lithofacies record the transition from a subaerial environment close
to an active vent to a subaqueous, quiet depositional environment into which a partly extrusive
cryptodome was emplaced. The volcano-sedimentary stratigraphy was subsequently tilted ~
30° to the south, which is consistent with the regional tilt indicated by limestone platforms
throughout the TLTF island chain. The hydrothermal alteration and vein mineral assemblages
refl ect the evolution from porphyry (biotite-stable) to epithermal (adularia-stable) conditions
in both the Minifi e and Lienetz ore zones. Minifi e epithermal gold mineralisation is hosted
predominantly in hydraulic breccias and veins that are characterised by quartz-calcite-adularia and adularia-quartz-pyrite assemblages.
The Lienetz ore zone is hosted within a magmatic-hydrothermal breccia that has
cross cut polymictic breccias and sandstones (L1), pyroxene-phyric coherent and clastic
basalt (L2) and a shallow-level microdiorite pluton (L3). The bulk of the gold mineralisation
in the Lienetz ore zone is related to porphyry-style features, specifi cally an anhydrite-biotiteorthoclase-cemented
breccia facies association (L4) that is associated with a feldspar-phyric
syenite intrusion (L5). The L4 breccia and L5 syenite are interpreted to have been emplaced
in a near-vertical orientation, but are now north-dipping as a result of regional tilting to the
south. Lienetz epithermal facies are characterised by pyrite-cemented breccias that form a
layer between an argillic and advanced argillic clay blanket and the underlying porphyrystyle
L4 lithofacies. Following the porphyry stage, volcanic-sector collapse, and the main
epithermal stages, emplacement of breccia in a discordant, subvertical body (L7) destroyed
the western margin of the ore zone and the argillic blanket. The L7 breccias were intruded
by plagioclase-phyric andesite dykes (L8).
The volcano-sedimentary lithofacies of the Luise volcano demonstrate an evolution
that includes volcanic cone growth, intrusions, subsidence, regional uplift and tilting, and
volcanic sector-collapse. The hydrothermal facies record four gold depositional events related
to an (a) early gold-rich (1 to 4 g/t gold with bonanza grade up to 180 g/t Au) porphyry
environment that has been overprinted by low-sulfi dation epithermal (>4 g/t Au) conditions
in both the Minifi e and Lienetz ore zones. Prior to and during regional uplift and tilting, gold
was deposited by boiling hydrothermal fl uids in the porphyry and (b) transitional epithermal
environments. After dissection of the Luise volcano (<190 ka), a (c) homogeneous layer of
refractory sulfi de ore (4 to 6 g/t Au) gold was deposited, possibly by seawater-quenching of
hydrothermal fl uids, in the Minifi e ore zone. In Lienetz, (d) epithermal gold (>4 g/t Au) was
deposited in a zone of open space produced by acid leaching in the geothermal environment.
The western margin of the Lienetz ore zone is cross cut by a discordant matrix-rich breccia
body that may have formed during a phreatic explosive eruption. The breccia body was
subsequently cut by andesite dykes. Geothermal activity continues today, locally remobilising
and leaching gold in the near-surface clay alteration zone and driving steam explosions, both
of which produce signifi cant challenges to mining.

Item Type: Thesis (PhD)
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Date Deposited: 15 Jul 2011 00:34
Last Modified: 17 Aug 2016 06:29
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