Application of Hydrogeochemistry in Mineral Exploration: A Systematic Review of Global Practices, Emerging Trends, and Future Directions

Abstract

Hydrogeochemistry is a practical and low-impact tool for mineral exploration that relies primarily on groundwater as sampling media. It is particularly valuable for blind or deeply buried deposits where surface geochemical methods are ineffective, as groundwater acts as a natural integrator of geochemical signals from depth. This study presents a PRISMA 2020-compliant systematic review of hydrogeochemical exploration practices published between 1946 and 2025, synthesizing 118 empirically screened case studies from diverse geological and climatic settings. The review evaluates the geochemical processes governing aqueous dispersion halos, including sulphide oxidation, water–rock interaction, redox controls, and physicochemical speciation, and assesses how these processes influence pathfinder behaviour and anomaly expression. Quantitative synthesis highlights consistent patterns in hydrogeochemical footprints across major mineral systems and demonstrates the effectiveness of thermodynamically informed and multivariate interpretation strategies over simple concentration-based approaches. Emerging trends identified include the growing application of non-traditional stable isotope fractionation, nanoparticle geochemistry using single-particle ICP-MS, and integration of hydrogeochemical datasets with GIS, geophysics, and machine learning-based prospectivity modelling. Unlike recent narrative reviews, this study provides a fully reproducible, structured evaluation of the global evidence base and formalizes a standardized end-to-end workflow.

1. Introduction

Hydrogeochemistry has emerged as an essential tool in modern mineral exploration [1,2,3]. This is particularly in terrains where conventional methods such as soil sampling, stream sediment surveys, and geophysical techniques are constrained by thick overburden, deeply weathered regolith, or limited surface exposure [4,5]. Traditional surface-based geochemical methods rely on the chemical signature of rocks or sediments that are in direct contact with the mineralized zone [1]. However, in covered or transported terrains, surface materials are often dissociated from the underlying ore bodies, making detection of buried mineralization challenging. Owing to that, hydrogeochemistry provides a cost-effective and practical alternative by analyzing the chemical and isotopic composition of groundwater and surface water, which can transport diagnostic signals from concealed deposits to accessible sampling points [3,6,7,8,9]. Several recent narrative and authoritative reviews have comprehensively documented the historical development, empirical case studies, and methodological evolution of exploration hydrogeochemistry (e.g., [2,3,10]). These reviews provide an expert-driven synthesis of key concepts, case histories, and emerging analytical tools.

The principle underlying hydrogeochemical exploration is straightforward yet powerful. As water moves through rocks and sediments, it interacts with minerals and acquires dissolved elements and isotopic signatures that reflect the surrounding geology [3,4,5,11]. When these waters encounter ore bodies, selective leaching and transport of metals and associated pathfinder elements occur. The resulting chemical anomalies are dispersed into the surrounding hydrosphere, forming three-dimensional geochemical halos that extend far beyond the source deposit [3,12]. These secondary dispersion halos often span several kilometres, providing explorers with targets that are significantly larger than the discrete orebody itself. Case studies have documented detectable anomalies up to 10 km from mineralization, illustrating the potential for regional-scale reconnaissance using relatively low-density sampling networks [2,3,13].

Hydrogeochemical surveys employ a variety of elements as pathfinders, including base metals (Cu, Zn, Pb), precious metals (Au, Ag), and critical or strategic elements such as uranium, molybdenum, lithium, and rare earth elements [14,15,16]. Their mobility in groundwater is governed by complex geochemical processes, including mineral dissolution, adsorption–desorption reactions, redox transformations, complexation, and pH-dependent solubility [6,17]. Understanding these processes is critical to interpreting hydrogeochemical anomalies accurately and avoiding false positives caused by natural background variations or anthropogenic contamination. Thermodynamically based indicators such as mineral saturation indices are commonly applied to characterise groundwater evolution, assess water–rock interaction, and interpret geochemical dispersion processes relevant to mineral exploration [18,19].

Globally, hydrogeochemistry has demonstrated its effectiveness across a wide range of geological and climatic settings. In Canada, groundwater surveys have helped locate gold and base metals beneath glacial cover in shield areas [6]. In Australia, hydrogeochemical methods have been applied in arid regions with deep lateritic regolith, aiding exploration in greenstone belts and other complex terrains [20,21]. Also, in South America, hydrogeochemistry has contributed to the detection of porphyry copper and lithium brine systems in both mountainous and basin environments [22]. Similarly, studies in Africa and Asia have shown that groundwater chemistry can reveal concealed mineralization even in areas with extensive transported cover or limited surface outcrop exposure [3,23]. These examples illustrate the adaptability and robustness of hydrogeochemical approaches under varying environmental, geological, and climatic conditions.

The methodological evolution of hydrogeochemistry has paralleled technological advancements in analytical instrumentation. Traditional colorimetric and spectroscopic methods have largely been replaced or complemented by ultra-trace, multi-element analysis using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), high-resolution isotopic techniques, and complementary mineralogical characterization [17,24]. These tools enhance the detection of subtle geochemical signals and allow the differentiation of true mineralization from natural background variability. The integration of machine learning algorithms and geospatial analysis further enables the interpretation of complex datasets, improving targeting accuracy and reducing false positives [25,26]. Beyond its technical advantages, hydrogeochemistry is environmentally benign and logistically straightforward. Water sampling is minimally invasive, rapid, and suitable for coverage of large areas. This aspect aligns with the growing emphasis on Environmental, Social, and Governance (ESG) principles in mineral exploration programs, where minimizing environmental impact is increasingly prioritized [27,28]. The dual benefit of exploration and environmental monitoring reinforces the strategic value of hydrogeochemistry in contemporary mineral projects.

Despite its proven benefits, hydrogeochemical methods are not uniformly applied worldwide. Countries with well-developed geoscientific infrastructure tend to integrate hydrogeochemistry into national exploration programs, supported by long-term datasets, advanced laboratories, and trained personnel [3,29]. In contrast, frontier regions often face challenges including limited baseline data, technical skill gaps, and weak institutional support [29]. This hinders the widespread adoption of hydrogeochemistry, even in areas where it would be most beneficial [30,31]. Addressing these disparities is critical to ensuring equitable access to modern exploration tools and improving the efficiency of mineral discovery globally. The contemporary relevance of hydrogeochemistry is amplified by a shift in the mineral exploration industry. Near-surface, outcropping deposits in mature exploration provinces are becoming increasingly scarce, prompting the search for resources concealed under post-mineral cover [10,13,23,32,33,34]. Challenging environments, including glacial deposits, deeply weathered regolith, and alluvial plains, often render traditional surface-based techniques ineffective, as the surface material is decoupled from the bedrock [3,10,13,23]. Hydrogeochemistry addresses this limitation by using groundwater and deep spring water as natural sampling media, carrying diagnostic chemical and isotopic signatures from concealed mineralization to accessible points. This ability to “see through cover” provides a three-dimensional perspective on ore distribution and has become indispensable for modern exploration [3,35,36,37].

The global demand for minerals including copper, gold, lithium, and rare earth elements, further underscores the importance of hydrogeochemistry in supporting resource security, high-technology applications, and the transition to sustainable energy [3,14,38]. By combining hydrogeochemical data with GIS, remote sensing, and real-time monitoring, researchers can develop more accurate models, improve targeting efficiency, and reduce exploration risks [2,7,24,37,39,40].

In contrast to recent narrative reviews [3], which synthesize expert knowledge and selected case studies, the present study adopts a PRISMA-2020-compliant systematic review framework to quantitatively evaluate the global evidence base for hydrogeochemical mineral exploration. This approach enables transparent study selection, reproducible data extraction, and formal assessment of methodological quality and bias across a large corpus of empirical studies. By standardizing analytical methods, hydrogeochemical media, pathfinder suites, and interpretive strategies across 118 independently screened studies, this review moves beyond narrative synthesis to provide a defensible, reproducible, and quantitatively grounded framework for evaluating hydrogeochemical exploration effectiveness.

Therefore, the objectives of this review are to: (1) synthesize the global applications of hydrogeochemistry across various mineral systems; (2) unravel and establish fundamental geochemical principles governing anomalies: their formation, key pathfinder elements, analytical techniques, and practical case studies across diverse geological and climatic settings; and (3) examine emerging trends, technological advancements, and global challenges, offering insights into the future trajectory of hydrogeochemistry as a core tool in modern, sustainable, and efficient mineral exploration.

2. Literature Search and Methodology

This study was conducted as a systematic review in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines [41,42]. A comprehensive and reproducible literature search was undertaken to identify peer-reviewed studies examining the application of hydrogeochemistry in mineral and critical mineral exploration, with particular emphasis on exploration effectiveness, geochemical dispersion processes, and emerging approaches that support sustainable exploration workflows. This systematic structure enables direct comparison of hydrogeochemical practices, analytical approaches, and exploration outcomes across studies in a manner not achievable through a narrative review alone. The study followed the 27-item PRISMA 2020 checklist for systematic reviews, which is provided in Supplementary Table S1.

2.1. Search Strategy and Data Sources

A comprehensive and systematic literature search was conducted across major international scientific databases, including Scopus, Web of Science Core Collection, ScienceDirect, and Google Scholar, covering publications from 1946 to March 2025. The search strategy was designed to capture peer-reviewed empirical studies at the intersection of hydrogeochemistry and mineral exploration and combined controlled vocabulary with free-text keywords. Core thematic concepts included hydrogeochemistry, groundwater and water geochemistry, mineral and critical mineral exploration, geochemical dispersion processes, hydrogeochemical anomalies, pathfinder elements, multivariate geochemical analysis, and sustainable exploration technologies.

For the Scopus database, searches were applied to the title, abstract, and author-keywords fields using a nested Boolean structure as follows: TITLE-ABS-KEY (“hydrogeochemistry” OR “groundwater geochemistry” OR “water geochemistry”) AND TITLE-ABS-KEY (“mineral exploration” OR “ore exploration” OR “critical minerals”) AND TITLE-ABS-KEY (“geochemical dispersion” OR “hydrogeochemical anomalies” OR “pathfinder elements” OR “multivariate geochemical analysis”).

In the Web of Science Core Collection, the search was conducted using the Topic field (TS), which queries titles, abstracts, and keywords. The applied search string was: TS = (“hydrogeochemistry” OR “groundwater geochemistry” OR “water geochemistry”) AND TS = (“mineral exploration” OR “ore exploration” OR “critical minerals”) AND TS = (“geochemical dispersion” OR “hydrogeochemical anomalies” OR “pathfinder elements” OR “multivariate geochemical analysis”).

For ScienceDirect, which does not support fully nested Boolean logic across indexed fields, keyword combinations consistent with the core conceptual groups were applied through the full-text search interface using the following structure: (“hydrogeochemistry” OR “groundwater geochemistry”) AND (“mineral exploration” OR “critical minerals”) AND (“geochemical dispersion” OR “hydrogeochemical anomalies”). While syntax constraints varied across databases, conceptual consistency was maintained to ensure comparable retrieval of relevant studies.

Google Scholar was used selectively to capture grey literature, early access publications, and relevant sources not indexed in subscription-based databases. Owing to platform limitations, Google Scholar does not support fully reproducible database-specific Boolean logic, field-restricted queries, or controlled vocabulary searches. Accordingly, simplified keyword combinations reflecting the primary search concepts were employed using phrase-based queries and basic Boolean operators. The exact search string applied in Google Scholar was: “hydrogeochemistry” AND (“mineral exploration” OR “critical mineral exploration”) AND (“groundwater” OR “water chemistry”).

The database search yielded 235 records in total, comprising ScienceDirect (n = 101), Google Scholar (n = 56), Scopus (n = 46), and Web of Science (n = 32). After manual removal of 24 duplicate records, 211 publications were screened based on titles and abstracts (Figure 1). During this screening stage, 18 records were excluded (10 on the basis of title and 8 based on abstract content), leaving 193 records for full-text retrieval. Of these, 7 articles could not be accessed, and 182 full-text articles were assessed for eligibility.

Following full-text evaluation, 64 studies were excluded, including papers irrelevant to the scope of the review (n = 6), publications lacking original empirical data (n = 57), and short or workshop papers (n = 1). Ultimately, 118 empirical studies met all inclusion criteria and were retained for qualitative synthesis. The study identification, screening, and selection process is summarized in the PRISMA flow diagram (Figure 1), which also presents the reasons for exclusion at the full-text stage; individual excluded studies are not listed but are available upon request.

2.2. Eligibility Criteria and Study Selection

After removal of duplicate records, the remaining studies were screened at the title and abstract level to assess relevance to the objectives of the review. Studies were retained for further consideration only if they clearly addressed the use of hydrogeochemical data or methods in the context of mineral or critical-mineral exploration and demonstrated relevance to exploration workflows, such as reconnaissance surveys, target delineation, or geochemical vectoring under cover. Eligible studies were required to present original hydrogeochemical data, methodological frameworks, or modelling approaches that contributed interpretive value to understanding geochemical dispersion systems in an exploration context.

Records were excluded at this stage if they consisted solely of conference abstracts without full peer-reviewed papers, lacked sufficient methodological clarity, or focused exclusively on groundwater quality, contamination, or environmental and health impacts without explicit exploration relevance. Publications centred on analytical method development, machine-learning or GIS techniques, or conceptual reviews without application to hydrogeochemical mineral exploration case studies were similarly excluded from extraction. Non-English publications were also excluded. Studies meeting the initial screening criteria proceeded to full-text assessment.

Full-text evaluation further refined the selection by excluding papers that did not present substantive hydrogeochemical datasets, did not demonstrate direct applicability to mineral exploration objectives, or addressed hydrogeochemical processes solely within environmental or hydrogeological frameworks without a link to exploration decision-making. The background literature of this nature was retained for contextual discussion but excluded from formal data extraction and synthesis. The final set of eligible studies constituted the evidence base for the systematic synthesis presented in this review, and the complete study selection process is summarized in the PRISMA flow diagram (Figure 1).

2.3. Review Protocol and Screening Procedure

A structured review protocol defining the search strategy, eligibility criteria, screening procedures, and synthesis objectives was established prior to data extraction. Titles and abstracts were independently screened for eligibility by two reviewers, followed by full-text assessment.

The review protocol was not registered in a public repository, as currently available registration platforms such as PROSPERO are primarily designed for health-related systematic reviews and do not provide tailored pathways for geoscience-focused evidence syntheses. All stages of screening and eligibility assessment were conducted manually, without the use of automation tools. Study selection followed a two-stage process consisting of title and abstract screening followed by full-text evaluation, and any uncertainties regarding eligibility were resolved through discussion. These procedures were implemented to ensure transparency, consistency, and adherence to PRISMA 2020 methodological standards.

2.4. Data Extraction and Quality Assessment

Data extraction was conducted using a standardized template developed a priori to ensure consistency across studies. Extracted information included bibliographic details, geographic and geological settings, hydrogeochemical media, analytical techniques, exploration objectives, key findings, and reported limitations. One reviewer performed the initial data extraction, which was independently verified by a second reviewer to ensure accuracy and consistency; discrepancies were resolved through discussion and consensus, following a primary extractor–secondary verifier approach. Extracted data were subsequently harmonized within a consistent framework by standardizing terminology related to hydrogeochemical media, analytical methods, and mineral system classifications. Where necessary, reported values and interpretations were conceptually normalized to facilitate cross-study comparison. Missing or incomplete data were not imputed but were instead documented and considered qualitatively during synthesis. The complete extracted dataset for all included studies is provided in Supplementary Table S2.

To evaluate the robustness of the evidence base, each included study was further assessed for methodological quality and potential risk of bias using an adapted framework appropriate for hydrogeochemical and exploration-geochemistry research. Assessment considered sampling design and representativeness, analytical reliability and QA/QC reporting, transparency of data interpretation, and clarity of the linkage between hydrogeochemical signals and mineralization. Risk of bias was assessed qualitatively, with overall study quality considered during evidence synthesis rather than used as a basis for exclusion. Potential reporting bias was evaluated by examining the diversity of study locations, mineral systems, and publication sources. Although formal statistical assessments of reporting bias were not applicable, inclusion of studies spanning a broad temporal range (1946–2025) and multiple geographic regions was used to reduce the risk of selection bias. Summary results of the bias and quality assessments are presented in Supplementary Table S2.

As this study is a qualitative systematic review without meta-analysis, no standardized effect measures (e.g., risk ratios or mean differences) were defined. Instead, outcomes were synthesized based on reported hydrogeochemical patterns, geochemical indicators, and exploration effectiveness metrics described in the included studies. Also, sensitivity analyses were not conducted, as the review did not involve quantitative synthesis or statistical modelling. Instead, robustness of findings was evaluated qualitatively through consistency of reported patterns across multiple independent studies and geological settings.

The certainty of evidence was assessed qualitatively based on consistency of findings across studies, methodological rigor, analytical reliability, and strength of linkage between hydrogeochemical signatures and mineralization. Given the observational and heterogeneous nature of the included studies, a formal grading framework (e.g., GRADE) was not applied.

Possible causes of heterogeneity among study results were explored qualitatively through comparative analysis of key study characteristics. This included examination of differences in geological settings, mineral system types, hydrogeochemical parameters measured, analytical techniques, spatial and temporal sampling scales, and study objectives. Variability in environmental context (e.g., climatic conditions and hydrogeological regimes) and methodological approaches were considered when interpreting differences in reported hydrogeochemical signatures and exploration outcomes. Formal statistical assessment of heterogeneity was not undertaken due to the qualitative and descriptive nature of the evidence base.

2.5. Data Synthesis and Analytical Approach

The synthesis of results was conducted using a qualitative, narrative approach, appropriate for the heterogeneous and predominantly observational nature of the included studies. Extracted data were systematically organized and grouped according to key thematic domains, including mineral system types, hydrogeochemical processes, analytical techniques, and exploration outcomes. Comparative analysis was employed to identify recurring patterns, consistencies, and divergences in hydrogeochemical signatures across different geological and climatic settings. Where appropriate, findings were further structured into process-based and commodity-specific categories to facilitate interpretation and cross-study comparison. This approach was selected in preference to quantitative meta-analysis due to substantial variability in study design, sampling media, analytical methods, and reported variables, which precluded meaningful statistical aggregation. The qualitative synthesis enabled integration of diverse evidence while preserving contextual geological and methodological nuances critical for interpreting hydrogeochemical exploration effectiveness.

2.6. Reporting and Transparency

This systematic review adheres fully to the PRISMA 2020 reporting standards. The completed PRISMA checklist is provided in Supplementary Table S1, and the flow diagram illustrating the identification, screening, eligibility, and inclusion of studies is presented in Figure 1. Collectively, these procedures ensure transparency, reproducibility, and methodological rigor consistent with best practices for systematic reviews in hydrogeochemistry and mineral exploration research.

3. Systematic Review Findings

3.1. Study Selection and Characteristics

The database search (1946–2025) retrieved 235 records; after deduplication, title/abstract screening, and full-text assessment under PRISMA, 118 studies were included for the synthesis (Figure 1). Screening and eligibility were conducted manually using predefined criteria [41,42].

3.2. Analytical Techniques and Detection Capabilities

Eligible studies are clustered into six analytical families (

Table 1. Comparative review of analytical techniques.

Technique	Principle	Typical Detection Limits	Throughput	Matrix Tolerance (TDS)	Relative Cost	Primary Exploration Application	Reference
Flame AAS	Atomic Absorption	ppm–high ppb	Single element (Sequential)	High	Low	Targeted analysis for base metals in contaminated or high-concentration settings	[14,50]
GFAAS	Atomic Absorption	ppb–low ppb	Single element (Sequential)	Moderate	Low-Medium	Analysis of specific trace elements (e.g., Pb, Cd, Au) where ICP is unavailable	[14,50]
ICP-OES	Optical Emission	high ppb–low ppb	Multi-element (Simultaneous)	Very High (up to 30%)	Medium	Regional screening for major and trace elements; analysis of high-salinity waters (brines, wastewaters)	[10,45]
ICP-MS	Mass Spectrometry	ppb–ppt	Multi-element (Simultaneous)	Low (~0.2%)	High	Standard for most exploration surveys; ultra-trace pathfinders (Au, PGE), REE, isotopes	[10,36,43,45]
HR-ICP-MS	Mass Spectrometry	ppt–sub-ppt (ppq)	Multi-element (Simultaneous)	Low (~0.2%)	Very High	Research applications; resolving complex interferences; ultra-trace analysis for concealed deposits	[36,39,44]
Isotope MS	Mass Spectrometry	Isotopic Ratios (%)	Varies	Varies	High	Source and process fingerprinting; direct vectoring to ore	[2,43,46,51]
TEM/spICP-MS	Electron Microscopy/Mass Spectrometry	Nanoparticle analysis	Single particle	Low	Very High	Direct detection and characterization of ore-related solid particles in groundwater	[48,49]

). ICP-OES is a robust atomic emission spectroscopic technique that is particularly well suited for the analysis of high-total dissolved solids (TDS) matrices such as saline groundwater, brines, and salt-rich waters, owing to its high matrix tolerance and reduced susceptibility to plasma instability. While its detection limits are higher than those of ICP-MS, ICP-OES remain an effective and widely applied screening and quantification tool for major and trace elements in exploration settings where sample salinity exceeds the practical tolerance of ICP-MS and extensive dilution would compromise analytical efficiency [14,43]. HR-ICP-MS provides sub-ppt (ppq) sensitivity and improved interference resolution [39,44]. ICP-OES is preferentially used for high-TDS matrices (tolerance to ~30%) with high- to low-ppb detection limits [10,45]. MC-ICP-MS/isotope-ratio MS supports high-precision radiogenic/stable isotopes (Pb, Sr; δ⁶⁵Cu, δ⁹⁸Mo) for source/process tracing [2,33,43,46,47]. Single-particle ICP-MS (spICP-MS) and TEM are reported for detecting/characterizing ore-related nanoparticles in groundwater [48,49]. Ion Chromatography remains standard for the quantification of major anions including Cl⁻ and SO₄²⁻, as well as halide tracers such as Br⁻ and I⁻ that are useful for distinguishing fluid sources and salinity evolution [5,14].

3.3. Empirical Patterns Consistently Reported

Across climates and deposit types, three observation-based patterns occur.

(i)

Zoned halos: Proximal enrichment of less mobile cations (Cu, Pb, Zn) and distal enrichment of more mobile oxyanions (Mo, Re, As, Se) are commonly mapped; element ratios between species of contrasting mobility are frequently used to sharpen vectors [2,4,6,10,52].

(ii)

Systematic association with field chemistry: higher dissolved Cu/Zn are reported in acidic waters, whereas Mo/As/U are commonly elevated in near-neutral to alkaline, carbonate-buffered waters; sharp dissolved U step-changes coincide with redox boundaries [2,5,10,53,54].

(iii)

Salinity/climate effects on noble metals: in arid, saline terrains, shallow groundwaters frequently contain detectable Ag (and locally Au), whereas oxidizing, near-neutral freshwaters more often show short-range, subtle dissolved Au anomalies [10,14,55]. Attenuation of cationic metals along flow paths via adsorption to Fe–Mn (oxyhydr)oxides and secondary mineral precipitation is widely documented [4,14,56]. Deposit-specific examples are illustrated in Figure 2.

3.4. Commodity-Specific Hydrogeochemical Footprints

3.4.1. Porphyry Cu–Mo–Au Systems

Deposit and district-scale surveys frequently document a proximal Cu–Zn–Co halo with an enveloping distal Mo–Re–As–Se plume over kilometers scales (Figure 2) [2,6,10,52]. At Pebble (Alaska), acidic waters over thin cover show Cu–Cd–Re–SO₄²⁻, whereas near-neutral waters over deeper cover exhibit Mo–As–Sb–W at ultra-trace levels [39,58,59,60]. At Spence (Chile), reported groundwater ranges include Cu commonly ~10³ µg/L (local maximum 28,991 µg/L), Mo frequently >100 µg/L, with pH ≈ 4.7 and TDS > 25,000 mg/L in parts of the plume; Re is detected down-gradient at ultra-trace levels [6,10,61].

3.4.2. Volcanogenic Massive Sulphide (VMS) Systems

Shield-lake surveys in permafrost/boreal settings consistently note very low dissolved metal backgrounds (often < 2 ppb) and stronger Zn than Cu anomalies in the dissolved fraction [4,62].

3.4.3. Sediment-Hosted Pb–Zn (SEDEX/MVT) Systems

In neutral, carbonate-buffered terrains, studies report the utility of normalizations (e.g., Zn/Cl⁻, SO₄²⁻/Cl⁻) and the mapping of radiogenic Pb-isotope outliers beneath cover; representative models are summarized in

Table 2. Summary of global hydrogeochemical exploration models.

Commodity/Deposit Type	Location/Environment	Sample Medium	Key Indicators/Pathfinder Suite	Dominant Geochemical Process	Key Innovation/Outcome	References
Base Metals
VMS (Cu, Zn)	Canadian Shield (Permafrost, Pristine)	Lake Water	Zn > Cu	Sulphide oxidation in low-salinity, acidic-to-neutral water; Adsorption on Fe-oxides.	Helicopter-based rapid sampling system; Model for detailed follow-up surveys.	[4]
Porphyry Cu	Puerto Rico (Tropical, High Weathering)	Stream Water	Regional: SO42− Detailed: Cu, Zn, F−	Tiered approach using regional (SO42−) and detailed (Cu, Zn) pathfinders are effective.	Pathfinder Element Suite model for high-weathering environments.	[65]
Pb-Zn	North Wales, UK (Temperate, Complex)	Stream Water	Multi-element suites (Pb, Zn, Cd, As)	Multiple overlapping signals: Mineralization, bedrock weathering, atmospheric, and anthropogenic inputs.	Model for exploration in complex terrains; Call for integration of multi-source data in GIS.	[5]
Cu-Au (IOCG)	Mumbwa, Zambia (Deeply weathered)	Groundwater	As, Mo, Fe, Mn, Zn, δ98Mo, δ65Cu	Leaching of alteration halo and ore; Isotopic fractionation during weathering.	Highlighted the utility of non-traditional stable isotopes for fingerprinting and vectoring.	[2]
Precious Metals
Epithermal Au-Ag	Great Basin, USA (Arid, Saline)	Shallow Groundwater	Ag	Supergene oxidation; Transport as chloride complexes in saline water.	Predictive modelling of mineral solubility; Ag in groundwater as a pathfinder for buried deposits.	[55]
Epithermal Au	Sardinia, Italy (Semi-Arid, Neutral pH)	Stream Water	As, Sb > Au	Limited mobility of Au in near-neutral freshwaters; greater mobility of As and Sb.	Model for prospect-scale targeting using robust pathfinders for a restricted Au halo.	[14]
Critical Minerals
Roll-Front U	Texas/Wyoming, USA (Semi-arid)	Groundwater	Saturation Index (SI) for Uraninite	Redox-controlled dissolution and precipitation of uranium at a mobile front.	Thermodynamic Modelling (SI maps) are superior to raw concentration maps for target delineation.	[66]
Unconformity U	Pine Creek, Australia (Monsoonal)	Groundwater	Normalized Magnesium (NMg)	Leaching of distinctive Mg-rich alteration halo around the orebody.	Alteration Geochemistry Signature (NMg ratio) is a robust indicator, especially where U is immobile.	[67]
Lithium Brine	Andean Plateau, South America (Hyper-arid)	Brine	Direct Li concentration; Low Mg/Li ratio	Evaporative concentration in a closed basin with a Li source.	Direct resource evaluation model based on major ion ratios for economic viability.	[22,68]
REE	Carbonatite-Hosted (Sardinia, Italy)	Groundwater	REE Fractionation Patterns (HREE enrichment)	HREE enrichment via Carbonate complexation provides a vector to mineralization.	Use of normalized REE patterns as a vectoring tool.	[69]

[35,38,63,64].

3.4.4. Epithermal Au–Ag Systems

In arid, saline basins (e.g., parts of the Great Basin), shallow groundwater surveys report broad Ag halos [55]. In oxidizing, near-neutral systems (e.g., Sardinia), dissolved Au anomalies are typically ≤ ~0.5 km, while As/Sb provide more consistent aqueous footprints [14].

3.4.5. Orogenic and Carlin-Type Au Systems

Continental- to camp-scale surveys (e.g., Yilgarn Craton) report background dissolved Au on the order of single-digit ng/L, with local maxima up to ~2 µg/L (Carosue Dam) and ~52 ng/L (St Ives); co-occurrence of As, Sb, Te, W near Au systems is frequently documented [20,29,70]. In Carlin-type terrains, groundwater suites of As, Sb, Hg, Tl are consistently mapped where dissolved Au is weak; representative hydrochemical facies distributions are shown for Getchell [71,72,73,74].

3.4.6. Uranium Systems

For roll-front deposits, mapping of calculated Saturation Index (SI) for uraninite is repeatedly reported as delineating ore trends more effectively than raw dissolved U [66,75]. For unconformity-related systems, elevated ²³⁴U/²³⁸U activity ratios, Ra-isotope anomalies, and Normalized Magnesium (NMg) signals are documented as hydrogeochemical indicators [46,67,76,77].

3.4.7. Lithium and Rare-Earth Elements (REE)

In lithium brines, appraisal emphasizes absolute Li and Mg/Li as primary evaluation metrics; dispersion of Li, Rb, Cs is reported in shallow groundwater proximal to LCT pegmatites [22,68,78,79,80,81,82]. For REE, vectors are described using normalized REE patterns with LREE/HREE fractionation and Eu/Ce anomalies [69,83,84].

3.5. Sampling, QA/QC, and Data-Handling Practices Reported

Field protocols commonly include acid-rinsed containers, 0.45 µm filtration for the dissolved fraction, acidification of cation/trace aliquots, dedicated bottles for anions, and in situ measurement of pH, Eh, EC, and temperature [16,76,85]. UAV-enabled sampling and peristaltic pumps for point-source filtration are reported in remote settings [24,86,87]. Data workflows frequently acknowledge the compositional nature of aqueous geochemical data and therefore apply log-ratio (clr) transformations prior to PCA or cluster analysis. Thermodynamic modelling by calculation of saturation indices and metal speciation by codes, e.g., PHREEQC [88], is also widely used. These approaches are often combined with GIS-based integration of other prospectivity layers. Such workflows are well documented in the literature (a summary of interpretation approaches is provided in

Table 3. Comparison of data interpretation methodologies.

Methodology	Primary Function	Strengths	Limitations	Data Requirements	References
Single-Element Thresholding	Identify statistical outliers in a single variable’s distribution.	Simple, fast, and easy to visualize (on maps)	Highly prone to false anomalies; ignores covariance and underlying processes	Single-element concentration data.	[5,89,90]
Pathfinder Ratios	Normalize for background effects or highlight specific processes.	Can reduce effects of dilution/evaporation; can enhance mineralization signature	Can be misleading if the denominator is not a true conservative tracer for the process being corrected	At least two element concentrations.	[10,63,89]
Thermodynamic Modelling (SI)	Model mineral-solution equilibria and element speciation.	Provides a process-based chemical framework; can identify prospective waters near saturation (SI ≈ 0) even with low concentrations	Assumes equilibrium, which is often not met; highly sensitive to quality of Eh, pH, and temperature data	Full major and minor analysis, plus field parameters (pH, T, Eh).	[46,66,76,89]
Multivariate Statistics (PCA/Cluster)	Deconstruct complex datasets to identify dominant processes and group samples by genetic type.	Identifies underlying processes (weathering, pollution); allows for context-specific anomaly definition, reducing false anomalies	Requires a large, complete dataset; results can be abstract and require expert interpretation	Comprehensive multi-element dataset for a large number of samples.	[19,38,89]
Isotopic Analysis	Trace the source of solutes and the processes they have undergone.	Provides direct, unambiguous information on source (e.g., radiogenic Pb) and process (e.g., UAR); can provide temporal data (e.g., Ra isotopes)	Higher analytical cost; requires specialized laboratory facilities and expert interpretation	High-precision isotope ratio data.	[2,46,91]
Geospatial Integration (GIS)	Spatially integrate and analyse multiple georeferenced datasets to identify converging evidence.	Reveals spatial relationships between anomalies and geological features; enhances target confidence	Effectiveness depends on the quality and relevance of the integrated datasets.	Georeferenced hydrogeochemical, geological, geophysical, and remote sensing data.	[5,92,93,94]

; see Section 4.6.3) [19,88,89].

3.6. Evidence Synthesis

Across the compiled studies, evidence supports that: (i) zoned dispersion halos with predictable element partitioning are observed across hydrogeological settings; (ii) ultra-trace analytical capability routinely resolves subtle footprints; and (iii) standardized field/laboratory protocols coupled with multivariate and geospatial synthesis are widely implemented for aqueous anomaly delineation [3,4,10,39].

Overall, the certainty of evidence is considered moderate to high for well-established hydrogeochemical processes (e.g., sulphide oxidation, pH–Eh controls, dispersion halos), supported by consistent observations across diverse geological settings. Emerging approaches such as isotope geochemistry and nanoparticle analysis show promising but comparatively lower certainty due to limited datasets and evolving methodologies.

4. Discussion

4.1. Principles and Evolution of Hydrogeochemistry in Mineral Exploration

The effectiveness of hydrogeochemistry as an exploration tool is rooted in a series of interconnected geochemical processes that transfer the chemical signature of a mineral deposit into the surrounding aqueous environment [1]. A thorough understanding of these processes, from the initial release of ore-related elements at the source, through their transport and modification along a groundwater flow path, to their eventual attenuation, is fundamental to the design of effective survey strategies and the accurate interpretation of hydrogeochemical data [95,96,97]. These mechanisms also explain why groundwater can detect deeply buried or concealed mineralization, as demonstrated by studies in hyper-arid terrains and complex geological settings [7].

The application of hydrogeochemistry as a distinct scientific discipline has a history spanning over 70 years, marked by cycles of intense interest and relative dormancy, each cycle defined by the alignment of exploration challenges with available technology [3,12,98]. The discipline’s modern origins can be traced to the systematic work of Russian geochemists in the 1930s and 1940s, who pioneered concepts of “physicochemical prospecting” by formally testing the use of water chemistry to detect secondary dispersion from ore bodies [62,98,99,100,101,102,103]. One of the first documented case studies was conducted in 1941 by A. Reznikov, who used dithizone-based colorimetric reagents to estimate trace concentrations of base metals in water, successfully identifying anomalies related to polymetallic deposits in Altai, Russia [23,99].

A significant boom in its application occurred during the global search for uranium in the 1970s, driven by a clear “problem-technology fit” [3,12,24,96,104,105]. The target, uranium, is highly mobile in aqueous environments, and the analytical methods of the time, such as fluorometry and delayed neutron counting, were sufficiently sensitive to detect its anomalies [3,12]. Large-scale programs like the Hydrogeochemical and Stream Sediment Reconnaissance (HSSR) in North America proved highly successful, cementing the technique’s value for specific commodities [3,12,96,104].

Following this period, the discipline’s broader application waned, particularly as exploration shifted to less mobile commodities like gold [3,98]. The existing analytical technologies, such as Atomic Absorption Spectrometry (AAS), often lacked sensitivity to detect the extremely subtle anomalies generated by these elements, limiting the technique’s perceived utility [3,98,106]. This led to a “crisis of confidence” in the early 1980s, when researchers began to recognize that simply measuring the concentration of a metal in water was often insufficient and led to inconsistent or misleading results [66,67]. Early methods frequently failed because they relied on a simple anomaly threshold for an element like uranium or copper, without accounting for the profound influence of the local geochemical environment [37,66,67,107]. For example, studies found a poor correlation between dissolved uranium and a known orebody because local redox conditions and pH had a stronger effect on uranium’s solubility than proximity to the source [37,66,107]. This crisis necessitated a move beyond simple anomaly hunting and forced a fundamental paradigm shift. The scientific focus evolved from asking what the concentration was to a more sophisticated inquiry: why it was that value [66]. This shift from empirical measurement to process-based understanding gave rise to new interpretive philosophies that remain central to the discipline today [65,66,67,107]. Three distinct but complementary strategies emerged to overcome the limitations of simple concentration data: the use of computer models to calculate a mineral’s Saturation Index (SI), a thermodynamic measure of stability; a focus on the chemical fingerprint of alteration halos using major ion ratios; and the employment of a suite of mobile pathfinder elements to identify the broader signature of a mineralizing system [37,65,66,67].

The contemporary resurgence of hydrogeochemistry has been driven by a new problem–technology fit. The modern challenge of discovering deposits concealed deep beneath post-mineral cover created a pressing need for depth-penetrating methods, and a concurrent revolution in analytical geochemistry provided the solution [3,13]. The development and commercialization of Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) in the 1980s, and its subsequent refinement, fundamentally transformed the field [7,23,108,109]. The ability of ICP-MS and its high-resolution variants to provide rapid, multi-element analysis at extremely low detection limits down to parts-per-trillion (ppt) or nanograms-per-litre (ng L⁻¹) levels has dramatically expanded the detectable footprint of mineral deposits [3,23,24,37,39,58,109]. This technological leap overcame historical limitations and enabled the discovery of subtle anomalies from deeply buried systems that were previously undetectable, making hydrogeochemistry a routine and cost-effective tool for modern exploration [3,23,37,39,96,110]. This history reveals a clear pattern: the viability of hydrogeochemistry as a solution to exploration problems has been paced by technology, suggesting that future breakthroughs will also be driven by the next generation of analytical and computational tools [3,23,98].

Figure 3 illustrates the global distribution of major mineral commodities relative to Köppen–Geiger climate zones, highlighting clear concentrations in temperate and cold continental regions as well as secondary clusters across arid and tropical belts. This climatic diversity is directly relevant to the historical development of hydrogeochemistry. The application of hydrogeochemistry as a scientific discipline, spans more than seven decades, marked by cycles of strong adoption and periods of decline, each driven by how well analytical technologies matched the geochemical challenges presented by different climatic settings [3,12,98]. Early successes such as those documented by Russian geochemists in the 1930s and the uranium exploration boom of the 1970s were primarily achieved in environments where groundwater flow and physicochemical conditions reliably mobilized target elements. However, as exploration expanded into tropical and arid terrains, where complex hydrology and extreme weathering altered metal dispersion pathways, the limitations of early technologies became apparent, contributing to the “crisis of confidence” of the 1980s [66,67]. The climatic patterns in Figure 3 therefore reinforce a central historical theme: hydrogeochemistry has always been constrained by both environmental setting and technological capability. The subsequent development of high-sensitivity, multi-element ICP-MS analysis has now largely overcome these limitations, enabling the detection of ppt-level anomalies across a far broader range of climatic environments [3,23,37,58,109]. As a result, the modern resurgence of hydrogeochemistry reflects a renewed “problem–technology fit,” aligning advanced analytical tools with the challenges posed by concealed mineral systems in diverse climatic zones.

4.2. Geochemical Processes Influencing Groundwater Composition

The formation of a hydrogeochemical anomaly begins with the direct interaction between water and minerals, a process that includes dissolution, hydrolysis, and oxidation [11,111]. While the background chemical composition of any groundwater sample is a composite signature derived from multiple sources, including meteoric recharge and dissolution of non-mineralized host rock, the dominant process in the context of mineral exploration is the selective leaching of elements from an orebody [10,112].

In many of the world’s most significant base and precious metal systems, the primary engine for this element mobilization is the oxidation of sulphide minerals [2,4,33,55,65]. When sulphide minerals such as pyrite (FeS₂) or chalcopyrite (CuFeS₂) are exposed to oxygenated groundwater, they undergo oxidative weathering [2,33,54,113,114]. This reaction is critical because it releases three key components into the surrounding water: dissolved metals (such as Cu, Zn) and sulphate (SO₄²⁻), a highly mobile and stable indicator of the process, and acidity (H⁺), which dramatically alters the local chemical environment and accelerates the dissolution of other minerals [2,4,65,115,116,117,118,119,120]. This process is often exothermic, a factor of particular importance in cold environments like the Canadian Shield, where the heat generated can create thawed channels within permafrost, enhancing water flow and element dispersion even in sub-zero conditions [4,116,121].

4.3. Physicochemical Controls on Element Mobility and Transport

Once an element is released from its host mineral, its ability to travel in water, its mobility, is governed by a suite of external physicochemical conditions [10,12,66]. The two master variables controlling most geochemical reactions are pH (the measure of acidity or alkalinity) and Eh (the oxidation-reduction potential) [2,122]. The pH of the water is a critical control determined by the balance between acid generation from sulfide oxidation and the natural buffering capacity of the host rocks [4,115,123]. In regions dominated by base-poor lithologies such as certain granites, the water becomes acidic (pH < 5.5), which significantly increases the solubility and mobility of many metal cations, including copper (Cu), zinc (Zn), and aluminium (Al) [5,116,124,125]. Conversely, where the geology includes carbonate-bearing rocks like limestone, the acid is neutralized, resulting in near-neutral or even slightly alkaline waters that limit the mobility of these cations but favor the mobility of elements that form stable, soluble oxyanions, such as molybdenum (Mo), arsenic (As), and uranium (U) [2,4,5,10,89,126,127].

The redox potential (Eh) is paramount for elements that can exist in multiple valence states [2,66]. The classic example is uranium, which is highly soluble and mobile as the hexavalent uranyl ion (UO₂²⁺) under oxidizing conditions but becomes highly insoluble and precipitates as U(IV) minerals like uraninite (UO₂) in reducing environments [53,66,77,114]. Exploration for certain uranium deposit types is therefore fundamentally a search for these redox boundaries [77]. The mobility of metallic ions is also significantly enhanced by the formation of soluble aqueous complexes with various ligands naturally present in water [66,111,128,129]. This process of complexation can keep a metal dissolved in conditions where it would normally precipitate [66,129]. The nature of the dominant ligand creates a natural geochemical separation of elements during transport. For instance, in the low-salinity, acidic waters of the Canadian Shield, the relative mobility of base metals is largely controlled by pH, making Zn a more mobile pathfinder than Cu [4,124]. In stark contrast, in the arid, saline groundwaters of the Great Basin, the high concentration of chloride (Cl⁻) becomes the dominant control. Here, metals like gold (Au) and silver (Ag) form stable, soluble chloride complexes (for example, AuCl₂⁻ and AgCl₂⁻), which dramatically increases their mobility and allows them to be transported far from their source [3,55]. This demonstrates that the best pathfinder is not an intrinsic property of an element but is a context-dependent variable dictated by the local hydrogeochemical environment.

Formation of Dispersion Halos and Reduction Processes

The distance an element can travel from its source, its dispersion train, is ultimately limited by reduction processes, or sinks, that remove it from solution [4,12,35,46,89,130]. The interplay between mobilization, transport, and attenuation results in the formation of structured, zoned geochemical anomalies, or dispersion halos, in groundwater and surface water systems [3,10]. Geochemical dispersion is not a simple dilution process; it is an active process of geochemical fractionation that spatially separates elements based on their chemical properties, creating predictable patterns that can be used for vectoring [2,3].

The most significant attenuation process in many freshwater systems is the adsorption of dissolved metal cations onto the surfaces of solid particles, particularly freshly precipitated hydrous iron (Fe) and manganese (Mn) oxides [4,14,56,131,132,133]. This process is highly pH-dependent and further contributes to the geochemical separation of elements. For example, Cu tends to adsorb at a lower pH than Zn, which explains why Cu anomalies are often less extensive than Zn anomalies originating from the same sulphide source [4,132]. The other primary attenuation mechanism is the precipitation of secondary minerals, which occurs when the water becomes supersaturated with respect to a particular mineral phase as physicochemical conditions change along a flow path [10,12,35].

These attenuation processes are often viewed as a limitation, but they also represent a concentrating mechanism that transfers the anomaly from the dissolved phase to the solid phase. The adsorption of metals onto Fe-Mn oxides, for example, relocates the anomaly to the suspended particulate matter or the active stream sediment, creating a new exploration opportunity [14]. This dynamic interplay results in characteristically zone geochemical footprints. Less mobile elements that are readily removed from solution, such as Cu, lead (Pb), and Zn, tend to form proximal anomalies closer to the orebody [2,10]. In contrast, highly mobile elements that form stable, soluble oxyanions, such as Mo, rhenium (Re), and As, are less affected by these processes and can travel significant distances, forming extensive distal anomalies that create a much larger exploration target [2,10]. The most valuable information in a hydrogeochemical survey is therefore often found not in the absolute concentration of a single element, but in the ratios between elements of differing mobility, which can point directly toward the source [2,3].

4.4. Hydrogeological Controls on Hydrogeochemical Vectoring

A physically consistent interpretation of hydrogeochemical anomalies in mineral exploration requires explicit consideration of groundwater flow system theory, particularly the framework developed by Tóth, which establishes groundwater as a fundamental geologic agent responsible for mass transport across a hierarchy of spatial scales [134,135]. Within this framework, groundwater flow occurs in nested local, intermediate, and regional systems, each characterized by distinct flow path lengths, hydraulic gradients, residence times, and degrees of geochemical evolution. This distinction provides the hydrogeological context necessary to understand both the spatial resolution and the exploration significance of hydrogeochemical anomalies.

In local groundwater flow systems, groundwater circulates over relatively short distances and timescales, allowing dissolved constituents to remain closely coupled to their source lithologies. Under these conditions, hydrogeochemical signatures primarily reflect proximal water–rock interaction with altered and mineralized zones. Thermodynamically meaningful indicators such as mineral saturation indices, aqueous speciation, and redox conditions can therefore directly record equilibrium or near equilibrium with ore-related mineral assemblages [122,126]. As a result, hydrogeochemical anomalies identified within local flow regimes provide high-resolution vectoring toward concealed mineralization and represent direct exploration targets suitable for focused follow-up.

In contrast, regional groundwater flow systems operate over basin-scale distances and long residence times, during which groundwater chemistry is progressively modified through reactions with multiple lithologies, dilution, mixing, and buffering processes. Metals and pathfinder elements released from mineralized sources may be transported tens to hundreds of kilometers before discharging at topographic or structural lows. Hydrogeochemical anomalies observed in such discharge zones therefore integrate geochemical signals over large source areas and prolonged transport histories, reflecting basin fertility rather than spatial proximity to specific ore bodies [134,135]. While regional anomalies are valuable for identifying metallogenically favorable basins, they inherently provide low spatial resolution for direct vectoring to concealed deposits.

Failure to differentiate between local and regional groundwater flow regimes commonly leads to misinterpretation of hydrogeochemical data, particularly the expectation that all groundwater anomalies should directly indicate ore body locations. Incorporating Tóth’s framework clarifies that the effectiveness of hydrogeochemical vectoring is fundamentally governed by flow system scale and residence time. Local systems support precise mineral targeting, whereas regional systems support strategic, basin-scale exploration assessment [135]. This distinction also explains why thermodynamic indicators retain strong diagnostic value in local flow systems but become increasingly overprinted by integrated basin processes in regional settings.

Accordingly, this review frames hydrogeochemical mineral exploration within a mechanistic model that integrates thermodynamic geochemistry with groundwater flow system theory, recognizing groundwater as a geologic agent operating across nested spatial scales. This framework provides a coherent explanation for contrasting exploration outcomes reported in the literature and establishes a robust conceptual basis for interpreting hydrogeochemical data in covered and basin-hosted mineral systems [53,122,134,135].

4.5. Key Geochemical Indicators in Exploration

The practical goal of a hydrogeochemical survey is to identify a chemical anomaly, a change in water chemistry that stands out from the local background and indicates the presence of mineralization [3,4,5,136]. The evolution of geochemical indicators reflects the core challenge of discipline, moving from simple detection of an anomaly to confident attribution of its source. Each step up the hierarchy of indicators, from simple elements to complex isotopic systems, is designed to reduce interpretive uncertainty [2,33].

4.5.1. Elemental Pathfinders and the Definition of Geochemical Anomalies

Hydrogeochemical exploration targets can be categorized as either direct indicators (anomalous concentrations of the ore elements themselves) or, more commonly, pathfinder elements, associated elements that are often more abundant, more mobile in the local environment, or easier to detect, thereby creating a larger and more robust chemical halo [5,10,14,16]. A classic example is the use of As and Sb for certain types of gold deposits; a study in Sardinia found that while dissolved gold created a faint and restricted anomaly (~500 m), As and Sb were present in much higher concentrations and formed a clearer, more widespread pattern that pointed directly to the gold-bearing veins [14,16]. Other broad-spectrum pathfinders, such as high concentrations of sulphate (SO₄²⁻) and low pH, point to the larger mineralizing system rather than a specific metal, making them excellent tools for regional surveys [38,65].

An anomaly is formally defined as a concentration that is statistically significant relative to the regional geochemical background [4,137,138]. A critical principle is that the threshold for what constitutes an anomaly is entirely context dependent. In the pristine terrane of the northern Canadian Shield, background levels for Zn in lake water are often less than 2 parts per billion (ppb), making any value consistently above this level potentially significant [4,110,137,138,139,140]. In contrast, in a region with complex geology and a history of industrial and agricultural activity like the UK, anomalies must be defined against a much higher and more variable background [5]. This environmental complexity necessitates a multi-element approach to avoid false positives. A single high metal value could be part of a lithological signature or derive from anthropogenic sources; by analyzing a broad suite of elements, geochemists can recognize these distinct geochemical fingerprints and differentiate them from true mineralization-related anomalies, which often present as a specific, coherent suite of elements [5,19,89].

4.5.2. Process-Based Indicators Integrating Alteration Signatures and Thermodynamic Modelling

The recognition that simple concentration data could be misleading led to the development of more robust, process-based indicators that fingerprint the entire mineralizing system [65,66,67]. These indicators are more powerful because they fingerprint a system (a thermodynamic state, an alteration halo), which is less ambiguous than a single element concentration. One such approach is the use of alteration-related indicators, which come from the chemical signature that water acquires from interacting with the large alteration halo surrounding an orebody [3,67,141]. The classic example is the Normalized Magnesium (NMg) ratio, developed for unconformity-related uranium deposits in Australia. The rocks around these deposits are intensely altered and enriched with magnesium; groundwater leaching these rocks acquires a distinct high-Mg signature, which is a robust indicator that works even in local chemical conditions where uranium itself is immobile [67]. A second, more quantitative approach involves the use of model-derived indicators calculated using geochemical computer models such as WATEQ4F or PHREEQC [66,142,143]. The most well-known of these is the Saturation Index (SI), which measures how close a water sample is to being in thermodynamic equilibrium with a specific ore mineral [66]. A positive SI suggests the water is supersaturated and the mineral could precipitate, while a negative SI suggests it is undersaturated and the mineral could dissolve [66,143,144,145]. The SI is powerful because it integrates the effects of pH, Eh, temperature, and complexation into a single, meaningful number [66,145]. A map of the SI for an ore mineral like uraninite can provide a much clearer exploration target than a map of raw uranium concentrations, as it effectively corrects for local variations in the physicochemical environment that control solubility [46,66,144].

4.5.3. Advanced Tracers Integrating Isotopic Systems and Nanoparticle Geochemistry

In recent decades, the exploration toolkit has been expanded with advanced tracers that provide unprecedented levels of information on the source of elements and the specific processes they have undergone, providing a direct, often unambiguous, link to the source [2,46,49,51]. Radiogenic isotopes are used for source tracing. For example, the analysis of Pb isotopes can identify the distinctive radiogenic signature (²⁰⁶Pb/²⁰⁴Pb) of groundwater that has interacted with a uranium deposit, while strontium (Sr) isotopes (⁸⁷Sr/⁸⁶Sr) can be used to trace water-rock interaction pathways [2,38,46,146]. Uranium-series disequilibrium provides several tools for process tracing. The ²³⁴U/²³⁸U activity ratio (UAR) can distinguish between reducing conditions near an orebody and oxidizing zones where ore is actively dissolving, while Radium (Ra) isotopes (²²³Ra/²²⁶Ra) can act as a geochemical clock to estimate groundwater travel time from the source, providing a vector toward mineralization [46,91,147].

Traditional stable isotopes, such as Sulphur (δ³⁴S) and oxygen (δ¹⁸O) in dissolved sulphate, are used to trace the process of sulphide oxidation. Since hydrothermal sulphide minerals typically have a mantle-like Sulphur signature, this method allows explorers to distinguish a true ore-related sulphate anomaly from background sulphate derived from other sources, such as marine evaporites or agricultural contamination, as demonstrated at the Vaquillas deposit [2,10,51,148]. Non-traditional stable isotopes represent an emerging frontier, focusing on heavy metals like copper (δ⁶⁵Cu) and molybdenum (δ⁹⁸Mo) [2,33]. Research has shown that redox processes during the oxidative weathering of copper-sulphide minerals cause isotopic fractionation, preferentially releasing the heavier ⁶⁵Cu isotope into solution. This creates a distinct positive δ⁶⁵Cu anomaly in groundwater that can serve as a direct, process-specific vector toward the orebody [33]. Nanoparticle geochemistry has introduced a new paradigm of physical dispersion alongside chemical dispersion [49]. Studies have demonstrated that ore-related metals can be transported not only as dissolved ions but also as solid-phase nanoparticles (e.g., native metals, sulphides like Ag₂S) suspended in groundwater. This provides a viable transport mechanism for elements with very low solubility (like Au and Ag) and means the anomaly is a direct physical sample of the ore and its weathering products, retaining an unambiguous source fingerprint [49]. This opens new avenues for exploration, such as analyzing the unfiltered or colloidal fraction of water samples using advanced techniques like single-particle ICP-MS (spICP-MS) [48,49]. The future of high-stakes exploration targeting will increasingly rely on these diagnostically powerful tools to justify the cost and risk of drilling.

4.6. Methodologies from Field Data to Exploration Targets

The successful execution of a hydrogeochemical exploration program hinges on a meticulously planned workflow that encompasses survey design, rigorous sampling, appropriate analytical techniques, and sophisticated data interpretation strategies. Each stage must be conducted with precision to ensure the integrity of the data and the validity of the resulting exploration targets [3,149,150].

4.6.1. Survey Design, Sampling Protocols, and Field Measurements

The design of a hydrogeochemical survey is dictated by the exploration scale, regional climate, and the local hydrogeological context [150]. A critical first step in any program is the execution of an orientation survey over a known mineral occurrence within the target terrain [10,151]. This preliminary work is essential for understanding local dispersion mechanisms, identifying the most effective pathfinder elements, establishing the expected scale of the anomaly footprint, and validating protocols [10,151]. A wide array of sample media can be utilized, including water from streams, lakes, springs, seeps, and wells [85]. Regardless of the source, proper sampling protocols are paramount to data quality [16]. Standard procedure involves the use of clean, acid-rinsed plastic bottles, on-site filtration through a 0.45 µm membrane to separate the dissolved fraction, and preservation of cation and trace-element aliquots by acidification with ultra-pure acid to prevent adsorption onto container walls, following established international protocols for water sampling and analysis [76,85,152]. A separate, unacidified sample is collected for anion analysis. Equally critical are the in-situ field measurements of unstable physicochemical parameters such as pH, redox potential (Eh), temperature, and electrical conductivity (EC), which can change rapidly once a sample is exposed to the atmosphere and must be measured at the time of sampling using calibrated portable meters [76,85]. Obtaining reliable field Eh measurements can be particularly challenging and is complicated by the frequent ambiguity in the literature between probe-measured oxidation–reduction potential (ORP) and true Eh values referenced to the standard hydrogen electrode (SHE). Modern redox probes can be calibrated using solutions of known redox potential, allowing correction for temperature effects and improving the reliability of Eh estimates; however, contamination from atmospheric oxygen during sampling remains a significant source of uncertainty. To minimize this effect and obtain more representative in situ redox conditions, the use of down-hole probes, flow-through chambers, and closed measurement systems is recommended, and in some cases Eh may be constrained indirectly from measured redox couples for high-stakes interpretations [46,76]. It is important to distinguish between field oxidation–reduction potential (ORP) readings obtained using probe-based sensors and true Eh values referenced to the standard hydrogen electrode (SHE). ORP measurements are instrument-specific and require calibration against redox standards to derive meaningful Eh estimates. Atmospheric oxygen contamination during sampling can further bias redox measurements; therefore, the use of flow-through cells and closed measurement systems is recommended to better approximate in situ groundwater redox conditions.

Recent technological innovations in sampling are transforming field logistics. The development of a helicopter-mounted water sampling system in the Canadian Shield was a pioneering approach to cost-effective surveying in remote terrain [4]. More recently, the advent of Unmanned Aerial Vehicles (UAVs), or drones, offers disruptive technology for water sampling, providing enhanced safety and significant cost savings [24,86,87]. Methodological comparisons have shown that advanced, closed-loop systems featuring a peristaltic pump are superior, as the pump’s ability to perform point-source filtration is essential for obtaining a true dissolved fraction and ensuring the accurate analysis of redox-sensitive elements [24].

4.6.2. Advanced Analytical Techniques and Instrumentation in Hydrogeochemical Exploration

The effectiveness of hydrogeochemistry is intrinsically linked to the advancement of analytical technology capable of detecting ultra-trace concentrations of elements in water [10,24,98]. The modern workhorse for most hydrogeochemical exploration programs is Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) Table 1 [10,43]. This technique offers superior sensitivity, with routine detection limits in the parts-per-billion (ppb, or μg L⁻¹) to parts-per-trillion (ppt, or ng L⁻¹) range, combined with high-throughput, simultaneous analysis of more than 50 elements [10,14]. This sensitivity is essential for detecting subtle anomalies of critical pathfinder elements, particularly gold, which often occur at concentrations of just a few nanograms per litre [14,21]. The primary limitation of ICP-MS is its low tolerance for high total dissolved solids (TDS), typically requiring dilution for samples with more than ~0.2% dissolved solids [45]. For analyzing high-salinity waters like brines, the more robust ICP-OES is often the instrument of choice due to its exceptional matrix tolerance Table 1 [10,45]. In contrast, ICP-MS performance deteriorates rapidly in high-salinity matrices due to signal suppression, cone fouling, and increased polyatomic interferences, making ICP-OES the preferred technique for routine analysis of brine and saline waters in both hydrogeochemical exploration and lithium-brine resource evaluation.

At the cutting edge of analytical science are more specialized instruments. High-Resolution ICP-MS (HR-ICP-MS) provides even lower detection limits (sub-ppt) and superior capabilities for resolving problematic spectral interferences Table 1 [39,44]. Multi-Collector ICP-MS (MC-ICP-MS) allows for the extremely precise isotope ratio measurements required for non-traditional stable isotope geochemistry Table 1 [2,33,43]. For the direct analysis of solid-phase indicators, techniques such as Transmission Electron Microscopy (TEM) and Single Particle ICP-MS (spICP-MS) for nanoparticles are employed Table 1 [48,49]. Finally, Ion Chromatography (IC) remains the standard method for the precise measurement of major anions like chloride (Cl⁻) and sulphate (SO₄²⁻), which are essential for classifying water types and identifying alteration halos Table 1 [5,14,19,36].

4.6.3. Modern Data Interpretation Strategies

The transformation of raw analytical data into geologically meaningful information is a critical and complex step [66,89]. A fundamental first step is to establish the geochemical baseline and define context-specific anomaly thresholds, ideally by partitioning the dataset by the underlying bedrock geology and defining separate thresholds for each lithology (Table 3) [10,89,90].

Geochemical data are inherently compositional, meaning the variables are proportions of a whole, which can create spurious correlations if not handled correctly [153]. The modern approach is to apply log-ratio transformations (centered log-ratio, clr) to the data prior to statistical analysis, which opens the data and allows for valid calculations [153].

With properly transformed data, a range of interpretive tools can be applied. Thermodynamic modelling, using software such as PHREEQC, provides a process-based, physical chemistry framework for interpreting data [66,142,143]. By calculating element speciation and the Saturation Index (SI) for various minerals, this approach can assess whether a groundwater sample is in equilibrium with an ore body, providing a powerful interpretive tool that is less sensitive to simple concentration variations [46,55,66].

Complementing this theoretical framework, multivariate statistical analysis offers a data-driven approach to understanding complex hydrogeochemical systems [19,38,89,154]. Techniques such as Principal Component Analysis (PCA) and Factor Analysis (FA) are used to reduce the dimensionality of large, multi-element datasets and identify the underlying processes (weathering, pollution, mineralization) that control the water chemistry [19,63,89]. Q-mode Cluster Analysis can then be used to group individual water samples into distinct water types based on their shared genetic history, allowing for the definition of separate background and threshold values for each water type and isolating the subtle signature of mineralization [89].

Ultimately, exploration is a spatial science, and the Geographic Information System (GIS) is the essential platform for the final stage of data integration and visualization [5,92,93,94,155]. Plotting hydrogeochemical data on geological and topographic maps allows for direct comparison with known mineral occurrences, faults, or specific rock types [19]. The true power of this approach is realized when hydrogeochemical anomaly maps are overlaid with other prospectivity layers such as alteration patterns from remote sensing or subsurface structures from geophysics to identify areas where multiple lines of evidence converge, significantly increasing confidence in a potential target [5,94]. A summary and comparison of modern hydrogeochemical data interpretation techniques is given in Table 3.

4.7. Global Applications in Critical and Strategic Mineral Commodities

The scientific principles of hydrogeochemistry have been validated through successful application in a wide range of geological and climatic environments across the globe [3,37]. The following case studies, organized by mineral system type, illustrate how the technique is applied in practice and highlight the specific chemical signatures and exploration models associated with different kinds of mineralization. A summary of the global hydrogeochemical exploration models is also presented in Table 2.

4.7.1. Base Metal Systems

Hydrogeochemical methods are particularly well-suited for exploring for large-tonnage base metal deposits, such as porphyry copper, Volcanogenic Massive Sulphide (VMS), and sediment-hosted systems, which frequently generate strong and widespread chemical anomalies in water upon weathering [3,4,65].

Porphyry Copper (Cu-Au-Mo)

Research on world-class deposits in the hyper-arid Atacama Desert of Chile has defined a classic hydrogeochemical footprint featuring a zoned dispersion halo with a proximal anomaly of less mobile cations (Cu, Zn, Co). It contains a more extensive distal plume of highly mobile oxyanions (Mo, Re, As, Se) that can be detected many kilometers down-gradient [2,10,52]. At the giant, concealed Pebble deposit in Alaska, a study demonstrated the power of high-resolution ICP-MS to detect ultra-low (ng/L) anomalies under glacial cover and revealed a crucial principle of pH-dependent zonation. Over the thinly covered, sulfide-bearing Pebble West zone, the waters were acidic with a Cu-Cd-Re-sulphate anomaly, while over the more deeply buried Pebble East zone, waters were near-neutral, and the anomaly was defined by elements more mobile under those conditions (Mo, As, Sb, W) [39,58,59,60]. The integration of non-traditional stable isotope molybdenum and copper isotope compositions (δ⁹⁸Mo and δ⁶⁵Cu) has been shown to be a powerful tool for fingerprinting and vectoring at Iron Oxide Copper Gold (IOCG) deposits in Zambia and sediment-hosted Cu deposits in Australia [2,33].

Cobalt generally exhibits only weak anomalism in most documented studies [6], while fluoride can serve as an indicator of mineralized zones under low-pH conditions [58]. Molybdenum commonly functions as a reliable vector toward mineralization, except in acidic plume environments [65,156] or in stream drainages where its concentrations tend to be inconsistent [2,157]. Arsenic consistently emerges as a key pathfinder element across nearly all case studies (Figure 3). Rhenium, typically occurring as a trace constituent in molybdenite and generally present in groundwater at <1 µg/L [6,61], has proven to be a sensitive indicator for porphyry Cu–Mo systems in both groundwater and surface-water surveys [2,6,10,157,158] (Figure 3). Its potential as a vector was first recognized at the Spence Cu–Mo deposit in northern Chile, where pronounced downgradient Re enrichment was identified (Figure 3). In some investigations [58], Re anomalies are detectable only due to the very low detection limits of HR-ICP-MS, implying that Rhenium may be a more significant pathfinder than previously appreciated.

At the Spence deposit, metal mobility is strongly influenced by variations in salinity and ligand availability. Elevated salinity results from the dissolution of buried evaporite horizons, remnants of ancient salars, by circulating groundwater [159]. These “recycled brines” can be transferred upward into shallower aquifers along structural pathways, particularly during seismic events [13,52,61,160,161,162]. Local mixing between these deep, reduced brines and more oxidized near-surface waters produces extreme salinity, with TDS commonly exceeding 25,000 mg/L (relative to ~2500 mg/L up-gradient of the deposit), and creates a sharp redox transition [3,61]. This process generates an acidic geochemical footprint close to the mineralized system, characterized by pH values down to 4.73, Cu concentrations typically on the order of 1000 µg/L (locally reaching 28,991 µg/L), and Mo values frequently surpassing 100 µg/L [3,61]. Groundwaters at Spence reach equilibrium or supersaturation with respect to several secondary Cu minerals, including atacamite, malachite, azurite, brochantite, antlerite and chalcanthite [3,61].

Volcanogenic Massive Sulphide (VMS) Deposits

Foundational work in the permafrost landscape of the northern Canadian Shield demonstrated the effectiveness of lake water sampling in pristine environments with exceptionally low background metal concentrations (<2 ppb) [4,62]. Zinc (Zn) was established as a superior pathfinder to Cu due to its greater chemical mobility in typically acidic-to-neutral surface waters [4]. Exploration in the Abitibi Greenstone Belt faces the challenge of deep, saline groundwaters that can mask mineralization signatures, requiring sophisticated modelling to correct for salinization effects and reveal subtle but diagnostic alteration-related anomalies [163].

Sediment-Hosted (SEDEX/MVT) (Pb-Zn)

A pilot study in North Wales showcased the challenges of exploration in complex brownfields settings, demonstrating the absolute necessity of a comprehensive, multi-element approach to deconvolve overlapping signals from bedrock geology, mineralization, and anthropogenic inputs [5]. In carbonate-buffered, neutral-pH systems in India and Australia where metal mobility is limited, more sophisticated indicators are required, such as normalizing ore-related elements to a stable element like chloride (e.g., calculating Zn/Cl⁻ and SO₄²⁻/Cl⁻ molar ratios) or using highly radiogenic Pb-isotope outliers to delineate prospective areas under cover [35,38,63,64].

4.7.2. Precious Metal Systems

Exploring precious metals with hydrogeochemistry presents a significant analytical challenge due to their extremely low solubility and mobility, but modern ultra-trace techniques have made it a viable method [3,14,55].

Epithermal Gold–Silver (Au-Ag)

Two primary exploration models have been developed. In arid, saline environments like the Great Basin, USA, the investigation of the Sleeper deposit provides a classic model where Au and Ag are transported as stable, soluble chloride complexes (AuCl₂⁻ and AgCl₂⁻) [55]. Silver precipitates upon dilution, creating a broad, detectable halo in shallow groundwater that serves as a powerful vector to blind epithermal deposits [55]. In contrast, a study of an epithermal system in the oxidizing, near-neutral freshwaters of Sardinia found that dissolved gold mobility was very limited, creating a subtle anomaly of only about 500 m [14]. In this environment, hydrogeochemistry is best used for prospect-scale targeting, relying on more robust and widespread pathfinder elements like As and Sb [14].

Orogenic and Carlin-Type Gold (Au)

Hydrogeochemical surveys in the deeply weathered Yilgarn Craton of Australia have shown that while dissolved Au can be an effective pathfinder, exploration more consistently relies on a suite of pathfinder elements, notably As, Sb, Te, and W, which form more extensive halos than gold itself [20,21]. For Carlin-type gold deposits in Nevada, where gold mobility is very limited in the carbonate-hosted environment, exploration relies on detecting the highly mobile pathfinder suite of As, Sb, Hg, and Tl [71,72,73,74,164].

Orogenic (mesothermal) gold systems have also been the subject of substantial hydrogeochemical investigation in Australia, particularly across the Yilgarn Craton [165]. These efforts have underpinned several continent-scale hydrogeochemical mapping initiatives [16,28,163,166,167]. Owing to Australia’s arid climate, hydrogeochemical exploration is largely restricted to subsurface environments where groundwater is plentiful, and the physicochemical variability between groundwater types exerts a critical influence on the solubility, transport, and stability of Au and associated pathfinder elements.

Studies conducted around known mineralized systems within the Yilgarn Craton commonly reveal sharp geochemical contrasts when compared to the regional baseline of approximately 3 ng/L Au [20,29,168,169]. At the Carosue Dam deposit, oxygenated and acidic groundwaters contain up to 2 µg/L Au [70], while concentrations near Au mineralization in the Black Flag district reach about 7 ng/L [20]. In the St Ives system, acidic brines interacting with Au-bearing units generate pronounced Au anomalies of up to 52 ng/L, although these signals are spatially restricted and show weak associations with common pathfinders such as Sb, Bi, and Te [168]. Ref. [168] attributed this behavior to adsorption onto amorphous Fe (oxy)hydroxides, driven by a shift in Au speciation from AuCl₃ to mixed chloro-hydroxide complexes as pH rises from 4 to 7.

As with other Au deposit classes, leaching of mineralized zones can release a broad suite of pathfinders capable of forming extensive hydrogeochemical halos. At the Agnew deposit, elevated concentrations of dissolved Au, Ag, As, W, Co, and Mn have been documented [29], corresponding to major components of the Au ore system [29]. These anomalies commonly exceed regional background levels by two to five orders of magnitude, with some values representing the highest recorded in the northern Yilgarn region (Figure 4) [20]. Comparable multi-element signatures occur at the Harmony deposit, where Mo, W, and Rb anomalism have been reported [170], and Au enrichment aligns closely with hydrothermal alteration zones [20]. In the Black Flag district, pathfinders such as As, Mo, and Sb occur alongside Au mineralization [169]. Conversely, in South Australia, Ag and As are inconsistently distributed and only sporadically anomalous near Au camps. Groundwaters near the Tunkilla deposit exhibit elevated Au together with V and Te anomalies [70].

4.7.3. Critical Mineral Systems

Hydrogeochemistry is playing an increasingly vital role in the search for critical minerals essential for the green energy transition and high-tech industries [3].

Uranium (U)

The exploration model for sandstone-hosted roll-front deposits is fundamentally a search for a mobile redox interface, where uranium is leached by oxidizing groundwaters and precipitates at a reducing barrier [66,75,77]. Here, thermodynamic modelling has proven powerful, with maps of the calculated Saturation Index (SI) for uraninite accurately outlining ore trends where maps of raw dissolved uranium were ineffective [66]. Exploration for deeply buried unconformity-related deposits requires a multi-faceted approach, utilizing a combination of direct U anomalies, mobile pathfinders (Ra, Rn, He, As), advanced isotopic tracers (high ²³⁴U/²³⁸U activity ratios), and the chemical fingerprint of the surrounding alteration halo (the Normalized Magnesium ratio) [46,63,67,76].

Lithium (Li)

The role of hydrogeochemistry differs dramatically between deposit types. For lithium brine deposits in arid, closed tectonic basins like the Andean Lithium Triangle, hydrogeochemistry is the primary method of resource evaluation, as the ore is the lithium-enriched saline groundwater itself [22,68,78,171]. Exploration involves directly analyzing brines for economic concentrations of Li and for key economic viability ratios, particularly a low magnesium-to-lithium (Mg/Li) ratio [22,171]. For hard-rock Li-Cs-Ta (LCT) pegmatites, exploration relies primarily on lithogeochemistry (e.g., K/Rb ratios in rock), but hydrogeochemistry can detect dispersion halos of Li, Rb, and Cs in shallow groundwater from associated alteration zones [80,81,82,172].

Rare Earth Elements (REEs)

Hydrogeochemical exploration for REEs is a newer field that relies on interpreting the unique chemical behavior of the lanthanide series [69,83,84,173,174]. Exploration focuses not on the absolute concentration of a single element but on the patterns of the entire REE group when normalized to a standard like chondrite [83,84]. These patterns provide a powerful fingerprint of the water’s interaction history. Key diagnostic features include the degree of fractionation between light REEs (LREE) and heavy REEs (HREE), and the presence of specific elemental anomalies (e.g., Eu, Ce), which can provide a vector toward a source like a carbonatite [69,83,84,175].

4.8. Synthesis of Trends, Challenges, and Future Directions

Unlike narrative syntheses that rely on expert selection and thematic emphasis, the PRISMA-based approach adopted here enables quantitative evaluation of trends, consistency, and gaps across the global literature. The systematic extraction of analytical methods, hydrogeochemical indicators, and interpretive frameworks reveals not only dominant practices, but also under-represented approaches and emerging techniques. This distinction is critical for objectively assessing methodological maturity and guiding future research priorities.

The evolution of hydrogeochemistry reveals a clear trajectory toward more sophisticated, integrated, and predictive methodologies. This synthesis also highlights a set of persistent challenges that continue to shape research and practice, while pointing toward significant opportunities for the future of the discipline. The central challenge has matured from one of detecting whether a subtle anomaly can be measured to one of confident attribution and unambiguously determining what that measurement means [33].

4.8.1. Emerging Trends in Data Integration, Predictive Modelling, and Technological Innovation

Among the most significant emerging trends identified through this systematic review are the increasing application of non-traditional stable isotope fractionation (e.g., δ⁶⁵Cu, δ⁹⁸Mo) and nanoparticle geochemistry using spICP-MS, which provide process-specific and source-diagnostic signals not accessible through conventional dissolved-phase analysis.

A dominant trend is the shift away from simplistic, single-element anomaly hunting toward integrated exploration frameworks [2,5,94]. The early call to merge water chemistry with geological maps, stream sediment geochemistry, and remote sensing imagery within a GIS platform was a forward-looking vision of the multi-layered, data-driven approach that is now fundamental to modern exploration targeting [5,94]. This holistic approach allows for more robust target validation, as anomalies can be supported by multiple, independent lines of evidence.

Concurrently, the discipline has moved toward a more data-driven and predictive science [26,40,55,66]. This trend has accelerated dramatically with the rise of Artificial Intelligence (AI) and Machine Learning (ML) [26,40,176,177,178]. Algorithms like Random Forest and Neural Networks can analyze massive, multi-parameter datasets to find the subtle, non-linear patterns that often signal a geochemical anomaly, generating quantitative prospectivity maps that represent a leap beyond traditional anomaly detection [25,26,38,40].

This evolution is enabled by new frontiers in tracing and sampling technologies. The development and application of advanced tracers, particularly non-traditional stable isotopes and nanoparticles, provide unprecedented, process-specific information that reduces ambiguity [2,33,49]. At the same time, new sampling platforms like UAVs and the development of in-field sensors for real-time monitoring are revolutionizing data acquisition [24,179,180,181]. This move toward continuous monitoring transforms the practice from taking a single “snapshot” to creating a “movie” of the hydrogeochemical system, incorporating the fourth dimension of time to reveal powerful, transient anomalies that would be missed by conventional surveys.

4.8.2. Persistent Challenges in Hydrogeochemical Exploration

Despite its successes, hydrogeochemistry is subject to inherent challenges. A primary manifestation of this is seasonal and temporal variability. Spring snowmelt, for example, can drastically alter water chemistry, variously diluting, concentrating, or physically displacing anomalies from their source [4]. This temporal instability necessitates strict sampling protocols, such as collecting all samples within a narrow time window during stable, low-flow conditions, to ensure data comparability [4,5]. Technical and logistical hurdles, such as the ever-present risk of contamination at ultra-trace levels and the difficulty of obtaining representative samples from deep, fractured-rock environments, also remain significant challenges [4,76,85]. Background concentrations in many pristine environments are often in the low-ppb or sub-ppb range, so even minor contamination from sample containers, preservatives, or handling can lead to misleading anomalies [4,85].

Perhaps the greatest and most persistent challenge is the robust interpretation of anomalies against a complex and highly variable geochemical background [5,19,89,136,182]. Disentangling a subtle anomaly related to mineralization from the strong chemical signatures of different rock types, industrial pollution, or agricultural runoff requires sophisticated statistical analysis and a deep understanding of all potential sources within a catchment [5,19,89]. Distinguishing a true mineralization-related anomaly from a “false” anomaly generated by these other processes remains a primary focus of hydrogeochemical interpretation and is the driver behind the adoption of more advanced interpretive tools [2,89,182].

4.8.3. Exploration Workflow for Hydrogeochemistry

The workflow presented in Figure 5 provides a structured framework for the application of hydrogeochemistry in mineral exploration. It illustrates systematic progression from defining sampling objectives to the final stages of data interpretation and reporting, ensuring that each step is scientifically coherent and operationally robust. The process begins with sampling objectives, which establish the purpose of the survey, whether to define geochemical baselines, detect mineralization signals, or identify hydrogeochemical anomalies. These objectives guide subsequent decisions on site selection and pre-survey checks, including hydrogeological mapping, accessibility and safety assessments, equipment calibration, and GPS logging. Such preparatory steps are critical for ensuring both data quality and field efficiency.

Sample recovery and processing are central to the workflow. Groundwater is typically collected using bailers or pumps (peristaltic, bladder, inertial, or submersible), while stream water is recovered using syringes. Filtration methods, paper vacuums, cartridges, or in-line systems, are employed to separate dissolved and particulate fractions. Rigorous sample labelling and metadata recording (ID, coordinates, time/date, well depth or stream stage, collector name) ensure traceability and reproducibility.

To maintain analytical integrity, the workflow incorporates QA/QC procedures, including field blanks, trip blanks, duplicates, calibration standards, and chain-of-custody documentation. Samples are categorized into physical, dissolved, and isotopic types, with specific volumes and preservation requirements tailored to each analytical target. For example, δ¹³C requires airtight amber bottles, while isotopes such as δ⁶⁵Cu, δ⁹⁸Mo, and ⁸⁷Sr/⁸⁶Sr demand precise volumes and acidified preservation.

Field measurements and data acquisition are conducted in situ, using bailer readings, flow-through cells, or submerged probes. Parameters such as temperature, pH, redox potential (Eh), conductivity, total dissolved solids, dissolved oxygen, turbidity, and hydrological metrics (water table depth, stream velocity, stream dimensions) provide essential contextual information for interpreting chemical anomalies.

The workflow emphasizes sample transport and preservation, requiring insulated coolers, temperature control (<4 °C), shock-protected bottles, and same-day delivery where possible. Preservation strategies, airtight sealing, refrigeration, and acidification, are applied as appropriate to maintain sample integrity until laboratory analysis (Figure 5).

In the laboratory, advanced techniques such as ICP-MS and ICP-OES (for trace and major elements), ion chromatography (for anions), isotope ratio mass spectrometry, and TOC analysis are employed. QA/QC validation metrics, including relative percent difference, relative standard deviation, and spike recovery, ensure analytical reliability (Figure 5).

The final stages of the workflow focus on data interpretation and integration. Hydrochemical facies are classified using Piper, Schoeller, and Stiff diagrams, which are appropriate for characterizing groundwater types and dominant geochemical evolution trends relevant to mineral exploration, while multivariate statistical approaches (PCA, clustering) are applied to log-ratio transformed datasets (Figure 5). These methods facilitate anomaly identification and geological correlation. Results are then synthesized into technical reports and GIS-based hydrogeochemical maps, ensuring that findings are archived and accessible for long-term exploration planning.

4.8.4. Outlook and Future Research Priorities

The foundational research and emerging trends point toward several key opportunities for advancing the field. The most significant future direction is the deepening of multi-disciplinary integration. The next frontier involves moving beyond simple 2D map overlays to incorporating hydrogeochemical data into dynamic 3D and 4D models of geological systems, combining water chemistry with deep-penetrating geophysical methods to trace fluid pathways from large-scale crustal structures to the surface [10,112]. There is also a clear opportunity for the expansion of the geochemical toolkit. As the demand for critical minerals grows, research is needed to define reliable hydrogeochemical pathfinder suites and isotopic systems for under-explored commodities like cobalt, indium, and gallium [2,37]. The continued development and validation of non-traditional stable isotope systems and nanoparticle analysis across different deposit types will be crucial [2,33,49].

The accelerated adoption of new technologies will be critical. The exploration industry should continue to embrace the power of Artificial Intelligence (AI) and machine learning (ML), not just for post-survey data analysis, but for guiding exploration strategy in real-time [26,178,183]. To enable these data-intensive approaches, there is a pressing need for greater international collaboration and data-sharing initiatives. The creation of comprehensive, standardized, publicly accessible geochemical databases is crucial for establishing robust global baselines and for training the powerful AI models that will drive the next generation of mineral discovery [163,184]. This fundamentally changes the skillset required of an exploration geochemist; today, core competencies must include data science, multivariate statistics, isotopic geochemistry, and integrated 3D thinking [32].

The evidence base is subject to several limitations, including variability in sampling density, analytical methods, and reporting standards across studies. In some cases, incomplete metadata or limited QA/QC documentation restricts the ability to fully assess data reliability. Additionally, geographic bias toward well-studied regions may limit the generalizability of findings to underexplored terrains.

5. Conclusions

The PRISMA-2020-compliant systematic review synthesized evidence from 118 empirically screened studies published between 1946 and 2025, providing a structured evaluation of hydrogeochemical methods used in mineral exploration across diverse geological, hydrogeological, and climatic settings. The systematic synthesis demonstrates that hydrogeochemistry is most effective when interpreted as a process-based exploration tool, rather than as a simple concentration-driven anomaly detection method.

Across the reviewed studies, consistent empirical patterns were identified, including the development of zoned aqueous dispersion halos, predictable partitioning of mobile and immobile elements, and strong dependence of anomaly expression on groundwater flow regime, redox state, pH, and ligand availability. These findings, repeatedly documented across base-metal, precious-metal, critical mineral, and REE systems, confirm that hydrogeochemical anomalies reflect integrated mineralization processes rather than isolated geochemical signals.

The systematic comparison of methodologies shows clear convergence toward thermodynamically informed interpretation, multivariate compositional data analysis, and explicit consideration of groundwater flow systems. Studies employing mineral saturation indices, alteration-related ratios, isotopic tracers, and flow-system context consistently achieved higher interpretive confidence than those relying on single-element thresholds alone. Emerging approaches identified through the extracted literature, including non-traditional stable isotope fractionation and nanoparticle geochemistry using spICP-MS, provide enhanced source specificity and represent a significant methodological advance supported by a growing empirical evidence base.

A key contribution of this review is the formalization of a standardized, reproducible exploration workflow, grounded in the practices most consistently reported across the screened studies. By explicitly linking survey design, sampling protocols, analytical strategies, and data interpretation within a single framework, this synthesis moves beyond narrative reviews to provide an evidence-based foundation for reproducible hydrogeochemical exploration.

Collectively, the systematic evidence confirms that hydrogeochemistry is a scalable, low-impact, and scientifically robust exploration approach for detecting concealed mineral systems. When implemented using process-based interpretation and modern analytical tools, hydrogeochemistry provides reliable vectoring information that complements geophysical and geological datasets, supporting efficient and sustainable mineral exploration under increasing cover conditions.

Future research should prioritize quantitative inter-comparison of emerging isotopic and nanoparticle tracers across deposit types, expanded validation in under-represented climatic regions, and integration of hydrogeochemical datasets into dynamic 3D and 4D prospectivity models. These directions directly follow from gaps identified through the systematic synthesis rather than from conceptual extrapolation.