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Monday, July 8, 2019


Sorry for the lack of posts, I've been working on a large project that has been consuming all of my time.

It is now complete-ish, so I have some time to waste on this blog!

Buritica - Narrow veins and potential issues

Continental Gold have been releasing some juicy assay data from Buritic√°, here is an example from last month (link).

I hate companies that report strike length samples, to me they are trying to deceive you into misreading the table and believing that they have wide zones of gold mineralization:
  • e.g. 31m @ 24.09 g/t Au and 76.22 g/t Ag at 2.26m width
translates to:
  • A series of samples taken from a vein where a 31m long portion averaged 24.09 g/t Au and 7.66 g/t Ag over an average width of 2.26m 

However, what interested me most about this PR were the maps and this statement:

In Summary:
  • No capping
  • Weighted averages
  • No dilution
The issue with no using capping and using a weighted average is that a few high-grade samples can increase the overall average of the mineralized zone lead to companies calculating resource blocks with higher grades than actually occur. This is especially problematic this methodology continues through to mine development (this often happens when companies do resource calculations internally), when they get into a situation where reserve/resource grades >> mined grades (e.g. several mines operated by First Majestic).

I’m using the maps from the March 9th PR (link) as they give a breakdown for each channel sample compared to just a series of colored rectangles used in later PRs.

We’ll focus on the areas YR_1444_CX05 - Stope Areas 1 and 2 – I’ve added a grade legend as the one on the original maps is quite hard to read.

We know that Continental plan to mine all of this vein in this area as they have them shown as potential stopes, but if you look closely at this map you see 2 things:

1. Grade

The average weighted, uncapped grade for this area = 16.95 g/t Au, or red according to the grade legend, so:
  • How many samples are red (or better) from this section?
    • 7 out of 26 or 27% assayed >10 g/t Au or
    • 73% of the channel samples assayed less than 10 g/t Au
In high-grade gold mine you get a lot of variability, but it would be reasonable to expect that the modal value would be close to the reported average grade of this area? Let us look closer at the map.

At the far west end of the “2” stope area, we see an ultra-high-grade sample (cyan rectangle) that is grading 2.26m @ 119.6 g/t Au, what would happen to the average grade for the entire block if we remove this sample?
    • Grade drops from 16.95 g/t Au to 9.11 g/t Au, or by 46%
Typically mining companies cap ultra-high-grade samples to avoid this issue, where a couple of high-grade samples can significantly increase the overall grade of a zone, we'll do the same here:
  • Uncapped – 2.23m @ 16.95 g/t Au over a 78m strike length
  • 50 g/t Au Cap – 2.23m @ 11.08 g/t Au – a 35% reduction
  • 30 g/t Au Cap – 2.23m @ 9.72 g/t Au – a 43% reduction
A ‘traditional’ way that resources were calculated in underground mines (some mines still use this method (link)), is that they extended the mineralization above and below the level by a distance equal to half-strike length, so for this area we can guesstimate the contained gold for the 2 stopes and see the impact of applying a cap:

  • Strike length = 78m
  • Vertical extent = 78m (39m above and 39m below the level)
  • Average width = 2.23m
  • Tonnage factor/specific gravity = 3.13 t/m3 (from 2019 technical report – table 11.1)

Guesstimate contained gold:
  • Uncapped = 21,150 oz
  • 50 g/t Au cap = contained ounces drop to 15,128 oz
  • 30 g/t Au Cap = Au decreases to 12,273 oz Au
Just applying a small cap, we've reduced the average grade and therefore the contained gold by a lot.

You can clearly see the impact that a few high-grade samples can have! This is why companies apply a cap, it makes the results less sexy, but is important, as if you don't, you can start to include a lot of waste with the ore and it works it way into the mine plan, which looks like it is happening here as we can see that the limits of the stopes (the black lines) include everything, including marginal (yellow) and waste (green, grey and white) portions of the vein.

Essentially, based on the channel samples gold grade distribution in the stope is more like this:

We have some narrow high-grade zones (red), a large waste zone in the middle (green) and the majority of the block is low grade (orange) or marginal (yellow), which is <<16.95 g/t Au as originally advertised in the PR.

Why is this an issue? We can see that using a weighted average can skew the grade significantly (in this case by up to 40%), and in several mines that I know, they continue to use this 'traditional method' to define resources and zones to be mined.

A couple of high-grade samples/drill-holes can quickly add a chunk of mathematical resources that are have significantly higher grades than reality leading to large ares of sub-economic or marginal blocks being included in the resources and eventually mined.

This results in lower than expected head grades, lower gold production, lower revenues, higher per ounce production costs and reduced profits.

These are all things that shareholders love.

How many mines do you know where grades haven't met expectations or feasibility studies?

2. Vein widths

One thing that I found amusing was that all the samples thicknesses range from 2.06 to 2.27m. Remarkably consistent!

However, when you look at the level plans, you see:

That the vein is represented by a very thin red line, that is significantly narrower than the channel sample rectangles. That is the vein...

Here is a photo from the Fire Creek Mine in Nevada. 

See how they are not just sampling the vein, but the surrounding country rock (this is standard practice, you want to know the grade of everything you will be mining). In this photo the vein is ~90cm wide, and we can see that they are sampling ~1.5m either side of the vein, or a total channel sample width of ~3m. We can also see that a separate sample was taken from the small vein on the left. This is standard sampling practice.

So, if Continental are doing something similar, we would expect that they are sampling the vein, and 1m either side, which indicates that the vein is actually 5-27cm wide, which is similar to the drilled vein thicknesses (link).

High-grade but narrow, with a bit of gold in the country rock around the vein

Why is this an issue? Well in narrow vein mines, controlling dilution is critical, if you have excessive dilution, you will have:
  • difficulty in maintaining a consistent head-grade
  • difficulty in using modern mining equipment
  • low production rates
You can use highly selective mining methods (e.g. Resue (link)), but they have low production rates. You can increase production by either using mechanized methods (increase dilution), or extracting ore from many heading (increased labor costs)

If you have a large mill, you need to mine a lot of material and therefore you can't be very selective be very selective. At Buritica, the mining width (based on the width of the mine level) appears to be ~2.5m, whereas the the vein widths (in this section of the mine)  around 0.25m, so you already have massive dilution.

Minor changes in thickness of the vein or the inability to minimize mining widths and therefore minimize dilution can cascade through an operation, significantly impacting mine production and profitability.

How many mines have you read about that have had issues with grade control, or throughput?

We've see with Pretium at Brucejack where initial resource and reserve calculations appear to have struggled with accurately re-conciliating head-grades to estimated reserve/resource grades, that led to them implement a new comprehensive grade control program (with associated costs) to try and better understand the gold distribution.

I'm not saying that the same is happening at Buritic√°, but history rhymes, we have a situation where a company wants to mine a series of narrow, high-grade veins and some of the PRs (obviosuly promotional in the way that the data is shows) appears to suggest that several areas could have significantly different grades as thought.

As we have seen at Brucejack (and several other underground mines), there is a desire to define as large a resource as possible, and as quickly as possible, which can lead to the situation where mine plans, scale of mining, are being advanced before the true nature of gold distribution in the veins is properly understood.

Monday, May 6, 2019

Chakana Copper

We've been getting a few eye-watering drill results from Chakana Copper (link and link) from their various breccia pipes, which has led to them snagging Goldfields into pumping a chunk of change into the company (link).

I've been casually following them and Soledad and I've always though that the breccia pipes are very small, and if there is a porphyry on the project, it would be too deep to do anything with.

So, I've finally managed to compiling the data into leapfrog (link).

We can see that Chakana have drilled 25Km of drilling but appear to have 4 of the ~27 breccias identified so far on the project.

Breccias everywhere
If we look a bit closer at the pipes....

Essentially the breccia pipes are very, very small, typically with a 50m x 50m surface expression, based on the 5 breccias that we have maps for.


This has received the most drilling and has produced the best results.

We can see that lots and lots of holes have been drilled down the guts of the BX1. However, unlike BX5 and 6 below, a few of the holes intersected a blind breccia body to the SW.

It also appears the the grades are decreasing with depth, but this may be a result of the holes leaving the breccias and drilling country rocks, but we can clearly see, the breccia body is small (75m x 25m with 300m vertical extent - of the >2% CuEq mineralization), but high-grade.


More drilling down the guts of the system, it is 50m x 50m, and has been drilled to 200m depth. It is lower grade than BX1, but consistently over 1% Cu. However, I'm not sure that this is good enough for a body that will be mined from underground, it may need to be >2% to be viable.


BX6 is another small breccia body, with more drilling down the middle. We have a high-grade (>3% CuEq) close to surface (supergene enrichment), before the hole pass into moderate grade breccias below. Nothing special here

I'm going to stick with my original opinions, even though BX1 shows that there are additional breccias that don't reach surface, I'm not convinced that Chakana will be able to define a significant resource at the project. They have enough drilling into the BX1, BX5 and BX6 breccias to give the market something, but my back of the envelope notes come up with ~2Mt, which isn't very much.

I can understand them flogging the porphyry potential. This is a valid model, but on their own sections they suggest that any porphyry body will be very deep and essentially unexplorable.

However, they have money from Goldfields, and would like them to move away from BX1 and drill a few holes into 5-10 different breccias to see they have other high-grade breccias similar to BX1 (e.g. Faro and Corral in the Western Breccia zone).

Unless they find something amazing, and demonstrate that they can find enough breccias to define ~5Mt of material grading >2% CuEq, I don't see much value in the project on its own. However, with Hercules next door, could there be potential synergies with Lincuna? If the breccia bodies could be mined profitably from underground, maybe Lincuna would like some highly profitable ore to process (as long as it isn't too expensive to add a copper circuit to their mill)?

Saturday, April 13, 2019

Pretium - Lies, damn lies and Reserves

I've been bashing Brucejack frequently over the 'difference' been mine grade and reserve grade, and with the release of the update resource and resource statement, I decided to look at how the reserves and resources have changed overtime (just for the VoK):

Note: I've added dashed lines to the charts where data is missing (the Dec 2016 resource press release didn't include information for M&I and inferred resources).


We can see that that we had an initial increase in resources from 2011 to 2013 and then a leveling off as drilling focused on improving resource quality and converting resources to reserves. We see that recently drilling has also focused on increasing resources.

Contained Ounces

Ounces have stayed relatively flt over time, we had very large growth in resources from 2011 to mid-2012, then the focus has been on improving the quality of the resources (from Inferred to Measured and indicated) and to reserves in 2016.

Average Grade

I've added a lines for the Life of mine (LOM) mine grades in the 2014 Feasibility Study (FS - orange line) and the new 2019 proposed mine grade (purple line), and one for the 2018 actual mine head grade (grey dashed line).

  • Average grade (all resources) decreased from 22 g/t Au (2012) to 15.8 g/t Au (2019)
  • M&I resources remained constant since 2012 at ~17.1 g/t Au
  • Inferred resources decreased from 35 g/t Au (2012) to 17.7 g/t Au (2019)
  • Reserve grade decreased from 16.1 g/t Au to 13.8 g/t Au
Obviously the reserve and resource grades have dropped because of all the high-grade mineralization mined in 2018......

You will see that in many 43-101 reports the Inferred resources typically higher grade than the other resource categories, they are normally based on the lowest quality data (relatively speaking), the widest spaced drill-holes and can be heavily skewed by a few ultra high-grade holes. However, it is surprising that the grade of the inferred resources have fallen by such a large amount.

This figure was also interesting in the press release:

There was a lot of technical crap in the section accompanying this table, but in I think it means they mined 20% more material than planned, now this will be interesting to see going forward, especially with the mine being expanded to 3800 tonnes per day.

Brucejack produces a lot of gold, but it keeps falling short.

Data for these charts came from:

Feb 2011 resources - link

Nov 2011 resources - link

Apr 2012 resources - link

Sept 2012 resources - link

Nov 2012 resources - link

Dec 2013 resources - link

July 2016 resources - link

Dec 2016 reserves - link

Apr 2019 reserves and resources  - link

Friday, April 12, 2019

Garibaldi - The final cuts!!!!


Nothing special, nothing new, but after 4 months of waiting, Garibaldi have manged to publish the results from the final drill holes at Nickel mountain (link).

We get a smattering of regional exploration holes (all hit nothing), some hole exploring the new (i.e. not found by Silver Standard in the 1960s) and some nice promo-BS holes drilling down the guts of the NE and NW zones, which, I'm guessing weren't as good as expected and would have had no impact on the releasing the results.....

I'm only going to focus on the holes that were drilled in the NW and NE zones. A few holes have been drilled regionally, but they hit nothing, so we can forget about them. However, it would have been nice to have known what the holes were targeting (and maybe which geophysical anomaly), but that isn't the way that GGI operate.

You can find my Leafrog model here and I've also incorporated the surface samples form the Crevasse massive sulfide (link).

Northwest Zone - Explored by holes 36, 37 and 46.

Hole 36 
Drilled to explore for the southern continuation of the NW zone, it intersected a 42 cm zone of massive sulfides just 17m south of the DDH_05 drilled by Silver Standard in the 1960s that intersected 9.5m @ 2% Ni.

Looking small

Hole 37
Vertical hole drilled 22m south of hole EL-17-12 (18m @ 1.15% Ni and 1.09% Cu), that hit a couple of low grade zones.

Thick intercepts of nothing

Hole 46
Unsurprisingly with the poor results from holes 36 and 37, the final hole in the area (and the 2018 program) was a nice BS-hole, designed to drill right through the guts of the mineralization to get as long an intercept as possible.

Should have made the hole a bit steeper

The irony is, the hole did hit a wide zone of <1% Ni disseminated mineralization, but no high-grade and actually returned lower grades compared to adjacent holes (EL-17-05, 17-06,  and DDH_05).

Nice one guys!

NE and Crevasse Zone - explored by 5 holes.

I'm grouping these zones together as they all appear to be part of the same system.

We see that the massive sulfides are found at the contact between the E&L intrusion (purple) and the Hazelton sediments (lime green), and by some amazing fluke (GGI would never have deliberately designed them to be to be BS-promo holes) all 5 holes have been accidentally collared right in top of the of the mapped massive sulfides.

Unsurprisingly, all holes hit mineralization at surface, with each hole hitting a narrow (up to 1.5m core length) massive sulfide zone with minor (up to 18m) disseminated mineralization.

Should have drilled a metric-fuck tonne of 50m holes and got lots of intercepts

With nearly 40 holes drilled into an area that measures 250m by 125m, I am keenly waiting for GGI to commence a resource calculation for Nickel Mountain. I'm sure that they have enough information.

Joking aside, basically we see nothing new, the mineralization is restricted to small zones, there are areas where it can be expanded, but they are limited (and not everywhere as shown on the plan map accompanying the press release), but you need to remember:

Mineralization needs E&L Intrusion + Hazelton Sediments
Missing either = no mineralization

Why is this important - if GGI haven't rocks similar to the E&L Intrusion elsewhere on the property, then potential to discover additional mineralization is essentially zero.

Note - for the pedants

Wednesday, March 27, 2019

Some Sugar on you Porphyry, Sir?

Ahhh, mi viejo amigo El Cobre.

The Poliquins have been releasing some juicy results from El Cobre over the last few weeks (link and link)

Again, all we see is that they are drilling and re-drilling the same zones, it is now starting to get silly.

To the surprise of no-one, they got similar results to the previous dozen or so holes drilled into the same area. We do see that holes 65 and 70 are exploring the eastern and western limits to mineralization, but there has been plenty of holes poked into those areas as well.

I like the way that they drilled a nice shallow hole to make sure that they reamed the high-grade core rather than drilling below it and potentially adding some tonnes of hypothetical resources.

If we go back to the Feb release, where they did go and drill some of the other targets, all we see is Azucar drilling the known mineralization from a different angle, and getting nothing that is really any good.

Generally, crap, narrow (for a porphyry) low grade mineralization. No-one in the porphyry world is going to get an erection over 0.2 g/t Au and 0.2% Cu (except for ValOro’s Tepal project where that would be high-grade).

Azucar, you’ve put ~40+ holes into El Cobre, a resource calculation is due, all you are doing now is dicking everyone around. All you are doing is demonstrating that you have a small moderate grade porphyry that isn’t going anywhere. The regional targets are generally crap, and that you re demonstrating you lack of imagination by drilling the same areas time and time and time again.
What a waste.

One thing I hate in exploration is waste. Either wasting time (just by sitting on projects and doing nothing with them) or wasting money. 

Azucar are wasting money, they could have easily reduced the amount of drilling at El Cobre by at least 50% (and probably by 75%) and have no meaningful reducing in the quality of information, and will all that extra cash, could have easily put a few holes into some of the other targets that showing in every presentation and press release.

If those targets are any good, why don't you stick a fecking hole in them!