Variography in Micromine 2014 – Part 2: The Variogram Map Modelling Workflow

In Part 1 I showed you how to set up a variogram map. In this post I'll show you how to use and interpret it.

Background

Suppose you have an orebody striking roughly north-south and dipping around 30° to the east:




Intuitively you expect grades to be more continuous along strike than they are down-dip, and more continuous down-dip than they are across its thickness. The variogram map quantifies those directions.




Tic marks around the outside of the map provide a directional reference, dis­playing strike, dip or pitch values for the relevant map. Dip and pitch tics are only shown in the lower hemisphere, and these maps also include labels showing the left- and right-handed dip directions. At all times the status bar displays the lag distance and azimuth/plunge of the mouse pointer.

The workflow

The variogram map uses a three-stage workflow to identify the strike and dip of the plane containing the mineralisation, and then measure the pitch of the dominant axis in that plane (which hopefully defines the direction of greatest grade continui­ty).

You display each map (strike, dip and pitch) in turn, starting with the strike. (To help you visualise the workflow the maps are shown in their true 3D orientation in the following screenshots.They always appear flat in Micromine.)


The strike map with the measured strike direction shown by the magenta (purple) line

Like any variogram, low values indicate a stronger correlation and better continuity between sample pairs. Interpreting the map is simply a case of locating the zone of low values running through its centre, paying most attention to the region nearest the centre. (The hard part is configuring the map to show that zone in the first place.)

Once you’ve found the zone of low values, use the Select Direction button to measure its strike. Always use your knowledge of your project geology to ensure the direction is meaningful.

Click the Next button to proceed to the dip map:


The dip map, with the strike map shown in grey. The measured dip is shown by the magenta (purple) line

Micromine automatically aligns this map vertically at right-angles to your measured strike direction, a bit like drawing a cross section in Vizex (but without clipping).

Refine the processing options if neces­sary and then locate the zone of low values passing through its centre. As before, use Select Direction to measure its dip, ensuring it is geologically meaningful.

You have now defined the strike and dip of the plane containing the orebody, as represented by its grade continuity. The style of mineralisation will determine how closely this orientation should match the orebody as represented by its geology.

Click the Next button to proceed to the pitch map:


The pitch map with the dip map shown in grey. The measured pitch is shown by the magenta (purple) line

Micromine automatically aligns this map parallel to the strike and dip you measured in the two preceding maps. Because this plane is usually inclined the measurement you make here is pitch or rake, not plunge. However, the status bar always displays the azimuth and plunge of the mouse pointer.

As before, refine the processing options and then locate the zone of low values passing through its centre. Use Select Direction to measure its pitch, once again ensuring it is geologically meaningful.

The workflow is finished once you measure a direction in the pitch map, and the Next button changes icon to indicate that it is the last step.

The line you measured here represents the 3D direction of greatest continuity, known as the main axis or Axis 1. Micromine uses this information along with the strike and dip of the plane to calculate the orientation of Axis 2, which is positioned at right-angles to Axis 1 whilst staying within the orebody plane, and Axis 3, which is at right-angles to Axes 1 and 2.


The final axes, which are determined from the strike and dip of the plane and pitch of Axis 1. Axis 3 (blue) has been flipped to make it clearer. The pitch map (aligned to the strike and dip of the orebody plane) is shown in grey.

Lastly, use a variogram control file to pass this information to the variogram chart display or search ellipsoid. Simply click the Create Variogram Control File button and enter a filename. You can view the file contents once it has been created by re-clicking the button and then right-clicking the filename:



From here you pass the variogram control file to the chart semi-variograms (via Stats | Semi Variograms) for more detailed modelling. I'll cover that in Part 3.