Geology & GeoMetallurgy

Geology & GeoMetallurgy2017-04-04T06:58:01-04:00
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Open Pit Grade Control (6 replies)

4 years ago
(unknown) 4 years ago

RC Open pit grade control - how many benches? How far ahead in space and time
What's a workable number of benches to drill using RC or grade control? Say the benches to mine are 5 m. Three benches or four in one pass? Why? How much to drill in front of production 2 months or 3?

Helena Russell
4 years ago
Helena Russell 4 years ago

The grade control data must be available in time to inform and be integrated into the mining plan – if it does not do that, it is too late.

In practical terms, the answer will relate to the vertical rate of advance of mining, overall pit geometry and the grade control-mine planning cycle time. That is often a function of how well drilled out the deposit was/is prior to starting mining (and the competency the 'resource development team' was, or if there was one at all).

In practical terms,

I have found it useful to drill staggered lines of RCGC so that I am getting as much advanced 'warning' of potential issues (or but perhaps I was unlucky with the places I worked at).

Depending on ground conditions, restrict the hole depths to <36m to keep the holes as straight as possible. If you are getting too much downhole wander (oxidized or rubbly ground etc), you will need to shorten the holes.

When starting the RCGC process, downhole survey the initial holes/ground combinations to ensure the holes are straight and modify the planned depth as required) - but you have to keep the hole straight (or know where they wander to).

As much as everyone will hate the cost and time burden, stem the RCGC holes. Nothing like seeing a good blast start to rifle up open RCGC holes; followed by uneven blast heave, oversize, broken digger teeth and toe.

4 years ago
Gruppen 4 years ago

Depend upon the situation such as; the ore body, size of the pit, production rates, pit design, amount of groundwater etc, so it is hard to give a definitive answer. One project I worked on (a small shallow -40m deep pit) we drilled out the whole pit from surface. There was the additional cost of drilling through a lot of 'waste' but to have the whole pit modeled and estimated very early in the mining cycle was a definite advantage.

One other thing that I have found useful in the past is to stagger the drilling, so that even when you mine to a bench that then requires grade control, there is double spaced drilling several benches below. This also reduces the time that the rig is on the floor (hence disruption to mining) and also provides an initial estimate of grade and tonnes below.

Marshal Meru
4 years ago
Marshal Meru 4 years ago

Thirty meters (6 benches) is a good depth as hole deviation should not have a major impact at this depth and it is often more efficient than shorter holes. Tramming takes up a lot of time while drilling GC patterns so by adding an additional ten meters to each there is an efficiency gain that only takes a little extra time and does not usually impact the rigs performance. The longer holes also mean that more data is available for modeling with fewer edges between data sets resulting in better models.

Six benches is usually equivalent to 3-4 months production so this high quality GC data can be used in quarterly forecasting processes. By including GC information in the forecast you can eliminate and Reserve to GC reconciliation issues and fine tune ore selection criteria and blend plans to account for the actual GC defined ore this adds significant value to the business.

4 years ago
(unknown) 4 years ago

I would be interested to know what proportion of the semi-variance is targeted for grade control - 50% of the sill or less. And at what point (depth) do you decide you cannot rely on the resource drilling holes as part of the grade control process?

4 years ago
Gruppen 4 years ago

As long as the resource holes have gone through the usual QAQC process and most importantly they have good quality donwhole surveys they can be used at any depth.

Bob Mathias
4 years ago
Bob Mathias 4 years ago

Provided the Resource holes are done with a similar drill method and hole size as your grade control drilling (ultimately QQ plots to ensure no bias between drill programmes) then Yes use your Resource drill data.

With regard to semi-variogram range, similar ranges as defined your measured resource, (between 1/2 and 2/3rds) and I would like to consistently see conditional bias slopes above 0.8. 

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