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Life cycle of a mine

1. Exploration: Generative stage

Minerals exploration is the process of finding commercially viable quantities of minerals to mine. The methods used for exploration vary at different stages of the process depending on the size of the area being explored, as well as the density and type of information sought.

In the generative stage, the general area to be explored is identified. The first activity is the analysis of public geoscientific information. In Australia, this includes a range of detailed pre-competitive geophysical data such as magnetic, radiometric and contour maps, images and survey data produced by Geoscience Australia and the State and Northern Territory Geological Surveys and mines departments. Historical exploration data are also reviewed including results publicly reported to the Australian Securities Exchange.

Evaluation of publicly available data is supplemented with private reconnaissance field work including prospecting, mapping and geochemical sampling activities. With the survey material, analysis of the properties of other established deposits and an understanding of the earth’s physical and chemical processes, an exploration model is developed to guide more detailed exploration activity.

2. Exploration: Primary exploration stage

In this stage, the targeted area is subjected to exploratory drilling or trenching to delineate likely zones of mineralisation. If a potentially viable deposit is discovered, more intensive sampling and geochemical analysis is conducted in order to fully map and define its size, grade and geometry. This may involve intensive drilling, trenching or underground sampling. An exploration shaft, decline or adit may be excavated to gain underground access to the deposit. To undertake this stage, a mining company must secure an exploration licence and other relevant approvals. Exploration activities are supported by professional expertise in safety and risk management, community and environmental management and project, information and financial management.

3. Exploration: Evaluation stage

If the early stages of exploration have discovered a mineral deposit, exploration proceeds to the evaluation stage to determine whether it is economically viable to develop. This stage involves comprehensive technical and socio-economic analysis to determine whether or not mining is economically feasible. It includes detailed assessments of geoscientific and engineering operational factors including mine design infrastructure availability, assessments of environmental impacts and stakeholder issues influencing a project’s social license to operate and detailed financial analysis of costs and revenue projections and market assessment.

The evaluation stage includes a number of separate, but related, phases:

  • Scoping studies – Scoping studies are commonly the first economic evaluation of a project and may be based on a combination of project data together with assumptions borrowed from similar deposits or operations. They typically identify technical issues that will require additional examination and are commonly used internally by companies for comparative and planning purposes.
  • Preliminary feasibility study – Preliminary feasibility or pre-feasibility studies are a comprehensive study of a range of options for the technical and economic viability of a mineral project including mine design, production schedule, gold recoveries, plant design and expenses. These studies identify but do not finalise the preferred mining, processing, and infrastructure requirements and capacities.
  • Feasibility study – Feasibility studies examine comprehensively the selected development option for a mineral project and include detailed assessments of the operational, financial, social and regulatory factors relevant to demonstrating that extraction is economically viable. Feasibility studies are of a higher level of rigour than pre-feasibility studies and normally serve as the basis for an investment decision or decision to proceed to the development stage. They contain mining, infrastructure and process designs and may address implementation issues such as detailed mining schedules, construction ramp up and project execution plans. Social, environmental and governmental approvals, permits and agreements will be in place, or will be approaching finalisation.

The evaluation stage of exploration involves the investigation of the total environmental and socio-economic impacts of going from resource to mine and through to post-mining reclaimed land. This assessment of the risks and benefits of mining is a critical input to government decision-making in relation to the mine’s development. Permits are required across the entire life cycle of a mine. During this process, the company will present comprehensive documents to Federal and State/Territory Government agencies outlining predicted impacts associated with the development and how these will be mitigated or managed over the life of the mine. The nomenclature of this study varies, but is generally known as an Environmental Impact Study (EIS).

The EIS must be completed before a miner can be granted a license to construct a mine. In this study, impacts on wildlife, habitats, noise, air and water are assessed and the strategies to manage them described. The EIS is disclosed to agencies making the decision about permission to build and is also subject to community consultation. If the EIS is rejected, the company is usually given the opportunity to make adjustments to take account of the concerns raised by the community or government. In some instances, it may be determined that the impacts are unacceptable or the strategies to manage them too costly. In such circumstances, the development will not proceed.

Reclamation and remediation is a separate process, but is always considered as part of the evaluation process. Mining companies must address how they will reclaim the land when the mine closes and ensure that there are no ongoing social or environmental legacies. Financial surety is often a requirement to ensure funds remain for future environment works to remediate the mine site in the event that the company closes the mine prematurely.

4. Development stage: Mine construction

The development stage includes pre-construction and mine construction work and commences after all necessary permits and approvals have been obtained.

Pre-construction prepares the mine site for construction. Steps include removing old buildings, developing infrastructure and building camps for workers if there is insufficient availability of local accommodation. Mines can grow large enough to support towns, with schools, medical facilities and recreation areas.

Mine construction generally takes a few years, depending on the mine location, complexity and regulatory requirements. There are two main types of gold mines: open pit (surface) and underground. Many mines are a hybrid of both types.

Construction generally takes a few years depending on the nature and location of the mine and related infrastructure requirements. Significant environmental requirements apply at this stage in relation to the management of flora, fauna, topsoil and water quality, and the prevention of erosion. Miners must adhere to the rigorous environmental standards laid out in their permits during this phase of development.

5. Production phase

The mining process includes controlled blasting, hauling, crushing, leaching, processing and beneficiation for the recovery of gold. The process varies depending on the individual mine, however an indicative operational mining and processing circuit is as follows:

  1. Ore extracted from the mine, some of which has required blasting to loosen the rock prior to excavation by hydraulic diggers, is hauled by dump truck to the Run of Mine (ROM) pad to create feed for the crushing and grinding circuit.
  2. The ROM feed is crushed by primary crusher to a size of less than 300mm to enable transport by conveyor.
  3. The material is transported by conveyor to the coarse ore stockpile to provide feed to the secondary crushing circuit.
  4. Further size reduction of the primary crusher product in the secondary crushing circuit occurs using a range of different methods, including cone crushers, rod milling, mineral sizers and high-pressure grinding rolls or, more commonly, a semi-autogenous grinding (SAG) mill to reduce feed to a size for a ball mill (around 80% passing 1 to 4mm).
  5. Ball mills are tumbling grinding mills in which metallic balls are used as the grinding media. Most frequently the balls are made of cast or forged steel or cast iron. Ball mills are used to grind products finer than 80% passing 0.5mm. The water used in the grinding circuit will typically include water from dewatering the mine, from embankment underdrains and water that is decanted from the tailings storage facility. Excess water is pumped into a water treatment plant and treated to the required standards before discharge to the environment.
  6. Slurry of ground ore, water and a weak cyanide solution is fed into large steel leach tanks where gold and any silver present are dissolved into solution.
  7. Following this leaching process, the slurry passes through a series of absorption tanks containing carbon granules which absorb the gold and silver.
  8. The loaded carbon is fed into an elution or desorption column where the bullion is washed off. The barren carbon is recycled. The wash solution – pregnant electrolyte – is passed through electrowinning cells where gold and silver is won onto stainless steel cathodes. The remaining components of the slurry mainly finely ground rock are pumped as tailings to a tailings storage facility where solids settle and compact. Water is decanted off and the facility is carefully managed to prevent release of any materials. The processing circuit may include a cyanide destruction unit to neutralise any cyanide before material is released to the tailings storage facility (TSF) or, if the concentrations are low, may allow any residual cyanide to be denatured by exposure to sunlight within the confines of the TSF. This process is subject to stringent environmental regulation.
  9. The loaded cathodes are rinsed to yield a gold bearing sludge which is dried, mixed with fluxes and put into the furnace. After several hours the molten material is poured into a series of moulds producing bars of doré bullion. Some gold mines may instead produce a gold-rich copper concentrate which is pumped as slurry to a filtration plant where it is then de-watered and transported by rail to a port for export. These mines may also use a gravity circuit or other technology to recover a proportion of the gold which is smelted on site to produce gold doré.

6. Mine closure and rehabilitation

Once a gold reserve has been exhausted, the owner must rehabilitate the site. The intent of mine closure and rehabilitation is to return the land, as close as is reasonably possible, to its pre-disturbance condition suitable for use by traditional owners and as habitat for flora and fauna. Infrastructure not requested to remain in place by traditional owners or government will be removed for sale or disposal.

A comprehensive rehabilitation program has many clearly stated objectives which may include:

  • ensuring public health and safety
  • minimising environmental effects
  • removing waste and hazardous material
  • preserving water quality
  • stabilising land to protect against erosion
  • establishing new landforms and vegetation

To establish a safe and stable post-mining landform, overburden rock is placed either in engineered storage areas or used to backfill mine areas according to the approved reclamation plan.

After re-grading, stored topsoil is replaced and fertilisers and other soil additions are incorporated to create a suitable growing medium for seeds and tube stock. The topsoil also includes some original seed bank which continues to be viable. The re-sloped and topsoiled areas are direct seeded with local grass and shrub species to establish annual ground cover to prevent erosion and to provide a suitable environment for direct planting of tube stock of local tree species.

7. Monitoring and evaluation

To ensure that a safe and stable post-mining landscape has been created, the company monitors the extent of vegetation regrowth, including species diversity and abundance. This occurs over several years.

The company will also monitor the quantity and quality of runoff from the mined areas and ensure that measures are taken to prevent any adverse environmental impacts. Mines are deliberately constructed to drain internally to ensure no off-lease impact.

8. Lease relinquishment

This final phase of the mining operation involves the relevant State/Territory government undertaking an assessment of the extent to which the rehabilitation outcomes meet agreed mine rehabilitation criteria.

Once an agreed outcome has been reached, the mining company is permitted to relinquish tenure and liability for the site.

Any remaining financial surety held by the relevant government department is also returned at this time.