Editorial for Special Issue “Novel Methods and Applications for Mineral Exploration, Volume II”

1. Introduction

As already exemplified by the highly successful first volume of this Special Issue, both the theoretical understanding of mineral exploration and its practical application in the field are undergoing a profound and significant transformation. This transformation is primarily driven by the numerous societal, economic, and environmental challenges facing the mining industry and our society; some of these challenges have always been present (e.g., economic sustainability, equitable distribution of profits), while others are rather new, such as the dramatically increased demand for the so-called critical minerals as defined by different legislations principally in the Western world. For instance, the European Union, the USA, and Canada, among others, have designed and implemented national programs with the purpose of enhancing the exploration for and the production of a number of commodities deemed crucial not only for economic security, but also and particularly for the implementation of the transition to a low-carbon economy (based on “clean technology”), itself driven by depleting hydrocarbon resources and the urgent need to reduce greenhouse gas emissions in order to address the unfolding climate catastrophe. What is considered as a critical mineral varies somewhat from country to country as a function of national priorities, but typically includes 20 to 30 commodities dominated by lithium, graphite, nickel, cobalt, copper, and the rare earth elements. As we will see later, several papers in this volume deal with critical minerals, specifically lithium and copper.

Another significant factor for the observed transformation of the mineral exploration theory and practice is the unprecedentedly robust technological development the world has experienced: it is—among other things—a major impetus, instrument, and symptom of the changes observed. This is of course logical and has been a constant since the scientific and industrial revolution of the 18th century: better theoretical understanding leads to improved technological advancement, which in turn provides the means necessary for further theoretical growth. Such an interdependency, and indeed a mutually beneficial relationship between science and industry, is what drives the technological progress that strongly affects the theory and practice of mineral exploration today. Specifically, this progress is most visible in two areas: novel computing capabilities (e.g., machine learning and artificial intelligence) and novel analytical methodologies (e.g., laser-induced breakdown spectroscopy, increased portability, and the automation of data collection and interpretation). Combined, these two aspects of technological progress are contributing to a vast paradigmatic change, with orders of magnitude more data (obtained faster and cheaper), treated and indeed interpreted by more powerful computers utilizing artificial intelligence models and machine learning. Not surprisingly, several papers in this volume deal specifically with the application of novel technologies, both analytical and computational, to mineral exploration.

This second installment of this Special Issue entitled “Novel Methods and Applications for Mineral Exploration” represents a brief but representative cross-section of the advancements mentioned above and in particular of the progress made since the first volume was published three years ago. The papers published in this volume are cutting-edge and indeed pioneering contributions to our understanding of mineral exploration, both theoretical and in practice and, as such, will be of particular interest to both the academia-based scientist and the practicing exploration geologist, who will undoubtedly find them interesting and useful.

2. Review of the Papers in This Special Issue

The papers published in this Special Issue are diverse, but are strongly dominated by geophysical methods, as in the first volume, with seven papers. Three papers deal with novel analytical technology, one paper deals with WebGIS technology, and one paper deals with lunar navigation. The individual contributions will be briefly presented here.

2.1. Geophysics

Most of the contributions in this volume using geophysical methods combined with novel computational applications are applied papers focusing on commodities such as copper, gold, coal, nickel, and hydrocarbons, and these will be described first.

The first contribution we will consider, by Liu et al. [1], is strictly applied and describes the integration of new high-precision airborne gravity and magnetic data with data from geological exploration and drilling, and the resulting improved understanding of the Jurassic–Cretaceous and the Upper Paleozoic strata of the Dayangshu Basin, China, and their potential to host oil and gas reservoirs. The conclusion of this contribution is that three specific zones, the Liuhe Sag in Dayangshu Basin, the depression in NE Longjiang Basin, and the northern parts of the Taikang swell, have high potential for oil and gas exploration.

Another contribution, by Wang et al. [2], describes the utilization of novel passive electromagnetic methods, the super-low-frequency alternating magnetic component method (SLF) and the audio frequency magnetotelluric method (AMT), to detect and interpret different types of coal mine goaf. Their conclusion, after validation of the method with real-world data from the Henan Province in China, is that these passive electromagnetic interpretation methods have the potential to identify fault structures and water-filled gauf areas with depth of up to 400 m. This paper provides the theoretical background for the application of this novel gauf detection method as a fast and low-cost alternative of traditional methods such as seismics, gravity, logging, radiation and electromagnetic methods.

The next contribution, by Xiang at al. [3], deals with electrical exploration of shale gas reservoir exploration and evaluation. This study measures the resistivity and characterizes the induced polarization parameters of 34 shale samples from southern Sichuan, China; it also analyzes the electrical anisotropy response characteristics under different temperature and pressure conditions, thus defining the specific factors that affect the complex resistivity anisotropy of shales. The major significance of this study is in providing the theoretical basis and describing a set of testing methods that are useful in understanding the electrical anisotropy characteristics of shale gas reservoirs.

Another applied geophysics study, reported by Du et al. [4], also utilizes a passive method, passive seismic in this case. Ambient noise tomography, employing a dense array of 100 temporary detectors, is used to study the shallow crustal structure of Karatungk Mine, the second-largest Cu-Ni mine in China, down to almost 1.3 km in depth. Surface wave dispersions at 0.1 to 1.5 s are obtained and inverted for a 3D S-wave structure, which helped identify several zones with distinct velocities. Some of these are identified as intrusive host-rocks and others as highly sulfide mineralized areas. The principle contribution of this study is demonstrating the value of using a very dense array of detectors in the study of ore deposits.

A detailed and extensive geophysical mapping, using gravity and magnetics, is reported for two new IOCG mineral systems in the Archaean São Francisco Craton, Brazil, in the Vale do Curaçá and Riacho do Pontal copper districts (Hühn et al. [5]). Several high-density (3.13 g/cm³) and high magnetic susceptibility (SI > 0.005) areas are identified corresponding to the Caraíba, Surubim, Vermelho, and other minor copper deposits. The mapping also indicates that the deposits are controlled by NNW and NW-SE structural trends.

The next contribution is more theoretical: Wang et al. [6] report a methodological innovation, specifically the imaging method of dispersion curve variability function. This method follows a precise procedure, based on the breakpoints of the dispersion curve produced by an anomalous body in a seismic study. These breakpoints are classified, counted, and assigned a weighing factor, after which the variability function for the dispersion curve is constructed and then used to visualize the anomalous body. The method is validated by applying it to a real-world and known example, the working face of coal mine 090606 in China, and seems to provide a more accurate image of the known anomaly.

The last geophysics contribution in this Special Issue, by Speczik et al. [7], reports the use of historic seismic and gravimetric data in the exploration for Cu and Ag deposits, with an example from the Nowa Sól deposit in SW Poland. The major significance of the paper is to demonstrate the effectiveness of the new method of effective reflection coefficients (ERC), applied on archival data. Compared to amplitude-based seismic section, the ERC-generated section boasts much higher imaging resolution, allowing the use of the historic data, in particular when combined with drill core information.

2.2. Analytical Innovations

Two contributions in this Special Issue report the use in exploration of a relatively new geochemical exploration tool, hand-held laser-induced breakdown spectroscopy (LIBS), which has the advantage of measuring light elements in situ, lithium in particular. The first of these papers, by Wise et al. [8], describes the application of the method in exploration for Li-Cs-Ta rich pegmatites, with, as an example, the Carolina Lithium Project, situated in the Carolina Tin-Spodumene Belt, which extends from South Carolina into North Carolina, USA. The paper thoroughly describes the method employed—it is indeed mostly a methodological paper—and discusses the strength, weaknesses, limitations and applications of the method in the context of geochemical exploration, including the challenges related to obtaining fully quantitative analyses and the ways to overcome these challenges. The major conclusion of the paper is that hand-held LIBS can indeed be advantageously used in geochemical exploration of pegmatites.

The second contribution using the hand-held LIBS method is by the same team (Harmon et al. [9]) and demonstrates how the tool can be used to analyze muscovite from a lithium-bearing pegmatite. The example is that of the Carolina Lithium Prospect in North Carolina, USA, where more than 130 muscovite samples are analyzed for Li, K, and Rb. It is found that Li contents varies from 0.04 to 0.74 wt% and the K/Rb ratios from 8 and 63. Higher Li is observed in spodumene-bearing pegmatites than in spodumene-free ones. Although the method is described in significant detail, the analytical uncertainty and the detection limits of this method are not apparent.

A very interesting paper by Chisambi et al. [10] describes, in significant detail, the integration of X-ray computed tomography (XCT) with textural, mineralogical and chemical imaging using SEM and optical microscopy, in order to produce a precise 3D image of gold distribution in samples from Manondo-Choma gold prospect in Malawi. This contribution emphasizes the importance of the correlative approach of combining 3D XCT data with 2D mineralogical and chemical data to understand a complex orogenic gold mineralization. The use of XCT in combination with microscopy provides an excellent tool for understanding gold mineralization. Further, this visualization method seems particularly useful in obtaining a better understanding of the complex structures controlling the gold mineralization (and possibly other types of deposits), mostly at the microscopic and mesoscopic scale.

2.3. Other Topics

An interesting contribution by Cardoso-Fernandes et al. [11] presents the results of the INOVMINERAL4.0 project, consisting of a WebGIS-based integration and visualization of various data from the Aldeia pegmatite deposit in Portugal. The data are obtained by means of unmanned aerial vehicle (LIDAR), on the ground (radiometry), and in the lab (spectral data on 11 samples). The resulting images are compared with those obtained by Landsat 9. The main conclusions are that radiometry can effectively delineate pegmatites from their host rocks; that a spectral data library can be very useful in space-based exploration; and that using a user-friendly webGIS platform facilitates data integration, visualization, and sharing.

Finally, we come to a somewhat futuristic space exploration-related contribution (Ignjatović Stupar et al. [12]). It describes, in considerable depth, a proposed Lunar Regional Navigation Transceiver System (LRNTS), to be used for the accurate—to the centimeter—positioning of facilities and structures that would be placed on the Moon’s surface during a potential future settlement mission. The system, using lessons learned from mining, consists of at least nine transceivers, each containing both navigation transmitters and receivers, and placed on the Moon’s surface. The case study presented in this paper is a computer simulation of the implementation of this system on the Shackleton Crater. The best of 12 models, using 9 nodes, achieved a horizontal accuracy of 6.7 mm, a vertical accuracy of 11.5 mm, and availability over 95.5% of the study area. These impressive results open up tantalizing prospects for planet exploration and much more.

3. Conclusions

As the reader will without doubt appreciate, exploration theory and practice are advancing at a very high pace. This acceleration is particularly noticeable when we compare this volume with the first volume of this Special Issue, published a mere three years ago, and this reflects the rapid changes in a fast-evolving exploration and mining industry. Some of the drivers in this evolution are recent advances in information and communication technology and computational methods (specifically artificial intelligence and computer learning) and the development of novel analytical methods (e.g., LIBS [8,9]) and the increased portability of existing ones (e.g., PIMA), combined with increased automation of data collection and data processing.

These innovations are most noticeable in geophysical exploration, which, because of its high dependence on computational power, is the major beneficiary [2,3,6]. Clear opportunities for future developments and opportunities can be perceived, as well as for novel applications [11,12]. Interestingly, new understanding and novel methodologies allow us to re-visit and re-process historical geophysical data [7], which is, in itself, interesting and full of potential.

In conclusion, the second volume of this Special Issue provides a brief but representative cross-section of the exciting developments in our understanding and practice regarding mineral exploration and, as such, will no doubt be highly useful, interesting, and thought-provoking for both the academia-based and the practicing exploration geologist.