Open-pit, Block size selection and its impact on design and mine planning

This work evaluates and characterizes the impact of the support size and mining dilution of a block model in the operation and selection of equipment at an open pit mine. An exploratory analysis and
geostatistical modelling of the grades assayed on a set of drill hole samples is performed, which allows constructing a grade model on a dense grid by means of a conditional simulation. The grades are then averaged to greater selective mining unit supports in order to proceed with the study and methodology.
First, the metal-tonnage curves are used to analyse the loss of selectivity on the in situ resources caused by a change of support.
Concerning the ore reserves and their mining sequence, defined in the pit optimization and analysis process, the same curves prove that the loss of selectivity is more accentuated. Second, the importance of the block boundary dilution produced during mining operations is assessed. Its value depends on the amount of ore in contact with waste, on the loading error of the equipment and on
the ore loss/waste dilution criterion assumed by the planner. The dilution percentage and the differences between waste and ore grades become higher when the block size decreases. For these
reasons, considering the dilution in the production plans makes mining a small support more constraining. An adequate delimitation of the orebody can improve the mining selectivity, leading to greater income. Finally, a study of the increase in mining cost that provides a zero difference in NPV between the different block size options is carried out, in order to determine the maximum increase in mining costs for which it remains profitable to mine at a smaller block size.
Keywords: Mining selectivity, support effect, block boundary dilution, selective mining unit size, block model.

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