The Porgera gold deposit is located at an altitude of 2500 m in
the remote Enga Province of western Papua New Guinea, 130 km west of
Mount Hagen and 600 km northwest of Port Moresby.
The operation, which was commissioned in 1990, was based on the lower
grade, open pit, Waruwari bulk gold orebody (originally calculated at 54
Mt @ 4.3 g/t Au) and the high grade, underground, tabular Zone 7
deposit (5.9 Mt @ 27 g/t Au).
The orebody is associated with the Late Miocene (6.0±0.3Ma),
silica-poor, K-rich, shoshonitic Porgera Intrusive Complex (PIC) of
micro-gabbro to diorite and feldspar porphyry, which was intruded at an
estimated palaeodepth of ~2.0 to 2.5 km, into previously thrusted late
Jurassic to Cretaceous pelitic sediments deposited on the shelf of the
Australasian plate margin. Intrusion of the PIC magmas occurred
slightly prior to, or during, the uplift of the Papuan Fold and Thrust
Belt that was formed as a result of the collision of the Indo-Australian
plate with the Bismarck Sea and Caroline oceanic plates to the north
and east. The mineralising event occurred immediately after the
emplacement of the PIC and was locally accompanied by intense phyllic
alteration that affected the shallow-level intrusions. A late, steeply
dipping normal structure, the Roamane fault, crosscuts all intrusions
and hosts the richest gold bearing veins and breccias.
The host shelf sequence includes the Jurassic Om Formation, a thinly
bedded, dark grey carbonaceous siltstone with pyritic calcareous
nodules, overlain by the Cretaceous Chim Formation which comprises
bioturbated and laminated grey calcareous siltstone and mudstone with
local sandy horizons. Eocene to Miocene limestone crops out further
south and east of the mine.
Regionally, these sediments are broadly folded about east to ESE
trending axes, but dip steeply around the intrusive contacts, adjacent
to which they are altered. The earliest faults are flat dipping and may
be related to early mineralisation. Steeper NE and E-W faults post date
the intrusive complex and are also mineralised. The main gold ore is
associated with the E-W trending, 60 to 75° dipping Romaine Fault and
steeper associated footwall and hangingwall fractures.
The immediate host package comprises Cretaceous sediments including
mudstones, shales, siltstones and sandstones, which are often
calcareous. Syn-mineral thrusting has placed calcarenite in contact with
black shale, the main host rock in the mine area, while post-mineral
thrusting on the Boundary Fault has resulted in the emplacement of the
calcarenite - black shale package over fissile brown shale.
The gold mineralisation at Porgera displays a staged geological
development associated with evolution of the PIC as a differentiating
alkaline intrusion, intruded as stocks (<500 m diameter) and dykes.
Aeromagnetic data indicate that the intrusions exposed at the mine occur
as apophyses to a much larger (3.5 km diameter) buried magmatic source,
which is inferred to have domed the host sedimentary rocks.
Two main mineralising events have been recognised, namely:
Stage 1 associated with the emplacement of generally porphyritic intrusions of, i). initial augite hornblende diorite, ranging from equigranular to porphyritic, with associated 'andesite' dykes and sills; ii). subsequent hornblende diorite with radiating hornblende rosettes in a fine-grained matrix.
Stage 2 mineralisation was associated with quartz-feldspar
porphyry stocks and dykes emplaced as the most differentiated later
stage of PIC magmatism and was initiated as localised crosscutting
milled matrix fluidised breccia dykes, containing fine silica-pyrite,
formed by explosive hydrothermal activity at an elevated crustal
Three types of gold-bearing vein assemblages are recognised:
(i) Pre-Stage I magnetite-sulphide-carbonate veins with
economically insignificant gold, localised in the central and deep parts
of the mine, some 1000 m below the current workings. Magnetite is
dominant, with associated secondary biotite and lenses of pyrite,
pyrrhotite, chalcopyrite, and rare inclusions of gold in pyrite;
(ii) Stage I base metal-sulphide±gold-carbonate veins.
Sulphide minerals are pyrite, sphalerite and galena, with rare
chalcopyrite, arsenopyrite, marcasite, freibergite and
proustite/pyrargyrite. The gangue consists of complex Mn-Ca-Mg
carbonates and subordinate quartz. The veining carries refractory Au in
arsenical pyrite, disseminated in zones of intense phyllic alteration,
plus minor Au in the associated base metal sulphide veins localised by
NE- to NNE-trending pre-mineral faults and the margins of intrusive
bodies. Gold ocurs in pyrite, either in an invisible form, or as
microscopic inclusions of native gold. This stage predominates in the
Waruwari open pit area; and
(iii) Stage 2 quartz-roscoelite-pyrite-gold and breccia
veins, which host the highest gold grades and are the economically most
significant vein assemblage. Some pyrite is arsenian, with local
arsenopyrite, although chalcopyrite is rare. The Ag-Au tellurides
petzite, krennerite and calaverite, and the Ag telluride hessite are
present, as well as coloradoite and altaite. Gold dominantly occurs in
the native form and as electrum. Veins are strictly fault controlled, in
particular within the late normal Romaine Fault.
Gold grades and grain size increase from Stages 1 to 2. In addition,
Stage 2 mineralisation is characterised by high silica and low sulphur,
and consequently by higher Au:S (10:1) ratios compared to Stage 1 (2:1)
which has low silica and high sulphur.
The majority of gold is present as extremely fine grains contained
within pyrite. The Romaine Fault is around 4 km long, with the main
mineralised zone being over a strike length of 1 km, with a width of up
to 20 m. The highest grade ore has a vertical extent of 240 m, and is
found below a depth of 400 m from the surface. At depth the roscoelite
mineralisation gives way to a gypsum-rich zone with less sulphide.
Mineralisation is commonly associated with the altered contact zones of
the intrusive complex.
At December 2007, reserves and resources were:
Proven + probable reserves of 97.96 Mt @ 3.98 g/t Au
Measured resources of 34.8 Mt @ 2.55 g/t Au
Indicated resource of 24.0 Mt @ 1.96 g/t Au
Inferred resource of 11.1 Mt @ 2.89 g/t Au
In 2010, proven + probabe reserves contained 230 t (7.4 Moz) of gold
(Barrick website, 2012), presumably at similar grades to the 2007
Barrick is the owner of a 95% joint venture interest and is the operator of the Porgera gold mine.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2012)