The dilemma of geological control for underground ore development

by Steve Rose, Principal Mining Geologist at Rose Mining Geology Consultants

From the beginning of underground mining, it has been the practice to develop ore drives along the mineralized structure with decisions on the direction of that development made by the miner (pre-19th Century; see Agricola, 1556), the engineer or the surveyor (19th Century). Once mining geology became a profession (Mckinstry, 1948), this decision has become the responsibility of the geologist.

My first job as an underground mine geologist (and it’s usually the domain of the junior geos on the team) was to go underground each day to map the new headings and provide a call on which direction to drill the next cut. There was a certain thrill in being put on the spot each day and being the only one on the team with the intimate knowledge of the orebody and that ‘special’ insight to give the development crew the instruction. Mostly, of course, the development crew already knew where to go, and we were just playing along in the geologist-miner charade. Usually, this resulted in zigzag drives following a surprisingly straight lode.

↑ Figure 1 – Example of face painted up, with the blue arrow showing the direction for the next cut

In narrow-lode mines, where a mineralized structure has a defined hangingwall (e.g., narrow vein gold deposits such as Norseman or Kundana in Western Australia), the usual arrangement is:

Development round is bogged out. The heading is barred down and supported.
The mine geologist maps the face, takes samples across the face.
Mine geologist measures the position of the face from the last survey peg.

↑Figure 2 – Example of a face marked up for sampling, with the right-hand markers showing directions for the next cut

Mine geologist then sprays an arrow or some other instruction on the backs or the face for the direction of the next round (see Figures 1 and 2).
Back in the office, the mine geologist completes the face map and shares it with the production team.

The benefits of having ore drive development under geological control are:

Reduction in sample drill meters to define the lode. Only enough drilling is needed to justify the decision to carry out the development.
Reduction in drill cuddy requirements.
The decision to develop can be made earlier; and
Minimizes delays in the developing turn-outs from the crosscuts, and then the ore drives.

The pitfalls:

The development drive becomes part of the resource definition/grade control process. It is serving as both an access for sampling and mapping, and also as the platform for subsequent stoping.
There is a tension between the need to develop the vein as perfectly as possible for geological sampling and mapping, and the need to have economically useful ore coming out of the development (on narrow vein mines, development can be 40-50% or even more of the production source), and the need to have the development drive in the right place to allow effective stoping (Figure 3).

↑ Figure 3 – Schematic of the conflicting aims of ore drive development

Making the wrong call and following a promising splay (Figure 4). For a couple of cuts the face grade is high, but then peters out, so need to restart development on what is now recognized as the main lode. The splay development can cause problems for subsequent stoping, due to an increase in drive width.

↑ Figure 4 – Example plan of an ore drive, with grades from face sampling, showing the drive was veering too far to the right, with need to adjust and turn to the left.

Making the wrong call and chasing a splay that leads to unplanned breakthrough in a neighboring drive; and
Delays in the development drive schedule due to:

1. Mining extra meters in order to follow a splay;
2. Extending the development drive because still in ore;
3. Needing to slash out the side of the drive to expose the vein; and
4. Putting the development on hold while waiting on assays because unsure if the face is still in ore.

Data

Analysis was carried out from level plans from five different narrow lode gold mines. All mines were developed in the last 15 years. Four of the mines were developed with conventional jumbo drill rigs, with the mining method planned to be longhole open stoping, and one was developed using airlegs, with the planned mining method to be a variant on shrink-stoping. Four mines are in Australia, and one is in Asia. Measurements were taken of ore drive development length, and simply the length of any errors during that development. In this simple first run no account was taken for apparent mistakes like ore drives not pushed to natural extent, as these situations could be due to planning or scheduling constraints rather than problems when developing. A summary of the measurements is shown in Table 1. The total measurements are 9462 m (31 042 ft), of which an additional 741 m (2431 ft) was developed, this is an average error of 8%, but on individual drives, the highest was 46%.

Analysis

A simple approach has been used: ‘Is it an economically better strategy to reduce drilling costs by carrying out ore development under geological control?’

Assumptions

Development cost per meter: AUD 7500.
Existing drilling is already to 25 m (82 ft) spacing. Halving this spacing to 12.5 m (41 ft) would reduce uncertainty in interpretation and grade estimation.
The average length of infill drill hole is 75 m (246 ft).
Drilling cost per meter: AUD 150.

From this

Cost of development errors: AUD 5.6 million
Cost to infill drill: AUD 4.2 million

Conclusion

Development of ore drives in narrow-lode underground mines is often carried out under geological control, as a means of reducing infill drilling costs and to maximize the grade of the development ore. There is a tension inherent in this strategy where the ore drive is then to be used as the platform for stoping.

Analysis of nearly 9500 m (31 168 ft) of ore drive development plans from five different gold mines shows that on average this can lead to 8% more meters of development being carried out. On individual drives, however, the extra development can be as high as 46%. On a simple cost comparison between the value of the extra development meters and the cost of infill drilling there is clear support for carrying out the infill drilling to reduce drive development meters.

Where ore drives are developed for the purpose of stoping, it is recommended that the priorities should be:

Design and mine the drive for stable and effective stoping;
Have the drive under survey control;
Geologists interpret a mineralization model that is the basis for mine design, and then carry out mapping and face checks to confirm the interpretation;
If there are significant differences between the mineralization interpretation and what is being mapped in the drive, then update the interpretation and issue a new survey instruction.

References