When the Exploration Department is satisfied that a certain deposit has potential for further drilling and possible mining, they will hand over all the Drill data, Assays, Collar Surveys and any Deposit Models to the Resource Definition Department.
The aim of Resource Definition Department is as follows;
Close off the ore body in all directions
Sterilisation drilling of surrounding area for mine infrastructure (e.g. waste dumps and haul roads)
Infill drill the deposit to a confidence level upon which mining can commence (e.g. Measured)
Geotechnical drilling to determine pit slope stability, Pit Access Design and Dewatering
Grade Control Drilling
Grade Control is the final phase of drilling before mining commences. At this point you will be fine tuning the drill data set that you already have (to within 5m), defining the boundaries of the ore body and its Grade/Tonnage component.
Some mines have a separate team of drillers and Geologists drilling out the entire deposit in one hit, this is usually done in shallow deposits with minimal overburden.
The other form of Grade Control is the use of sampling the blast hole drilling chips, this is done in deeper pits with multiple phases of drilling as the deepens. The drilling is contained in a much smaller pattern and easier to manage data set for modelling.
Below is a Mindmap to help in the planning process for GC drilling
The Grade Control model comes in the form of a Block Model, with each block having a calculated Grade and Tonnage. The aim of the “Blocking Out” process is to group all the ore into pre-defined categories, for example:
Waste <0.2 Cu%
Low Grade 0.2 – 0.4 Cu%
Medium Grade 0.4 – 0.6 Cu%
High Grade >0.6 Cu%
The pre-defined categories of ore are based of Regression Modelling and Metallurgical test work maximising the metal recovery.
As the project starts producing product (metal) it’s imperative to continue the regression work as the category grades may shift as the ore coming out of the pit changes with depth.
All mines encourage Business Improvement Initiatives to maximise profit, you can either cut costs which can be detrimental to staff retention and project viability or increase Revenue. It has been proven that “Regression Work” on “Metal Recovery” will always be the biggest revenue generator with a zero cost to implement and lowest risk to project viability.
Stockpile Management
The daily truck sheets capture how many tonnes went to each stockpile, so the Geologist, Engineer or Metallurgical Accountant knows exactly how many tonnes are on each stockpile and at what grade.
The data is absolutely vital for the optimal running of the mine and processing plant. The majority of processing plants run on basic chemical reactions, so when a known grade/tonnes of ore enters the plant the chemist must add exactly the right quantity of extractive agent. Too much extractive agent is a waste of resources, whilst too little will send unprocessed ore (e.g. Gold) out to the tailings dam and lost forever.
Resource Modelling
The exploration models are generally in the low confidence JORC compliance class (eg Inferred/Indicated) and require much more work before mining can commence. The Resource Drilling is amalgamated with all the previous phases of drilling and re-modelled.
Due diligence is critical at this juncture, validating that all previous drill holes, collar surveys and assays are correct and removing any that are not before the modelling process begins.
The main aim is to produce a Resource Model with a high enough JORC confidence to start mining.
Grade Control Modelling
The drill data set for Grade Control Modelling has a much higher drill density allowing the modeller to accurately model the resource to a confidence level of Measured. The GC models are the final optimisation of the deposit before mining commences.
In conjunction with assay results you can apply rock types and structures to the model to get an even greater definition of the ore zone.
Mining Blocks
The grouped ore blocks are mapped digitally as “Strings” (Strings are Geo-spatial data points joined by lines) and set out by survey directly on top of the ore. The points are joined together with coloured flagging tape and white marker paint for good visualisation should the tape be destroyed by excavator tracks.
Each ore block is given a unique alpha/numeric code denoting it category (eg HG.67) for reconciliation purposes. The excavator operator is given a map and instructions on how to dig the ore blocks and where the different categories of ore should be stockpiled.
The truck drivers hauling the ore record the following data
Load & Drop times
Block code
Tonnage
Destination
The recorded data is captured either on paper or computer and finally stored in the mine data base for daily reporting purposes and reconciliation.
Reconciliation
All mining projects can look like they are running well at face value, especially when Expit tonnage targets are being exceeded and operating costs reduced. It’s not until the ore body starts to get reconciled from Pit to Sold Product can the true success of the mine be revealed.
Tonnage is a worthless calibration of success without the Grade component. It has always been an uphill battle with the mining fraternity explaining that high dig rates are ineffectual unless they are accompanied by the correct grade. The correct grade can be achieved by digging the ore blocks to perfection, removing waste and sending the material to the correct stockpile.
The reconciliation process is an ongoing event which will continually adjust the mining and processing variables, maximising the grade, tonnage and the bottom line.
