Table 1: Minerals likely to interfere or
graphite assay by LOI method
Source: Mitchell, 1993 (BGS)
Graphite exploration follows a similar path to other
minerals, often from discovery of an outcrop, which is then
explored by methods such as field mapping, trenching,
geophysics, drilling, assaying of the graphite content and
mineralogical and metallurgical testing.
Data generated in this way, if successful, can lead to the
estimation of a mineral resource. At the bare minimum, this
defines the geometry, tonnage and the graphite content of the
Most of the recent exploration activity has focused on flake
graphite deposits which generally occur in tabular or lens-like
bodies; these may vary greatly in thickness and range from
sub-horizontal to steep-dipping. The purpose of this article is
to provide an overview of a road-map to success, based on some
fundamental steps through the exploration process for flake
Geophysical techniques are an indirect way of tracing
geological and/or mineralisation trends across an exploration
project. Given that graphite and associated metal sulphide
minerals – for example pyrite and pyrrhotite
– are conductors, various electromagnetic (EM) methods
can be highly effective exploration tools for graphite
mineralisation. EM surveys can be carried out on the ground,
downhole or from the air.
Ground surveys can be performed by several methods,
including fixed loop (FLEM) or moving loop. Downhole EM surveys
(DHEM) can be used to locate conductive targets that may have
been missed by a drill hole. Airborne EM surveys, such as
versatile time-domain electromagnetics (VTEM), are commonly
used during the early stages of exploration as large areas can
be covered quickly and relatively cost-effectively.
An example of VTEM anomalies that have been successfully
explored by drilling is given in Figure 1.
Exploration trenching and drilling
Outcrops of weathered graphite schist may be sampled by
excavating trenches, or by cutting channels across un-weathered
outcrops, using a portable disk grinder (Figure 2).
These methods provide a reasonably inexpensive way to trace
mineralisation across a property before drilling.
There are two main methods of drilling for graphite, namely
reverse circulation (RC) and diamond core drilling (DD), each
of which has its own advantages and disadvantages. Auger
drilling may occasionally be used to explore highly-weathered
RC is a type of percussion drilling that uses a hammer to
pulverise the rock into powder and chips, which are brought to
the surface by compressed air (Figure 3). RC is a
useful way of infill drilling between DD section lines to
demonstrate geological and grade continuity, as it is quicker
and less costly than DD.
DD is however the preferred method of exploration drilling
for graphite, as the graphite flakes and host rock are
relatively undisturbed when retrieved as core (Figure
4). Many exploration companies employ both RC and DD
methods to optimise drilling density.
Quality assurance and quality control
The old computing adage of "garbage in, garbage out" is
highly appropriate in the mining industry, which relies on
miniscule sample sizes (such as drill cores) to make important
decisions regarding the set-up of a mining project.
Quality assurance (QA) is put in place to prevent problems,
while quality control (QC) aims to detect them in the event
that they occur. It is necessary to
insert samples known as standards, blanks and duplicates into
the sample sequence that is submitted to a laboratory. A set of
samples should also be submitted to another (external)
laboratory for check, or umpire assays.
QC data may be visualised in a number of ways including
tables, control charts (Figure 5), histograms,
scatterplots or quantile-quantile (QQ) plots.
Twinned holes are traditionally drilled for verification of
historic data or confirmation of drill hole data during
geological due diligence studies. When drilling out a resource,
a selection of RC holes must be twinned with DD holes, as the
soft and low density graphite may be lost as fine dust from RC
samples and cause an assay bias.
Bulk density is a measure of mass per unit volume of rock.
In the mining industry, this is generally referred to as metric
tonnes per cubic metre, or pounds per cubic foot.
Graphite resources are typically modelled as volumes in
three-dimensional space, after which the estimated volume is
converted to mass using density values. Density can be expected
to vary across a graphite deposit, from low density weathered
mineralisation near surface through a denser transitional zone
and finally into the densest fresh (un-weathered) rock.
Determining bulk density from small samples is something
often faced by geologists, who might only have drill core
samples to use for density measurement. There are several
methods available to determine volume of a sample, including
water displacement (the "Archimedes" method) or the caliper
method, in which core diameter and length are measured and from
which the volume can be derived.
Highly weathered drill core presents a challenge, as it may
not be possible to remove the core intact from the core tray
for volume measurement. In this case, an entire tray with core
can be weighed, from which the weight of an empty tray is
subtracted to give the weight of core. The core volume is then
determined using the caliper method.
Assaying for graphitic carbon
Carbon may be present in rocks in several different forms,
including organic carbon, carbonates or graphitic carbon.
Depending on the method used, carbon in rocks may be reported
as total carbon (organic carbon + carbon in carbonate minerals
+ carbon as graphite) or as total graphitic carbon (TGC) (total
carbon – (organic + carbonate carbon)).
The simplest way to analyse a sample for graphite is by loss
on ignition (LOI), in which case a sample is heated to
1,000oC and the graphite content is determined as
the percentage weight loss. However, other minerals
– such as calcite that contain carbon dioxide
(CO2), or clay and mica that contain structural
water – will contribute to weight loss, resulting in
apparently higher graphite content than anticipated (Table
Therefore, when TGC is reported, organic carbon and carbon
in carbonate minerals such as calcite should be removed before
assaying TGC. Different laboratories use different
procedures for measuring graphitic carbon, which is the reason
for inserting standards into the sample stream and also for
sending a set of samples to an external, or umpire
Samples may be analysed for graphitic carbon and other
elements, in addition to examining thin sections under a
petrographic microscope. Petrographic examination of polished
thin sections (Figure 6) using an optical microscope
is a relatively affordable and quick way of estimating the in
situ graphite flake size distribution and likely liberation
Polarised-light microscopy is usually complemented by
methods such as X-ray diffraction (XRD), QEMSCAN (quantitative
evaluation of minerals by scanning electron microscopy) and
mineral liberation analyser (MLA, or automated SEM).
Mineralogical examination helps with geometallurgical
domaining of graphite deposits and selection of composites for
metallurgical testing. Sulphide minerals such as pyrite
are common impurities in graphite deposits. Thin section
petrography can help define areas or specific lithologies where
sulphides are interleaved within graphite flakes and therefore
may be difficult to liberate (Figure 7).
Graphite deposits may be weathered near surface, in which
case sulphide minerals may be replaced by sulphates, or
silicate minerals such as sillimanite may be replaced by
kaolinite. The volume increase brought about during the
kaolinisation process may cause graphite flakes to split
Assaying for graphitic carbon quantifies the amount of
graphite contained within a deposit, but does not indicate the
amount of graphite that may be recoverable; the purity of such
graphite; the particle size distribution of recovered
(liberated) graphite; the process required to liberate and
produce a graphite concentrate; or likely markets for that
Therefore, it is essential to test representative samples of
mineralisation from a deposit to confirm appropriate
metallurgical processes and likely product mix. Samples should
be taken to a specialist laboratory, which would typically run
mineralogical, crushing, assay by size and other
characterisation tests before embarking on flotation or gravity
tests (Figure 9).
Mineral resource estimation
The next step in the exploration process is to estimate a
mineral resource, which is usually done after importing a
validated geological and assay database into 3D modelling
software. During this process, the resource geologist (someone
specialised in modelling resources) models the limits of
mineralisation taking into account structural controls such as
folding or faulting, spatial distribution of graphite grades,
lithological variations or other attributes. The model should
be domained where possible according to grade (graphite
content), flake size, lithology and weathering which can all
have an impact on mining and processing methods.
The final outcome should be a block model (Figure
10) from which a mineral resource may be estimated; this
is typically reported in terms of tonnes and grade (%
Mineral resource classification
Publicly-listed companies should report mineral resources
(and reserves) according to accepted codes such as JORC
(Australia); SAMREC (South Africa); NI 43-101 (Canada); SME
Guide (US); PERC (Europe); NAEN (Russia); or CRIRSCO
The different categories of mineral resource classification
reflect increasing geological confidence (Table 2) and
the link to economic viability and the importance of continuity
of both geology and grade (or product quality) as is
emphasised, for example, in the 2012 edition of the JORC
Reporting of industrial mineral
Industrial mineral resources such as graphite should be
reported in terms of product specifications, as noted by the
JORC Code (2012) which requires that industrial mineral
resources or reserves must be reported in terms of mineral
The code states that: "For minerals that are defined by a
specification, the mineral resource or ore reserve estimation
must be reported in terms of the mineral or minerals on which
the project is to be based and must include the specification
of those minerals."
Exploration for graphite is likely to follow a similar track
to exploration for other minerals, for example:
• Discovery of a mineralised outcrop
• Field mapping and trenching
• Geophysical survey
• Drilling and assaying for graphite content
• Mineralogical and metallurgical testing
Successful exploration following the above steps should
result in the definition of a mineral resource, which may be
classified according to geological confidence and must take
account of product specifications and markets.
*Dr Andrew Scogings is a consultant for
Industrial Minerals and principal
consultant at CSA Global, based in Perth, Australia.
Mitchell, CJ, 1993. Flake Graphite. Industrial Minerals
Laboratory Manual. British Geological Survey, Technical Report
Scogings, AJ and Coombes, J (2014). Quality Control and
Public Reporting in Industrial Minerals. Industrial Minerals
Magazine, September 2014, 50-54.
Scogings, AJ (2015). Bulk Density: neglected but essential.
Industrial Minerals Magazine, April 2015, 60-62.
Scogings, AJ, Hughes, E., Salwan, S and Li, A, 2015. Natural
graphite report. Strategic outlook to 2020. Industrial Minerals
Research, October 2015.