A: by Dr. M.K.Devarajan, Geovale
To answer this question we need to discuss the term cut-off grade (COG) along with two other terms, Resource and Reserve. A rock in which no metal is present above it’s background value may be considered as a ‘waste’ rock in mining parlance. However, every other rock with concentration of elements above it’s background value can not be called as an ore. COG is the minimum grade required in order for a metal concentration in a rock to be considered for it’s eventual economic extraction. This is determined by studying a number of comparable mining costs, milling and smelting costs for mines which are operated with a profit. For example 0.5 g/t may be the initial COG for Au in a deposit while defining it’s Resource. This comes from our knowledge from many working mines. However, out of this Resource defined by 0.5g/t, only that part of the ore above 0.6g/t may eventually get classified as Reserve in a specific mine, after detailed assessment of options for mining , milling and smelting in a Feasibility study conducted specifically for that mine .
So, COG while defining the Resource is decided by taking into consideration our experience in a number of profitably run mines while COG for defining the Reserve is decided on the basis of specific studies carried out for a particular mine in a feasibility study.
Obviously COG is not decided by looking at a sample with a hand lens.
A: By Sri Pramod Kumar, AMD (retd)
Cut off is minimum grade that can be mined out with profit. It varies from country to country based on need, strategic importance, availability etc. Indian cut off for iron is 60% but Japan imports even 30% –the murram soil of India thrown on the village roads. Our Uranium cut off is 0.03%, whereas Canada, Australia have more than this in rejected tailings…. cut off is very wide in concept. In ore body examination we only assess the average grade only.
A: Sri Om Prakash Somani, MSPL
(a)The cut off grade is basically a function of economics of the mineral deposit which is defined as the grade above which an ore or minerals can be mined economically. A definite minimum chemical grade or some metrics is important to arrive at recovery of economic minerals above which ore or minerals can be sold in the market profitably depending upon the market price of the material being mined, thus it can vary. With higher prices of mineral commodities the cut off grade can be lowered.
(b) However, experienced people can say something about grade of the particular metal in the hand specimen but not cut off grade.