Mine Builders vs Mine Vendors

Date: October 15, 2023 Author: Ken Kuchling Comments: Leave a reply

Normally Major and Intermediate miners advance their projects through the study stages with the ultimate intent to actually build the mine. Sometimes they may opt to sell their project if it no longer fits in their long term strategy or if they desperately need some cash. However, selling the project when it was first acquired was likely not their initial intent. They want to be on a Mine Builder path.

On the other hand, Junior miners will follow one of two paths. They are either on (a) the Mine Builder path, or (b) the Mine Vendor path (i.e. sell the project to someone who will build it). In this article, I will present some examples of companies on each path.

There will also be some discussion on whether the engineers undertaking early stage studies (e.g., PEA’s) should be considering the path their client is intending to follow.

The Mine BUILDER Path

The Mine Builder generally follows a systematic approach, as sketched out in the image below. The project advances from drilling to Mineral Resource Estimate (MRE), scoping study (PEA), then through the Pre-Feasibility Study (PFS) and/or Feasibility Study (FS) stages. Environmental permitting is normally proceeding in conjunction with the engineering work. Once the FS is complete, with permits in hand, the next hurdles for the Mine Builder are financing and construction. The entire path is fairly orderly and can be aggressive.

The amount of exploration drilling is only needed to define an economic resource to the Measured and Indicated classifications. There is usually no requirement to delineate the entire mineral resource on the entire property. There will be plenty of time to do that during production. Demonstrating an economic resource, with some geological upside potential, is often sufficient for the Mine Builder. If the resource grows significantly, they can always introduce an expansion phase. They don’t need to have everything fully optimized at the start.

Three examples of companies on the Builder path are shown below; Orla Camino Rojo gold project (in operation), SilverCrest Las Chispas gold project (in operation), and Nexgen Rook uranium project (financing stage). One may question the inclusion of Rook on this list, since its progress appears to be flattening off.

The three plots show the five Lassonde Stages and the dash line highlights the upward path through each stage (shown on the right axis). Individual milestones are on the left axis. For similar charts for other projects, visit the DrillingDown website Timeline Viewer: https://sites.google.com/view/drillingdown/timeline-viewer

Although the duration of each timeline is different due to different project complexities, the development paths are consistent. Most junior miners would not consider themselves on the Builder path.

Mine Vendor type organizations have the primary goal of selling their project. These companies may consist of management teams that don’t have the desire, comfort, or capability to put a mine into production. For example, this is often the case with companies founded by exploration geologists, whereby their concept is to explore, grow the resource, and then sell all (or part) of the project. There is nothing wrong with this approach.

In other cases the Junior miner realizes their project is too large with a massive capital cost. That capital cost is beyond the financial capability of the company. Hence a deep-pocket partner is needed or an outright sale is preferred.

The Mine Vendors tend to follow a different development path than the Mine Builders. They don’t have the same fast track objective as the Mine Builder. Vendors want out at some point.

The Vendor path can be more irregular, with multiple exploration programs interspersed with studies undertaken at different levels of detail. Sometime studies will step back to lower level as more information is acquired. Their object is to make the project look good to potential buyers, and look better than their junior miner competitors also for sale. This ongoing project improvement process is termed “de-risking”.

Not only must the Vendors demonstrate an economic resource, they must demonstrate a highly valuable resource to maximize the acquisition price for the shareholders. They will try to do this through multiple drill campaigns followed by multiple studies, each one looking better than the prior one. Rising metal prices helps, inflation hurts.

Sometimes you will see a management team state, if the project isn’t sold, they are going to put it into production themselves. This may be true in some cases, or simply part of the negotiating game to try to maximize the acquisition price.

Two quick examples of companies on the Vendor path are shown below: Western Copper Casino project and Seabridge KSM project. The duration of their development timelines are extensive and expensive, while waiting for an interested buyer. During these periods, the companies may continue to spend money de-risk the project further. The defining line tends to be the Feasibility Study – some will delay this stage as long as possible since it can signal the end of the road. Either build or move on.

The hope is that the company can eventually make the project attractive or that changing market conditions will make it attractive for them. Unfortunately, there is always the possibility that no buyer will ever come along. In that case, find another project and work on that while hanging on to the original.

An QP Engineer’s Perspective

A pertinent question is whether the independent geologists and engineers working on the advanced studies should be aware of the path the company is intending to follow. Is the company a Builder or a Vendor?

Some may feel that the focus of the technical work should be independent of the path being followed. Based on my experience as both an owner’s representative and independent study QP, I have a somewhat different opinion. The technical work should be tailored to the intended path.

The Engineer on the MINE BUILDER Path

If an engineer understands that a Mine Builder’s project will move from PEA to PFS to FS in rapid succession, then there is more incentive to ensure each study is somewhat integrated.

For example, a PEA will use Inferred resources in the economics. However, if the project will advance to the PFS stage, where Inferred cannot be used, then it is important for the PEA to understand the role that Inferred plays in the economics. How much drilling will be needed to upgrade Inferred resource to Indicated for the PFS, if needed at all?

Typically, capital costs tend to increase as advancing studies get more accurate due to greater levels of engineering. A Builder wants to avoid large cost increases when moving from PEA to PFS to FS. Therefore, when costing at the PEA stage, one may wish to increase contingency or use conservative design assumptions. Be realistic. After all, one is not trying to sell or promote the project, but rather move it towards production.

There is no value to the Mine Builder by fooling themselves with low-balled cost estimates. (Although some may argue there is still a desire to low ball costs to get internal management to approve the project, and maybe this does happen sometimes). Conversely Mine Vendors do have some incentive to low ball the costs.

Perhaps some of the recent project capital cost over-runs we have seen is that the Vendor mentality was used at the PEA stage to optimistically set the capital cost baseline. Subsequent studies were then forced to conform to that initial baseline cost estimate. Ultimately construction will be the arbiter on the true project cost. Hence there is no real value in underestimating costs, ultimately making management appear incompetent if costs over-runs do occur.

The Mine Builder will also be advancing environmental permitting simultaneously with their baselines and advanced studies. Hence at the early stage (PEA) it is important to properly define the site layout, processing method, production rate, facility locations, etc. since they all feed into the permitting documents.

Changing significant design details in the future will set back the permitting and construction timelines. Hence, for the Mine Builder, the engineers should focus on getting the design criteria mostly correct at the PEA stage. For the Mine Vendor, this is not as important since multiple studies are being planned for in the future anyway.

The Engineer on the MINE VENDOR Path

The objective of the Mine Vendor is to make the project attractive to potential buyers. There is less urgency in fast tracking towards detailed engineering.

On this path it is typical to see multiple drilling programs, followed my multiple studies of scenarios with different size, production rate, and layout. The degree of engineering conservativeness in design and costing is less critical since future studies may be on substantially different sized projects. Open pit only studies may move to combined open pit and underground operations as future drill changes the nature of the deposit.

The role that the Inferred resource plays in the economics is also less important at this time, since a lot more infill drilling may be coming. The Vendor’s objective tends to be on maximizing resource size not necessarily optimizing resource classification.

While the Mine Vendor may also be advancing environmental permitting as another way to de-risk the project, the project design may still be in flux as the resource size changes. Major modifications to the plan may cause permitting to stop and re-start, leading to an extended project timeline and wasted money.

There is also risk in starting the permitting with a project definition that isn’t of economic interest to future buyers. Sometimes the Vendor may be making regulatory commitments or design revisions that constrain the operating flexibility of future mine operators. Its easy to commit to environmental conditions when you aren’t the one having to live up to them.

The Mine Vendor will also de-risk the project by moving from PEA to PFS and even to FS. The caution with completing a FS is that it is a costly study and essentially brings one to the end of the study line. Feasibility studies have the dual role of trying to boost the share price and market cap with a great NPV, as well as frame the project for potential buyers. What does the company do now if there is still no buyer?

Feasibility studies also have a shelf life, with the cost estimates and economics becoming inaccurate after a few years. Some companies may re-examine the project, re-frame it, and jump back to the PEA or PFS stages. One can create an on-going study loop, requiring continued funding with no guarantee of a salein sight. Consultants love it.

Conclusion

As an engineer, it is helpful to understand the objectives of the project owner and then tailor the technical studies to meet those objectives. This does not mean low balling costs to make the study a promotional tool. It means focusing on what is important. It means recognizing the path, and what doesn’t need to be engineered in detail at this time. This may save the client time, money, and improve credibility in the long run.

In many cases, the precise size of the deposit is less important than understanding the site, access, water supply, local community issues, the environmentally acceptable location for dumps and tailings, etc.. It can be more important to focus on these issues rather than having a detailed mine plan with multiple pit phases that immediately becomes obsolete in a few months after the next drilling campaign.

Potential buyers will have their own technical team that will develop their own opinions on what the project should be and what it should cost. Just because a Mine Vendor has a Feasibility Study in hand, doesn’t mean a potential buyer will believe it.

Note: You can sign up for the KJK mailing list to get notified when new blogs are posted. Follow me on Twitter at @KJKLtd for updates and other mining posts. The entire blog post library can be found at https://kuchling.com/library/

For some free mining calculator apps, including project timelines and a simplified cashflow modeller, check out this website https://sites.google.com/view/drillingdown

Don’t Cut Corners, Cut Cross-Sections Instead

Date: October 4, 2023 Author: Ken Kuchling Comments: Leave a reply

This article is about the benefit of preparing (cutting) more geological cross-sections and the value they bring.

Geological sections are one of the easiest ways to explain the character of an orebody. They have an inherent simplicity yet provide more information than any other mining related graphic.

Some sections can be simple cartoon-like images while others can be technically complicated, presenting detailed geological data.

Cartoon-stylized sections are typically used to describe the general nature of the orebody. The detailed sections can present technical data such as drill hole traces, color coded assays intervals, ore block grades, ore zone interpretations, mineral classifications, etc.

Sections provide a level of clarity to everyone, including to those new to the mining industry as well as those with decades of experience.

This article briefly describes what story I (as an engineer) am looking for in sections. Geologists may have a different view on what they conclude when reviewing geological sections.

I will describe the three types of geological sections that one can cut and what each may be describing. The three types are: (1) longitudinal (long) sections; (2) cross-sections; (3) bench (level) plans. Each plays a different role in helping to understand the orebody and mining environment.

There is also another way to share simple geological images via3D PDF files. I will provide an example later.

Longitudinal (Long) Sections

Long sections are aligned along the long axis of the deposit. They can be vertically oriented, although sometimes they may be tilted to follow the dip angle of an ore zone.

Long sections are typically shown for narrow structure style deposits (e.g. gold veins) and are typically less relevant for bulk deposits (e.g. porphyry).

The information garnered from long sections includes:

The lateral extent of the mineralized structure, which can be in hundred of metres or even kilometers. This provides a sense for how large the entire system is. Sometimes these sections may show geophysics, drilling to defend the basis for the regional interpretation.
Long sections will often highlight the drill hole pierce points to illustrate how well the mineralized zone is drilled off. Is the ore zone defined with a good drill density or are there only widely spaced holes? As well, long sections can show how deep ore zone has been defined by drilling. On some projects, a few widely spaced deep holes, although insufficient for resource estimation purposes, may confirm that the ore zone extends to great depth. This bodes well for potential development in that a long life deposit may exist.
Sometimes the long section drill intercept pierce points can be contoured on grade, thickness, or grade-thickness. This information provides a sense for the uniformity (or variability) of the ore zone. It also shows the elevations of the higher grade zones, if the deposit is more likely an open pit mine, an underground mine, or a combination of both.

Cross-Sections

Cross-sections are generally the most popular geological sections seen in presentations. These are vertical slices aligned perpendicular to the strike of the orebody. They can show the ore zone interpretation, drill holes traces, assays, rock types, and/or color-coded resource block grades.

As an engineer, my greatest interest is in seeing the resource blocks, color coded by grade. Sometimes open pit shells may be included on the section to define the potential mining volume. The engineering information garnered from block model cross-sections includes:

Where are the higher-grade areas located; at depth or near surface?
If a pit shell profile is included, what will the relative strip ratio look like? Are the ore zones relatively narrow compared to the size of the pit?
How will the topography impact on the pit shape? In mountainous terrain, will a push-back on pit wall result in the need to climb up a hillside and create a very high pit slope? This can result in high stripping ratios or difficult mining conditions.
Does the ore zone extend deeper and if one wants to push the pit a bit deeper, is there a high incremental strip ratio to do this? Does one need to strip a lot of waste to gain a bit more ore?
Are the widths of the mineable ore zones narrow or wide, or are there multiple ore zones separated by internal waste zones? This may indicate if lower-cost bulk mining is possible, or if higher cost selective mining is required to minimize waste dilution.
How difficult will it be to maintain grade control? For example, narrow veins being mined using a 10 metre bench height and 7 metre blast pattern will have difficulty in defining the ore /waste contacts.
Cross-sections that show the ore blocks color coded by classification (Measured, Indicated, Inferred), illustrate where the less reliable (Inferred) resources are located and how much relative tonnage may be in the more certain Measured and Indicated categories.

When looking at cross-sections, it is always important to look at multiple cross-sections across the orebody. Too often in reports one may be presented with the widest and juiciest ore zone, as if that was typical for the entire orebody. It likely is not typical.

Stepping away from that one section to look at others is important. Possibly the character of the ore zones changes and hence its important to cut multiple sections along the orebody.

Bench (Level) Plans

Bench plans (or level plans) are horizontal slices across the ore body at various elevations. In these sections one is looking down on the orebody from above.

Level plans are typically less common to see in presentations, although they are very useful. The level plans may show geological detail, rock types, ore zone interpretations, ore block grades, and underground workings.

The bench plan represents what the open pit mining crews would see as they are working along a bench in the pit. The information garnered from bench plans that include the block model grades includes:

Where are the higher-grade areas found on a level? Are these higher grade areas continuous or do they consist of higher grade pockets scattered amongst lower grade blocks?
Do the ore zones swell or pinch out on a bench? A vertical cross-section may give a false sense the ore zones are uniform. The bench plan gives an indication on how complicated mining, grade control, and dilution control might be for operators.
Do the ore zones on a bench level extend out beyond the pit walls and is there potential to expand the pit to capture that ore?
On a given bench what will the strip ratio be? Are the ore zones small compared to the total area of the bench?

As recommended with cross-sections, when looking at bench plans, one should try to look at multiple elevations. The mineability of the ore zones may change as one moves vertically upwards or downwards through a deposit.

Never mind cross-sections – give me 3D

While geological sections are great, another way to present the orebody is with 3D PDF files to allow users to view the deposit in three-dimensions. Web platforms like VRIFY are great, but I have been told they sometimes can be slow to use.

3D PDF files can be created by some of the geological software packages. They can export specific data of interest; for example topography, ore zone wireframes, underground workings, and block model information. These 3D files allows anyone to rotate an image, zoom in as needed and turn layers off and on.

You can also create your own simplistic cross-sections through the pdf menus (see image).

A simple example of such a 3D PDF file can be downloaded at this link (3D DPF File Example). It only includes two pit designs and some ore blocks to keep it simple.

The nice thing about these PDF files is that one doesn’t need a standalone viewer program (e.g. Leapfrog viewer) to view them. They are also not huge in size. As far as I know 3D PDF files only work with Adobe Reader, which most everyone already has. It would be good if companies made such 3D PDF files downloadable along with their corporate PowerPoint presentations.

Conclusion

The different types of geological sections all provide useful information. Don’t focus only on cross-sections, and don’t focus only on one typical section. Create more sections at different orientations to help everyone understand better.

In 2019 I wrote an article describing the lack of geological cross-sections in many 43-101 technical reports. The link to that article is her “43-101 Reports – What Sections Are Missing?”

Geological sections are some of the first items I look for in a report. Sometimes they can be hidden away in the appendices at the back of the report. If they are available, take the time to actually study them since they can explain more than you realize.

Note: You can sign up for the KJK mailing list to get notified when new blogs are posted. Follow me on Twitter at @KJKLtd for updates and other mining posts. The entire blog post library can be found at https://kuchling.com/library/

Grade-Tonnage Curves – Worthy of a Good Look

Date: July 16, 2023 Author: Ken Kuchling Comments: Leave a reply

Most of us have seen the typical “grade-tonnage” table or graph, showing ore tonnes and grade at varying cutoff grades. It is usually part of every 43-101 technical report in Section 14. We may glance at it quickly and then move on to more exciting chapters. Section 14 (Mineral Resources) can be a very complex chapter to read with statistics, geostatistics, and mathematical formulae. However the grade-tonnage curve aspect isn’t complicated at all.

The next time you see the grade-tonnage relationship, I suggest taking a few seconds to study it a bit further. There might be some interesting things in there.

Typical Grade-Tonnage Information

Typically, one will see grade-tonnage data in 43-101 Technical Reports towards the back of Section 14 "Mineral Resources". The information is normally presented in either of two ways; (i) a grade-tonnage table or (ii) a grade tonnage graph. Examples of each are shown below. The grade tonnage graph typically has the cutoff grade along the bottom x-axis and the two separate y-axes representing the ore tonnes above cutoff and the average ore grade above cutoff.

Rarely do you see both the table and curve in the report, although ideally one would want to see both. Given the option, I would prefer to see the graph more than the table of numbers. The trend of the grade-tonnage information is just as important as the values, maybe even a bit more important. Unfortunately, a data table by itself doesn’t illustrate trends very well.

Useful Grade-Tonnage Curve Information

When I am undertaking a due diligence review or working on a study, very early on I like to have a look at the grade-tonnage information. This could be for the entire deposit resource, within a resource constraining shell, or in the pit design.

The grade-tonnage information gives an understanding of how future economics or technical issues may impact on the mineable tonnage.

An example of a typical grade-tonnage curve is shown here.

The cutoff grade along the x-axis will be impacted by changes in metal price or operating cost. The cutoff grade will increase if metal prices decrease or if operating costs increase.

The question is how sensitive is the mineable tonnage to these economic factors. The slope of the tonnage and grade curves will help answer this question.

In the example shown, the tonnage curve (blue dots) is fairly linear, meaning the ore tonnage steadily decreases with increasing cut-off grade. That is expected and is reasonable.

However, if the tonnage curve profile resembled the light blue line in this image, with a concave shape, the ore tonnage is decreasing rapidly with increasing cutoff grade. This is generally not a favorable situation.

It indicates that a significant portion of the tonnage has a grade close to the cutoff grade. If that’s the situation, the calculation of the cutoff and the inputs used to generate it are important and worthy of scrutiny. Are they reasonable? Over the long term, is the cutoff grade more likely to increase or decrease?

The same logic can be used with the ore grade curve in the graph. As shown in this example, the ore grade increases steadily as the cutoff is raised. This is because lower grade ore is being shifted from ore to waste, and hence the remaining ore has better quality. If the cutoff is raised from 0.4 g/t to 0.5 g/t, then some material with a grade of about 0.45 g/t is moved from ore to waste.

I also like to compare the ratio of the average grade to the cutoff grade. Its nice to see a ratio of 4:1 to 5:1 to ensure the overall average grade isn’t close to the cutoff. In this example, the cutoff grade is 0.5 g/t and the average grade is 4.5 g/t, a ratio of 9:1.

The tonnage curve and grade curve provide information on the nature of the mineral resource. Study them both.

Reporting Waste Within a Shell

One complaint I have about reporting mineral resources inside a resource constraining shell is the lack of strip ratio information. This applies whether disclosing a single mineral resource estimate or variable grade-tonnage data.

In my view, the strip ratio is even more important to be aware of when looking at grade tonnage data.

The strip ratio within a shell will climb as an increasing cutoff grade results in a decreasing ore tonnage. Sometimes the strip ratio will increase exponentially. The corresponding amount of waste remaining in that pit shell increases, hence the ratio of the two (i.e. strip ratio) can escalate rapidly.

Regarding mineral resources, one should be required to disclose the waste tonnage and strip ratio when reporting resources inside a constraining shell. The constraining shell and cutoff grade are both based on defined economic factors such as unit mining costs, processing cost, process recoveries, and metal prices. With respect to the mining cost component, the strip ratio is a key aspect of the total mining cost, yet it normally isn’t disclosed.

Its common to see mention that the mining cost is (say) $2.50/t, but if the strip ratio is 10:1, that equates to an effective mining cost of $27.50 per tonne of ore. That’s an important cost to know, especially if one is pushing a pit shell deep to maximum the mineral resource tonnage.

Each mineral deposit resource model can behave differently. Hence, in my view, the waste tonnage should be included when reporting mineral resource tonnages (or presenting grade-tonnage data) within a constraining shell. This waste tonnage or strip ratio can be in the footnotes to the mineral resource summary table.

Spider Diagram Downsides

In 43-101 technical reports, the financial Chapter 22 normally presents the project sensitivities expressed in a spider diagram or a table format.

In a previous blog post I had discussed the flaws in the spider diagram approach. That article link is at “Cashflow Sensitivity Analyses – Be Careful”. The grade-tonnage curve helps explain why that is.

In the spider diagrams, we typically see sensitivities related to +/- 20% on metal prices and operating costs. If either of these factors change, then in reality the cutoff grade would change.

If the metal price decreases by -20%, or the operating cost climbs by +20%, the cutoff grade must increase. This adjustment is normally not made in the sensitivity analysis because it requires a lot of re-work.

Elevating the cutoff grade would shift the pit ore tonnage towards the right on the grade-tonnage curve, showing a decrease in mineable tonnes. However, in the spider diagram logic, the assumption is that production schedule in the cashflow model is unchanged and simply the metal prices or operating costs are adjusted. Therefore, the spider diagram can be a misleading representation of the downside risk, showing a more positive situation than in reality.

Conclusion

The grade-tonnage information is always presented in technical reports. It examines the sensitivity of the orebody size to changes in cutoff grade. The next time you see grade-tonnage data, don’t skip over it. Take a minute to study it further to see what can be learned.

Note: You can sign up for the KJK mailing list to get notified when new blogs are posted. Follow me on Twitter at @KJKLtd for updates and other mining posts. The entire blog post library can be found at https://kuchling.com/library/

Polymetallic Drill Results – Interesting or Not?

Date: June 30, 2023 Author: Ken Kuchling Comments: Leave a reply

A while ago I posted an article about how one can evaluate the economic potential of a gold deposit using early-stage exploration intercepts. That article can be found at this link. Doing the same evaluation for a polymetallic deposit is a bit more challenging. There will be different metals of interest, with variable grades, prices, and process recoveries.

When disclosing polymetallic drill results, many companies will convert the multiple metal grades into a single equivalent grade. I am not a big proponent of that approach.

I prefer using the rock value, whether calculated as a recoverable “NSR dollar value per tonne” or as an “insitu value per tonne”. Either rock value is fine for my purposes.

Interestingly NI 43-101 prohibits the disclosure of insitu rock value but allows the use of metal-equivalents. In my view this is a bit counter-intuitive since the equivalent grade can be more misleading than rock value.

What can drill intercepts show

The three aspects that interest me the most when looking at early-stage drill results are:

The economic value of the rock (in $/t tonne). This can either be “insitu value” (assuming 100% recovery, 100% payable) or the “NSR value” incorporating recovery and payable factors (if available). Personally, the 100% insitu value is simpler to calculate and assess.
The depth to the top of the economic zone, which indicates if this deposit would be a lower cost open pit mine or must be a higher cost underground mine.
The length of the economic intervals, which indicates whether bulk mining approaches are viable versus the need to selectively mine narrow ore zones. The economic interval lengths also give a sense for the potential tonnage size (i.e. is it a big deposit or a small one).

There are two types of early-stage exploration data that can be examined with respect to the three items of interest described above. They are (i) the drill hole assay data and (ii) the drill hole "intercepts of interest". I will show an example of each in this post using sample data from an actual exploration program.

One can examine individual drill hole assays to calculate the rock value profile along each drill hole. One can also examine the rock values for the major and minor intervals of interest reported in company news releases.

I normally like to examine both, but the intervals of interest data is publicly disclosed and more readily available. Drill hole assays are often a bit harder, if not impossible, to track down.

Economic Parameters

In a polymetallic deposit, the insitu rock value is simply the summation of value of the individual metal, based on their respective assay grades. An NSR rock value would apply an adjustment for metal recoveries and smelter payables, thereby lowering the insitu rock value somewhat. However the insitu value is fine if there is no metallurgical or process data to rely upon.

Next one must determine what insitu rock value is deemed potentially economic, i.e. the breakeven cutoff.

One can estimate a processing cost and G&A cost. In an open pit scenario, one doesn’t include the mining cost since the goal is to decide whether to send a truck to the waste dump or to the crusher. Only the processing and G&A cost musts be recovered by the ore value. In an underground mining scenario, one would include the mining cost in the cutoff calculation.

In our example, lets assume a unit processing cost of $12/t and a G&A cost of $$2/t, for a combined cost of $14/t. If we envision a metal recovery range of 75%-95%, we can assume 85% for now.

If we envision a smelter payable range of 75% to 95%, we will assume 85% for that also.

The “NSR factor” would now be 85% x 85% or 75%. Therefore, if the breakeven cost is $14/t, then one should target to mine rock with an insitu value greater than $20/tonne (i.e. $14 / 0.75). This would be the approximate ore vs waste cutoff. It is still only ballpark estimate at this early stage, but good enough for this type of review.

Normally it would be nice to see the average head grade (or rock value) at 3 to 4 times greater than the cutoff grade. This is not a necessity but it is a positive factor.

For example, in a gold deposit with a 0.3 g/t cutoff, one would like to see average head grades at least 0.9 to 1.2 g/t or more. If the average head grade is close to the cutoff grade, then possibly the orebody tonnage may be very sensitive to changes in cutoff. This may not be a good thing.

In our example, with a breakeven cutoff rock value of $20/t, one would like to see some ore zones with insitu values 3-4x higher, or above $60 - $80/t. We can target >$70/t rock as a "nice to have" with $20/t as the cutoff.

So far, its all pretty simple. Let’s look at some actual exploration data to see how to apply this approach.

Our example will be a polymetallic deposit containing four metals of interest; copper, gold, cobalt, and iron. One can examine a few drill holes as well as the intervals of interest.

Metal prices used in this example are Cu = $4/lb, Au = $1980/oz, Co = $15.50/lb, Fe concentrate = $100/tonne, assuming 100% recovery and 100% payable for everything.

Drill Hole Assays Examples

The following three graphs show down hole profiles for Drill Holes A, B, C. For each hole there are two plots. One plot shows the insitu rock values down the hole. The second plot is the same, except the x-axis minimum has been set to the breakeven cutoff value of $20/t. This is done simply to highlight the potentially economic zones.

Normally I would not spend a lot of time examining holes with little to no grade. Some may consider this as a biased view. However, every orebody has its limits, and what is occurring along the edges isn’t that critical in my view.

My objective is to understand what is happening in the core of the orebody, since that is what will dictate the overall economics. Is the core of the orebody marginal value, or does it consist of high value rock? Ultimately it will be the exploration company's task to keep drilling to define if there is sufficient tonnage of this higher value rock to justify a mine. However this shows that at least the grades are there.

Intervals of Interest Example

The next series of plots examines the insitu rock values over drill intervals typically published in a company news releases. The intervals of interest will composite the individual assays over larger widths based on the company’s technical judgement.

It is interesting to see whether the larger intervals have good economic potential. The following charts combine both major intervals with minor zones, often referred to as “including” in news releases. Both major and minor intervals can provide useful information.

Insitu Rock Value vs Depth:

This chart shows the rock values for multiple report intervals versus their depth (top) along the hole.

One can see multiple intervals at open pit depths (<250 m) with insitu values above the $20/t cutoff and above the $70/t threshold.

Within the upper 250 metres, we are seeing multiple intervals with good value. That is a positive sign.

Note that these depths are not depths from surface, but distance along the drill hole. In reality the intervals may be slightly closer to surface, depending on the hole inclination.

Insitu Rock Value vs Interval Length:

The next question to ask is whether the higher value zones are narrow or wide?

In the example here one can see some wide zones (70 to 90m) with rock values in the range of $40-70/t. These are good open pit mining widths.

There are numerous higher grade zones ($70-$200/t) in the 5m to 20m width range. These widths are still fine for open pit mining.

Some intervals are quite narrow (<5m), being a bit more difficult to mine. Since many of these are higher grade, they will tolerate some mining dilution.

Conclusion

Although publishing insitu rock values is prohibited by NI-43-101, I find them important in my understanding the economic potential of a deposit. Reviewing the insitu rock values spatially is not difficult and can shed light on what is there. Even at a very early stage, one can get a sense of economic character of the orebody. This is a great approach to use when doing an acquisition due diligence on an exploration stage project consisting mainly of drill hole data.

In my view, it would be beneficial if all polymetallic drill results were reported with the individual grades and using a standardized industry wide insitu rock value formula. Then one could compare projects (or even different zones on the same project) on an equal basis. The cutoff to be applied to different projects would vary but the insitu value is what it is.

This might be better than each company applying their own unique equivalent grade calculation to their exploration results.

The equivalent grade calculation still requires assumptions on the metal prices and recoveries. The result is, unfortunately, presented as a grade value rather than a dollar value.

The intervals of interest published in news releases are usually not available for download. Great Bear is (was) one example where the data was available. It would be nice if more companies followed suit by releasing their interval data in CSV or Excel format. It worked out well for Great Bear!

Perhaps the detailed hole assay data may be too complex or voluminous to release. Maybe this level of information is not useful except to the more technically driven investors. Nevertheless it would still be nice to have access to this drill data in electronic form, at least in the core of the orebody.

For further light reading, the two previous articles referenced above are “Gold Exploration Intercepts – Interesting or Not?" and "Metal Equivalent Grade versus NSR for multi-metals – Preference?"

Note: You can sign up for the KJK mailing list to get notified when new blogs are posted. Follow me on Twitter at @KJKLtd for updates and other mining posts. The entire blog post library can be found at https://kuchling.com/library/

Mining Under Lakes – Part 2: Design Issues

Date: June 11, 2023 Author: Ken Kuchling Comments: 3 Replies

This is Part 2 of a blog post related to open pit mining within bodies of water. Part 1 can be found at this link “Mining Under Lakes – Part 1“, which provides a few examples where this has been done successfully. Part 2 focuses on some of the social and technical issues the need to be considered when faced with the challenge of open pit mining within a water body.

The primary question to be answered is whether one can mine safely and economically without creating significant impacts on the environment.

The answer to this question will depend on the project location and the design of the water retaining structure.

I have worked on several projects where dike structures were built. I have also undertaken due diligence reviews of projects where dikes would be required. Most recently I have participated in some scoping level studies where mining within a lake or very close to a river were part of the plan.

In some instances, the entire orebody is located in the lakebed. In others, the orebody is mainly on land but extends out into the water. Each situation will be unique. In northern Canada, given the number of lakes present, it would be surprising if a new mining project isn’t close to a river or lake somewhere.

Dike concepts consider many factors

Different mining projects may use different styles of dikes, depending on their site conditions. Some dikes may incorporate sheet piling walls, slurry cutoff walls, low permeability fill cores, or soil grouting. There are multiple options available, and one must choose the one best suited for the site.

The following is list of some of the key factors and issues that should be examined.

ESG Issues

One’s primary focus should be on whether building a dike would be socially and environmentally acceptable. If it is not, then there is no point in undertaking detailed geotechnical site investigations and engineering design. One must have the “social license” to proceed down this path.

Water Body Importance: Is there a public use of the water body? It could be a fresh water source for consumption, used for agricultural or fishery purposes, or used as a navigable waterway, etc. Would the presence of the dike impact on any of these uses? Does the water body have any historical or traditional significance that would prevent mining within it?

Lake Turbidity: Dike construction will need to be done through the water column. Works such as dredging or dumping rock fill will create sediment plumes that can extend far beyond the dike. Is the area particularly sensitive to such turbidity disturbances, is there water current flow to carry away sediments?

At Diavik, a floating sediment curtain surrounding the dike construction area was largely able to contain the sediment plume in the lake.

Regional Flow Regime: Will the dike be affecting the regional surface water flow patterns? If the dike is blocking a lake outflow point, can the natural flow regime be maintained during both wet and dry periods?

Location Issues

If there are no ESG issues preventing the use of a dike, the next item to address is the ideal location for it.

Water depth: normally as the dike moves further away from land, both the water depth and dike length will increase. The water depth at the deepest points along the dike are a concern due to the hydraulic head differential created once the interior water pool is pumped out. The seepage barrier must be able to withstand that pressure differential, without leaking or eroding. A low height dike in shallow water may be able to use a simpler seepage cutoff system than a dike in deep water.

Islands: Are there any islands located along the dike path that can be used to shorten the construction length and reduce the fill volumes? Is there a dike alignment path that can follow shallower water zones?

Pit wall setback: Given the size and depth of the open pit, how far must the dike be from the pit crest? Its nice to have 200 metre setback distance, but that may push the dike out into deeper water.

If the dike is too close to the pit, then pit slope failures or stress relaxation may result in fracture opening and increase the risk of seepage flows or catastrophic flooding. The pit wall rock mass quality will be the key determining factor in the setback distance.

Maximizing ore recovery: If the ore zone extends further out into the lake, maximizing ore recovery may require using a steep pit wall along the outer sections of the pit. This may require positioning haulroads with switchbacks along other sides of the pit rather than using a conventional spiral ramp layout.

At Diavik (see image), the A154 north open pit wall was pushed to about 60 metres of the dike to access as much of the A154N kimberlite ore as possible. Haulroads were kept to the south side of the pit.

It may be possible to recover even more ore by pushing out the dike even further. However, this may result in a larger and costlier dike or even require a different style of dike. There will be a tradeoff between how much additional ore is recovered versus the additional cost to achieve that. There will be a happy medium between what makes both technical sense and economic sense.

Design Issues

Once the approximate location of the dike has been identified, the next step is to examine the design of the dike itself. Most of the issues to be considered relate to the geotechnical site conditions.

Lakebed foundation sediments: What does the lakebed consist of with respect to soft sediments? Soft sediments can cause dike settlement and cracking, or mud-waving of fill material.

Will the soft sediments need to be dredged prior to construction, and if so, where do you dispose of this dredge slurry, and what impact will dredging have on the lake turbidity?

Lakebed foundation gravels: Are there any foundation gravel layers that can act as seepage conduits beneath the dike? If so, will these need to be sub-excavated, or grouted, or cut off with some type of barrier wall? Sonic drilling, rather than core drilling, is a better way to identify the presence of open gravel beds.

Upper bedrock fracturing: Is the upper bedrock highly fractured, thereby creating leakage paths? If so, then rock grouting may be required all along the dike path to seal off these fractures.

Major faults: Are there any major faults or regional structures that could connect the open pit with the lake, acting as a source of large water inflow?. At Diavik, we attempted to characterize such structures with geotechnical drilling before construction. Upon review, I understand there was one such structure not identified, which did result in higher pit inflows until it was eventually grouted off.

Water level fluctuations: In a lake or river one may see seasonal water level fluctuations as well as storm event fluctuations. The height of the dike above the maximum water level (i.e. freeboard) must be considered when sizing the dike.

Ice scouring: In a lake or river that freezes over, ice loads can be an important consideration. During spring breakup as the ice melts, large sheets of ice can be pushed around and may scour or damage the crest of the dike. The dike must be robust enough to withstand these forces.

Construction materials available on site: Is there an abundance of competent rock for dike fill? Is there any low permeability glacial till or clay that can be used in dike construction? If these materials are available on site, the dike design may be able to incorporate them. If such materials are not available, then a alternate dike design may be more appropriate, albeit at a cost.

Conclusion

Each mine site is different, and that is what makes mining into water bodies a unique challenge. However many mine operators have done this successfully using various approaches to tackle the challenge.

Even at the exploration stage, while you are still core drilling the orebody through the ice, you can start to collect some of this information to help figure it all out.

The bottom line is that while mining into a water body is not a preferred situation, it doesn’t mean the project is dead in the water. It will add capital cost and environmental permitting complexity, but there are proven ways to address it.

On the opposite side, I have also seen situations where a dike solution was not feasible, so ultimately there are no guarantees that engineers can successfully address every situation. Lets hope your project isn’t one of them.

There could be a 3rd part to this post that discusses issues associated with underground mining beneath bodies of water; however that is not my area of expertise. I would be more than happy to collaborate on a article with someone willing to share their knowledge and experience on that subject.

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Mining Under Lakes – Part 1: Examples

Date: May 28, 2023 Author: Ken Kuchling Comments: 2 Replies

Springpole Project

I recently saw an investor presentation from First Mining Gold about their Springpole Project. The situation is that their open pit is located within a lake and will require the construction of a couple of small cofferdams to isolate the pit area from the lake. The concept is shown in this image.

Over the last couple of years I have been involved in a few early-stage studies for mining projects in which nearby bodies of water play a role in the design. In Canada’s north there are thousands of lakes and rivers, so its not surprising to find mines next to them.

That got me thinking about how many other mines are in the same situation, i.e. projects that may be located very close to, or within, a lake, river, or ocean. Hence I have compiled a short list of a few such mines.

I have been directly involved with some of those in the list, while others are only known to me with limited detail. Some mines I had never heard of before, but their names were provided to me by some Twitter colleagues.

My observation is that building a mine within, or adjacent to, a body of water is nothing new and this has been done multiple times successfully.

Some of these projects may refer to the dams as “dikes”, “cofferdams”, “sea walls” but I assume they are all providing roughly the same function of holding back water for the life of the project. They are not viewed as permanent dams.

This is Part 1 of a two-part blog post. Part 1 provides some examples of projects where water bodies were involved in the design. Part 2 provides a discussion on specific geotechnical and hydrogeological issues that would normally have to be examined with such projects.

Some Lake Mining Examples

The following are some examples of operating mines involving lakes. I have captured a few Google Earth images, unfortunately some have only low resolution vintage satellite imagery.

This is a project I was working on with in 1997 to 2000 while it was still at the design and permitting stage. My role focused on pit hydrogeology and geotechnical as well as mine planning.

The project would require the construction of three dikes in sequence to mine four lakebed kimberlite pipes.

The three dikes were named after the associated kimberlite pipe being mined inside it; A154, A418, and A21.

The first dikes were built in 2002 and the last dike (A21) was completed in 2018.

The total dike length for the three dikes is about 6.2 km.

For those interested in learning a bit more about Diavik, I have posted an earlier article about the open pit hydrogeology there, linked to at " Hydrogeology At Diavik – Its Complicated".

This is a project I was involved with several years ago. The bauxite deposit extends beneath the Suriname River and the goal was to mine as much ore as possible.

Given the flow rates in the river, especially during the wet season, it would be difficult to maintain a cofferdam out into the river.

The shoreline overburden consisted of sands and soft clays, so the decision as made to construct a sheet piling wall along the river bank to protect the pit from river erosion. This was mined out successfully and eventually reclaimed.

Conclusion

As one can see, the idea of mining into a body of water is nothing new. Its not a preferred situation, but it can be done economically and safely. The technical challenges are straightforward, and engineers have dealt with them before. However there also are instances where the design could not economically address the water issue, and thus played a role in the mine not getting built.

If you know of other mines not listed above that have successfully dealt with a water body, please let me know and I can update this blog post.

This concludes Part 1. Part 2 can be read at this link " Mining Under Lakes – Part 2: Design Issues" discusses some of the concerns that engineers need to consider when building a mine in these situations.

Finally, the worst-case scenario is shown in this grainy video of a tin mine (Pantai Remis Mine) pit slope failure. It seems they mined too close to the ocean. Watch to the end, its hard to believe. Its looks like something out of a Hollywood disaster movie.

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NPV One – Cashflow Modelling Without Excel

Date: May 21, 2023 Author: Ken Kuchling Comments: Leave a reply

From time to time, I encounter interesting software applications related to the mining industry. I recently became aware of NPV One, an Australian based, cloud hosted application used to calculate mineral project economics. Their website is https://npvone.com/npvone/

NPV One is targeting to replace the typical Excel based cashflow model with an online cloud model. It reminds me of personal income tax software, where one simply inputs the income and expense information, and then the software takes over doing all the calculations and outputting the result.

NPV One may be well suited for those not comfortable with Excel modelling, or not comfortable building Excel logic for depreciation, income tax, or financing calculations. These calculations are already built in the NPV One application.

I had a quick review of NPV One, being given free access to test it out. I spent a bit of time looking at the input menus and outputs, but by no means am I proficient in the software after this short review.

Like everything, I saw some very good aspects and some possible limitations. However, my observations may be a bit skewed since I do a lot of Excel modelling and have a strong comfort level with it. Nevertheless, Excel cashflow modelling has its own pro’s and con’s, some of which have been irritants for years.

NPV One – Pros and Cons

Pros

NPV One develops financial models that are in a standardized format. Models will be very similar to one another regardless of who creates it. We are familiar with Excel “artists” that have their own modelling style that can make sharing working models difficult. NPV One might be a good standard solution for large collaborative teams looking at multiple projects while working in multiple offices.
NPV One, I have been assured, is error free. A drawback with Excel modelling is the possibility of formula errors in a model, either during the initial model build or by a collaborator overwriting a cell on purpose (or inadvertently).
With NPV One, a user doesn’t need to be an Excel or tax modelling expert to run an economic analysis since it handles all the calculations internally.
NPV One allows the uploading of large input data sets; for example life-of-mine production schedules with multiple ore grades per year. This means technical teams can still generate their output (production schedules, annual cost summaries, etc.) in Excel. They can then simply import the relevant rows of data into NPV One using user-created templates in CSV format.
As NPV One evolves over time with more client input, functionality and usability may improve as new features are added or modified.

Cons

Like anything, nothing is perfect and NPV may have a few issues for me.

Since I live and breathe with Excel, working with an input-based model can be uncomfortable and take time to get accustomed to. Unlike Excel, in NPV One, one cannot see the entire model at once and scroll down a specific year to see production, processing, revenue, costs, and cashflow. With NPV jump to. If you’re not an avid Excel user, this issue may not be a big deal.
In Excel one can see the individual formulas as to how a value is being calculated. Excel allows one to follow a mathematical trail if one is uncertain which parameters are being used. With NPV One the calculations are built in. I have been assured there are no errors in NPV One, so accuracy is not the issue for me. It’s more the lack of ability to dissect a calculation to learn how it is done.
With NPV One, a team of people may be involved in using it. That’s the benefit of collaborative cloud software. However that means there will be a learning curve or training sessions that would be required before giving anyone access to the NPV One model. Although much of NPV One is intuitive, one still needs to be shown how to input and adjust certain parameters.
Currently NPV One does not have the functionality to run Monte Carlo simulations, like Excel does with @Risk. I understand NPV One can introduce this functionality if there is user demand for it. There will likely be ongoing conflict to try to keep the software simple to use versus accommodating the requests of customers to tailor the software to their specific needs.

Conclusion

The NPV One software is an option for those wishing to standardize or simplify their financial modelling.

Whether using Excel or NPV One, I would recommend that a single person is still responsible for the initial development and maintenance of a financial model. The evaluation of alternate scenarios must be managed to avoid it becoming a modelling team free for all.

Regarding the cost for NPV One, I understand they are moving away from a fixed purchase price arrangement to a subscription based model. I don’t have the details for their new pricing strategy as of May 2023. Contact Christian Kunze (ck@npvone.com) who can explain more, give you a demo, and maybe even provide a trial access period to test drive the software.

To clarify I received no compensation for writing this blog post, it is solely my personal opinion.

Regarding Excel model complexity mentioned earlier, I have written a previous blog about the desire to keep cashflow models simple and not works of art. You can read that blog at “Mine Financial Modelling – Please Think of Others”.

As with any new mining software, I had also posted some concerns with QP responsibilities as pertaining to new software and 43-101. You can read that post at the appropriately titled “New Mining Software and 43-101 Legal Issues”.

Note: You can sign up for the KJK mailing list to get notified when new blogs are posted. Follow me on Twitter at @KJKLtd for updates and other mining posts. The entire blog post library can be found at https://kuchling.com/library/

Steeper Pit Slopes Can Save Money

Date: April 14, 2023 Author: Ken Kuchling Comments: Leave a reply

We likely have all heard the statement that increasing pit wall angles will result in significant cost savings to the mining operation.

What is the potential cost saving?

The steeper wall angles reduce waste stripping volumes, which also provide other less obvious benefits.

I was recently in a situation where we undertook some comparative open pit designs using both 45 and 50 degree inter-ramp angles (“IRA”). I would like to share some of those results and discuss where all the benefits may lay.

Comparative Pit Designs

In this project, four separate open pits were designed with 45 and 50 degree IRA’s in an area with hilly topography. Some of the pits had high walls that extended up the valley hillsides. Its not hard to envision that waste stripping reductions would be seen along those areas with steepened walls.

The results of applying the increased inter-ramp angle to each of the four pits is shown in the Bar Chart. Note that the waste reduction is not necessarily the same for each pit. It depends on the specific topography around each pit.

However, on average, there was an overall 15% reduction in waste tonnage.

The Table shown below presents the cumulative tonnage for all four pits. The 50 degree wall results in a waste decrease of 25.4 million tonnes (15%), with a strip ratio reduction from 5.8:1 to 5.0:1.

There is also a very minor decrease in ore tonnage. This is because the 50 degree slopes did lose some ore behind the walls that is being recovered by the 45 degree slope.

In both scenarios the project life would be about 10 years at an assumed ore processing rate of 3 Mtpa.

4 Positive Impacts of Steeper Walls

In general one can typically see four positive outcomes from adopting steeper pit walls. They are as follows:

1. Cost Savings: The waste tonnage reduction over the 10 year life would be about 25.4 million tonnes. At a mining cost of $2.00/tonne, this equates to $50.8 million tonnes spent less on stripping. This could move the project NPV from marginal to profitable, since most waste is normally stripped towards the front part of the mining schedule with less discounting.

The next time you are looking at the NPV from an open pit project, take a quick look to see if the pit slope assumptions are conservative or optimistic. That decision can play a significant role in the final NPV.

2. Equipment Fleet Size: Over the 10 year life, the average annual mining rate would range from 20.5 Mtpa (45 deg) to 18.1 Mtpa (50 deg). On a daily basis, the average would range from 56,100 tpd (45 deg) versus 49,700 tpd (50 deg). While this mining rate reduction is not likely sufficient to eliminate a loader, it could result in the elimination of a truck or two. This would have some capital cost saving.

3. Waste Dump Size: The 15% reduction in the waste tonnage means external waste dumps could be 15% smaller. This may not have a huge impact but could be of interest if waste storage sites are limited on the property. It could have a more significant impact if local closure regulations require open pit backfilling.

4. Pit Crest Location: The steeper wall angles result in a shift in the final pit crest location. The Image shows the impact that the 5 degree steepening had on the crest location for one of the pits in this scenario.

Although in this project the crest location wasn’t critical, there are situations where rivers, lakes, roads, mine facilities, or public infrastructure are close to the pit. A steeper wall could improve ore recovery at depth while maintaining the same buffer setback distance.

Conclusion

Steeper pit walls can have multiple benefits at an open pit mining operation. However, these benefits can all be negated if the rock mass cannot tolerate those steeper walls. Pit wall failures could be minor or they could have major impacts. There are the obvious worker safety issues, as well as equipment damage and production curtailment concerns with slope failures. Public perception of the mining operation also comes into play with dangerously unstable slopes.

Steepening of the pit walls is great in theory, but always ensure that geotechnical engineers have confirmed it is reasonable.

It is relatively easy to justify spending additional time and money on proper geotechnical investigations and geotechnical monitoring given the potential slope steepening benefits.

When designing pits, there is some value in looking at alternate designs with varying slope angles to help the team understand if there are potential gains and how large they might be.

In closing, I previously wrote a related blog post about how pit walls are configured to ensure safe catch bench widths and decisions as to whether one should use single, double, or triple benching. That earlier post can be read at this link. Pit Wall Angles and Bench Widths – How Do They Relate?

Feel free to share your personal experiences if you are aware of other benefits (or even downsides) to steeper pit walls that I did not mention.

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Life as an Engineer – Read All About It

Date: December 9, 2022 Author: Ken Kuchling Comments: 2 Replies

One of the interesting aspects of being an engineer in the mining industry is travelling around the globe (or even) around your own country. I have been to over a dozen countries as part of my career and this only makes me a small-time traveller compared to other engineers I know. Travelling and experiencing the world is often part of the job, whether working for a junior miner, a major, a financial house, a consulting firm, or an equipment vendor. It is actually quite difficult to avoid travel if you work in mining.

Diavik Project

Recently a former colleague of mine on the Diavik Diamond Diavik project has published book that describes his life as an engineer. The book is titled Roseway: a Life of Adventure and is available on Amazon.

Its the story of John Wonnacott, a Canadian professional engineer who was involved in the construction of several projects, including the Diavik Diamond mine in Canada, a nickel smelter in China, a gold mine in Brazil, and a titanium mine in Madagascar to list a few.

John has a broad background, having conducted engineering studies in the jungles of Indonesia, the cold of Greenland, the sands of the desert, the heat of Australia, the altitude of the Andes. He has documented his engineering career in his new book.

Disclaimer: I have not yet read the book since it has only recently been published. However John has kindly sent me some excerpts that I have reprinted below to provide everyone with a sense for the content and style.

Some Excerpts

Introduction

At one time or another, I have been a professional paper-boy, forest worker, tree planter, market gardener, food processing equipment operator, lobster fisherman’s helper, commercial dragger deckhand, short-order cook, military engineering officer, computer system installer, greenhouse worker, permafrost researcher, marine oil spill cleanup specialist, pyrometallurgy researcher, garbage landfill operator, project manager, construction company general manager, regional director, open pit diamond miner, underground gold miner, corporate vice-president, design consultant, company owner, private corporation president and for 50 years, a damn good engineer. I have also been happily married to my wonderful wife Carole Anne for more than 52 years and we have 2 outstanding children. So I can add “husband”, “father” and “grandfather” to the list – but making lists like this is boring. Let me tell you my story.

Newfoundland

I remember in the late fall of that year, the company had a chance to bid on a larger project in Gros Morne National Park, Newfoundland. So our President, Frank Nolan (he was a brother to Fred Nolan, the infamous land-owner at Oak Island, by the way), decided he wanted to see the site and he chartered a Bell 106 helicopter to fly us there from Deer Lake. It was December (they say “December month” in that province) and when we got close to the Park, we ran into a sudden snow squall.

From bright sunny weather we were suddenly flying in heavy wet snow. I was sitting in the back of the chopper, with Frank sitting in the left front passenger seat. We were chatting with the pilot, via the radio headsets, when suddenly there was a loud “BEEP BEEP BEEP” sound coming from the front of the aircraft, and a number of the instrument lights started flashing. The engine had cut out – we learned later that wet snow had blocked the air intake and the engine had stalled – and we started descending pretty fast. Most people don’t realize that a helicopter will glide (quite steeply, at a glide angle of about 10 to 1) provided the pilot gets the torque off the rotor and he makes the correct feathering adjustments.

Our pilot did that instinctively and when we passed through the squall he calmly explained to us what was happening as he looked around for an open, flat spot to land. We didn’t have many options as we were flying over a densely wooded forest, with the mountains of Gros Morne and a deep fiord up ahead. But the pilot spotted a snow-covered frozen bog that was not a lot bigger than the helicopter and he put us down there as smoothly as if the engine hadn’t stopped. Maybe the deep snow cushioned our impact, because I felt nothing. But the instant we landed, Frank Nolan wrenched his door open, and he bolted out of the machine, straight ahead, in front of us.

The rotor was still spinning rapidly, and just as Frank ran ahead, the chopper settled further into the snow, tilting the machine forward in the process. With the chopper blades almost skimming the top of the snow, both the pilot and I expected Frank to be cut into pieces by the rotor, but he was just past their reach and he ran on, unaware of his narrow escape. When the spinning parts stopped, the pilot and I climbed out of the chopper to catch up with Frank. Examination of the machine showed us how the snow had plugged the air intake. The pilot cleared away the snow, and walked around the chopper once and then we took off again. We continued our aerial inspection of the National Park project and later that afternoon we flew back to Deer Lake.

Madagascar

The QMM field office In Port Dauphin, Madagascar was located near the edge of town, and I typically walked from my lodging to the office each morning when I was there, about the time when school started for the children. Typically I passed dozens and dozens of tiny bamboo huts with corrugated metal roofs, and dirt floors each about 2 meters square.

I was constantly amazed by the flocks of young boys and girls walking to school – each child aged from 6 to 15 years old, I suppose – dressed in immaculate white shirt or blouse and blue shorts or skirts. I never saw a dirty child, and how they could have kept clean clothes while living in those small crude huts was something I never could figure out. Even more amazing, were the genuine, wide smiles and frequent greeting as we passed the children: “bonjour monsieur, bonjour monsieur”.

It brings tears to my eyes even now, thinking about those children. If they were girls, they could look forward to a life expectancy of 48 years, according to the town officials we talked to. If they were boys, they could expect to live to an age of only 40 years. The perils of fishing in the ocean in dugout canoes made life even harder for the men.

The next morning, we arrived at the Astana international airport, to find that the check-in arrangements were quite different from what we were used to. Instead of checking our luggage at a desk and then walking through Security to get to our departure gate, everyone was expected to wheel their luggage and handbags through security, as the airline check-in desks were located inside.

The mechanics of rustling our luggage weren’t difficult, but as I passed through the check-point, suddenly a strange-sounding alarm went off. As the alarm rang and rang, my mind raced – what did I have in my bag that would trigger the alarm, I wondered? It didn’t help that the Security guards only spoke Russian, and they were dressed in military uniforms with ridiculously large military caps, which made them look imposing (and silly). But what began to worry me more, was that the look on the Security guards’ faces was not the usual one that happens when a piece of metal sets off an alarm. The guards looked frightened and angry, at the same time.

Fortunately for us, the commotion caught the attention of the clerks at the Turkish Airlines desk inside the terminal building, and one English-speaking fellow approached, speaking to the security guards in Russian first, then saying to us: “I speak English, may I help”? Well, he helped, but it took a while, because it turned out that a rarely used hidden nuclear radiation detector had been triggered when I came through the gate, and the guards were concerned that I had some kind of radioactive material in my suitcase.

For a moment my mind went blank, and then I remembered a card that I was carrying in my wallet. I had had a bout of prostate cancer the previous fall, and my brachytherapy treatment had involved inserting over a hundred tiny radioactive pellets in and around my prostate – designed to kill the cancer. The pellets decay naturally in a fairly short time, and by now, 9 or 10 months after my operation, I would have bet that the radioactive material had all decayed to an undetectable level. But my doctor had given me a card to carry, which explained the medical procedure, just for circumstances like this. When I pulled out the card, it was like a “Get out of Jail Free Card” from the Monopoly game. Instantly the guards’ attitudes changed from fear and suspicion, to sympathy and smiles. One of the big fellows wheeled my luggage over to the Turkish Airlines desk where the Good Samaritan clerk reverted back to his normal job of checking us in.

**** end of excerpt ****

Conclusion

It is one thing to briefly visit a remote project as part of a review team. It is another thing to be there as part of a design team trying to solve a problem and engineer a solution. I know of many engineers and geologists that would have similar work life experiences as part of their careers. However John has taken the initiative to write it all down.

The author is available to be contacted on LinkedIn if you have any questions or just want to say hello (at https://www.linkedin.com/in/john-wonnacott-84aa461a/).

The book can be found on Amazon at this link: Roseway: a Life of Adventure.

This is a story from the life of an experienced engineer working in the mining industry. If you want to read the perspective from a new mining engineer graduate, check out this post “A Junior EIT Mining Story“. There is no book deal yet here.

If you find stories about working as an engineer of interest, I have written a 2 part blog post on my adventures in the potash industry in Saskatchewan. You can read that post at this link “Potash Stories from 3000 Feet Down – Part 1“

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Clays and Mining – Friends or Foes?

Date: August 24, 2022 Author: Ken Kuchling Comments: 3 Replies

Overburden is a generalized termed used to describe unconsolidated material encountered at a mine. It can consist of gravels, sands, silts, and clays and combinations of each. Usually overburden is not given much focus in many mining studies. Very often, the overburden as a unit, is not adequately characterized.

This blog will explain why proper characterization can be an important issue, particularly the clay component.

I also want to share some personal mining experiences with clays, all types of clays. There is more to them than meets the eye; a fact often not apparent to those involved only in hard rock mining.

Clays have unique geotechnical properties that can make for challenging situations and require special consideration in project design. Many simply view clay as a sticky cohesive material – no big deal. So let’s examine this a bit further. I tried to avoid geotechnical lingo where possible, since this blog isn’t being written for geotechnical engineers.

There are several types of clays, or clay-like materials that can be encountered in mining. Here are the ones that I have been lucky (or unlucky) enough to have dealt with over the years.

Normally consolidated clays
Over-consolidated clays
Sensitive (or quick) clays
Swelling clays
Saprolite clays
Kimberlite clays (muds)

What are the challenges?

Each of the clays listed above can be found in different locations, have unique properties, will behave differently, and can create specific mining challenges. Clays can also cause problems in process plant circuits, but that is a subject outside my area of expertise.

Normally consolidated clays

These are the clays most people are familiar with, i.e. a sedimentary deposit of very fine particles that have settled in a calm body of water. Normally consolidated clays are generally not a problem, other than having a high moisture content. As such, they can be very sticky in loader buckets, truck boxes, and when feeding crushers.

When wet, they can become sloppy and difficult to handle efficiently. They can creep and run when placed into waste dumps. For these reasons, engineers must be aware if a large proportion of the overburden will consist of clays so they can avoid surprises.

Over-consolidated clays

These clays have undergone greater vertical compression in their history than in their current condition. For example, perhaps they were once pre-loaded and compressed by a mile of glacial ice sheet during an ice age, which has subsequently melted.

Clays in general consist of very fine plate like particles, as shown in this sketch. In over-consolidated clays, these particles have been flattened and tightly compressed as in the right image. The result is that the clay may be dense, have a good cross bedding shear strength, but very low shear strength along the plates. This characteristic is analogous to the lubricating properties of graphite, which is facilitated by sliding along graphite plates.

My experience in working with over-consolidated clays was at the Fort McMurray oil sands mining operations. In that region the Clearwater clays formed part of the overburden sequence above the oil sands. Stripping these clays with trucks and shovels was not exceptionally challenging. They had low moisture content and were stiff. The challenge really came when needing to build on top of them, for example building a waste dump or tailings dam.

The cross-bedding shear strength was good, with peak friction angles exceeding 25 degrees. However after any creep or deformation, the peak shear strength was gone and the residual friction angle would now control stability. The residual friction angles could drop as low as 6 degrees (very weak) and, upon surcharging the clay could maintain high internal pore pressures. Due to these factors it was not uncommon to see tailings dams or waste dumps with 15:1 (H:V) downstream slopes. This compares to the 3:1 slopes one may normally see at hard rock mine sites.

Building a 15:1 dam or dump is much less volume efficient than building a 3:1 embankment. It also doesn’t take much instability to cause an embankment to creep along a foundation with only a 6-degree friction angle. Hence the over-consolidated clays presented a unique engineering challenge when working in the oil sands.

Sensitive (quick) clays

Referring to the clay particle sketch shown above, quick clays represent a card house structure (on the left image). These clays were often deposited in a quiet marine environment, where electrical charges prevented the clays from settling uniformly. Instead, the clay particles tend to stack up like a house of cards. The large void spaces are filled with water, whereby moisture contents can exceed 100% by weight.

When these clays are disturbed by vibration or movement, the house of cards structure collapses. Combined with the excess void water, these clays will flow…. and flow a lot. This video shows a slope failure in quick clays in Norway. Try to stop that failure once it has initiated.

My experience with sensitive clays was at the former BHP bauxite mining operations along the northern coast of Suriname. There were Demerara clay channels up to 20m thick over top of many of their open pits. The bucketwheel excavators used for waste stripping would trigger the quick clay slope failures, sometimes resulting in the crawler tracks being buried and unfortunately also causing some worker fatalities.

I recall walking up towards a bucketwheel digging face as the machine quietly churned away. About 70 metres from the machine, we would see cracks quietly opening all around us as the ground mass was starting to initiate its flow towards the machine. Most times the bucketwheel could just sit there and dig. Instead of the machine having to advance toward the face, the face would advance towards the machine.

To address the safety issue, eventually mine-wide grids of cone penetration tests were used to define the Demerara clay channels. Dredges were then brought in to remove these channels before allowing the bucketwheels to strip the remaining sands and normally consolidated clays.

Swelling clays

In some locations, mines may contain swelling clays. The issue with these clays is that they can absorb water rapidly, swell by 30%, and become extremely soft to operate on. If they form part of the ore zone and find their way to the tailings pond, one may find they don’t want to settle out in the pond. Water clarification and clean water recycle to the plant can become an operational issue. Mineralogy tests will indicate if one has swelling clays (smectites, montmorillonites, bentonite). The swelling clays do have a functional use however, discussed later.

Kimberlite clays (muds)

The formation of the diamond deposits in northern Canada often involved the explosive eruption of kimberlite pipes under bodies of water. The lakebed muds and expelled kimberlite by the eruption would collapse back into the crater, resulting in a mix of mud and kimberlite (yellow zones in the image). This muddy kimberlite could be soft, weak, and difficult to mine with underground methods.

Normally as one descends deeper into the kimberlite pipe, the harder primary kimberlite dominates over the muddy material. An upside is that the muddy kimberlite can be scrubbed fairly easily during processing, with the very fine clay particles being washed away.

Clays can’t be all bad?

Encountering clays at a mine site can’t be always negative? There must be some benefits that clays can provide? Well there are a few positive aspects.

Saprolite clays

At many tropical mining operations (west African gold projects for example) the upper bedrock has undergone weathering, resulting in the fresh rock being decomposed into saprolite. This clay-rich material can exceed 50 metres in thickness, can be fairly soft and diggable without blasting. This is an obvious mining cost benefit.

As well, grinding circuits can easily deal with saprolite. For example, if a 1000 tpd grind circuit is designed for the underlying deeper bedrock, it may be able to push through 1400 tpd of saprolite. This would yield a 40% increase in mill throughput for little added cost. This will boost early gold production. However as the blend of saprolite to fresh rock declines over the years as the pit deepens, the plant throughput will decrease to the original design capacity.

One concern with saprolite sometimes is its sticky nature. A truck load of saprolite ore dumped on a crusher grizzly may just sit there. Possible some prodding or water flushing may be required to get it moving. Nevertheless, this is normally an easily resolvable operating issue.

Clay core dams

One of the ways miners build water retention or tailings dams is to use mined waste rock. The issue with this is that a dam built solely with waste rock will leak like a sieve, which can lead to piping failure. One solution is to build an internal clay core in the center the dam to act as a seepage barrier. Having on-site access to good quality clean clay fill is a benefit when such dams are required. If the clay fill isn’t available at site, then more complex synthetic liners or internal seepage control measure must be instituted.

Compacted clay fill can also be used as a pond liner material for water retention ponds.

One can also purchase rolls of geosynthetic clay liners (GCL), whereby a thin layer of dried swelling clay is encapsulated between two sheets of geotextile. Once the liner is laid out and re-hydrated with water, the clays swell and will act as an impervious liner. The installation approach is somewhat simpler than for HDPE liners and such liners can be self-healing if punctured. A downside is that the transport weight of these GCL liners can be significant.

See, there are some positives with having clay at site.

Conclusion

All clays are not the same. The mining of clays can create unique challenges for mining engineers and operating personnel. Whenever I see study happily mention that their open pit mine waste consists of “free digging overburden”, I say to myself “Be careful what you wish for”.

One must ensure that the overburden is properly characterized, even though it may be viewed as an unimportant or uninteresting material. Determine whether it consists of gravels, sands, silts, tills, or clays, or combinations thereof. It can make a big difference in how it is mined, disposed of, and whether it can have any secondary uses on site. In many studies that I have reviewed, the overburden tends to be forgotten and does not get the technical respect it deserves.

Please feel free to share any thoughts on your experience in working with clays.

We had to build mine haul roads across large swamps underlain by soft clays. One option was to use geo-textiles or swamp vegetation to assist us. Another option was to place the sand fill hydraulically. You can read how we did this at the following blog post “Using Pumped Sand to Build Mine Roads“.

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On the other hand, Junior miners will follow one of two paths. They are either on (a) the Mine Builder path, or (b) the Mine Vendor path (i.e. sell the project to someone who will build it). In this article, I will present some examples of companies on each path.

There will also be some discussion on whether the engineers undertaking early stage studies (e.g., PEA’s) should be considering the path their client is intending to follow.

The Mine BUILDER Path

Although the duration of each timeline is different due to different project complexities, the development paths are consistent. Most junior miners would not consider themselves on the Builder path.

The Mine Vendor Path

In other cases the Junior miner realizes their project is too large with a massive capital cost. That capital cost is beyond the financial capability of the company. Hence a deep-pocket partner is needed or an outright sale is preferred.

The Mine Vendors tend to follow a different development path than the Mine Builders. They don’t have the same fast track objective as the Mine Builder. Vendors want out at some point.

Sometimes you will see a management team state, if the project isn’t sold, they are going to put it into production themselves. This may be true in some cases, or simply part of the negotiating game to try to maximize the acquisition price.

An QP Engineer’s Perspective

A pertinent question is whether the independent geologists and engineers working on the advanced studies should be aware of the path the company is intending to follow. Is the company a Builder or a Vendor?

Some may feel that the focus of the technical work should be independent of the path being followed. Based on my experience as both an owner’s representative and independent study QP, I have a somewhat different opinion. The technical work should be tailored to the intended path.

The Engineer on the MINE BUILDER Path

If an engineer understands that a Mine Builder’s project will move from PEA to PFS to FS in rapid succession, then there is more incentive to ensure each study is somewhat integrated.

The Engineer on the MINE VENDOR Path

The objective of the Mine Vendor is to make the project attractive to potential buyers. There is less urgency in fast tracking towards detailed engineering.

The role that the Inferred resource plays in the economics is also less important at this time, since a lot more infill drilling may be coming. The Vendor’s objective tends to be on maximizing resource size not necessarily optimizing resource classification.

Conclusion

Potential buyers will have their own technical team that will develop their own opinions on what the project should be and what it should cost. Just because a Mine Vendor has a Feasibility Study in hand, doesn’t mean a potential buyer will believe it.

Note: You can sign up for the KJK mailing list to get notified when new blogs are posted. Follow me on Twitter at @KJKLtd for updates and other mining posts. The entire blog post library can be found at https://kuchling.com/library/

For some free mining calculator apps, including project timelines and a simplified cashflow modeller, check out this website https://sites.google.com/view/drillingdown

This article is about the benefit of preparing (cutting) more geological cross-sections and the value they bring.

Geological sections are one of the easiest ways to explain the character of an orebody. They have an inherent simplicity yet provide more information than any other mining related graphic.

Some sections can be simple cartoon-like images while others can be technically complicated, presenting detailed geological data.

Cartoon-stylized sections are typically used to describe the general nature of the orebody. The detailed sections can present technical data such as drill hole traces, color coded assays intervals, ore block grades, ore zone interpretations, mineral classifications, etc.

Sections provide a level of clarity to everyone, including to those new to the mining industry as well as those with decades of experience.

This article briefly describes what story I (as an engineer) am looking for in sections. Geologists may have a different view on what they conclude when reviewing geological sections.

I will describe the three types of geological sections that one can cut and what each may be describing. The three types are: (1) longitudinal (long) sections; (2) cross-sections; (3) bench (level) plans. Each plays a different role in helping to understand the orebody and mining environment.

There is also another way to share simple geological images via3D PDF files. I will provide an example later.

Longitudinal (Long) Sections

Long sections are aligned along the long axis of the deposit. They can be vertically oriented, although sometimes they may be tilted to follow the dip angle of an ore zone.

Long sections are typically shown for narrow structure style deposits (e.g. gold veins) and are typically less relevant for bulk deposits (e.g. porphyry).

The information garnered from long sections includes:

The lateral extent of the mineralized structure, which can be in hundred of metres or even kilometers. This provides a sense for how large the entire system is. Sometimes these sections may show geophysics, drilling to defend the basis for the regional interpretation.

Cross-Sections

Cross-sections are generally the most popular geological sections seen in presentations. These are vertical slices aligned perpendicular to the strike of the orebody. They can show the ore zone interpretation, drill holes traces, assays, rock types, and/or color-coded resource block grades.

As an engineer, my greatest interest is in seeing the resource blocks, color coded by grade. Sometimes open pit shells may be included on the section to define the potential mining volume. The engineering information garnered from block model cross-sections includes:

Where are the higher-grade areas located; at depth or near surface?

If a pit shell profile is included, what will the relative strip ratio look like? Are the ore zones relatively narrow compared to the size of the pit?

How will the topography impact on the pit shape? In mountainous terrain, will a push-back on pit wall result in the need to climb up a hillside and create a very high pit slope? This can result in high stripping ratios or difficult mining conditions.

Does the ore zone extend deeper and if one wants to push the pit a bit deeper, is there a high incremental strip ratio to do this? Does one need to strip a lot of waste to gain a bit more ore?

Are the widths of the mineable ore zones narrow or wide, or are there multiple ore zones separated by internal waste zones? This may indicate if lower-cost bulk mining is possible, or if higher cost selective mining is required to minimize waste dilution.

How difficult will it be to maintain grade control? For example, narrow veins being mined using a 10 metre bench height and 7 metre blast pattern will have difficulty in defining the ore /waste contacts.

Cross-sections that show the ore blocks color coded by classification (Measured, Indicated, Inferred), illustrate where the less reliable (Inferred) resources are located and how much relative tonnage may be in the more certain Measured and Indicated categories.

When looking at cross-sections, it is always important to look at multiple cross-sections across the orebody. Too often in reports one may be presented with the widest and juiciest ore zone, as if that was typical for the entire orebody. It likely is not typical.

Stepping away from that one section to look at others is important. Possibly the character of the ore zones changes and hence its important to cut multiple sections along the orebody.

Bench (Level) Plans

Bench plans (or level plans) are horizontal slices across the ore body at various elevations. In these sections one is looking down on the orebody from above.

Level plans are typically less common to see in presentations, although they are very useful. The level plans may show geological detail, rock types, ore zone interpretations, ore block grades, and underground workings.

The bench plan represents what the open pit mining crews would see as they are working along a bench in the pit. The information garnered from bench plans that include the block model grades includes:

Where are the higher-grade areas found on a level? Are these higher grade areas continuous or do they consist of higher grade pockets scattered amongst lower grade blocks?

Do the ore zones swell or pinch out on a bench? A vertical cross-section may give a false sense the ore zones are uniform. The bench plan gives an indication on how complicated mining, grade control, and dilution control might be for operators.

Do the ore zones on a bench level extend out beyond the pit walls and is there potential to expand the pit to capture that ore?

On a given bench what will the strip ratio be? Are the ore zones small compared to the total area of the bench?

As recommended with cross-sections, when looking at bench plans, one should try to look at multiple elevations. The mineability of the ore zones may change as one moves vertically upwards or downwards through a deposit.

Never mind cross-sections – give me 3D

While geological sections are great, another way to present the orebody is with 3D PDF files to allow users to view the deposit in three-dimensions. Web platforms like VRIFY are great, but I have been told they sometimes can be slow to use.

You can also create your own simplistic cross-sections through the pdf menus (see image).

A simple example of such a 3D PDF file can be downloaded at this link (3D DPF File Example). It only includes two pit designs and some ore blocks to keep it simple.

Conclusion

The different types of geological sections all provide useful information. Don’t focus only on cross-sections, and don’t focus only on one typical section. Create more sections at different orientations to help everyone understand better.

In 2019 I wrote an article describing the lack of geological cross-sections in many 43-101 technical reports. The link to that article is her “43-101 Reports – What Sections Are Missing?”

Geological sections are some of the first items I look for in a report. Sometimes they can be hidden away in the appendices at the back of the report. If they are available, take the time to actually study them since they can explain more than you realize.

Note: You can sign up for the KJK mailing list to get notified when new blogs are posted. Follow me on Twitter at @KJKLtd for updates and other mining posts. The entire blog post library can be found at https://kuchling.com/library/

The next time you see the grade-tonnage relationship, I suggest taking a few seconds to study it a bit further. There might be some interesting things in there.

Typical Grade-Tonnage Information

Useful Grade-Tonnage Curve Information

When I am undertaking a due diligence review or working on a study, very early on I like to have a look at the grade-tonnage information. This could be for the entire deposit resource, within a resource constraining shell, or in the pit design.

The grade-tonnage information gives an understanding of how future economics or technical issues may impact on the mineable tonnage.

An example of a typical grade-tonnage curve is shown here.

The cutoff grade along the x-axis will be impacted by changes in metal price or operating cost. The cutoff grade will increase if metal prices decrease or if operating costs increase.

The question is how sensitive is the mineable tonnage to these economic factors. The slope of the tonnage and grade curves will help answer this question.

In the example shown, the tonnage curve (blue dots) is fairly linear, meaning the ore tonnage steadily decreases with increasing cut-off grade. That is expected and is reasonable.

However, if the tonnage curve profile resembled the light blue line in this image, with a concave shape, the ore tonnage is decreasing rapidly with increasing cutoff grade. This is generally not a favorable situation.

I also like to compare the ratio of the average grade to the cutoff grade. Its nice to see a ratio of 4:1 to 5:1 to ensure the overall average grade isn’t close to the cutoff. In this example, the cutoff grade is 0.5 g/t and the average grade is 4.5 g/t, a ratio of 9:1.

The tonnage curve and grade curve provide information on the nature of the mineral resource. Study them both.

Reporting Waste Within a Shell

One complaint I have about reporting mineral resources inside a resource constraining shell is the lack of strip ratio information. This applies whether disclosing a single mineral resource estimate or variable grade-tonnage data.

In my view, the strip ratio is even more important to be aware of when looking at grade tonnage data.

Its common to see mention that the mining cost is (say) $2.50/t, but if the strip ratio is 10:1, that equates to an effective mining cost of $27.50 per tonne of ore. That’s an important cost to know, especially if one is pushing a pit shell deep to maximum the mineral resource tonnage.

Spider Diagram Downsides