Page 247 - Introduction to Mineral Exploration

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

Before drilling a hole it is recommended that collected from several sampling programs as
a section be drawn along its projected length, described above, including trenching, pitting,
allowing for hole deviation if this is possible. percussion drilling, reverse circulation drilling,
As the hole progresses the section is modified. and diamond drilling, as well as during trial
Drilling is an expensive and a time-consuming mining. The object of this sampling is to pro-
part of mineral exploration. The objective is vide a mineral inventory. Once the mining
to drill a precise number of holes, within bud- engineers and financial analysts have estab-
get, safely, and provide the exact number of lished that the mineral can be mined at a profit,
intersections needed to demonstrate the grade or in the case of industrial minerals can be
and tonnage (dimensions) of the mineralisa- marketed at a profit, it can then be referred to
tion at an appropriate level of accuracy and as an ore reserve.
precision. Unfortunately, such optimization The stock market controversies that sur-
is rarely, if ever, possible. At second best an rounded the discovery of the Poseidon nickel
iterative procedure is followed whereby a deposits in Western Australia in the 1960s
series of successively closer spaced drilling pro- led the Minerals Council of Australia to es-
grams are completed with each followed by a tablish a committee to resolve these issues.
re-assessment of all existing data – a process of When the Australasian Institute of Mining and
successive approximations. The main problem Metallurgy joined it, the resulting committee
is that in the calculation of mineral resources was called the Australasian Joint Ore Reserves
the zone of influence of each sample is not Committee (JORC). Other controversies, in-
known until a minimum amount of work is cluding that surrounding the Bre-X deposit in
completed. If two adjacent samples (taking a Indonesia in the 1990s (see section 5.4), pro-
drillhole as a sample) cannot be correlated at an mpted Australian and north American stock
acceptable confidence level, then neither has exchanges to adopt rigorous procedures for
an acceptable zone of influence in the inter- companies to report their resources. The leader
vening space and further sampling is necessary in determining these procedures has been
(see section 10.4.1). Conversely where adjacent JORC, which has defined the JORC Code (Aus-
samples show appropriate correlation further tralasian Joint Ore Reserves Committee 2003)
sampling is not required. that has been taken up by a number of compan-
There are several methods of assigning ies and stock exchanges, and forms the basis for
zones of influence to either successive samples most international reporting systems.
or individual boreholes: the mean-square (dn) As early as 1976 the United States Geolo-
successive difference test, the use of correla- gical Survey (USGS 1976) published defini-
2
tion coefficients (R ) or geostatistics using the tions of reserves and resources for mineral
range (a) of a semi-variogram. However, a relat- and coal deposits. These publications take into
ively large nugget effect (section 10.4.3) in the account the increasing degree of confidence
semi-variogram indicates an element of un- in the resources and the financial feasibility of
certainty that would call for a reduction in mining them.
spacing. Such an effect is apparent in the evalu- Between 1972 and 1989, a number of reports
ation of low-grade disseminated gold deposits were issued by JORC which made recom-
where drilling may be required on a ±50 m grid mendations on public reporting, and resource
basis. and reserve classification. These gradually
developed the principles now incorporated in
the JORC Code. In 2004, JORC published its
10.4 MINERAL RESOURCE AND ORE RESERVE latest version of the code.
ESTIMATION, GRADE CALCULATIONS The JORC Code is currently used as a model
for reporting codes of other countries with
During exploration and initial evaluation of a appropriate modifications to reflect local con-
base metal, industrial mineral, or coal deposit, ditions and regulatory systems. Examples
the principal emphasis is placed upon its geo- include the USA (SME 2004), South Africa
logy and the estimation of the quality and (SAIMM 2004), and UK (IOM3 2004). Agree-
quantity of the resources present. Data are ment has also been reached with the United