IAMG 2001--Cancún Technical Program--Session D

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Grade Tonnage Curve: How Far Can it Be Relied Upon? by Francisco Silva, Somincor--Sociedade Mineira de Neves-Corvo, S.A, Portugal, and Amílcar Oliveira Soares, CMRP--Instituto Superior Técnico, Portugal In the mining industry, from the geology and mine planning areas to management, the grade tonnage curve has been applied for decades now in economic and financial analysis probably being one of the most important tools in determining the volume and grade of material in face of variations of the cut-off grade. The grade tonnage curve is usually calculated with the in-situ reserves and shows always the 'best' or 'optimistic' scenario, i.e., at a given cut-off grade the respective tonnage depicted by the curve presents the ideal situation where all the material would be mined coming from the assumption that the shape of the orebody at this cut-off is continuous and every block is amenable to be mined. Usually the grade tonnage curve is not calculated with mineable reserves which depends on the mining method: geometry, spatial location and topological relations between ore and waste blocks and spatial location of main mining structures (galleries, main shaft, etc.). For a given cut-off, the difference between in-situ and mineable reserves is as much evident as the mining method is selective. But to design and run a simulation of the mining method on the entire orebody for different cut-offs is a tedious and cumbersome practice. On the other hand, in most situations at the early stages of a mine, the mining method is only known in a broad sense without the details regarding the mining sequence of blocks, the mining rates, etc. Hence in this paper it is proposed the following approximative procedure to convert the in-situ into mineable reserves: a given block of ore (above the threshold) is considered as mineable if it belongs to a connected set of ore blocks, otherwise it is considered waste if it is isolated by waste blocks. Another important issue addressed regards the uncertainty, i.e. the spatial uncertainty of the ore grades and the corresponding uncertainty of grade tonnage curve of mineable reserves. To accomplish the mentioned objectives the grade tonnage grade curve is calculated by the following methodology: Maps of grades are generated by stochastic simulation. Copper, zinc and tin grades are simulated by using the direct sequential simulation and co-simulation. After averaging the grades of each map in blocks (equivalent to the dimension of mining stopes) an image analysis algorithm is applied to calculate connected sets of ore blocks and consequently, the mineable reserves for the different cut-offs. A grade tonnage curve is obtained for each simulated map, giving rise to an uncertainty measure of that mine planning tool. The proposed methodology was conducted in the Neves Corvo sulphide copper mine. Paper in PDF format Silva and Soares, Acrobat PDF, 329 k.