This ‘Drilling 101’ for mining investors will help you evaluate exploration results [Twitter thread unrolled]

Drilling is the most significant exploration method in mining.

But all drilling is not created equal.

Here’s your quick guide to drilling, to help you understand reports and news releases:

Let’s start with definitions.

Drilling is simultaneously a geological and a geochemical exploration tool, where rock is recovered from beneath the surface for examination and analysis by geologists and laboratories.

The goal is to intersect a mineralised body.

A mineralised body is simply the shape, and size where mineralisation exists at a location.

Other methods study these from surface:

• mapping
• sampling
• trenching
• geophysics

(I’ll cover each in future threads)

But drilling ‘gets to the core’ (yes, pun, but true).

There are 2 key considerations:

• Reason to drill: what is the explorer trying to achieve?
• Technique to be used: which method would be most cost-effective way to reach the above goal?

Both condition each other, as well as the specific terrain and other conditions.

Let’s start with goals.

They vary with the different stages of a mining project.

There are basically 3 objectives:

• Discovery drilling
• Expansion drilling
• Infill drilling

Let’s define each.

(We’ll go back to techniques in a second)

1) Discovery

Follows surface geological, geochemical and/or geophysical techniques, to test anomalies or mineralisation at depth.

Good results may indicate discovery of a new mineral deposit.

CAN garner significant market interest.

This leads to…

2) Expansion

Comes after discovery drilling, and it tests for expansion of a deposit at depth or along strike.

Good results indicate that the deposit is growing in size, or increasing in grade.

Both CAN create a positive buzz in the markets.

This leads to…

3) Infill

Comes at an advanced stage and aims to drill in between existing holes to better define a known deposit.

Usually used to upgrade a resource or to prepare for economic studies.

Normally, WON’T cause many market movements.

By the time a project reaches pre-feasibility, drilling takes up to 80-90% of the exploration budget.

So all these decisions are impactful.

Now, how do you decide which technique to use?

By a combination of terrain, goals to achieve & availability of capital.

Main techniques are:

• Air core or AC
• Reverse circulation or RC
• Diamond drilling or DD 📷 @global_lithium

Each step increases in costs and capabilities.

Traditionally, contractors such as @MajorDrilling drill but some explorers do directly.

Let’s break each down (!)


Uses 3-bladed steel or tungsten drill bits to bore holes into unconsolidated ground.

Commonly used in first-pass exploration drill programs to test targets, as it’s the cheapest of these.

Depth of up to 300m.

Very common in Australia, not much in Latam.


A rotating bit attached to the end of steel rods uses percussion to break the rock into small chips.

At surface a splitter separates samples and bags them, while a small portion is stored in boxes, with markings showing depth.

More expensive than AC and can reach 500m.

C) Diamond

A hollow drill bit impregnated with industrial diamonds is attached to the end of steel rods. Rotation forces it to go down into the rock, producing a rock cylinder that geologists can describe & analyse.

The most expensive yet the most used.

Can exceed 1,500m

A geologist will inspect the core or chips at site, and work with field assistants to prepare samples in bags.

These are later sent for assaying at labs such as @ALS_Global .

And then we wait for results!

Fun fact: hole designation shows the type of drilling used.

DDH is Diamond Drill Hole, and so on.

And that’s your quick guide. Does it all make more sense now?

Here’s a primer from @StockheadAU if you want to keep reading.

If this was useful, please retweet the start of the thread.

Follow me @paola_rojas for more on metals, markets and tech.

#GeologyForInvestors #MiningForInvestors

Originally tweeted by Paola Rojas 🐝 (@paola_rojas) on July 30, 2022.

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