The main objective of ore reserve estimation is to acquire as much information as possible form sampling and estimate the tonnage and grade (value and extent) or the ore deposit. In order to achieve this, geologist organize detailed sampling procedures to follow.


Conclusive evidence of the existence of an ore body requires that samples, representative of the whole, be collected and analysed. Mineral samples are obtained by excavation or drilling. If the deposit is exposed or located at shallow depths of burial, an opening is excavated from the surface in the form of a channel, pit or trench. Drilling small circular holes from the surface into the deposit is by far the most expeditious and economical mean of sampling an ore body that extend to depth. Over 95% of mineral sampling today is done by drilling.

The three most common drilling methods employed purpose today are:
  1. Diamond drilling
  2. Auger or Roller Bit rotary and
  3.  Percussion drilling
There are basically 2 forms of sampling grid. These are;
  • Regular (rectangular/square, triangular and polygonal)
  • Irregular
Sampling on surface excavations, the deposit geometry generally dictates the pattern and spacing of sampling points. On the other hand, sampling in underground excavation are usually dependent on development works, accessibility and circumstances prevailing in the underground environment. Furthermore, sampling through drilling is done by choosing an optimal array and number of holes by considering geologic, economic and statistical input. An optimal sampling grid is one that provides the maximum information from least expenditure of drill holes.

Initially geologic conditions may favor an irregular grid but a regular grid is to be adopted as early as feasible. The reason is that it provides uniform level of knowledge of all parts of deposits and enhances visual interpretation of results.

Reserve Estimation

Upon completion of sampling and sampling results become available, the next step is estimate as accurately as possible the ore reserve. Ore Reserve estimation is a procedure that starts at exploration and continues throughout the life of a mine with constant improvement and refinement in the methods employed. Thus increasing the confidence and establishing a competent geologic database of the ore deposit. Data from the ore reserve estimation are the basic input for feasibility study of a proposed mining project.
There are two broad techniques employed in ore reserve estimation;
  1. Classical
  2. Geo-statistical

  1. Classical Technique
Classical methods use simple geometric principles like (area, volume) to estimate the grade and tonnage of ore deposit. Different classical procedures are used in calculating reserve mainly differ in the ways in which they combine the sample data. These are;
  • Polygon method 
    • areas of influence polygons are assigned to drillholes either on plan or cross section and reserve calculations are done thereon.
  • Triangle method
    • a modification of the polygon method, differs from it by constructing triangles with the drill holes forming the apices of the triangle. The advantage is the the data in three point holes enter into the calculations and then proceed the same manner as in polygon method
  • Section method
    • blocks of ore are outlined on regularly or evenly spaced cross sections of the ore body. Areas of influence are calculated from the same way and all calculations are tabulated.
By selecting/employing one of these methods, the calculation of tonnage and grade for an ore body could be done with simple geometric and arithmetic functions.

     2. Geo-statistical
This technique involves the application of mathematics of random functions to the reconnaissance of mineral deposits. Usually this procedure entails these steps;
  • Study of geologic controls on mineralization and any zones of the deposit
  • Computation of variograms (graphical correlation of mineral characteristic) for each geologic zone
  • Division of ore body into matrices of blocks for kriging (calculation of block grade related to adjacent samples); blocks are then classed as measured, indicated and inferred
  • Estimate of tonnage and grade of each block at given cut-off-grade
  • Printing of recoverable grade distribution plans by level or bench in the proposed mine.
While there will always be doubts as to whether the added sophistication and cost are justified, geostatistics does provide a comprehensive suite of tools-not only to find the total ore reserve parameters but also, the distribution of recoverable grade throughout the deposit.

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