Page 249 - Introduction to Mineral Exploration

232 M.K.G. WHATELEY & B. SCOTT

JORC defines ore and reserves as the ally in discussions with the mining engineers,
economically mineable part of a measured or it is possible to start the evaluation procedure.
indicated mineral resource. Reserves include It is therefore important to understand the
diluting materials and allowances for mining cut-off grade theory and grade tonnage curves.
losses. Reserves are usually assessed during
feasibility studies and include consideration
of and modification by realistically assumed 10.4.2 Cut-off grade theory
mining, metallurgical, economic, marketing, Geologists will be required to understand
legal, environmental, social and governmental the concept of cut-off grade theory (Lane 1988,
factors. These assessments demonstrate at the King 2000), because they will be required to
time of reporting that extraction could reason- identify only the material above the lowest
ably be justified. Ore reserves are subdivided in grade that will be included in the potentially
order of increasing confidence into probable economic part of the deposit. This is defined
and proved ore reserves. A probable ore reserve as the cut-off grade, which separates the miner-
is the economically mineable part of an indi- alized rock from barren or low grade rock.
cated, and in some circumstances measured, Deposits with only one valuable metal, such as
−1
mineral resource. A probable ore reserve is gold, would normally use grade (g t ) to rank
the economically mineable part of a measured the resource. In a simple process, the highest
mineral resource. grade material is processed to recover the gold
Geologists are interested in the amount and and the low grade material sent to a waste
quality of in situ mineralisation within a dump.
geologically defined envelope. In situ mineral The resource potential will therefore be
resources tend to be slightly greater than the determined by the cut-off grade. It is common
ore reserves. Discussions of the subject are practice to calculate the resource tonnage at a
given by the Australasian Joint Ore Reserves series of cut-off grades. This information is
Committee (JORC 2003), Whateley and Harvey plotted on a grade–tonnage graph (Fig. 10.15).
(1994), and Taylor (1989). Eventually the material above cut-off will be
Before resource calculations can proceed, used to develop a mine plan and the blocks
a study of the mineralisation envelope is within the mine will be scheduled for extrac-
required. This envelope can often be defined tion. The schedule will be affected by location
by readily identifiable geological boundaries. In and distribution of ore in respect to topography
some cases inferences and projections must be and elevation, mineral types, physical charac-
made and borne in mind when assessing con- teristics, and grade–tonnage distribution, and
fidence in the resources. Some deposits can direct operating expenses associated with min-
only be delineated by selecting an assay cut-off ing, processing, and converting the commodity
to define geographically and quantitatively the into a saleable form. The ultimate aim is to
potential mineralized limits (see Chapter 16, calculate the net present value (NPV; see sec-
where an assay cut-off is used to define the tion 11.5.3).
outline of the Trinity Silver Mine and where All this is based upon the tonnage and grade
waste blocks are identified within the deposit calculations made by the geologist. Although
for separate disposal). Initially this is highly cut-off grades are used to classify material into
subjective. In later stages when confidence is streams for processing, stockpiling, or being
higher, the ore limits will be carefully calcu- sent to waste dumps, Fig. 10.16 shows how
lated from conceptual mining, metallurgical, errors in predicting properties of the resource
cost, and marketing data (see section 11.2). may result in misclassification of this material
Geologists must provide the basis for invest- (Wellmer 1998). The ellipse represents a band
ment decisions in mining using the data pro- of confidence between the estimated and ac-
vided from the above sampling. It is their tual values (typically 95% confidence band).
responsibility to ensure that the database that The same cut-off has been applied to both axes.
they are using is valid (see section 9.1). Once It is apparent that there are two areas where
the geological and assay (grade) cut-offs (Lane material is classified correctly and two areas
1988, King 2000) have been established, usu- of misclassification. Misclassified waste has a