Pb Model Ages Research Articles

Lead Isotopes Constrain Precambrian Crustal Architecture, Thermal History and Lithospheric Foundering in Laurentia

ABSTRACTLaurentia (ancestral North America) records nearly 4 billion years of crustal evolution. Here, a newly compiled continental‐scale Pb isotopic database is used to evaluate the Precambrian crustal evolution of Laurentia. Pb model ages yield a 2.7 Ga peak, a 2.5–1.8 Ga minimum and 1.8–0.9 Ga continuum. Pb model ages yield thermochronometric data and track crustal growth via arc‐related magmatism and accretionary orogenesis. Model 232Th/204Pb and 238U/204Pb broadly correlate with mapped crustal domains. More homogeneous and less radiogenic 238U/204Pb and 232Th/238U after 2.7 Ga suggests a shift to more juvenile sources, loss of early isotopic reservoirs and greater crustal reworking. U and Th are fractionated from Pb in Proterozoic orogens with abundant ferroan and anorthosite–mangerite–charnockite–granite(AMCG)‐suite magmatism. This fractionation suggests the removal of Pb‐rich lower crust, supporting petrogenetic models involving lithospheric foundering and magmatic underplating. Lithospheric thinning and associated magmatism may have contributed to high middle Proterozoic geothermal gradients.

Mixed lead sources in tin metal: Implications for using lead isotopes to study tin artifacts

Methods for provenancing copper, lead, and silver using the isotopic composition of lead are well-established. Lead isotope analysis holds promise for the study of tin metal as well, as long as one accounts for the U-Th-Pb systematics of cassiterite (SnO2) and chaîne opératoire of tin production. Although Precambrian cassiterite may contain 10s of ppm Pb or more (predominantly radiogenic 206Pb), Phanerozoic examples typically contain only a few parts per million Pb. However, all but one of the 133 raw tin ingots excavated from European Bronze Age shipwrecks contains more Pb than could have come from cassiterite alone, as do six of the twelve analyzed tin objects interpreted to have been derived from the ores of southern Africa. Accordingly, almost all tin objects must contain Pb from external cassiterite sources and interpretation of LIA must account for this contamination. The nature of the contaminant (sulfides, U-Th-bearing minerals, silicates) can be inferred from patterns in Pb concentration and LI values. The 3 major sources of Pb that can typically be identified in tin artifacts are original Pb from the tin ore, radiogenic Pb produced in-situ due to U decay, and external Pb added during the cassiterite smelting and ingot production. As cassiterite has high U/Pb but low Th/Pb, the 208Pb/204Pb may be representative of the initial Pb incorporated in the mineral. This is assuming either that no external Pb is added during the ore processing or that the added Pb is from coeval sulfides from the same Pb ore provenance. In such cases 208Pb/204 Pb can be used to estimate a Pb model age, which in turn can be used for provenance estimate of the ingots. If the addition of Pb is from U-Th-mineral contaminants to the ore concentrate, then this will also increase 208Pb/204Pb and point to erroneously young model Pb ages. In such cases, the problem would be evident in positively correlated values of 206Pb/204Pb and 208Pb/204Pb. If Pb concentrations are above a certain threshold (approximately 5 ppm). LIA typical common Pb isotope ratios will be clear indication that external, non-cassiterite Pb, is added to the tin artifact. This tin could be from impurities in the ore (e.g., inclusions in cassiterite, impurities in the ore concentrate, or added during ore smelting and/or metal processing. Overall, elevated Pb concentrations accompanied with non-radiogenic Pb isotopes typical for common Pb, is a clear indication that significant amount of external (contaminant) Pb is added to the tin artifact.

Genesis of Tertiary Akçakışla vein-type Pb-Zn-Cu mineralisation (Central Anatolia, Turkey): Evidence from fluid inclusion and O, H, S, Pb-isotope compositions

The Akçakışla Pb–Zn–Cu vein type deposit is one of the active mining deposits located in the Akdağmadeni (Yozgat) metallogenic province in Turkey. The sulfide vein deposit, trending E-W, is hosted by calcschist of the Akçakışla and Topaktaş formations of Akdağmadeni metamorphites, which are intruded by the Central Anatolian granitoids and overlain by sedimentary and Tertiary volcanic rocks. Microthermometric measurements indicate that the Pb-Zn-Cu deposit formed by hydrothermal fluids containing dissolved CaCl2, MgCl2 and NaCl. A mixing of fluids with greatly varying salinities and temperatures is shown by the wide range of calculated salinity (0.2–10.4%) and temperature (170–400 °C) values. Isotope ratios of oxygen (δ18OH2O = −5.09 to +4.71‰) and hydrogen (δD = −67‰ and −82‰) suggest that water in hydrothermal fluid might be either mixing of meteoric and magmatic waters or meteoric water that oxygen isotope composition was modified by fluid-rock interaction. Sulphide minerals exhibit δ34S values ranging from 5.51‰ to 8.40‰, which assume a magmatic sulphur source modified by assimilation of upper crustal material (possibly the host metamorphites). In galena from the studied Akçakışla Pb–Zn–Cu deposit ratios of 206Pb/204Pb (18.783–18.821),207Pb/204Pb (15.654–15.700) and 208Pb/204Pb (38.874–39.039) suggest a mixture of lead sources from orogenic-upper crust materials. We assume that hydrothermal fluids dissolved the sulphur from the intruded granitoids and the lead from the host metamorphites. The galena Pb model ages values point out two different ore forming ages; suggesting initial formation in upper Cretaceous (87 Ma) and highly reworking in Eocene–Oligocene (23 and 39 Ma).

Feldspar Pb isotope evidence of cryptic impact-driven hydrothermal alteration in the Paleoproterozoic

Hypervelocity impacts throughout Earth's history have profoundly affected the evolution of the continental crust. Accessory minerals like zircon are typically used to date impact events and rock-forming minerals like quartz are routinely used as shock barometers. However, feldspar group minerals – a major constituent of most crustal rocks – are generally underutilized in the documentation of impact-induced deformation and alteration. Alkali feldspar contains appreciable amounts of Pb and analysis of Pb isotopes in feldspar may offer the opportunity to identify impact-related isotopic modifications of shocked crustal target rocks and estimate their timing. Here, we apply a combination of laser ablation inductively coupled plasma (LA-ICP) and thermal ionization mass spectrometry (TIMS) Pb isotope analysis with imaging techniques, including electron backscatter diffraction (EBSD), cathodoluminescence (CL), and time of flight secondary ion mass spectrometry (ToF-SIMS), to shocked alkali feldspar from monzogranite in the oldest confirmed terrestrial impact structure (2229 ± 5 Ma) at Yarrabubba, Western Australia. Alkali feldspar preserves microstructures such as sub-planar and irregular fractures, sets of planar deformation bands that accommodate misorientations of up to ∼20°, sets of damage lamellae, and broad domains of lattice damage that can be linked to impact-related deformation. The Pb isotope compositions in alkali feldspar correlate with variations in electron diffraction band contrast – a proxy for crystallinity – and also the degree of misorientation and CL response. Less damaged alkali feldspar yields Pb model ages similar to the igneous zircon U–Pb crystallization age of the host monzogranite (∼2650 Ma), whereas younger Pb model ages correspond to zones of damage (high relative misorientation, low crystallinity, weak CL response). The observed Pb isotope behaviour implies radiogenic ingrowth of Pb, from decay of U and Th within damaged alkali feldspar, and therefore mixing with a grain-scale Pb reservoir that formed at the time of impact. The U and Th zonation in some shock-deformed alkali feldspar is broadly similar and follows the orientation of sub-planar fractures and damage lamellae. Detailed imaging reveals the zones of U and Th enrichment are associated with trains of monazite micro-inclusions, in conjunction with magnetite and/or hematite in places, which are inferred to have precipitated during impact-induced hydrothermal circulation. Hence, the Pb isotopic data record grain-scale hydrothermal alteration in superficially weakly altered monzogranite target rocks.

Archean Pb isotope variability tracks crust-mantle fractionation, granite production, and ore deposit formation

Various geological processes that affect Earth's crust may be encoded into isotopic tracers preserved in rocks and minerals. The enhanced sensitivity of U, Th, and Pb to crustal fractionation processes allows Pb isotopes to complement information from the Nd and Hf isotope systems. However, melt fractionation, crustal contamination and recycling, hydrothermal fluid flow and fluid-rock interaction, and other processes in the continental crust can lead to mixing of Pb isotopic signatures. Here, we report new Pb isotopic data from granite-K-feldspar and integrate these data with published Pb isotope ratios from granite K-feldspar and Pb-rich ores across the Yilgarn Craton in Western Australia. The aim of this study is to explore how the variability of Pb isotope ratios and derivative parameters can be used to gain information on specific geological processes occurring throughout the crustal column. We develop a model that subdivides different sampling media into chemical process groups and links their initial Pb signatures to Pb source regions and fractionation processes at various locations within the crust and upper mantle. Equilibration of Pb signatures with a primary mantle source reservoir (in part represented by volcanic-hosted massive sulfides) is contrasted with granite formation in the mid to lower crust (granite K-feldspar), and mineralization of ore deposits in the mid to upper crust (Pb-rich ores). Spatial trends similar to those in Nd and Hf isotopic data are recorded by Pb isotopic derivative parameters (μ = source 238U/204Pb, ω = 232Th/204Pb, and Δt - the difference between true sample age and Pb model age) calculated for komatiite-hosted Ni sulfide ores, granite K-feldspar, and volcanic-hosted massive sulfide (VHMS) ores. The significance of subtle differences in absolute values of derivative parameters is supported by the diversity of Pb isotope ratios, Pb model ages, and ∆t as tracked by a statistical metric, quantifying the variability of Pb sources involved in the formation of different chemical process groups. Generally greater variety in an older terrane (Youanmi) documents more ancient and recycled continental crust as compared with more homogeneous Pb isotopic signatures in a younger terrane (Eastern Goldfields Superterrane). The Pb signatures are interpreted, in part, to relate to the timing of source fractionation in the upper mantle with a legacy of this source signal preserved through various depths in the lithospheric section. The least radiogenic VHMS ore samples appear to provide a good approximation of mantle Pb signatures, indicated for example by a deficit in 206Pb and 208Pb relative to the other process groups. A significant heterogeneity recorded in Pb isotopic data from Pb-rich gold ores is explained by the interplay of hydrothermal fluids with diverse sources leading to the mineralization of gold deposits (e.g., leaching of Pb from surrounding rocks or fluid mixing). Such gold ore Pb signatures are distinct from other process groups, which together track sources less heterogeneous in age and/or U and Th.

Fluid inclusion and stable isotope (H–O–S–Pb) constraints on the genesis of the Haxi gold deposit, west Junggar, China

The Haxi gold deposit located in the Hatu–Saertuohai metallogenic belt of the west Junggar region, China, is a newly discovered gold deposit with proven reserves of ∼10 tons of gold and an average grade of 6.70 g/t. Primarily, the deposit is hosted in the early Carboniferous Tailegula Formation, controlled by the NEE-trending Anqi fault. The gold ore bodies contain gold-bearing quartz veins and altered basalt. The ore mineral is dominated by abundant sulfides, such as pyrite, arsenopyrite, chalcopyrite, and a little gersdorffite. To date, three types of fluid inclusion in quartz veins have been identified: liquid–gas (V-type), CO2–H2O (C-type), and liquid–rich (L-type). The ore-forming fluids exhibit medium homogenization temperatures (Th) of 226 °C–325 °C with low salinities of 2.24–9.34 wt% NaCl equivalent and low densities from 0.72 to 0.88 g/cm3. The δD values range from −115.3‰ to −100.7‰, with an average value of −109.1‰; the δ18O values range from 11.3‰ to 12.2‰, with an average value of 11.8‰; and the δ34S values in sulfide range from 0.3‰ to 1.8‰, with an average of 1.2‰. The average values of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in sulfide are 18.059, 15.547, and 37.996, respectively. To summarize, the metallogenic fluids were derived not from a single source but from late magmatic–hydrothermal fluid mixed with paleo-atmospheric water; furthermore, the ore-forming materials principally originated from a crust–mantle mixed source. The Pb model age calculated using Hautman’s formula is 347–365 Ma, leading to the speculation that the metallogenic period may be Devonian–Carboniferous.

New findings of ancient Greek silver sources

Over the last 60 years, much analytical research has sought to determine the ore sources of ancient Greek silver artefacts. Lead isotopic analysis has played a key role in this endeavor. While most studies so far have limited their search to places mentioned in historical sources, the present study takes a different approach by first identifying Ag-bearing ore sources in the Aegean world based on their geological characteristics and then using Pb isotopes to determine whether they were exploited in antiquity. To this end, we have geolocated, sampled, and measured high-precision Pb isotopic compositions of 17 Ag-bearing mineralizations in Greece for which we have evidence of ancient mining activity, and a further 10 exhibiting minor Ag occurrences that may also have been exploited in ancient times. We found that Pb model ages provide better discrimination of ore sources than the more conventional plots of raw Pb isotope data.Our study establishes Lavrion, northeast Chalkidiki, Pangaeon, Thasos, Siphnos, Palaea Kavala, Angistron, and south Euboea as the most important ancient silver mining districts in Greece. Two previously undiscovered ancient mining areas in Pelion and in the Kroussia mountain range are also documented. The latter may be identified with ancient Mount Dysoron, from which King Alexander I of Macedon reportedly extracted the fabulous sum of a talent of silver per day. For the first time, we isotopically differentiate some of the mining districts in Thraco-Macedonia, and show that the mines of Thasos include geologically different silver-bearing ore sources. We further identify the hitherto unrealized importance of Euboean silver mines and demonstrate that they isotopically overlap those of Siphnos, with major implications for our understanding of ancient Greek history.

The significance of galena Pb model ages and the formation of large Pb-Zn sedimentary deposits

In an attempt to clarify the significance of Pb model ages in Pb-Zn sedimentary deposits, we report high-precision Pb isotopic compositions for 64 galenas and 52 K-feldspars, the former from ores and the latter separated from granites. All samples are from Spain and the French Pyrenees. Lead from galena ores is of unequivocal continental origin. With few exceptions, Pb model ages systematically exceed emplacement ages by up to 400 Ma, a gap which is well outside the uncertainties of ~ 30 Ma assigned to the model. The histogram of the new high-precision Pb isotope data shows prominent peaks of galena Pb model ages at 94 ± 38 Ma and 392 ± 39 Ma. When the data are consolidated with literature data and examined in 3-dimensional Pb isotope space, cluster analysis identifies five groups. The model ages of the peaks occur, in order of decreasing peak intensity, at 395 ± 40 (Middle Devonian), 90 ± 34 Ma (Middle Cretaceous), and 613 ± 42 Ma (Neoproterozoic), with two minor peaks at 185 + 26 Ma (Jurassic) and 313 ± 41 (Upper Carboniferous). To a large extent, the model ages centered around these peaks correspond to distinct localities. The ages of the peaks do not coincide with any of the Betic, Variscan, or Pan-African tectonic events, which are the main tectonic episodes that shaped Iberian geology, but instead match well-known global oceanic anoxic events. It is argued that surges of metals weathered from continental surfaces scorched during anoxic events accumulated and combined in anoxic water masses with unoxidized marine sulfide released by submarine hydrothermal activity to precipitate the primary Pb-Zn stock. Frozen Pb isotope compositions require that galenas from black shales are the source of the final ores. The sulfides were later remobilized by large-scale convective circulation of basinal and hydrothermal fluids. The peaks of K-feldspar Pb model ages are distinct from those of galenas and do not correlate with magmatic emplacement ages. It is suggested that they instead reflect local circulation in Paleozoic sediments surrounding individual plutons. While Pb isotopes can be used as a regional provenance tool, such an approach requires that the data are considered in a fully 3-dimensional space.

Decratonization by rifting enables orogenic reworking and transcurrent dispersal of old terranes in NE Brazil

Dispersion and deformation of cratonic fragments within orogens require weakening of the craton margins in a process of decratonization. The orogenic Borborema Province, in NE Brazil, is one of several Brasiliano/Pan-African late Neoproterozoic orogens that led to the amalgamation of Gondwana. A common feature of these orogens is that a period of extension and opening of narrow oceans preceded inversion and collision. For the case of the Borborema Province, the São Francisco Craton was pulled away from its other half, the Benino-Nigerian Shield, during an intermittent extension event between 1.0–0.92 and 0.9–0.82 Ga. This was followed by inversion of an embryonic and confined oceanic basin at ca. 0.60 Ga and transpressional orogeny from ca. 0.59 Ga onwards. Here we investigate the boundary region between the north São Francisco Craton and the Borborema Province and demonstrate how cratonic blocks became physically involved in the orogeny. We combine these results with a wide compilation of U–Pb and Nd-isotopic model ages to show that the Borborema Province consists of up to 65% of strongly sheared ancient rocks affiliated with the São Francisco/Benino-Nigerian Craton, separated by major transcurrent shear zones, with only ≈ 15% addition of juvenile material during the Neoproterozoic orogeny. This evolution is repeated across a number of Brasiliano/Pan-African orogens, with significant local variations, and indicate that extension weakened cratonic regions in a process of decratonization that prepared them for involvement in the orogenies, that led to the amalgamation of Gondwana.

Provenance of metasedimentary rocks of the western Dom Feliciano Belt in Uruguay: Insights from U–Pb detrital zircon geochronology, Hf and Nd model ages, and geochemical data

New isotopic and geochemical data for the Paleo- Meso- and Neoproterozoic metasedimentary cover of the southern Dom Feliciano Belt (Brasiliano/Pan-African) are presented and evaluated combined with published information. Whole-rock major and trace element geochemistry indicates that the dominant source for all the units had a composition similar to average upper continental crust. The geochemistry is similar even for late Ediacaran successions with a source component from slightly older Ediacaran granites, due to the crustal origin of these granites. Age constraints based on detrital zircon, fossil content, interbedded volcanic rocks and isotope geochemistry confirm Paleo-, Meso-, and Neoproterozoic successions, despite uncertainties remain in some cases. Detrital zircon data show the dominance of Archean (3.3–2.8 Ga) to Paleoproterozoic (2.2–1.9 Ga), and subordinated Mesoproterozoic (1.5–1.3 and 1.1 Ga) and Neoproterozoic (0.5–0.6 Ga) ages. Lu–Hf zircon and Sm–Nd whole-rock model ages confirm that the Archean and the Paleoproterozoic were the major crustal growth periods for the source areas. U–Pb detrital zircon age distributions and model ages demonstrate that the Nico Pérez Terrane and not the Río de la Plata Craton was the source for most metasedimentary rocks of the western Dom Feliciano Belt. Likewise, comparison of Archean, Paleo-, Meso- and Neoproterozoic events supports that the Nico Pérez Terrane could represent a fragment of the Congo Craton.

Two stage of gold-bearing sulfide ores of early Proterozoic gabbroids in the north Ladoga area

In a study based on Pb isotopes, it has been shown that the formation of gold-sulfide mineralization in the Early Proterozoic gabbros of the Northern Ladoga region is associated with different tectonic phases. This conclusion is based on a comparative study of isotopic data for feldspars and lead sulfides in two Early Proterozoic intrusions; Velimäki and Alattu-Päkylä. The investigated intrusions have similar geological settings at the junction of the Svecofennian accretionary complex and the Karelian craton. Both massifs are characterized by manifestations of noble metal sulfide mineralization associated with zones of local shear deformations. The formation of the studied ore massifs is attributed to the Svecofennian and Caledonian tectonic stages. Feldspars of Velimäki intrusion have parameters and model age of Pb significantly older than U-Pb age of zircon (1.9 Ga), although they correspond to the Svecofennian time of magmatic crystallization of gabbro and clinopyroxenites, while sulfide pyrite-pyrrhotite mineralization is associated with gold in these rocks with the Caledonian stage of fluid-thermal processing of the Early Proterozoic magmatic mineral paracenteses. Feldspars and sulfides of the Alattu-Päkylä intrusion have similar lead isotopic parameters and Pb model age (~ 2 Ga), also somewhat older than the U-Pb age of zircon, but at the same time indicating the formation of rock-forming and ore sulfide associations with gold during the Svecofennian (Early Proterozoic) plutonic and tectonic-thermal events. The source of lead sulfides of the Velimäki massif is characterized by the parameters of the upper crust with high µ2 = 238U / 204Pb, while the µ2 parameter of the minerals of the Alattu-Päkylä massif is slightly lower, which indicates the participation of the mantle-lower crustal reservoir material in the formation of magma.

New Findings of Ancient Greek Silver Sources

Over the last 60 years, much analytical research has sought to determine the ore sources of ancient Greek silver artefacts. Lead isotopic analysis has played a key role in this endeavor. While most studies so far have limited their search to places mentioned in historical sources, the present study takes a different approach by first identifying Ag-bearing ore sources in the Aegean world based on their geological characteristics and then using Pb isotopes to determine whether they were exploited in antiquity. To this end, we have geolocated, sampled, and measured high-precision Pb isotopic compositions of 17 Ag-bearing mineralizations in Greece for which we have evidence of ancient mining activity, and a further 10 exhibiting minor Ag occurrences that may also have been exploited in ancient times. We found that Pb model ages provide better discrimination of ore sources than the more conventional plots of raw Pb isotope data. Our study establishes Lavrion, northeast Chalkidiki, Pangaeon, Thasos, Siphnos, Palaea Kavala, Angistron, and south Euboea as the most important ancient silver mining districts in Greece. Two previously undiscovered ancient mining areas in Pelion and in the Kroussia mountain range are also documented. The latter may be identified with ancient Mount Dysoron, from which King Alexander I of Macedon reportedly extracted the fabulous sum of a talent of silver per day. For the first time, we isotopically differentiate some of the mining districts in Thraco-Macedonia, and show that the mines of Thasos include geologically different silver-bearing ore sources. We further identify the hitherto unrealized importance of Euboean silver mines and demonstrate that they isotopically overlap those of Siphnos, with major implications for our understanding of ancient Greek history.

Geochemistry and petrogenesis of Paleoproterozoic rhyolite-hosted zinc-rich metamorphosed volcanogenic massive sulfide deposits in the eastern Betul Belt, central India

The Palaeoproterozoic Betul Belt in central India contains several small base metal (Zn-Cu and Zn-Pb-Cu) deposits that are hosted by felsic volcanics which form part of a bimodal volcanic suite. Although a volcanogenic massive sulfide (VMS) origin has been attributed to these deposits, they are not well-defined based on modern classification criteria. Regionally, the volcanic facies association around the Bhuyari zinc prospect includes massive rhyolite, pillowed basalt, rhyolitic autobreccia, hyaloclastite and peperite that indicate volcanism and sedimentation in relatively deep water. At the deposit-scale, the mineralization is hosted by intensely altered massive quartz-porphyritic rhyolite and associated tremolite-carbonate rock. Alteration facies mapping show a zoned alteration system with an inner phlogopite-biotite-chlorite-garnet-rich core (BPG schist; interpreted as having a chlorite-rich protolith) and an outer quartz-muscovite-K-feldspar zone (QMK schist; sericite-rich protolith). Metamorphism was largely isochemical and the mineral assemblages are amphibolite facies equivalents of hydrothermally altered protoliths. Tremolite-carbonate rocks are spatially close to mineralization and are interpreted either as metaexhalites or intensely altered rhyolite and probably had talc-dolomite protoliths. Carbon and oxygen isotope signatures of carbonate show derivation from seawater-dominant hydrothermal solutions. Tremolite-carbonate and some BPG schist show enrichment in HREE indicating hydrothermal mobilization of REE. Geochemically, the least-altered rhyolite has transitional magmatic affinities with moderate negative Nb anomaly and strong negative Sr, and Ti anomalies. Mantle upwelling and the associated high-heat source generated high-temperature, high-silica rhyolites with elevated HSFE and REE contents. Emplacement of felsic melts at depth provided heat that drove hydrothermal fluid circulation and formation of zinc-rich mineralization and associated hydrothermal alteration zones. Pb-model ages of galena give ~1800 Ma using the Stacey and Kramers (1975) model for VMS mineralization in Bhuyari. The Bhuyari deposit possibly formed in a continental back-arc rift environment during the amalgamation of Bastar and Bundelkhand cratons along the Central Indian Tectonic Zone. The high HFSE-REE contents of Bhuyari rhyolites are similar to other highly prospective VMS districts formed by rifting of evolved continental crust. The recognition of metamorphosed VMS deposit at Bhuyari has implications for further exploration of such deposits along the Paleoproterozoic Betul Belt and similar-aged deformed terrains in India and elsewhere.

First evidence for Neoproterozoic magmatism in the Quadrilátero Ferrífero of Minas Gerais, Brazil, and geotectonic implications

A decimetre-thick, deeply weathered sill intrusive in the Itabira iron formation and exposed at the historical Veloso underground mine in Ouro Preto, Minas Gerais, contains three populations of magmatic zircon grains: (i) xenocrysts with Archaean Pb–Pb and Hf model ages; (ii) xenocrysts of Palaeoproterozoic crystallisation ages of ca. 2200 Ma, with superchondritic εHf2.2 Ga values between 1.5 and 11.6; (iii) autocrysts and overgrowths formed at ca. 620 Ma, with εHf0.62 Ga values between −17.6 and 11.5 (mean = 1.5 ± 6.9, n = 15). Furthermore, there is evidence for zircon age reset at ca. 565 Ma. The three magmatic zircon populations indicate that the sill emplaced during the Neoproterozoic and probed different magmatic sources at depth, comprising anatectically reworked Palaeoproterozoic and Archaean felsic rocks. Variable Hf-isotope compositions of zircon rims and whole-rock geochemical data additionally suggest a hybrid origin for the sill magma. Our results provide the first evidence that the southern edge of the Quadrilátero Ferrífero is underlain not only by Archaean, but also by juvenile Palaeoproterozoic rocks, similar to those exposed in the adjacent Mineiro belt, and that the latter became accreted to the São Francisco craton prior to lower crustal anatexis during an early extension at ca. 620 Ma.

A miner's perspective on Pb isotope provenances in the Western and Central Mediterranean

The conventional approach to ore provenance studies of ancient silver coins and artifacts has been to first analyze and then try to match them to published data about mining districts, a difficult task given our incomplete knowledge of these. While literary sources are useful to identify possible provenances, they potentially bias interpretations proper because of a variety of limitations of their time. Archeological evidence in the form of mining shafts, galleries, spoil heaps, and tools also provides a tangible and reliable record of mining, but dating such activity can be problematic and the record is inconsistent. Here we propose a new approach driven by Pb isotopic data rather than numismatic groups. Statistical analysis of Pb isotopic data is used to identify ore-defined isotopic clusters. This new method is based on an algorithm that predicts the number of isotopic clusters necessary to fulfill the simple condition that variance within isotopic clusters is minimized whereas inter-cluster variance is maximized. Since each cluster reflects a discrete geological episode within a particular environment broadly datable to a specific Pb model age, it can be identified as a potential source exploited by ancient miners. We explore the potential of this method in two examples using data from coins and ores respectively. In the first example, Roman Republican silver coins form three ‘end-member’ clusters sourced in mining districts with Cenozoic, Mesozoic, and Paleozoic Pb model ages. The example demonstrates how sources of silver used to mint coinage of the Roman Republic shifted within 50 years of the end of the Second Punic War in 201 BCE. In the second example using Aegean galena samples, Pb isotopes distinguish components with model ages datable to the Hercynian basement, the recent Aegean tectonic province, and Cyprus, noting that significant silver mining districts may remain unidentified in either Spain or the Aegean world. We further clarify a number of potential analytical issues and advocate that users of Pb isotopes for tracing archeological artifacts measure all four lead isotopes and inspect the 12 proposed isotope combinations in order to select those that provide the best representation of the data. We also emphasize that full advantage should be taken of the geologically informed parameters (model age and Th/U/Pb relationships) to identify the geological context of metal sources.

Geochemistry of Gold Ores Mined During Celtic Times from the North-Western French Massif Central

The Celtic culture of Western Europe left magnificent gold objects, such as jewellery and weapons from nobility graves and hoarded coins, as well as field evidence of pre-Roman gold mining and metallurgical workshops that attest to the mining of local ores. This is the case of Central France where many precious metallic ores have been mined throughout the ages from the Prehistoric times onwards. One of the lingering problems in assessing the provenance of gold artefacts and coins is the lack of relevant data on the isotope geochemistry and mineralogy of ore sources. Forty gold ores samples were collected and studied from Limousin (French Massif Central), a very significant gold mining district from the Celtic times. Their Pb isotope compositions clearly show a local dichotomy i.e. two distinct groups of ores, one of Late Proterozoic to Early Paleozoic Pb model age and another associated to Variscan ages and consistent with field relationships, mineralogy and elemental analyses. The use of Cu and Ag isotopes, and their coupling with Pb isotopes, will refine the tracing of future metal provenance studies, but also highlight some metallurgical practices like deliberate metal additions to gold artefact or debasement of gold coins. The newly acquired Pb, Ag, and Cu isotopic data on gold ores improves our understanding of ore deposits geology and provide clarifications on the provenance of Celtic gold from this area and its economic importance.

Ages and sources of mantle eclogites: ID-TIMS and in situ MC-ICPMS Pb-Sr isotope systematics of clinopyroxene

Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction mélanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (3–6 ppm), unradiogenic Pb isotopic compositions (206Pb/204Pb = 13.57–13.52) and low 238U/204Pb (1.0–1.5) give single-stage model Pb dates of 2.90–2.84 Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18 ± 0.45 Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic 87Sr/86Sr (0.7016). Similarly, the age for samples from Orapa (2.20 ± 0.54 Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin.Gabbroic eclogites (Eu/Eu* > 1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model μ (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0 Ga addition of a sedimentary component, possibly derived from reworking of a high-μ basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work.

Ancient volcanism on the Moon: Insights from Pb isotopes in the MIL 13317 and Kalahari 009 lunar meteorites

Lunar meteorites provide a potential opportunity to expand the study of ancient (>4000 Ma) basaltic volcanism on the Moon, of which there are only a few examples in the Apollo sample collection. Secondary Ion Mass Spectrometry (SIMS) was used to determine the Pb isotopic compositions of multiple mineral phases (Ca-phosphates, baddeleyite K-feldspar, K-rich glass and plagioclase) in two lunar meteorites, Miller Range (MIL) 13317 and Kalahari (Kal) 009. These data were used to calculate crystallisation ages of 4332±2 Ma (95% confidence level) for basaltic clasts in MIL 13317, and 4369±7 Ma (95% confidence level) for the monomict basaltic breccia Kal 009. From the analyses of the MIL 13317 basaltic clasts, it was possible to determine an initial Pb isotopic composition of the protolith from which the clasts originated, and infer a 238U/204Pb ratio (μ-value) of 850±130 (2σ uncertainty) for the magmatic source of this basalt. This is lower than μ-values determined previously for KREEP-rich (an acronym for K, Rare Earth Elements and P) basalts, although analyses of other lithological components in the meteorite suggest the presence of a KREEP component in the regolith from which the breccia was formed and, therefore, a more probable origin for the meteorite on the lunar nearside. It was not possible to determine a similar initial Pb isotopic composition from the Kal 009 data, but previous studies of the meteorite have highlighted the very low concentrations of incompatible trace elements and proposed an origin on the farside of the Moon. Taken together, the data from these two meteorites provide more compelling evidence for widespread ancient volcanism on the Moon. Furthermore, the compositional differences between the basaltic materials in the meteorites provide evidence that this volcanism was not an isolated or localised occurrence, but happened in multiple locations on the Moon and at distinct times. In light of previous studies into early lunar magmatic evolution, these data also imply that basaltic volcanism commenced almost immediately after Lunar Magma Ocean (LMO) crystallisation, as defined by Nd, Hf and Pb model ages at about 4370 Ma.

Geochronology of the DeGrussa volcanic-hosted massive sulphide deposit and associated mineralisation of the Yerrida, Bryah and Padbury Basins, Western Australia

The Palaeoproterozoic Yerrida, Bryah and Padbury Group sedimentary rocks are located on the northern margin of the Archaean Yilgarn Craton and are host to epigenetic gold (Peak Hill, Fortnum and Horseshoe), volcanic-hosted massive sulphide (VHMS; Horseshoe Lights, Red Bore and DeGrussa) and epithermal copper deposits (Thaduna). The DeGrussa Cu–Au–Ag VHMS deposit (12.4Mt @ 4.7% Cu and 1.8g/t Au) comprises four lenses and is hosted in turbiditic sedimentary rocks and basalts of the DeGrussa host sediments/Karalundi Formation, the lowest mafic volcano-sedimentary unit of the Bryah Group and crosscut by intrusive dolerite of Narracoota Formation age. Immediate wall rocks to the ore lenses are commonly altered to chlorite and talc-carbonate.A number of geochronological techniques were employed to date mineralisation in the DeGrussa deposit. These include rhenium–osmium (Re–Os) of molybdenite, Pb isotopes of galena and pyrite, and U–Pb on zircon. Re–Os geochronology of molybdenite resulted in ages of 2027±7Ma and 2011±7Ma and 2013±7Ma for mineralisation, similar to Pb–Pb model ages for galena of 2060±50Ma and 2075±50Ma. U–Pb for magmatic zircons from intrusive dolerite within the mine sequence provided ages of 1991±7Ma, 1999±7Ma and 2003±7Ma with granodiorite rocks providing ages of 2014±7Ma and 2018±9Ma. Remobilisation of DeGrussa sulphides occurred at approximately 1980Ma given Re–Os ages of pyrrhotite (1982±7Ma and 1984±8Ma) and Pb–Pb pyrite ages of 1980±30Ma.Re-examination of the Pb isotopic evolution models for this area and the Capricorn Orogen suggests that the model of Stacey and Kramers (1975) is more appropriate than Cumming and Richards (1975); using the former model leads to revised model ages as follows: Horseshoe Lights VHMS 2000±35Ma; Nathans 1820±30Ma, Peak Hill (Main pit: 1610Ma, 1620Ma, 1705Ma, and 2070±30Ma; Fiveways: 1955Ma; Mt. Pleasant: 1770Ma), Horseshoe/Cassidy 1940±30Ma, Mikhaburra 1915±30Ma, and Labouchere 1800±30Ma. These new ages coincide with major regional orogenic events – the Glenburgh from 2005 to 1960Ma, the Capricorn from 1830 to 1780Ma, the Mangaroon from 1690 to 1620Ma, and the Edmundian from 920 to 850Ma. Although orogenic events are compressional, the Yerrida/Bryah Group sedimentary rocks, along with mineralisation at the DeGrussa and Horseshoe Lights deposits, were formed in an extensional basin setting during the Glenburgh Orogeny.These new ages assist in constraining stratigraphic and mineralising events of the Palaeoproterozoic Yerrida, Bryah and Padbury Group with regional orogenic events and may provide guidance for exploration for significant gold and base metal mineralisation, not only in the Bryah, but also throughout the larger Capricorn Orogen.

New data of the Bayan Obo Fe–REE–Nb deposit, Inner Mongolia: Implications for ore genesis

The Bayan Obo deposit in the Inner Mongolia, North China Craton, is the largest rare-earth element (REE) resource in the world. The deposit is hosted in Mesoproterozoic sediments of the Bayan Obo Group, the lithological sequence of which has been divided into seventeen units (H1–H17). The REE mineralization is mainly concentrated in the H8 dolomite unit, and therefore the depositional age of this unit is important in addressing the debate over the genesis of the REE–Fe–Nb ore deposit. Here we present U–Pb and Hf model ages of zircons from the adjacent H9 slate. Our data suggest that the H9 slate formed at 1505±12Ma and the H8 dolomite was deposited during the Mesoproterozoic. We also present Ar–Ar ages of riebeckite in veins cutting the H8 dolomite and biotite in the ore bodies which show 389.5±3.0Ma and 289.1±1.8Ma, respectively. The U–Pb measurements from overgrowth zircon in H9 slate give an age of 518.8±7.5Ma. These new age data indicate that the Bayan Obo deposit was disturbed during thermal events at 520Ma to 390Ma, generating a second phase of REE–Nb mineralization. The δ7Li values of the REE mineralized dolomite are similar to those of carbonatite dykes. Combined with ɛNd features, we provide geochemical evidences that REE mineralized dolomite has undergone fluid interaction. Results reveal that the carbonatite magmatism occurred within the Bayan Obo rift at ca. 1.2 Ga, provided the primary source for REE–Nb mineralization or to fertilize the Bayan Obo crust, and the later was remobilized during the Caledonian to generate the second phase of mineralization within a convergent plate margin regime.