Geoscience Reference
In-Depth Information
13
Short-term Models
Abstract
Most mineral resource estimates are not final. They are interim estimates modified by more
information as it becomes available. At the time of actual mining, or just before mining, the
nature and requirements of estimation is different. Results that are accurate over a longer
time scale are no longer sufficient. This Chapter explains considerations for short and me-
dium term mine plan models.
13.1
Limitations of Long-term Models
for Short-term Planning
ticularly for disseminated-type deposits, deposits with very
simple geology, and grade variables that do not exhibit high
spatial variability. In the case of a new operation, the long-
term model will generally be based on relatively tight drill
hole spacing (infill) covering the initial years of operation,
designed to accurately estimate the payback period.
Updating the long-term model is required virtually in all
mine operations for several reasons. The most important
reason is the need to improve accuracy for Medium- and
Short-term mine plans. These plans would correspond, for
example, to yearly budgets and quarterly forecasts of mine
production and corresponding cash flows.
For month to month mine planning, the model's reliabil-
ity is increased through infill drilling. The additional drilling
will result in improved accuracy of the resource model for
the near-term mine operation. Updating the long-term model
with the new data and subsequently updating the correspond-
ing mine plans results in less uncertainty about the opera-
tion's short-term cash flow.
The definition of “medium” and “short” term models
varies widely from one mining company to the next, and
also from one geographic area to the next. In many cases,
a “short-term” model is in fact a grade control model, the
daily ore/waste selection process. In this topic, a medium-
term model will be any model that is meant to provide esti-
mates on much smaller volumes than the long-term resource
model, and is also short-lived. It generally means a volume
equivalent to one to six months production, although it de-
pends on the type of mining performed. The models devel-
oped for daily ore/waste selection and weekly mine plans are
always called here short-term or grade control models.
Resource models are said to be long-term when they are used
for long term mine planning, such as Life of Mine (LOM)
plans. When a feasibility study is prepared for a new mining
project, a mining schedule needs to be prepared to estimate
future cash flows from the operation. The LOM plan is based
on a reserve model, in turn converted from the resource
model. It provides estimates of tonnage and grade for each
period involved through to the end of the life of the mine.
Often, the LOM plan is scheduled according to variable units
of time. For example, it may be that for the first two years
of the operation, the schedule is monthly; the following two
years it may be based on semi-annual volumes; and from the
fifth year until the end of the mine life it may be yearly.
Long-term models are based on widely-spaced drilling,
which is gradually filled in as the project advances. The long-
term models are usually updated on a yearly basis with infor-
mation gathered from new drill holes. More accurate fore-
casts in the short term are often needed as well. It is tempting
to use the existing long-term resource model for shorter term
predictions. However, because of the dynamics of the opera-
tion, the long-term model quickly becomes outdated.
Long-term models are by construction designed to
provide global estimates with acceptable accuracy. Global
estimates are understood to correspond to volumes equiv-
alent to a year or longer. Therefore, it cannot be expected
to perform as well on a block by block basis, or even for a
small volume. Sometimes reasonable accuracy is obtained
from long-term models for volumes smaller than a year, par-
