Applications of Lead Isotopes to Ore Geology, Metallogenesis and Exploration

Abstract

Although lead isotopes are most commonly used to date geological events, including mineralizing events, they also can provide information on many aspects of metallogeny and can be directly used in mineral exploration. Lead isotope data are generally reported as ratios of radiogenic isotopes normalized to the non-radiogenic isotope ²⁰⁴Pb (e.g. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb). These ratios can be used in exploration to characterize the style of mineralization, metal (i.e. Pb) source and as vectors to ore. When combined with lead isotope evolution models, the data can be used to indicate the age and tectonic environment of mineralization. The raw ratios and evolution models enable calculation of derived parameters such as μ (²³⁸U/²⁰⁴Pb), κ (²³²Th/²³⁸U) and ω (²³²Th/²⁰⁴Pb), which provide more information about tectonic setting and can be contoured to identify crustal boundaries and metallogenic provinces. In some cases, tectonic boundaries, mapped using gradients in μ and other derived parameters, are fundamental controls on the distribution of certain deposit types in space and time. Moreover, crustal character, as determined by lead and other radiogenic isotopes (e.g. Nd) can be an indicator of province fertility for many deposit types. The development of cost effective analytical techniques and the assembly of large geo-located datasets for lead and other isotope data has enabled significant advances in understanding the genesis and localization of many deposit type, particularly when the isotopic data are integrated with other independent datasets such as potential field, magnetotelluric, passive seismic and geochemical data.

1 Introduction

Radiogenic isotopes have had an important use in the geological sciences as chronometers, providing constraints on the ages of rocks and geological events, including mineralizing events (see also Chelle-Michou and Schaltegger 2023; Chiaradia 2023). Another important, though less widespread, use of radiogenic isotopes is as a tracer of geological processes, including mineralizing processes. The use of radiogenic isotopes as tracers takes advantage not only of radioactive decay between parent and daughter isotopes, but also geochemical partitioning between the parent and daughter elements: the isotopic evolution of a given volume of rock is not only governed by radiogenic decay, but also by geochemical processes within the rock volume and between it and other rock volumes. In this chapter we review how spatial and temporal patterns in lead isotope data have been used in metallogenic and tectonic studies. Similar reviews for the Sm–Nd and Lu–Hf isotopic systems are provided in other chapters of this volume (Champion and Huston (2023) for Sm–Nd, Waltenberg (2023) for Lu–Hf). These three reviews illustrate that large scale variations in the spatial and temporal characteristics of radiogenic isotopes can be used to identify tectonic and metallogenic processes at the global to province scale—information important for regional target area selection in exploration. At the smaller scale, isotopic signatures and patterns may be used more directly for local target definition.

2 The Uranium–Thorium-Lead Isotopic and Geochemical System

The lead isotope composition of a particular mineral, rock or geological reservoir is the sum of the isotopic composition of lead present in the geological entity (mineral, rock or reservoir) when it formed and the isotopic composition of lead formed subsequently by radioactive decay of uranium and thorium within the entity. ²³⁸U decays through a complex reaction chain to ²⁰⁶Pb; similarly ²³⁵U decays to ²⁰⁷Pb and ²³²Th decays to ²⁰⁸Pb. The decay constants for the chains are 0.155125 × 10^–9 yr⁻¹, 0.98485 × 10^–9 yr⁻¹, and 0.049475 × 10^–9 yr⁻¹, which correspond to half-lives of 4.468 Gyr, 0.704 Gyr and 14.05 Gyr, respectively (LeRoux and Glendenin 1963; Jaffey et al. 1971).

For the Earth as a whole, the isotopic composition at formation is generally taken as that of Canyon Diablo meteorite (Tatsumoto et al. 1973) at 4.54 Ga, with radioactive lead growth dependent on time, the decay constants of ²³⁸U, ²³⁵U and ²³²Th, and chondritic abundances of uranium, thorium and lead at 4.54 Ga. The relative abundances of uranium, thorium and lead can be expressed as ²³⁸U/²⁰⁴Pb (μ), ²³²Th/²³⁸U (κ) and ²³²Th/²⁰⁴Pb (ω). Ancient values of these ratios can be calculated using measured lead isotope ratios from lead isotope evolution models. Although uranium, thorium and lead are all concentrated in the crust relative to the mantle (Wedepohl and Hartmann 1994; Palme and O’Neill 2003), the relative fractionation of these elements differs (Fig. 1), implying that the isotopic evolution of a reservoir is dependent upon the geochemical processes through which the reservoir formed. Figure 2a shows the effect on lead isotopic growth of a hypothetical geochemical differentiation event in which an isotopically homogeneous reservoir produced two separate reservoirs. As shown in the figure, the isotopic characteristics of the two reservoirs diverge, with the reservoir with higher μ producing ever higher ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios over time relative to the second lower μ reservoir. The concentrations of lead isotopes that grow radiogenically (i.e. ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb) are generally normalized to the concentration of ²⁰⁴Pb, which does not grow radiogenically, to allow comparison.

Fig. 1

Variations in the concentrations of uranium, thorium and lead and in U/Pb, Th/U and Th/Pb ratios between major terrestrial reservoirs (element concentrations for the major reservoirs are from McDonough 2003; Palme and O’Neill 2003; Rudnick and Gao 2003). The abscissa on the main diagram indicates the depth range that the various reservoirs occupy

Fig. 2

Diagrams showing lead isotope evolution models. a ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb diagram showing the changes in lead isotope composition in a hypothetical two stage lead evolution model. The model assumes lead growth in an isotopically homogeneous Earth until 3.5 Ga at which time a (crust-forming) event produces two reservoirs that evolve separately. ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb (b) and ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁸Pb/²⁰⁴Pb (c) diagrams showing evolution of uranogenic and thorogenic lead in major terrestrial reservoirs according to the plumbotectonic model of Zartman and Doe (1981). Parts (a) and (b) are reproduced with permission from Zartman and Doe (1981); Copyright 1981 Elsevier

The above scenario reflects crust formation. Because uranium partitions more strongly into the crust than lead (Fig. 1), the upper trajectory in Fig. 2a, which is associated with higher μ, reflects the crustal reservoir, whereas the lower trajectory reflects the residual mantle reservoir. The scenario suggests that an important control of lead isotope evolution is crust formation, a process that, on modern Earth, occurs mostly along plate margins. As U/Pb, Th/Pb and Th/U also vary within the crust (Fig. 1), lead isotopic ratios not only reflect crust formation, but also geochemical processes that occur within the crust.

3 Lead Isotope Evolution Models, Model Ages, and μ, κ and ω Values

Once an isotopic reservoir has been identified, growth of lead within the reservoir can be modelled using lead isotope evolution models. These mathematical models simulate radiogenic isotope growth within the closed reservoir as a function of geological time. Holmes (1946) and Houtermans (1946) independently proposed a model whereby the lead isotope characteristics of galena (and other Pb-rich and U-poor minerals) reflects lead growth in a reservoir until time t, at which time the lead was extracted from the reservoir and was crystalized into galena. Because the galena contains virtually no uranium the uranogenic isotopic ratios are frozen in and closely reflect “initial” ratios at the time of crystallization. Using the measured ratios of the galena and the isotopic ratio of the reservoir when it formed, a two-point isochron can be determined and the time since formation of the reservoir can be estimated. Holmes (1946) and Houtermans (1946) treated the Earth (or solar nebula) as this reservoir and the age of the Earth as time when the reservoir formed. Hence, the age t, as determined from the isochron is the age since formation of the reservoir, in this case the age of the Earth. Subsequently it has been shown that the Holmes-Houtermans model produces erroneous ages, and other models for reservoir lead evolution have been developed to allow estimation of ages from galena isotopic ratios.

These models are quite varied and make different assumptions. In “single-stage” models the reservoir evolution is modelled using initial lead isotopic ratios, and initial μ, κ and ω values. The isotopic evolution as a function of time is then calculated simply using these initial values and the decay constants. A single-stage model was used in Fig. 2a until the point at which chemical differentiation occurred. Most single-stage models are geologically unreasonable except at the broadest scale as they do not consider later geological/geochemical events during which uranium and thorium are fractionated relative to lead.

Maltese and Mezger (2020) recently proposed a more geologically reasonable single stage model for the evolution of bulk silicate Earth (i.e. the crust and mantle). This model was developed to resolve the so-called “future lead paradox” (Sinha and Tilton 1973) in which major crust and mantle lead reservoirs are more radiogenic that expected assuming lead growth from a chrondritic initial reservoir. It infers that a relatively small chondritic impactor, Theia, collided with a volatile-rich proto-Earth at ca 4.50 Ga (~70 Myr after formation of the solar system). This collision not only resulted in the formation of the moon, but the formation of a bulk silicate Earth reservoir that was modelled by the Maltese and Mezger (2020) model. This model, a significant departure from previous two stage models (see below), explains the isotopic evolution of bulk silicate Earth relatively well.

Two-stage models begin like single-stage models, but at some point during the evolution, a geochemical or geological event is assumed to cause fractionation between uranium, thorium and lead, and two isotopic reservoirs form with different isotopic evolution trajectories. At the point where geochemical differentiation of Earth’s crust and mantle occurred (at 3.5 Ga), the evolution model shown in Fig. 2a becomes a two-stage model. Interaction or exchange between the two reservoirs can be modelled as mixing between to the two evolution curves that formed after differentiation by varying μ (i.e. using an intermediate value of μ). The most commonly used global lead isotope evolution model, that of Stacey and Kramers (1975), is a two-stage model, which models the evolution of “terrestrial” lead after geochemical differentiation. Another global model (Cumming and Richards 1975) assumes that fractionation between uranium, thorium and lead occurs continuously and models lead growth assuming U/Pb and Th/U change continuously through time. Although these models approximate the evolution of lead at the global scale reasonably well, the geological processes that form reservoirs are more complex than these relatively simple models.

Doe, Zartman and co-workers (Doe and Zartman 1979; Zartman and Doe 1981; Zartman and Haines 1988) developed a more complex model, in which the evolution of four major isotopic reservoirs was modelled assuming periodic events when uranium, thorium and lead were exchanged between reservoirs. During each event, the fourth reservoir, the so-called orogene reservoir, was created by recycling and extraction from the other three reservoirs—mantle, lower crust and upper crust. The model was based upon the concepts of modern plate tectonics—hence the name ‘plumbotectonics’—although Zartman and Haines (1988) indicated that the modelling would remain valid for geodynamic processes other than plate tectonics if such processes involved a similar magnitude of reservoir interaction.

The earlier versions of plumbotectonics assumed an orogene event every 400 Myr (Zartman and Doe 1981), with the mantle input decreasing through time. The later versions (e.g. Zartman and Haines 1988) increased the frequency of events and the complexity of the models, although the overall trends were similar. It must be noted that the term orogene, as used by Doe, Zartman and co-workers, is not a volume of rock that has undergone orogenesis, but rather quantifies the global distribution of crust formation (Zartman and Haines 1988).

Figure 2b, c illustrate the evolution of the four major reservoirs based on plumbotectonics version II (Zartman and Doe 1981). The plumbotectonic model replicates modern isotopic characteristics of the four main reservoirs well, and the general evolution of each reservoir matches natural secular variations reasonably well. The plumbotectonic model highlights important differences in the four major reservoirs, particularly the lower crust in which uranogenic lead growth has been retarded relative to other reservoirs (although thorogenic lead growth is similar to other reservoirs). As a consequence the plumbotectonic evolution curves have been widely used to infer reservoir sources for many mineral deposits, as discussed below. Doe and Zartman (1979) assessed the correspondence of a range of (mainly young) deposits to the plumbotectonic model. Like other general models, model ages can be quite inaccurate, but with a few notable exceptions (e.g. some Mississippi Valley-type deposits) the modelled source of lead in the deposits is consistent with the geological environment inferred for ore formation.

Kramers and Tolstikhin (1997) used a similar concept of isotopic and elemental exchange between isotopic reservoirs to model lead isotope evolution. Their approach differed from that of Doe and co-workers in that it involved nine—instead of four—different reservoirs, including the Earth’s core, and used crustal growth rates as an input into the model. Like Doe and co-workers, Kramers and Tolstikhin (1997) were able to model present-day crustal and mantle reservoirs reasonably well.

Due to inaccuracies of global lead isotope evolution models, some workers (Thorpe et al. 1992; Thorpe 1999; Carr et al. 1995; Sun et al. 1996) have produced evolution models that model lead growth at the province scale. These models can very accurately and precisely model isotopic growth, but direct comparison of these models or extending the models outside of their calibrated reservoirs are problematic. Hence, for global or large-scale comparisons, use of a global model (e.g. Stacey and Kramers 1975; Cumming and Richards 1975; Zartman and Doe 1981) is more effective.

As mentioned above, lead isotope model ages assume that no radiogenic growth has occurred since crystallization of the analysed mineral—that is the concentrations of uranium (and Th) are so low that the additional radiogenic lead produced since crystallization is negligible. Other minerals also meet the criteria of being Pb-rich and U- and Th-poor. The most common of these minerals is potassium feldspar. In many cases measured ratios of potassium feldspars are virtually identical to the initial ratios inherited from crystallization (see discussion regarding analytical techniques below). Less common lead selenide (clausthalite), telluride (altaite) and sulfosalt (beaverite, bournonite and cosalite) minerals also can be analyzed (Thorpe 2008) and retain initial ratios.

The discussion above assumes that the mineral of interest has been a closed system since crystallization; if the system has been open (i.e. due to recrystallization, alteration or a second mineralizing event), the isotopic ratios will reflect either the ratios at the time when the system opened or a mixture between lead produced during initial crystallization and lead introduced during the resetting event.

4 Analytical Methods

For metallogenic studies and exploration, the most common methods of lead isotope analysis are thermal ionization mass spectrometry (TIMS) or multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) on solutions derived by the dissolution of mineral separates and rock powders. Over the last two decades, the development of MC-ICP-MS has seen a revolution in lead isotopic analysis, producing high quality analyses for a much lower cost in comparison with TIMS. This is largely because MC-ICP-MS analyses are more rapid and less labour-intensive than TIMS analyses.

For both TIMS and MC-ICP-MS analyses, samples are dissolved using a range of chemical attacks and then elutriated using ionic distillation columns to remove matrix elements. For conventional TIMS analysis, which was the most common analytical method until the early-mid 1990s; lead from the elutriant was loaded onto a rhenium filament in a silica gel-phosphoric acid suspension (Cameron et al. 1969), in some cases after addition of a ²⁰²Pb or ²⁰⁷Pb spike.¹ A current is passed through the filament and the lead is ionised, accelerated using an electrical potential gradient, forming a beam which is split by mass using and electromagnet, and then analyzed using Faraday cup detectors. Mass fractionation during analysis is then corrected by normalization using fractionation factors determined from standards. External analytical precisions (2σ) for conventional TIMS analysis are typically 0.05% for ²⁰⁶Pb/²⁰⁴Pb, and 0.1% for ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb (cf. Carr et al. 1995).

As precisions of conventional analyses are insufficient to resolve many geological problems, new methods were developed over the last 20–30 years to improve precision. The first of these, which came into widespread use in the mid-1990s, was TIMS analysis in which spikes are added to an aliquot of the elutriant prior to loading onto the filament. The spiked and unspiked aliquots are then loaded and analyzed as described above, with the unspiked analyses corrected for mass fractionation using the spiked analyses and mathematical equations developed by Dodson (1963). Double-spike TIMS analyses involve spikes containing two isotopes (²⁰²Pb-²⁰⁵Pb: Todt et al. 1993; ²⁰⁴Pb-²⁰⁷Pb: Woodhead et al. 1995), whereas triple-spike analyses, which are even more precise, involve spikes containing three isotopes (²⁰⁴Pb-²⁰⁶Pb-²⁰⁷Pb: Galer and Abouchami 1998). Typical external analytical precisions (2σ) for double and triple spike analyses are 0.008–0.034% for ²⁰⁶Pb/²⁰⁴Pb, 0.010–0.047% for ²⁰⁷Pb/²⁰⁴Pb and 0.012–0.057% for ²⁰⁸Pb/²⁰⁴Pb (based on repeated analysis of SRM981 from a range of labs: Thirlwall 2000). The extra time and expense of double spike analyses (two mass spectrometer runs are required for each sample) is justified for many geological studies by the significant improvement in precision. For more detailed descriptions of TIMS analytical methods the reader is referred to the above references as well as Tuttas and Habfest (1982).

For MC-ICP-MS analysis, an aliquot of doped tracer solution containing thallium isotopes (²⁰³Tl and ²⁰⁵Tl: Hirata 1996; Belshaw et al. 1998; Rehkämper and Halliday 1998; Woodhead 2002) of known concentrations and ratios is added to the sample during dissolution. The resulting solution is purified by ionic distillation and the elutriant is then aspirated into the ionization chamber of the MC-ICP-MS and analysed. Instrumental mass fractionation is corrected using mass fractionation factors determined from the doped isotopes added during dissolution; this produces more precise analyses than conventional TIMS analyses but less precise analyses than double or triple spike TIMS analyses. Typical external analytical precisions (2σ) for MC-ICP-MS analyses are 0.018–0.053% for ²⁰⁶Pb/²⁰⁴Pb, 0.030–0.047% for ²⁰⁷Pb/²⁰⁴Pb and 0.012–0.057% for ²⁰⁸Pb/²⁰⁴Pb (Thirlwall 2000; R Maas, 2021, pers. comm.). More details of the methods used by MC-ICP-MS analysis are described in the references above as well as Albarède et al. (2004) and Baker et al. (2004).

Because potassium feldspars are common rock forming minerals that strongly concentrate lead relative to uranium and thorium, these minerals can provide information on the initial isotopic composition of their host rocks, such as granites. However, as potassium feldspars are readily altered or recrystallized, they can exhibit open system behaviour. To minimize these effects and exclude the contribution of radiogenic lead, potassium feldspar and, less commonly, clinopyroxene are analysed with using a sequential acid leach Typically, the first leach uses a relatively weak acid or combinations of acids (HCl-HNO₃) followed by leaches using stronger acids (concentrated HF-HNO₃) (e.g. Carr et al. 1995). The earlier leaches removes labile, radiogenic lead, whereas the later leaches dissolved common lead held in the mineral lattice and are taken as the closest approximation of the initial ratios that characterize the mineral.

Material for isotopic analysis can also be extracted directly from samples using either of two microanalytical techniques—secondary ion mass spectrometry (SIMS) and multi-collector laser-ablation inductively-coupled-plasma mass spectrometry (MC-LA-ICP-MS). In both cases the analytical spots are generally below 100 μm in diameter, with SIMS analyses, in some cases, less than 10 μm in width. Methods involving SIMS, which includes the sensitive high-resolution ion microprobe (SHRIMP), have most application in U–Pb geochronology and are described by Chelle-Michou and Schaltegger (2023). Gigon et al. (2020) describe a study that has documented micro-scale variations in lead isotopes from SIMS analyses galena, as discussed in the section on future developments below. Methods involving MC-LA-ICP-MS can be less precise and require larger spot sizes (Zametzer et al. 2022), although these disadvantages are set against more rapid analysis, lower costs and the ability to analyze a greater range of sample types. For instance, Pettke et al. (2010) have analysed lead isotopes in fluid inclusions and used these data to infer the source of lead in the Bingham Canyon porphyry copper district (see below). Typical external precisions (2σ) for MC-LA-ICP-MS analyses are 0.10–0.18% for ²⁰⁶Pb/²⁰⁴Pb, 0.084–0.17% for ²⁰⁷Pb/²⁰⁴Pb and 0.085–0.16% for ²⁰⁸Pb/²⁰⁴Pb (based on repeated analysis of internal feldspar standard “Albany”: Zametzer et al. 2022).

5 Application of Lead Isotopes to Metallogenic Studies and Exploration

Over the last few decades, lead isotope characteristics have found many uses and potential uses in both academic metallogenic studies and practical exploration. The most common academic use has been identifying metal source regions using isotopes as a fingerprinting tool, but more recently spatial variations in isotopic properties have been used to identify and map the extent, character, endowment and origin of tectonic and metallogenic provinces. For practical exploration, district-scale variations in lead isotope ratios may serve as direct vectors to ore, lead isotope ratios can be used to discriminate different styles of mineralization in some provinces, and lead isotope characteristics can be indicative of fertility for some deposit types and, in some cases, size potential. Examples of all of these uses are presented below. These studies can use both analyses of mineral separates and whole rocks, although interpretation of analyses from Pb-poor samples (both mineral separates and whole rock) can be problematic as determining initial ratios, which are essential in many studies, requires information on the age and the concentrations of lead, uranium and thorium of the analysed sample. Moreover, open system behaviour is more difficult to determine in such analyses. Nonetheless, some of the case studies below use whole rock samples in addition to mineral separates for analyses.

5.1 Determining Lead Sources

Unlike most elements for which isotopic data is collected in ore genesis studies, lead is commonly extracted as an ore metal from many deposits for which lead isotope data are collected. Hence, no assumptions are required to link the isotopic system with the ore metal assemblage in these deposits. Metallic stable isotope systems such as iron (Troll et al. 2019; Lobato et al. 2023), copper (Li et al. 2010; Wilkinson 2023) and zinc (John et al. 2008; Mathur and Zhao 2023) also have this advantage. On the other hand, relating the Nd-Sm, Re–Os, Hf–Lu radiogenic systems and most light stable isotope systems to ore minerals and assemblages, is often not straightforward. Lead is commonly present, but not extracted, in other deposit types as a trace or minor element in the ore assemblage or in ore-related fluid inclusions. In these instances, lead isotopes can also be used as a tracer to determine metal sources, although assumptions are required regarding the relationship between lead and the minerals/metals of economic interest. Below we give a few examples of studies where lead is recovered and where lead is present, but not recovered. It must be stressed that the literature on this topic is extensive, so only a select number of studies are included in this review. Russell and Farquhar (1960), Cannon et al. (1961), Doe and Stacey (1974), Doe and Zartman (1979) and Gulson (1986) summarize other examples.

5.1.1 Studies of Deposits Where Lead is an Ore Metal

Although lead is recovered from many deposit types, the most relevant sources of lead are shale-hosted Zn–Pb–Ag, Mississippi Valley-type Zn–Pb (MVT; including Irish-type) and volcanic-hosted massive sulfide (VHMS) deposits. The origin of lead (and by inference Zn) in these deposit types has been historically contentious, with hypotheses involving crustal or mantle, and magmatic-hydrothermal or leached country rock sources advocated by different authors over time. As examples of the use of lead isotopes to resolve these questions, the studies of Brevart et al. (1982), Fehn et al. (1983), Relvas et al. (2001), Leach et al. (2005) and Vaasjoki and Gulson (1986) are summarized below. These examples were chosen to cover all of the major types of lead deposits and to illustrate different approaches to determine lead sources using lead isotopes.

The Massif Central in south-central France contains a relatively large number of small Zn–Pb deposits and occurrences including stratiform shale-hosted deposits, epigenetic carbonate-hosted and cross-cutting vein deposits. These deposits are hosted by Phanerozoic metasedimentary rocks that have been intruded by Hercynian granites. Brevart et al. (1982) used analyses of galena from these deposits to identify two discrete populations, a less radiogenic population (population 1) dominated by carbonate-hosted, stratiform deposits in Paleozoic sedimentary successions, and a more radiogenic population (population 2) dominated by vein deposits but also including the stratiform, sandstone-hosted Largentiére deposit (Fig. 3a). The second, more radiogenic population overlaps with the isotopic composition of Hercynian granites as determined from K-feldspar analyses (Vitrac et al. 1981; Brevart et al. 1982; Fig. 3a). These results suggest two mineralizing events, a syngenetic or early diagenetic event, and a second event associated with Hercynian magmatism and deformation. During the latter event, lead may have been sourced either via magmatic-hydrothermal processes associated with intrusion of the granites or, alternatively, through leaching of the granites subsequent to their intrusion and crystallization.

Fig. 3

²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb diagrams showing a the lead isotope composition of Pb-rich minerals from mineral deposits and occurrences from the Massif Central, France (Brevart et al. 1982) in comparison with the initial isotopic composition of local Hercynian granites (Vitrac et al. 1981; a composite of diagrams in Brevart et al. 1982); b the lead isotope composition of Pb-rich minerals from the Hokuroku district of Japan illustrating differences in the lead isotope characteristics of yellow (Cu-rich) and black (Zn-rich) (modified after Fehn et al. 1983); c variations in the lead isotope composition of cassiterite from the Neves Corvo deposit in comparison to the lead isotope composition of sulfides from the Neves Corvo and other Iberian Pyrite Belt massive sulfide deposits (modified after Relvas et al. 2001; Iberian Pyrite Belt sulfide data from Marcoux 1998); d the lead isotope compositions of galena and altaite from volcanic-hosted massive sulfide deposits and orogenic gold deposits in the Youanmi Terrane overlain on isochrons calculated from the Cumming and Richards (1975) lead evolution model (modified after Browning et al. 1987)

As a consequence of systematic studies by Japanese and international researchers in the late 1970s and early 1980s, Zn–Pb–Cu–Ag-Au deposits in the Miocene Hokuroku district of Japan are among the best understood deposits in the world (Ohmoto and Skinner, 1983). This research has been key to the understanding of ore forming processes of VHMS deposits. As part of this research program, Fehn et al. (1983) undertook a systematic lead isotope study of deposits (using mostly galena analyses, with subordinate whole-rock ore analyses) in this district to determine lead source(s). The authors found a consistent pattern in individual deposits in which samples of yellow (Cu-rich) ores are systematically less radiogenic than black (Zn–Pb-rich) ores (Fig. 3b). Fehn et al. (1983) interpreted these results and data from potential source rocks to indicate that the lead in the Zn–Pb-rich ores was mainly sourced from the host volcanic succession (or from related intrusive rocks), whereas there was a significant component of basement lead in the Cu-rich ores. The difference in sources was interpreted to be the result of deeper penetration of the ore fluids into the basin as the hydrothermal system waxed from lower temperature Zn–Pb-rich mineralization to higher temperature Cu-rich mineralization (Fehn et al. 1983).

The early Carboniferous Neves-Corvo deposit in Portugal is unusual for VHMS deposits in that tin, along with copper, zinc and lead, are present in the ores, raising questions as to the source of the anomalous tin. Lead isotope studies by Relvas et al. (2001), supported by results from the Sm–Nd and Rb–Sr isotopic systems, suggest the presence of two types of lead at Neves Corvo. The ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb plot in Fig. 3c identifies two trends in the isotopic data, a steeper trend that corresponds with other VHMS deposits in the Iberian Pyrite Belt, and a shallower trend characterized by cassiterite analyses. The apparent age associated with the cassiterite trend is unrealistic (~934 Ma). The host rocks to the deposit have a late Strunian age (354.8–354.0 Ma) based on faunal assemblages (Oliveira et al. 2004), and a sulfide-cassiterite Rb–Sr errorchron and a pyrite Re-Os isochron yielded ages of 347 ± 25 and 354 ± 29 Ma, respectively (Relvas et al. 2001; Munhá et al. 2005). Relvas et al. (2001) suggested that the cassiterite Pb–Pb trend reflected the mixing of a much more radiogenic lead source into the mineralizing system possibly from a magmatic source, although such a source is not exposed in the region.

In their synthesis of sediment-hosted Zn–Pb deposits, Leach et al. (2005) documented and compared the lead isotope characteristics of both shale-hosted and MVT Zn–Pb deposits around the world, leading to several important conclusions relating to the origin of lead in these deposits. The most important conclusion was that the lead in both deposit types was derived almost entirely from crustal sources, with little or no mantle input. The authors also noted several important differences between these two major types of sediment-hosted Zn–Pb deposits. They found that most of the large range in lead isotope ratios observed in sediment-hosted deposits is accounted for by MVT deposits. Phanerozoic MVT deposits commonly have future model ages no matter the model used to calculate the age.

Nier (1938) originally recognized the anomalously radiogenic character of many MVT ores, later termed Joplin-type, or J-type, lead by Houtermans (1953) after the Joplin mine in the Tri-State MVT district in the United States. Cannon et al. (1961) found that whereas many North American MVT ores had J-type lead, Many European MVT deposits did not; Vaasjoki and Gulson (1986) noted that Australian MVT deposits had J-type lead. In addition to the radiogenic character of many MVT deposits, Vaasjoki and Gulson (1986) two types of MVT deposits with respect to their lead isotope characteristics: (1) deposits with isotopically homogeneous lead, and (2) deposits with highly heterogeneous lead that define linear arrays on ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb diagrams. Vaasjoki and Gulson (1986) found that the former type of deposits occurs in basins in which the basement is only slightly older than the basin, whereas the latter deposits occur in basins that are significantly younger (to 1300 Myr) than the underlying basement (Fig. 4).

Fig. 4

Reproduced with permission from Vaasjoki and Gulson (1986); Copyright 1986 Soceity of Economic Geologists. Note also change in scale at 1000 Myr

Relationship of excess radiogenic lead (Δ²⁰⁶Pb/²⁰⁴Pb = difference between measured ²⁰⁶Pb/²⁰⁴Pb and average lead calculated using the Cumming and Richards (1975) model) to age difference between crystalline basement and host rocks (ΔT = age of basement—age of host unit). Note the increasing radiogeneity with increasing ΔT for all deposits except Pine Point.

Mechanisms to explain these arrays and their highly radiogenic character are problematic. Cannon et al. (1961) suggested either (1) incremental accumulation of radiogenic lead over a protracted period, or (2) mixing of radiogenic lead with common lead during a short period. The key for both processes is the separation of radiogenic lead from common lead. Chiaradia and Fontboté (2003) demonstrated that radiogenic lead is preferentially leached from rock powders during weak acid attack, whereas common lead is preferentially leached during stronger acid attacks; a similar response has been noted in K-feldspar (cf. Carr et al. 1995). These results should also apply to lead source rocks (Ströbele et al. 2012), hence, radiogenic lead is more easily leached than common lead.

The combination of the Chiaradia and Fontboté (2003) and Ströbele et al. (2012) results may account for the two types of MVT deposits (Vaasjoki and Gulson 1986). Extraction of highly radiogenic lead from old, crystalline basement (i.e. granite and high-grade metamorphic rocks) during non-pervasive alteration of variable intensity would lead to highly heterogeneous lead that characterize type (2) deposits of Vaasjoki and Gulson (1986). If the alteration was more intense and pervasive and affected low-grade metasedimentary and mafic rocks, common lead would be extracted, leading to the homogeneous lead characteristic of type (1) MVT deposits.

5.1.2 Studies of Deposits Where Lead is a Minor or Trace Element

Lead is present as an anomalous minor to trace element in many types of mineral deposits, occurring as galena or substituted into other ore and gangue minerals. As discussed above, although lead isotope data can be used to infer the sources of lead in these deposits, caution must be exercised in inferring the source of ore metals from the lead isotope data. Below case studies of orogenic gold deposits in Western Australia, the Bingham Canyon porphyry copper deposit in Utah (United States) and the Dahu Au-Mo deposit in China are used to illustrate the potential use and pitfalls of lead isotopes to determine metal source when lead is not a major ore component.

The Yilgarn Craton in Western Australia is one of two major Archean orogenic gold provinces, the other being the Abitibi Subprovince in eastern Canada. Resources and production from the Yilgarn Craton total approximately 8 kt contained gold (Robert et al. 2005). Given the controversy at the time regarding the timing of gold mineralization (i.e. syngenetic versus epigenetic) Browning et al. (1987) undertook lead isotope analyses mostly of galena from orogenic gold, VHMS and komatiite-associated Ni-Cu-PGE deposits from the Murchison and Southern Cross Provinces (Youanmi Terrane in current terminology) and Eastern Goldfields Province (Superterrane) to determine relative and absolute ages of mineralization. In the Youanmi Terrane, Browning et al. (1987) estimated model ages of between 3050 and 2970 Ma for VHMS deposits and mostly between 2865 and 2755 Ma for orogenic gold deposits using the evolution model of Cumming and Richards (1975) (Fig. 3d). Although these model ages are 100–200 Myr older than currently accepted ages for these deposits (~2950 Ma for VHMS deposits; Wang et al. 1998) and 2670–2620 Ma for orogenic gold deposits (Robert et al. 2005)), the difference in model ages supported the interpretation that the gold deposits had an epigenetic rather than syngenetic origin. Results from the Eastern Goldfields Superterrane were ambiguous: the isotopic characteristics of the lode gold were not sufficiently different from the VHMS deposits to confidently ascribe a younger, epigenetic origin to the lode gold deposits. The Eastern Goldfields data suggested that there was significant variability in μ, which Browning et al. (1987) ascribed to the presence of old crust in parts of the Eastern Goldfields Superterrane, an inference highlighted by Oversby (1975) and later lead isotope mapping.

In the first study of its type, Pettke et al. (2010) used high precision MC-LA-ICP-MS analysis to determine the lead isotopic composition of well-characterized fluid inclusions from the Bingham Canyon porphyry copper deposit. This study differs from others presented here in that the lead isotope analyses are of the ore fluids. The fluid inclusions analysed, from ore-related quartz, were interpreted as pseudosecondary and were located on planes that contained ore minerals such as molybdenite. These fluid inclusions, which can contain hundreds to thousands ppm Pb, were preferred to analysis of Pb-bearing minerals in the deposit as the latter were considered by Pettke et al. (2010) to be more susceptible to post-depositional isotopic disturbances. The fluid inclusion analyses define a tight group that plots at the more radiogenic end of an array in ²⁰⁸Pb/²⁰⁶Pb versus ²⁰⁷Pb/²⁰⁶Pb space defined by fluid inclusion, K-feldspar and galena data (Fig. 5a). This array trends toward less radiogenic present day values of MORB-source mantle. In ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb space, the fluid inclusion data also define a tight array, that lies between the trend of sub-continental lithospheric mantle defined using data from regional (relative to Bingham Canyon) metasomatized mantle xenoliths and a tight grouping of galena analyses (Fig. 5b). In combination with Monte-Carlo simulations, Pettke et al. (2010) interpreted the data to indicate that the Bingham Canyon magmas and mineralization were derived through the melting of sub-continental lithospheric mantle that was metasomatized during the Paleoproterozoic during north-dipping subduction below the Wyoming Craton. They further suggested that such ancient metasomatized mantle may be the key to formation of major porphyry Cu–Mo and molybdenum deposits in the western United States. The importance of pre-existing metasomatized mantle is increasingly being recognized as a key in forming porphyry copper (Richards 2009) and other deposit types around the world.

Fig. 5

²⁰⁸Pb/²⁰⁶Pb versus ²⁰⁷Pb/²⁰⁶Pb (a) and ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁸Pb/²⁰⁴Pb (b) diagrams showing the lead isotopic composition of fluid inclusions, galena and magmatic K-feldspar from the Bingham Canyon deposit and associated magmatic rocks (modified after Pettke et al. 2010). The field showing the composition of present-day MORB-source mantle (A) and MORB-source mantle evolution (B) are from Kramers and Tolstikhin (1997)

The study of Ni et al. (2012) on the Dahu deposit in China illustrates an evolving trend to use multiple radiogenic isotope systems, in this case U–Th–Pb, Rb–Sr and Sm–Nd, to determine metal and fluid sources. The Dahu Au–Mo deposit is a structurally controlled quartz vein deposit hosted by migmatite and biotite-plagioclase gneiss of the Archean to Paleoproterozoic Taihua Supergroup in the Qinling Orogen of central China. SHRIMP U–Pb analysis of hydrothermal monazite intergrown with molybdenite indicated an age of 218 ± 5 Ma (Li et al. 2011) for the main mineralizing event; an isotopic disturbance towards 125 Ma is also indicated by the data. Initial lead isotope ratios (and ⁸⁷Sr/⁸⁶Sr_218 Ma and ε_Nd, 218 Ma) from the Dahu deposit differ from those of the host Taihua Supergroup (Fig. 6), suggesting that the lead (and Sr and Nd: Fig. 6a), and possibly the gold and molybdenum, were sourced, at least in part, exogenously. The lead isotope data from the ores define a gross trend from more radiogenic host rock towards a less radiogenic source (hypothetical fluid in diagram), particularly on the ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb diagram (Fig. 6b). Based on the three radiogenic isotope systems used, Ni et al. (2012) argued that the lead, strontium and neodymium—and by inference the ore-forming fluid, Au and Mo—were derived from either depleted mantle or a refractory, subducted oceanic slab. The isotopic composition of the ores indicate mixing of this fluid with a wall rock reservoir represented by the Taihua Supergroup.

Fig. 6

Diagrams showing isotopic characteristics of the Dahu deposit, China: (a) ⁸⁷Sr/⁸⁶Sr_218 Ma versus ε_{Nd, 218 Ma} diagram, and (b) ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb diagram (modified after Ni et al. 2012). In both diagrams, isotopic values have been corrected to the time of ore formation (218 Ma); the isotopic compositions of the ores and wall rocks are shown as fields, with individual analyses shown as symbols. Hypothesized ore fluids, prior to mixing with the wall rock reservoir at the site of deposition, are also shown as fields. Part (b) also includes plumbotectonic evolution curves of Zartman and Doe (1981)

5.1.3 Determining Metal Sources with Lead Isotopes—Buyer Beware

Like other isotopic systems and elements used as tracers of geochemical and geological processes a number of factors must be considered in assessing metal sources as determined using lead isotopes. These include the potential that lead isotope ratios can change after mineralization, the possibility that the sources of the lead and other ore metals might be different, and the ambiguity of resolving sources.

For robust interpretation of lead isotope systematics in mineral systems, the signature of the isotopic system must be that at the time of mineralization. As discussed above, lead can be introduced into a mineralized sample by either the addition of common lead during a subsequent mineralizing event or by post-mineralization ingrowth of radiogenic lead from decay of uranium and thorium. When interpreting the lead isotope data as a source tracer, both processes must be considered and, if possible, corrected.

In deposits where lead is recovered, lead isotopic characteristics directly reflect the provenance of an ore metal, but in deposits in which lead is not recovered, an implicit assumption that the source of lead is similar to that of the ore metal (e.g. Cu, Au or Mo in the cases discussed above) is made. As documented by Browning et al. (1987) and McNaughton et al. (1993), and highlighted by Huston et al. (2014), the lead isotopic signatures of orogenic gold deposits in the Eastern Goldfields Superterrane are very provincial, suggesting local crustal sources of lead. Although it is tempting to infer a crustal source for gold based on the lead isotope data, it is possible that the lead and gold had different sources and a crustal gold source cannot be definitively inferred.

The data of Ni et al. (2012) highlight the ambiguity of using lead isotopes to identify metal sources. Notwithstanding the question as to whether the source(s) of gold, molybdenum and lead in the Dahu deposit were the same, the origin of the lead is somewhat ambiguous. As shown in Fig. 6b, the lead isotopic composition of the ore fluid lies between the Zartman and Doe (1981) lower crust and mantle reservoirs, consistent with the possibility that the ore fluids sourced a significant component of their lead from the lower crust in addition to the preferred interpretation of Ni et al. (2012) of a mantle or ocean slab source. Perhaps a more important result to be taken from this type of study is what reservoirs can be excluded as metal sources. In the case of the Dahu deposit, both the upper crust and orogene reservoirs can be excluded as potential sources of lead (and, possibly, Mo and Au), which places important constraints on genetic models.

5.2 Model Ages, μ and the Age of Mineralization

Lead isotope evolution models can be used for a given lead isotope analysis to determine the model age, μ, κ and ω. It must be stressed that these parameters are dependent upon the model used—model ages, μ κ and ω determined using, for example, the Stacey and Kramers (1975) model differ from those determined using the Cumming and Richards (1975) model. Model ages must not be considered equivalent to geochronological ages—there are many processes that affect lead evolution models and, hence model ages. In fact spatial variations in the difference between model ages and independently-established mineralizing ages can be used to infer these processes.

Of parameters produced from evolution models, the model age and μ are most relevant to metallogenic studies. Although the use of locally calibrated lead evolution models can produce model ages that are accurate and precise (e.g. Thorpe et al. 1992; Thorpe 1999; Carr et al. 1995; Sun et al. 1996), a significant proportion (~20% based on the application of the Thorpe (1999) model to volcanic-hosted massive sulfide deposits in the Abitibi Subprovince in Canada) of the model ages can be inaccurate; global models are even less accurate. Model ages should be considered indicative only, and followed up using more reliable techniques to determine the age of mineralization. Model ages are inaccurate for a number of reasons, for example application of an inappropriate model. Another possible reason for inaccurate ages is the use of isotopic ratios that are not initial ratios. Analysis of samples with low U/Pb and Th/Pb ratios (e.g. galena or other Pb-rich minerals, and some whole rocks) yield initial ratios. As a general rule, samples with lead concentrations over 1000 ppm yield initial ratios unless they also have unusually high concentrations of U and/or Th. Leachate analysis of potassium feldspars can also yield initial ratios, but analyses of Pb-poor samples (e.g. most whole rocks and many pyrite) need to be corrected for post-crystallization ingrowth of lead from uranium and thorium decay. This can only happen if the age of lead introduction and U/Pb and Th/Pb are known. If there has been a second lead introduction event, measured ratios, even of Pb-rich material, will not be initial ratios. When undertaking lead isotope studies, initial ratios should be sought, and it is preferable to have multiple analyses of a deposit or rock unit.

5.3 Isotopic Mapping Using Lead Isotope Data

The development of GIS spatial data analysis packages and the availability of large sets of radiogenic analyses has enabled isotopic mapping at regional to continental scales, and maps thus produced have implication not only to metallogenesis, but also to tectonics. Even before the development of GIS analysis, however, many workers had recognized that isotopic data could map tectonic boundaries. For example, Rb–Sr data had been used to identify and map Proterozoic margins both in the Western Cordillera of North America (Armstrong et al. 1977; Kistler and Peterman 1978; Burchfiel and Davis 1981) and in eastern Australia (Webb and McDougall 1968) many decades ago. Bennett and DePaolo (1987) mapped the extent of Proterozoic crust in the Western Cordillera using neodymium isotopes, and Zartman (1974) defined broad lead isotope provinces in the western United States. Wooden and co-workers (Wooden and Aleinikoff 1987; Wooden and Miller 1990; Wooden and DeWitt 1991) used detailed variations in lead isotope ratios in Arizona and California to identify a boundary zone between the Proterozoic provinces that in a broad sense corresponded to boundaries defined by neodymium isotope data (Fig. 7). In addition, Chiaradia et al. (2006) identified distinctive lead isotope provinces and boundaries in the Altaid Orogen of central Asia, and Tessalina et al. (2016) defined systematic decreases in μ from VHMS deposits from west to east across the southern Urals orogenic zone. In the latter study, the higher μ zones are associated with a back-arc zone, whereas the lower μ zones are associated with an island arc. Tosdal et al. (1999) summarized many of the results of earlier studies, illustrating the potential of lead isotope mapping in tectonic and metallogenic studies.

Fig. 7

Map (a) showing lead isotope and neodymium isotope provinces of the western United States (modified after Wooden and DeWitt 1991; Nd provinces from Bennett and DePaolo 1987). The colours in (a) indicate the extent of lead isotope provinces defined by Wooden and deWitt (1991). The heavy brown lines indicate boundaries between neodymium isotope provinces as defined by Bennett and DePaolo (1987); the heavy blue line define the boundary between Central Arizona and Southern Arizona lead isotope subprovinces (Wooden and deWitt 1991). The inset (b) shows spatial variations in ΔJ (Delta Jerome) in the Central Arizona and Mohave lead isotope provinces from (mostly) northwestern Arizona (modified after Duebendorfer et al. 2006). The ΔJ parameter was defined by Wooden and DeWitt (1991) as follows: “In order to emphasize the differences in ²⁰⁷Pb/²⁰⁴Pb among these samples a normalization technique is used. A model 1.70 Ga ²⁰⁷Pb/²⁰⁴Pb—²⁰⁶Pb/²⁰⁴Pb isochron was calculated that passes through the galena lead isotopic data from the United Verde mine at Jerome (²⁰⁶Pb/²⁰⁴Pb = 15.725, ²⁰⁷Pb/²⁰⁴Pb = 15.270, ²⁰⁸Pb/²⁰⁴Pb = 35.344). For any given ²⁰⁶Pb/²⁰⁴Pb measured for a sample a ²⁰⁷Pb/²⁰⁴Pb ratios can be calculated that falls on this model isochron. The ²⁰⁷Pb/²⁰⁴Pb measured from the sample is compared to the calculated value by subtracting the model value from the sample value and multiplying by 100. This derived number is referred to as the Delta Jerome value.”

These studies, however, did not produce maps showing variations in lead isotope ratios and derived parameters, but rather produced either generalized maps showing province boundaries (e.g. Figure 7a), general statements about the location of provinces or broad transects (e.g. Chiaradia et al. 2006). Although results illustrated in this manner produce useful trends, contouring (or mapping) the results on map images can show subtleties not evident using other techniques. Some of the earliest studies that mapped spatial variations in lead isotope parameters include Duebendorfer et al. (2006) and Huston et al. (2005). Duebendorfer et al. (2006) showed that in detail the boundary zone between the Central Arizona and Mojave lead isotope (sub)provinces, originally defined by Wooden and Miller (1990) is relatively complex with a number of discrete zones of more juvenile crust (Fig. 7b) interpreted as rift basins. As shown by Duebendorfer et al. (2006), mapping of derived lead isotope parameters (in this case the ΔJ (delta Jerome) parameter; see caption to Fig. 7 for definition) can identify subtle variations in patterns not shown in the more generalized approach. Moreover, these more subtle variations may have metallogenic and exploration significance, as discussed below. It should be noted, however, that the details mapped out by studies such as Duebendorfer et al. (2006) require a relatively high density of data. It is important to stress that data density has a marked effect on patterns produced by isotopic mapping; these patterns are unreliable in areas of low data density.

Below we describe a number of recent studies in which maps showing variations in parameters derived from lead isotope data have been produced at the province and continental scales. These include studies by Huston et al. (2005, 2014) in the Archean Yilgarn Craton in Western Australia and Abitibi-Wawa Subprovince in Canada, Blichert-Toft et al. (2016) in Europe, and Huston et al. (2016, 2017) in the Tasman Element in eastern Australia. Champion and Huston (2016, 2023) present additional examples of lead (and Nd) isotope mapping.

5.3.1 Lead Isotope Mapping of the Eastern Goldfields Superterrane, Western Australia and Abitibi-Wawa Subprovince, Canada

Huston et al. (2005, 2014) used lead isotope data from Neoarchean orogenic gold and VHMS deposits in the Eastern Goldfields Superterrane and the Abitibi-Wawa Subprovince in Canada to map spatial variations in μ using the Abitibi-Wawa lead isotope evolution model (Thorpe et al. 1992; Thorpe 1999). The results from the Eastern Goldfields Superterrane define an internal, north trending zone of low μ that corresponds to a zone of low T_2DM (T_2DM is the modelled age of extraction from depleted mantle using a two stage neodymium evolution model (see Champion and Huston (2023) for details) mapped using granite neodymium data (Fig. 8; see also Champion and Cassidy 2008; Champion and Huston 2016, 2023). The data also defined a major gradient in T_2DM across the Ida Fault, which separates the Eastern Goldfields Superterrane to the east from the Youanmi Terrane to the west. In the Abitibi-Wawa Subprovince (not shown) the data indicate an east–west trend marked by an internal lower μ zone that grades to higher μ margins both to the north and south. Importantly, the Abitibi-Wawa Subprovince is characterized by significantly lower μ (7.69–7.96) than the Eastern Goldfields Superterrane (8.05–9.05), with only the north-trending low-μ zone (8.05–8.15) in the latter terrane approaching the values observed in the Abitibi-Wawa Subprovince. Huston et al. (2014) interpreted these low μ zones as extensional zones with a greater mantle input, and found that most VHMS deposits were localized within the low-μ zones, commonly along gradients. They found that Archean and Proterozoic VHMS deposits were preferentially associated with juvenile crust as determined from lead and neodymium isotope data, with juvenile zones and provinces having significantly higher endowment than more evolved zones (Fig. 9: Champion and Huston 2016). In contrast, komatiite-associated nickel-sulfide (KANS) deposits are more strongly associated with more evolved crust. Although this relationship is particularly well developed in the Eastern Goldfields Superterrane (Fig. 8), where KANS deposits are common, limited data from the Abitibi-Wawa Subprovince suggests that the few KANS deposits in this juvenile terrane are associated with more evolved signatures. Barrie et al. (1999) found that KANS ores in the Abitibi-Wawa Subprovince have Abitibi-Wawa model μ values of 7.94 (Alexo deposit: Tilton 1983) and 8.28 ± 0.12 (Marbridge deposit: Deloule et al. 1989), much higher than the range in local VHMS ores.

Fig. 8

Variations in T_2DM (a: from granite analyses) and μ (b: from volcanic-hosted massive sulfide and orogenic gold deposits) in the Eastern Goldfields Superterrane (modified after Huston et al. 2014). The location and deposit types of analyses (mostly from Browning et al. 1987) are shown as different symbols. The location of major komatiite-associated nickel sulfide deposits are also shown; data from these deposits were not used in determining variations in μ

Fig. 9

Diagram showing the relationship of the endowment of total base metal (Zn + Pb + Cu) from volcanic-hosted massive sulfide deposits in selected Archean terranes classified according to crustal character (juvenile or evolved) based on lead and neodymium isotope data (modified after Champion and Huston 2016)

More recently, Zametzer et al. (2022) undertook MC-LA-ICP-MS analysis of potassium feldspar from granites along a transect that crossed the boundary between the Eastern Goldfields Superterrane and the Youanmi Terrane. This study used the same granite samples used in the original neodymium T_2DM study (Champion and Cassidy 2008), and demonstrated that like the T_2DM data, parameters derived from lead isotope data, such as μ and ω, also varied across the Ida Fault, with granites from the Youanmi Terrane characterized by more evolved (higher μ) lead isotope signatures relative to the Eastern Goldfields Superterrane. The Zametzer et al. (2022) study suggested that spatial variations seen in neodymium and lead data are genetically linked.

5.3.2 Lead Isotope Mapping of Europe

Blichert-Toft et al. (2016) used ore lead isotope analyses from a range of deposits in Europe to determine model ages and calculate μ and ²³²Th/²⁰⁴Pb values using the Stacey and Kramers (1975) model. The calculated model ages matched the major tectonic cycles in Europe (Svecofennian, Pan-African, Variscan, Caledonian and Alpine: Fig. 10) and were consistent with the ages of major metallogenic epochs in Europe. Maps showing μ and ω (²³²Th/²⁰⁴Pb) values of individual analyses by Blichert-Toft et al. (2016) show systematic patterns, particularly when compared with tectonic subdivisions of Europe. Figure 11 uses the data of Blichert-Toft et al. (2016) to calculated a μ map similar to ones presented for the Eastern Goldfield Superterrane and Abitibi-Wawa Subprovince (see above) and the Tasman Element (see below).

Fig. 10

Histogram of lead model ages (using Stacey and Kramers 1975 model) from Europe showing correspondence with major tectonic cycles (modified after Blichert-Toft et al. 2016)

Fig. 11

Variations in μ, as determined mostly from the analyses of Pb-rich mineralized samples, overlain by European tectonic boundaries (lead isotope data used to construct image from Blichert-Toft et al. 2016; tectonic boundaries from Plant et al. 2005; Lahtinen et al. 2005; Richards 2015; Faure and Farrière 2022). Significant volcanic hosted massive sulfide districts are also indicated

Overall, southern Europe is characterized by higher μ (Fig. 11: Blichert-Toft et al. 2016) than northern Europe, but there are zones within this broad subdivision that correspond to tectonic boundaries and metallogenic domains. In northern Europe, the Caledonian Orogen is characterized by low μ. In the northern British Isles, the boundary between the Caledonian Orogen and Avalonia is marked by a μ gradient with which carbonate-hosted Zn-Pb deposits of the Irish Midlands are strongly associated (Hollis et al. 2019). Avalonia is characterized by moderately high μ values of 9.6–9.9 (Fig. 11). Moderately low values of μ (mostly 9.5–9.6) characterize the northern Variscan Orogen, but in southern France, this tectonic province is dominated by high μ values of 9.7–9.9. This gradient broadly corresponds to the northern boundary of Gondwanan terranes within the Variscan Orogen (c.f. Plant et al. 2005). The Alpine Orogen is dominated by values of 9.7–9.9, though with discrete low-μ zones. μ decreases strongly along the suture between the Alpine and Variscan orogens in central Europe.

In northern Europe, Svecofennia has highly variable μ with discrete zones of both low and high μ. Gradients to lower μ mark the western and northwestern margins, with the Trans-Scandinavian granite-porphyry belt and the Caledonian Orogen, respectively. Within Svecofennia, the Skellfteå, Kaitele and Bothnia domains have lower μ than the Ulmeå. Bergslagen and Tavastia domains. The differing μ characteristics of these terranes is consistent with the involvement of multiple microcontentents in Svecofennia assembly (Lahtinen et al. 2005).

Like Archean and, possibly, Paleoproterozoic provinces globally, Paleoproterozoic and Phanerozoic VHMS districts and provinces in Europe are associated with low μ zones. The Skellefte, Norwegian Caledonide, Pontide and Troodos VHMS provinces, are located in zones with μ < 9.6 (mostly < 9.5: Fig. 11). Even the Iberian Pyrite Belt has μ values lower than domains to the south and west. Hence, the association of VHMS deposits with juvenile (low μ) crustal domains, as noted in the Archean (Huston et al. 2014), is also seen in European Phanerozoic domains.

The relationships of μ gradients to tectonic boundaries and the μ characteristics to tectonic domains suggest that lead isotope evolution in Europe (and elsewhere) reflects, and can be used to map tectonic domains. Moreover, modelling of the evolution of lead in these domains may provide insights into the processes that produced the domains (Blichert-Toft et al. 2016). The data suggest that, metallogenic provinces can be defined using lead isotope characteristics and/or gradients (c.f. Hollis et al. 2019).

5.3.3 Lead Isotope Mapping of the Tasman Element, Eastern Australia

Huston et al. (2016, 2017) mapped the largely Phanerozoic Tasman Element of eastern Australia using a range of parameters derived from lead isotope analyses of Pb-rich samples from mineral deposits and occurrences. This study followed an earlier study of the same area by Carr et al. (1995) that presented much of the data used and an evolution model to account for the isotopic variability. Figure 12a shows that variations in μ reflect province boundaries as well as mineral provinces. For example, a strong gradient in μ occurs along much of the boundary between the Eastern and Central Lachlan. The high-μ Central Lachlan hosts tin deposits of the Wagga tin province, whereas the low-μ zone in the central part of the East Lachlan corresponds very closely with the Macquarie porphyry Cu-Au province. Huston et al. (2017) discuss other patterns present in the μ map of the Tasman Element, and the broad similarity in this map to the T_2DM map constructed using neodymium isotope data from granites (Fig. 12c).

Fig. 12

Modified from diagrams in Champion (2013) and Huston et al. (2017). Province and zone boundaries are from Glen (2013)

Diagrams showing variations in μ (²³⁸U/²⁰⁴Pb) (a), Δt (b) and T_2DM (c) in the Tasman Element and surrounds. Parts (a) and (b) were determined using the Cumming and Richards (1975) lead evolution model.

Lead isotope variations in the Tasman Element cannot be fully explained by the Carr et al. (1995) evolution model, which was developed for the Lachlan Orogen and models the spread in isotopic data as the resulting of the interaction of two lead sources that evolved through time, Lachlan crust and Lachlan mantle. Figure 13a compares the isotopic characteristics of Cambrian (520–490 Ma) deposits and Silurian (435–420 Ma) deposits from the southern Tasman element with the evolution of the two lead sources proposed by Carr et al. (1995). For the Silurian deposits that the model was designed to explain, it works well, with independently-determined ages of mineralization similar to model ages. When expanded outside of the Lachlan Orogen, the model breaks down: the Lachlan crustal curve models the age of Cambrian deposits (Kanmantoo, Eclipse and Cymbric Vale) to the west of the Lachan Orogen well, but model ages of Cambrian deposits in the Mount Read Volcanics, western Tasmania (Rosebery, Que River, Hellyer, Mount Lyell and Henty) are ~200 Myr too young. Some Cambrian deposits along the western edge of the Tasman Element (Mount Ararat and Ponto) yield model ages more than 200 Myr too old using the Lachlan mantle model (Fig. 13a).

Fig. 13

Comparison of lead isotope data from selected deposits from the Tasman Element with lead isotope evolution curves: a Cambrian volcanic-hosted massive sulfide and Silurian porphyry-related deposits with the Lachlan crustal and Lachlan mantle curves of Carr et al. (1995); and b Cambrian volcanic-hosted massive sulfide and related deposits with a four component isotopic evolution model consisting of the two component proto-Pacific system (Lachlan mantle curve of Carr et al. (1995) and contemporaneous crustal curve) and a two component proto-Australia system (Lachlan crustal curve of Carr et al. (1995) and contemporaneous mantle curve). For both the proto-Pacific and the proto-Australian systems, the second curve was calculated by changing the μ associated with original Carr et al. (1995) curve. The dashed lines are mixing lines between the Lachlan crustal and Lachlan mantle curves

These discrepancies can be resolved by expanding the Carr et al. (1995) model so that the Lachlan crust and Lachlan mantle sources are considered parts of two lead isotopic systems. If the μ of the Lachlan mantle curve is increased to values of 10.2, the resulting evolution curve yields model ages that closely match the ages of western Tasmania deposits. Similarly, if the μ of the Lachlan crustal curve is decreased to 13.0, the resulting evolution curve produces ages that match the ages of western Tasman Element deposits (Fig. 13b). Based on these relationships, Huston et al. (2016, 2017) suggested that the formation of the Tasman Element involved the interaction of two isotopic systems that include both crustal and mantle components: one that characterized a proto-Pacific plate that was consumed during largely west-dipping subduction, and a second that characterized pre-existing proto-Australian crust in the over-riding plate. The vast majority of the variability in lead isotopes in the Tasman Element can be effectively explained by mixing between these four sources (Fig. 13b). Geologically, the West Tasmania Terrane forms part of the Selwyn Block (or Vandieland), which is considered an exotic block accreted onto Australia (Cayley 2011), consistent with the model of two separate isotopic systems.

Figure 12b illustrates variations in the parameter Δt, the difference between the age and model age of mineralisation. As the Cumming and Richards (1975) model used to calculate Δt has an evolution similar to that of the Lachlan crustal curve of Carr et al. (1995). The negative to near zero values in the western mainland part of the Tasman Element indicate a dominant proto-Australia influence, whereas strongly positive Δt values, particularly in western Tasmania and the New England Orogen, indicate a strong proto-Pacific influence. Huston et al. (2017) interpreted the low-μ, positive Δt signature of the Macquarie Cu-Au province to reflect the influence of volatiles from the subducting Pacific plate. As both isotopic systems have both low-μ and high-μ components, patterns in Δt are independent of variations in μ: some low-μ zones have positive proto-Pacific Δt signature whereas others have a near-zero to negative proto-Australia Δt signature.

5.3.4 Potential Problems with Lead Isotope Mapping—Some Traps for Young Players

A significant observation from this review is the variety of parameters that have been used to map or model variations in lead isotopes. Many of these parameters (e.g. ΔJ and Δ²⁰⁷Pb/²⁰⁴Pb) are locally defined and, although they work in the local areas in which they were defined, do not have universal application. In addition, in many cases isotope mapping uses parameters derived from local lead evolution models and not more universal models such as Stacey and Kramers (1975) or Cumming and Richards (1975). The use of local models makes direct comparison between maps difficult. It should be stressed, however, that although absolute values of parameters derived from different evolution models differ, the observed patterns should be similar.

Like other geochemical data, the contouring of lead isotope data is strongly influenced by data density. In areas of low data density or no data, commonly used contouring packages can produce artefacts, hence patterns in areas of low data density should regarded critically, and areas with no data masked.

6 Applications of Lead Isotope Data to Exploration

Although not widely recognized as a direct tool, lead isotope data can and has been used in mineral exploration, from the province to the deposit scale. At the province scale, lead isotopes can be used assess the fertility of a particular region for different styles of mineralization. At the district to deposit scales, lead isotope data have many uses including assessing potential deposit type and potential size and defining vectors to ore.

6.1 Inferring Metallogenic Fertility

As discussed above, at the province scale, lead (and similarly Nd) isotopes can indicate fertility for some deposit types. This is best shown for VHMS deposits, where the most fertile provinces occur within isotopically juvenile terranes. In contrast, more fertile provinces for KANS deposits are isotopically more evolved. Although this was first documented for Archean provinces, emerging data suggest the relationship of fertile VHMS provinces with juvenile terranes also applies to younger provinces (see above).

At the large scale, Hollis et al. (2019) and Huston et al. (2020) have documented the association of major Irish-style and shale-hosted massive sulfide deposits, respectively with gradients in μ (and also κ and ω), in the Irish Midlands, the North Australian Zinc Belts and the Northern Cordillera of North America. These gradients are interpreted to mark cratonic edges and are also visible in geophysical datasets (Huston et al. 2020).

6.2 Deposit Type and Size

Since the pioneering work of Cannon et al. (1961), many workers have suggested that lead isotope characteristics can indicate the type and, potentially, the size of mineral deposits/occurrences at an early stage of exploration. Many mineral provinces are characterized by the presence of different types of deposits, and determining deposit type is important to develop and apply exploration models, although limited geological data can make this difficult. As lead isotope data reflect both the source of the lead and the age of mineralization, they can provide an early indication of the origin of mineralization. Below we give examples of the use of lead isotope data to categorize mineral occurrences and gossans at an early stage of exploration. Then we discuss suggestions that lead isotope characteristics can indicate size potential.

6.2.1 Fingerprinting Deposit Types, a Case Study from Western Tasmania

An example of lead isotope fingerprinting during an exploration program was described by Gulson et al. (1987) in western Tasmania, Australia. Western Tasmania, one of the most richly mineralized provinces in Australia, has a range of mineral deposit types, including Cambrian VHMS and related deposits, Ordovician Irish-style or MVT deposits and Devonian granite-related tin deposits. Gulson et al. (1987) showed that major deposits of these groups have different lead isotope signatures (Fig. 14). These signatures were then used to classify and rank mineral occurrences from a range of sample media (steam sediments, soils, gossans/exposure, drill chips and drill core). Another example where lead isotope data have been used to determine the origin of deposits is in the Coeur d’Alene region in Idaho and Montana, United States, where lead isotope data distinguish Mesoproterozoic Cordilleran-type veins deposits from vein deposits associated with Tertiary intrusions (Zartman and Stacey 1971).

Fig. 14

²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb diagram comparing the fields of mineral prospects from the Elliott Bay area with major mineral deposits from western Tasmania, Australia (compiled from Gulson et al. 1987). For this study, which targeted Cambrian VHMS deposits, prospects overlapping with major Cambrian deposits were given priority over prospects overlapping major Devonian deposits

6.2.2 Assessing the Origin of Gossans

An important task during early exploration is determining the origin of surficial ironstones—are they true (i.e. related to mineralisation) or false gossans? Based on data from stratiform (VHMS and shale-hosted) Zn–Pb deposits, Gulson and Mizon (1979) found that true gossans usually had very uniform lead isotope characteristics that plotted close to locally determined growth curves and gave model ages consistent with expected ages. In contrast, false gossans were characterized by variable ratios that yielded model ages significantly younger than the expected ages. Like the deposit fingerprinting described above, lead isotope data have potential for early-stage ranking of surficial gossans, with resulting savings in time and money.

6.2.3 Inferring Potential Size of Deposits

One of the earliest suggested uses of lead isotope data in exploration was the inference that the data could be used to indicate deposit size (e.g. Cannon et al. 1961). Early workers (Cannon et al., 1961; Stacey et al. 1967, Stacey and Nkomo 1968; Zartman and Stacey 1971; Doe 1978; Stacey and Hedlund 1983) found that in intrusion-centred hydrothermal systems, the larger deposits near the causative intrusion tended to have less radiogenic and more homogeneous isotopic signatures than peripheral deposits. The larger deposits formed in the centres of these systems may be dominated by homogeneous, less radiogenic lead associated with the intrusion, with the smaller, peripheral deposits incorporating a larger component of more radiogenic lead from the wall rocks.

Another characteristic that has been used to infer deposit size is the homogeneity of the isotopic data: large deposits have more homogeneous lead isotope ratios than small deposits or occurrences. This relationship has been suggested for a range of deposit types, including orogenic gold (Browning et al. 1987), MVT (Foley et al. 1981) and shale-hosted massive sulfide (Carr et al. 2001; Large et al. 2005) deposits. The homogeneous isotopic character of the large deposits may be the consequence of large hydrothermal systems that homogenize lead isotope signatures, whereas the heterogeneous isotopic character of the small deposits may indicate the predominance of smaller hydrothermal systems tapping different metal sources.

In a critical assessment of the utility of lead isotopic data as indications of deposit size, Gulson (1986) cites several examples of small deposits characterized by uniform lead isotope data. Moreover, in the Zeehan district (Tasmania, Autralia), the largest deposit, at the core of the system, is characterized by a more radiogenic isotopic signature (Huston et al. 2017). Hence, the relationships between deposit size and lead isotope characteristics do not appear to be consistent and have limited exploration utility in greenfields provinces, although they may have utility in brownfield provinces where lead isotope characteristics are better established.

6.3 Vectors to Ore

In addition to discriminating deposit type, lead isotope ratios have also been used as vectors to ore from a range of sample media. For example, in the Snow Lake district, Manitoba, Canada, Bell and Franklin (1993) found that the least radiogenic lead isotope ratios from the fine-grained fractions of till samples were located down ice from the Chisel Lake deposit, suggesting that lead isotope data from tills could be used as vectors in glaciated terranes. In addition, lead isotope ratios may provide vectors toward ore in ground and surface waters in mineralized provinces, as described below. It must be stressed that use of lead isotopes as vectors to ores and as discriminators of deposit type is strongly provincial: the signatures differ according to mineral province and deposit type, so local orientation studies are required for use.

6.3.1 Lead Isotopes in Surface and Groundwater, Restigouche Deposit, New Brunswick, Canada

Leybourne et al. (2009) undertook a combined study documenting the variations in lead isotope compositions of ore samples from the Ordovician Restigouche VHMS deposit in the Bathurst district, New Brunswick. They found that the isotopic composition of ores from this deposit were similar to those of other VHMS deposits in the district but different from volcanic and sedimentary host rocks to the deposit, which were significantly more radiogenic (Fig. 15). They also found that surface and ground water sampled proximal to the deposit had an ore signature, but distal groundwater had more radiogenic lead isotope compositions due to weathering of host volcanic and sedimentary rocks (Fig. 15). Similarly, in the Broken Hill district, New South Wales, de Caritat et al. (2005) found that in the vicinity of deposits groundwater lead isotope compositions reflected the composition of the deposit; moreover, the groundwater data could be used to identify the style of mineralisation.

Fig. 15

²⁰⁶Pb/²⁰⁴Pb versus lead concentration (ppm) diagram comparing the isotopic composition of surface and ground waters from in and around the Restigouche deposit, New Brunswick, Canada (modified after Leybourne et al. 2009)

6.4 Summary and Potential Traps for Young Players

As described above, lead isotopes have a large range of potential uses in exploration, including determining fertility of mineral provinces, identifying crustal boundaries that controls major deposits, fingerprinting deposit (and gossan) types and as direct vectors to ore. Another suggested use is prediction of potential deposit size. Although the criteria to determine deposit size have not lived up to their early potential (e.g. Gulson 1986), it does appear that heterogeneous lead isotope signature is indicative of a small deposit for many (though not all) deposit types (i.e. shale-hosted massive sulfide, VHMS and orogenic gold deposits, for instance, but not MVT deposits).

The use of lead isotopes in exploration at the district to prospect scale is largely empirical, and site or district specific. Although particular patterns might be expected in district and prospect-scale datasets, it is important that orientation studies are undertaken to validate the utility of lead isotopes prior to undertaking major data collection. Like other uses of lead isotopes, it is preferable to determine initial ratios, if possible, when using lead isotopes directly in exploration.

7 Conclusions and Future Developments

As with many other scientific fields, the field of lead isotopes has changed dramatically over the last two decades. Using new analytical techniques such as MC-ICP-MS and MC-LA-ICP-MS, the cost of analyses has dropped, the precision has improved, and types of materials analysed has diversified. These developments, combined with the increasing availability of large datasets and new methods of data representation and analysis (e.g. GIS) has allowed significant advances in the use of lead and other radiogenic isotopes in metallogenesis and exploration. In particular, development of regional lead isotope evolution models has improved the accuracy of model ages, micro-scale analytical techniques and the combination of lead isotope data with other radiogenic and stable isotope systems can better constrain the sources of ore and related metals, and the combination of large datasets and GIS has allowed construction of maps showing variations in lead isotope ratios and derived parameters such as μ and Δt that have both metallogenic and tectonic significance.

In exploration, lead isotopes have many and varied uses. Regional lead isotope (and Nd isotope) mapping can identify zones with higher potential for certain types of deposits. For example, in Archean and Proterozoic terranes, VHMS deposits tend to be more abundant in juvenile crust, which can be mapped using lead (and neodymium) isotopes. In addition, lead isotope characteristics can distinguish between different types of mineralization, for example VHMS versus granite-related tin in western Tasmania or fertile versus barren gossans. There is some suggestion that isotopically homogeneous deposits are larger than isotopically heterogeneous deposits. Finally, lead isotopes can be used as empirical vectors to ore both at the camp to deposit scale, using a range of media ranging from galena in minor occurrences, through groundwater and tills.

The recent developments in analytical techniques and GIS open up a range of applications of lead isotopes to metallogenic, ore and tectonic studies. In particular, high precision laser ablation lead isotope analysis of fluid inclusions (Pettke et al. 2010) offer the opportunity of direct sampling of ore fluids, and, if the fluid inclusions are well constrained, direct insights into the behaviour of lead during fluid flow and ore deposition. The opportunity for microscale sampling of ore minerals presents similar opportunities to those seen with the development of micro-analytical capabilities for other isotopic systems such as oxygen, sulfur and boron (Sharp 1990; Eldridge et al. 1987; Foster et al. 2018; Troll et al. 2019). For example, Gigon et al. (2020) documented apparent micro-scale variations in lead isotopes in galena from the HYC (McArthur River) deposit in the Northern Territory, Australia from SIMS analysis. They interpreted these variations as mixing between two lead sources, but “bulk” high-precision double spike galena analyses did not show the same trends (e.g. Carr et al. 2001); reasons for this dichotomy in results have not been resolved. As spatially resolved micro-analyses of lead isotopes in a range of media become more widely available, new insights will emerge.

However, the opportunities for advancement extend well beyond microanalytical studies. In the regional context isotopic mapping of thorogenic lead and combination of the U–Th–Pb isotopic system with other radiogenic isotope systems such as Sm–Nd and Re–Os has potential to provide new insights into the role of different parts of the crust and mantle in metallogenesis and tectonics. For example, the combination of uranogenic and thorogenic lead may constrain the role of the lower to middle crust in metallogenesis. Radiogenic isotope studies from the continent- to fluid inclusion-scales offer the potential to understand province to ore-shoot controls on ore formation, data that will assist in developing genetic and exploration models, both for area selection and vectoring.