Saturday, March 25, 2017

Grade vs thickness

You often get companies reporting some really high grade assays from underground channel sampling or from drilling, but you need to check the true widths. We often see companies smearing out grade from narrow high-grade veins to bulk up their appearance, but another extreme is going for grade.

In this scenario, you report all the narrow high-grade zones to show that you have lots and lots of great hits so your assay table is full of nice bold and red text (remember - red = good)

Here I'm using the latest results (10 holes) from Orex Silver and the JV drilling at Coneto as an example (link). The drilling intersected multiple veins, including several previously unknown structures.

You can see that they highlight all the assays that are:
  • >1 g/t Au
  • >100 g/t Ag
Lets do some excelling

  • results from 59 samples reported
  • 19 samples >100 g/t AgEq
  • 11 samples >1g/t Au
  • 10 samples >100 g.t Ag
This is what you would expect to get from an initial phase of drilling on a narrow vein deposit 
Note: A total of 83 holes has been drilled at Coneto, so this isn't an early stage project.

We also can see that Orex have reported everything as we see some stunning results like:

  • 0.65m @ 0.01 g/t Au and 0 g/t Ag
  • 27m @ 0.02 g/t Au and 2 g/t Ag
  • 3.66m @ 0.03 g/t Au and 2 g/t Ag
Typically in press releases companies report a summary (or sub-set) of the results - for example assays containing >0.25 g/t Au or 100 g/t Ag or something similar. I'm not sure why Orex reported everything, it does show that they have multiple structures, but most are very narrow and low grade.

Why don't we have a closer look at the 'high-grade samples' (the ones grading >100 g/t AgEq)

some monsta hits
Look closely at the true thicknesses. As you would expect, the highest grade samples are typically the narrowest, but at Coneto they are very narrow:

  • 950 g/t AgEq over 10cm
    • that is the length of a flaccid penis
    • or for people without penises (or penii) - that is the width of your palm.
  • The average thickness for a sample containing >100 g/t Ag is 0.59m
  • If you calculate a AgEq grade over a 1.5m mining width (last column)
    • only 2 intercepts (highlighted in yellow) are >200 g/t AgEq cut-off grade for an underground operation.
Here it is graphically, you basically want the results to be in the top right corner (wide and high-grade). If they aren't, then it could be time to look elsewhere.

top right = good
So we see here that even though there are a reasonable number of high grade samples, the majority come from very narrow structures, and nothing is indicating the potential for thick-high grade intercepts.


  1. Super post! This doesn't look like a viable deposit but at least the company gives us the data to make that determination. Often we aren't provided true widths and sometimes a company will even try to drill parallel to the plane of mineralization in an attempt to fool the market.

  2. Hello Tom,
    Orex have been very accommodating by providing assays for all the structures intercepted by this drilling so you can pull it apart and do some simple things like checking to see if the grades are economic over mining widths.

    Fortunately, Fresnillo are spending the money to explore this project, so it will be interesting to see if they continue onto a 6th phase of drilling.

    It will be interesting to see what you think - there are multiple veins, and several have been well tested (there have been 83 holes drilled to date), when do you start to draw the line, call it a day and go home?

    1. I have to believe Fresnillo are seeing some teases in the results like areas that may be swelling or showing classic quartz textures, multiple pulses of mineralization or good vertical longevity suggesting intense, telescoped boiling points, etc etc etc. I suspect deciding when to give up is more art than science.