Orex - Sandra Escobar - Metallurgical Testing

Orex have announced that they will be conducting some Metallurgical testing on the Sandra Escobar project.

Link to the PR here

Some points that I've noticed:
They still go on about it being a flat lying disseminated deposit. I disagree with this, as you have read in early posts, I feel that it is horizontal, low grade disseminated halo around vertical, high grade veins/stock work.


Here is a photo from Sandra Escobar the La Posta Stock work

Not many horizontal veins!

However, in the PR they give us 2 important pieces of information

1. The different metallurgical sample grades (50, 100, 150, 200, 250g/t Ag)
2. Different grind sizes (i.e. the sizes of the particles being tested - 75, 106 and 250 micron).

Sample grades

This is good, I explained before that a common trick is to just send high grade ore for testing, and this often skews the results as high grade ore is typically significantly different from low grade ore. Here are some photos from Sandra to demonstrate:

They look different, and they will have different metallurgical properties.

So reading between the lines:

• 50 g/t Ag - this will probably be the lower cut-off used in the upcoming resource calculations
• 250 g/t Ag - this material will probably be dominated by veining or disseminated mineralisation very close to the veining, so will have different metallurgical properties.

Orex are essentially testing all logical grades, but the critical results will be from the 50 g/t and 100 g/t samples as these areas will form the bulk (>50%) of the deposit. If they get crap recovery from the low grade zones, then there is no deposit here, time to move on.

Grind sizes.

General rule of thumb:

• coarse grind (e.g. 150 micron) - poor recovery due to poor liberation (i.e. unable to recover the very fine grains and encapsulated mineralisation).
• Fine grind - good recovery, but more expensive grinding costs, lower mill throughput

More simply:

From Runge et al (2013).

The finer the grind, the higher the cost

If you look at the sizes selected by Orex (150, 106 and 75 microns) that immediately tells us that the silver cannot be recovered by a heap leaching (particles sizes from 12-100mm), they need to crush and grind the ore to recover the silver.



My guess is:

• low recoveries (around 55%) from the 50 g/t zone
• good recoveries (around 75-80%) from the 200 and 250 g/t zones

The results from this study will make or break Sandra Escobar. Will we get some BS with only the most favourable results being released (along with the headline - 80% recovery from Sandra ore), and carefully relegating any poor results to the bottom of the PR?

Time will tell