Ore Grade Determination Techniques

Ore Grade Determination Techniques

Ore grade is the single key variable in establishing mine economics. It defines the quality of materials to be extracted and processed. Ore grade defines the state of ore deposit as resources or reserve which further equates to its economic value that can be utilised in mine planning, pit optimisation, production scheduling, setting production target etc.

There are several methods in which ore grade can be obtained (pit samples, channel sample, chip samples and from drill cores) however employing a suitable techniques to estimate a representative grade is crucial.

A quick solution would be to reduce the sampling density (<5m) to increase confidence in the grade values and to fully define the inherent variation of grade between sampling points but this is a very tedious and costly exercise. Hence, how can we fully understand this key information through sampling data obtained from widely spaced (>50m) sampling points?

In this post I will be discussing the method of vertical section to estimate grade values that is representative of the orebody/sections of the orebody. Several techniques have been employed and accept usage around mines in the world and here are two simple but common techniques that are utilised namely; Method of Vertical Section and Method of Horizontal sections.

1. Method of Vertical Section

This technique employs the traditional method of ore reserve estimation through the use of sections. It has a number of advantages (Hustrulid & Kuchta, 2006):
  • Can be done by hand
  • Can be easily depicted and visualised
  • Easily understood and checked
The materials required are:
  1. Current up-to-date plan map detailing location of drill holes, pit cross section and pit condition and structures.
  2. Complete set of cross section containing all drilling and sampling information

The general procedure followed to estimate the grade of a section is as follows (Hustrulid & Kuchta, 2006):
  1. Drill sections evaluated on cross section and ores of different types of materials identified for convenience. Any pit operation that may disprove drilling samples must be taken into consideration.
  2. Limits of ore material transposed into pit area. Note pit limit and layout.
  3. Pit design transposed to the plan and account optimum pit slope angle, haul road and any in-pit permanent infrastructure. (permanent infrastructures implies to any mine infrastructure to be erected within the mine pit that will last for the duration of the mine (LOM))
  4. Assume the extent of each material one half of the sampling distance and 3m to 4m for end sections.
  5. Compute volume and average grade for each section
  6. Tonnage of each section to be totaled to give the overall minable reserve/resource.
  7. Weighted average of grade analysis is computed to produce the final grade.
Example 1:

Figure 1 shows the cross section of a drill program. Ore was intersected at an average depth of 35m in which different grades were determined through laboratory analysis and are denoted with “g”. Diamond drill holes (DDH) were spaced 150m apart. Determine the average grade using method of vertical section.

(DDH – Diamond Drill Holes, g – chemical analysis grade value at a point; Thick black lines – Drill holes; light brown – pit layout with haul road; green lines  different lithology (geological domains))

 Figure 1: Drill Section (Drawing not to scale)
For ease of calculation following assumption has been made;
  • The surface is flat
  • Pit outline and optimum pit slope angle has ignored
  • Lithological variations and inherent variation in inter-bench angle has been ignored
  • The optimum economic pit limit will be able to mine the entire deposit.
However is convenient at this stage to assume that you can clearly vasualise the effect of these features and how they tend to affect the overall reserve and minable grade. That mean anything (ore) that lies outside of the optimum pit limit will ignored as they fall into resource category. In addition, in competent lithologies, the inter-bench angles may be increased and in incompetent lithologies the inter-bench angles may be reduced and so forth resulting in the final pit slope varying affecting the resource estimate.

The average grade of the section plotted above is calculated as follows:
According to the rule Step 7, the weighted average grade of the section shall define the grade of this section. Using the formula for composite grade by length:

Note: The numbers above indicate the length (depth) between chemical analysis points (distance between g1 and g9, g1 and g2 etc where there is known grade value: i.e. g1, g2, g3 ... g9) and the distance extends one half of the distance between analysis points.

Recommended reading:  Classical Methods of Ore Reserves

 “Method of Horizontal Section” will be discussed in the next post update. Don’t miss out, Subscribe to news feed to receive post by email. Like and Share to show your support.

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Ore Grade Determination Techniques Ore Grade Determination Techniques Reviewed by CEDRICK KAU on October 26, 2017 Rating: 5

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