Critical metals: Their mineral systems and exploration

Abstract

Complex geological formation processes under a variety of tectonic regimes have led to a heterogeneous global distribution of the critical metal resources that will be increasingly in demand due to Net Zero and the clean energy transition. Although brownfield exploration around existing mines can add to inventories of some critical metals, including copper in the short to medium term, all mines have a finite life with many historical giants already exhausted. Therefore, without a revolution in metal recycling, successful global greenfield exploration is of key importance.

If greenfield exploration is to supply a significant proportion of critical metals to meet future supply for a sustainable economic future, it is imperative that superior conceptual models are employed for initial ground selection. This requires an understanding of the critical components of the wide variety of metallic mineral systems that contain these critical metals and their temporal distribution and tectonic settings. Greenfield exploration faces numerous challenges as new discoveries are commonly concealed by younger sedimentary cover and they occur in increasingly remote terrains and at greater depths, resulting in declining discovery rates despite rising budgets.

Some mineral systems have high preservation potential and are distributed within well-established temporal ranges in Earth history. These systems include orogenic gold, VMS-type Cu-Pb-Zn, intrusion-hosted Ni-Cu±PGE, carbonatite REE, and SEDEX, MVT, Broken Hill-type, and Zambian-type base-metal systems. Understanding the temporal range and tectonic setting of such critical mineral systems aids conceptual targeting to define new exploration spaces. However, many of the Precambrian critical mineral deposits within these systems are now situated in subdued topography and have experienced regolith development such that they have thick cover ranging from desert sands to thick complex regolith, to glacial till. All scenarios represent technical and financial challenges in terms of successful exploration using increasingly sophisticated remote sensing, geophysical, and geochemical survey methodologies.

A contrasting conceptual targeting scenario is provided by critical mineral systems that formed in convergent margin arc settings or in marginal terrestrial sedimentary basins. These include widespread porphyry-skarn Cu-Au-Mo systems, both high- and low-sulfidation epithermal Au-Ag systems, and more geographically restricted IRGD and Carlin-type systems. Where tectonic uplift rates are high, critical metal deposits of these systems are rapidly eroded and have low preservation potential and hence are largely confined to the Cenozoic with only rare systems beyond the late Mesozoic. Conceptual exploration is less challenging for this group as they are commonly situated in mountainous terrains where remote sensing spectral surveys are effective and deposit footprints are large due to vertically and/or laterally extensive hydrothermal alteration haloes. The downside is that most near-surface deposits and their related hydrothermal alteration zones have been discovered already as shown by declining discovery rates.

Clearly, exploration for critical metal mineral systems faces numerous severe geoscientific challenges. Mineral exploration is also challenged by the risk-averse nature of major mining companies coincident with low exploration budgets for junior exploration companies, strict environmental and indigenous population issues, and a negative public perception of mining in most western societies. Future exploration will increasingly need to utilize more sophisticated conceptual exploration models based on knowledge of the critical mineral systems summarized in this paper. In the future, this may be enhanced by interpretation of geological Big Data sets supported by artificial intelligence (A.I.), as discussed elsewhere in this Special Issue. There is a clear imperative for mineral exploration to increase critical metal supply and enhance the possibility of a sustainable future for our materials-based civilization if the clean energy transition continues to consume critical metals at ever increasing rates.