Copper is crucial for decarbonisation, yet we’ve been underinvesting for over a decade.
To be copper-ready we must step up.
Here's the skinny on porphyry deposits to help you take advantage of this lifetime opportunity.
Today I'll cover some copper porphyry basics including:
• must & nice to haves
• mining methods
• size and mineralisation
• minerals present
Let's dive in.
Porphyry copper deposits are:
• low to medium in grade
• host copper, gold & molybdenum
• hydrothermal deposits related to igneous intrusions
• dominant mineralisation is Cu, Mo sulphide in fractures, stockworks, veins and disseminations
Let's break it down.
Porphyries are the world's major source of copper and molybdenum and a significant source of gold.
Found around the world but most deposits are in the circum-pacific belt, also known as the 'ring of fire' due to its volcanic activity (makes sense, huh?).
Each deposit type has an associated grade and tonnage range that can be economic, i.e. mined at a profit.
Typical grades for porphyry* are:
• Copper: 0.25-1%
• Gold: 0.1-1 g/t
• Molybdenum: 0.01-0.1%
• Silver: 1-3g/t
*may also host tin & tungsten.
On the other hand, the ore tonnage range for porphyries is 10-1000MT, with a typical ~100MT.
Larger examples are also known.
Do not confuse tonnage with resources or reserves.
There are some examples in the range in this list, including @resolutioncu
A worthy porphyry:
• grades in range
• intersections start near surface ~100m like $ABRA.V 📊
• hundreds m in all directions
• tens to hundreds of m in length
• open at depth
• large zones of mineralisation at 200-400m
• 10-50m zones at top of grade range
Given their low grade and large size, porphyry deposits are mined via open pit.
Like Kennecott, also known as $RIO's Bingham Canyon mine in Utah.
A few use block caving, which despite being an underground method permits high tonnage and low-cost production.
The ideal deposit is oval or spherical in form, while some are pipe-shaped.
Can extend hundreds to thousands of m in each direction.
Minerals are disseminated within host rock or in fractures, veins and breccia.
A common style of mineralisation is stockwork.
Dominant economic minerals are sulphides including chalcopyrite, bornite, chalcocite and molybdenite.
Gold is found in its native state and in electrum (natural alloy of gold + silver).
In deposits with a supergene zone, copper oxide minerals and native copper may be present.
Thought to originate from hydrothermal fluids on top of a cooling magma at depths between 1-5km.
Hot fluids move upwards.
Minerals crystallise out of fluids mainly as a result of cooling.
Cause a fracture network and the characteristic stockwork texture.
📊 Stephen Marshak
Considerable gold or silver-gold deposits are formed on top of porphyries.
CEOs often say they are looking to hit the porphyry.
Even if it's too deep to target directly, its genesis helps predict how it may present and where.
Predicting is essential when you can't see!
Among producing and closed copper mines we can mention:
• El Teniente
• Alumbrera (c)
• Bingham Canyon
For other advanced projects in the pipeline see my thread
And that's a wrap!
By digesting these concepts you can more confidently allocate capital to copper explorers.
My absolute favourite is deposit formation. Think like a geologist but with an investor's perspective.
Keen to dig deeper? Visit my blog:
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#GeologyForInvestors #MiningForInvestors #ExplorationForInvestors
Originally tweeted by Paola Rojas 🐝 (@paola_rojas) on September 19, 2022.