by Nick Lisowiec, Exploration Manager (Eastern Australia) at Gold Road Resources
According to the CSIRO, approximately 80% of the Australian landmass is under post-mineralization cover and regolith, but most mineral discoveries to date have occurred where mineralization is located at or near the surface. Exploration through un-mineralized cover is substantially more difficult and costly, especially as the depth of cover increases. This is reflected in the discovery rate, where new discoveries, concealed under more than 50 m (164 ft) of cover, are rare.
This pattern is not unique to Australia and is relevant globally because the easy, outcropping discoveries have mostly been made. A significant quantity of future mineral supply will come from extensions/expansions and/or mining lower-grade ore within existing deposits, but the shortfall must come from new or ‘greenfield’ discoveries. Exploring under cover represents an opportunity to test new areas that haven’t previously been prospected or assessed.
This article aims to break down some of the challenges associated with it and present some possible ways the industry can improve.
Defining cover
Source: BHP Operation Review for year ended 30 June 2023.
Cover refers to either consolidated or unconsolidated rocks, gravels, or sands that have been deposited over the top of a mineral deposit after the mineralization event occurred (e.g. Figure 1). This could be thousands, millions, or billions of years after the deposit formed. The nature of the cover material can be extremely variable and can include any rock type deposited or formed at the surface (sediments, volcanic rocks, etc.). In geologically complex settings, there could be multiple phases of cover rocks overlying a deposit.
Regolith is a type of cover associated with the weathering of rocks at or near the surface and includes soil, alluvium, saprolite (weathered bedrock), duricrusts, etc. Regolith can be either in situ (in place) or transported from its source, usually via gravity, glacial or aeolian processes. This article focusses on material deposited on top of and subsequent to the bedrock mineralization and not on in situ weathering.
The issue with cover
Cover rocks effectively reduce the exploration toolkit for geologists. In areas of outcrop or in situ regolith, traditional exploration methods, such as geological mapping and surface sampling (soils, rocks, stream sediments, etc.), can be used to assess the prospectivity of an area. Whilst economic mineralization may not occur at the surface, geologists may detect evidence of veining or alteration, related to the distal expression of a hydrothermal system. With increasing data, the geologists can then focus their efforts and vector towards the deposit.
Without the surface bedrock exposure, geologists are effectively blindfolded and critical geological information can only be inferred through geophysics or obtained via drilling. Geophysics such as magnetics, gravity, seismic and electromagnetics (EM) can be used to extrapolate areas of known geology (from outcrop or previous drilling) into the unknown areas under cover. However, the degree of confidence will depend on the geological terrain and the distance from the known geological reference points.
The style of mineralization sought will also impact the type of geophysics used. Many mineral systems can be directly detected or inferred by geophysical techniques, due to sulfides, magnetic susceptibility, density contrasts, etc., which may be associated with the mineralization itself or altered host rocks. Other mineralization styles are more cryptic and have a subtle response that is much harder to confidently detect, or they have no geophysical response at all.
Whilst geophysical techniques are essential to exploration undercover, many methods are impacted by the physical characteristics and/or depth of the cover. For example, saline groundwater and high porosity within the cover sequence can impact electrical geophysical methods and mask the bedrock response. The depth of the cover can also weaken the response/signal, making subtle anomalies more difficult to distinguish above the background noise.
When a target or anomaly has been identified via geophysics, the only way to test is to drill holes and recover samples. Cover rocks can often be difficult to drill, especially in poorly consolidated ground. Unconsolidated ground may be more likely to collapse, resulting in more time conditioning holes or trying to free stuck drill rods, etc. Porous formations may contain significant aquifer(s), which need to be controlled or contained. The depth and physical characteristics of the cover will influence the type of drilling required (e.g. Aircore vs RC vs Mud Rotary vs Diamond), as well as site preparation and cost. Due to the lack of geological information available prior to drilling, exploration for concealed mineralization may require multiple drill campaigns to understand and/or test the target prior to reaching a decision point.
There are also the economic costs associated with mining and converting a potential discovery into an orebody. Many factors are involved, such as grade, geometry, geotechnical, metallurgy, etc., but in general, the deeper the deposit, the more likely it is that it will be mined from underground. Whilst mining methods shouldn’t impact the likelihood of discovery, they may influence the styles of mineralization that companies will explore for and the depth of cover that is worth exploring.
The final issue relates to funding. Irrespective of the company size, the risks associated with exploration under cover limit the resources allocated to this pursuit, particularly when compared to brownfields projects that have a higher chance of delivering a resource or exploration intercept. A significant number of companies avoid greenfields exploration altogether and rely on brownfields exploration and/or acquisitions to replace and grow their portfolios. Whilst this may be successful in the short to medium term, it won’t (in theory) be sustainable as the pipeline of projects will eventually dry out. Junior companies are also generally underfunded and are often drawn into mature terrains where there is a higher probability of reporting exploration results that make headlines and attract investors. Unfortunately, this often results in old projects being recycled and limits the opportunities to make new discoveries.
Success stories
The previous section outlined many of the challenges, but there are many success stories both in Australia and globally. A graphic from BHP in their 2024 Commodity Outlook (Figure 2) shows the increasing depth of copper discoveries since 1900. Further investigation of this chart reveals that only a handful of these discoveries are true greenfields discoveries and even less are under any significant cover (>50 m or 164 ft).
Source: BHP Economic and Commodity Outlook, August 2024.
Examples that meet this criterion include Olympic Dam (approx. 350 m/ 1148 ft cover), Carrapateena (500 m/ 160 ft), and Oak Dam (650 m/ 2133 ft) in the Gawler Craton in South Australia as well as Spence in the Antofagasta Region of Chile (80-100 m cover or 262-328 ft). Resolution in Arizona is by far the deepest discovery on the chart, but it is adjacent to and below previously known mineralization.
Other examples of greenfields discoveries under significant cover include Havieron Au-Cu (420 m/ 1378 ft) and Admiral Bay Pb-Zn (1.2 km/ 0.75 mi) in Western Australia and Neves-Corvo Cu-Zn (200 m/ 656 ft) in the Iberian Pyrite Belt in Portugal.
It’s no coincidence that the majority of these systems are associated with magnetic and/or gravity anomalies and geophysical surveys played a major role in the discovery. Admiral Bay is different as it was discovered as part of an oil exploration program in the Canning Basin and was not identified directly through geophysics. The discovery of the Olympic Dam deposit in 1975 is well documented and won’t be repeated here, but remains a compelling success story considering the era, location, depth and extent of cover. The unique combination of alteration, mineralization and scale of the hydrothermal system resulted in a broadly coincident magnetic and gravity anomaly that was identified and drilled by WMC. This discovery also defined a new deposit style (Iron Oxide Copper Gold) and opened up a new search space for world-class copper deposits.
The Havieron deposit in the Paterson Province of Western Australia (Figure 3) is a more recent example of success but also highlights the difficulties associated with exploration under cover. The deposit, which lies under 420 m (1378 ft) of Canning Basin sediments, is also associated with a broadly coincident magnetic and gravity anomaly, which was first identified in the late 1980s. The anomaly was first drilled by Newcrest in 1991, and low-grade Au-Cu mineralization was encountered. Further drilling failed to return an economic intercept and considering the depth and location, the project was relinquished in 2009. Greatland Gold later acquired the ground, re-evaluated the target and intersected high-grade Au-Cu mineralization in their first drill hole in 2018. It was later determined that an early Newcrest drill hole had missed this high-grade zone by only 40 m (131 ft)!
Increasing the chances of discovery
In countries like Australia, the potential under-cover search space is significant and generally easier for explorers to acquire ground. Australia has excellent pre-competitive datasets, including solid and surface geology, geophysics and depth-to-basement models. Continued government funding in this area is critical and should also focus on improved basement geological interpretations, geochronology, acquisition of higher-resolution geophysics and more geological (stratigraphic) drilling. Examples of the latter include the MinEX CRC National Drilling Initiative (NDI), which is increasing the regional geological understanding of various under-cover terrains around Australia.
Co-funded drilling initiatives (e.g. CDI, EIS, AIS) have been commonplace in Australia for many years, funded via state geological surveys. Whilst these initiatives should continue, the funding model should be reviewed, with a higher percentage directed towards genuine frontier-type exploration in new and/or undercover terrains. One criticism of the NDI is that the selection and prioritization of drill sites is influenced by government objectives and program execution and data release is slow. To supplement the NDI, a selection of industry applications could be fully funded by the government, as opposed to the current 50-50 model, to encourage more drilling through cover.
Improvements in drilling and geophysical techniques to more efficiently and effectively explore through cover would help increase the chance of discovery. There has been continual research in both fields for many years, but the processes and methods used today are mostly incremental improvements on the technology developed in the last century. Collaborative research programs such as Deep Exploration Technologies CRC and, more recently, MinEx CRC were created to solve many of these problems. Developments such as the Coil Tube Drill Rig (Figure 4) show promise to drill effectively and efficiently through cover but are yet to be fully commercialized. From explorers’ perspective, drilling should ideally get through the cover as quickly and cheaply as possible, while maintaining hole integrity (including deviation), controlling water and minimizing the environmental footprint. This allows more exploration dollars to go towards actually testing targets and concepts. Advancements in geophysical methods to look deeper while maintaining resolution would also improve targeting by prioritizing or rejecting targets and/or drilling the right areas earlier.
Source: MinEx CRC / LinkedIn.
Increased collaboration between companies may also increase the chances of success. Whilst there will always be commercial sensitivities and competitive advantage arguments, discoveries have a positive outcome for the whole industry, especially in a new terrain (e.g. Olympic Dam). This collaboration may be in the form of exploration alliances with shared regional data collection, sharing of technical information and results, etc. One example is the BHP Xplor program, where junior companies work collaboratively with BHP to help advance exploration projects that are of interest to the company. More initiatives like this may help the industry as a whole make more discoveries.
Changes to the way that exploration results are reported externally may also assist with this. Some of these changes are already in progress in Australia, partly driven by proposed revisions to the reporting code (JORC), and partly by companies that see the benefits of sharing their exploration story. For many years, publicly released exploration reports have often contained the bare minimum information to ensure compliance with relevant mineral resource/exploration reporting codes. The more that was reported, the more work was required to ensure compliance, etc. Early-stage exploration is often associated with data and results that are deemed ‘not material’ for the market and hence not reported. Reporting more technical findings and geological interpretations can better demonstrate the work being completed, where and how the company is spending money, as well as help non-technical investors understand the exploration process. Other companies can also potentially leverage this information to assist their own exploration activities rather than spending time and money to achieve the same outcome.
Consistent funding is also critically important. Most companies that undertake greenfields exploration programs understand that consistent funding over many years allows exploration teams to work on projects over the timeframes required to build up and test targets. Companies that make the strategic decision to conduct greenfields exploration need to commit to at least a five- to ten-year timeframe before any return on investment is to be expected. Smaller companies usually lack the funding and may need to follow the ‘project generator’ model by working up projects to a point in which larger companies are more likely to invest or acquire. Alternatively, with the right backers and/or private equity, junior companies may be able to persist for longer to make their own discoveries.
Finally, good geology is essential throughout all stages of the exploration process. Exploration geologists must be equipped with the skills to recognize and understand the cryptic geological and geochemical indicators of fertile hydrothermal systems and be able to formulate logical geological models from limited information/data. They must also know when to persist (drill more) and when to move on to other targets. The majority of these skills are acquired via experience and/or postgraduate studies. As such, industry, academia and, most importantly, individual geologists must work together to ensure that the exploration teams have the skill set to succeed.
Summary
The next generation of mineral deposits will be concealed under cover and, as an industry, we need to improve how we explore in these terrains. If we don’t, then the discovery rate will continue to decline. The task is difficult, but there are several ways we can increase the chances of success. None of these are a silver bullet, but a combination of ongoing and more targeted government assistance, continued R&D funding into drilling and geophysical technology, increased collaboration and better external reporting can encourage more investment into greenfields exploration and hopefully lead to more discoveries. Countries like Australia have significant mineral wealth, but there are vast areas that have not been explored. The potential is massive and for the exploration industry, it’s the ultimate challenge.
For more information: Get in touch with Nick on LinkedIn
