June 2016

Modern mining geology

  • By Steve Rose FAusIMM(CP) and Gerry Fahey FAusIMM, CSA Global Pty Ltd, Australia

An overview of the modern mine geologists’ role, and a discussion of professional best practice and pitfalls to avoid

Although mining has occurred for thousands of years, geology as a profession did not have wide recognition until the 1780s. Up until the 1920s geology tended to be an academic pursuit. The identification and discovery of mineral deposits was by professionals primarily trained as engineers or surveyors (Park, 1906).

As well as the need to find and mine ores more efficiently, the cause of mining geology was considerably advanced by litigation. In the USA, the ‘Right of Apex’ mineral laws meant that pegging the outcrop of a vein gave the leaseholder the rights to mine the down-dip extents of the vein. Geologists were needed to verify if veins found underground were the same as the vein exposed in outcrop or ‘apex’. Details of these exploits are described by Sales (1964).

In 1948, HE McKinstry wrote his influential textbook Mining Geology. The book details all of the tasks of the staff geologist. By 1948, most mining and exploration projects were carried out under geological guidance (McKinstry, 1948).

Ranta (2008) defines that the ultimate role of the mine geologist is to ‘keep the mine in ore and to see that geological factors are fully considered throughout the life of the mine’. The value of the persistent and consistent application of mining geology methods is documented in Campbell (1990).

McKinstry sets out the tasks for mine geologists as:

  • drilling information – keep mapping up to date, plot up current assays
  • daily conversation with the mine foreman
  • recommending development
  • input to stope design
  • ore estimation
  • mineralogical aids in ore treatment
  • publication of scientific findings.

Computers were first introduced at major mines in the late 1960s and early 1970s. The early applications were typified by high throughput, repetitive tasks, and were usually related to facets of accounting (Hughes, 1974). By the end of the 1980s the use of computers, in particular microcomputers, had become widespread in many mines (Scott and Visnovsky, 1983). This encouraged the development of commercial software that could handle the tasks of assay database, plotting assays and maps, grade estimation and mine planning.

The task list of McKinstry remains the basis of the mine geologist’s routine, but advances in computer methods, assaying and surveying productivity have tended to skew the attention that the geologist can give to the basic tasks. Below is an outline of a modern mining geology system, with some thoughts regarding best practice and common pitfalls to avoid.

A mining geology system

A modern mining geology system should have the components shown in Figure 1. The reason for describing this as a ‘mine geology system’ is that all components need to work together. It should be well understood and also communicated to other departments within a mining operation. The results are reduced if parts of the system are missing or carried out to a different standard. In this paper, the details of the components have purposefully been kept generic.

graph44
Figure 1. A generic mining geology system.

Sampling

The sampling method needs to allow for the collection of representative samples within the constraints of the mining schedule. Generally, the decision to develop a mine is based on resource/reserve estimates utilising a set of samples (‘wide spaced sampling’). This sampling is generally not suitable for short-term mining decisions. The wide spaced sampling is therefore infilled (‘close spaced sampling’) prior to mining. The specifics of what sort of sampling and the spacing will be a function of the mining method, the commodity, the mineralisation style, cost and the risk of making incorrect decisions.

Any contractors or third parties used to carry out sampling (eg drillers, assay laboratories) should be treated as partners, so that they share the same goals of consistent, reproducible samples.

Pitfalls

  • The sample spacing is not optimised for the orebody. For example, during a case study review at an open pit mine the authors recommended increasing the spacing of reverse circulation (RC) grade control drilling by 20 per cent. The mine was using 1 m sampling on a 10 x 15 m pattern. The authors carried out a review and recommended a change to using 12.5 x 15 m on a staggered grid orthogonal to the main orebody strike. In addition, it was recommended that samples be taken as 3 m composites. These recommendations were based on:
    • an understanding of the dimensions and geometry of the orebody
    • an understanding of the minimum mining width and bench
    • using the methods discussed in Coombes (2008) and by reviewing semi-variograms created from the
      sample data.
  • Often too much effort is put into check sampling at the expense of predictive sampling. For example, at an open pit mine the mine geology team were spending nearly an hour each day collecting grab samples from the stockpiles on the run-of-mine (ROM) pad. Grab sampling from large stockpiles is difficult as it is nearly impossible to meet any of the conditions recommended by sampling theory (Holmes, 2008). At this particular operation the results of the sampling broadly agreed with expectations, so in fact no decisions were being made based on stockpile sampling. The authors recommended that routine stockpile sampling be discontinued as it was not adding any value and had an assaying expense. There were further indirect costs through taking up a significant amount of geology team time and adding further work to a stretched assay laboratory.
  • Not develop and test heterogeneity of geological units.
  • Not calculating the fundamental errors for each sampling protocol.

Sampling and assaying is generally a major expense for the mining geology department. It is important that mine geologists are competent to plan and implement the sampling program.

Mapping and logging

Samples need to be placed into their geological and spatial context, hence the reason for having a mapping and logging system that supports consistent and regular geological data collection, storage, and interpretation. The geological information needs to be published in formats that are useful to other users (eg geotechnical engineers, mining engineers).

Pitfalls

  • The authors’ experience is that mapping and logging is typically the first task to be dropped when mining geology teams become too busy, particularly in open pit mines. Haren and Williams (2000) set out the cost benefits of mapping at the Sunrise Dam gold mine. A cost of US$300 000 gives benefits of US$5 250 000.
  • Mapping is often done on an ad hoc basis and not collated onto base maps.
  • Logging rather than mapping. The benefits of mapping are that it gives structural information, and the true relationship of lithological boundaries.
  • Photographs can provide a good visual record of faces and walls, but there still needs to be a method which allows interpretation by the geologist to reduce the image to lines and orientation readings.
  • Collecting too much data. An understanding of the controls on mineralisation will determine what features the geologist should concentrate on.
  • Mixing geotechnical and geological mapping. Generally, it is better to carry out geological mapping which can then allow the definition of geological domains. Then targeted geotechnical mapping can be carried out within those domains.
  • Carrying out mapping on a campaign basis. The authors have seen occasions where external geologists were brought in to rapidly map an open pit without involving any of the mine geologists working there. While the goal of producing a map was achieved, there was no system to allow for continuingthe mapping once the campaign was completed.
  • The mapping is not collated into a format that is convenient to use forassay interpretation.

Mapping and logging has probably one of the best cost-benefit ratios of the activities that can be carried out by a mining geology team.

Quality assurance/quality control system

A simple quality assurance/quality control (QA/QC) system for samples and assays is needed to monitor sample quality, with a defined response for out of specification results.

Pitfalls

  • Keep the QA/QC system simple – itdoes not need to be run to the same level of detail as that used during the resource phase.
  • Not checking QA/QC results as each batch of results is returned. Assays should only be loaded into the main database once the checks have been reviewed.
  • Using standards that are not related to the cut-off grades being used to define ore and waste.
  • Using internal standards that are not properly prepared and certified.
  • Failing to collate results into a monthly report. This can be distributed and discussed with the assay laboratory.

Assay database

Use a single master database to store clean sampling, assaying and geology information. The better solutions are digital relational or flat file databases with limits on the data that can be entered. There should be tools to allow the direct importation of collar, survey, geology, sample and assay data without requiring that data to be manipulated.

Pitfalls

  • The biggest pitfall that the authors have come across at mine sites is where insufficiently trained geologists are administering databases on an ad hoc basis. On one open pit mine the geologists were entering the entire collar, survey and sampling data into separate tabs on an Excel spreadsheet. When the assays were returned in csv format, they would open the file in Excel, and then cut and paste the assays to match with the sample details. The data was then cut and pasted into relevant tables in an Access database. This process was extremely time consuming and prone to errors.
  • Many companies have well thought out and executed drillhole databases for their exploration data. Cut down versions of these form an excellent basis for mine geology data.
  • No library or lookup tables to enable deciphering of abbreviations or codes.
  • Developing complex in-house databases (eg based on Access or SQL) which do not have adequate documentation or support if the in-house expert leaves the company.

Because of the potential for errors and inefficient use of time, it is worth nominating a specialist database manager.

Assay interpretation

This component of the system is where assays, geological and other relevant data are processed and interpreted. This will generally be carried out using one of the general mine planning (GMP) packages. There is a need for a consistent method of exporting, visualising, and processing assay data to create mining blocks. The input data for interpretation will include sample and geological information from the assay database and mapping. Mineralisation domains can then be interpreted using all of the relevant geological information in addition to assay data. Domains can be created using strings, planes or wireframes.

Pitfalls

  • Ignoring the exploration data in interpretations. The authors have seen mine sites where the mine geology assay database does not include the exploration or resource definition drilling. This makes it difficult to compare the close spaced sampling with the wide spaced sampling.
  • Concentrating on sophisticated geostatistical methods and ignoring the interpretation of geological domains. Geology domains will always need to be used to assist interpretation.
  • Not understanding the parameters being used for grade estimation. The geologist is then not able to validate
    the results.
  • Failing to document estimation parameters, or documenting the method. This can result in different results depending on who on site carries out
    the estimate.
  • Using an ad hoc file management system. A good solution is to ensure the current version of the file always has the same name. When it is updated, a copy is made of the old version with a date tag in the file name.
  • Failing to target the reserve head grade. The authors have seen several mines recently where the mine geologists were diligently creating ore blocks to ensure recovery of all mineralisation above the marginal cut-off grade, but in doing so were not achieving the budgeted or reserve head grade.

Assay and geological interpretation can take up considerable amounts of mine geology time. Routine and repetitive parts of the task should be automated as much as possible.

Development planning and sign-off

There are daily, weekly and monthly mining plans that will need input from mine geologists. This will include things such as development, stoping, sampling activities, the location of ore, and its tonnes and grades.

Once plans are prepared, they should be signed off before being issued. Other regular plans that will need mine geologist input will include:

  • crusher feed blend
  • ore destinations (these can be separated by grade, physical characteristics or deleterious minerals)
  • waste destinations (eg when the waste has deleterious components).
  • A pitfall is not to have documentation for these decisions.

Production tracking and implementation

This component deals with implementation of the mining plan. This covers how mining shapes are laid out and mined. Mine geologists will need to take a proactive role in ensuring ore is mined as planned. This can be as simple as regular visual checks of the mining face during the shift and visiting stockpiles.

A production tracking system records what ore and waste volumes have been moved from the mining face to any intermediate stockpiles, the ROM pad and into the crusher. This system is shared with the mining production team.

Pitfalls

  • failing to reliably record production tallies
  • mine geology running their own set of production values separate from the production engineer
  • failing to take due care during mining in order to minimise dilution (waste taken with the ore).

Production reporting

Production reporting is a system for daily, weekly and monthly reporting of tonnes and grades and relevant production activities. It is important that all departments are using the same set of figures. This can be easier if reports are generated from a single mining production database.

Material movements should cover all movements, not just from the mine, but also from the stockpile into the processing plant.

There should be a monthly report that collates all the weekly information and includes a reconciliation of what was mined compared to the results from the concentrator.

Pitfalls

  • geology report different figures compared to engineering (poor communication)
  • stockpiles are not monitored and reset to zero when they are depleted.

Reconciliation

Reconciliation compares resource, reserve, grade control, mined and milled. One of the most important measures of how a mine is performing is how it reconciles each month. Reconciliation should be carried out at the end of each month. The relationship between the mine reserve and grade control models is critical to assessing the efficiency of the reserve (long-term model) in estimating future production scenarios. The mine to mill comparison must take account of all material and stockpile movements, and provide an assessment of the efficiency of the grade control model and mining implementation in predicting feed to the process plant.

Pitfalls

  • producing a reconciliation report, but failing to use the findings to improve the mining process
  • failing to include all material movements
  • not understanding the differences between bank cubic metres (BCM) and loose cubic metres (LCM), and their respective densities
  • not taking account of moisture
  • producing mine call factors (MCF) should be avoided.

Documentation

The mine geology team should have a clear statement of purpose.

There should be simple but effective descriptions of how the grade control system works, who has responsibilities and authorities. It is important the documents are easy to maintain, and are kept up to date.

Related to documentation is publication. As scientists, mine geologists have a responsibility to record observations about the geology or other facets of the mine, and publish them in scientific journals.

The authors rarely find satisfactory documentation on the sites that they have visited.

Training

To support the mine geology system there needs to be training in order that the geology team can become competent; this should include an understanding of operating costs and risk decisions. Mine geologists will need to ensure that they undertake activities as part of their obligations to continuing professional development. It would appear that in-house or external training has taken a back seat within mining companies over the past decade; this has to change if we are to improve our standards. 

Conclusions

It can be seen that the tasks needed by the modern mine geologist are broadly similar to those listed by McKinstry in 1948. The major changes are to do with technology, specifically:

  • digital databases
  • use of GMP software to process and interpret assay data
  • higher volume and cheaper assay determinations
  • higher productivity surveying.

On the face of it, these advances should speed up the mine geology process. However, unless these processes are well configured and the mine geologist is competent, it can lead to the tasks of assay database and assay interpretation taking up a substantial amount of time.

Mapping and logging, QA/QC, the geology part of assay interpretation, reconciliation, documentation and training tend to be the tasks that are neglected when time is short. Balancing the work so effort can be directed to these displaced tasks will ensure integrity of the mine geology process, and will undoubtedly improve the operation of the mine.

Acknowledgements

The authors wish to acknowledge the support of CSA Global Pty Ltd in preparing this paper.  

References

Campbell, JD (1990) Hidden Gold: The Central Norseman Story Volumes 1 and 2, AusIMM, Melbourne.

Coombes, J (2008) ‘The Art and Science of Resource Estimation’, Coombes Capability, Perth, WA. Pp 182

Haren, E and Williams, P (2000) ‘Mine Geology Practices at the Sunrise Open Pit’, Fourth International Mining Geology Conference, Coolum, Qld 2000

Holmes, RJ (2008) ‘The Importance of Sampling in Grade Control’, Fourth International Mining Geology Conference, Coolum, Qld, 2000

Hughes, G (1974) ‘Computers – A Management Curse or an Industry Hope’, The AusIMM Conference, Southern and Central Queensland.

McKinstry, HE (1948) Mining Geology, Prentice-Hall, New York.

Park, J (1906) A Textbook of Mining Geology for the use of Mining Students and Miners, Charles Griffin and Company Limited.

Ranta, DA (2008) ‘Project and Mining Geology’, Mining Engineering Handbook, Second Edition, Section 5. SME

Sales, RH (1964) Underground Warfare at Butte, World Museum of Mining. Caxton Printers, Caldwell, Idaho

Scott, A and Visnovsky, CS (1983) ‘The Application of Desktop Computers to Open Pit Mine Planning’, Computers in Mining Symposium, AusIMM South Queensland Branch.


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