Stuart Smith*, Paul Burrell!,
Andrew Lye#, Phil Jones@, Lennard Kolff van Oosterwijk# & Mick Sharry*
*GeoDiscovery Group, Level1 14 Primrose
St, Sherwood, Qld, 4075
#Northparkes Mines, PO Box 995, Parkes,
NSW 2870
!Burrell Exploration Services Pty Ltd,PO
Box 31, Cowra, NSW, 2794
@Phil Jones & Partners Pty Ltd, PO
Box 565, Yass, NSW, 2582
Key Words: copper, gold, Northparkes, porphyry, exploration
Introduction
The
Northparkes Mines (NPM) Cu-Au operation is located approximately 27 kilometres
NW of Parkes in Central West NSW.
Mining commenced in 1995 and continues with approximately 12 years of
mine life remaining. Mining has been
conducted from two open pits (E22 & E27) and one underground block cave
mine (E26) with approximately 5 Mtpa milled.
Active
exploration programmes are underway at NPM with the goal of adding value to the
operation. The NPM district (within approximately 10 kilometres of the
operation) is a mature exploration province with approximately 30 years of
continuous exploration activity.
This
paper outlines the exploration history and recent exploration activities in the
vicinity of NPM. The contribution
provides the geological and business framework within which exploration is
conducted and discusses the exploration history of the currently economic
deposits. Selected current and recent
exploration projects are discussed in order to illustrate the central theme
that a healthy near mine exploration portfolio consists of projects from “grass
roots” to resource definition. It is
our view that to apply the term “Brownfields Exploration” is an
oversimplification and that the boundaries between “Greenfields” and
“Brownfields” exploration are extremely blurry.
Geological Setting
The
Northparkes deposits occur within the Junee-Narromine Volcanic Belt (JNVB) of
the Lachlan Orogen (Glen et al.
1998). The JNVB is an early Ordovician
to early Silurian related igneous belt that forms the western part of the
postulated Macquarie Arc; an early Palaeozoic intra-oceanic arc (Glen et al. 1998).
The
Northparkes deposits are hosted by the Goonumbla Volcanics of the Goonumbla
Volcanic Complex (GVC) (Simpson et al
2000). Outcrop in the vicinity of NPM
is poor with transported cover up to 30 metres thick and saprolite developed to
depths of up to 80 metres in places.
Geological information is gleaned from outcropping portions of the GVC
to the south of NPM, from drilling and mine exposures and geophysical data.
The
lower Goonumbla Volcanics contain a predominantly fine to medium grained
volcaniclastic sequence with interbedded limestone units. Conformably overlying these, the middle
Goonumbla Volcanics are dominated by mafic to intermediate coarse grained
volcaniclastics and trachyandesite to latite coherent units. The uppermost Goonumbla Volcanics contains
most of the stratigraphy previously forming the Wombin Volcanics (Bowman et al, 1982) and consists of a mixed
sequence of trachytic volcanics and coarse grained volcaniclastic breccias and
conglomerates.
The
volcanic rocks of the GVC are intruded by at least three suites of monzodiorite
to quartz monzonite and syenite that range in age from approximately 480 Ma to
438Ma (Butera et al 2001; Blevin
2002).
The
Northparkes deposits are hosted by all lithologies in the mid to upper
Goonumbla Volcanics and a close temporal and genetic link can be demonstrated
between the monzonite-quartz monzonite to syenite intrusives (Perkins et al 1990, Butera et al 2001, Lickfold et a.l
submitted).
The
GVC is characterised by prehnite-pumpellyite to lower greenschist facies
metamorphism and has been relatively unaffected by regional post-early Silurian
tectonic events. The volcanic package
is characterised by gentle to moderate dips and the absence of penetrative
fabrics away from fault zones. Pre-,
syn- and immediately post-mineralisation brittle faults and open folds have
been documented and have been correlated regionally with the Benambran Orogeny. Post early-Silurian strain has been strongly
partitioned into the margins of the GVC with minor brittle faults and gentle
folds typical. An exception to this is
a major east dipping thrust (the Altona Fault) that exists east of E26 and
truncates the top of E48. The
distribution of these structures is poorly known due to lack of outcrop and
their subtle expression in magnetic data.
Geology of the NPM Deposits
Four
economic deposits (E22, E27, E26 and E48) occur within the NPM district. The total indicated resource for these
deposits at 30 June 1997 was 115.3 Mt @ 1.1 % Cu and 0.5 g/t Au. The deposits are characterised by their
pipe-like geometry with vertical extents in excess of 900 m known from E26 and
E48. At a 0.5 % Cu cut-off the deposits
range from approximately 200 to 400 metres in diameter.
Mineralisation
is contained in quartz-sulphide (+/- carbonate, anhydrite-gypsum) stockwork,
disseminated sulphide and sulphide fracture coatings. Bornite is the dominant sulphide in ore, grading outward to
chalcopyrite-dominant zones at lower grades.
The highest grade ore is contained within the most intense quartz
stockwork in the central regions of each deposit. At underground cut-offs {0.8% eCu; where eCu = Cu % + (approx.)
0.5*Au g/t} almost all ore is contained within quartz stockwork whereas at open
cut cut-offs (0.5%eCu) much of the ore is contained within disseminated and
fracture coating sulphides.
The
central part of each of the deposits consists of zoned porphyry complexes
ranging in composition from mafic monzonite to quartz syenite (Lickfold 2000;
Lickfold et al. submitted). Ore is hosted by porphyry, volcanics and
equigranular monzodiorite to quartz monzonite intrusive lithologies.
Alteration
is complexly zoned around the central porphyries with the inner most zone
consisting of an irregularly distributed intense sericite +/- alunite
assemblage typically restricted to porphyries (Wolfe 1994, Hooper et al 1996, Harris & Golding
2002). Distinctive and often texturally
destructive K-feldspar alteration of early porphyries and adjacent volcanics
forms the majority of the inner portion of E26 where it forms a zone up to 200
metres in diameter. This style of
alteration is much more restricted at E48, E22 and E27 where it can be absent
or form narrow (<10 metres) zones around porphyries.
Outboard
of the K-feldspar zone at each deposit is a biotite-magnetite zone that can
form a zone up to 200 metres wide. At
E48, hypogene hematite alteration can be developed at the expense of magnetite.
Highly variable and irregularly distributed sericite +/- albite, chlorite,
magnetite, pyrite alteration exists peripheral to the E26 deposit.
The
distribution of alteration facies is strongly influenced by host rocks with
both silicate and oxide species sensitive to primary host rock compositions.
Three
distinct mineral camps have been defined in the Goonumbla Volcanic Complex on
the basis of geographic clustering and geological similarities. The most significant of these is the
McClintocks Camp that contains the E26 deposit and the E26 South, E20 and E51
prospects. The systems within the camp
are linked by the presence of significant phyllosilicate and albite alteration
and widespread (although not universal) gypsum-anhydrite veining.
The
Braeside Camp contains the E22, E27 and E22North porphyry systems. These systems have remarkably similar
alteration zoning and grade distribution.
All systems have relatively minor K-feldspar alteration and well
developed biotite-magnetite alteration.
The systems are characterised by the absence of sulphate veining and
peripheral phyllosilicate-albite alteration.
The
Adavale Camp contains the Veedas, E37 and E37West systems and is the only
mineral camp outside the immediate NPM operation. The systems in this camp are remarkably similar to the Braeside
systems in the alteration and grade characteristics, lacking sulphates and
extensive phyllosilicate or albite alteration.
The
E48 is the only porphyry system in the district that occurs as an isolated
occurrence. It is also geologically
distinct from the other systems lacking the widespread phyllosilicate and
K-feldspar alteration of the McLintocks systems, but is higher grade than the
Braeside or Adavale systems.
Exploration History
Exploration
by Geopeko (exploration arm of the then Peko Company) commenced in the
Northparkes district in 1972 with the submarine volcanic succession considered
to hold potential to host VHMS-style Pb-Zn deposits. Initial exploration was concentrated to the south west of the
current Northparkes operation and consisted of mapping and geochemical sampling
in this area of outcrop and residual soils.
Aeromagnetic
surveys were flown in 1974 in order to aid exploration and extend the search
under cover to the north. These surveys
were complemented in 1975 by regional scale auger-core drill traverses
undertaken along public roads orthogonal to regional strike. During the summer of 1976 a traverse was
undertaken along the Adavale Lane with holes spaced at one kilometre centres. One of these holes intersected pink
K-feldspar alteration and minor chalcopyrite-bornite mineralisation in bedrock
on the northern side of what is now the E22 deposit. Follow-up RAB drilling defined a large Cu-Au anomaly and in 1977
the “discovery” diamond hole was drilled, intersecting disseminated and vein sulphides
over its entire 319 metre length (G Jones, written comm., 1995). Follow-up of weak copper anomalism in the
auger-core hole one kilometre to the east of the E22 discovery was undertaken
shortly after and resulted in the E27 discovery (G Jones, written comm., 1995).
Exploration
continued with RAB grids extended and mapping undertaken in the area of the
current NPM operation. During the
course of this mapping intense quartz-sericite alteration was noted in the
vicinity of E26 but returned low Cu values and no further work was
undertaken. The discovery of E26 was
not made until an extension to the E28 RAB grid returned strong Cu values in
the area now known as E26.
With
intensive exploration in the ensuing 15 years further porphyry systems (and numerous
other zones of anomalous Cu-Au) were discovered at E22North, E20, E37, E37West
and E31North. Although a range of
exploration techniques were applied, all of these systems were discovered by
RAB drilling.
In
1992 a targeting exercise based on newly acquired aeromagnetics (120 metre line
spacing) was undertaken (Stolz, 1992).
One target (MTA9) selected from this exercise was accompanied by
anomalous Cu and occurred approximately half way between the E26 and E27
deposits. Follow up RC and diamond drilling
in 1993 intersected strong Cu-Au mineralisation in what was to become the E48
orebody (Hooper et al. 1996).
Recent Exploration Activities
In
the period from the original discoveries until December 1998 exploration in the
NPM area was the responsibility of the corporate exploration group (Geopeko,
then North Limited Exploration Division) with the management, funding and
strategic direction the responsibility of this group. However in December 1998 the North Limited Exploration Division
regional office in Parkes closed.
Senior NPM management considered that exploration could add value to the
operation and that the area remained prospective. A decision was made that NPM should conduct exploration in its
own right.
An
initial five year strategy was developed with exploration funding and
management the direct responsibility of NPM.
The emphasis was on building the project portfolio through a combination
of developing new targets and re-evaluation of known prospects. The underlying strategy was that a healthy
exploration portfolio should consist of projects at all levels of
advancement. Building strong technical
fundamentals and testing many targets was seen as critical to building the
portfolio and ultimately to adding value to the NPM operation.
Important
to ensuring that exploration had the maximum chance to add value to the
operation was linking the exploration strategy to the mine plan schedule. To this end, critical periods in the
life-of-mine plan were identified and exploration projects prioritised on their
ability to impact in a positive way on these.
Projects
undertaken during this period have included essentially grass roots projects
developed from newly defined targets.
One example of this project type is the Veedas prospect, approximately 5
kilometres west of the NPM mill. Veedas
is a previously unknown porphyry system within the Adavale Camp discovered as a
direct result of high quality magnetic data acquired in 2000 and was the first
new porphyry system discovered in the district since the E48 discovery in
1993. The system was the focus of
reverse circulation (RC) and diamond drilling (DD) during 2000 and early
2001. It has all the elements of the economic
deposits in the district but has limited near surface extent and, at the levels
currently tested, grades are too low to be economic as an underground
operation.
Although
uneconomic, the discovery of the Veedas system by application of advanced
technology to a tried-and-tested technique (aeromagnetics) demonstrates that
even in mature terranes proven techniques should never be dismissed. This is particularly true if advances in
technology allow enhancements. In the
Veedas case this enhancement was the ability to collect magnetic data at 25 m
line spacing and 30 m ground clearance compared to the pre-existing data
acquired at 120 m line spacing and 80 m ground clearance.
Although
the Adavale Camp systems remain prospective their distance from the NPM
underground infrastructure dictates that any underground operation needs to be
a “stand alone” operation. This
significantly downgrades their position in the NPM exploration portfolio and no
work is currently underway.
Another,
although very different, grass roots project example is recent drilling from
the E26 Lift 2 development infrastructure.
This infrastructure extends to the north of E26 for approximately 2 km
at depths ranging from approximately 450 to 800 metres. This area has had essentially no drilling at
these depths and is highly prospective.
Drilling has been undertaken to take maximum advantage of the new
infrastructure with a series of flat underground holes drilled laterally. Traditional targeting techniques, such as
surface geology, geochemistry, magnetics and IP have no applicability at these
depths. Therefore this programme is in essence a geochemical sampling programme
akin to the RAB programmes that were used to great effect in the early
exploration success in the district.
Second phase follow-up generated by this programme is currently
underway.
At
the opposite end of the project spectrum is the E26 North East Extension
project. The exploration phase of this
project consisted of 1947.7 metres of underground diamond drilling. The project arose with the recognition that
previously known extensions to the E26 orebody were poorly drilled and had the
ability to contribute high grade ore during the transition period between the
E26 Lift 1 and E26 Lift 2 mines.
Although tonnages were known to be relatively modest, the definition of
an additional high grade ore source for mining during this important period has
contributed to NPM exploration fulfilling its brief of adding value to the
operation.
Conclusions
An
active exploration programme continues at NPM and a healthy portfolio of
projects at all levels of advancement exists.
Although the next “Big One” is yet to be found a combination of
persistence, innovation and technical excellence places the Exploration Team in
a good position to fulfil its goal of adding value to the operation.
Acknowledgments
We
thank the dedicated geologists and geophysicists that have worked in the
district over many years and particularly acknowledge the contribution of those
active in the area in the 1970s and 1980s.
Northparkes Mines senior management (past and present) is thanked for
supporting exploration and allowing publication of this abstract.
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Bowman, H.N., Richardson, S.J., and Dolanski, J., 1982, Narromine 1:250 000 Metallogenic Map SI 55-3 - mine data sheets and metallogenic study: New South Wales Geological Survey, pp 337.
Butera, K.M., Williams, I.S., Blevin, P.L., and Simpson, C.J., 2001, Zircon U-Pb dating of Early Palaeozoic monzonitic intrusives from the Goonumbla area, New South Wales: Australian Journal of Earth Sciences, v. 48, p. 457-464.
Glen R.A., Walshe, J.L., Barron, L.M. and Watkins, J.J., 1998, Ordovician convergent margin vocanism and tectonism in the Lachlan sector of east Gondwana: Geology, v. 26, p. 751-754.
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Lickfold V., 2000, The intrusive history of the Goonumbla porphyry Cu-Au deposits, NSW: in Skilbeck, C.G., and Hubble, T.C.T., eds., Understanding planet earth: Searching for a sustainable future: Abstracts for the 15th Australian Geological Convention, University of Technology, Sydney, Australia, 2000, p. 303.
Perkins, C., McDougall, I., and Claoué-Long, J., 1990, 40Ar/39Ar and U-Pb geochronology of the Goonumbla porphyry Cu-Au deposits, New South Wales, Australia: Economic Geology, v. 85, p. 1808-1824.
Simpson, C., Cas, R.A.F., and Arundell, M.C., 2000, The Goonumbla Caldera, Parkes, NSW: fact or fiction?: in Skilbeck, C.G., and Hubble, T.C.T., eds., Understanding planet earth: Searching for a sustainable future: Abstracts for the 15th Australian Geological Convention, University of Technology, Sydney, Australia, 2000, p. 452.
Stolz, N., 1992, An aeromagnetic interpretation of the Goonumbla area: Unpublished confidential report to Geopeko, Report No; PK92/75/1.
Wolfe, R.C., 1994, The geology, paragenesis and alteration geochemistry of the Endeavour 48 Cu-Au porphyry, Goonumbla NSW: Unpublished BSc Honours thesis, Hobart, Tasmania, University of Tasmania, 102 p.