New Advances, Challenges, and Illustrations in Applied Geochemistry

1. Introduction

The Special Issue ‘New Advances and Illustrations in Applied Geochemistry in China’ was organized for the presentation of ideas from the 9th National Conference on Applied Geochemistry in China held in Chengdu, Sichuan Province, in October 2023. A new Special Issue, ‘New Advances, Challenges, and Illustrations in Applied Chemistry’, is currently being organized, which aims to facilitate the academic exchange and presentation of new ideas in the field of applied geochemistry, particularly those presented at the 10th National Conference on Applied Geochemistry, which was held in Kunming, Yunnan Province, in November 2024. This Special Issue is edited by the Committee of Applied Geochemistry of the Chinese Society for Mineralogy, Petrology, and Geochemistry (CSMPG).

This Special Issue contains 12 scientific papers that reflect various advances, challenges, and illustrations in applied geochemistry. These articles cover to the following three broad fields: (i) geochemical exploration, including traditional and non-traditional exploration methods and determination methods for geochemical background values; (ii) environmental geochemistry, including risk assessment, provenance tracing, and the remediation of heavy metals; and (iii) basic applications in geology, including magmatism, mineralization, and tectonic thermal evolution, etc.

2. Geochemical Exploration

2.1. Methods for the Geochemical Exploration of Mineral Resources

Geochemical exploration mainly focuses on prospecting for mineral resources; its methods are commonly divided into the following two groups: traditional methods and non-traditional methods [1].

Traditional methods have developed from the denudation principle of mineralized exposed materials, and the commonly used sampling materials for traditional methods include stream sediments, soils, and rocks. For example, the RGNR (regional geochemistry–national reconnaissance) project in China collects stream sediment from mountainous and hilly areas [2]; the NMPRGS (national multi-purpose regional geochemical survey) project in China collects soils from plains and basins [3]; and primary halo surveys on a specific deposit mainly focus on rock samples taken from an outcropped area [4]. On the contrary, few geochemical survey projects have been conducted in deserts and sandy areas. Wen et al. (contribution 1) reported the results of a geochemical survey, including the geochemical background and baseline values of 61 indicators for 344 composite samples in 12 desert and sandy areas in China. These background and baseline data greatly enriched the elemental abundance database within applied geochemistry [5].

Non-traditional methods have developed from the penetration principle of mineralized concealed materials, the common sampling materials for which include gas, water, plants, and other materials which are analyzed differently from traditional total concentrations [6]. Geogas [7], geoelectrochemical technology [8], gas analyzers [9], and partial extraction methods [10] are commonly used approaches for prospecting buried ore deposits. Zhou et al. (contribution 2) compared traditional and non-traditional exploration methods on a concealed gold deposit. In their illustration, non-traditional methods, including the chemical form analysis of gold, a soil halogen survey, and a heat-released mercury survey, were utilized, which were found to be applicable and efficient in targeting potential gold mineralization zones. Sun et al. (contribution 3) introduced a geoelectrochemical method, but they aimed to remove the heavy metal Cd from a sample of paddy soil rather than to target mineral resources. This illustrates the wider applications of geoelectrochemical technology.

2.2. Methods for the Determination of Geochemical Background Values

Contribution 1, a study on geochemical background values, is an important foundational work in earth sciences. The types of methods used to determine geochemical background values can be classified as fixed-value and unfixed-value methods. Fixed-value methods define a fixed value as an elemental background value in a specific area, whereas unfixed-value methods indicate that different values are determined as the elemental background values within a specific area, with even each background value corresponding to each sample within a specific area.

Utilizing fixed-value methods, Wen et al. (contribution 1) introduced three ways in which to determine geochemical background values, as follows: the iterative method [11], the frequency histogram method, and the multifractal concentration-area method [12]. In this paper (contribution 1), the final geochemical background value was obtained by averaging the values calculated using the three methods for each analytical item.

For the unfixed-value method, which is the ideal method to use, each geological sample has a different geochemical background value due to different degrees of weathering and lithology in each sample, as proposed by Gong et al. [13]. In their study [13], the geochemical background values of 27 trace elements in a sample could be calculated from the sample’s major oxide contents using empirical equations. Recently, the equations for Li [14] and Cr [15] were further improved, to help determine their geochemical backgrounds more accurately.

3. Environmental Geochemistry

Studies on environmental geochemistry mainly focus on aspects of pollution risk assessment, elemental bioaccumulation, and heavy metal remediation.

3.1. Methods for Heavy Metal Risk Assessments

Methods for assessing the pollution risk of heavy metals in soils have been used and discussed for several decades and can be classified into two types: those with standards and those without standards.

Assessment methods without standards for heavy metal soil pollution were reviewed and summarized by Gong et al. [16] and are currently widely used, albeit with some new revised versions. For example, Luo et al. (contribution 4) adopted the indices of the Pollution Load Index (PLI) [17], the Geoaccumulation index (I_geo) [18], and the Nemero index [19] to assess the pollution risk of heavy metals in the soils of an industrial park in Kunming, China.

Methods with standards are also commonly used to assess the pollution risk of soils and other geological materials, and are based on promulgated standards, such as GB15618-2018 [20] in China, among others. Xu et al. (contribution 5) proposed single and integrated indices for the assessment of the pollution risk of heavy metals in soils and crops based on the Chinese standards GB15618-2018 and GB2762-2022 [21], respectively. They found two inconsistent assessments, indicating a new challenge in maintaining consistency between the assessment standards for soils and crops. This inconsistent assessment was also found by Sun et al. (contribution 3). In contrast, concerning atmospheric deposition, Wei et al. (contribution 6) adopted the criteria of non-carcinogenic and carcinogenic risk assessment methods used by the United States Environmental Protection Agency (USEPA) in order to assess the risk of potential toxic elements (PTE) in a ion-adsorption rare earth mining area in Ganzhou City, Southeast China.

3.2. Elemental Bioaccumulation

Elemental bioaccumulation refers to transfer of elements from soils to crops or the crop uptake of elements from soils, which is measured by conducting a geochemical survey on the soils and the crops grown. The bioconcentration factor (BCF) is often used to quantitatively calculate the accumulation of heavy metals in plant tissues relative to the environmental soils [22].

Li et al. (contribution 7) studied the bioaccumulation of arsenic in the soil–rice system, using paired soil–rice samples from karst regions in Guangxi, China. They indicate that concentrations of As in the rice are inconsistent with the total concentrations of As in the soils, and the low bioavailability of As by rice in the karstic paddy soil was attributed to its residual form being present in Fe-Mn oxides. This low bioavailability of heavy metals can explain the inconsistent assessment results for soils and their crops (contribution 5).

Although inconsistent assessments and uncorrelated concentrations between soils and crops are widespread findings, many studies aiming to predict heavy metal concentrations in crops based on their concentrations in soils have been carried out. Geng et al. (contribution 8) established a prediction model for zinc concentration in rice from farmland soil, using 371 paired rice–soil samples from the Pear River Delta and Heyuan regions in Guangdong, China, and machine learning models. Ma et al. [23] also presented a prediction model for cadmium concentrations in paired rice–soil samples, using paddy soil samples from Guangxi Province, China, and machine learning algorithms. Besides rice, heavy metal contamination levels in other crops, such as maize [24] and peanut [25], have also been predicted on the contamination levels of the soil in which they were grown. However, these prediction models are only pilot studies and primarily data-driven, and knowledge-based models have not been well developed.

3.3. Remediation of Heavy Metals

Alongside the risk assessment of heavy metals, the remediation of heavy metals in soils is another important topic in environmental geochemistry. Remediation methods are categorized as agronomic control or soil amendment techniques. The agronomic control methods include crop variation, optimized fertilization, and foliar obstruction control ([26] and contribution 3), and the soil amendment methods include chemical leaching, in situ passivation, plant enrichment, and electrokinetic techniques ([27] and contribution 3).

Sun et al. (contribution 3) introduced the geo-electrochemical technology to remove the heavy metal Cd from paddy soil in Guilin, China. The results indicated that the new method is effective, not only at removing the Cd from the paddy soil but also at reducing the contents of Cd in the tissues and organs of the rice. This contribution is a good illustration of new in situ electrokinetic remediation methods.

4. Basic Applications in Geology

The main sample materials used in geochemical exploration and environmental geochemistry are soils and stream sediments [2,3], while rock and mineral samples are mainly used within basic geology, specifically when studying magmatism, mineralization, and tectonic thermal evolution, etc. [28,29].

Abd El-Naby and Dawood (contribution 9) studied peralkaline granites and related pegmatites in the Arabian shield in order to discover their geochemistry, petrogenesis, and rare-metal mineralization. Liu et al. (contribution 10) analyzed the trace elements in the minerals chalcopyrite and cassiterite, taken from the Shuangjianzishan super-large silver deposit in Inner Mongolia, in order to shed light on the genesis of Cu-Sn mineralization. In addition to the geochemistry of minerals, basic geology is also concerned with minerals’ signatures. Wu et al. (contribution 11) analyzed the apatite fission-track and zircon fission-track in the Helanshan Mountain tectonic belt, Northwest China, to reveal the tectonic activities in this area. Huang et al. (contribution 12) studied the characteristics of luminescence in high-temperature- and high-pressure-treated diamonds in order to explore a new rapid and nondestructive identification method. These contributions are very interesting and attractive for applications in the field of basic geology.

As a conclusion, the new advances and studies in applied geochemistry are mainly focused on the typical topics covered in the field, namely geochemical exploration and environmental geochemistry; some new challenges are also found and proposed in this Special Issue. The innovative studies presented here have great application potential in the fields of resources, environments, and social services. We thank the authors and reviewers for their contribution to this Special Issue and hope that more in-depth research in applied geochemistry will be illustrated in the future National Conferences on Applied Geochemistry.