The ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os Radiogenic Isotope Systems: Techniques and Applications to Metallogenic Systems

Abstract

Rhenium, Os, and Pt are redox sensitive elements that are concentrated in highly reducing environments such as those associated with black shales but mobile under more oxidizing conditions such as those associated with arc volcanism. They are chalcophile in many terrestrial ore-forming environments, and their isotopic systematics provide unique opportunities to date the formation of sulfide ore deposits and understand their petrogenesis. Fractional crystallization of magmatic sulfide ores generates primary variations in Re/Os and Pt/Os that allow mineral and whole rock isochron ages to be determined and discrimination of crustal and mantle sources based on initial Os isotopic compositions. Molybdenite is especially well suited for geochronology due to its high Re/Os and resistance to resetting. Rhenium concentrations in molybdenite tend to reflect the composition or provenance of the ore-forming fluids, with higher concentrations associated with more primitive sources or more oxidized fluids and lower concentrations with more evolved and/or reduced conditions, although local and regional factors also have a significant influence. Many studies have used pyrite for dating but its typically low Re concentration, variable initial Os isotopic composition (reflecting fluid mxing), and susceptibility to re-equilibration makes its use as a geochronometer problematic in many cases. Other sulfide minerals such as bornite and arsenopyrite have shown promise for Re–Os isotope geochronology but additional studies are needed to evaluate their broader applicability for dating of ore deposits. The isobaric beta decay of parent isotope ¹⁸⁷Re to ¹⁸⁷Os has restricted investigation of this system by microbeam techniques such as ion microprobe or laser ablation mass spectrometry, especially for geochronology. This requires either chemically processing the sample to separate the elements or novel techniques such as collision-cells that preferentially ionize the Re and Os during the analysis. Thermal ionization mass spectrometry (TIMS) and inductively-coupled plasma mass spectrometry (ICPMS) are the most widely applied techniques for Re-Pt-Os isotopic analyses. Specialized techniques for sample digestion to ensure redox equilibrium between Os in the sample and the isotopically enriched spikes used for isotope dilution measurements are typically required. This chapter briefly reviews development of the ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os isotopic systems for earth science, physico-chemical controls on their behavior in ore-forming environments, and applications to metallogenic systems.

1 Introduction

The ¹⁸⁷Re-¹⁸⁷Os isotopic system has a number of characteristics that distinguish it from other commonly applied radiogenic isotopic systems and make it especially useful for ore deposit research. The system is based on the long-lived β-decay of ¹⁸⁷Re to ¹⁸⁷Os. Both Re and Os tend to be chalcophile or siderophile when sulfide or metallic melts are present, leading to pronounced enrichments of these elements in magmatic sulfide deposits. During partial melting of the Earth’s mantle, Os behaves compatibly and is retained in the depleted residue. In contrast, Re is moderately incompatible and is progressively extracted into the crust. This fractionation of Re/Os between the crust and depleted mantle has produced large variations in the proportion of ¹⁸⁷Os (expressed as ¹⁸⁷Os/¹⁸⁸Os) in various reservoirs through time, with the crust evolving to highly radiogenic values (high Re/Os and ¹⁸⁷Os/¹⁸⁸Os) and strongly depleted mantle evolving to less radiogenic compositions (Fig. 1).

Fig. 1

Time versus ¹⁸⁷Os/¹⁸⁸Os ratios for various reservoirs in the Earth. The line labeled ‘primitive mantle’ is representative of much of the Earth’s fertile convecting mantle through time. The black box shows representative compositions of modern mid-ocean ridge basalts (MORB) and ocean islnd basalts (OIB). Recently erupted island arc basalts (IAB) have ¹⁸⁷Os/¹⁸⁸Os compositions that range from MORB-like to highly radiogenic values. ‘Crust’ includes basaltic and continental crust. The stippled field is occupied primarily by strongly depleted lithospheric mantle. The dashed line illustrates Re depletion model ages (T_RD)

Osmium isotopes are therefore sensitive tracers of the relative proportions of crustal and mantle inputs to ore systems, as well as being useful for geochronology. In addition, both elements are redox sensitive, being immobilized in organic-rich reducing environments such as those associated with black shales, but mobile in more oxidizing environments such as those associated with arc volcanism and near-surface magmatic outgassing. As a consequence of these diverse characteristics, Os isotopes have been applied to a wide variety of problems in earth science. However, both elements occur in very low concentrations in most common rock types (ppb or less), making the analysis and broader application of the Re–Os system a challenging task. This chapter briefly reviews aspects of the ¹⁸⁷Re-¹⁸⁷Os system that are especially relevant to ore deposits, including sulfide-related geochronology and source tracing. The ¹⁹⁰Pt-¹⁸⁶Os decay scheme is mentioned, although it has so far found relatively limited applications to ore systems (Sun et al. 2003d). The articles cited here are intended to be illustrative rather than a comprehensive review of the literature. Reviews of the Re–Os isotopic system of particular relevance to ore deposits have been presented by Shirey and Walker (1998), Lambert et al. (1999), Chesley (1999), Carlson (2005), Stein (2014), and Stein and Hannah (2015). Barnes and Ripley (2016) review the application of Re–Os isotopes to magmatic sulfide deposits, and considerable relevant information is provided by other chapters in that volume.

2 Background

Osmium has seven naturally occurring isotopes (Table 1; Rosman and Taylor 1998), all of which can be considered stable (¹⁸⁶Os decays to ¹⁸²W with a half-life of 2 × 10¹⁵ years; Arblaster 2004). The minor isotopes ¹⁸⁶Os and ¹⁸⁷Os are radioactive decay products of ¹⁹⁰Pt and ¹⁸⁷Re, respectively, and these isotopic systems provide the basis for geochronology and source tracing as applied to ore deposits and related systems. Naldrett and Libby (1948) established the weak radioactivity of rhenium and showed that it was a beta decay of ¹⁸⁷Re to ¹⁸⁷Os. Hintenberger et al. (1954) showed that the osmium in a molybdenite was ≥ 99.5% radiogenic ¹⁸⁷Os, in contrast to the ≤ 2% in common Os, thereby establishing the ¹⁸⁷Re-¹⁸⁷Os system as a possible geochronometer. Subsequent studies showed that geologically plausible ages could be obtained from some molybdenites (Herr et al. 1968) but the methods then available were hindered by poor sensitivity, the resulting requirement for very large samples (1–10 g), and uncertainties in both the isotopic measurements and the ¹⁸⁷Re half-life. Isotope dilution measurements of Re concentrations in a large suite of Australian molybdenites (Riley 1967) demonstrated an extremely wide range of Re concentrations (0.25–1690 ppm), and neutron activation analyses of Australian molybdenites by Morgan et al. (1968) confirmed their high Re/non-radiogenic Os ratios and pointed to unusually low Re concentrations of molybdenites from Tasmania compared to those from mainland Australia.

Table 1 The naturally occurring isotopes of osmium

The modern era for application of Re–Os isotopes to source tracing and ore deposit geochronology began with Allègre and Luck (1980) and Luck and Allègre (1982). Allègre and Luck (1980) established the Os isotopic evolution trend of the mantle based on compositions of samples of osmiridium with a range of assumed ages, and they showed that the much of the mantle apparently has evolved with a near-chondritic Re/Os since the origin of the Earth. This was surprising considering the complex history of mantle dynamics that was already known from Sr, Nd, and Pb isotopes, and it provided the basis for calculating mantle evolution model ages based on initial Os isotopic compositions. Luck and Allègre (1982) showed that geologically reasonable dates with useful precision could be obtained from molybdenite with a wide range of Re concentrations, although a few exceptions were noted.

3 The ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os Isotope Systems

The ¹⁸⁷Re decay constant has been constrained by both direct counting and calibration against other geochronometers (reviewed by Selby et al. 2007a). Direct counting experiments have yielded uncertainties of several percent. The most widely used value in geosciences is 1.666 × 10^–11 y⁻¹, based on a best-fit for iron meteorites with an assumed age of 4557.8 ± 0.4 Ma (Smoliar et al. 1996). Selby et al. (2007a) re-evaluated the λ¹⁸⁷Re based on a comparison of molybdenite ages against U–Pb ages of magmatic zircons that could reasonably be considered as coeval with the molybdenite. They proposed a value of 1.6668 ± 0.0034 × 10^–11 y⁻¹, within uncertainty of the Smoliar et al. (1996) value. Begemann et al. (2001) also reviewed the half-lives for commonly applied isotopic geochronometers.

Macfarlane and Kohman (1961) established ¹⁹⁰Pt as an alpha emitter using enriched isotopes. Walker et al. (1997) demonstrated the potential of the ¹⁹⁰Pt-¹⁸⁶Os system for dating platinum-group mineralization through a study of ores from the Noril’sk region of the Siberian flood basalts. Begemann et al. (2001), Cook et al. (2004), and Tavares et al. (2006) reviewed the status of the ¹⁹⁰Pt decay constant and concluded that there are significant discrepancies between values obtained by direct counting and theoretical approaches compared to calibrations against other isotopic geochronometers such as ¹⁸⁷Re-¹⁸⁷Os and U–Pb, and that these discrepancies are much larger than the precision of ~1–2% typically obtained by the latter. Begemann et al. (2001) recalculated the λ¹⁹⁰Pt proposed by Walker et al. (1997) to a value of 1.477 × 10^–12 y⁻¹ in order to account for a revised atomic percentage of ¹⁹⁰Pt. Cook et al. (2004) found that a value of 1.41 × 10^–12 y⁻¹ with an uncertainty of ± 1–2% provides the best match between ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os isochron ages of magmatic iron meteorites. Additional work in this area is needed to refine λ¹⁹⁰Pt and account for the discrepancy between the geological versus experimental and theoretical calibrations.

4 Analytical Methods

Reisberg and Meisel (2002) and Meisel et al (2003) provide comprehensive reviews of analytical techniques used for Re–Os isotopic analysis. See also Walker and Fassett (1986) and Shirey and Walker (1998) for useful summaries of historical developments. Meisel and Horan (2016) provide a recent review of analytical methods for platinum-group elements more generally that includes information about Re, Os, and Pt. Here we provide a brief review of the development of analytical techniques for Re–Os isotopic analysis.

4.1 Instrumental Techniques

As the parent and daughter isotopes both have effectively the same mass, either chemical separations or differential ionization of the Re and Os are essential for geochronology. This isobaric relationship effectively rules out commonly available microbeam techniques such as ion microprobe or laser ablation ICPMS for in-situ dating of high Re/Os phases such as molybdenite. Those techniques have, however, been applied to source tracing of osmiridiums and other PGE-bearing minerals with low Re/Os ratios (Allègre and Luck 1980; Hart and Kinloch 1989; Hattori and Hart 1991; Meibom et al. 2002, 2004; Hirata et al. 1998; Pearson et al. 2002; Ahmed et al. 2006). Hogmalm et al. (2019) presented a novel technique using collision cells and ICP tandem mass spectrometry for the on-line separation of Re from Os during laser ablation analyses of molybdenite, but the method has so far not been widely applied to ore deposit geochronology.

The low concentrations and high ionization potential of both Re (7.9 eV) and Os (8.7 eV) presented particular challenges to the development of geochemically useful techniques. A recurring theme in the development of Os isotopic analysis is to utilize the volatile character of the compound OsO₄ (osmium tetroxide). The boiling point of OsO₄ is ~130 °C but it can be distilled at much lower temperatures. It is also highly poisonous and reacts rapidly with organics such as those comprising various body parts of laboratory analysts. Early studies ionized OsO₄ by electron bombardment for introduction into a magnetic sector mass spectrometer (Nier 1937). Herr et al. (1968) applied this technique to measure ¹⁸⁷Os in molybdenites for Re–Os dating, with Os concentrations quantified by isotope dilution using a ¹⁹⁰Os spike and Re concentrations measured by neutron activation (e.g., Morgan 1965). These pioneering techniques provided reasonable precision (~3% for ¹⁸⁷Os/¹⁹²Os quoted by Nier 1937) but they required large sample sizes (e.g., 1–10 g) and ppm quantities of Os for the measurement.

A significant advance was the development of techniques for Re–Os isotopic analysis by secondary ionization mass spectrometry (SIMS or ion microprobe), either in situ for phases such as osmiridium or following chemical separations for whole rock analysis (Luck et al. 1980; Luck and Allègre 1983). They obtained precision on the isotope ratios of ≤ 1% relative and substantially reduced the amount of Os needed for analysis. This was an important development because it revitalized interest in Re–Os as a geochemically important isotopic system. Luck and Allègre (1982) applied a modified version of this technique to determine ¹⁸⁷Re/¹⁸⁷Os ages of a suite of molybdenite samples. Building on this success and a variety of technological developments, several innovative techniques for Os isotopic analysis were trialled. These included laser ionization mass spectrometry (Englert and Herpers 1980; Simons 1983), resonance ionization mass spectrometry (Walker and Fassett 1986; Blum et al. 1990), inductively-coupled plasma mass spectrometry (Russ et al. 1987; Dickin et al. 1988; Richardson et al. 1989) and accelerator mass spectrometry (Fehn et al. 1986; Sie et al. 2002). Each of these instruments had certain advantages, but all of them became obsolete after development of the negative-ion thermal ionization technique (N-TIMS) described below. Notable here, however, is the concept of introducing OsO₄ gas directly into an inductively-coupled plasma mass spectrometer (ICP-MS) as described by Russ et al. (1987). That step has been revived for some applications to ore deposit geochronology, as also discussed below.

Thermal ionization mass spectrometry is based on the formation of ions of the elements or oxides during desorption from a hot filament, followed by focussing of the ions into a beam by electrostatic lenses and separation of the isotopes according to their mass/charge by an electromagnet or mass analyzer. The ion current for each mass is then measured by some type of detector, typically either an electron multiplier or a Faraday cup. Isotope ratios can be measured using a single collector by cycling the magnet to focus each mass in the detector, or all masses of interest can be collected simultaneously using a multi-collector array. Multi-collection provides better precision and faster analysis times, and is considered as state-of-the-art for high-precision isotope ratio analysis. Conventional isotope ratio analyses of elements like Sr, Nd, and Pb are based on creating positive ions but the high ionization potential of Os makes this impractical. In contrast, Os readily forms negative ions such as OsO₃⁻ at relatively low T (800–900 °C) when loaded on the filament with a Ba or La activator (Völkening et al. 1991; Creaser et al. 1991; Walczyk et al. 1991). Analytical precisions of < 0.01% on isotope ratios can be obtained from very small amounts (few ng) of Os by N-TIMS using multiple collection. A potential disadvantage to N-TIMS is the need to obtain highly purified aliquots of Os and Re for analysis. Reisberg and Meisel (2002) provide additional advice and details regarding analytical conditions.

An alternative approach is ionization via an inductively-coupled plasma (ICP). Argon plasmas have a first ionization potential of 15.8 eV compared to 8.7 eV for Os and 7.9 eV for Re, so Re and Os will be efficiently ionized. For isotopic analysis, ICP ionization sources are coupled with either a quadrupole mass analyser or a magnetic sector mass spectrometer. Sample introduction can be by solution aspiration via a nebulizer, a dry gas stream produced by laser ablation, or vapor phase sample introduction. Osmium is prone to memory effects during solution aspiration and chemical procedures have been developed to offset this (Schoenberg et al. 2000; Nowell et al. 2008a). In contrast, vapour phase introduction of OsO₄, also known as sparging, produces better intensities and fewer memory effects as well as providing an efficient separation of ¹⁸⁷Os from ¹⁸⁷Re without additional chemistry (Russ et al. 1987; Richardson et al. 1989; Gregoire 1990). Hassler et al. (2000) further developed the use of sparging for Os isotopic analysis by single-collector magnetic ICP-MS and obtained a precision similar to that of Russ et al. (1987).

A significant technological advance was the commercial availability of multi-collector magnetic sector ICPMS instruments (MC-ICPMS) as these provided the efficient ionization of an ICP with higher measurement precision provided by multiple Faraday cup collector arrays. Schoenberg et al. (2000), Norman et al. (2002), and Nozaki et al. (2012) describe sparging experiments for Os isotopic analysis using MC-ICPMS. Because the ICP effectively decomposes the oxide molecules, these measurements are made on the nominal masses of the element, rather than the oxides as for N-TIMS. While N-TIMS remains the method of choice for low-level Os isotopic analysis, MC-ICPMS can be useful for certain applications. For example, molybdenite samples from a variety of settings have been dated using a combination of sparging and solution aspiration for isotope dilution determinations of ¹⁸⁷Os and Re concentrations, respectively (Norman et al. 2004a; Armistead et al. 2017; Kemp et al. 2020; Wells et al. 2021). Magnetic sector ICP-MS instruments can be also be configured with multiple electron multiplier array detectors for measurement of less intense beams (Birck et al. 2016; Zhu et al. 2019).

Microbeam techniques such as electron microprobe, laser ablation ICPMS, and secondary ionization mass spectrometry (SIMS) have been used to examine the spatial distribution of Re and Os within individual grains and have found that many grains are remarkably heterogeneous (Zaccarini et al. 2014; Barra et al. 2017; Hnatyshin et al. 2020; Wells et al. 2021).

4.2 Chemical Procedures

As both Re and Os are typically trace constituents of geological materials (osmiridium and rheniite being notable exceptions), chemical methods for concentrating these elements from appropriate volumes of samples are usually necessary for the isotopic analysis. This requires recipes for dissolving or digesting the sample without losing volatile Os compounds or incompletely attacking potentially important phases like spinels. Because OsO₄ is volatile at low T, samples cannot be simply dissolved on a hotplate using oxidizing acid solutions such as HF-HNO₃, as commonly done for Rb–Sr or Sm–Nd isotopes. For geochronology and age-corrections, it is necessary to ensure complete equilibration between the enriched isotope spikes and natural Re and Os from the sample; this is especially challenging for Os due to its multiple oxidation states and different behaviours when analysed in oxidized versus reduced forms.

The most widely applied method of sample digestion for Re–Os isotopic measurements uses Carius tubes and mixed HNO₃–HCl acids in proportions 2:1 (inverse aqua regia). Carius tubes are thick-walled borosilicate glass tubes, sealed at both ends by a glassblower (Shirey and Walker 1995). When heated to temperatures of 240–260 °C, they generate large internal pressures and conditions that ensure spike-sample equilibration through complete oxidation of the Os and Re. An advantage of this method is that the Os (as OsO₄) can be sparged or distilled directly from the resulting solution. A disadvantage is that sample size typically is limited compared to other methods of decomposition such as fire assay. For example, < 0.5 g aliquots of sulfides are commonly used (e.g., Selby et al. 2009; Morelli et al. 2010; Hnaytshin et al. 2020) due in part to practical limits on the volume of acid required to fully oxidize the spike and sample (Frei et al. 1998), whereas 1–3 g of silicate samples can be digested by this technique (Shirey and Walker 1995; Meisel et al. 2001a). A variant of the technique is the high-pressure asher, which achieves even higher T and internal pressures resulting in faster and more complete sample dissolution (Meisel et al. 2001b). For most samples encountered in ore deposit studies, however, conventional Carius tube digestion seems adequate.

Rhenium-Os geochronology and isotope tracing can also be applied to sediment-hosted deposits and hydrocarbon systems through analysis of black shales and associated organics (e.g., reviews by Stein and Hannah 2015; Zeng et al. 2014; Gao et al. 2020), but these kinds of studies require specialized techniques to disentangle hydrogenous components associated with the organic phases from mixtures with detrital components. Selby and Creaser (2003) showed that the hydrogenous Re and Os can be extracted selectively from black shales using a carius tube digestion with CrO₃–H₂SO₄ instead of reverse aqua regia, and procedures for dating petroleum using extracted organic fractions were developed by Selby et al. (2007b), Sen and Peucker-Erinbrink (2014) and Georgiev et al. (2016). Economic applications of these techniques have been primarily to hydrocarbon systems (e.g., Cumming et al. 2014; Liu et al. 2019; DiMarzio et al. 2018; Meng et al. 2021; Li et al. 2021; Georgiev et al. 2021) although they appear to have considerable potential for application to sediment-hosted metallogenic systems as well, especially those associated with organic matter (e.g., Saintilan et al. 2019).

Two other approaches to sample digestion that have been applied to ore deposit studies are alkali fusion (Morgan and Walker 1989; Markey et al. 1998) and NiS fire assay (Hoffmann et al. 1978; Fehn et al. 1986; Ravizza and Pyle 1997; Savard et al. 2010). While the alkali fusion method can produce accurate and precise results, it is a complex, multi-step procedure that can be prone to incomplete spike-sample equilibration, incomplete dissolution of the sample, and higher procedural blanks compared to Carius tube digestion. In addition, the resulting fusion cake must be dissolved for separation of the Os and Re, and the procedure for this does not appear to be suitable for direct analysis. The NiS fire assay can cope with large samples (> 10 g), thereby overcoming nugget effects and sample heterogeneity, but yields and procedural blanks can be variable. Although Re can be poorly recovered by NiS fire assay, good results for isotope dilution measurements for Re by this method have been reported (Ravizza et al. 1991; Park et al. 2013). Maintaining a reducing environment during the fusion appears to be a key step to obtaining useful Re data by this method (Reisberg and Meisel 2002).

For N-TIMS analysis, it is necessary to obtain pure aliquots of Re and Os. Purification of the Os is a moderately complex process involving solvent extractions and microdistillation, the latter specifically designed for Os purification utilising the volatile nature of OsO₄ (Birck et al. 1997; Morgan et al. 1991; Cohen and Waters 1996; Shirey and Walker 1998; Reisberg and Meisel 2002). In this respect, sparging the OsO₄ directly into an ICP mass spectrometer is a more efficient approach as it provides a chemical separation of Os from the Re and matrix elements and an isotopic analysis in a single step, provided that the samples are appropriate. Purification of the Re is more straightforward as it is not volatile under ambient conditions and can be separated from matrix elements using conventional anion exchange chromatography.

4.3 Data Handling

Thermal ionization and ICP mass spectrometers rarely measure exactly the correct isotope ratio for any element. Thermal ionization is based on evaporation of a finite mass of the element of interest, which tends to produce a Raleigh distillation effect as light masses evaporate preferentially to heavier masses. This often causes the measured isotopic composition to change with time during a run, or to be offset from the actual values. The physics behind this type of process is reasonably well understood and can be corrected for by assuming a nominal ratio of two stable isotopes in the analyte and some form of the distillation law, typically exponential (Habfast 1983, 1998; Andreasen and Sharma 2009). For Os isotopic analysis by N-TIMS, the fractionation correction uses the oxide equivalent masses of either ¹⁹²Os/¹⁸⁸Os or, less commonly, ¹⁹⁰Os/¹⁹²Os. This method also requires corrections for oxygen isotopic compositions. In contrast to TIMS, ICPMS isotopic analysis usually relies on consumption of a constant composition sample, either as a solution or bulk extraction of a homogeneous phase as for sparging. In this case, the composition of the sample does not change during the analysis. However, ICP mass spectrometers typically measure isotopic compositions that are offset from the actual value by factors of ~1% and that vary as a regular function of mass (Albarede et al. 2004). The physical processes that are responsible for this mass bias are less well understood (e.g., Andrén et al. 2004), but corrections assuming fractionation laws similar to those used for TIMS appear to be adequate for most applications.

If the sample contains a sufficient amount of common Os, then the fractionation or mass bias corrections can be made based on the isotopes intrinsic to the sample. In this case, concentrations can be determined by spiking the sample with a single enriched isotope such as ¹⁹⁰Os for a conventional isotope dilution measurement. Alternatively, for samples with negligible amounts of initial Os, such as molybdenite, the intrinsic Os in the sample will be essentially all radiogenic and therefore monoisotopic (¹⁸⁷Os). In this case, the sample can be spiked with a well-calibrated solution of common Os or with a specially prepared double spike containing, for example, ¹⁸⁸Os and ¹⁹⁰Os in known proportions. Either approach allows both mass bias corrections and concentrations to be determined by isotope dilution. The advantage of the double spike method is that it allows a monitor of any initial Os in the sample using other masses, which can be useful if the sample is young or contains low concentrations of Re (Markey et al. 2003). A potential complication is mass-independent isotopic fractionation during MC-ICPMS analysis. Zhu et al. (2018) measured a series of Os standard solutions and suggested biases both positive and negative of up to ~0.5% on ¹⁸⁷Os relative to ¹⁸⁸Os and ¹⁹⁰Os. However, such effects have not been found in other studies (Nanne et al. 2017) and the topic requires further investigation with particular attention to potential interferences (Birck et al. 2016). Fractionation or mass bias corrections for Re are more difficult because it has only 2 isotopes (mass 185 and 187). In this case, the corrections can be applied by normalization to a reference solution run under the same conditions as the samples (also known as “standard-sample bracketing”), or by spiking the sample with another element of known isotopic composition and using that element for the fractionation or mass bias corrections. For example, Miller et al. (2009) used W for mass bias corrections of Re isotopes analysed by MC-ICPMS.

Modern geochemical studies typically report Os isotopic data as ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os ratios as these reflect the time-integrated parent/daughter ratios. The ¹⁸⁷Os/¹⁸⁸Os ratios, either measured or age-corrected to initial values, are often reported as the quantity γ¹⁸⁷Os (gamma), which is the percent deviation of the data from the chondritic evolution curve. In contrast, ¹⁸⁶Os/¹⁸⁸Os ratios are reported as ε¹⁸⁶Os (epsilon), or deviations from chondritic in parts per 10,000. As discussed below, this is useful because the Earth’s mantle appears to have evolved with near-chondritic Re/Os and Pt/Os since at least 3.8 Ga (Bennett et al. 2002). A source of potential confusion was the reporting of Os isotopic data as ¹⁸⁷Os/¹⁸⁶Os ratios in some of the early literature. This practice was discontinued once high-precision measurements demonstrated variations in ¹⁸⁶Os due to decay of ¹⁹⁰Pt.

5 Geochemical and Mineral Systems: Controls on Re–Os Behaviour in Magmatic and Hydrothermal Environments

5.1 Silicate Mineral and Oxide Partitioning

The partitioning behavior of Re, Os and other PGE during mantle melting is controlled primarily by the presence or absence of sulfide in the source and to a lesser extent by partitioning into other silicate and oxides phases (Brenan et al. 2016). Rhenium becomes increasingly incompatible under more oxidizing conditions, such as those applicable to volcanic arcs, due to the larger fractions of sulphate relative to sulfide, and Re⁶⁺ relative to Re⁴⁺ (Brenan et al. 2003; Fonseca et al. 2007; Mallmann and O’Neill 2007). Osmium is only sparingly soluble in basaltic melts under all conditions, with partitioning into alloys and spinel contributing to its retention in the mantle even after sulfide is exhausted. A long-standing observation is the high concentrations of Os, Ir, and other refractory PGE and low concentrations of Re in Cr-spinels relative to silicates (Page and Talkigton 1984; Zhou et al. 1998; Park et al. 2012; Lorand and Luget 2016). This appears to reflect a combination of mineral-melt partitioning, and preferential inclusion of platinum-group minerals such as laurite-erlichmanite (RuS₂–OsS₂) into the spinel, possibly due to small-scale redox gradients at the crystal-melt interface during melting (Brenan et al. 2016; Lorand and Luget 2016; O’Driscoll and Gonzalez-Jimenez 2016). As Cr-spinel is resistant against hydrothermal alteration, it has been used to recover the initial Os isotopic compositions of serpentinites, greenstones, and metamorphosed ultramafic rocks where the isotopic composition of the whole rock has been compromised (Bennett et al. 2002; Standish et al. 2002; Walker et al 2002).

Osmium is largely incompatible in silicate minerals (Brenan et al. 2016), but it follows other compatible elements such as MgO, Ni, and Cr during basalt fractionation (Hart and Ravizza 1996). This leads to very low concentrations (≤ 0.01 ppb) in moderately evolved basalts, making them prone to compositional modification by crustal contamination or alteration (Lassiter and Luhr 2001; Gannoun et al. 2007, 2016; Pitcher et al 2009). As noted above, the compatibility of Os seems to reflect a physico-chemical association with Cr-spinel, although the exact mechanism responsible for this association remains somewhat obscure (Lorand and Luget 2016; Gannoun et al. 2016).

As an incompatible element, Re concentrations tend to increase during fractional crystallization of mafic and intermediate magmas (Righter et al. 2008; Pitcher et al. 2009; Li 2014). It can, however, be lost from these magmas as a volatile species during near-surface degassing (Lassiter 2003; Sun et al. 2003a, b, c, 2004; Norman et al. 2004b; Righter et al. 2008; Pitcher et al. 2009). Failure to recognize Re loss due to outgassing could lead to incorrect inferences regarding source composition and sulfide saturation (Hauri and Hart 1997; Bennett et al. 2000). In more evolved magmas, Re can be compatible in FeTi oxides, probably due to substitution of Re⁴⁺ for Ti⁴⁺ (Righter et al. 1998; Mallmann and O’Neill 2007; Li 2014; Park et al. 2013). Righter et al. (1998) estimated magnetite/melt partition coefficients of 20–50 in the absence of sulfide, plausibly accounting for the systematic decrease in Re contents of evolved arc magmas (Li 2014). Crystallization of magnetite can also promote fluid exsolution (Sun et al. 2004) or the saturation of Cu-Au sulfides from oxidized arc magmas through conversion of sulfate in the melt to sulfide via the reaction: SO₄²⁻ + 8Fe²⁺O = S²⁻ + 8Fe³⁺O_1.5, a process that has been referred to as the ‘magnetite crisis’ (Jenner et al. 2010).

5.2 Sulfide Melt—Silicate Melt Partitioning

The siderophile behaviour of Re, Os, and Pt is clearly demonstrated by partitioning experiments that yield liquid metal/liquid silicate partition coefficients of 10⁹ to 10¹² at 1200–1400 °C and fO₂ = IW (regression of Brenan et al. 2016). Temperature and redox (fO₂) appear to be the primary parameters that control metal-silicate partitioning, with silicate melt composition as a secondary control and pressure not very important. Metal-silicate partitioning is applicable primarily to planetary scale processes such as core formation.

In crustal magmatic environments, sulfide melt/silicate melt immiscibility and solid sulfide/liquid sulfide partitioning is more relevant. The partitioning of Re, Os, Pt, and other platinum-group elements (PGE) is complicated, however, because fS₂, fO₂, and silicate melt composition all exert significant controls on the oxidation states of the elements as well as the composition and stability of the sulfide phase. For example, Re and Os become increasingly soluble in silicate melts with increasing fO₂, consistent with predominantly Re⁶⁺ and Os⁴⁺ species in most terrestrial magmas (Brenan et al. 2016). In contrast, a variety of Pt species may be present in silicate melts, including Pt⁰, Pt²⁺, and Pt⁴⁺ with increasing fO₂ (Brenan et al. 2016). The solubility of Pt appears to decrease with increasing SiO₂ in the melt (Borisov and Danyushevsky 2011; Brenan et al. 2016) but similar experiments have not been done for Re and Os.

Experimental measurements of sulfide melt/silicate melt partition coefficients (D_SUL/SIL) for Re have demonstrated a strong influence on relative fO₂ and fS₂ (Fonseca et al. 2007; Brenan 2008; Brenan et al. 2016). In relatively reducing and sulphur-rich magmas, possibly relevant to MORB, Re is chalcophile with D_SUL/SIL on the order of ~10²–10³, whereas in more oxidizing conditions it appears to be lithophile with D_SUL/SIL <<1. Brenan et al. (2016) describe this as an exchange reaction between oxide and sulfide species:

$${{{\text{Re}}_{\text{x}}}} {{{\text{O}}_{\text{y(silicate)}}}} + {\text{z}}/2\times{\text{S}}_2 = {{{\text{Re}}_{\text{x}}}} {{{\text{S}}_{\text{z(sulfide)}}}} + {\text{y}}/2\times{\text{O}}_2 .$$

in which the fO₂ and fS₂ (expressed as 0.5logfS₂–0.5logfO₂) controls the metal/sulfur composition of the sulfide melt. Kiseeva and Wood (2013) proposed an alternate approach that makes use of an exchange reaction involving the element of interest and Fe in the sulfide or silicate. Their formulation does not have an explicit dependence on fO₂ or fS₂.

Experimental determinations of D_SUL/SIL for Os, Pt, and other PGE have been plagued by concerns over nuggets and other experimental effects, with ‘best guess’ estimates of ~10⁴–10⁷ (see discussion in Brenan et al. 2016; Barnes and Ripley 2016). Similar D_SUL/SIL values have been inferred from the Re and Os compositions of sulfide globules in MORB and submarine OIB (Roy-Barman et al. 1998). These D_SUL/SIL values imply the potential for substantial depletion of Re relative to Os and Pt during melting and/or crystallization when sulfide is present, consistent with the compositions of basaltic magmas and mantle xenoliths as described in a subsequent section. However, uncertainties over bulk sulfide contents, redox states, and possible sulfide mobility in the mantle (Gaetani and Grove 1999; Rehkämper et al. 1999) have complicated attempts to model the magmatic behaviour of these elements precisely.

5.3 Sulfide Mineral-Melt Partitioning

Once a sulfide melt forms, fractional crystallization of that melt can produce large variations in the relative abundances of Re, Pt, and Os, which can be exploited for geochronology. On crystallization of a homogeneous sulfide liquid, the first phase to form is a Fe-rich monosulfide solid solution (MSS) that is typically enriched in Os, Re, Ir, Ru, and Rh. Further crystallization of the residual melt produces a Cu-rich intermediate solid solution (ISS) that becomes enriched in Pt, Pd, and other incompatible chalcophile elements. Experimentally determined values of D_{MSS/sulfide melt} show that Os is more compatible than Re in the MSS (D_Os ~5–20, D_Re ~2–10) whereas Pt is incompatible (D_Pt~0.03–0.2) (Brenan et al. 2016). This implies that MSS cumulates will have relatively high Re and Os concentrations but only moderate fractionations of Re/Os and very low Pt/Os ratios.

Fractional crystallization of sulfide melts has been recognized in Ni–Cu–PGE deposits associated with the Sudbury Igneous Complex (SIC; Naldrett et al. 1982, 1999), basaltic intrusions in the Noril’sk district (Czamanske et al. 1992), and some komatiite-related ores (Barnes et al. 1997). Smaller scale examples of these process have also been observed in sulfide globules from MORB and mantle xenoliths (Brenan et al. 2016; Barnes and Ripley 2016). Sulfide ores in the Noril’sk district are mineralogically and chemically complex with broad ranges and strong spatial zonation in the absolute and relative abundances of Cu, Ni, PGE and related mineral assemblages (Czamanske et al. 1992; Stekhin 1994; Torgashin 1994; Distler 1994; Naldrett et al. 1994; Zientek et al. 1994). This compositional diversity is reflected in the Re–Os-Pt isotopic data, with, for example, ¹⁸⁷Re/¹⁸⁸Os ratios of the ores (S > 30 wt%) ranging from ~4.3 to 930 (Walker et al. 1994; Malitch and Latypov 2011).

While the isotopic data provide information about ages and sources of magmatic sulfide deposits, constraints on the geochemical evolution of sulfide melts can be obtained from the concentrations of Re and Os, with the Noril’sk district ores being especially suitable for this due to their wide compositional variations. As shown in Fig. 2, Re/Os increases whereas concentrations of both Re and Os decrease by 2–3 orders of magnitude with increasing Cu content (data from Walker et al. 1994). Following the approach of Naldrett et al. (1994), these compositional variations were modelled as fractional crystallization of an MSS phase from an evolving sulfide melt (Fig. 3). In this model, the ores represent predominantly cumulus MSS with variable proportions of trapped sulfide melt. The models presented in Fig. 3 assume initial melt compositions of 5 wt% for Cu, 500 ppb for Re, and 250 ppb for Os, and D_{MSS/sulfide melt} = 0.1 for Cu, 3.5 for Re, and 4 for Os. The values of initial Cu concentration and D_Cu are from Naldrett et al. (1994). For Re and Os, the values provide empirical fits to the data; however, these values for Os are similar to those inferred by Naldrett et al. (1994) for the geochemically similar element Ir, and the D_Re and D_Os values are similar to values measured experimentally by Brenan (2002) and those used by Lambert et al. (1999) to model the evolution of magmatic sulfide deposits. Overall, the observed variations of Re and Os in the Noril’sk ores are broadly consistent with previous conclusions based on other PGE that the massive ores are predominantly MSS cumulates with variable proportions of trapped sulfide melt (Naldrett et al. 1994; Zientek et al. 1994). The strong increase in Re/Os at constant Cu concentration of ~30 wt% probably reflects crystallization of an intermediate solid solution (ISS) phase from the highly fractionated sulfide liquid as this would increase D_Cu dramatically and prevent further enrichments of Cu in the melt (Naldrett et al. 1994).

Fig. 2

Copper (wt%) in Noril’sk district massive ores versus a Os concentrations, b Re concentrations, and c ¹⁸⁷Re/¹⁸⁸Os ratios. d ¹⁸⁷Re/¹⁸⁸Os versus ¹⁸⁷Os/¹⁸⁸Os isochron for Noril’sk district massive ores. Data from Walker et al. (1994)

Fig. 3

Rhenium-osmium compositions of Norilsk massive ores modelled as fractional crystallization of an MSS phase from an evolving sulfide melt. Model trends show the composition of the MSS cumulate (+), the complementary sulfide residual liquid (open diamonds), and a mixture of MSS plus 50% trapped residual sulfide liquid (X). Markers indicate 1% increments of residual melt remaining for F = 0.12–0.99. Initial contents of the sulfide melt are assumed to be 5 wt% Cu, 500 ppb Re, and 250 ppb Os. MSS-sulfide liquid distribution coefficients are D(Cu) = 0.1, D(Re) = 3.5, and D(Os) = 4 (after Naldret et al. 1994). Noril’sk data (open circles) from Walker et al. (1994)

5.4 Diffusion and Closure Temperatures

Limited data for diffusion and closure temperatures are available for Re and Os in geological environments and ore-forming systems. Brenan et al. (2000) conducted a study of Os diffusion in pyrite and pyrrhotite. They calculated closure temperatures of ~300–400 °C for 10–1000 micron grains of pyrrhotite, and ‘core retention times’ of < 0.5 Ma for a 500 micron grain of pyrrhotite. This implies that pyrrhotite will tend to equilibrate rapidly during relatively mild thermal events, and so may be unsuitable for Re–Os dating. In contrast, pyrite appears to be more robust. Brenan et al. (2000) observed Os uptake by pyrite only during new growth with no evidence of diffusion at temperatures of 400–600 °C, and they calculate a minimum ‘core retention time’ of > 10 Ma for Os in pyrite. Suzuki et al. (1996) estimated a closure temperatures for molybdenite of ~500 °C, similar to that of Rb–Sr and considerably higher than that of K–Ar in granitic systems.

Stein et al. (1998) showed that the primary crystallization ages of molybdenite associated with 2.7 Ga intrusions in the Fennoscandian Shield were preserved during subsequent thermal metamorphism and deformation. In that study, pyrite analyses recorded a younger age, reflecting post-metamorphic recrystallization. Bingen and Stein (2003) noted that molybdenite that formed during granulite-facies metamorphism at 973 Ma was not affected by subsequent contact metamorphism associated with anorthosite intrusion at 930 Ma, implying closure at conditions of 4.7 kb and 710 °C. Stein et al. (2003) reached a similar conclusion regarding the immunity of molybdenite to subsequent metamorphism. As emphasized by Stein (2014), however, chemical exchange may be more important than a thermal threshold for Re–Os isotopic closure. When it is present, molybdenite typically contains orders of magnitude more Re and Os than other phases, so while there would be a large chemical gradient, diffusive equilibration would be impaired by the relative stability of Os in molybdenite compared to many other sulfides (Stein et al. 2003; Takahashi et al. 2007). In some situations, minerals with low Os concentrations can acquire ¹⁸⁷Os by diffusion from coexisting molybdenite; examples of unrealistically old ages of chalcopyrite attributed to this process are described by Stein et al. (2003).

5.5 Transport and Deposition of Re and Os in Volcanogenic Hydrothermal Systems

Rhenium is readily transported in chloride-rich solutions at high fO₂ and neutral to acidic pH (Xiong and Wood 1999, 2001; Xiong et al. 2006). Its solubility increases with ionic strength of the solution, consistent with a chloride complex. In contrast, Re precipitates from sulfide-rich solutions such that mixing of oxidised, saline solutions with reduced sulfur should be an effective mechanism for depositing rhenium in sulfides, either in solid solution or incorporated as nanoparticles. A prediction would be that the Re contents of molybdenite (MoS₂) and related phases should be related to the fO₂ and salinity of the transporting fluid. Few studies have addressed the stability of Os in hydrothermal solutions, in part due to lack of thermodynamic data. In contrast to Re, Os-chloride complexes appear to be stable only at very low pH (≤ 3) with oxygen-bearing, neutral, and sulfur-bearing species stable under relatively oxidizing to more reducing conditions, respectively (Mountain and Wood 1988). In addition, Os-bearing alloys and sulfides can form during hydrothermal alteration (e.g., 350–800 °C; Petrou and Economou-Eliopoulos 2009), sequestering the Os and further restricting its mobility in these environments.

Numerous Re–Os isotopic studies of active or recent ocean-floor hydrothermal systems have demonstrated mixing between seawater and crustal components, with seawater typically dominating the Os isotopic composition of the sulfides (Ravizza et al. 1996; Brügmann et al. 1998; Cave et al. 2003; Zeng et al. 2014). Osmium and Re concentrations also reflect mixing between oxidised seawater and reduced hydrothermal fluid, as well as variations in redox conditions within the system, creating variations in ¹⁸⁷Re/¹⁸⁸Os that can be useful for geochronology (Zeng et al. 2014). However, variations in initial ¹⁸⁷Os/¹⁸⁸Os related to mixing of seawater and primary hydrothermal fluids as well as post-depositional redistribution of Os and loss of Re during sulfide oxidation (Ravizza et al. 2001) would tend to introduce scatter and degrade any isochron relationship (Zeng et al. 2014). Nonetheless, Re–Os isochron ages have been obtained from a number of ancient volcanogenic massive sulfide (VMS) deposits, including the Iberian Pyrite Belt (Mathur et al. 1999; Munhá et al. 2005), the Urals VMS deposit (Gannoun et al. 2003; Tessalina et al 2008a), the Iimori Besshi-type Cu deposit (Nozaki et al. 2010), the Hitachi VMS deposit (Nozaki et al. 2014), and the Gacun Ag-rich VMS deposit (Hou et al. 2003) among others. At the Iimori and Hitachi deposits, the Re–Os system was not affected by later regional metamorphism and allowed the primary depositional age of the deposit to be established (Nozaki et al. 2010).

Both Re and Os can also be mobile as volatile species in magmatic environments depending on conditions. Osmium appears to be volatile primarily under oxidizing conditions and at high temperatures (> 1000 °C) (Wood 1987; Fleet and Wu 1993) whereas Re volatility is enhanced by the presence of Cl ± S as well as high fO₂ (MacKenzie and Canil 2006; Johnson and Canil 2011). A number of studies have documented Re and Os volatility in natural environments. Tessalina et al. (2008b) found high concentrations of both Re and Os in magmatic gasses collected at Kudryavy volcano, and they determined a Re–Os isochron age of 79 ± 11 years based on replicate analyses of the mineral rheniite (ReS₂). Rhenium loss by magmatic outgassing has also been demonstrated for arc volcanics (Sun et al. 2003a; Righter et al. 2008) and subaerial ocean island basalts (Lassiter 2003; Norman et al. 2004b).

5.6 Subduction Recycling

The efficiency of Os and Re recycling through subduction zones, which has implications for sources of metals in ore deposits associated with volcanic arcs, remains a topic of some debate. Brandon et al. (1996) and McInnes et al. (1999) provided evidence for incorporation of crustal Os into subduction-modified mantle xenoliths, but the genetic relationship between those xenoliths, arc basalts, and associated ore systems such as Lihir is unclear. Oxidizing conditions clearly favors mobilization of metals from the mantle to the crust, but the suggestion that slab-derived brines transport Os (Brandon et al. 1996; Borg et al. 2000) is not strongly supported by the experimental data on Os complexes (previous section). Alternatively, stabilization of Os in the mantle wedge might contribute to the recycling of crustal Os into the source regions of OIB (Borg et al. 2000; Suzuki et al. 2002).

Many arc basalts have radiogenic Os isotopic compositions but also very low Os concentrations, which makes them prone to modification by even small degrees of crustal interaction. Alves et al. (2002) recognized a broad negative correlation between Os concentrations and Re/Os ratios for a global set of arc lavas that parallels similar trends in MORB and OIB. They also found systematically higher ¹⁸⁷Os/¹⁸⁸Os in the lavas with individual arcs forming discrete linear arrays. This implies a mixing process between a less radiogenic component similar to the MORB source and crustal components that vary from arc to arc, but the authors were not able to distinguish between subduction contamination of the mantle versus magmatic assimilation. Subsequent studies have tended to favour high-level crustal contamination during evolution of arc magmas as the primary process responsible for their radiogenic Os isotopic compositions rather than a subduction modified mantle source (Hart et al. 2002; Woodhead and Brauns 2004; Turner et al. 2009; Bezard et al. 2015). In this case, the Os isotopic compositions of ore deposits associated with subduction environments may reflect crustal processes more than source characteristics.

6 Isotopic Dating and Source Tracing Using the ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os Isotopic Systems

6.1 Isotopic Dating

As mentioned earlier, the ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os systems are unique among commonly used radiogenic isotope decay schemes because the parent and daughter isotopes of both systems are chalcophile or siderophile when sulfides or metal are present. This provides an opportunity for dating the formation of ore minerals directly rather than relying on associated phases such as zircons or micas, whose relationship to the mineralisation might be unclear or affected by later events. Like most long-lived radiogenic isotope systems, ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os can be used for geochronology and source tracing. Conventional isochrons are based on the correlation of parent/daughter isotopes versus radiogenic isotopic compositions with age, e.g., ¹⁸⁷Re/¹⁸⁸Os versus ¹⁸⁷Os/¹⁸⁸Os. The slope of this correlation is proportional to the time since the system closed and the y-intercept provides the initial ¹⁸⁷Os/¹⁸⁸Os of the system. Fundamentally, any process that geochemically fractionates Re or Pt from Os can be dated using this approach if cogenetic samples with a range of parent/daughter ratios are available or using individual phases with high Re/Os or Pt/Os provided the initial Os isotopic is known or can be assumed. Such processes could include sulfide immiscibility, fractional crystallization of either the sulfide melt or the silicate melt, or the in-situ crystallization of phases, typically sulfides, with high parent/daughter ratios. Because the half-life of ¹⁸⁷Re is relatively fast (similar to ⁸⁷Rb), measurable differences in ¹⁸⁷Os/¹⁸⁸Os can develop quickly depending on the Re/Os. For example, a basalt with a ¹⁸⁷Re/¹⁸⁸Os of 100, which would be representative of modern MORB or OIB, would develop measurable differences in ¹⁸⁷Os/¹⁸⁸Os relative to its initial composition within ~10⁴ years, assuming an analytical precision of 0.01%. An impressive example of the ability to date young events is the absolute age of 79 ± 11 years based on fumarolic rheniite (ReS₂) by Tessalina et al. (2008b). Current uncertainty on the ¹⁸⁷Re decay constant is ~0.2% (Selby et al. 2007a), similar to that for ²³⁸U (Villa et al. 2016).

Isochrons can be based on either ‘whole rock’ samples of ore (e.g., Foster et al. 1996; Mathur et al. 1999; McInnes et al. 2008; Nozaki et al. 2010) or separates of minerals such as pyrite (Stein et al. 2000; Barra et al. 2003; Gao et al. 2020; Hnatyshin et al. 2020), arsenopyrite (Morelli et al. 2005, 2010), and chalcopyrite (Zhimin and Yali 2013) (Fig. 4). Selby et al. (2009) reported highly precise multi-mineral isochron ages based on pyrite, chalcopyrite, and bornite separates but inclusions of cogenetic minor phases such as renierite ((Cu, Zn)₁₁(Ge, As)₂Fe₄S₁₆) or germanite (Cu₂₆Fe₄Ge₄S₃₂) in those samples might be the actual hosts for the Re. Pyrrhotite appears to be susceptible to disturbance and does not appear to be generally suitable for Re–Os dating (Frei et al. 1998; Brenan et al. 2000; Morelli et al. 2010).

Fig. 4

A Re–Os isochron based on pyrite (black filled symbols), chalcopyrite (open symbols), and bornite (diagonal filled symbols) for the Ruby Creek, Alaska, deposit. Reproduced with permission from Selby et al. (2009); Copyright 2009 Society of Economic Geologists. The insert shows model ages obtained from each analysis. Size of the symbols on the isochron is fixed and not proportional to analytical uncertainties; these are indicated by the length of the bars shown for the model ages

Single-mineral ages can also be determined for phases such as molybdenite that form with very high Re/Os ratios, analogous to U–Pb dating of zircon or Rb–Sr dating of biotite. As discussed in Sect. 7.2 of this chapter, molybdenite (MoS₂) is especially suitable for Re–Os dating because of its typically high Re concentrations (often 10’s-1000’s of ppm), negligible initial Os, and resistance to subsequent resetting (Suzuki et al. 1993; Stein et al. 2001; Chiaradia et al. 2013). In this case, the age is given directly by the measured ¹⁸⁷Re/¹⁸⁷Os ratio assuming negligible initial Os (i.e., y-intercept = 0). In contrast, most other common sulfides such those mentioned above typically contain about a thousand times less Re, often in the low ppb range, but their initial Re/Os ratios are often still sufficiently high that they are suitable for dating. Stein et al. (2000) referred to this type of sulfide as Low Level Highly Radiogenic (LLHR) and suggested that plots of ¹⁸⁷Re vs ¹⁸⁷Os concentrations (rather than parent/daughter ratios as for conventional isochrons) are more suitable for determining their ages so as to avoid introducing correlated errors based on poorly determined initial Os compositions, although this may not always be necessary (e.g., Barra et al. 2003; Selby et al. 2009; Hnatyshin et al. 2020). Ideally, the age calculated from this type of isochron is consistent with the ¹⁸⁷Re/¹⁸⁷Os model age of each phase (Fig. 4). Sulfide mineral separates can be obtained either by handpicking coarsely crushed ore in an attempt to maximize the variation in Re contents (e.g., Barra et al. 2003; Majzlan et al. 2022) or by a combination of magnetic and density separations, minimizing contact with metal to avoid contamination (e.g., Hnatyshin et al. 2020; Saintilan et al. 2020).

The ¹⁸⁷Re-¹⁸⁷Os system has also been used to date titanomagnetite (Morgan et al. 2000; Lambert et al. 2000; Mathur et al. 2002; Zhou et al. 2005; Huang et al. 2014), deposition of black shales (Ravizza and Turekian 1989; Creaser et al. 2002; Hannah et al. 2004; Selby and Creaser 2005a), formation of bitumen and petroleum (Selby and Creaser 2003, 2005b; Selby et al. 2007b; Georgiev et al. 2016), eruption and alteration of komatiites and related ores (Foster et al. 1996; Gangopadhyay and Walker 2003; Gangopadhyay et al. 2005; Puchtel et al. 2004), gold deposits (Kirk et al. 2002; McInnes et al. 2008; Schaefer et al. 2010), sulfide inclusions in diamonds (Harvey et al. 2016), graphite (Toma et al. 2022), and a variety of minor phases including arsenides and sulfarsenides (e.g., Saintilan et al. 2017; Majzlan et al. 2022).

The ¹⁹⁰Pt-¹⁸⁶Os system has not been as widely applied for geochronology due to the low relative abundance and long half-life of the parent isotope. Consequently, variations of ¹⁸⁶Os/¹⁸⁸Os are generally much less than those of ¹⁸⁷Os/¹⁸⁸Os, which results in relatively large uncertainties on ages calculated from ¹⁹⁰Pt-¹⁸⁶Os isochrons compared to those obtained using ¹⁸⁷Re-¹⁸⁷Os (e.g., Puchtel et al. 2004). Nonetheless, it has been applied with some success to dating of mineralization in the Bushveld Complex (Coggon et al. 2012) and alluvial grains that are otherwise difficult to date (Nowell et al. 2008b; Coggan et al. 2011).

6.2 Source Tracing and Re–Os Model Ages

The Os isotopic composition of a source reflects its long-term Re/Os and Pt/Os. At the global scale, the fundamental process that fractionates Re from Os is crust formation. Continental crust has a highly radiogenic ¹⁸⁷Os/¹⁸⁸Os isotopic composition reflecting its high time-integrated ¹⁸⁷Re/¹⁸⁸Os (Esser and Turekian 1993, Saal et al. 1998; Peucker-Ehrenbrink and Jahn 2001; Fig. 1). However, the behavior of Re, Os, and Pt in the mantle differs from other commonly applied isotopic systems based on incompatible lithophile elements such Rb–Sr, Lu–Hf, and Sm–Nd in that they are not strongly fractionated by small to moderate degrees of mantle melting. As a consequence, fertile to moderately depleted mantle (i.e., MORB-source) has an Os isotopic composition consistent with long-term evolution at near-chondritic ratios of Re/Os and Pt/Os (Meisel et al. 2001a; Gannoun et al. 2007), and this seems to have been the case for at least the last 3.8 billion years (Bennett et al. 2002). Komatiite-hosted magmatic sulfide ores often have ¹⁸⁷Os/¹⁸⁸Os isotopic compositions consistent with this type of mantle source, precluding assimilation of significant amounts of older continental crust (Lambert et al. 1998, 1999). Assuming chondritic Re–Os and Pt–Os isotopic evolution for the mantle, therefore, provides a useful baseline for evaluating the relative contributions of crustal and mantle inputs to an ore system.

Model ages relative to a reference composition such as the mantle evolution curve can be calculated. Two types of model ages have been widely applied. One of these assumes that the Re and Os in the sample are both primary and the measured ¹⁸⁷Os/¹⁸⁸Os is corrected for in-situ decay until the isotopic composition intersects that of the mantle evolution curve. This “age” is referred to as T_MA and is analogous to the T_DM ages often calculated from Sm–Nd isotopic data. It can be applied to any material regardless of its Re/Os ratio but requires the assumptions of a normal mantle source and a single Re/Os fractionation event. A second type of model age assumes that the sample contains no Re (i.e., Re/Os = 0) such that the measured Os isotopic composition provides the mantle value directly, and therefore the time of Re depletion, presumably in a melting event. This Re-depletion model age (T_RD; Fig. 1) is suitable only for materials with low Re/Os ratios, such as highly depleted mantle xenoliths or chromite. Both of these model ages (T_MA, T_RD) assume that the sample was derived directly from the mantle, and they are therefore most commonly applied to materials such as mantle xenoliths and basalts. Neither type of model age is especially useful for most ore-forming systems, which often contain a crustal component and may have had multi-stage histories. Crustal model ages have not been widely applied, in part because the crust is highly radiogenic and appears to be heterogeneous in its Os isotopic composition (Johnson et al. 1996).

In contrast to the continental crust and fertile upper mantle, the subcontinental lithospheric mantle, as sampled by mantle xenoliths, preserves evidence of ancient melt depletion events in their low Re/Os ratios, subchondritic ¹⁸⁷Os/¹⁸⁸Os, and correlations of these parameters with geochemical indicators of melt extraction such as Al₂O₃ abundances (Meisel et al. 2001a; Carlson 2005; Aulbach et al. 2016; Harvey et al. 2016). A common application of Os model ages is constraining the timing of ancient melt depletion events (Shirey and Walker 1998; Carlson 2005; Rudnick and Walker 2009; Dijkstra et al. 2016; Harvey et al. 2016). Osmium isotopic evidence for ancient depletion events has also been found in abyssal peridotites (Brandon et al. 2000; Harvey et al. 2006) and peridotite xenoliths from arc (Parkinson et al. 1998) and mantle plume settings (Bizimis et al. 2007). Kimberlites, lamproites, and diamonds are often linked to such sources (Lambert et al. 1995; Carlson et al. 1996; Graham et al. 1999; Aulbach et al. 2016; Harvey et al. 2016).

While such data are generally considered to have broad geochronological significance, it should be emphasized that these are model ages analogous to those obtained from common Pb isotopic compositions of galena or the depleted mantle ages (T_DM) calculated from ¹⁴³Nd/¹⁴⁴Nd isotopic compositions, rather than crystallization ages. The significance of model ages depends on the applicability of the model and the time-integrated history of parent/daughter isotopic evolution in the measured materials and their source regions. In this case, Os isotopic model ages can be affected by a variety of processes such as melt-rock reactions, refertilization, metasomatism, metamorphism, Re volatility, or intrinsic isotopic heterogeneity in the source (Burton et al. 2000; Griffin et al. 2004; Gangopadhyay et al. 2005; Rudnick and Walker 2009; Kochergina et al. 2016; Aulbach et al. 2016).

Volcanogenic base-metal deposits such as porphyries and manto-type deposits often show clear evidence for involvement of older crust in the elevated initial ¹⁸⁷Os/¹⁸⁸Os isotopic compositions of the ores (Ruiz and Mathur 1999; Mathur et al. 2000a, 2005; Barra et al. 2003). However, the relative contributions of crustal and mantle sources remain contentious and variable signatures may reflect the tectonic and/or thermal evolution of the system (Freydier et al. 1997; Mathur et al. 2000b; McBride et al. 2001; Zimmerman et al. 2014; Saintilan et al. 2021). Gregory et al. (2008) concluded that the elevated ¹⁸⁷Os/¹⁸⁸Os isotopic compositions of Cu ore from the sediment-hosted Mt. Isa deposit reflects extraction of the Cu from older basalts that had developed radiogenic isotopic compositions through a combination of in-situ decay (high Re/Os) and crustal contamination or alteration during their emplacement. In contrast, the isochrons obtained by McInnes et al. (2008) for gold and copper–gold mineralization in the Proterozoic Tanami and Tennant Creek districts of the Northern Territory, Australia, have mantle-like initial ¹⁸⁷Os/¹⁸⁸Os isotopic compositions despite their emplacement into radiogenic crustal settings.

Interestingly, few ore deposits show clear isotopic evidence for involvement of ancient, Re-depleted mantle. While a hydrated and metasomatised peridotite might be an attractive source for the ‘boninitic’ (high Si, Mg) characterstics of some layered intrusion magmas (Lambert et al. 1994; Carlson 2005), the Os isotopic signature expected for an ancient, highly depleted mantle is typically absent in these intrusions. An exception is the Proterozoic Ipuera-Medrado chromite deposits in Brazil (Marques et al. 2003). Stratigraphically lower chromitites have the unusual combination of unradiogenic Os (Re-depleted) and unradiogenic Nd (LREE-enriched) isotopic compositions, which appears to be a unique signature of the subcontinental lithospheric mantle. As for many other layered intrusions, the Nd-Os isotopic compositions at Ipuera-Medrado change upsection in a way that is consistent with increasing degrees of crustal contamination.

7 Applications to Metallogenic Systems

7.1 Magmatic Sulfide Deposits

Osmium isotopic studies of magmatic sulfide deposits, which are significant resources for PGE, Ni, Co, Cr, and other metals, have provided definitive information about the timing of ore formation and the sources of ore metals. Examples include komatiite-associated NiS deposits such as those in Western Australia and the Canadian Shield (Walker et al. 1988; Foster et al. 1996; Lambert et al. 1998, 1999; Lahaye et al. 2001; Hulbert et al. 2005), layered mafic intrusions such as the Bushveld (Hart and Kinloch 1989; Schoenberg et al. 1999; Reisberg et al. 2011; Coggon et al. 2012), Stillwater (Martin 1989; Marcantonio et al. 1993; Lambert et al. 1994; Horan et al. 2001), Voisey’s Bay (Lambert et al. 2000; Hannah and Stein 2002), Muskox (Day et al. 2008), Great Dyke (Schoenberg et al. 2003), Portimo (Andersen et al. 2006), and Duluth complexes (Ripley et al. 1998, 2008; Williams et al. 2010), and Ni–Cu–PGE ores associated with flood basalts such as those in the Noril’sk region of Siberia (Walker et al. 1994) and the Xinjie layered intrusion in the Emeishan large igneous province of China (Zhong et al. 2011).

Komatiite-associated NiS deposits have yielded geologically reasonable Re–Os isochron ages although precision is often degraded by Re mobility related to subsequent deformation and alteration. Perhaps the most significant contribution that Re–Os isotope geochemistry has made to understanding these deposits is the clear definition of a mantle signature in the Os isotopic compositions. Although some komatiite flows may have experienced crustal assimilation (Lahaye et al. 2001), the data clearly show that the Ni and PGE (including Os) in these ores were derived predominantly from the mantle. This has allowed a detailed evaluation of volcanological processes and sources associated with emplacement of these magmatic systems, in particular by comparison with mass-independent variations in Δ³³S (Lambert et al. 1999; Bekker et al. 2009; Fiorentini et al. 2012).

In contrast to komatiite-hosted ores, Os isotopic compositions of mineralization in layered mafic intrusions show that mixing of crustal and mantle components is a ubiquitous process. The proportions of crust and mantle components vary widely (Shirey and Walker 1998), and the mechanism(s) by which the enriched (crustal) component becomes incorporated is not always clear. Assimilation of country rocks, magma mixing, mantle metasomatism, and overprinting by late hydrothermal fluids have all been suggested as possible mechanisms to explain the Os isotopic heterogeneity observed within many layered intrusions. In the Bushveld complex, initial ¹⁸⁷Os/¹⁸⁸Os ratios increase with stratigraphic height through the Critical Zone, reaching a maximum at the level of the Merensky Reef before decreasing sharply again within a few meters (Fig. 5; Reisberg et al. 2011, and references therein). In that scenario, ore grades in the Merensky Reef may have been enhanced by transient assimilation of PGE-rich black shales (Reisberg et al. 2011). Sproule et al. (2002) showed how Nd and Os isotopes can be decoupled depending on the timing of sulfide saturation relative to crustal assimilation. The magma-fluid-country rock interactions documented at the Duluth Complex (Ripley et al. 2001; Williams et al. 2010) illustrate how complex these processes can be. An implication of these studies would be that binary mixing or assimilation-fractional crystallization (AFC) calculations are unlikely to capture the physico-chemical reality of these processes. Late-stage mobility of Re due to hydrothermal activity is commonly inferred (Marcantonio et al. 1994; Andersen et al. 2006; Schoenberg et al. 2003; Day et al. 2008).

Fig. 5

Initial Os isotope stratigraphy of the Bushveld complex, after Reisberg et al. (2011). Filled black symbols are samples from the Merensky Reef. The vertical hashed bar shows the mantle composition at 2.054 Ga. Crustal host rocks would have had ¹⁸⁷Os/¹⁸⁸Os composition of ~0.5 at that time. Increasing initial ¹⁸⁷Os/¹⁸⁸Os ratios with stratigraphic height indicates progressively greater contributions of crust in the magma

The Sudbury Igneous Complex provides another example of the use of Re–Os isotopes to define the timing of ore formation and metal sources. Walker et al. (1991) and Morgan et al. (2002) showed that the Sudbury ores formed contemporaneously with crystallization of the igneous complex at 1850 Ma, followed by minor resetting at ~1770–1780 Ma. However, the limited range of ¹⁸⁷Re/¹⁸⁷Os and scatter introduced by either small-scale heterogeneities in initial ¹⁸⁷Os/¹⁸⁸Os isotopic compositions or minor post-crystallization disturbance limited the precision on some of the isochrons (e.g., 1825 ± 340 Ma, MSWD = 228 for Falconbridge and 1835 ±  70 Ma, MSWD = 45 for McCreedy West; Morgan et al. 2002). Notable in these data are the much higher Pt and lower Re and Os concentrations, and higher ¹⁸⁷Re/¹⁸⁸Os and ¹⁹⁰Pt/¹⁸⁸Os ratios in the Deep Copper Zone (DCZ) of the Strathcona mine compared to those from Falconbridge, McCreedy West, and the other Strathcona ores. This is consistent with an origin of the DCZ ores predominantly as residual sulfide melt and the Falconbridge, McCreedy West and other Strathcona ores predominantly as MSS-dominated cumulates. Morgan et al. (2002) also report ¹⁹⁰Pt-¹⁸⁶Os data, which show that the DCZ ores have highly radiogenic ¹⁸⁶Os/¹⁸⁸Os isotopic compositions as expected from their high Pt/Os ratios. However, the low Os concentrations in these ores and apparent variability in initial ¹⁸⁶Os/¹⁸⁸Os isotopic compositions precluded a precise age determination based on this system. Although the initial ¹⁸⁷Os/¹⁸⁸Os isotopic compositions of Sudbury ores are heterogeneous they are all totally dominated by radiogenic crust, possibly reflecting variable mixing of the crustal target rocks during formation of the SIC by impact melting (Morgan et al. 2002). The predominantly crustal composition of the SIC contrasts with that of many other layered intrusions such as Bushveld, Stillwater, Muskox, Rum, and Xinjie, which have Os isotopic compositions indicating mixtures of crustal and mantle components (Reisberg et al. 2011; Marcantonio et al. 1993; Lambert et al. 1994; Day et al. 2008; O’Driscoll et al. 2009; Zhong et al. 2011).

A somewhat different style of magmatic sulfide deposit is associated with flood basalts such as those in the Noril’sk region of Siberia and the Emeishan large igneous province of SW China. At Noril’sk, sulfide ores yield Re–Os isochron ages of ~245–248 Ma (Fig. 2), somewhat younger than the canonical eruption age of the Siberian flood basalts (250–252 Ma; Renne 1995; Kamo et al. 2003). However, uncertainties on the isochrons are large (±~ 2–10%) and the exact age obtained depends on which samples are included in the regressions. As for Sudbury, excess scatter in the data is indicated by the large MSWD’s of these isochrons (e.g., MSWD = 45–288 for the four isochrons presented by Malitch and Latypov 2011), possibly reflecting local variations in the ¹⁸⁷Os/¹⁸⁸Os of the ores at the time they formed. Alternatively, the Re–Os isotopic systematics of the ores might have been disturbed by subsequent events such as the minor silicic magmatism that followed emplacement of the flood basalts (Kamo et al. 2003; Malitch et al. 2010). In contrast to Sudbury, the initial ¹⁸⁷Os/¹⁸⁸Os isotopic compositions of Noril’sk district ores indicate a slightly enriched mantle source, consistent with the formation of the Siberian flood basalt from a mantle plume. The Emeishan large igneous province appears to be part of a broad episode of flood basalt eruption in the Late Permian with the Xinjie mafic–ultramafic layered intrusion yielding a ¹⁸⁷Re-¹⁸⁷Os isochrons age of 262 ± 27 Ma and an initial ¹⁸⁷Os/¹⁸⁸Os consistent with a moderately enriched mantle plume source (Zhong et al. 2011).

7.2 Molybdenite

Molybdenite (MoS₂) has been widely used for Re–Os geochronology because it often contains 10’s–100’s of ppm of Re and essentially no initial Os (Stein et al. 2001; Stein 2014). Precipitation of molybdenite from hydrothermal solutions is favored by decreasing fO₂, salinity, and T, and high fS₂ (Cao 1989; Ulrich and Mavrogenes 2008). It has a high closure temperature and is robust against subsequent thermal events and metamorphism (Fig. 6). Because Mo partitions into condensed fluids relative to vapour, primary molybdenite will tend to occur closer to the source of hydrothermal fluids compared to more vapour-mobile elements such as Au and Cu (Zajacz et al. 2017). Although a few examples of molybdenite with compositionally distinct cores and rims have been reported (Stein et al. 2004; Aleinikoff et al. 2012), this is rare. Rather, multiple episodes of molybdenite crystallization tend to form as discrete crystal populations such that age distributions can sometimes be at least partially resolved through analysis of multiple aliquots of different grain size fractions (Wilson et al. 2007; Aleinikoff et al. 2012). Variations in Re contents of molybdenite can also contribute to recognition of multiple mineralization events (Selby and Creaser 2001; Barra et al. 2005; Wilson et al. 2007; Wells et al. 2021). Controls on Re concentrations in molybdenite are discussed further in the next section.

Fig. 6

Diagram illustrating approximate closure temperatures for various mineral-isotope systems relevant to ore deposit geochronology. Modified after Chiaradia et al. (2013)

While the reliability of Re–Os molybdenite dating is now well established, care is needed when sampling the molybdenite because small-scale decoupling of ¹⁸⁷Re/¹⁸⁷Os within individual crystals has been inferred from apparent variations in the spatial distribution of ¹⁸⁵Re/Σm187, where Σm187 = the net signal intensity measured on mass 187, comprising ¹⁸⁷Re plus radiogenic ¹⁸⁷Os, measured by laser ablation ICPMS (Stein et al. 2003; Selby and Creaser 2004; Košler et al. 2003). Such decoupling might be due to intra-crystal migration of ¹⁸⁷Os as a consequence of the misfit of Os in the crystal structure of molybdenite whereas Re substitutes for Mo as a limited solid solution of ReS₂ in MoS₂ (Takahashi et al. 2007; Drabek et al. 2010). In contrast, imaging of the spatial distribution of Re and Os in molybdenite using nano-SIMS (Barra et al. 2017) and a laser ablation study using a MC-ICPMS equipped with a collision cell to separate Re from Os during the analysis (Hogmalm et al. 2019) did not observe such decoupling, although both of those studies have been criticised as inconclusive (Zimmerman et al. 2022). Clearly, further work is necessary to address this question.

Molybdenite can also form as a primary magmatic phase, often as small inclusions in quartz phenocrysts (Audétat et al. 2011; Sun et al. 2014). It seems to occur more frequently in within-plate or rift-related rhyolites compared to arc rhyolites, probably due to the higher fO₂ and resulting higher proportion of sulphate versus sulfide in the latter. Audétat et al. (2011) developed a thermodynamic model that estimates magmatic fO₂ and fS₂ provided that the melt is saturated in molybdenite + pyrrhotite, and the Mo concentration and T can be constrained. Alternatively, fS₂ can be calculated if fO₂ can be constrained independently, for example, through FeTi-oxide thermobarometry. Molybdenum solubility in silicic melts is enhanced by oxidizing conditions and depolymerized (e.g., strongly peralkaline) compositions (Sun et al. 2014).

7.3 Controls on Re Concentrations in Molybdenite

Bulk rhenium concentrations in molybdenite display a broad correlation with the lithologic association and style of the deposit (Barton et al. 2020). Terada et al. (1971) showed that rhenium contents of molybdenite generally decrease in the sequence: volcanic sublimate, porphyry copper deposits, contact-metasomatic (skarn) deposits, disseminated greisen or stockwork deposits, and quartz veins, which suggests a broad gradient with temperature and/or source of the metals. An extreme example of high-T molybdenites are those associated with fumaroles at Kudryavy volcano in the Kurile volcanic arc, which have ~1.5 wt% Re (Tessalina et al. 2008b). Although rheniite (ReS2) has also been found at this volcano, the Re content of this molybdenite appears to be primary and reflects the high concentrations of Re and other volatile metals in the volcanic gases (Tessalina et al. 2008b). Rhenium volatility during magmatic outgassing appears to be a common process that has been documented at a variety of settings that include ocean island and island arc volcanoes (Sun et al. 2003a; Lassiter 2003; Norman et al. 2004b; Righter et al. 2008).

Porphyry deposits often carry molybdenite with high Re concentrations although this appears to vary with the specific style of mineralization (Berzina et al. 2005; Barton et al. 2020). In general, molybdenite associated with Cu and Au porphyries tend to have systematically higher Re contents (100’s to 1000’s of ppm) than that in Mo-dominated systems (10’s to 100’s of ppm), possibly due to dilution of the Re by the large volume of molybdenite in the latter (Stein et al. 2001; Stein 2014). Molybdenite in deposits associated with compositionally intermediate granitic systems often contain 10’s to 100’s of ppm Re whereas those related to highly evolved felsic rocks, and especially Sn-W deposits, tend to have low Re contents (<1–10 ppm) (e.g., Morgan et al. 1968; Selby et al. 2007a; Barton et al. 2020). It has been suggested that the general trend of higher Re in molybdenite associated with more mafic systems to lower Re in more evolved systems reflects a transition from mantle- to crustally-dominated systems (e.g., Stein et al. 2001), but it may also be due to co-factors such as redox and/or sulfidation state, with more oxidized, higher sulfidation systems producing molybdenite with higher Re contents (Berzina et al. 2005; Barton et al. 2020).

In addition to these broad lithologic associations and physical–chemical controls, there may also be a degree of underlying geological influence on the composition of molybdenite in some deposits. For example, some provinces appear to produce molybdenite with systematically lower concentrations (<20 to sub-ppm; Morgan et al. 1968; Stein et al. 2001) possibly due to derivation of the system by metamorphic dehydration melting of mid-crustal rocks, a process that should be reflected in other isotopic and compositional characteristics (Stein 2006; Voudouris et al. 2013). In contrast, Voudouris et al. (2013) suggested that the regional distribution of molybdenite with high to very high Re contents in northeastern Greece may indicate a Re-enriched source, perhaps related to its tectonic history.

7.4 Case Study: Crystallization and Uplift of the Boyongan and Bayugo Cu–Au Porphyry Deposit

While Re–Os isotopic studies of molybdenite are typically aimed at establishing the age of mineralisation, the study by Braxton et al. (2012) provides an example of the added value that molybdenite dating can provide when placed in a broader geological and geochronological context. Braxton et al. (2012) studied the Pleistocene age Boyongan and Bayugo Cu–Au porphyry deposits, which are located in the Surigao district of the Phillipines, using multiple radiogenic isotope systems. The Boyongan complex is characterized by an exceptionally thick (600 m) oxidation profile.

Porphyry magmatism occurred between 2.09 and 2.31 Ma based on U–Pb isotopic ages of zircons from the earliest and latest intrusive phases of the complex. Rhenium-Os ages of 2.115 ± 0.008 and 2.120 ± 0.007 Ma on late molybdenite are consistent with a close genetic link between igneous intrusion and the mineralization, while K–Ar ages of 2.09–2.12 Ma on hydrothermal illite inferred to post-date the molybdenite shows that the system cooled rapidly following the final stage of porphyry emplacement and mineralization. In contrast, (U–Th)/He apatite thermochronology provided ages between 1.49 and 1.98 Ma, with ages decreasing systematically with increasing depth into the intrusive complex. Those cooling ages provide constraints on the uplift and exhumation of the complex. Finally, the timing of late extension and sedimentation was established from ⁴⁰Ar-³⁹Ar ages of post-mineralization volcanics and ¹⁴C dating of wood and plant material, which indicate burial began at ~0.6 Ma and continued to ~1.6 ka. These relationships are summarized in Fig. 7.

Fig. 7

Diagram illustrating the sequence of igneous, hydrothermal, and cooling ages at the Boyongan and Bayugo porprhyry deposits, Philippines (after Braxton et al. 2012)

The geochronological data on the Boyongan-Bayugo complex imply porphyry mineralization at ~2.1 Ma was followed by rapid uplift at a rate of 1–5 km/Ma, with exposure and intense supergene alteration occurring predominantly between ~0.6 and 1.5 Ma. These uplift, alteration, and burial rates are considerably greater than the global averages but similar to other sites with analogous tectonic and climatic conditions. Braxton et al. (2012) interpret the uplift and subsequent burial history of the complex to reflect a transition from strongly compressional tectonism related to collision of the Philippine arc with the Eurasian plate to the present-day transtensional setting in the mid-Pleistocene, highlighting the dynamic tectonism of the Philippine Mobile Belt and related geological environments.

8 Conclusions

The ¹⁸⁷Re-¹⁸⁷Os isotope system is well suited for direct dating of sulfide minerals. Molybdenite is especially valuable for ore deposit geochronology because of its relatively high Re concentrations, high initial ¹⁸⁷Re/¹⁸⁷Os, high closure T, and resistance to resetting, but it is not always present; many other sulfide minerals have been dated successfully. Initial ¹⁸⁷Os/¹⁸⁸Os isotopic compositions of magmatic sulfide deposits have established a predominantly mantle provenance for the PGE in these systems, whereas the ore-forming fluids responsible for hydrothermal deposits carry a strong signature of crustal Os. The ¹⁹⁰Pt-¹⁸⁶Os system has so far been useful only for a limited set of applications, mainly in the dating of alluvial nuggets with high Pt/Os. Future developments can expect to see increasing applications to the dating of other mineral systems, and a more integrated approach to place mineralisation ages within a broader geotectonic context and the duration of ore-forming systems. Recent efforts to measure stable isotope variations of Re (Miller et al. 2009), Os (Nanne et al. 2017), and Pt (Creech et al. 2014) may find future applications to ore deposits. Further studies of Os isotopes coupled with stable isotopic compositions of S and O will continue to provide new insights into the sources of mineralising fluids and conditions of mineralization.