Grades can also be interpolated into an empty cell block model generated (or imported) earlier.
Block Model Cell Size
Model cell size should be selected depending on the following parameters:
Drilling density Variability of grade Smallest mining unit SMU Final model size
The cell size should be sized to be small enough to produce a grade map for grade distribution and big enough that it reflects available data.
When block model cell size is selected and the extent of model is calculated, an empty cell block model should be generated using Modelling| 3D Block | blank block model. The block sizes should be saved in a form and restrict to wireframe option should be used to save only the blocks in the mineralisation.
Flagging
If wireframe solids and DTMs are modelled, then they should be used to flag the block model. Basically, flagging should be carried out the same way as described in the section 6. The only difference would be if sub blocking is required.
When the block model is flagged for all possible domains / zones / ore bodies etc, all other cells (unflagged) should be deleted from the model to reduce the size of the file and number of records (File | Filter | Subset). That will also help to control the interpolation process.
The process should not take more than several hours.
Checklist
The model should not have too many cells. An average block model has several hundreds of thousands cells.
Generate sub blocks to represent volume more accurately.
Generate as many new fields for flags, as there are wireframe/string/outline types that are used for flagging. It is easy to combine them later, if necessary.
Notes:
How to decide the block size
The block size of 10 metres east, 20 metres north and 5 metres in rl is displayed at the centre of the search ellipse. This block size for iron estimation is appropriate given the sample spacing of 100 metres. The blocks must reflect grade distribution, showing a local map of grade occurrence, so the block cannot be too big as the grade change will not be shown and cannot be too small because the file will be unnecessarily large and the grade estimate will become less reliable.
The block size for an iron deposit will be bigger than the block size for more densely spaced shear hosted gold deposits or VMS hosted base metals because the samples are more closely spaced and the geology is far more variable.
Notes:
If the sample spacing for a gold deposit was 25m between sections, 10m between the drillholes along the section and 1m sample intervals then an appropriate block size would be 5m by 2m by 2.5m in rl.
Subcelling
Subcelling is the creation of smaller blocks on the edge of the wireframe when the parent cell is not fully inside the wireframe. The numbers entered into the sub block boxes are how much the parent cell is divided by to define the subcell in metres. If the parent cell is 10 metres in east and the sub blocks east is entered as 5 then the subcells will be 2 metres in the easterly direction. A sub block factor is different, the cells are not subblocked, rather a number between zero and one is defined for the percentage of the block inside the wireframe.
Notes:
Exercise 22.12 Creating a Blank Block Model
1. We know we are going to be asked for the extents of the Iron wireframe, so firstly open that in Vizex in Plan view. Jot down values to encompass the wireframe in Eastings and Northings. You should have jotted down something like: 19000mE-19900mE and 35000mN-36500mN.
2. Switch to Looking North view and note the RLs as well. You should have something like -650mRL-0mRL.
3. Select Modelling | 3D Block Estimate | Blank Block Model and fill in as below. The Output entries are all typed in as we are creating a new file.
4. Under Restrictions | Wireframes click on the More button. Fill in the details as shown below. Again, the Block Factor Field is going to be created and the Sub-block values represent the number of times you want the block sub-divided not the size to which you want it sub-divided.
5. Close the Restrict With Wireframes form.
Notes:
6. Click on the Block Definitions button. This is where we use the values that we jotted down earlier.
7. Enter the values but notice that you are asked for the Block Centre. To accommodate this and keep our blocks on round number co-ordinates, add half the relevant block size (Spacing) to each Origin Block Centre and subtract half the relevant block size from each End Block Centre.
8. Select Forms | Save As and save the form for later use as Iron OBM.
9. Close the Block Defintions form.
10. Select Forms | Save As and save the form as Iron OBM.
11. Click the Run button. This may take a couple of minutes and progress is shown in the bottom left of the screen.
12. Right-click on Iron OBM in the Output File box and firstly select Min/Max. You will notice various details including the creation of about 110,000 records and that the data does not start until 19025mE.
13. Right-click on Iron OBM a second time and select view. Notice that the BF field stores a value between 0 and 1 representing the number of virtual sub-blocks inside the wireframe as a fraction.
Notes:
Lesson Summary
This lesson has introduced the concepts of
Good Practice
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