How to convert gold drill results into ounces
Terminology
The calculation of the
amount of gold in the ground (gig) is surprisingly simple.
The calculation involves determining the volume of earth (a
3-dimensional rectangle) that contains gold and multiplying it by the
density of the gold in that earth.
It’s that simple. The rectangle in the earth uses the
following terminology:
Notice
that the term “width” and “depth” are used in a manner contrary to
regular use. The reason for using “width” to indicate the extent of the
depth of a gold region is because the core samples that are pulled out
of the ground are laid on their side, and this (core) width is measured.
The core width is the depth.
The core width can vary, but 10 meters is not unreasonable.
To add to the equation, the core sample length is referred to in
context as core depth.
The
Strike (or Length) and Depth indicate the extent of the gold region in
terms of area. They can both
be on the order of hundreds of meters or multiple kilometers.
Determination of gold region
How do gold mining
companies determine the Strike, Depth and Width?
They drill holes into the ground and measure the density of gold
in their drill samples. The
density of gold is in units of grams per tonne (one tonne = 1000 kg).
This density is known as the gold grade.
The term in the periodic table that is used for gold is “Au”.
You may see “Au g/t” which is grams of gold per tonne.
What
can happen is that the core sample can be drilled at an angle.
When this is corrected to the horizontal, this core width becomes
a “true width”. For example,
assume that a core sample was drilled at an angle from zenith (a dip
angle) of 75°. Let’s assume
that the length of this drill sample (a long tube of dirt is extracted;
this is called a core sample) is 120 meters long and the gold was found
to start at a core depth of 85 meters and end at a core depth of 92
meters. This is shown
diagrammatically below.
Because the core sample
was taken at a dip angle of 75°, its length needs to be corrected to get
the true depth in the ground (and recover the true core width).
This means that the core width from 85m to 92m (a 7 meter length)
is really 7 meters multiplied by sin (75°) which is 6.34 meters.
Because there are typically several drill samples taken throughout a
gold region, it is likely that the samples will experience not only
different gold density measurements, but multiple gold regions within
each sample. This complexity
is managed by calculating a weighted average of the gold grade and true
core widths across all drill samples, as shown below.
How does the calculation work?
The
Strike length (m) and Depth
(m) in units of meters must be entered by the user.
From this, the Volume (m3) field can be calculated once the
Average width (m) field is
calculated (this is explained later).
The Volume (m3)
field is calculated by:
[Volume (m3)] = [Strike length (m)] X [Depth (m)]
The
volume in cubic meters is the volume of all the rock and soil that
contains gold to be mined (or potentially mined).
The
Specific gravity (t/m3) is a field that the user must
enter. Typically, this is
2.76 tonnes per m3 (a typical value for quartz-rich mineral
rock).
Once
the Volume (m3) has been calculated and the
Specific gravity (t/m3)
has been entered by the user, the
Tonnage (t) can be calculated.
The Tonnage (t) field
is calculated by:
[Tonnage (t)] = [Volume (m3)] X [Specific gravity (t/m3)]
The
tonnage in tonnes is the total mass of all the rock and soil that
contains gold to be mined (or potentially mined).
For
each drill hole result (1-10) the user can enter a
Drill hole name (column 1).
This is not used in the calculation, but it is handy for the
operator to keep track of drill hole numbers.
The
user is expected to fill in both the width and grade only in metric
(columns 2 and 3) or only those units in Imperial units (columns 5 and
6) for a given row.
The
Width (m) field is the true width of the core sample.
The
Grade (g/tonne) is the density of the gold found in the given core
sample in grams per tonne.
The user is expected to enter this value.
The
Grade X width field is the grade in grams per tonne multiplied by
the core sample width in meters.
This gives units of gram-meters per tonne.
The calculation of Grade X
width is shown below:
[Grade X width]
= [Width / length (m)] X [Grade
(g/tonne)]
For
the imperial units columns, the calculation is essentially identical.
The
Width / length (ft) field is the true width of the core sample.
The user is expected to enter this value.
One meter is approximately 3.28084 feet.
The
Grade (oz/ton) is the density of the gold found in the given core
sample in grams per tonne.
The user is expected to enter this value.
Note that a unit of oz refers to troy ounces (used in the
measurement of precious metals).
One troy ounce is 480 grains; a normal (avoirdupois) ounce is
437.5 grains. One troy ounce
is approximately 31.1034 grams.
A ton is 2000 lbs.
One pound of mass is approximately 2.20462 kg.
The
Grade X width field is the grade in troy ounces per ton multiplied
by the core sample width in feet .
This gives units of troy ounce-feet per ton.
The calculation of Grade X
width is shown below:
[Grade X width]
= [Width / length (ft)] X [Grade
(oz/ton)]
Let’s
assume that the user has entered all values in metric units.
The
Average width (m) field is calculated by summing all the
Width (m) values and dividing
it by the number of rows that have values.
The
(weighted) Average Au grade (Au g/t) is the sum of the entire
Grade X width values in
gram-meters/tonne divided by the sum of all the
Width (m) values.
The
Gold contained (oz) field is calculated by taking the average gold
grade in grams per tonne and multiplying it by the tonnage in tonnes,
and converting the grams-value to troy ounces:
[Gold contained (oz)] = [Average Au grade (Au g/t)] X
[Tonnage (t)] / [31.1034 grams per troy ounce]
The
Gold price $(US)/oz is a field required to be entered by the user.
It is the price of gold per troy ounce in US dollars.
It could be the spot gold price (the current price gold is
trading at) or a 3-year average price, or for that matter any price the
user wishes to see used in the calculations.
The
M&I value $(US) is calculated using the following:
[M&I value $(US)] = 0.22 X [Gold price $(US)/oz] X [Gold contained (oz)]
The
Inferred value $(US) is calculated using the following:
[Inferred value $(US)] = 0.055 X [Gold price
$(US)/oz] X [Gold contained (oz)]
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