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Re: AI-GEOSTATS: stope optimisation

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  • Digby Millikan
    Hi David, I have designed the Gecko underground mine optimally from a block model (Gecko Mine N.T. 80 stopes). It is easier than pit optimisation because you
    Message 1 of 1 , Jan 4, 2003
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      Hi David,
      I have designed the Gecko underground mine optimally from a block
      model (Gecko Mine N.T. 80 stopes). It is easier than pit optimisation
      because you do not have to consider waste removal in the manner as
      an open pit. I will outline the process here assumming you have one
      metal and you are using a cutoff grade. If you have two metals and a
      sale to a concentrator you use a dollar value cutoff and a Net Smelter
      Return for revenue, i.e. you must set the dollar values in the block
      model. If you want details of this please contact me.

      1. The first step is to calculate cutoff grade. This is based on operating
      costs. e.g. gold mine with cip plant.

      cutoff = (drill&blast $/t + haulage $/t + admin $/t + cip $/t) /
      (gold price $/g * cip recovery%)

      Note that development costs are not included in this calculation,
      development costs are capital and will be considered later.
      The cutoff is the grade of material which will make profit once it has
      been accessed by development.
      The next step is to identify your ore zones(those above cutoff) and
      draw in your decline and development.

      2. Once drawing in your development you outline the ore zones based
      on your cutoff grade. Say you have identified a region to design a
      stope and you have two ore zones marked based on the cutoff

      3. You must then design your stope around these two ore zones. You
      have three options for your stope. You can mine ore zone 1, ore
      zone 2 or both ore zones. To decide which option you choose for
      your stope you must select the option which returns the maximum
      profit. So you must design each of the three stopes and evaluate
      each profit individually.

      4. The profit figure is simply

      Profit = Revenue - Operating cost.
      = Stope tonnes*stope grade*metal price*recovery -
      Stope tonnes*(drill&blast $/t + haulage $/t + admin $/t +
      cip $/t).

      So the option of the three which returns the maximum profit figure is
      selected. Note that it can be easier to calculate the profit figure if
      place the profit figure into each block of your block model, then
      depending on what software you are using you just total the profit
      figure for the blocks within each stope. Now your stope is designed
      and you have a profit figure calculated.

      5. You must now compare the profit of your stope against your
      development costs. This must be done in a number of stages e.g.
      firstly if the stope will pay for the level development access and
      secondly if all stopes will pay for the decline access.
      So say you have a decline with one access drive to your stope. The
      profit figure for your stope must be greater than the cost of the access
      drive. If the profit figure for the stope is less than the cost of
      development of the access drive then the stope is not economic.

      6. Repeat this process for all stopes and then develop a cashflow based
      on the economic stopes to determine if the mine is economic.

      Stopes are generally designed using wireframes and evaluating each
      wireframe against a block model.
      Software is available which will crudely perform this automatically only
      creating stopes which are RECTANGULAR in shape so may have little
      or no use to your situation. Genrally for a final mine design you should be
      creating the wireframes.
      The other thing to consider is Net Present Value this may be ignored or
      you may wish to raise cutoff grades in early years to increase early year
      cashflow, however this is entirely dependent on each case and different
      for each mine.
      Also relevant is the decision of which blocks you are going to design
      against e.g. normally you would design against blocks which are classified
      as Measured and Indicated, to produce a Proven and Probable reserve.
      If you design against blocks which are inferred these will be in the
      category of your reserve and would not usually be considered in your
      financial evaluation of the project.

      Regards Digby Millikan
      Geolite Mining Systems


      >Does anyone know of an undeground alternative to
      >open pit optimisation? How would you derive underground
      >reserves from a 3-dimensional resource block model?


      >David Boakye

      >Ashanti Goldfields Company Ltd

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