olympic dam geology

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    Olympic Dam – Expanding and Changing the Geological Interpretations of a Major Deposit and Its Implications

    Mr James Hodgkison
    Mine Geologist, WMC Resources Limited

    INTRODUCTION

    The WMC Olympic Dam mine is located 550 kilometres north of Adelaide in the mid-north of South Australia. The mine is serviced by the township of Roxby Downs which has a population of 4,000 and is located 16 kilometres south of the mine.

    Production at the underground mining operation commenced in 1988. The current annual production rate is 3 million tonnes to produce 85,000 tonnes of copper, 1,700 tonnes of uranium, 1 tonne of gold and 15 tonnes of silver. Copper therefore represents over 90 percent of the total commodities produced at the operation. Access underground is gained by a service decline and there are currently two vertical shafts which are used to either hoist ore or waste to the surface and to convey people in and out of the mine.

    The mine is fully mechanised using hydraulic jumbos, loaders and trucks involving a cyclic drilling, blasting and mucking operation. The development drives are generally 6 metres wide and 5 metres high and there has been a total of 150 kilometres of lateral development to date. Long hole open stoping with backfill is the mining method used. The dimensions of a typical stope are 30 metres along strike, 35 metres across strike and 120 metres in height. The deepest extraction level is 680 metres below the surface.

    The current $1.6 billion expansion, which is the largest single private development being undertaken in Australia, is nearly complete. It will see annual production rates increase to 9 million tonnes to produce 200,000 tonnes of copper and 4,600 tonnes of uranium.

    A third shaft, which will hoist all the ore to the surface, has been completed and is about to be commissioned. An automated electric rail haulage system is being established 740 metres below the surface to collect and move the broken ore to the main underground crusher. This system will rely on loaders to dump the ore into ore passes and the number of underground diesel trucks will be markedly reduced, which will result in a reduction of over 80% in haulage costs and also provide benefits for ventilation in the mine.

    GEOLOGY

    The deposit, which contains 2.1 billion tonnes at 1.4% copper extends for 5 kilometres in a northwest-southeast orientation and is 2 kilometres wide. Olympic Dam is the world’s sixth largest copper resource and the world’s largest uranium deposit. Copper sulphide mineralisation occurs beneath 350 metres of flat-lying, barren sedimentary cover within the 1.6 billion year old Olympic Dam Breccia complex (Reeve et al, 1990). The host rock to the ore is the Roxby Downs Granite. One interpretation has the mineralised fluids containing copper, uranium, gold, silver and iron deriving from a source at depth, possibly from a basaltic magma. The fluids migrated upwards along a major fracture within the Earth’s crust, causing violent explosive reactions with the granite in a near surface environment, similar to the crater lake districts seen in some parts of New Zealand.

    Unmineralised granite occurs near the margins of the deposit, while towards the centre of the deposit the rock type comprises clasts or fragments of granite and iron oxide in the form of hematite in variable proportions. The very centre of the deposit is hematite rich with up to 60 percent iron and does not contain copper mineralisation. The core of the deposit represents a leached zone where sulphide material has possibly been scavenged. Within the mineralised zones surrounding the barren core, the four commodities do not occur in discrete zones but are mixed together and cannot be mined selectively.

    EXPLORATION

    For many years, mine exploration has focused on searching for extensions of known ore positions along the main northwest-southeast trend. This has proved successful to a certain degree, with an addition of ore in mine areas F and FN (Eldridge, 1998). Surface diamond drilling continued to follow this trend in mine area FNW until unmineralised granite was encountered and the edge of mineralisation was assumed to have been located, however the interpretation had been left open. Due to the discouraging results in the drill program to the north and success elsewhere within the deposit, priorities soon changed and the program to the north was suspended for several years.

    Gravity geophysical surevys

    One of the exploration tools used to help locate mineralisation at Olympic Dam is the geophysical gravity data set. Gravity data, collected from aerial and ground surveys measure the densities of the different rock types. A model can be produced highlighting the rocks that contain hematite, which is almost twice as dense as the surrounding granite. This is important as the copper mineralisation at Olympic Dam is closely associated with hematite. Previous modelling has shown a large circular anomalous gravity high located over the deposit reflecting the high density contrast with the surrounding granite. This model has limited use as it does not highlight discrete potentially mineralised bodies but rather shows the entire area as being a density high.

    In 1997, Jim Hanneson, a WMC geophysicist, was able to enhance the gravity data set, removing background noise to reveal discrete anomalies that better reflect the distribution of the rocktypes and mineralisation. To the north, in mine area FNW, the gravity data appears to deviate from the previously defined northwest trend to an east-west trend. This has provided the opportunity for a new interpretation, with the potential to find more mineralisation.

    The increase in underground diamond drilling (72 km) and development (38 km) in the northern part of the deposit, which were required to support the expansion, have also resulted in a better geological understanding of the deposit. Structural features such as faults and joints identified from the drilling and mapping are oriented in a similar direction to the new gravity anomaly and provide support for the new interpretation (Hodgkison, 1997).

    A barren quartz-hematite breccia, similar to the rocktype located at the centre of the deposit, was identified in the new drilling information. The orientation of the barren quartz-hematite breccia is also parallel to the new gravity anomaly and possibly represents another site of localised hydrothermal activity.

    A metal zonation pattern has been recognised at Olympic Dam with chalcocite - bornite - chalcopyrite - pyrite occurring laterally outwards and vertically downwards from the main barren core in the centre of the deposit (Haynes et al, 1995). This metal zonation pattern provides a useful predictive tool for exploration in terms of the mineralisation species likely to be encountered. This is important as the different species have different effects on the metallurgical extractive process. Chalcopyrite and pyrite are less friendly to the smelter due to the higher sulphur content, while chalcocite and bornite are preferred. The same metal zonation pattern has been recognised surrounding the smaller barren quartz-hematite breccia identified in FNW. The surface diamond drilling program, testing the gravity anomaly in FNW is still in progress and continues to show encouragement with intersections of bornite-chalcocite zones as well as chalcopyrite-pyrite zones.

    Transient electromagnetic geophysical surveys

    WMC is currently trialling the use of other geophysical techniques at Olympic Dam such as downhole Transient Electromagnetic (TEM) surveys to indicate the presence of sulphide bodies. While this is not new technology, improvements within the field have made the technique more attractive.

    A suite of downhole geophysical surveys including TEM were conducted in February 1998 using a surface diamond drillhole located 4 kilometres northeast of the Olympic Dam deposit, which had been drilled to test a residual gravity anomaly (Hanneson, 1998). Copper sulphide mineralisation was encountered in the drillcore and the objective of the geophysical surveys were to determine if a response to the mineralisation could be detected and to also utilise the geophysical information to help determine the location of subsequent diamond drillholes.

    The survey measures the electrical conductivity of the ground surrounding the drillhole, up to a radius of 200 metres. The method involves arranging a series of electrical wire loops near the drillhole at the surface. The wire loops transmit a current and primary magnetic field into the earth, generating a secondary magnetic field in the vicinity of a conducting body and the response is recorded by a receiver in the drillhole (McNeill, 1980). To energise the ground, five square loops were arranged around the drillhole. The largest loop was centered on the drillhole and had sides 800 metres in length. The other four loops had sides 400 metres in length representing the four quadrants of the largest loop. By changing the loop location and varying the current source with time the different responses can be measured and the orientation of the conductive bodies can be estimated to give a qualitative interpretation of the potential mineralisation.

    The TEM survey produced encouraging results detecting the sulphides encountered in the drillcore and several other conducting bodies away from the drillhole which could represent mineralisation and warrant further drill testing. Other techniques which have the potential to work with future improvements include the use of Spectral Induced Polarisation (IP) to help identify and discriminate between different sulphide species.

    Data capture

    Technical advances have also been made in the way that geological information can be captured into the database. Previous pen and paper methods have been replaced by the use of portable computer notebooks with pen based input known as the Geological Logging System (GLS). The GLS provides a powerful and innovative
    tool that allows much more detail to be captured, compiled and stored in an easy to retrieve format, removing the need for manual data entry, increasing the efficiency of the mine exploration process. Future projects include implementing a similar system underground for geological mapping.

    CONCLUSIONS

    The geological interpretation of the Olympic Dam deposit has evolved over the last 10 years, particularly during the recent expansion. The current interpretation has benefited from the use of geophysical techniques, particularly the enhanced gravity dataset, which has had a large influence on the exploration process and the future ore potential at Olympic Dam. The increase in drilling and development rates to support the expansion has led to an increase in information in the northern part of the deposit. The geological observations from the drillcore and underground exposures have supported the geophysical data leading to a paradigm shift with respect to mineralisation trends and has implications for the strategies used to explore for new ore positions.

    Future technology in mine exploration will include improvements in the various geophysical techniques, particularly techniques used to identify mineralisation and to discriminate between the various mineral species. Other advancements will be gained by cost reducing initiatives such as digital data capture of information and improvements in analytical techniques.

 
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