June 2017

Resource characterisation of ore, waste and rehabilitation materials: tools for geologists

  • By Dr G A Maddocks MAusIMM, Principal Geochemical Engineer and A Robertson MAusIMM, Director and Geochemist, RGS Environmental Pty Ltd

Total deposit knowledge can help mining companies to make informed and reliable decisions that will lead to significantly better environmental and social outcomes

Mining is an international business and vital contributor to national and global economies that depends on the trust and confidence of investors and stakeholders for its financial and operational well-being (CRISCO, 2016).

Social licence to operate (SLTO) is a multi-faceted and multi-stakeholder risk that can impact investment decisions (EY, 2016; Deloitte, 2017 and KPMG, 2017). Stakeholder concerns, while multi-faceted, are often unified around perceived or real risks to land and water resources (EY, 2016; Deloitte, 2017 and KPMG, 2017). Community challenges to broader political and economic decisions have given rise to protest, delaying or stopping projects or closing mines (EY, 2016).

With billions of dollars in project investment at stake, ongoing engagement, collaboration and effective communication with stakeholders is crucial and mutually-beneficial solutions are increasingly expected (Ernst and Young, 2016, Deloitte, 2017 and KPMG, 2017). It is therefore essential that the industry communicates the risks (and opportunities) associated with investment effectively and transparently in order to earn the level of trust necessary with investors and stakeholders to underpin its activities (CRISCO, 2016).

Some governments are now giving greater powers to stakeholders to make the final decision on approving mining and metals activities (EY, 2016). In Australia in the last 12 months, stakeholder concerns have translated into an ongoing Australian Senate inquiry into the rehabilitation of mining and resource projects and the enabling of the Environmental Protection (Chain of Responsibility) Amendment Act in Queensland.

To resolve poor environmental outcomes that lead to local community – or broader social – angst, there is a need to produce better technical outcomes that include improving the way mines are designed, operated, decommissioned and rehabilitated (Robertson et al, 2015). In order to do this, the full range of issues faced over the full life of the asset from exploration through to closure, decommissioning and relinquishment need to be understood.

Total deposit knowledge

In geological circles, there is increasing discussion about ‘total deposit knowledge’ that can be considered in terms of geo-environmental models developed by the USGS and others (USGS, 2016). Total deposit knowledge is the characterisation of the ore deposit and its surrounds at all scales and should include: comprehensive geological assessment (eg lithology, oxidation, alteration and weathering, the spatial orientation of a range of geological features or discontinuities and rock mass fabric knowledge); metal grade and distribution data supporting geo-metallurgy; hydrogeology inclusive of permeability and porosity; geophysics; geomechanics; geochemistry; and geostatistics to understand the variability of the ‘geo-data’ to inform decision making processes (Chitombo, 2011; Scott, 2012).

In regard to material characterisation, the mining sector continues to make advances in developing mandatory mineral reporting codes. However, the characterisation classification and reporting of ‘other mined materials’ inclusive of soil, construction material, rehabilitation material, waste rock and tailings is managed by government or industry guidelines, eg the Australian Commonwealth AMD Guideline and the International Network for Acid Prevention (INAP) Global Acid Rock Drainage (GARD) Guide respectively. The shortcoming of guidelines is that there is no requirement to implement programs that meet the intent of the guidelines: eg that at pre-feasibility stage there will be ‘sufficient data points to develop a preliminary block model with a reliable distribution of static geochemical data for ore, waste and wall rock’ (DITR, 2016).

The business case for innovation and obtaining deposit knowledge

  • Without clear, concise, definitive and robust technical plans to develop and operate a mine, earning and maintaining the SLTO will become an increasingly difficult obstacle for the sector to overcome.
  • The SLTO is becoming the third leg on the tripod to obtain project approval and can transcend and influence regulatory (eg environmental impact statement) and financial sector (eg ASX) processes.
  • In the absence of successful rehabilitation of tailings storage facilities (TSFs), waste rock storage facilities (WRSFs) and open pits, where mining legacies remain, mining companies will see increasing levels of financial liability, financial assurance or bonds, difficulty in on-selling assets, and may face perpetual management of their site(s).
  • Reliable life-of-asset financial assessments with accurate ‘modifying factors’ for environmental and social aspects are likely (almost certain) to be underestimating the actual cost of rehabilitation and the potential for perpetual management. This will become increasingly relevant and a mining company’s reputation and financial status may be adversely affected as the number of assets they hold get closer to closure.

A good news story for the mining sector and our stakeholders – new approaches for exploration geologists

In the past 12 months, RGS Environmental has seen three mining companies develop and implement an operational procedure for exploration and resource geologists to identify, characterise, classify and quantify volumes of mine waste and each major type of rehabilitation material within unmined ground ahead of mining. This can then be scheduled for containment or use by short-, medium- and long-term planners to improve environmental outcomes. The same approach is also being developed by two other companies. Geologists are taking the lead on this, and with a little thought and planning, it doesn’t cost much to get the job done.

The exploration program, the rehabilitation toolbox and the planning process

The toolbox and planning process for geologists and mine planners working on their resource projects, which is directly transferable to rehabilitation planning, fits into five general phases of work:

  • drilling, sampling and analysis by geologists
  • geological modelling and production of material balances
  • mine planning to develop and test planning options
  • environmental evaluation to determine the risks and opportunities of each option
  • financial evaluation to work out the most effective (and best overall) option, not the just the cheapest option of mining and placing the waste.

Drilling, sampling and analysis

Considerations for the geologist can include:

  • At greenfield sites the geologist could use geo-environmental models in the exploration planning stage to gain an understanding of the environmental issues associated with the deposit (USGS, 2016). The potential list of elements or parameters that could be of environmental concern can be shortlisted and added to the analytical suite, eg sulfur will be a given at any coal mine and is usually assayed on every resource sample.
  • At operating or brownfield sites the potential elements of environmental concern should already be known. While there is a somewhat simplistic realisation that there is waste within the deposit and that non acid forming (NAF) and potentially acid forming (PAF) waste needs to be identified so that it can be managed, what is still lacking in the sector is the realisation that there are potential chemical issues that may occur (eg saline, sodic and metalliferous material) and be just as significant for rehabilitation.
  • The exploration program shouldn’t just be about the ore/coal; there will be mine waste requiring management and good material that can be used for construction or rehabilitation.
  • Implement the standard drilling program to define the resource. Log the drill holes using standard practices, but pay some additional attention to sulfur minerals beyond the ore and the presence or absence of acid consuming minerals, some of which may not necessarily be identified in standard geochemical test programs.
  • Consider new techniques to characterise the mineralogy (eg HyloggerTM analysis) within the ore, waste and rehabilitation materials that may be useful on entire drill holes and add as little as $5 per linear metre to the drilling program; outputs include high resolution photographs of the core and full range of spectral data (Figure 1). The data from this type of analysis is useful to the geologist, geochemist, engineer, hydrogeologist and metallurgist.
  • Sample and package up core or chips from the holes. If the core or chips are being put into chip trays, collect duplicate or triplicate trays. This will provide sufficient sample distribution (typically 1 m intervals) and sample mass for immediate or future geochemical analysis to define the waste and rehabilitation material and saves the geologists time and money redrilling or resampling later.
  • Chip samples can also be used for basic (but nevertheless very useful) physical analyses, such as Atterberg Limits and Emerson Aggregate testing and even specialist testing for soil cover modelling, such as soil water characteristic curve measurements.
Click for larger image.

Geological modelling

As the exploration program progresses, the geologists will develop and refine a geological model of the orebody that is likely to be required to meet some level of prescribed quality (eg JORC requirements). The geologist will not have used the geological logs to produce a model of the major waste and rehabilitation units in the deposit, but this can be done with very little extra effort with some suitable guidance on what units or strata should be grouped together and where boundaries may need to be established.

Throughout the exploration drilling program, the chip or core samples put aside during drilling can be retrieved from the core farm and analysed for chemical and/or physical properties. The sampling and analytical program developed at this stage will most likely adopt a staged analytical program, whereby the waste and potential rehabilitation material is screened initially for acid base accounting or net acid generation testing to classify the net acid producing potential.

The output of the screening will identify a range of waste classes in the broad waste and rehabilitation material zones. Supplementary analyses may include sulfide/sulfur analysis to quantify the reactive sulfide content, and total elemental analysis using acid digests or handheld XRF analysis using pulped pelletised samples in the waste. Within the potential rehabilitation materials, supplementary analyses may include cation exchange capacity to define the potential for salinity and sodicity, and physical analyses such as Atterberg Limits and Emerson Aggregate tests. The chemical and physical data analysis may identify that some of the clay units (for example) are sodic and may not be suitable for use in a soil cover, and despite being NAF should be classified as waste because of the physical properties of the material.

In the absence of having these samples on hand to provide some of the deposit knowledge required today, there will almost certainly be a need to re-sample additional drill core/chip materials at great expense, particularly if the project is in a remote area. It is the expense of doing things retrospectively that puts constraints on the number of drill holes and samples that can be obtained and used to characterise the mine waste and rehabilitation materials.

As early as the concept study stage, the outcome of the work program outlined above could be that the geologist could provide a geological model of the waste and rehabilitation materials with reasonable material balances for the major units such as the volume of:

  • soil
  • extremely or heavily weathered (non-mineralised) overburden
  • extremely or heavily weathered (mineralised) overburden
  • weathered or transitional overburden
  • fresh rock.

Including chemical and physical data will supplement the geological model. The value in this type of information cannot be underestimated. It does not take much thought for a TSF design team to realise that a large volume of soil, clay and rock will be required for the rehabilitation of a TSF and that unless these materials are identified and stockpiled throughout the life-of-mine it will be an expensive process to source the materials at the point of closure.

Depending on the type of deposit that is being evaluated (and the amount of data), it may be possible to start developing the following maps or models.

Stratigraphic models

Coal deposits assay total sulfur on all coal samples and some sites analyse the roof and floor separately as discrete units. There are typically thousands of sulfur assay points within the coal, and these can be plotted using isopach maps to show the distribution of sulfur in the coals seams, plies and roof and floor units. This enables site to:

  • infer the potential risk associated with acid drainage (supplementary analyses such as sulfur speciation using the chromium reducible sulfur method can be used to quantify the sulfide content in the coal)
  • provide a very clear picture of whether particular seams have a higher sulfur content and potentially higher risk
  • track changes in sulfur content throughout the deposit over time.

Coal deposits are somewhat contiguous in nature and the stratigraphic geological models developed in the overburden and interburden can be coded with general physical properties (eg degree of weathering, hardness, durability, etc).

Block models

Metalliferous deposits utilise block models to define the location of ore, low-grade ore and waste. The block models can be extended into the waste zones to provide qualitative models of materials based on logged oxidation, weathering, alteration, clay content, etc. As chemical and physical analyses are added to the model it will become quantitative and increasingly reliable.

At this point the geologists pass on their combined deposit knowledge to the mine planning and engineering teams. There is likely to be new content here (and changes in thought process) for the long-medium- and short-term planners, but this is how the geological data should be utilised at pre-feasibility stage and beyond.

Conclusion

Environmental outcomes must be improved to offset community and social opposition to operating mines or new mining projects. To do this, better technical solutions must be identified and implemented so that adverse impacts to land water resources are reduced. Better technical solutions require total deposit knowledge.

Mineral deposits contain ore and deleterious mine waste and beneficial material that may be suitable for construction and rehabilitation. Material that is suitable for construction and rehabilitation (this could include tailings) therefore has an intrinsic financial value to the project, and we must pay more attention to quantifying this resource and utilising it to maximum benefit. Early exploration drilling usually includes drill holes at the extent of the deposit to define its margins. These drill holes can often be in the zones where a lot of the potential construction material and material for rehabilitation is likely to be found; hence they are a valuable addition to rehabilitation resource knowledge in the deposit. Collecting additional sample mass during exploration or resource drilling can allow subsequent geochemical and physical analysis of the materials and prevents the need to re-drill and sample at a later date, saving money and time.

The level of engineering, science and planning applied to the development of a TSF, WRSF or open pit exceeds the amount of work applied at the back end of an operation. To date, the value of the work that could be done at very little expense throughout exploration and the resource definition phases, and the connection with the closure decommissioning and rehabilitation planning process, has generally not been made. There is a need to break down internal silos and make use of existing data we or our colleagues may have, obtain more data earlier and more data as the project develops, and implement efficient data management to improve the way we design, operate, decommission and rehabilitate mines.

Geologists do a great job of defining the geology of the deposit and the orebody. Geologists have the qualitative data on hand within the geological database to develop geological models of the waste and rehabilitation materials. The inclusion and visual presentation of data is becoming increasingly relevant and much easier to do, and allows project teams to have more time to analyse data and focus on options, optimisation and innovation (Snodgrass and Cockerill, 2017). The visual presentation of data is also a valuable tool to support community engagement and participation. The use of the current generation of software packages provide a visual platform to demonstrate to our stakeholders that we understand the deposit and that we have the knowledge to make informed and reliable decisions that will lead to significantly better environmental and social outcomes.

References

Chitombo G, 2011. Total deposit knowledge: geology, underground mass mining and the future. A mining engineer’s perspective. In Proceedings Eighth International Mining Geology Conference, pp 11-14 (The Australasian Institute of Mining and Metallurgy: Melbourne).

CRIRSCO, 2016. Committee for Mineral Reserves International Reporting Standards [online]. Available from: http://www.crirsco.com/welcome.asp.

DITR, 2016. Preventing Acid and Metalliferous Drainage. Leading Practice Sustainable Development Program for the Mining Industry. Commonwealth of Australia, May.

Deloitte, 2017. Tracking the trends 2017: The top 10 trends the mining companies will face in the coming year [online]. Available from: www2.deloitte.com/au/en/pages/energy-and-resources/articles/tracking-the-trends-2017.html.

EY, 2016. Business Risks Facing Mining and Metals 2015-2016 [online]. Available from: www.ey.com/GL/en/Industries/Mining—Metals/Business-risks-in-mining-and-metals.

KPMG, 2017. Australian Mining Risk Forecast 2017 [online]. Available from: https://home.kpmg.com/au/en/home/insights/2017/01/australian-mining-risk-forecast-2017.html.

Robertson A M, Maddocks G A and Swane I, 2015. Understanding Mine Waste Geochemistry in the Bowen Basin: From Exploration to Mine Closure. In Proceedings of the Bowen Basin Symposium 2015: Bowen Basin and Beyond (ed: JW Beeston), pp473-480. Brisbane, Queensland, 7-9 October.

Scott M, 2012. Developing total deposit knowledge – New approaches to manage uncertainty and mining risk in complicated operational environments. Mining 2012, 5th International Conference on Innovation on Mine Operations.

Snodgrass C and Cockerill T, 2017. Improving productivity by integrating software developers into resource engineering, AusIMM Bulletin, February.

USGS, 2016. Geoenvironmental model refinement and advancement [online]. Available from: http://minerals.usgs.gov/science/geoenvironmodel/index.html#overview.

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