A lot of gold veins are hosted within shear zones (faults) that impart stress on the surrounding rocks, and generally, rocks respond to stress in one of two ways: they break, or they bend.
- When a rock breaks, it is called brittle deformation.
- Wide damage zone = wide vein
- When rocks bend or flow, like clay, it is called ductile deformation.
- Produces a narrow deformation zone and a narrow vein
Here is an example:
This is the amount of low (or zero) grade material that has to be mined with the ore. Occasionally veins are surrounded by a low grade halo, but this is more often the case:
Great grades in the white and grey banded vein, and virtually nothing in the rock below. Using this example:
- Using a 1.2m mining width - average grade is 34.2 g/t Au
- Over a 2m mining width - average grade drops to 20.5 g/t Au
You can see how applying a mining width can significantly decrease the grade from high grade structures decrease significantly, and that it is critical for mining operations to minimise the amount of low grade material being mined.
External Dilution - the material outside the ore zone that you have to mine with the ore.
|Low angle gold vein, similar angle as Curraghinalt?
It can be hard to selectively mine a narrow vein, so all of the surrounding waste (A and C) will need to be mined with the ore (B).
For very narrow ore-bodies, (0.5-2m wide), selective mining needs to be used as to avoid mining too much waste with the ore. Slim‐sized machines (drill rigs, jumbos, LHDs) are available to working in drifts as narrow as 2.0 m. However, in such narrow veins, the use of machines produces waste which dilutes the ore. The alternative is to use manual techniques to extract only the higher grade material in the vein, but require more high trained miners and generally manual techniques are not efficient for mining at a commercial scale.
This is the reason in resource reports companies use a term minimum mining width or composite widths in resource calculations.
Internal dilution - fragments/beds of low/zero grade material within the vein.
As you can see, vein angle of dip can have a huge impact on dilution, compare a vertical vein (below) with the shallowly dipping veins above.
|Hishikari Gold Mine, Japan
Vertical vein (Hishikari Gold Mine, Japan). Here we can see that the vein is the width of 3 Japanese geologists (1 sumo?), probably around 2m wide. You can see that vertical veins can be mined much more efficiently with minimal dilution compared to low angle veins.
We all love high grade drill-intercepts and samples, but gold is effected by the nugget effect, i.e. it isn’t evenly distributed in a deposits, it clumps together in grains, and therefore there can be significant variation in gold values over small scales (i.e. between samples and drill-holes).
|Core scale (length of drill core is approx 10cm) - Tyhee Clan drill-core (link)
|At the deposit scale
This is the reason that high grade samples are capped (often at 30 or 50 g/t Au or 500 or 1000 g/t Ag) in resource calculations, so to minimize the bias the highest grade samples will have on a deposit.
Core length vs true thickness.
Many drill intercepts reported in press releases will be core length, for example:
Some companies report core length (or thickness or interval), but also include a column called "true thickness".
What does this mean? For many exploration stage projects the companies do not know the orientation or dip of the mineralised zone, so don't know its true thickness, so can only provide the "length over which mineralisation was found" in drilling (the core length) in the table. However, a typical trick is to report interval length as it is always wider than the actual mineralized zone.
Here is an example:
Mineralisation is the white vein and the white lines are hypothetical drill-holes
- the Red interval represents the true thickness of the deposit as the holes was drill perpendicular (at 90 degrees) to the vein
- core interval = 60cm or 100% of the true vein thickness
- Yellow - intersected the vein at ~60 degrees
- core interval = 72cm or 120% of the true thickness
- Cyan - intersected the vein at ~45 degrees
- core interval = 126cm or 210% of the true thickness
- Green - intersected the vein at ~30 degrees
- core interval = 156cm or 260% of the true thickness
You can quickly see how intersections in drilling can be quickly multiplied. Rarely do you see companies deliberately trying to maximize their thickness of the drill intercepts, but many companies will refrain from providing true thicknesses even when they know and understand the orientation of mineralisation. You'll often see a catch all quote: