Figure 15 - Percentage of the world’s copper hosted by key deposit types
Porphyry Deposits
The most important deposit style for primary copper extraction is porphyry (porphyritic) copper, that accounts for 70 per cent of the world’s copper supply, along with significant amounts of molybdenum and gold. These porphyry deposits are typically low-grade deposits containing up to billions of tonnes of ore, associated with structurally controlled vein networks that are spatially and genetically related to porphyry intrusions. Particular tectonic regimes (subduction zones related to convergent plate margin settings) are considered a requirement for forming porphyry copper deposits. These regimes create the conditions for developing plutons at depth, which comprise the source of heat, metals and mineralising fluids for the copper deposit, which is emplaced at a shallower depth.
The largest deposits are found at convergent tectonic margins in the United States, Canada, the western Pacific, Chile, and the Indonesian archipelago. Ore formation in these deposits has shaped modern methods of primary copper extraction. Today, the bulk mineralised rock averages 0.5–2 per cent copper and is amenable to extraction by large-scale mining methods as shown in Figure 16 below.
Figure 16 - Image of Bingham Canyon Mine, Utah, USA
Sediment-hosted Copper Deposits
Figure 17 - Copper deposits present in sediment-hosted and porphyry deposits around the world (USGS)
Sediment-hosted copper deposits account for 12 per cent of the world’s copper production, and as indicated by USGS in Figure 17 above., These deposits are laterally extensive and relatively thin bodies of disseminated, cementing and veinlet-hosted copper mineralisation that are nearly conformable with their sedimentary or metasedimentary host rocks in sedimentary basins. These include shale-hosted sulphide deposits like the Kupferschiefer shale, which underlies an area extending from northern England to Poland, and the most important of these deposits, the Copper Belt, which extends from the DRC into Zambia (these deposits are also currently the most important global source of cobalt). The formation and location of sedimentary basins vary over geological time with changes in plate tectonic activity, but their distribution is mainly controlled by paleolatitude; the host sedimentary rocks were deposited within 20-30 degrees of the paleoequator.
These deposits are amenable to mining by surface open pit and underground methods, depending on deposit depth and geometry. Tonnages range from 10 to 100 Mt of ore (smaller than typical porphyry copper deposits), and copper grades are typically 1-3 per cent (higher than the typical grade for porphyry copper deposits).
Iron Oxide-Copper-Gold Deposits
Iron oxide–copper–gold (IOCG) deposits supply about 5 per cent of world copper production, most of which comes from the giant Olympic Dam deposit in South Australia. These are a diverse family of mineral deposits characterised by the presence of copper, plus or minus gold as economic minerals, hydrothermal ore styles and strong structural controls, abundant magnetite and/or hematite, iron oxides, and no clear spatial associations with igneous intrusions (as displayed by porphyry deposits). Most IOCG deposits display space-time associations with batholith granitoids and occur in crustal settings with extensive and pervasive alkali metasomatism.
Uranium-rich IOCG deposits in which uranium is an economic metal are an important but as yet uncommon subset of the IOCG family. Currently Olympic Dam is the only known IOCG with economic uranium.
Volcanogenic Massive Sulphide Deposits
Five per cent of the world’s copper production comes from volcanogenic massive sulphide (VMS) deposits. These deposits form at or near the seafloor, where circulating hydrothermal fluids driven by magmatic heat are quenched through mixing with bottom waters or porewaters in near seafloor lithologies. Massive sulphide lenses vary widely in shape and size and may be pod-like or sheet-like. They are generally stratiform and may occur as multiple lenses.
VMS deposits range in size from small pods of less than a tonne to supergiant accumulations like Rio Tinto (Spain), 1.5 Bt (billion metric tonnes). VMS deposits range in age from 3.55 Ga (billion years) to zero-age deposits actively forming in extensional settings on the seafloor, especially mid-ocean ridges, island arcs, and back-arc spreading basins.
Magmatic Deposits
Magmatic copper deposits supply about 5 per cent of world copper production. These occur with mafic and/or ultramafic bodies emplaced in diverse geological settings and range in age from Archean to Tertiary. Deposits occur on most continents, although ore deposits of sufficient grade to be economical to mine are relatively rare. In these deposits, copper is almost always associated with nickel and minor amounts of platinum-group elements, particularly at Kambalda in Australia, Sudbury in Canada, and Norilsk in Russia. Nickel-copper sulphide ore deposits can occur as single or multiple sulphide lenses within mafic and/or ultramafic bodies, with clusters of such deposits comprising a district or mining camp. Typically, deposits contain ore grades of between 0.5 and 3 per cent nickel and between 0.2 and 2 per cent copper. Tonnages of individual deposits range from a few tens of thousands to tens of millions of metric tonnes (Mt) of bulk ore.
Secondary and Supergene Deposits
Consideration is also given to the mineralogical complexity of deposits, which occurs due to alteration around porphyry deposits. Alteration zones display different mineralogy and texture and thus influence different mineral processing characteristics and methods. This mineral and textural diversity can be made more complex through processes of weathering, leaching, and supergene enrichment.
Most copper sulphide minerals are unstable at the Earth’s surface and oxidise during weathering. Where weathering acts on hydrothermal or magmatic copper deposits exposed at the surface, it can form two types of deposits. If the dissolved copper precipitates as copper oxides and carbonates, secondary copper deposits are formed. If it percolates downward into underlying unweathered rock and precipitates new sulphide minerals, supergene copper deposits are formed. Both processes have contributed to the grade of some copper deposits and have formed separate ore bodies.
Copper deposits can be classified based on their geological setting and the processes involved in their formation.
EXPLORATION METHODS
Greenfield Exploration
Greenfield exploration seeks to discover mineral deposits in previously unexplored areas (i.e. away from the immediate vicinity of existing mining activity) or in areas where a particular type of deposit was not known to exist.
Driving greenfield exploration at scale requires an understanding of the mineral systems that host various copper deposit styles alongside the elements that must come together to form these mineral systems – fertility (source for metal); transporting agency (moving metal from source to eventual deposition); geodynamic trigger; architecture/geometry of the system; site of deposition (what caused the metal to accumulate); preservation.
The exploration search method undertaken will vary and depends on assumptions regarding the target deposit type and is informed by an understanding of the geological controls on source, transport and deposition at varying scales. This process is usually successful in locating the right geological belt for exploration; however, finding the mineralisation in that belt remains a complex exercise as depth, type of cover and size of the deposit are highly variable within the upper two kilometres of the Earth’s crust. Further limitations on exploration arise relating to available funding and supporting infrastructure.
The process of delineating the presence of the mineral system requires a focus on larger scale geographic areas, while direct detection technologies are deployed at the prospect site. An outcome of greenfield exploration is an early understanding of the location and potential size of a copper discovery and indicators of the mineralising system’s fertility.
Greenfield exploration typically follows a refinement process from ‘Large Scale’ to ‘Belt and Camp’ scale and then further to the ‘Project and Prospect’ scale. Each stage may deploy various geophysical and geochemical tools and techniques, which are applied along with geological mapping to highlight prospective areas and provide vectors for potential mineralisation. Drilling is used to further enhance understanding of the geological setting and confirm the presence of mineralised copper discoveries.
Greenfield exploration work programs and tool kits may include a range of datasets such as remote sensing, geophysical, geological information (e.g. lithology, structure), geochemical and geochronological. These datasets are typically consolidated and integrated into an exploration model. Table 2 shows how the exploration toolkit may be deployed in greenfield exploration programs.
Various remote sensing and geophysical techniques are applied during the exploration process to better understand the petrophysical properties of rocks, where they relate to rock-forming processes and mineralogy. While typical and theoretical responses of deposits are often known, these may vary according to the petrophysics of the host rocks. Figure 18 and Figure 19 shows the application of various geophysical techniques according to the multiple scales of exploration and the effectiveness of some of these methods as depth increases., The application of these techniques is particularly useful at the ‘Belt and Camp’ scales of exploration.
Figure 18 - Application and effectiveness of various geophysical methods for a whole-of-lithosphere approach to exploration showing emphasis on scale of lithospheric architecture
Figure 19 - Application and effectiveness of various geophysical methods for a whole-of-lithosphere approach to exploration showing the effectiveness of geophysical technique with depth
Greenfield exploration teams visualise the datasets to support potential interpretations and work to reduce the exploration focus further, refining understanding and targeting through the acquisition of additional datasets.
In addition to the exploration activities described above, supplementary stages are required to progress greenfield exploration to discovery. The specifics may depend on jurisdictional requirements; however, the additional steps generally include the following:
Stakeholder engagement: Engagement with the landholders, local communities and First Nations people, including seeking approval for access to the land for on-ground activities. Increasingly, jurisdictions require written approval for access, and in some jurisdictions, this is a compulsory activity when applying for a mineral exploration permit. Engagement with a range of stakeholders early in the exploration process can lay the foundation for a good working relationship and open communication is important to minimise issues as exploration and discovery proceeds.
Resource estimation: To assess the size and quality of a resource, focused drilling is generally required, usually detailed drilling in grid formation to enable definition of the orebody and modelling.
Feasibility studies: A detailed understanding of the orebody and location are undertaken through feasibility studies (usually a pre-feasibility study to assess mine-ability followed by a more detailed definitive feasibility study). These studies assess the economic potential of a deposit and the information required to progress project development to final investment decision (FID).
Environmental and social assessments: The development of the mine plan includes environmental and social assessments to understand the baseline state and model and mitigate the impact of the proposed development.
Government approvals: These studies are used to progress and meet relevant government requirements that vary with each jurisdiction.
Modern greenfield exploration programs deploy a range of advanced exploration technologies to improve either the understanding of the geological setting or the potential for mineralisation. Examples include the use of satellite data and the use of drones and aircraft to undertake geophysical surveys. The application of advanced technologies can reduce surface impact at the early stages of exploration and can overcome accessibility challenges such as areas of thick vegetation or high elevation.
Advancements in geochemistry have aided porphyry exploration in particular. The recognition of fertile belts of igneous intrusions and prospective areas of hydrothermal alteration can now be assisted using porphyry indicator minerals (PIMS) such as zircon, plagioclase, apatite, magnetite and tourmaline. These are useful to target the location of porphyry copper deposits under cover. The minerals used in PIMS studies are resistate minerals, i.e. those resistant to erosion, preserving signals of their source environment. The signals are detected using scanning electron microscope (SEM) automated mineralogy technology.
At the district scale, far-field detection of concealed mineralised centres in porphyry districts can now be enabled by applying porphyry vectoring and fertility tools. These tools detect low-level geochemical anomalies preserved in hydrothermal alteration minerals such as epidote, chlorite or alunite. This new generation of geochemical exploration tools has evolved due to advances in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analytical techniques.
Advanced exploration activities in greenfield environments are often supported by exploration camps, which, depending on the activity may be small or large and located in remote areas.
Brownfield Exploration
Brownfield exploration aims to discover new deposits or extensions of known ones, often near existing mines. This typically means that infrastructure is already established to support various exploration activities, such as energy supply and accommodation, as well as operations to transport and process new ore. For these reasons, brownfield exploration is considered a lower-risk activity compared to greenfield exploration.
Brownfield exploration typically does not include production geology, which involves activities that ensure an operating mine continues to extract ore according to the planned schedule, tonnages, and grade. Brownfield exploration can begin just outside a company’s current production pit boundary.
There is generally a better understanding of the search space in brownfield exploration than in greenfield exploration for a number of reasons, including:
The barren background rocks have been characterised by their petrophysical and geochemical signatures. This means exploration efforts can identify what an anomaly looks like in these datasets and can better identify near-misses to previous attempts at detecting a deposit.
The structural trap for the deposition of mineralisation is known and can be mapped by in-field activities (structural mapping) and/or from interpretations of geophysical data (magnetics, gravity +/- seismic).
Equipped with the knowledge of a pre-existing exploration target and its contrast with the host rock, brownfield exploration geologists generally have a better under- standing of which methods will effectively detect that target. Datasets from previous exploration efforts or pre-competitive surveys may also be available at the appropriate resolution to aid the exploration effort. However, deeper deposits may not be adequately imaged by shallower investigations.
The presence of a known deposit nearby has led to the development of deep learning (DL) systems that can be trained to support the recognition of similar deposits within the available data and search space, provided this data can be integrated and transformed into 3D systems.
Equipped with foundational knowledge of the geology and appropriate exploration methodologies, brownfield exploration is considered less speculative than greenfield exploration. Near-surface prospecting can also guide more extensive or deeper discoveries, and activities like artisanal and small-scale mining (ASM) can inform exploration efforts, as these operations may occur before exploration or mining concessions are granted.
Many of the technologies and tools used in greenfield exploration are also applied in brownfield environments, focusing on advanced technologies that define mineralised zones and the host geological units.
The interpretation of geophysical and geochemical data helps drive the design of drilling programs and the search for buried deposits. Typical early-stage brownfield exploration activities include detailed geological mapping and 3D modelling. Figure 21 and Figure 22 show the general types of search methods used to progress exploration at the project and prospect scales. These figures show primary search methods for all non-bulk mineral discoveries, noting that copper accounts for 58 per cent of all base metal discoveries in these assessments.
Figure 21 - Primary search methods used at the project scale for all non-bulk mineral discoveries in the world 1900-2019
Figure 22 - Primary search methods at the prospect scale for all non-bulk mineral discoveries in the world 1900-2019
Tools and techniques typically applied in brownfield exploration include collecting geophysical datasets, remote sensing datasets (sourced from instruments on a range of platforms including satellites and drill core scanning equipment), geological information and knowledge (such as lithology and structure), as well as geochemical and geochronological datasets. These are consolidated into an increasingly refined exploration model.
As in greenfield exploration, exploration teams typically visualise the datasets and interpretations using software applications or packages that allow 3D visualisation and analysis. Here, they further refine their understanding and reduce uncertainty by acquiring and collating additional datasets. These may include remote sensing data, geophysical data, geochemical data, new mapping, and drilling (such as air core, reverse circulation (RC), diamond), along with downhole geophysical (including induced polarisation (IP)) and geochemical data (ncluding multi-elemental analysis and x-ray fluorescence (XRF) analysis), plus hyperspectral data.
Brownfield exploration is typically supported by extensive data and background information. Machine-learning (ML) algorithms are increasingly applied to aid in the processing of geological data, generating exploration targets, assessing their potential value, and optimising exploration strategies. Applications using deep learning algorithms are also emerging to manage and connect hyperspectral data from drill cores with the vast amounts of associated data to inform interpretations for geological modelling.
The role of brownfield exploration extends beyond discovery. After a deposit is found, ongoing exploration is crucial for reducing risks at each stage of the mining cycle - from feasibility and development to production, closure, and rehabilitation. This process involves enhancing the understanding of the deposit’s mineralogy, including the final products and waste streams. Additionally, the use of mineralogical and metallurgical data helps refine resource and reserve models and optimise mine planning.
