Here’s a plain language guide explaining how miners ‘grade’ mineral discoveries
It’s always exciting when an ASX-listed explorer announces a “high-grade” — or even “bonanza grade” — mineral find.
The problem is not many people have any idea what defines a high-grade from a low-grade find, and ASX announcements rarely give an explanation.
This is the first in a series of “explainer” articles that aims to help investors nut out the complex workings of the ASX resources sector — starting with mineral gradings.
When an explorer makes a find, a number of factors influence whether it’s worth the cost of digging it up — such as how deep you have to dig to get to it, the consistency of the mineral grades and the size of the hits.
What is considered profitable to mine can also vary from one country to another depending on the type of geology you have to contend with. It also depends on the price of a commodity at the time.
“What investors want to look out for is whether the company claiming to have made a discovery has drilled enough holes into [an orebody] at the right spacing,” Jon Dugdale, the boss of graphite explorer Peninsula Mines (ASX:PSM) told Stockhead recently.
“You want to have multiple drill holes intersecting the orebody – you don’t want to have to rely on one, because that could mean anything.
“It could just mean a little patch of mineralisation that doesn’t go anywhere.”
Here’s a quick overview of what is generally considered high and low grade for some key commodities:
Precious metals are reported in grams per tonne (g/t).
“For an average gold orebody anything over 6g/t would be high grade, as a rule of thumb,” Mr Dugdale says.
Less than 1.5g/t is low grade for gold.
Silver (Ag) – less than 10g/t is low grade, over 50g/t is high grade;
Platinum (Pt) – less than 1g/t is low grade, over 2.5g/t is high grade;
Palladium (Pd) – less than 1.5g/t is low grade, over 5g/t is high grade.
Base metals, meanwhile, are reported as a percentage per tonne along with iron ore (a bulk commodity) and uranium.
“A high-grade nickel orebody would be plus 3 per cent per tonne, and a high-grade copper orebody – worth half as much as nickel – would be high grade at 6 per cent per tonne,” Mr Dugdale said.
“And a high-grade zinc orebody would be plus 12 per cent per tonne.”
For copper (Cu) less than 0.5 per cent is low grade, for nickel (Ni) less than 1 per cent is low grade and for zinc (Zn) less than 2.5 per cent is low grade.
Lead (Pb) – less than 2.5 per cent is low grade, over 10 per cent is high grade;
Iron ore (Fe) – less than 25 per cent is low grade, over 35 per cent is high grade;
Uranium (U) – less than 0.15 per cent is low grade, over 0.4 per cent is high grade.
While anything less than 1 per cent cobalt (Co) is low-grade, the high price of the battery metal at the moment is making large deposits with grades of as low as 0.1 per cent economic.
Grades of 0.3 per cent are currently considered very good with scale.
In general though, less than 1 per cent is low grade and over 2 per cent is high grade.
When it comes to some battery metals it is sometimes not just as simple as looking at the grade of the ore.
As is the case for vanadium.
The keys to determining a good vanadium deposit come down to the overall magnetite recovery and the grade of the vanadium you get from that magnetic concentrate.
Canada’s Largo Resources produces the highest-grade vanadium pentoxide (V2O5) in the world of 3-3.2 per cent from its Brazilian mine.
But it does that from a typical magnetite recovery of 30-35 per cent, which is considered pretty good.
The Rhovan mine in South Africa’s Bushveld Complex also has a 30-35 per cent magnetite recovery and it produces 1.6-1.8 per cent V2O5.
Meanwhile, the typical grades of a hard rock lithium mine range between 0.9 and 1.6 per cent lithium oxide, according to European Lithium (ASX:EUR), while the highest lithium concentration from brines is 0.07-0.16 per cent.