The construction of plausible geological models

We had the opportunity to discuss with Mike Richard, Chief Geological Officer, and Pablo Mejia-Herrera, VP Exploration, from ERO Copper Corp about the importance that integrated 3D multidisciplinary models have in the discovery of new mineralization in the Curaçá Valley. We discussed Mira Geoscience’s contribution to further understand the Mineração Caraíba project area using modern exploration techniques such as the generation of a regional geological model driven from potential fields and EM surveys, and improving unbiased mineralization prediction with a data-driven targeting workflow approach. These techniques proved to be helpful in generating new 2D and 3D models to help ERO move into new areas of interest throughout the valley.

Q: Mike, your collaboration with Mira Geoscience has been going on for a few years now. We got the chance to work with you at Teck, Lundin, and now with ERO Copper. In what way is the collaboration different this time around?

MIKE: Quite different. ERO Copper was founded in mid-2016 and the acquisition of Mineração Caraíba occurred at the end of 2016. We started working in 2017 so it’s a very young company. We started with 4 or 5 people and acquired Mineração Caraíba that had over 1,500 employees. The main difference initially, with an undertaking like that, is with support. At Teck, there’s a lot of specialized people in the organization to look at all kinds of things, do all kinds of work. Whereas at ERO, we were basically alone. Because our plan was to move very quickly both in operations and in exploration, Mira helped us get started on the right foot from day one without really having to slow down other priorities. The learning curve is incredibly steep, so having Mira on board allowed us to get a better handle on what had been done prior to the acquisition and define preliminary targets early on.

Q: ERO Copper seems to be committed to the Common Earth Model (CEM) approach. We’d like to know a little bit more why that is.

PABLO: The reason that we are committed to the CEM approach is because we are dealing with a lot of information. Mineração Caraíba has more than 40 years of exploration data. The CEM approach allows us to analyze and model all this information very quickly. [To do this, we use GOCAD Mining Suite.] I can’t think of another platform that can analyze and integrate all the information in order to query the data. It synthesizes what is important and streamlines our exploration programs.

MIKE: Yes. One of our biggest challenges is that the property is approximately 120 km long by 25 km wide. It took over a year to compile all that historic data together and we’re still finding data! It was mostly surface geochemical geophysical and drill data. There are a large number of [mineral] prospects throughout the valley. Prioritizing those prospects is really what we do every day. We’re very aggressive, so we wouldn’t be able to move at this speed without a data-driven approach [such as CEM].

Caption: Contribution to the exploration model obtained by filtering of constrained inversion Voxet. © ERO Copper Corp. All rights reserved

Q: Can you describe a little bit more how GOCAD Mining Suite (GMS) was valuable in the understanding your exploration data?

PABLO: This platform is not just there to allow us to visualize and interrogate the data. It enables us to create scenarios from the data or modelling, interpolate or extrapolate the data, and try to figure out what’s going on from the data itself. It’s not just integrating the data but creating all the possibilities that you have using the data as an input. It’s trying to understand the potential outcomes using the data. That’s one of the main advantages of GMS.

MIKE: Yes, for a non-expert like me, I would say that [if you need] versatility and the ability to query a vast amount of data, GMS is definitely one of the best platforms out there.

Q: Since 2018, ERO Copper has been using modern exploration techniques1 in the Curaçá Valley. Can you outline the valley’s potential to host major mineralization?

MIKE: Well, as we discussed, it’s a very large area [more than 120 km by 25 km] with 40 years of exploration based on immediate needs: [the mining reserve renewal] and the need to keep the property in good standing with the government. [The past owner] did IP and gravity, but on a “postage stamp” coverage basis. The idea was to add regional gravity and EM surveys, and then integrate all that into a model that allows us to prioritize the targets. In the southern end of the valley, there’s the Pilar mine that’s been operating for 40 years. The new Vermelhos mine that we constructed is located in the northern end. The footprint at the surface for Pilar is about 2 ½ km and I would say that tonnage [including historic production and current reserve] is over 100 million tons grading more than 1% copper. It’s a major deposit. If you compare Vermelhos [80 km up north] the deposit’s footprint on surface is over 5 km. We’re seeing copper for over 5 km with soil anomalies, gravity anomalies, IP anomalies, EM anomalies, etc. Drilling is generally to 300 metres depth in localized portions of that 5 km. We’ve found very high-grade mineralization. So, to answer your question, I think there’s already evidence of major mineralization! We’re starting to uncover new districts. So, one in the south, Pilar, one in north, Vermelhos, and we think we can identify more of these districts throughout the belt (between Vermelhos and Pilar).

PABLO: This is a 120 km long belt. We extend exploration to the upper part of the Curaçá Valley to see the potential with the [electromagnetic, magnetic and gravity] airborne surveys that we did recently. We can now model the upper part of the crust using constrained modelling of the geophysics. This allows us to see the potential in the ground by taking into account the architecture of the upper crust.

Q: Mira Geoscience has been helping ERO with a reinterpretation of the belt. What differences will the new model make to the geological understanding of the area? Have there been any major surprises so far?

MIKE: To me, the first surprise would be the work done on the gravity in identifying an important phase of lower density magmatism underplating the Vermelhos district. It seems to correlate with an alteration event we see regionally. The question is whether this event is late-magmatic and/or post-magmatic. The second thing would be the higher conductance targets in the BHEM. It’s important to add that Scott (Principal Geophysicist at Mira Geoscience) does all our BHEM interpretation, so Mira Geoscience does all that. It’s found pyrrhotite-dominated massive sulphide breccia mineralization and works very well. We had no idea, or nobody had any idea of that being down there prior to that BHEM modelling work. I would say those would be the two main surprises, or novelties, for me.

PABLO: Since ERO Copper started exploration, we implement modern techniques [e.g., BHEM] combined with a lot of historical data digitization, and we can say that it works pretty well in helping us find the high grades. I don’t think it’s a surprise, but it’s new. Now we can say that it’s actually very useful in Curaçá Valley or in other districts in the world. That’s the “surprise” – we can put more confidence in exploration using this model.

Caption: Curaçá Valley Region regional 3D model representation. Top left: residual gravity overlaid on modelled lithological domains; Top right: classification compartments to assign physical properties for further modelling; Bottom left: Total Magnetic Intensity (TMI) with lithological overlay; Bottom right: lithological domain bounderies overlaid on radiometric image to cross-check correct lithological classification viewed in GOCAD Mining Suite. © ERO Copper Corp. All rights reserved

Q: Mira Geoscience worked for you in our traditional role (integrated interpretation, investigative modelling and construction of a CEM), but also did on short-timeframe work such as QA/QC of the airborne geophysics and BHEM interpretation. How has this worked for you?

MIKE: Rapidly and effectively! It worked really well. We try to be as organized as possible, but things are changing all the time, so we try to adjust quickly. We are reviewing the plan all the time; even if that means moving drill rigs according to what the data is saying. [Working with Mira] has worked out very well because we can switch plans quickly. We have these projects appearing throughout the year. Specific projects for areas that are a bit out of our expertise or that we don’t have that much time to do. They are the kind of open-ended project that allows us to really move quickly, and switch things around and throw things at JP (Director, Global Consulting at Mira Geoscience) and Scott. You know we’d have to build up a much larger team in the head office, which we don’t want to do, to be able to do this kind of work.

PABLO: Yeah, ERO Copper has, maybe, one of the most aggressive exploration programs in the world. We need this kind of dynamic, versatility, and flexibility in decision-making. Mira Geoscience is on the same page. We can analyze the data today in one location or district, and the next week do another district and analyze the data again. We have this flexibility because we have Mira Geoscience’s team aligned with a work program that is also flexible.

Q: Is it accurate to say that we act as your external geophysical department?

MIKE: Yes, absolutely, but not just that. JP’s work has been on targeting2. It also allows us to have another set of eyes look at stuff. Some people think it’s better to have everything internal because you learn more. My thinking is, it’s better to have more people look at it because you learn more.

Q: According to you, is the CEM approach a game changer for mineral exploration?

MIKE: Like in many other industries, mining is starting to rely on data much more than it did in the past. I think the CEM is the reflection of this; the ability to query all those data sets. There is no doubt that everybody will be on board eventually because you can’t do maps and sections only by hand anymore; there’s just too much data. I think it’s a major step change, and you guys have been at it for a long time and I don’t see it stopping. It’s only going to get more powerful.

PABLO: I think that’s right. From our experience, the CEM approach has led us to some significant the discoveries at depth. Some of the best discoveries that ERO Copper has made in previous years!

MIKE: The deep extensions are maybe the best!

PABLO: Yes, the discovery of Siriema is in part due to the CEM. Trying to put together all the information: geology, geophysics… and we [ended up with] the discoveries. So yes, this approach is working for us.

MIKE: When you’re in a data-heavy environment, this is the perfect place for this kind of application.

Technical note:

1: Because some of the mineralization produced conductive responses, a systematic BHEM (200m diameter), and a district-wide state-of-the art airborne EM (200 m – 250 m depth) survey were conducted to try to image the locations of conductive bodies. The magnetic and gravity data was then used to generate a physical rock-property-driven geological model which, combined with the soil data, ASTER imagery, radiometrics, and other layers of information, was used to discriminate between EM targets and generate new areas of interest.

2: Structural interpretations supported the targeting work. General craton boundaries interpreted from the gravity and magnetic data were established and suggest deep crustal boundaries that might be interpreted as important heat-upwelling zones. The radiometric data, in combination with the magnetics and gravity, were processed using unsupervised learning (Hierarchical Clustering) to attempt an improved unbiased prediction of the lithological map. The different map units generated by this approach help to better confine the surface expression of the lithological boundaries. Lastly, a data-driven targeting workflow based on multiple exploration features proved to be helpful in generating new maps for the prediction of mineralization through the belt.

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