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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