EOM Reconciliation Bugbears Part 2

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This article was written by Snowden’s Mining Consulting Manager, Geoff Booth.

From some of the varied comments on my first set of bugbears, I might just have hit a nerve or two with friends and colleagues alike, and all to the good. I suspect it’s not too early to move to the next round of these EOM reconciliation blogposts. Now, if only to re-clarify, these “repeat offenders” reflect an incident frequency of ore shortfall or misclassification from material loss reports which have crossed my desk over a couple of decades. So, where forecasts were poor, and metal clearly went walkabout, these were the reasons (excuses?) why or what “root cause analysis” proffered after the fact for an EOM report.

Now, this begs the question: Did every material loss that you or mine geological co-workers witness over the years get reported, tracked, actioned and permanently rectified? Regrettably not, as there were often so many. Interestingly, even when I’ve returned years afterwards to sites that I used to help manage, I’m impressed, make that stunned, at how hard it is to curb bad habits, especially when geologists and engineers alike ought to know better. So, when looking in the mirror, have we all met the enemy at least at once in our careers? I’ll have to let readers decide as to how many times wheels may require re-invention, at least across the mining sector.

Bugbear Numbers 6 – 8

6) Poor Mining Practices

It’s a little challenging to know just where to start in this topic area, so rich is the selection of bad actors. However, if I limit myself to the chronic problem children, what do my incident logs tell us for open cut operations? Well, unsurprisingly, ore loss was most often linked to those perennial favourites:

  1. Digging without a mark-up.
  2. Under or over digging a mark-up.
  3. Mixing ore and waste across a mark-up (e.g. setting up on the footwall vs the hangingwall).
  4. Excessive sheeting +/- stemming.

Now, one has to ask: Is there anything surprising or new about any of these problems and accordingly, are they not all truly avoidable? Certainly, where visual grade control is possible, I’d like to believe such shortfalls are less likely, particularly on day shift. Where diggers require individual management, there is perhaps some rationale for ore loss over time. Meanwhile, are such simple operational shortfalls reflective of poor supervision or operator training? I mean did anyone actually calculate just how much waste was being used for road sheeting and why didn’t we use the ultra-low grade or mineralised waste as we all agreed? Actually, while we’re at it, has anyone seen the stemming loader since it went to the workshop over a week ago?

Now, for underground operations, unsurprisingly, my logs record waste stockpiles being bogged as ore and ore stockpiles bogged as waste, often despite some very clear signage, which somehow disappears during the shift? When this happens, it’s up to the ROM to separate those couple (?) of loads of hanging wall gneiss from the ore at 3:00AM with our favourite can of fluorescent orange spray paint, or off to the waste dump to retrieve the ore. For whatever reason, when ore is misclassified, it invariably seems to happen when you’re carting high grade! Interestingly, I’ve found when the paste-fill finally makes it onto the ROM, the really serious tourists drop by for a selfie or two with that big load of CAF. Interestingly, these kinds of incidents invariably start with a call from the ROM loader operator who states: “Hey Boothie - you’re not going believe what just came up”!

Meanwhile, when it comes to “bogging down” as opposed to “firing a hang-up” to notionally “preserve a brow”, this series of often repetitive incidents have ripple effects which can last a long time in a cave. Similarly, whether you believe in “interactive draw” or not, perpetually standing a hung drawpoint up, while continuing to bog from adjacent crosscuts, is also very dangerous. There is really no good reason for repeatedly trying to “tickle that boondie down”. Making it safe with a proper bund wall and bringing in the jumbo or airleg to pop it the same way you would any other oversize, is always the preferred solution. And if your brows tend to be a bit “fragile”, how about looking at some additional ground support?

Now, for someone who remembers a time before multi-channel radios, often with a dedicated chat frequency, when it comes to prevention, I have a hard time understanding why more people don’t call up when they can’t remember what should go where. Be advised, it’s a rare mine geologist who gets upset if someone asks them for help, even repeatedly throughout the shift. Most I know or have met, tend to be (seriously) anxious to help, since metal accounting is their premier responsibility. They understand only too well that putting the toothpaste back in the tube is invariably tedious, especially when prevention is a 10 second call away. Just saying……

7) “Errant” Densities  

Now, here’s very rich topic with lots of blame to share amongst geologist, engineer and metallurgist alike. While my files don’t record a huge number of density-specific incidents, where and when poor density estimates or assumptions bite, the impact is invariably large (i.e. ugly), and rarely is a single EOM affected.

For geologists, my density incident post-mortems cite “unrealistically high” or “overly uniform” values, with root causes linked to:

  1. An inadequate number or sometimes virtual absence of physical measurements, be it Archimedes immersion (with or without wax), downhole gamma logging, calliper core measurements, and/or RC sample mass measurements.
  2. A (serious) underestimation of moisture content.
  3. Poor core or RC sample recovery, linked to a variable and weathered rockmass.
  4. Dubious geological domaining.

So, where a rockmass is heavily altered and saturated, there’s always the potential for a double whammy! Hence, when consulting for companies in the tropics, I’ve sometimes had to give clients some rather bad news about just how low their density truly was, especially in near surface domains. Elsewhere, I’ve had to remind geologists that where their densities were clearly high relative to well established (i.e. literature-based) values by lithology, they were likely asking for trouble (i.e. just what were you thinking here?).

Amongst mining engineers, my files list more than a couple of post-blast (i.e. broken bulk) density estimates which to be candid, were completely unrealistic. Coupled with excessive truck factors which were regrettably never checked with a proper weighbridge study, ore tonnes were overestimated and improperly reconciled against parental ore blocks. By way of anecdote, I well remember a testy example of "silo behaviour" between mine geological and engineering teams, linked to this very issue. On the day, the only way to keep the peace and balance the metal, was to introduce a retrospective grade factor. Now that I think of it, might this just form the basis for yet another blog topic: “Who really owns the tonnes at a mine – the geologist or mining engineer?

Finally, and once again to be perfectly candid, when discussing density related incidents with colleagues at the mill, so changeable are the numbers sometimes, that it feels like I’ve joined Arthur Dent on the Heart of Gold and we’re being driven through space using the Improbability Drive! It may be the amount of back-calculation that is often involved, and the number of assumptions made regarding concentrate quality or cathode / bullion purity or even carbon loading etc. Alas, I tend to lose confidence with the amount of tweaking that occurs often using Goal Seek (i.e. Excel), even after an EOM is notionally closed out. Linked to my first “weighbridge” bugbear, I well understand that density and moisture are often the easiest factors to play with when reconciling your garden-variety mud factory. However, I suspect greater transparency is what’s most required to explain why feed density is often deemed lower rather than higher, once it hits the mill. Is this just inherent conservatism rearing its ugly head (i.e. God forbid we’ve overcalled?). Unsurprisingly, what often worries geologists most are those “calculated tail grades” and to be candid yet again, I’ve drilled enough tailings dams after the fact, to know how wrong some of these can be.

8) Resource Estimation

There may have been a few of you expecting this particular topic to come up a much sooner in this blog. As I tally up the incidents, this bugbear was not as common as you might think and that’s not just Geoff “being economical with the truth”. Now, it may be a reflection of those operations where I was employed and the attention to detail which the estimation and modelling process received under the resource development team’s collective skillset.

Irrespectively, for mine geologists who have inherited a defendable model, as an orebody is progressively developed and is better understood, changes are bound to occur and unsurprisingly, not all of them are good from a reconciliation perspective. In this regard, my incident reports three major areas for ore loss which are common across most mine sites with which I’ve been involved. By way of frequency, these include:

  1. Changes in Orebody Geometry
  2. Increased Internal Dilution
  3. Changes in Orebody Grade / Tenor

Now to remind engineers and metallurgists alike, there’s only so much drilling geologists can do and where assumptions are made about an orebody’s shape, we typically employ all of the information available at the time and validate them with care. When geometry / wireframes go south however, and orebodies split, move or suddenly pinch out for a myriad of reasons, occasionally linked to adverse structure, metal forecasts and EOM’s inevitably suffer.

In this regard, my material loss reports record massive sulphide orebodies that have shrunk from over two metres in width to less than half a metre in the space of two, 4 metre development cuts. When it was least expected, I’ve also seen an orebody’s margins seemingly sheared off the planet, only to be re-intersected several years and hundreds of metres away by an enterprising exploration team. As we say in the biz, “the old X-ray vision isn’t what it used to be”.

With internal dilution, I’ve seen barren crosscutting dykes suddenly appear and then disappear in an orebody without geological rhyme or reason. Regrettably, I’ve also intersected large rafts of unmineralized hangingwall sediment floating in what was thought to be a very uniformly mineralised deposit. On the day, we mine geologists watched every bucket of this grey rubbish come out of a crosscut and resolved grimly to sludge-drill the hell out of the area to determine how much more was likely to be present.

In terms of grade underperformance, whilst I have seen resource models which undeniably overestimated grade through use of an esoteric and previously untried mode of kriging, very often it’s poor domaining or truth be told, no domaining, which can seriously smear grade. In this regard, I’ve witnessed milleritic nickel ore swap quickly to pentlandite and wondered why after the fact, this high-grade domain was ever modelled so generously? From the standpoint of deleterious metals, regrettably, I’ve also seen more than my fair share of arsenic suddenly ruin a perfectly good high-grade lode, forcing us to feed it piecemeal to keep the con on-spec. Now, had we geologists been just slightly less naïve on the day, certainly some of this could have been better predicted.

Accordingly, for this class of incidents, I have to continually remind mine geologists to communicate why they occur and how a mine plan should be pre-emptively modified to include such potential shortfalls, since they’re bound to happen again. This is where geological credibility is often lost or won at a mine site and there’s nothing more frustrating than coming up short, knowing that prediction was possible, had one been proactive rather than reactive.

Now as always, I hope to find some entertaining comments from any interested readers by way of response to this blog and as always, I’d be happy to chat further with all and sundry who’d like to compare notes and/or share anecdotes on LinkedIn or my Snowden email. Please stay tuned for some more bugbears!


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