David Sleigh
Group Geologist, Selwyn Operations Pty Ltd.
Key Words : iron oxide-copper-gold; epigenetic; Mount Isa Inlier; resources; exploration
Selwyn Mines started production at the dormant Mt Elliott Cu-Au Mine in March 2000 with around three years of mine life in reserves. Urgent investigations to locate more reserves and high quality exploration targets were undertaken. This exploration involved re-organization, verification, validation, reprocessing of the substantial data base before re-interpretation of entire mineralized systems was undertaken. The principal mineralized systems identified included : 1) the Cu-Au bearing, calcic-skarn related Mt Elliott District; 2) the Cu dominated black shale breccia-hosted Mt Dore District; 3) the shear-hosted, iron oxide-copper-gold system of the Selwyn Line. Exploration and project focus was placed on the Selwyn Line due to its substantial infrastructure, already in place, associated with the five dormant mines. Resource modelling of the known mineralized system (~6km of strike) identified a large global resource (253mt @ 0.34% Cu and 0.48g/t Au at a 0.2% eCu cut off). Reworking of various mining options resulted in the definition, within this resource, of a Measured and Indicated resource of 22mt tonne @ 1.13% Cu and 1.81 g/t Au, using a 1.5% eCu cut off. Previously only 1.28mt @ 2.2% Cu and 1.8g/t Au had been reported. Exploration of the data also yielded numerous extension and new exploration drilling targets. At the 222 Mine eight strategically placed surface drill holes generated an additional 1mt of Measured and Indicated resources, and the ore zones are still open at depth and to the north. This evaluation phase and subsequent plant upgrade has doubled plant throughput and generated at least eight years of mine life, thereby giving the company time to evaluate and develop the other major projects within its tenure.
Selwyn Mines Ltd are currently operating four mines in the Eastern Succession of the Mount Isa Proterozoic Inlier, in NW Queensland (Figure 1). These include the skarn hosted Mt Elliott copper-gold mine, the iron oxide-copper-gold Selwyn 222 and 251 mines, and the breccia hosted copper-gold Lady Ella Mine. Total production from these mines has been ramped up to 2mt per annum. Selwyn Mines bought their way into highly prospective mining and exploration tenure covering over 1500 square kilometres during late 1999 and early 2000. They recommenced production from the Mt Elliott Mine located 20km north of the Selwyn processing plant in March 2000. The Mt Elliott Mine resource at that stage included Proven and Probable reserves of 1.89mt @ 3.2% Cu and 1.5 g/t Au (a reserve life of ~3 years at planned through-put rates of 0.52mtpa). No other reserves of significance (>0.5mt) were listed in their inventory at that time and it was clear that if the company was to succeed it would require a rapid organization and screening of the available data in order to locate and define new reserves. This paper outlines the process followed and the results obtained, with particular focus on the Selwyn Line of FeO-Au-Cu mineralization.
The bulk of the Selwyn tenements are underlain by Proterozoic meta-sedimentary units of the Kuridala Formation (Soldiers Cap Group), Staveley Formation (Mary Kathleen Group), Answer Slate Formation, and the older Gin Creek complex, of the Eastern Fold Belt of the Mt Isa Inlier (see Figure 2). These host late to post-tectonic A-type granitoids of the Williams batholith and to numerous elongate amphibolite bodies. The district contains major structures, including the prominent north-south trending multi-aged Mt Dore Fault zone, the adjacent Selwyn and Starra shears and the Answer Fault. Empirical observation on the distribution of mineral occurrences and deposits overwhelmingly supports the view that these regional structures provide the principal controls on the distribution of most of the observed metasomatic and mineralizing processes within the Selwyn tenements.
A variety of copper-gold mineralization styles are observed in the Selwyn District deposits, as listed below. For all deposit styles the age of formation is closely related to the late structures (post D2).
1) deposits associated largely with the Mt Dore shear system and the Kuridala
Formation black shales and phyllites:
§ pyroxene dominated skarn at Mt Elliott Mine :
(global resource @ 0.5% eCu cutoff of 20mt @ 1.11% Cu and 0.57g/t Au);
§ calc-silicate breccia hosted mineralization at the Swan deposit :
(global resource @ 0.5% eCu cutoff of 43mt @ 0.58% Cu and 0.38g/t Au); and the Lady Ella Mine :
(global resource @ 1.0% eCu cut off of 0.66mt @ 1.48% Cu and 1.33g/t Au);
§ black shale/phyllite breccia hosted copper “oxide” dominated mineralization at the Mt Dore deposit :
(global resource @ 0.2% eCu cut off of 78.3mt @ 0.5% Cu and 0.1g/t Au);
and the Victoria mine :
(2.4mt @1.3% Cu and 0.2g/t Au @ 0.5% eCu cut off); and
2) deposits associated with the Starra shear
§ The Selwyn Line shear hosted FeOx-Cu-Au mines (the 222, 244, 251, 257 and 276 mines).
Historically these mines exploited the high gold portions of the mineralized system and produced 6.84mt @ 2.1% Cu and 4.6g/t Au. However these deposits formed part of a larger pre-mining global resource of 95.1mt @ 0.74% Cu and 1.11g/t Au using a 0.5% eCu cut off (Sleigh 2002).
The Selwyn Line comprises an intensely alkali-iron-silica-carbonate altered section of the Starra shear zone, which is host to multiple high grade gold-copper shoots (Figure 3). These shoots are developed in structural loci within a larger copper-gold mineralized tabular system (Kary and Harley 1992; Sleigh 2002). The Starra shear here separates calcic psammites and siltstones of the Staveley Formation (Mary Kathleen Group) in the east, from the Answer Slate and Gin Creek gneiss complex to the west.
In the vicinity of the Selwyn Line the Starra shear is interpreted as an early ductile-brittle (regional D1 age) decollement zone (Blake 1987 and Laing et al. 1989), overprinted by vertical to steeply eastward dipping, north-south trending D2 and local D4 age shearing.
After initial scanning of the data, the association of alteration and distribution of Cu and Au mineralization with the main structures was apparent. The potential for Selwyn Mines Limited (as a junior mining company) to grow into a medium to large company, hinges in part on its ability to rapidly locate, define and exploit new deposits located within the huge database collected diligently and at significant expense, by previous owners of the tenements. In the prevailing exploration finance market Selwyn would have to generate sufficient mine life and cash flow before it could embark on exploration and development of major projects, and a sustained exploration programme on the highly prospective ground held under license. Hence, since the company was not encumbered by the problems of obtaining valid licences to explore and mine under the prolonged and uncertain process of native title negotiation and agreement, it focussed its efforts on the existing Mining Leases; in the “shadow of the plant”.
The hard copy and digital data bases were systematically re-organized, verified and validated, starting with the Mt Elliott Mine drilling data base, followed by the Mt Dore and Selwyn Line data bases. The Selwyn Line drilling data base contains over 1600 drill holes spread over 6 kms of strike. These were used in a global resource scoping study to determine the dimensions of the Selwyn Line system and the zones on which more detailed studies should be focussed. (post-mining global resource of 253mt @ 0.34% Cu and 0.48g/t Au at a 0.2% eCu cut off was calculated) The flat gradients of the grade tonnage curve (Figure 4) suggested there was a strong potential for mining and processing of residual ore; at higher through-puts and lower cut-off grades than had been mined historically. Through examination of various options, an initial processing plant through-put capacity upgrade from ~1mt to 2mt per annum per annum, was identified as being optimal for processing ore reserves defined by a 1.5% eCu cut off. More detailed geological and resource modelling of the Mt Elliott, Selwyn 222, 244, 251, 257 and 276 mines enabled an initial reserve definition of 16.61mt @ 2.37% Cu and 1.38g/t Au by June 2001 (Selwyn 2001 Annual Report. This has to be compared with the 1.89mt @ 3.2% Cu and 1.5 g/t Au quoted in the June 2000 Selwyn Mines Prospectus. Exploration of the data had now yielded eight years mine life and a basis for future project exploration and development.
In addition the intensive processing of the data had generated a number of high priority exploration targets within the Selwyn Line. These mostly comprised undrilled extensions to ore shoots, indicated by the resource modelling. Despite an intensive programme of drilling survey and mining data verification and validation, there were also a string of targets within the models that required checking and validation by exploration drilling. These included ore blocks adjacent to mined voids and ore zones in erratic positions relative to the general geological settings. Hence as always the limitations of the resource lay primarily in the quality of the data digested.
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At 222 Mine the gold and copper mineralization is associated with at least 5 “sites”, where roughly north-south trending ferruginized shears intersect with a north-easterly trending structure (the “hinge” zone, see Figure 3). These structures plunge to the north-north-east at >700 and this is the general orientation of the 222 ore zones. It was also the direction identified by the variography conducted on the re-modelled 222 ore zones (Selwyn Internal Report 2, 2002). The remodelling identified high grade shoots within broader lower grade zones of more intense brecciation, albitization and intrafolial iron oxide replacement (Figure5). The absence of drilling into the northern portions of these plunging zones, generated an opportunity to convert Inferred and untested presence of the plunging shoots and generated an additional 1.07mt of resources resources to Indicated category or better. The drilling of 8 holes during July to September 2001, confirmed the Measured and Indicated categories.
At the 276 Mine the development of the first pass resource model confirmed observations made by previous workers that unlike the other Selwyn Line mines with their northward plunging high grade shoots , 276 had an initial near surface eastward plunge to the high grade portions, which then swung to ~45o plunge to the SSE. The variography was considered to support this trend, and drilling of deep holes confirmed the extension of the ore zone to the south-south-east. Due to considerable depth and steep dip angles this depth extension will be further tested by underground drilling.
An example of exploration check drilling is the program conducted at the 251 Mine, where near surface blocks had shown the presence of high grade ore zones in the hanging wall of the main stope void. It also gave indications of medium grade ore in extensions southward and northward from the mined area. This drilling indicated the stope void had incorporated the hanging wall mineralization, but that the near surface southward and northward extensions had not been mined.
PUTTING ON WEIGHT
The exploration conducted in the Selwyn Line over the last 2 years has centred on verification, validation, reprocessing, reviewing, re-interpreting and testing of the available data. This process has generated ~16mt of ore reserves where < 0.5mt existed pre June 2000. It is not the hard core exploration we are familiar with, but has proved highly effective on this mine site for the mineralized systems and information available to us. Critical components of the exploration process included:
A) The recognition of the Selwyn Line as a major mineralized iron oxide-gold-
copper system. This is not new thinking but hitherto the Selwyn Line data base had not been treated as a single mineralized entity for resource evaluation purposes, at lower cut offs.
B) A revised ore measurement process, of starting from the boundaries of the mineralized system and working inward to the main mineralized zones. In this way a truer statistical reflection of the grade distribution, both numerical and spatial could be defined, rather than direct confinement to one or two tight high-grade domains.
C) A vigorous (but unfortunately not quite infallible) process of verification and validation of a large data base. This is a critical component of the exploration-by-numbers process in an old mining area, as there is often incomplete data (eg. survey data from voids stripped on retreat).
D) Recognition of the principal controlling geometries of the ore zones and confirmation that they are being reflected in the resource statistics;
E) Optimal economic evaluation of the available resource using a variety of options, and taking into account the existing infrastructure, to arrive at optimal reduced cut-offs. This was a major step to generating new resources and reserves. Due consideration to metallurgical, geotechnical and mining issues are critical;
F) Testing of targets generated during the development of the resource models, both for increases in resources, and for the checking of resources in the vicinity of old workings.
Selwyn has grown through exploration of the mine site data set followed by drill validation of the resultant ideas. The process has involved the combination of data organization and validation, reprocessing and reinterpretation of the entire set , identification of the appropriate economic options, identification of the relevant resources fulfilling those options and testing of them by drilling.
2. Not all the new resources identified by the data exploration process, were found to be factual due to problems with the quality or currency of the data available. Checking well in advance of mining activities will mitigate these shortcomings.
3. The results of this process on the Selwyn Line include the increase in resources from <0.5mt to >22mt. converting to reserves of >16mt.
4. Exploration in the shadow of the “Selwyn headframe” has involved extending the view to the boundaries of the reliable data set, recognizing the bigger picture, and contracting in toward the areas of best potential for the scale of operation planned. This has provided the basis for new exploration data collection and it is feasible that other entirely new high grade ore shoots will be identified.
5. The step toward definition and development of the major deposits in the Mt Dore and Mt Elliott Districts, and potentially on the exploration tenements, must be undertaken during the current mine life and will require a similar approach to that taken for the Selwyn Line. This exploration of the data could over time, ultimately lead to a further production increase, from the current ~20 000t of Cu per annum, to in excess of 100 000t Cu per annum.
ACKNOWLEDGMENTS
Selwyn Mines Ltd for releasing the relevant information. Selwyn Operations Pty Ltd Geological Department staff for their work in developing the understanding of the entire data base. Gavin Daneel, for his drive and vision which linked the data sets to the production requirements.
REFERENCES
Blake, D.H., 1987. Geology of the Mount Isa inlier and environs, Queensland and Northern Territory : Australian Bureau of Mineral Resources Bull. v. 225, p. 83.
Kary, G.L., and Harley, R.A., 1992. Selwyn Gold-Copper Deposits. Geology of the Mineral Deposits of Australia and Papua New Guinea (ed. F.E. Hughes) Institute of Australasian Mining and Metallurgy. p. 955-960.
Laing, W.P., Rubenach, M.J., and Switzer, C.K., 1989. The Starra gold-copper deposit: syndeformational metamorphic mineralization in a folded early regional zone of decollement. Geological Society of Australia Abstracts, 21, p. 229.
Selwyn Mines Internal Report 1., Selwyn Line Resource Estimation Study – Scoping Study Resource, by Snowden Mining Industry Consultants, November 2000.
Selwyn Mines Internal Report 2., Area 222 Resource Model Update, November 2002, by Snowden Mining Industry Consultants, February 2002.
Selwyn Mines Prospectus, June 2000.
Sleigh, D.W.W., 2002. The Selwyn Line Tabular Iron-Copper-Gold System, Mt Isa Inlier, NW Queensland, Australia. In Porter, T.M. (Ed) Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, Volume 2. PGC Publishing, Adelaide, pp 77-93.