The Outside-In Method—Herding Geologists Toward a Better Gold Model

This is ???????? ?? of a two-part series on the structural analysis of mineral deposits. Part 1 was a technical talk delivered by Brett Davis at the Australian Institute of Geoscientists (AIG) GOLD24 Symposium in Perth on 22 October 2024, which can be found here. Brett covered the fundamentals of the “inside-out” method of structural analysis of mineral deposits, while Part 2 is my talk from the same conference, presenting the exact opposite approach—the "outside-in" method discussed in Cowan (2020).

The "outside-in" method shifts the focus from small-scale details to big-picture patterns in drilling data. Instead of getting bogged down in the fine details—which can be overwhelming and takes decades of experience to decipher—we begin by identifying deposit-scale trends and then validate them in the field. Ideally, these interpretations are independently tested where exposures exist. I’ve collaborated with Brett, who conducted field investigations to confirm my deposit-scale interpretations.

In my talk (linked at the end of this article), I cover the application of the "outside-in" method for interpreting eight gold deposits at the deposit scale, but it’s important to understand how the method was developed and the context behind it. So, let me take you on a brief detour to explain where the "outside-in" method originated, before circling back to gold deposits.

An Outside Inspiration

The "outside-in" method traces back to my experience as a fluvial sedimentologist in the late 1980s during my MSc research in Gallup, New Mexico, under the supervision of Professor Andrew Miall at the University of Toronto. At the time, sedimentologists primarily conducted facies analysis by documenting and analysing vertical profiles—measuring vertical sections, focusing on grain size variations and sedimentary package thicknesses, all gathered from 1D vertical sections.

For decades, facies analysis was standardised around vertical profiles, and depositional environment models were built entirely from this 1D sampling approach, regardless of how extensive the rock exposure was. In the 1960s, John R.L. Allen, a highly respected researcher from the University of Reading, had presented the first complete statement of the fining-upward cycle as a definitive vertical sequence of grain size and primary sedimentary structures (Allen 1963). By the 1970s vertical profile analysis had become the accepted standard, as reflected in the widely used sedimentology textbook Facies Models (Walker, 1979).

Looking back, this rigid focus on 1D profile analysis—despite the availability of 3D exposures—seems baffling in hindsight. While the petroleum industry relied on 1D drilling data, sedimentologists studying sedimentary package geometry exposed in outcrop had no real reason to take the same approach—yet that’s the power of collective behaviour. When respected academics declare something the standard approach, few are inclined to challenge it!

The first signs of a shift away from 1D profiling emerged in October 1977, when Professor Allen delivered a keynote address at the First International Symposium on Fluvial Sedimentology in Calgary—an event organised by a young and ambitious Andrew D. Miall, a rising figure in fluvial sedimentology (Miall, 1978). Allen introduced the term "fluvial architecture", describing the geometry and 3D arrangement of channel and overbank deposits in a fluvial sequence —ironically beginning to dismantle his own earlier thesis, as all great scientists do!

Allen’s most influential contribution came in 1983, when he published a paper using large road-cut photographs to document and analyse the 3D architecture of fluvial deposits (Allen, 1983). His work later inspired Andrew Miall, by then a professor at the University of Toronto, to develop a framework for classifying fluvial styles based on large lateral exposures. In 1985, Miall coined the term “Architectural Element Analysis”—a methodology that shifted the focus to 2D lateral profile analysis, moving away from the traditional reliance on 1D vertical sections.

This shift from 1D to 2D and 3D perspectives sparked a quiet revolution—one that now seems so obvious in hindsight, yet at the time, it was anything but.

Miall’s review article became highly influential, with over 3,400 citations by 2025—including an impressive 50% increase in just the last six years (Figure 3). The impact was significant because it transformed facies analysis, extending its influence to other depositional environments. The new method revealed geologically meaningful large-scale patterns that were completely overlooked in the traditional 1D approach. The method was so effective it was used recently to interpret fluvial rocks on Mars (Salese et al., 2020)—a setting where small-scale details are inaccessible.

The Birth of "Outside-In" Analysis

So, what exactly is involved in the "outside-in" analysis in sedimentology? Let’s take a look.

The new approach to sedimentology emerged through a rigorous scientific process, expertly demonstrated by me and my field assistant, Paul Godin , in the images below. But first, let me set the scene for the 1980s—because, let’s be honest, most of you weren’t even born yet.

Back in 1988, when BIG hair reigned supreme and rogue shoulder pads caused wardrobe malfunctions—long before GPS, the internet, the Y2K apocalypse, mobile phones, and drones—geology was a whole different game. We navigated with paper maps, paced out distances one step at a time, used walkie-talkies instead of mobiles, and did our research in actual libraries instead of online. As for Paul and me, we waited a week for printed photos from the pharmacy, then painstakingly taped them together for our work—because "instant" drone scanning was pure science fiction.

“Outside-in” fieldwork in New Mexico, USA, 1988 Edition

Why the "Outside-In" Approach Matters

If there’s one thing I’ve learned over the years, it’s this: geologists often miss the forest for the trees. Whether in sedimentology or economic geology, the obsession with small-scale details has long overshadowed the need to understand deposit-scale geometries.

The "outside-in" method challenges this habit that focusses entirely on the small-scale details. Instead of getting bogged down in isolated observations, it forces geologists to first grasp the larger structural framework, then work their way down to finer details in context of the large-scale. This method has proven invaluable and revolutionised fluvial sedimentology.

A similar revolution can happen in mineral deposit studies, where ignoring structural controls since the 1950s has resulted in costly commercial mistakes (some listed in Reid and Cowan, 2023).

Unfortunately, blind geological modelling plagues the minerals industry, where software-generated models are treated as unquestionable truth, with little to no regard for deposit-scale structural context. Geostatistics is even worse—detached from geological reality, as most resource geologists seem oblivious to even the most basic structural principles. This isn’t just a theoretical problem—it has real-world consequences for exploration success and for Mineral Resource Estimation.

So what’s next for the minerals industry?

The mining industry has access to an abundance of drill data—far more than the petroleum industry could ever dream of—yet we are still not making full use of the data we have access to.

Applying the "outside-in" method in sedimentology is a straightforward process, as shown in the photos above. What took two people in 1988 can now be done by one person with a drone in a fraction of the time—though, of course, I’d still need an assistant for the beer run! In mining, all it takes is 3D software to visualise and analyse drilling data properly—unlocking the full potential of this approach.

The shift away from 1D interpretations in sedimentology took time, but it transformed the field, revealing insights that the inside-out 1D approach simply couldn’t provide. The mining industry is decades overdue for a similar shift. We have the drill data, the technology, and the structural insights to do better—but will we?

At GOLD24, I presented insights into multiple gold deposits that would not have been possible without applying the "outside-in" method. Two key findings from my talk were:

1. The shear zone model of gold—a model that has been accepted for decades—simply does not make sense viewed from the "outside-in" perspective; and
2. The external bounding geometry of mineralisation can only be determined using the "outside-in" method, and this is critical detail that must be considered by those applying the “inside-out” method (Figure 4).

These are important findings, and the details will be published in a paper co-authored with my mentor, Professor Bruce Hobbs , which is currently under review for the GOLD24 International Symposium Special Publication. If accepted, this Open Access paper will be published in the Australian Journal of Earth Sciences later this year, and I will share it with you once it is available for download (for the GOLD24 extended abstract see Cowan and Hobbs, 2024).

It’s time for geologists to stop blindly following the herd and start seeing the bigger picture by applying the "outside-in" method. If we do, the impact will be unparalleled—baaaa none.

Dedication

This talk, along with my paper with Bruce Hobbs on Perkins Discontinuities, is dedicated to Bill Perkins (W.G. Perkins), whose structural observations at Mount Isa, published in 1997, but ignored by economic geology academics, inspired my search for similar deposit-scale features across various deposit types, including gold. You were spot on, Bill—it just took the rest of the world 30 years to catch up!

References

[most papers can be found here]

Allen, J.R.L., 1963, Henry Clifton Sorby and the sedimentary structures of sands and sandstones in relation to flow conditions: Geol. Mijnbouw 42, 223-228.

Allen J.R.L., 1983, Studies in fluviatile sedimentation: bars, bar complexes and sandstone sheets (low-sinuosity braided streams) in the Brownstones (L. Devonian), Welsh Borders. Sedimentary Geology 133, 237-293.

Cowan, E.J., 2020, Deposit-scale structural architecture of the Sigma-Lamaque gold deposit, Canada—insights from a newly proposed 3D method for assessing structural controls from drill hole data. Mineralium Deposita 55, 217–240.

Cowan, E.J. and Hobbs, B.E., 2024, Perkins Discontinuities: Structurally controlled grade patterns diagnostic of late orogenic gold [and other] epigenetic mineralisation. Australian Institute of Geoscientists Bulletin 75, 29-38.

Miall, A.D., 1978, Fluvial sedimentology: an historical review. In Fluvial Sedimentology (A.D. Miall, Ed). Canadian Society of Petroleum Geologists, Memoir 5. 859pp.

Miall, A.D., 1985, Architectural-element analysis: a new method of facies analysis applied to fluvial deposits. Earth-Science Reviews 22, 261-308.

Perkins, W. G., 1997, Mount Isa lead-zinc orebodies: Replacement lodes in a zoned syndeformational copper-lead-zinc system? Ore Geology Reviews, 12, 61–110.

Reid, R. J. and Cowan, E. J., 2023, Towards quantifying uncertainties in geological models for mineral resource estimation through outside-in deposit-scale structural geological analysis. Australian Journal of Earth Sciences, 70(7), 990–1009.

Salese, F., McMahon, W.J. Balme, M.R., Ansan, V., Davis, J.M. and Kleinhans, M.G., 2020, Sustained fluvial deposition recorded in Mars’ Noachian stratigraphic record. Nature Communications. 11, 2067.

Sibson, R.H., Robert, F., and Poulsen, K.H., 1988, High-angle reverse faults, fluid-pressure cycling, and mesothermal gold-quartz deposits. Geology, 16, 551-555.

Walker, R.G., 1979, Facies Models. Geoscience Canada Reprint Series 1.

Acknowledgement

The Australian Institute of Geoscientists for providing the original video recordings.

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Jun Cowan is a structural geological consultant, specialising in interpreting mineral deposits at the deposit-scale. He is the conceptual founder of Leapfrog software, which is now used by many international mining and mineral exploration companies (Leapfrog software resulted from private R&D collaboration undertaken by a joint venture between SRK Consulting Australasia, where Jun worked, and New Zealand company, ARANZ). Out of his home in Fremantle, Western Australia, he consults to mineral industry clients around the world and enjoys sharing his crazy ideas with his clients, and with online colleagues. This and other articles, mainly focused on geological subjects, are available from LinkedIn.

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