Sampling for Mineral Resource definition – A pragmatic approach.SAIMM presentation. June 2013

Sampling for Mineral Resource
definition – A pragmatic approach
H F J Theart, SRK Consulting (South Africa) Pty Ltd

Some definitions
- Mineral Resource
The SAMREC Code (2007):
A ‘Mineral Resource’ is a concentration or occurrence of
material of economic interest in or on the earth’s crust in such
form, quality and quantity that there are reasonable and
realistic prospects for eventual economic extraction. The
location, quantity, grade, continuity and other geological
characteristics of a Mineral Resource are known, or estimated
from specific geological evidence, sampling and
knowledge interpreted from an appropriately constrained
and portrayed geological model. Mineral Resources are
subdivided, and must be so reported, in order of increasing
confidence in respect of geoscientific evidence, into Inferred,
Indicated or Measured categories.

Some definitions
- A Sample
• In statistics, a sample is a set of observations drawn from a
population by a defined procedure.
• In the Geological Profession we refer to the observation as
a sample and to a statistically defined sample as a set of
samples.
• With regard the Resource definition the geographical
location of the sample is very important. (spatial
statistics)
• It is therefore assumed that the sample is representative of
the properties of the Mineral Resource at the locality in the
resource from where it was collected and that the set of
samples is representative of the entire Mineral
Resource or that part of it, as clearly defined in the
description of the Mineral Resource

Description of a Mineral
Resource
• In practice a Mineral Resource is described in
terms of its:
– Quality
– Volume
– Relative Density
• From this the resource can be expressed in
terms of its tonnage and grade.

QA/QC
Quality Assurance and Quality Control in Mineral Resource
Definition
•The reporting codes only relate QA/QC to the quality of the
chemical analyses of the samples
•Should be a feature of all aspects considered in defining the
resource and the collection of the samples.
•All measurements for Resources and reserves.
This include amongst others:
•The geographical location (survey)
•Measurement (trenches, drill holes, channel samples)
•The geological context (logging)
•Measurements of density
•The sample preparation
•The sample analysis
•Database Management
•Interpretation of the geological model

The geographical location
(survey)
Appropriate accuracy:
• Definition of the elevation especially for near surface
deposits. The DTM.
• Modern advances Satellite images; and Lidar surveys;
Ground control; advanced GPS technology
Down hole surveys (the bearing, dip, and distance):
• North seeking (magnetic) – magnetic declination
• Gyroscopic
• Closing the loop (In and out surveys)
Conversion to appropriate geographic reference system.

Measurement
• Unlike grab samples, the samples used in resource
definition has to be representative of a specific width
over which it is collected. What is considered here
include:
– Trenches, pits, drill holes, channel samples
• How representative the sample is over the relevant sampling
width?
• In drilling: core or chip recovery –
– remedies
• Channel and trench sampling (not bulk sampling)
– remedies

MeasurementMeasurement
Even under
difficult
conditions the
sampling
should always
be
representative
of the material
sampled

Core recovery - Diamond drilling
• How is core recovery defined
• Also in terms of drilling contract
• Example Wits Gold
• Carbon seam related to highest
grades
• Shear-zone hosted gold
• Effect of rock fabric
• Appropriate drilling method

MeasurementAppropriate drilling method
By selecting the
correct drilling
method much
improved core
recoveries can
be achieved.
In situations
such as this a
double or triple
core barrel
design would
have given much
better core
recovery.

MeasurementSample recovery
Cutting channel
samples with a
diamond saw
can give much
better results –
but even here
control
measures are
required

MeasurementSample recovery
Sampling a
rock dump is
extremely
difficult unless
reverted to
bulk sampling
Note the pen
used for scale in
both cases
Samples were
rejected in audit
as they were not
representative of
the material
sampled.

The geological context
• The opinion of the Competent Person
• Is the sampling method appropriate for the style of
mineralization
• The need for an auditable relational (digital) database
• An independent permanent record
– The core itself
– Core photographs
– Chip boards
• I do not believe in remote core logging
• The need to store sample pulps under appropriate
conditions for repeat analysis

Is the sampling method
appropriate?
Vertical drill holes were used to
sample the deposit.

Measurements of Density
• Consider using the correct terminology and not
Specific Gravity.
• In situ dry bulk density is used to convert the
resource volume to resource tonnage.
• Density measurements should reflect an
auditable record of QA/QC as should any other
resource measurement.
• It is good practice to confirm the measurement
by at least one alternative method.

Dry Bulk Density
Some of the alternative measurements include:
– The simple volume and mass method
• Calliper measurement, ore recovery
– Water displacement method
• Difficulty to measure displaced volume
– Archimedes Principle
• (special care with porous or water absorbing materials)
– The gas pycnometer method
• Small sample (Is it representative)
• Porous rocks
– The Stoichiometric method
• For this the chemical composition and mineralogy should be known

Problem with Resource Statement
of an Iron ore deposit
• The company states in its Mineral Resource Statement
(JORC compliant) that the in situ bulk dry density is
2.80. And this is used for the tonnage conversion.
• A simple stoichiometric calculation indicates that the
Relative Density should be in the order of 4.94.
• Clearly there is a problem and one should carefully
consider the company’s defence that the lower density
is a conservative estimate.
• My client was the government who can be negatively
influenced by such an action.
Example

Overestimation that may be directly
related to RD and sample quality
Nevsun Resources Ltd. (NSU) on 7 February announced its expected “2012 outlook
for planned gold production is 190,000 to 210,000 ounces from the Bisha mine in
Eritrea”. This is about half of what Nevsun was previously expecting to produce
in 2012.
They “concluded that the resource estimate used for mine planning over-estimated
gold in portions of the Bisha Main Oxide Mineralization.” It goes on to add that it’s new
resource estimate “includes approximately 1.3 million tonnes @ 5.1 g/t Au of the
remaining 1.6 Mt of Bisha Main Oxide. This compares with 2.5 Mt that was previously
expected.”
Possible causes:
•Core recovery
•Inappropriate drilling method - reverse circulation drilling
•Gas pycnometer density determinations
Example

Sample preparation
• Sample preparation is often the cause of the
biggest QA/QC problems and this include:
• Splitting error
• Ni sulphide example
• Cross contamination
• Vein gold example
• External contamination
• Vertical spindle mills
• Consider the use of blank samples very
carefully, especially the point at which
they are introduced in the process

The chemical analysis of samples
• Most advanced in terms of QA/QC
– Laboratory accreditation ISO 17025
– Umpire laboratory analyses
– Blanks, duplicates and standards (in
house as well as Certified Reference
Materials).
• More important questions are:
– Is the sample dissolution appropriate
– Is the analytical method appropriate

Database Management
• The most important aspect that I want to address here is the
need to reduce human error as far as possible.
• Format the database and the laboratory report so that there is
a seamless import of the data and merging with various other
types of data to avoid input errors and column swaps.
• Ensure that there is adequate access control and database
security to prevent unauthorised access. Ensure that access to
the data base is limited, recorded and that there is sign-off by
responsible persons.
• Build automatic QC processing into the database so that
problems can be identified and appropriate measures taken as
soon as possible.
Design to
facilitate
easy import
and query
procedures

Interpretation of the geological
model
• Sampling design must be appropriate for the geological model
– Sample density
• Definition of the geological model is normally based on:
– 3-D distribution of information.
– Geological structures, and intrusions that will affect the distribution
of the ore zone – inferring continuity across such features may lead
to mistakes in the estimation of grade.
– Delineation of the ore zone often require assumptions with regard
cut-off grades, the definition of geological losses, and the certainty
with which the geographic position of sources of information.
– Sample information must be distributed in a manner that
demonstrates that they represent the overall characteristics of the
resource or zone within the resource. It should be clear what is
done with regard missing data.
• Example massive sulphides in a Ni-sulphide ore body

Sampling for Mineral Resource definition – A pragmatic approach.SAIMM presentation. June 2013

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Sampling for Mineral Resource definition – A pragmatic approach.SAIMM presentation. June 2013

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