Compare the grades of the metallurgical samples against the average grade of the deposit.
- Typically: high grade material = higher recoveries.
- Met samples are different (i.e. do not represent) from majority of the ore.
Companies will often just send high grade material for metallurgical testing, which will have different mineralisation styles, ore types and minerals causing it to have different metallurgical characteristics and recoveries when compared to the 'average ore' in a deposit
Example 1: Azure Minerals – Mesa de Plata Project, México.
Recently announced 70% recovery of silver by leaching from a disseminated silver deposit (link).
Diging into the PR we can see that they submitted 2 samples for testing:
- A master composite at 138 g/t Ag.
- A high grade composite: at 670 g/t Ag ( ~5 times more)
We can also see that the Master composite contains significant Arsenic (As). Will it go into the concentrate? This is important as many smelters will charge penalties on, or even not accept, concentrates containing significant amounts of arsenic.
- Master: Leaching = 50%, Flotation = 51-55%:
- High grade: Leaching = 70%; Flotation = 67-72%
- Using bad maths:
- Average deposit grade = 138 g/t Ag
- Using 51-55% recovery - only 69-76 g/t Ag is recovered, the rest is lost.
- @ $17/oz Ag the recoverable silver value at Mesa de Plata is $37.7-41.4/tonne
But here are some questions:
- At current metal prices, will Mesa de Plata the deposit be economic with these recoveries?
- Does the Arsenic go into the Silver concentrates?
Example 2: Golden Mineral – El Quezar, Argentina
From their 43-101 on El Quevar, they have the following Resources.
A reasonable resources, but we can see that they have 2 different ore types (oxide and sulphide) and are looking to mine the deposit by open pit (bulk mining) and underground (selective) mining. They have conducted some metallurgical testing, and here is some info on the sample submitted:
All of the composite samples contain considerably more silver (300-650 g/t Ag) than the average grade of the deposit (140-150 g/t).
The recoveries look fine, flotation gives between 60-90% recovery and leaching a bit less, but it raises the following questions:
- What is the recovery for average ore from El Quevar
- How does recoveries differ from the oxide and sulphide mineralisation?
- What is the transition zone from the oxide into to the sulphide resources?
- is it a sharp or do you have a significant transitional zone where the metallurgy of the ore is more complicated (and recovery can be significantly lower)..
- what is the recovery in this transitional zone?
When I see reports like these, it sometimes appears that companies go out of their way to specifically test high grade ore, that only represents a very small fraction of the deposit.
I can understand the desire to portray your project as being as good as possible, and I have no issues with companies testing high grade ore as it will be different from the majority of the ore in a deposit, but why do many companies insist on ONLY testing high grade material?
If you don’t test the modal/average ore, you won’t know about any metallurgical/recovery issues that could impact >75% of the project’s resources.
I’ll ask you this question – how many mines announce that production was lower than expected due to poor metallurgical recoveries?