Rustler's Roost Gold Project
Environmental Assessment Report and Recommendations
1.0 Introduction and Background
This report assesses the environmental impact of the proposal by Rustler's Roost Mining Pty Ltd to expand and upgrade the Rustler's Roost Gold Mine on the Old Mount Bundey Station, approximately 100 km south east of Darwin (Figure 1) The company currently operates an open pit, heap leach gold mine and proposes to deepen and join the existing pits, install a new treatment plant and construct a tailings dam. The current operations were subject to environmental impact assessment in 1994 in the form of a Preliminary Environmental report. This assessment deals specifically with the issues arising from the expansion and upgrading of the mine.
The report reviews the draft Environmental Impact Statement (EIS), comments received during the review and the company's response to the comments in the Supplement to the draft EIS. The draft EIS and the Supplement constitutes the final EIS.
1.1 Environmental Assessment Process
Environmental impact assessment is predicated on defining those elements of the environment which may be affected by a proposal, and on determining the significance, risks and consequences of the potential impacts of the proposal at a local and regional level.
This report assesses the adequacy of the EIS in achieving the above objectives and evaluates the undertakings and environmental safeguards proposed by the proponent to mitigate potential impacts. Further safeguards are recommended as appropriate.
The safeguards may be implemented at various levels within the planning framework of the proposal. These levels are:
- Site selection;
- Layout of facilities;
- Design of facilities;
- Processes used in facilities (inputs and outputs); and
- Management of facilities and processes.
This report forms the basis of advice to the Northern Territory Minister for Lands, Planning and Environment on the environmental issues associated with the proposal.
1.2 Environmental Assessment History
Rustler's Roost Mining Pty Ltd lodged a Notice of Intent (NOI) with the Department of Mines and Energy (DME) in August 1996 to expand the mine and upgrade the facilities at the Rustler's Roost Gold Mine. The NOI was considered by the Project Assessment Committee (PAC) comprising officers of the Department of Lands, Planning and Environment (DLPE) and DME. PAC considered the environmental issues associated with the proposal significant enough to warrant assessment under the Environmental Assessment Act at the level of an EIS. The Minister for Lands, Planning and Environment accepted the PAC's recommendation and on 31 October 1996 determined that an EIS would be required.
Draft guidelines for the preparation of a draft EIS were advertised for public comment and circulated to NT Government advisory bodies for comment on 3 December 1996. Two public comments were received. Final guidelines were prepared taking into account the comments received from the public and government agencies. The Minister issued the final guidelines and a direction to prepare the EIS to the proponent on 19 December 1996.
The draft EIS was submitted in February 1997 and placed on public review for 4 weeks until 10 March 1997. It was also circulated to government advisory bodies for review and comment. No public comments were received whilst the comments from advisory bodies were consolidated into one NT Government submission (see Appendix A). This submission was forwarded to the proponent at the close of the public review period for the proponent to prepare a supplement addressing the issues raised by the NT Government.
The Supplement was submitted to DLPE on 25 June 1997 and circulated to government advisory bodies for review and comment. Their comments have been incorporated in this report.
2.0 The Proposal
2.1 Alternatives to the Proposal
As the proposal is to expand and upgrade an existing mine there were no requirements for the proponent to consider or address alternatives to the proposal except to consider alternatives to components of the proposal.
2.2 The Mine
The proposal aims to optimise the gold available in the four existing pits by combining and deepening the pits to a maximum depth of 155 metres, and expanding the combined pit surface area from 34 hectares to about 46 hectares. The existing waste rock dumps will be combined and increased in surface area from about 30 hectares to about 50 hectares and increased in height by about 20 metres to 45 metres. Mining operations will continue as before, and be carried out by mining and earth moving contractors using hydraulic excavators and 85 tonne dump trucks. Production of ore will be expanded to around 2-3,000,000 tonnes per year over a 4-5 year period. Figure 2 shows the mine layout.
Processing of the ore will change from heap leach to a Resin in Leach (RIL) process, a new technology to Australia. Feasibility and design studies are underway for two other RIL plants in Australia. The change in process means that a ball mill will be required to treat the ore. Cyanide is still used but resin beads rather than carbon is used to adsorb the gold cyanide complex (ion exchange). The resin is required because of the carbonaceous material that occurs naturally in the Rustler's Roost primary ore. Kerosene is also required in the process to deactivate the carbonaceous material which would otherwise adsorb the gold cyanide complex. Dosage rates up to 1500 grams/tonne of ore are expected which at the maximum production rate of 3,000,000 tonnes of ore per year means up to 4,500 tonnes of kerosene will be used annually. To keep operating costs low it is in the interest of the proponent to ensure that a minimum of kerosene is used.
Because of the change in process, tailings will be produced which means a tailings dam will be required for tailings disposal. The new tailings dam will cover an area of about 62 hectares. Current land requirements at the mine are 104 hectares whilst the proposed expansion and upgrade will increase this requirement to 181 hectares. The water usage at the mine will increase considerably because of the new process. Water will be supplied from local bores, pit dewatering and surface water catchment. This water will be stored in Annie Dam, an existing dam adjacent to the mine that was previously used for pastoral purposes. Process water will be recycled within the plant or directed to the tailings dam for storage and subsequent reuse in the plant.
In addition the heap leach pads will be decommissioned and the mine infrastructure and workforce will expand. The mine and all its facilities are located within Minerals Lease North (MLN) 1083.
2.3 Major Issues
The major environmental issues associated with the proposed expansion and upgrading of the Rustler's Roost Gold Mine are:
- new technology, Resin in Leach;
- tailings dam;
- waste rock management; and
- water management including acid rock drainage.
3.0 Regional Setting
This section is based on information provided in the draft EIS.
3.1 The Region
The mine is located in the headwaters of Mount Bundey Creek, within the Mary River catchment, in a secluded location approximately 15 kilometres south of the Arnhem Highway. In this area, the main activities are pastoral, tourism and recreation, in particular recreational fishing. Mining, quarrying, horticulture and defence force activities are also carried out in the general area.
The area is sparsely populated. The closest community of any size is Humpty Doo, on the Arnhem Highway, about 60 kilometres west of the mine.
3.2 Biogeography
The area where the mine is located forms the divide between the catchments of the Adelaide River and the Mary River, although the mine is in the Mary River catchment. Topographically, the project area is characterised by low, flat-topped hills with moderate to gentle slopes. Relief in the immediate vicinity is approximately 40 metres but it ranges from 30 metres to 190 metres Australian Height Datum (AHD) in the region. The vegetation of the hills is low, open and largely deciduous. The dominant vegetation of the plateaus comprises taller and for the most part evergreen trees. Creek lines in the hills have a minimum of alluvium with the vegetation indistinguishable from the adjoining slopes. The lowermost basins and creek lines have varying degrees of silty alluvium and support a distinctive tree and grass flora.
Three plant communities with distinctive soil types have been identified:
- Eucalyptus dichromoploia-E.tintinnans Low Woodland with skeletal and gravelly yellow lithosols;
- E.miniata-E.tetrodonta Open Forest with shallow lithosols and deep red earths; and
- E.polycarpa Low Open Woodland/Grassland with alluvium.
The project area lies on the northern flank of the Pine Creek Inlier within the transgressive marine sequence dominated by folded shallow marine sediments of the Early Proterozoic South Alligator Group. In general, the Pine Creek Inlier consists of an extensive, but poorly exposed sequence of Early Proterozoic sediments of low to medium metamorphic grade, which were deposited in a shallow intra-cratonic geosyncline. Cratonic basements consists of Late Archaean granites. The sediments were intruded and overlain by late Early Proterozoic volcanism and by later platform sequences of Middle Proterozoic age.
Gold mineralisation at Rustler's Roost is related to varying forms of silification occurring within a structurally prepared environment. The gold is hosted by weakly sulphitic (pyrite-arsenopyrite) banded chert mudstone sequences with varying quantities of thin quartz veining.
The climate is tropical monsoonal with two distinct seasons, and transitional periods between the two seasons. The wet season, December to March, is characterised by high rainfalls and north to north westerly winds. The dry season, May to September, has almost no rainfall with predominantly south easterly winds. Temperatures are generally high with a daily maximum temperature range of 32-37°C and a minimum range of 18-24°C. Annual average rainfall in the area is of the order of 1200 mm.
3.3 Areas of Conservation Significance
There are a number of conservation reserves managed by the Parks and Wildlife Commission in the area. The closest is the Mary River Crossing Reserve on the Arnhem Highway about 15 km to the north east of the mine. The western boundary of Kakadu National Park is 60 km to the east.
4.0 Environmental Assessment
It is important for interpretation purposes that the recommendations (in bold) are not considered in isolation, as the text identifies concerns, suggestions and undertakings associated with the project.
It is acknowledged that during detailed implementation of proposals, flexibility is necessary and desirable to allow for minor and non-substantial changes to the design and specifications which have been examined as part of this assessment. It is considered that subsequent statutory approvals for this project could make provisions for such changes, where it can be shown that the changes are not likely to have a significant effect on the environment.
Subject to decisions which permit the expansion and upgrading of the Rustler's Roost Gold Mine (Stage 2 Mining) to proceed, the primary recommendation resulting from the assessment is:
Recommendation 1
The proponent shall ensure that the proposal is implemented in accordance with the environmental commitments and safeguards identified in the Rustler's Roost Mining Pty Ltd draft Environmental Impact Statement, or as modified in the Supplement to the draft EIS, this assessment report or an approved Mine Environmental Management Plan.
The environmental commitments and safeguards made by the proponent have been summarised by the proponent in the Supplement and are included in this report as Appendix B.
4.1 Major issues
The review of the final EIS and the comments from the NT Government advisory bodies has identified four major issues associated with the proposed expansion and upgrading of the Rustler's Roost Gold Mine.
These issues are:
- new technology, Resin in Leach;
- tailings dam;
- waste rock management; and
- water management including acid rock drainage.
4.1.1 New Technology, Resin in Leach
As a new, untried technology, the RIL process is considered to be a major issue because products (resin beads and kerosene) are not currently used in gold processing in the NT. Taking all the information provided in the final EIS into account, the new RIL process is well described. Both the Resin and the cyanide used in the process will be recovered and reused, although some cyanide will end up in the tailings dam. The proponent is committed to maintain cyanide levels in the tailings dam in accordance with current best practice technology.
The concern is with the use of kerosene in the process as it is not recovered but disposed with the tailings to the tailings dam. The proponent states in the draft EIS that the kerosene is effectively bound to the tailings and expected to remain in the tailings dam. It is also expected the kerosene will naturally breakdown over time, and that this breakdown can be accelerated by the addition of commercially available microbes. There is no data or information provided to verify this expectation. If there are such microbes available this implies that there must be some information available on the breakdown of kerosene. No attempt has been made to ascertain the effects of kerosene on the environment except that free kerosene floating on the surface of the tailings dam is expected to evaporate.
The proponent has committed to ensure free and insoluble kerosene is kept to a minimum but there is no information how this will be done. A commitment to undertake studies on the effects of kerosene on the environment once the plant has been commissioned has also been made. As part of such a study it will be necessary to ascertain whether the kerosene naturally breaks down in the tailings dam and how this breakdown occurs. The outcome of any studies and investigations should be incorporated into the management of the tailings.
Recommendation 2
It is recommended that the study into kerosene and its effect on the environment should include whether and how the kerosene breaks down in the tailings dam. Further, the study should commence prior to commissioning the plant. The proponent should also investigate the availability and effectiveness of microbial inocula that can be used to breakdown the kerosene.
4.1.2 Tailings Dam
As the new RIL process requires a dam for tailings disposal, six sites were initially considered. Four of the sites have been discarded as unsuitable whilst two sites are under further consideration. One of these sites (site 4, the preferred site) has undergone some site investigations to establish its suitability, and detailed design has been completed. There are, however, Native Titles issues that need to be resolved as the site is on an adjoining exploration lease and not on the mineral lease (MLN 1083). There is no gold ore underlying the site that should be extracted.
The other site (site 6) has not been investigated to ascertain its suitability for a tailings dam, although a preliminary design has been prepared based on the assumption that the site is geotechnically similar to site 4. Site 6 has not been sterilised. Although it is stated in the Supplement that the proponent cannot commit to either site at this stage due to the uncertainty of Native Title, it appears from the text and drawings in the final EIS that site 6 is the most likely site to be used. Site 6 is shown on Figure 2.
The preliminary design for site 6 is for a three year life although it is expected that the proposed expansion and upgrading under consideration will extend the mine life for 5-6 years. The only reference to this potential shortfall in tailings dam capacity in the final EIS is that the dam walls can be raised to accommodate the extra tailings if site 4 does not become available. It is not clear from the EIS whether a tailings dam will be constructed and used at site 4 if this site becomes available at some time during the life of the mine and a tailings dam is already in use at site 6.
The tailings dam is part of the water management system at the mine and the water management as described in the final EIS is based on a tailings dam at site 6. The uncertainty regarding the site for a tailings dam needs to be resolved and a decision to use either site 4 or site 6 needs to be made by the proponent prior to the preparation of any Mine Environmental Management Plan.
The proponent makes a number of commitments that would be appropriate to a tailings dam constructed at either site.
Recommendation 3
It is recommended that either site 4 or site 6 be used for a tailings dam and if it is to be site 6 then the necessary investigations be carried out to determine its suitability. Further, only one tailings dam should be constructed and used for the proposed expansion and upgrade at the mine as described in the final EIS.
4.1.3 Waste Rock Characterisation and Management
The proponent has drilled three holes, one in three of the four pits, and taken samples at one metre intervals from each hole for waste rock characterisation. Analysis of the samples confirms that the waste rock has both the potential for acid rock drainage (ARD) and acid neutralising capacity (ANC). Comparison is made to recent, similar waste rock sampling at Brocks Creek Gold Mine and at Ranger Uranium. Fewer samples were taken but more holes were drilled at Brocks Creek, giving a better understanding of the spatial distribution of the waste rock there. As regards the Ranger comparison, it is not stated whether this is to the existing Ranger mine or the Jabiluka No 2 underground mine that is currently undergoing environmental impact assessment. If it is the Jabiluka mine a comparison would be invalid because it is an underground mine whilst Rustler's Roost is an open cut mine.
Although samples were taken at one metre intervals the waste rock characterisation as carried out is considered inadequate because only three holes were drilled for this purpose. Appendix J (Waste Dump Design) of the Supplement acknowledges that there may be inaccuracies in the waste rock characterisation because of this limited drilling. Even though it may be inaccurate, the information obtained from the three holes has been used to estimate the quantities of the various components of the waste rock. It is estimated that there is sufficient non acid producing waste rock available to be used in the construction of the tailings dam wall as well as encapsulating the potential acid forming rock.
The Supplement (page 5) makes the point that the Rustler's Roost deposit is very heterogeneous in nature. Because of this heterogeneity, the three holes give very limited information about the spatial distribution and quantities of the waste rock and its various components: non acid producing; potentially acid producing; and acid neutralising capacity.
It is also stated in the Supplement (page 5) that the three ore formations or pits where the holes were drilled are separated by a relatively wide and barren sandstone band. No data or information is given to support this statement, nor is any information provided on the waste rock characteristics of this band. As the pits are to be combined and deepened it is assumed that at least part of the areas between the pits will have to be excavated and disposed of as waste rock.
Recommendation 4
It is recommended that further investigations be carried out to ascertain waste rock characteristics, the spatial distribution and quantities of the various waste rock components with potential for acid rock drainage, acid neutralising potential and the extent of non acid forming rock.
The proponent estimates that there is sufficient ANC waste rock to overcome the ARD potential. The basic design of the waste rock dump is to encapsulate potentially acid forming rock with non acid forming rock. The proponent makes a commitment that the "waste rock dump will be non-acid producing by encapsulating acid producing waste". In addition, the water courses at the waste dump site that flow through the dump, thus forming part of the underdrainage, will be lined with ANC waste rock to prevent water coming into contact with potentially acid forming rock. Upon completion of mining the waste rock dump will be covered with a layer of compacted, non acid producing rock and then a layer of topsoil. Each layer will be 0.5 m thick. The compacted layer will be of low permeability to minimise entry of rainwater into the dump. If monitoring of water quality during the operation of the mine indicates acid is draining from the dump it is proposed to incorporate a 50 mm layer of lime between the potentially acid forming rock and the compacted layer of non acid producing rock. How this is to be achieved where the compacted layer has already been put in place is not described.
The proposed design of the dump lacks the necessary evidence to support the commitment that the dump will be non-acid producing. To prevent oxidation of the sulphide waste both water and oxygen must be excluded. The design as described in the final EIS (Appendix D of the draft EIS and Appendix J of the Supplement) does not exclude water and oxygen but rather expects some water to percolate through the dump. The construction technique (end tipping) is likely to provide a preferential drainage path for both entry of water into the dump and the exit for any acid leachate from within the "encapsulated waste". If water can enter the dump one must expect that, in the long term, oxygen can also enter the waste rock leading to the formation of acid leachate from the dump.
Recommendation 5
It is recommended that the design of the waste rock dump and the construction technique be reconsidered to ensure that it will be non-acid producing or alternatively the dump be redesigned and managed to minimise and control any acid that may be formed and leached from the dump.
4.1.4 Water Management
The proponent describes a revised water management system in the Supplement that addresses some of the concerns raised in the NT Government submission. Both the original and the revised systems are based on site 6 being used for the tailings dam. This dam and Annie Dam are to be used for makeup water for the process plant. Process water will be recycled within the plant. In the original system clean surface runoff, bore water and water from the pit dewatering were to be directed to Annie Dam. Water from the underdrainage of the waste rock dump was also to be directed to Annie Dam after passing through a wetland filter.
One of the concerns with the water management system was the use of Annie Dam for storage of water from the pit dewatering, the waste rock dump as well as clean runoff and bore water. Annie Dam is not an engineered dam and therefore considered unsuitable because of the possibility for failure and subsequent discharge of contaminated water into the Marrakai Creek catchment. The Supplement describes a revised water management system in which Annie Dam will only be used for clean surface runoff and bore water. Figure 6A in the Supplement supposedly shows the revised water management system at the mine but it is exactly the same as figure 6A in the draft EIS. There is no indication in the Supplement on how pit water will be managed. The implication is that another storage will be required for water from the pit dewatering operations or Annie Dam will have to be reconstructed.
Another concern was with the treatment of the potentially acid water from the underdrainage of the waste rock dump. In the revised water management system this water will be collected by a dedicated drain discharging into a silt trap and treatment pond for ARD. Discharge from the trap and pond will be into a wetland filter. The dedicated drain is located between the waste rock dump and the expanded pit, and shown on Figure 1 of Appendix J of the Supplement. There appear to be some inconsistencies in the overall mine planning in that the drain as shown on Figure 1 seems to be in the path of a decline into the expanded pit as shown on Figure 3 in the draft EIS and the final waste dump and pit plan shown in Appendix J of the Supplement. The solution to this concern does not appear to take into account other aspects of mine planning and operations.
The revised system, with water from the waste rock dump being collected by a drain and treated in a silt trap, pond and wetland filter before discharge into Mt Bundey Creek rather than into Annie Dam, is an improvement on the original system. The revised system is enhanced by a proposed cut off drain directing clean surface water runoff into Annie Creek.
In the revised system overflow from Annie Dam and the contingency spillway on the tailings dam (site 6) discharge into a wetland filter that eventually drains into Mt Bundey Creek. It may prove difficult to establish this wetland filter as it is located in a narrow and incised creek. Excessive flows during wet weather may damage any constructed wetland unless allowance is made for diversion flows during high rainfall events. The use of wetland filters is generally acceptable provided they are operated and maintained as intended. However, the wetlands must be designed and managed so as to reduce the potential to create biting insect habitats.
The final EIS gives no indication of the water management system for the mine if site 4 becomes available for a tailings dam, in which case much of the system will have to be redesigned.
Recommendation 6
It is recommended that the water management system at the mine be further revised to take account of the need for storage of pit water, Annie Dam to be used for storage of clean surface water runoff and bore water only, the possibility of site 4 becoming available for a tailings dam, and the general mine layout.
4.2 Other issues
Four additional issues were raised during the assessment of the proposal:
- rehabilitation;
- weed and weed management;
- biting insects and biting insect habitats; and
- heritage matters.
4.2.1 Rehabilitation
Once mining is completed the proponent expects the land to revert to pastoral uses and the rehabilitation objective will be to leave the site in a safe and stable condition for such uses. Rehabilitation will commence while mining is still in progress with trials and monitoring of various techniques. The success of various techniques will be reported in the annual Environmental Reports required under the Mine Environmental Management Plan. The rehabilitation program will be revised as rehabilitation progresses through the mine's life.
Matters that need to be considered in the rehabilitation are the final batters of the waste rock dump, drainage, control of run off after decommissioning, and the long term monitoring of the rehabilitated mine site. Decommissioning and rehabilitation must include the wetland filters as there may be potential for these to dry out and there may be the potential for heavy metals to remobilise in early wet season storms.
4.2.2 Weed Management
The proposed weeds management as outlined in the draft EIS was considered very inadequate and it was recommended that the proponent consult with the Department of Primary Industry and Fisheries.. The department has since carried out an inspection of the mine site and made recommendations on the ongoing weed management at the site. The proponent has made a commitment to manage, where practical, weeds in accordance with the Department's recommendations.
Recommendation 7
It is recommended that weeds management at the mine be in accordance with the Department of Primary Industry and Fisheries recommendations to ensure the mine's operation does not cause the introduction or spread of weeds in the area.
4.2.3 Biting insect management
The proponent acknowledges the concerns with biting insects and, in particular, the need to manage and control mosquitos. A commitment to implement a program compiled by the Territory Health Services has therefore been made. Commitment 44 (Appendix B) incorrectly refers to the Agricultural Branch of the Department of Primary Industry and Fisheries. The correct agency is the Medical Entomology Branch of the Territory Health Services
4.2.4 Heritage
A heritage survey was carried out as part of the original proposal to develop the mine. Four sites were identified, three of which have been fenced and avoided by mining operations whilst the fourth has been destroyed under a permit issued in accordance with the Heritage Conservation Act. From the final EIS it appears that the three sites will remain undisturbed by the current proposal.
It is however, noted that the draft EIS indicates a further survey was to be conducted in February 1997 in areas where impacts were likely to occur which had not previously been subject to an archaeological assessment. The Heritage Conservation Branch of this Department has no record of receiving a report of such a survey.
Recommendation 8
It is recommended that the archaeological survey referred to in the draft EIS (section 4.2.2) be completed and a report submitted to the Department of Lands, Planning and Environment for consideration.
5.0 Conclusion
The outcome of the assessment of the proposed expansion and upgrading at Rustler's Roost Gold Mine is that the proposal can proceed. However, the assessment has raised a number of matters that need to be further addressed before any approval to proceed can be granted. These matters must be addressed in a Mine Environmental Management Plan that should be submitted to the Department of Mines and Energy for approval by that department and the Department of Lands, Planning and Environment. The matters that require further consideration by the proponent are outlined in this report and relate to the use of kerosene, the tailings dam site, waste rock characterisation and management, and the water management system.
