This new series will give caving professionals an overview of the factors affecting the calculation of a mineable ore reserve computation for a block cave mine. Discover in this post the first parameter to be considered in a new project.

By Tony Diering, Ph.D. VP Caving Business Unit Dassault Systèmes, GEOVIA

INTRODUCTION

This monograph provides a summary of the various factors affecting the calculation of a mineable block caving reserve using GEOVIA PCBC software. The software has been in use for over 25 years and during that time a wide variety of different block cave scenarios have been evaluated. Nothing is new or revolutionary in this monograph. Instead, the various sections should provide a check list for the reserve evaluation of a new block cave project. The basis of the paper is centred on the simple definition of an ore reserve (compared to resource) as follows: A Mineral Resource is a concentration of metal or other commodity in the ground, which has economic potential, while an Ore Reserve is that part of the Mineral Resource, which can be economically mined. If “ore” is material that forms part of the ore reserve, then perhaps dilution can be defined as mined material that is not ore? Thus, dilution is that material which you don’t want to mine, but need to due to the shape of the ore body and choice of mining method. The focus is specifically around block cave projects and mines as well as the specific “peculiarities” of block cave mining, which can make the ore reserve evaluation process more complicated. The broad areas covered are the effects of ore body geometry, mining geometry, material fragmentation and flow, economics and mining history/sequencing. The experience gained during the past 25 years has been with the use of the PCBC software package developed by Gemcom Software International Inc., now owned by Dassault Systèmes. The intent is not to say what is right or wrong about any process or work flow, but rather to provide information about the factors to consider and how they might affect the ore reserve estimation process for a block caving project. Some of the specifics covered in later sections are as follows:

• Dilution sources (internal vs external and geometric vs flow)
• Flow mechanisms
• Economics
• Excavation geometry
• Mining sequence and history
• Residual material in multi-lift situations
• Computational considerations
• Metal balance considerations and auditability
• How the software handles the above

It is tempting to try to provide typical values and guidance for working through the above. However, there are generally a variety of values and simple rule of thumb values, which should only be used with caution or in early stage evaluations. Another problem in the industry is the reluctance for mining companies to openly share the information. This may be due to intellectual property considerations or simply due to the difficulties of presenting this information at various conference venues. Thus, although PCBC has been widely used within the caving industry, the direct references to specific mines are limited.

TERMINOLOGY

For purposes of this monograph, the following terminology is used: • Metal is the commodity that is being extracted. This is grade * tonnes. Diamond mines are different, but for convenience, we will refer to them as “metal”.

• A draw point is typically one half of a draw bell.
• A draw column consists of the material located above a draw point
• Height of Draw (HOD) refers to the extraction of material up to a given height above a draw point.
• Haircut refers to the process whereby an irregular HOD profile is trimmed and smoothed.
• Peeling the potato refers to the process of smoothing the overall outline of block caving footprint to improve its cavability characteristics.

DILUTION SOURCES

There seem to be many different definitions of dilution and this monograph will not attempt to rigorously define what is or is not dilution. For our purposes, dilution is material, which you don’t really want to mine, but end up mining due to ore body shape and mining method. In block caving, three dilution sources can be listed as follows:

• Internal dilution within the ore body. This would include low grade inclusions within the main ore zone such as dykes or sills.
• External dilution around the fringes of the ore zone. In a block cave, these are conveniently separated into the bottom, sides and top of the cave zone.
  • Dilution around the sides depends upon the draw point layout and how the cave propagates whether this is vertical, less than vertical or expands beyond vertical limits.
  • Dilution around the bottom usually originates from the requirement for the draw points to be in a single plane (or almost planar) whereas the base of the ore body can have any shape.
  • Dilution from the top of the cave is the primary source for flow dilution to enter into the ore zone.
• Flow dilution. This is a term that is useful in the context of block caving. One may define a geometric outline and say that this is what is intended to be mined. The geometric outline may include dilution from the base, sides and interior as described above or the dilution may flow into the ore material during the extraction process. Flow dilution would (based on this definition) be essentially unique to cave mining methods. Flow dilution is, therefore, very dependent on the various flow mechanisms, some of which are described later.

People will often refer, in a general manner, to the amount of dilution expected for a block cave. Figures of 10% to 20% are common. It is useful to try to categorize this via the above sources of dilution.

In our next post​​​​​​​ we will focus on the flow mechanism and fragmentation parameters. Stay tuned!

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