Operational infrastructure
One underground complex, mining from two
separate ore bodies (kim south and main north)
accessed via declines. Centralised administrative office, engineering
workshops and one cip processing
plant (1.3 mtpa capacity).
Deposit type
Orogenic
greenstone gold deposits hosted in a number of different styles of lodes.
Although all of the agnew
deposits broadly occur at the intersections between structures and
stratigraphy, there are subtle
differences in alteration and mineralisation, that are controlled in part by
the local host rock chemistry.
Reporting
codes Gold Fields reports its Mineral resources and mineral reserves in accordance
with the 2007 Samrec code, the
south african codes for the reporting of mineral asset valuation (2009 Samval code) and other relevant
international codes such as the united states securities and Exchange commission (sec) industry
guide 7, the joint ore reserves committee (jorc 2012) Code and the national instrument (Ni) 43-101. The mineral
resources and mineral reserves are underpinned
by an appropriate mineral resource management process and protocol to ensure adequate corporate governance in
respect of the sarbanes-oxley act.
Geological
setting and mineralisation
Agnew is situated in the northern portion of
the Norseman- Wiluna greenstone
belt, which is part of the yilgarn craton,
A 2.6 ga granite greenstone terrain in Western Australia.
The rock types in the belt comprise abundant
tholeiitic and Komatiitic
volcanic rocks, chert, sulphidic and albitic
Sedimentary rocks, and a chain of discrete felsic volcanic Centres. The greenstones of the agnew
area have been Metamorphosed to
upper greenschist, lower amphibolite Metamorphic
grades. Gold mineralisation is found in quartz
Breccia lodes, quartz tensional veining and disseminated Arsenopyrite-pyrite-biotite
mineralisation. This is developed along
the lithological contact between a sandstone and ultramafic conglomerate and on other contacts within the ultramafic conglomerates package.
High-grade ore shoots Plunge
steeply to the north along these contacts. The controls on mineralisation are dilational zones at the intersection
of steeply dipping north-south
axial planar structures with the Stratigraphy.
The stronger the contrast in rock competency in these zones, the greater the potential for mineralisation. Much of the yilgarn craton is deeply
weathered and partially covered
by tertiary and quaternary regolith. Pre-tertiary Lateritic horizons are variably exposed, stripped or buried
by later deposits that have in
turn been lateritised. The depth of Weathering
is strongly controlled by original rock types, with mafic rocks generally being more susceptible to weathering than felsic rocks.
Although all of the Agnew deposits are broadly
hosted by the intersections between structures and stratigraphy, there are
subtle differences in alteration and mineralisation, controlled in part by the
local host rock chemistry. Songvang is unusual in its relatively high lead,
silver and fluorine content, possibly reflecting input from tonalite and
porphyry intrusions. There also appears to have been a slight decline in mineralization
temperatures from south to north through the Agnew area, associated with the
north plunge of the Lawlers Anticline and resultant erosion of the overlying
lower-temperature rocks to the south. This temperature change is reflected in
the changing mineralisation styles from south to north, with biotite/garnet
assemblages dominating to the south and quartz veining to the north. The
stratigraphy of the Agnew-Wiluna belt appears to be broadly similar to the
stratigraphy of the Kalgoorlie Region. The following section provides a
detailed description of the general stratigraphy for the Agnew mining leases
and the stratigraphic location of various ore bodies.
Mining methods
Access to the Waroonga underground mine is via
a portal and decline, located in the previously mined Waroonga open pit. All
primary infrastructures, including escape ways and ventilation shafts, are
located in the competent sandstone of the hanging wall. The dimensions of the
decline are 5.5 metres wide by 5.8 metres high, with arched backs to allow
high-capacity trucks to operate.
The Rajah Lode was mined in 2012 using the
modified Avoca Method. This is a variation of bench-stoping methodology where
the fill typically consists of uncemented waste rock that is introduced to
control wall stability at intervals from 20 to 30 metres. During 2012 the Main
Lode North ore body was extracted using transverse stoping, with mining
commencing at the footwall and retreating to the hangingwall. The stopes were
accessed via a tramming drive developed in the hangingwall sandstone. The final
northern cutback of the Songvang pit was completed in February 2012.
Mineral processing
The comminution circuit comprises a
contractor-owned and operated three-stage crushing plant, feeding a fine ore stockpile,
ahead of a two-stage closed circuit ball milling circuit. Milled ore proceeds to
a three-stage leaching train feeding the six-stage carbon in pulp (CIP)
circuit. Carbon elution is by pressure Zadra with gold being electro-won and smelted.
In February 2003 a gravity circuit was retrofitted. It comprises a Knelson
gravity concentrator and an intensive leach reactor (ILR) for intensive
cyanidation of the gravity concentrate. The gravity circuit was upgraded and
retrofitted in September 2011 to incorporate a second 26” Knelson gravity
concentrator, an ILR 2000BA and two Magscreen 1000 units. The processing
capacity at the plant is 1.3 Mtpa. Quantitative analysis of recovery
improvements will be evaluated and included in future Mineral Resources and
Mineral Reserve calculations in accordance with the grade/ recovery model
currently adopted. Tailings disposal and impoundment have historically been to a
conventional dam constructed using the upstream lift method sourcing wall
material from dried tailings. The original tailing storage facility (TSF) was
located in an area some 1.5 kilometres south-west of the Agnew plant.
Deposition to this facility ceased in early 2004 and a large section of this decommissioned
facility has been capped with waste rock.
The remaining exposed sections are being
harvested for use as underground paste fill. Following completion of mining activities
at the Redeemer Mine, the abandoned pit has been converted to a tailings
storage impoundment for tailings arising from the Agnew plant (TSF3). Although
situated seven kilometres south of the Agnew plant, the static head is
negative, and pumping of the tailings material is therefore done at no
additional cost from the previous deposition technique. The pontoon-mounted
decant pump recovers water from the supernatant pond, which forms due to liberation
of water from the tailings slurry as it settles and consolidates. TSF3 is
projected to last until 2015, based on deposition rates of 1.3 million tonnes
per annum and historical deposition levels from December 2004 to February 2011.
Mineral Resource classification
Mineral Reserve classification
Mine model of Agnew Waroonga ore
body
