Systematic ic Resource Estimation PracticeResource Estimation Practice
Our aims in resource estimation do not end at "getting the best estimate" from a statistical or numerical point of view, important though that aim is.
It is critical that we also formalise the estimation process, allocate responsibilities clearly, document the estimation adequately and take the geology into proper account.
Formalise Estimation Procedures Formalise Estimation Procedures
Making an estimate is a process, not an event. Estimation is a dynamic series of steps and we may wish to repeat certain steps, or incorporate new observations. To make the process of "revisiting" our estimates easier, formalisation is required. A good resource estimate needs to be thoroughly documented so that it can be repeated. Of course, a well-documented procedure also facilitates efficient resource audit and easier transitions if project staff change.
Kriging Kriging
Kriging is mechanically much like IDW. What differs is the way that the weights are derived. The kriging weights are not arbitrary:
they are based on data correlations.
Documentation Documentation
Quite apart from the professionalism of documenting the job, there are other good practical and legal reasons for competent documentation.
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The resource evaluation procedure starts with data collection, geological interpretation and validation. From this early stage, the procedure used should be documented and formalised:
Step Test/Control
Step 1 Collect samples Check representivity...
Step 2 Sample preparation Quality Control
Step 3 Assay Quality Control
Step 4 Hole survey Quality Control
etc... For major resource delineation programs, standards, control procedures etc. should be implemented and documented from the earliest stages of the project. A review of quality control in geochemical sampling programs is given by Thompson (1984). The best starting point for assessing sampling practices are Dominique François-
Bongarçon’s papers.
Formalisation and documentation should be implemented for each step of the data acquisition and resource estimation process.
Allocate Responsibilities Allocate Responsibilities
At each step, responsibilities need to be clear:who sites holes?who monitors assay quality control? etc.
Estimation begins with data collection, data validation and critical examination of the geological model. If there are important mining constraints to be accounted for early in this process, then a mining engineer must be ‘brought on board’ early on. Experience shows that team approaches are generally superior to lone efforts. Several people, in particular if their specialisations are different, tend to create a constructively critical environment that results in more objective decision making. Document Decision Making Steps Carefully
Document Decision Making Steps Carefully
Estimation of mineral resources involves many decision making steps. A geologist or mining engineer revisiting the estimate should be easily able to answer such questions as:
Which holes were used in the estimate? If some holes were not used, why?
What are the justifications of the key features in the geological
interpretation? Why was this particular model used? Are there rational alternatives?
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If populations were split (e.g. oxide versus primary or ‘northern end’ versus
‘southern end’), why? If different lithologies were combined in the estimate, why?
If different sampling techniques (e.g. RC and DDH) were treated in the
same way, how is this justified? If not, how were the differences quantified and accounted for?
If repeat assays were available were these used, and if so, how?
If grades were cut, how and why?
If a particular estimation methodology was selected, why?, etc.
For each deposit the specifics will be different, but the general scheme will still apply.
Geological Models Geological Models
Note thatall geological interpretations constitute models, whether this is explicit or not. The type of model used in estimation will be most reliant, in general, on the larger scale features that impact upon the spatial distribution of mineralisation. Genetic geological models and exploration models will often be a superset of the model required for resource estimation.
The critical features of the geological model used for resource estimation generally relate to geometry: stratigraphic contacts, folding, location of faults and discontinuities, identification of vein orientations etc. Knowledge of a genetic link between assayed elements (Au and As for example, or Pb and Ag) may be a useful part of the model.
We will see later that one of the most important (if notthe most important) decisions made in the estimation process is that of ‘stationarity’. We will rigorously define stationarity in subsequent chapters, but for the moment we can summarise the concept by: ‘stationarity decisions involve sub-setting (or re-grouping) data such that the resultant data sets may be legitimately employed for statistical analysis and estimation’. The geological model is a primary input to stationarity decision- making, and may be also influencedby that decision-making.
The practical consequence is that we must build geological models for the purposes of grade estimation by paying attention not only to the geology, but also to the grade distribution.
Update and Revise Estimation Models Update and Revise Estimation Models
As more sampling or geological information becomes available it is often necessary to update and revise our estimation. At these times, it may be possible to improve the estimation algorithm or the way in which we incorporate geological features into our model. The potential financial advantages (i.e. increased profits) that may result from optimal estimation can be significant. Staying with an estimation procedure that is not performing well, or simply applying "mine-call" factors as a
Geology Geology
Lack of good geological modelling is a detriment to any estimate. However, good geology is not enough, we must also estimate sensibly within geological
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band-aid solution will probably cost the mine, i.e. result in lost profits. During the life of a mine several estimation procedures are often used with the aim of constant improvement.
‘Black Box’ Approaches ‘Black Box’ Approaches
The whole point of formalisation, objectification and revisiting is to avoid so-called "black box" approaches. A ‘black box’ approach is one in which the assays are shoved into a computerised (or for that matter, manual) effectively fully automatic resource estimation procedure that no-one involved really understands. Classic answers to the question “why are you estimating this (or that) way?” from people using ‘black box’ approaches include:
This is the way it was done for the feasibility study and we're ‘locked in’. We have a policy from head office that this procedure be followed on all
deposits.
This is the only technique our software is set up to do.
The ore reserves system was set up by (insert name of long-departed
geologist/engineer) and we are unsure of how to go about changing anything.
If assumptions are challenged and decisions understood all the way through the process, a "black box" approach is not possible.