CUT-OFF GRADES AND GRADE TONNAGE CURVES

Backround

Not every portion of mineralized rock removed from an ore body can be turned to profit. Some portions simply contain insufficient metal to pay for the costs of mining, milling, and marketing the product. The grade and tonnages of rock containing the mineral or metal to be recovered must firstly have sufficient tonnage to support a long-term mining operation, and secondly must contain sufficient metal when sold at the current price to cover the cost of extraction and processing, as well as making a profit for the mining company. Establishing a cut-off grade for a mining operation, a delicate balance between maximizing the life of the mine while at the same time making as much money out of it as possible, is strongly linked to internal and external forces which shape company policy and procedure. While this might be an overarching principle, establishing an appropriate cut-off grade will also affect the mine design, mining layout and economic performance of the operation. For the majority of mineral deposits, the inverse relationship between tonnes available for mining and extraction, and the average grade of the contained metal, is shown in a grade-tonnage curve as a function of the cut-off grade. One should be reminded that grade tonnage curves provide a global and statistical appreciation of what the ore deposit could deliver in an ideal circumstance. It has been applied in financial analysis of the ore bodies providing an indication of the proportions (tonnages) and the grade at a range of different cut-off grades. It is always a best or optimistic estimation of what could be mined at a given cut-off assuming of course that the ore body is continuous, and every block is accessible for mining. It provides an estimation of global grade and tonnes and how they are related in the perfectly accessible and minable ore body. It takes no account of the spatial distribution of the actual SMU’s or panels that will be mined, and tells nothing about spatial continuity or about the location of specific blocks, it tells you nothing about scheduling or rates of extraction. It tells one nothing about the problems to be encountered in accessing the ore body or the mining layout or scheduling – it simply assumes all these are perfectly and easily manageable. There is a big difference between the minable reserves and in-situ reserves at any cut-off grade. The grade tonnage curve captures none of the uncertainty associated with an actual mining operation or the extractability of the ore. One thing we should consider is that once a cut-off grade is established it imposes a certain degree of continuity between blocks in the ore body. As the cut-off grade is increased this continuity between mining blocks breaks up, becoming less and less, to the point where there may be quite a number of payable blocks, but because they are surrounded by waste material, they are isolated and will never be mined. Nevertheless, an appropriate cut-off grade will ensure that the rate of production, the Life of Mine, and mass of metal delivered to the market from the deposit, will make appropriate returns to investment after costs, revenues, and profits, measured by NPV, IRR and Pay back periods, have been considered.