Isotopes in Economic Geology, Metallogeny and Exploration—An Introduction

Abstract

Although (Soddy, Nature 92:399–400, 1913) inferred the existence of isotopes early last century, it was not until the discovery of the neutron by (Chadwick, Nature 129:312, 1932) that isotopes were understood to result from differing numbers of neutrons in atomic nuclei. (Urey, J Chem Soc 1947:562–581, 1947) predicted that different isotopes would behave slightly differently in chemical (and physical) reactions due to mass differences, leading to the concept of isotopic fractionation. The discovery that some elements transformed into other elements by radioactive decay happened even before the recognition of isotopes (Rutherford and Soddy, Lond Edinb Dublin Philos Mag 4:370–396, 1902), although the role that different isotopes played in this process was discovered later. The twin, and related, concepts of isotopes and radioactive decay have been used by geoscience and other scientific disciplines as tools to understand geochemical processes such as mineralization, and also the age and duration of these processes. This book is a review of how isotope geoscience has developed to better understand the processes of ore formation and metallogenesis, and thereby improve mineral system models used in exploration.

1 Isotopic Research in the Geosciences

After the discovery of radiogenic decay and recognition of isotopes, largely by physicists and chemists, other disciplines—such as geology and biology—took up process-oriented research and began to apply isotopic studies to natural systems led largely by universities and government research organizations.

1.1 Radioactivity and Geochronology—Determining the Timing and Duration of Mineralization

Within a decade of the discovery of radiation by Henri Becquerel in 1896, Strutt (1905) first used radioactive decay (in this case the production of He (alpha particles) by Th decay) to estimate the age of a thorianite sample from Ceylon (present day Sri Lanka) at ca 2000 Ma. The dating of ore minerals followed soon after: Boltwood et al. (1907) used the U–Pb system to date uraninite samples, yielding uraninite ages of between 410 and 2200 Ma for different localities in Norway and North America. Holmes (1946) and Houtermans (1946) independently developed a method (in effect two-point isochrons) to estimate the age of lead-rich minerals. The old ages indicated by these studies were a key to understanding Earth’s evolution, indicating for the first time the extreme antiquity of Earth.

Although some of these early geochronology studies have been discredited for various reasons, they demonstrated the potential of radiogenic isotope systems to provide rigorous absolute ages for geological processes such as mineralization. Advances in analytical techniques and understanding of radiogenic isotope systems have continually improved so that for many mineral systems, absolute ages can be readily determined, and durations of mineralizing events can, in many cases, be robustly estimated.

1.2 Stable Isotopes—Tracers for Mineralizing Processes

Within two decades of the recognition of isotopes and within a decade of the theoretical prediction of isotopic fractionation, Epstein et al. (1953) calibrated a geothermometer based on measured carbon isotope fractionations from molluscs grown at different temperatures. Engel et al. (1958) first used stable isotopes in economic geology when they documented the effects of hydrothermal alteration on carbon and oxygen isotope characteristics of limestone in the Leadville district, Colorado, USA.

By the mid-1960s light stable isotopic studies had become a mainstream tool for understanding processes and sources of components in ore systems, and the variety and precision of analytical techniques and isotopic systems has since only increased. Isotopic studies were critical for the development of many modern ore genesis models, for example, porphyry (Sheppard et al. 1969, 1971; Field and Gustafson 1976; Wilson et al. 2007) and volcanic-hosted massive sulfide (VHMS) (Sangster 1968; Beaty and Taylor 1982; Ohmoto et al. 1983; Cathles 1993) systems. Light stable isotopes provide important constraints on the origin of, and interaction, between ore and ambient fluids, sulfur sources and chemical processes such as mixing, boiling, disproportionation and redox reactions.

As an example of the application of isotope research to mineral and other geological systems, Box 1 presents a summary of research undertaken at the Baas Becking Geobotanical Laboratory (BBL), a research laboratory supported by the Bureau of Mineral Resources (BMR, now Geoscience Australia), Commonwealth Scientific and Industrial Research Organisation (CSIRO) and industry. Although this laboratory was involved in many aspects of isotopic research, it is particularly recognized for having produced ground-breaking research on sulfur isotope variability in sedimentary rocks and mineral deposits in the Archean to Proterozoic. Although trends originally noted by the BBL have held up to subsequent research, the interpretation of some of these trends has changed because of continued acquisition of similar data and the availability of new types of data.

The development of inductively coupled plasma-mass spectrometry in the late 1970s to early 1980s (Houk et al. 1980) allowed analysis of new isotopic systems in the 1990s, including metallic, stable isotopes (Halliday et al. 1995; Zhu et al. 2002; Albarède 2004). Initially this work concentrated on major metals of economic interest—iron, copper and zinc (Zhu et al. 2000; Albarède, 2004; Mason et al. 2005; Johnson and Beard 2006)—but other work has studied the isotopic behaviour of minor metals such as antimony, molybdenum and silver (Rouxel et al. 2002; Arnold et al. 2004; Mathur et al. 2018), among others. The advantage of metallic stable isotopes is that they can track sources of and processes that affect ore metals themselves and do not require the frequent implicit assumption that there is a genetic relationship metals and the isotopic system (e.g., O, H, C or S) being analyzed. A challenge of metallic stable isotopes is that the degree of fractionation is much smaller than that of light stable isotopes, making the range in isotopic values much less and, therefore, more challenging to resolve analytically.

Box 1 Baas Becking Laboratory

The Baas Becking Geobiological Laboratory (BBL), which brought together geologists, geochemists, chemists, microbiologists and biochemists from the Bureau of Mineral Resources (now Geoscience Australia) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO), operated from 1963 to 1987. It was established, with mineral industry backing, to investigate the role of microorganisms in geological processes, particularly the formation of metal sulfide deposits such as sediment-hosted Zn–Pb–Ag deposits like Mount Isa, McArthur River and Broken Hill. Research at the BBL included studies that contributed to (1) determining sulfur isotope fractionation associated with bacterial sulfide reduction (BSR) in modern marine sediments, (2) defining and interpreting sulfur isotope trends in the Archean and Proterozoic, and (3) understanding changes in the carbon cycle leading up to the Cambrian explosion of life.

Sulfur isotope characteristics of modern marine sediments

Studies of sulfur isotope fractionation in modern marine sediments provided insights into the signatures of biological activity in metal sulfide ores in sedimentary strata. In modern sedimentary environments where sulfate availability is not constrained, biogenic pyrite exhibits a wide range of negative δ³⁴S, up to 60‰ less than δ³⁴S of source sulfate. In systems, where sulfate is not/slowly replenished, δ³⁴S values of biogenic sulfides trend to a range of positive values as sulfate is progressively depleted (Trudinger et al. 1972; Chambers and Trudinger 1979). The Permian Kupferschiefer base metal deposit in black shales in Germany, is a classic example of mineralisation exhibiting a typical biogenic S isotope signature.

Sulfur isotope trends in the Archean and Proterozoic

Documenting sedimentary sulfur isotopic trends through the Precambrian helped to define some major trends in biogeochemical evolution as well as providing insight into potential sulfur sources in mineral systems. The vast majority of iron sulfide minerals in Archaean metasedimentary rocks, collected near nickel, gold and iron ore deposits, have δ³⁴S in a narrow range of − 4 to 4‰, similar to the nearby mineral deposits (Donnelly et al. 1978; Lambert and Donnelly 1991). The δ³⁴S values are consistent with derivation directly or indirectly from magmatic sulfur, consistent with high levels of igneous activity. There were a few sites sampled, however, with more variability. Although these results had been interpreted previously (Goodwin et al. 1976; Ohmoto and Felder 1987) to indicate BSR, analyzed minerals were from veins and massive lenses and are closely associated with volcanics, leading to the conclusion that the anomalous δ³⁴S values were more likely generated by hydrothermal activity, and that there was no clear evidence of BSR, nor significant levels of sulfate, in the Archean hydrosphere. The low concentrations of sulfate in Archean seawater have largely been substantiated by later studies, although with some complexities (see below, Huston et al. 2023a, b and references therein).

This research also had implications for genesis of and, potentially, exploration for mineral deposits. In contrast with most Archean orogenic gold deposits, which have δ³⁴S_sulfide mostly in the range 1–4‰, Kalgoorlie samples have values of − 7 to − 1‰, interpreted as resulting from gold transport as reduced sulfur complexes and its precipitation as a result of sulfidation of magnetite-bearing host rocks, which resulted in partial oxidation of the ore fluids (Lambert et al. 1983). Based on this example, it was suggested that Archean epigenetic gold deposits of unknown size are likely to be very large if they have extensive alteration zones containing abundant pyrite with negative δ³⁴S—a conclusion supported by further studies (Phillips et al. 1986).

Although Archean oceans are generally thought to be sulfate-poor (Lambert and Donnelly 1991), there are localised occurrences of sulfate minerals in Paleoarchaean provinces, particularly the Pilbara (Western Australia) and Kaapvaal (South Africa) cratons. In the Pilbara, BBL studied bedded and discordant barite from the North Pole (Dresser) deposit, and barite from the volcanic-hosted massive sulfide (VHMS) Big Stubby deposit (Lambert et al. 1978). Barite from North Pole had δ³⁴S mostly of 3–4‰, tailing to weakly negative values; associated sulfide minerals had similar values. The barite was interpreted to have formed from Ba-bearing hydrothermal fluids introduced into a restricted (evaporative?) basin in which microbiological activities oxidised reduced sulfur, leading to barite deposition. In contrast, Big Stubby barite was found to have δ³⁴S values of 11–13‰, while associated sulfides had values in the range − 1 to − 5‰. This was interpreted in terms of the incorporation of sulfate-bearing basin waters into barium and base metal bearing hydrothermal exhalations related to felsic volcanism.

Research by the BBL contributed to the recognition of a sustained change in the sulfur isotope record from the early Paleoproterozoic, which was related to the Great Oxidation Event (GOE). Overall, Proterozoic sulfides in (meta)sedimentary rocks have wide ranges of negative to positive δ³⁴S values, with mean values well above both those of magmatic sulfur and modern marine sulfides (Hayes et al. 1992). Uncommon sulfate deposits have δ³⁴S values of 10–20‰. The δ³⁴S variations can be accounted for by sulfate becoming a stable component in low concentrations in the hydrosphere around the Archean-Proterozoic boundary, and its enrichment in ³⁴S as a result of bacterial and hydrothermal reduction processes (Hayes et al. 1992). The rising sulfate levels likely paralleled the stabilisation of oxygen in the atmosphere during the GOE, and proliferation of sulfate-reducing bacteria (Lambert and Groves 1981; Lambert and Donnelly 1991; Hayes et al. 1992). The predominance of positive δ³⁴S values in Mid to Late Proterozoic sulfides in stratiform base metal deposits and unmineralized strata were interpreted to indicate extensive reduction of sulfate-limited systems—restricted intracratonic marine or non-marine basins, possibly on supercontinents, where ³⁴S enriched sulfides formed as biological sulfate reduction proceeded.

Changes in the carbon cycle leading up to the Cambrian explosion of life

Carbon isotopic signatures of carbonate and organic carbon in strata straddling the Neoproterozoic-Cambrian boundary in China and Svalbard were studied in collaboration with the Proterozoic Paleobiology Research Group (Knoll et al. 1986; Lambert et al. 1987; see figure). These data and data from other sites around the world defined a trend of carbonate δ¹³C from the latest Proterozoic that were more positive than younger marine carbonates. Taken together with organic carbon δ¹³C data, this was interpreted to indicate major burial of organic carbon through this period, in some cases in closed environments and in others in open marine systems (Knoll et al. 1986; Lambert et al. 1987). This would likely have been accompanied by a major increase in atmospheric O₂, the Neoproterozoic Oxidation Event (Canfield 2005). The transition to the Cambrian was accompanied by a trend toward negative carbonate δ¹³C, indicating decreases in rates of accumulation and/or increased oxidation of organic matter—the latter would be expected as a result of the evolution of animals that effectively bioturbated the sediments, allowing ingress of oxygenated waters (Fig. 1).

Fig. 1

Reproduced with permission from Lambert et al. (1985); Crown copyright 1985 Geoscience Australia. The original figure used data summarized by Hsu et al. (1985)

Isotopic data for the carbonate-rich Upper Proterozoic to Early Cambrian succession from the Yangtze Gorges near Ichang, China.

2 Isotopes in Economic Geology, Metallogeny and Exploration

This book provides a review of the use of isotopes to understand mineralizing processes at scales ranging from microscopic to continental, including the timing of duration of these processes, regional controls on the localization of ore formation, sources of ore-forming components, and chemical and physical processes of ore deposition. The book is set out in four parts. Part I describes the use of radiogenic isotopes to determine the absolute timing and duration of mineralizing processes. Part II documents the use of radiogenic isotopes to determine metal sources, fingerprint deposit types and map tectonic and metallogenic provinces. Part III describes how light stable isotopes have been used to determine fluid and sulfur sources and to establish ore forming reactions and processes. Part IV examines the utility of stable metallic isotopes for the robust determination of metal sources and for understanding geochemical processes of mineralization like redox reactions.

2.1 Part I—Radiogenic Isotopes and the Age and Duration of Mineralization

Ever since the first attempt to date ore minerals by Boltwood (1907), the main use of radiogenic isotopes in economic geology, and in the geosciences in general, has been determination of the age of geological events. Largely because of the lack of analytical methods and uncertainties in the rates of radioactive decay, geochronology did not develop as a discipline until the latter half of the twentieth century, with the development and widespread adoption of thermal ionization mass spectrometry (TIMS) and secondary ion mass spectrometry (SIMS). The development and application of ICP-MS, particularly in conjunction with in situ sampling of individual minerals by laser ablation (LA-ICP-MS) from the 1990s to now, has revolutionized geochronology by enabling access to inexpensive and rapid analysis. This part of the book presents an overview of the use of radiogenic isotopes and geochronology to understand the timing and duration of mineralization.

The first chapter in Part I, by Chiaradia (2023), presents an overview of the theory and radiogenic isotope systems used in geochronology, including U–Th–Pb, Re–Os, K–Ar (Ar–Ar), Rb–Sr and Sm–Nd. An important aspect of geochronology here is the concept of closure, when a mineral ceases to undergo isotopic exchange with its surroundings. Potential problems related to the lack of closure were recognized from the start: Strutt (1905) observed that the amount of helium would give a minimum age of a Th-bearing mineral because “the helium may not have been all retained”. Closure depends upon many factors, including the mineral, the isotopic system, temperature and strain. Of these, the most important is temperature. Closure temperature is the temperature below which the mineral locks in isotopic composition. This varies not only with the mineral, but also the isotopic system (mainly limited by diffusion) and physical variables such as cooling rate and crystal size. Chiaradia (2023) presents closure temperatures for a range of minerals/isotope systems for which ages are commonly determined. Consideration of closure temperatures allows an interpretation of the significance of different ages (ages determined from mineral systems do not necessarily reflect primary ages of mineralization if the mineral remains open for an extended period of time or re-opens later due to processes such as heating or deformation), but also allows an understanding of the rates of cooling of mineral systems and possible constraints on subsequent events. Another important aspect of geochronology addressed by Chiaradia (2023) is the duration of mineralizing processes: individual mineralizing pulses associated with porphyry copper deposits last at most a few tens of thousands of years. Individual mineralizing pulses, however, can overprint each other, incrementally building up to total endowment to form world-class deposits.

Chelle-Michou and Schaltegger (2023) provide a description of the U–Pb isotopic system and how this system has been the main, and best understood, isotopic system to determine ages in most geological systems, including mineral systems. This overview provides information on theory, data presentation, processes that can disturb the system and complicate data interpretation, analytical methods, guidelines for data interpretation, minerals that can be dated as well as case studies. In addition, they give a brief summary of the Th–Pb system. Even though the U–Th–Pb isotopic system is well understood, there are many impediments to obtaining robust ages, and, because ore minerals are rarely dated directly, geological relationships between the dated mineral and the ore assemblage must be considered in data interpretation. Despite this, the U–Th–Pb system has been the workhorse for mineral system geochronology, and the high precision obtainable from this system has enabled robust studies on the duration of mineral systems.

The third chapter (Norman 2023) describes the theory and application of the Re–Os and Pt–Os systems to dating mineral systems. The Re–Os system is probably second in importance for mineral deposit geochronology after the U–Th–Pb system. In contrast to the U–Th–Pb system, many minerals dated by the Re–Os systems are ore minerals (e.g. molybdenite) or closely related to ore minerals (e.g. arsenopyrite and pyrite). The Pt–Os system, which can also be used to date ore minerals, is much less used owing the lower abundance of suitable minerals and analytical challenges. Molybdenite Re–Os dating has been particularly useful in age dating of mineral systems as, in addition to being an ore mineral in many systems, it is resistant to resetting. Despite these advantages, several processes can complicate data interpretation. Norman (2023) also describes how the Re–Os system can be used in understanding processes in orthomagmatic systems such as metal sources and partitioning of platinum group elements between sulfide and silicate melts during magma evolution.

Isotopic systems not described in detail in this book include K–Ar (Ar–Ar), Rb–Sr and Sm–Nd. Although the use of the latter two systems is limited in mineral deposit studies, the K–Ar (Ar–Ar) system is commonly used, particularly in dating K-bearing alteration minerals. Owing to complexities in data interpretation, low closure temperatures for many datable minerals (e.g. Chiaradia 2023) and uncertainties in relating alteration minerals to mineralizing events, care must be taken in interpreting the significance of K–Ar and Ar–Ar ages in mineral systems. More detailed descriptions and applications of the K–Ar (Ar–Ar) system can be found in Richards and Noble (1998), Vasconcelos (1999) and Kelley (2002).

2.2 Part II—Radiogenic Isotopes: Metal Source, Deposit Fingerprinting and Tectonic and Metallogenic Mapping

The second part of this book presents uses of radiogenic isotopic systems—Sm–Nd, Pb–Pb and Lu–Hf—to determine metals sources and to map tectonic and metallogenic provinces using variations in isotopic ratios. Although demonstrated as a method last century (Zartman 1974; Bennett and DePaolo 1987), recent advances in geographic information systems (GIS) and increases in the amount of data available have enabled isotopic mapping at the province to continental scales.

Champion and Huston (2023) and Huston and Champion (2023) discuss isotopic mapping using the Sm–Nd and Pb–Pb isotopic systems, respectively. Both papers present the theory behind the isotopic systems as well as analytical methods prior to describing applications of the data to metallogenic studies. The availability of large isotopic datasets combined with GIS visualization and contouring tools has enabled mapping of variations in isotopic parameters at the province to continental scales. Champion and Huston (2023) illustrate that variations in parameters derived from Sm–Nd isotopic data commonly coincide with boundaries between tectonic and metallogenic provinces. Moreover, they show that some types of mineral systems prefer different types of crust (i.e. isotopically juvenile versus evolved). Huston and Champion (2023) show that maps based on parameters derived from lead isotope analyses of ores (e.g., μ = ²³⁸U/²⁰⁴Pb) also define tectonic and metallogenic provinces that can be used to predict the metallogenic potential of mineral provinces. Moreover, they review previous literature that shows that lead isotope data can be used to determine the lead sources and to fingerprint deposit types in regions of complex metallogeny.

Waltenberg (2023) discusses the theory and analytical methods of the Lu–Hf isotopic system prior to considering its utility in metallogenic studies, particularly isotopic mapping. Although in many ways similar to the Sm–Nd system, Lu–Hf isotopic data, which are mostly derived from zircon analyses, also provide information on changes in isotopic patterns through time, a characteristic not available with the Sm–Nd or Pb–Pb systems. Like the Sm–Nd system, parameters derived from Lu–Hf data define tectonic and metallogenic provinces or boundaries, in this case at specific times. Understanding of the Lu–Hf system is evolving rapidly, hence the utility of the Lu–Hf system to metallogenic studies will increase in the future.

2.3 Part III—Light Stable Isotopes: Fluid and Sulfur Sources and Mineralizing Processes

Since their early application to metallogenic studies (Engel et al. 1958), light stable isotopes (initially H, C, O and S, but expanding to include B, Mg, Si, etc.) have played an important role in developing models for ore deposit and then mineral systems. Part III of this book presents the theory, analytical methods and processes that fractionate the most used light stable isotope systems (H, C, O, S and, in some cases, B) and then illustrates how these data have been used to develop mineral system models for VHMS, orogenic gold, shale-hosted Zn–Pb and iron ore deposits.

Huston et al. (2023a) present an overview of the most used stable isotopes in mineral systems studies. This chapter describes the general theory and conventions of these data, documents analytical techniques, including recent developments in microanalytical capabilities and interpretation of multiple sulfur isotope ratios, and discusses how light stable isotope data can be used to constrain possible fluid and sulfur sources, how isotopic patterns can be used to infer geochemical and geophysical processes, and, potentially, exploration. Huston et al. (2023a) stress that the interpretation of isotopic data is commonly ambiguous in that different sources and different processes can produce similar isotopic characteristics.

Volcanic-hosted massive sulfide deposits, the ancient analogues of modern black smoker deposits, form at or close to the seafloor in submarine volcanic successions. Because of this environment, VHMS mineral systems involve a complex interplay between component sources and processes. Huston et al. (2023b) review how stable isotope data, particularly oxygen, hydrogen and sulfur data, have been used to determine (or at least to delimit) fluid and sulfur sources, to infer processes, and to demonstrate that sources and processes have evolved with geological time. Oxygen-hydrogen data indicate that the main fluid was (evolved) seawater although magmatic-hydrothermal fluids are thought to have been present in some systems. Moreover, these data, particularly δ¹⁸O, define reasonably consistent patterns that have been used in exploration and discovery. Sulfur isotope data (including δ³⁴S, δ³³S and δ³⁶S) indicate that sulfur sources are a mixture between seawater sulfur (reduced sulfate) and igneous (leached or magmatic-hydrothermal) sulfur, with the proportion of seawater-derived sulfur increasing with geological time. Detailed deposit-scale sulfur isotope studies using in situ microanalysis demonstrate a complex mixture of igneous sulfur and that produced by thermochemical and biogenic sulfate reduction (TSR and BSR).

Orogenic gold deposits form in orogenic belts commonly intruded by igneous rocks, are mostly in the form of veins or stockworks. The association of these deposits with both orogenic belts and granitic rocks has led to controversies over the origin of ore fluids and sulfur. Quesnel et al. (2023) address these controversies using a database of over 8000 oxygen, hydrogen, carbon, sulfur, nitrogen, boron and silicon isotope analyses collected from deposits of all ages around the world. Based upon this dataset, Quesnel et al. (2023) conclude that the isotopic data for these deposits are most consistent with a dominant metamorphic fluid having a temperature of 360 ± 76 ℃ (1σ) and that this broad fluid type did not change significantly over geological time. Moreover, isotopic arrays can be interpreted to indicate mixing between this deep metamorphic fluid and upper crustal fluids. Secular variations were noted for nitrogen and sulfur isotopes. Quesnel et al. (2023) interpret the changes in nitrogen to reflect secular variations in δ¹⁵N values, whereas the changes in δ³⁴S ultimately reflect secular changes in δ³⁴S of seawater.

Williams (2023) also used an isotopic dataset compiled from clastic-dominated (shale-hosted or SEDEX) Zn–Pb deposits in the Paleo-Mesoproterozoic North Australian Zinc Belt and in the Paleozoic Northern Cordillera of Canada and the United States to determine sulfur and carbon sources and assess competing syngenetic and diagenetic/epigenetic timings of mineralization. An important conclusion of the study is that earlier bulk sulfur isotope studies using conventional analytical techniques blurred the isotopic pattern that became sharper with the advent of microanalytical methods beginning in the late 1980s. Carbon–oxygen data from carbonates suggest that the ore fluids were warm (> 150 ℃) basinal brines. Microanalytical sulfur isotope data in combination with observations of mineral textures suggest that the earliest, barren pyrite incorporated sulfide produced by BSR, whereas later-formed sphalerite and galena incorporated sulfide derived from TSR. The sulfur isotope and paragenetic data are most consistent with a diagenetic/epigenetic timing for mineralization, although in some deposits (e.g., Red Dog, Alaska) the data are indicative of protracted mineralization, including early syngenetic sulfide deposition.

Hagemann et al. (2023) document isotopic changes in the complex iron ore mineral system in which iron formation protore was progressively enriched via hypogene and supergene processes to form iron ore. Data from iron ore deposits in Australia, South Africa and Brazil, showed a consistent decrease in δ¹⁸O from values of 4–9‰ in iron formation protore to values as low as − 10‰ in high-grade iron ore. Hagemann et al. (2023) interpret this δ¹⁸O shift to indicate the incursion of ancient meteoric fluids along fault and fracture zones. They also interpret that upgrading involved magmatic fluids in the Carajás (Brazil), basinal brines in the Hamersley (Australia) and deep crustal (metamorphic or magmatic) fluids in the Quadrilátero Ferrífero (Brazil) provinces. The systematic decrease of δ¹⁸O values in iron oxides from the early to late paragenetic stages and from the distal to proximal alteration zones, including the ore zone, may be used as a geochemical vector. In this case, oxygen isotope analyses on iron oxides would provide a potential exploration tool (Hagemann et al. 2023).

2.4 Part IV—Metallic Stable Isotopes: Metal Sources and Mineralizing Processes

The development of ICP-MS has enabled stable isotope studies to be extended from the traditional light stable isotopes into transition metals such as copper, iron and zinc and other ore and related elements such as antimony, molybdenum and silver. Of these, studies of iron, copper and zinc in this part are reviewed in chapters by Lobato et al. (2023), Mathur and Zhao (2023) and Wilkinson (2023).

Lobato et al. (2023) compile existing data and present new data on the variability of iron isotopes in banded iron formation-hosted iron ore deposits from Australia and Brazil. This analysis found that although some characteristics of iron isotopes varied by metallogenic province, others, such as the association of hypogene ores with lower δ⁵⁶Fe and supergene ores with higher δ⁵⁶Fe, persist in all metallogenic provinces. In the Quadrilátero Ferrífero district (Brazil), iron isotopic characteristics in hematite ore differ between deformational domains: deposits in domains with low deformation have lower δ⁵⁶Fe than those in high-strain domains, possibly reflecting different fluid characteristics, such as temperature. In the Corumbá region (Brazil), δ⁵⁶Fe characteristics reflect the interplay of primary seawater, microbial activity and supergene alteration. In the Carajás district (Brazil), hypogene magnetite and hematite iron ores have lower δ⁵⁶Fe than their iron formation protores. In the Hamersley Province (Australia), δ⁵⁶Fe and δ¹⁸O values appear correlated during greenschist metamorphism and hypogene upgrading, but negatively correlated during subsequent supergene upgrading. Lobato et al. (2023) highlight that hypogene iron ores tend to have lower δ⁵⁶Fe values, whereas supergene overprints result in higher δ⁵⁶Fe values, although they note that more data are required to confirm these observations.

Mathur and Zhao (2023) review the development of copper isotopes over the last two decades in ore genesis, environmental monitoring and exploration. Compared to other metallic stable isotopes, δ⁶⁵Cu has a large range of 10‰, possibly due to copper being involved in redox reactions, during both hypogene and supergene mineralizing processes. After documenting analytic methods and reporting conventions, Mathur and Zhao (2023) provide an overview of the natural variability of copper isotopes followed by a discussion of how copper isotopes fractionate and evolve in mineral systems including magmatic Ni–Cu, porphyry copper (and related skarns), VHMS, sediment-hosted copper and supergene-enriched systems. They document that the greatest fractionations occur during low temperature redox reactions, such as supergene enrichment, and how this information can be used during exploration of, for instance, leached caps that form over copper-rich deposits at depth or in groundwaters.

For zinc isotopes, Wilkinson (2023) reviews analytical methods and reporting conventions, and then describes natural variability in rocks and mineral deposits. Compared to δ⁵⁶Fe and δ⁶⁵Cu, the variability observed in δ⁶⁶Zn (< 2‰) is small, probably because in zinc on Earth has only one valence state and, hence is not involved in redox reactions. Because of the newness of analytical methods, zinc isotope data from mineral deposits are uncommon. One exception is sediment-hosted zinc deposits, including both carbonate-hosted and shale-hosted deposits. Wilkinson (2023) demonstrates a reasonably consistent enrichment of δ⁶⁶Zn from the core to the peripheries of these deposits. For other deposit types for which data are available (VHMS, veins and porphyry and related deposits) currently available data are simply insufficient to establish consistent patterns.

Although not discussed in this book, significant natural variations have been observed in other metallic and semi-metallic isotopes, including antimony, molybdenum and silver (Rouxel et al. 2002; Arnold et al. 2004; Mathur et al. 2018). Because these elements are extracted from some ores or are closely associated with ore metals, they have potential to further understand ore genesis.

3 Conclusion

Isotope geochemistry has played an essential role in the development of mineral deposit and metallogenic models at all scales, providing constraints on sources (fluid, sulfur and metal), processes and ages. In particular, the data have provided critical information for mineral systems models for porphyry copper/epithermal, VHMS, orogenic gold, iron ore, sediment-hosted Zn–Pb and Cu, and orthomagmatic Ni–Cu deposits, among others. The development of rapid, inexpensive and robust geochronological methods for dating ore and ore-related minerals has provided the fourth dimension to economic geology and metallogenic research. Radiogenic isotopes also provide information on the sources of ore metals, including lead and rare earth elements; in addition, mapping of isotopic ratios and derived parameters provides new insights into tectonic and metallogenic provinces, including fertility and metal endowment. Light stable isotopes have been a workhorse in ore genesis studies since the mid-1960s, providing important constraints on fluid and sulfur sources as well as ore forming processes. Metallic stable isotopes provide information on metal sources and mineralizing processes, particularly redox processes. This isotopic research, like isotope mapping, is in the early stage of evolution and, when combined with other major development in isotopic research, such as the analysis of multiple isotope ratios in the oxygen (δ¹⁸O and δ¹⁷O) and sulfur (δ³⁴S, δ³³S and δ³⁶S) systems, will undoubtedly provide further invaluable insights into ore genesis.