Inclusion Suite Research Articles

Compositional Signatures of Gold from Different Deposit Types in British Columbia, Canada

A study of both in situ and detrital gold from different deposit types in British Columbia was undertaken to establish deposit-specific compositional characteristics in terms of alloy composition and suites of mineral inclusions. The study is based on 11,840 particles from 160 localities in which nine gold deposit types are represented, although there is a strong bias towards gold of orogenic, low-sulphidation epithermal, and alkalic porphyry origin. In general, Ag values in gold alloys are not a powerful discriminator for deposit type, but minor metals may prove useful where detectable, e.g., Cu in gold from ultramafic associations and Pd and Hg in gold from alkalic porphyry systems. The characterization of inclusion suites is far more illuminating, as they correlate strongly with the mineralogy of auriferous ores from different deposit types. This outcome has confirmed the validity of designing an indicator methodology based on inclusion suites and has permitted the prediction of inclusion suites for gold from other deposit types where data are more scarce. The compositional templates generated in the study were applied to identify the source deposit type(s) of gold from 41 localities (a total of 2916 detrital gold particles) where gold genesis was previously unknown.

The multi-component mantle source of Roman province ultrapotassic magmas revealed by melt inclusions

A key effect of subduction is the upward infiltration of slab-derived melts and fluids to create a chemically and lithologically heterogeneous region in the mantle wedge. Magmatism within the post-subduction setting of peninsular Italy is derived from a sediment-metasomatised lithospheric mantle source, with primitive products recording substantial temporal and spatial variations in the nature and extent of metasomatism. The Roman magmatic province, in central Italy, is host to ultrapotassic (HKS), leucite-bearing magmas that require source modification that is not yet well-constrained in terms of mineralogy and sediment provenance. Here, we present the chemistry and Sr-Nd-Pb isotope compositions of melt inclusions in forsterite-rich olivine (Fo88−92) from three key Roman volcanic centres (Vulsini, Sabatini, Alban Hills) to characterise the metasomatic imprint of their mantle sources. The melt inclusion suites (Vulsini HKS, Vulsini melilite-bearing HKS, Sabatini HKS, Alban Hills HKS) have clear differences in major-element (K, Ca, Al, P, Ti), volatile (F, Cl, B, Be), trace-element (e.g., Rb, Sr/Y, Th/Nb), and radiogenic-isotope (87Sr/86Sr) compositions that reflect the diversity of the primary melts beneath the region. For each suite, the extraction of melt batches from different domains in a heterogeneous (veined) source rock with different compositions and mineralogies can explain the chemical characteristics of the melts. At least three metasomatic vein types are required to form the respective parental melts and we infer phase assemblages consisting of phlogopite + Ca-amphibole + clinopyroxene ± apatite. The Sr-isotope differences between Vulsini melilite-bearing HKS and Vulsini and Sabatini HKS require at least two distinct metasomatic events to have occurred beneath the Roman province. These events need not be associated with two distinct subduction systems. Instead, we infer that different metasomatic agents could have originated under different P-T-t conditions from different domains in the same subducting slab.

Discrimination of thermodynamic and kinetic contributions to the heavy rare earth element patterns in metamorphic garnet

AbstractVariations of rare earth element (REE) concentrations in metamorphic garnet are an important source of information of geodynamic and geochemical processes in the deeper Earth. In order to extract this information, the thermodynamic equilibrium and kinetic contributions of the REE uptake in garnet must be distinguished and quantified. Utilizing high‐resolution trace element and μ‐Raman mapping together with combined thermodynamic–geochemical–diffusion models, we demonstrate that the equilibrium and kinetic aspects of the REE uptake in metamorphic garnet can be discriminated by interpreting 2D trace element mapping in a single sample. The heavy (H) REE (Tb to Lu) zoning in the investigated garnet from a high‐pressure blueschist comprises an inner part with an overall decrease from core to inner rim, followed by a concentric zone of HREE enrichment and a drastic HREE decrease towards the outermost rim. The central peak in the garnet core decreases in intensity with decreasing atomic number of the REE. The broad overall shape of this pattern resembles those often observed in metamorphic garnet from different rock types and tectonic settings. Superimposed on this trend is a concentric pattern of minor recurring fluctuations in the HREE concentrations with at least six regularly spaced sets of peaks and troughs along the entire garnet radius. Comparison of the observed inclusion suite, the trace element maps and thermodynamic–geochemical models show that the inner part with decreasing HREE concentrations results from fractional garnet growth in an unchanged mineral assemblage, whereas the REE enrichment zone is caused by the breakdown of titanite. We suggest that the width of the central peak is controlled by the bulk permeability of the interconnected transport matrix and the fraction of matrix minerals that the garnet equilibrates with. The superimposed REE fluctuations result from changing element transport properties of the host rock and mark recurring changes from equilibrium REE uptake to transport‐limited REE uptake in garnet. Such fluctuating element transport properties can be best explained by pulse‐like fluid fluxes that rhythmically change the interconnectivity of the intercrystalline transport matrix. Increasing numbers of published spatially highly resolved REE analyses show that such trace element fluctuations are common in metamorphic garnet indicating that recurring changes in rock permeabilities due to pulsed fluid fluxes are a common phenomenon during metamorphism.

Gold Provenance in Placers from Pureo Area, Southern Chile Coastal Cordillera, and Their Relationship with Paleozoic Metamorphic Rocks

Southern Chile placer gold deposits have been known and exploited since Spanish colonial times. Despite this, precise knowledge about their origin is scarce. This work aims to identify possible primary sources of the gold in the Pureo placers by studying the morphological and chemical characteristics of gold particles according to their spatial distribution. The former was determined by measurements and classification under a binocular microscope, allowing us to acquire a set of parameters related to the amount of transport that had affected the samples. The microchemical characteristics were determined by studying gold particles using optical microscopy, scanning electron microscopy (SEM) and electron microprobe (EMPA), where the native gold composition (in terms of major and minor elements) and the suite of mineral inclusions were obtained. The results regarding morphological characteristics suggest a low amount of transport from a primary source (<15 km). Microchemical data from gold particles indicate two compositional sub-populations, distinguished in both native gold composition (<15 Ag wt% with up to 4 Hg wt% and >15 Ag wt% with Hg bellow 1 wt%) and mineral inclusions (pyrite-galena rich and arsenopyrite rich, respectively), indicating two different primary gold sources. These results suggest a local origin of gold in the Coastal Cordillera, where the possible primary sources are associated with (i) massive sulfide deposits present in Paleozoic–Triassic metamorphic rocks and (ii) hydrothermal deposits associated with more recent Cenozoic intrusive activity. These conclusions have implications for the exploration of new placer deposits and of gold-bearing hypogene deposits (e.g., metamorphosed VMS deposits) in unexplored zones of southern Chile Coastal Cordillera.

Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland

Abstract The oxygen isotope composition of mantle-derived melts can place important constraints on how magmas are processed as they traverse the crust. Assimilation of crustal material is a crucial aspect of basalt petrogenesis, as it affects the chemical and rheological characteristics of eruptive magmas at active volcanoes. We report oxygen isotope (δ18O) and trace element (TE) data from a suite of well-characterized basaltic melt inclusions and groundmass glasses from the Bárðarbunga volcanic system in Iceland to assess how and where in the plumbing system crustal rocks interact with ascending magmas. While both melt inclusions and groundmass glasses record a large range in δ18O values (+3.2‰ to +6.4‰ and +2.6‰ to +5.5‰, respectively) groundmass glasses record lower values on average. Relationships between incompatible trace element (e.g., Zr/Nb) and oxygen isotope ratios are best explained with three-component mixing, where primary melts derived from depleted and enriched mantle components with distinct δ18O values mix and acquire a low-δ18O character upon progressive contamination with altered Icelandic crust. The majority (60%) of melt inclusions require 10–30% exchange of oxygen with the Icelandic crust. In addition, for the first time, we link the extent of oxygen isotope exchange with melt equilibration depths, showing that most of the contamination occurs at 1–2 kbar (3–7 km depth). We propose that a progressively assimilating, multi-tiered plumbing system is a characteristic feature of the Bárðarbunga volcanic system, whereby chemical modifications resulting from interaction with the crust systematically increase as melts migrate through higher crustal levels. We show that similar processes may also occur across the active rift zone in Iceland.

Diffusive equilibration of H2O and oxygen fugacity in natural olivine-hosted melt inclusions

Arc basalts are oxidized relative to mid-ocean ridge basalts; however, the mechanism through which arc basalts gain this elevated oxidative state is debated. This observation has been extended to include Fe3+/∑Fe measurements from olivine-hosted melt inclusions enclosed in relatively Fo-rich olivines (i.e., which have presumably trapped relatively unevolved liquid compositions as melt inclusions). Experimental data suggest, however, that Fe3+/∑Fe recorded by melt inclusions can diffusively equilibrate with the host magma on short timescales, perhaps as rapidly as timescales relevant for H+ diffusive equilibration. Here, we present major, minor, and trace element concentrations, H2O concentrations, and Fe3+/∑Fe of naturally glassy olivine-hosted melt inclusions from the Basalt of Round Valley Butte, a cinder cone in the Lassen Region of northern California. These melt inclusions record H2O loss through diffusion of H+ and can be used to constrain the extent of Fe3+/∑Fe equilibration in a suite of natural melt inclusions. The H2O concentrations of Basalt of Round Valley Butte melt inclusions decrease with decreasing melt inclusion radius, consistent with diffusive H+ loss. The Fe3+/∑Fe do not correlate with H2O or inclusion diameter, suggesting that Fe3+/∑Fe equilibration does not occur on the same timescale as H+ diffusion. Calculated oxygen fugacities (ƒO2) from the partitioning of vanadium between olivine and melt are ∼1 log unit higher than those calculated from Fe3+/∑Fe. One explanation for these combined observations is that Fe3+/∑Fe of the Basalt of Round Valley Butte melt inclusions diffusively equilibrated with a host magma at low pressure on shorter timescales than H+ diffusion. This scenario requires that the host magma decreased in ƒO2 after the time of melt inclusion entrapment to explain the offset between ƒO2 constraints from Fe3+/∑Fe of the melt inclusions and the partitioning of vanadium between olivine and melt. One volcanological process known to result in a decrease in ƒO2 of magmas stored at shallow pressure is the degassing of S, CO2, and H2O. We show that reasonable estimates for the extent of degassing of Basalt of Round Valley Butte magma, coupled with diffusive equilibration of both H+ and Fe3+/∑Fe, can reconcile Fe- and V-proxies for ƒO2. Additionally, we have compared estimates of ƒO2 from vanadium abundances of melt inclusions and their olivine hosts with recently reported measurements of Fe3+/∑Fe of the melt inclusions for several Mariana arc volcanoes and conclude that this process – small decreases in host magma ƒO2 driven by degassing and rapid equilibration of the Fe3+/∑Fe of olivine-hosted melt inclusions with this host magma - may be a widespread phenomenon.

Deciphering Degassing and Source Effects in Cl Isotopes in Melt Inclusions: The Possible Role of Amphibole in the Magma Source of Stromboli (Aeolian Island Arc)

Chlorine isotopes have emerged as a new geochemical tool over the past 15 years. Most of the data consist of bulk rock data, with a minority carried out in situ on melt inclusions using secondary ion mass spectrometry. More data are necessary to understand the relationship between δ37Cl measured in melt inclusions and that in bulk rocks from the same volcanic center. Here we have analyzed a suite of melt inclusions entrapped in olivine Fo63-85, as well as some from clinopyroxene crystals, from a single hand-sample from the Vancori unit of Stromboli, Aeolian Islands. The 27 selected melt inclusions have major element compositions ranging from high potassium alkali basalt to evolved shoshonite. Their δ37Cl vary from −2.6 ± 0.1‰ to +1.2 ± 0.2‰, a far larger range than for Stromboli bulk rocks. In this dataset, the δ37Cl variation in melt inclusions is not related to Cl degassing, or to fractional crystallization. Instead, correlations between δ37Cl and S/Cl, K2O and trace element ratios suggest mixing of two Cl endmembers with distinct δ37Cl signatures. A first endmember is characterized by high potassium alkali basalt compositions, high Ba/La (∼28), high S/Cl, and high δ37Cl (>1‰), confirming the influence in the mantle source of an aqueous fluid and providing a new constraint on its composition: that it derives from the breakdown of amphibole. The second endmember has a more evolved composition, high La/Yb, low S/Cl, and low δ37Cl (<−2‰), identifying the influence of a solute-rich component derived from subducted sediments. The δ37Cl data thus help refine the two sources initially identified from bulk rock studies and δ37Cl proves to be a potential tracer for amphibole.

Cr-pyrope xenocrysts with oxide mineral inclusions from the Chompolo lamprophyres (Aldan shield): Insights into mantle processes beneath the southeastern Siberian craton

AbstractPyrope xenocrysts (N = 52) with associated inclusions of Ti- and/or Cr-rich oxide minerals from the Aldanskaya dyke and Ogonek diatreme (Chompolo field, southeastern Siberian craton) have been investigated. The majority of xenocrysts are of lherzolitic paragenesis and have concave-upwards (normal) rare earth element (REEN) patterns that increase in concentration from light REE to medium–heavy REE (Group 1). Four Ca-rich (5.7–7.4 wt.% CaO) pyropes are extremely low in Ti, Na and Y and have sinusoidal REEN spectra, thus exhibiting distinct geochemical signatures (Group 2). A peculiar xenocryst, s165, is the only sample to show harzburgitic derivation, whilst demonstrating a normal-to-weakly sinusoidal REEN pattern and the highest Zr (93 ppm) and Sc (471 ppm). Chromite–magnesiochromite, rutile, Mg-ilmenite and crichtonite-group minerals comprise a suite of oxide mineral inclusions in the pyrope xenocrysts. These minerals are characteristically enriched in Cr with 0.6–7.2 wt.% Cr2O3 in rutile, 0.7–3.6 wt.% in Mg-ilmenite and 7.1–18.0 wt.% in the crichtonite-group minerals. Complex titanates of the crichtonite group enriched in large ion lithophile elements (LILE) are high in Al2O3 (0.9–2.2 wt.%), ZrO2 (1.5–5.4 wt.%) and display a trend of compositions from the Ca–Sr-specific varieties to the Ba-dominant species (e.g. lindsleyite). In the pyrope xenocrysts the oxides coexist with silicates (clino- and orthopyroxene and olivine), hydrous silicates (talc, phlogopite and amphibole), carbonate (magnesite), sulfides (pentlandite, chalcopyrite, breakdown products of monosulfide and bornite solid solutions), apatite and graphite. P–T estimates imply the inclusion-bearing pyrope xenocrysts have been derived from low-temperature peridotite assemblages that resided at temperatures of ~600–800°C and a pressure range of ~25–35 kbar in the graphite stability field. Pyrope genesis is linked to the metasomatic enrichment of peridotite protoliths by Ca–Zr–LILE-bearing percolating fluid–melt phases containing significant volatile components. These metasomatic agents are probably volatile-rich melts or supercritical C–O–H–S fluids that were released from a Palaeo-subduction slab.

Entrapment history of aqueous fluids in Archean cherts from the Barberton Greenstone Belt, South Africa

We performed fluid inclusion microthermometric analyses of four cherts and one silicified anhydrite from various successions of the Mesoarchean Barberton Greenstone Belt, South Africa, and constrained the entrapment conditions and provenance of fluids that were incorporated during different magmatic or tectono-thermal events. Major saline aqueous fluid inclusion assemblages in cherts from the 3.2 Ga old Mendon Formation (Barite Valley Syncline, Power Line Road) and from the 3.42–3.45 Ga old uppermost Hooggenoeg Formation (Buck Reef Chert) correspond to fluids incorporated during metamorphism. Their documented P-T conditions compare well with reported petrologic constraints. In Buck Reef chert we also found a distinctive suite of inclusions with high salinities and low homogenization temperatures (61–118 °C). This is in agreement with a hydrothermal formation of these fluids at subseafloor conditions. Finally, we investigated quartz in a silicified anhydrite and its sandstone host from the Stolzburg Syncline (Moodies Group, ca. 3.2 Ga). One major inclusion assemblage in the evaporite and in adjacent quartz within the embedding sandstone share the P-T range observed for the cherts from the Mendon Fm. Its incorporation is postdating the silicification process. We also found two higher temperature inclusion types that favor a magmatic origin. In particular, one of these inclusion suites is only observed in quartz within the matrix and likely mirrors trapping of brines associated with magmatic fluids predating sedimentation of the sandstone, e.g., from the potential magmatic source region of the quartz. Our results indicate the presence of coexisting aqueous fluids in cherts derived from distinct source environments, spanning an age range of several 100 Ma on a grain scale. The earliest hydrothermal aqueous inclusions were documented in a Buck Reef chert (3.42–3.45 Ga), whereas the majority of aqueous fluids are metamorphic in origin and likely related to the documented 3.23 Ga and 3.1 Ga tectono-thermal events. Our findings demonstrate that constraining environmental conditions by the volatile record of Archean cherts requires analyses on inclusion, not grain scale.

Detrital gold as an indicator mineral

Abstract Detrital gold fulfils the criteria of chemical inertia and physical durability required by indicator minerals but it has not found wide application in this role because it may be formed in different deposit types. This problem is soluble, because the generic compositional features of hydrothermal gold differ according to mineralization environment. The wide distribution of gold as a minor component of mineralization where other commodities are the principal exploration target extends the potential of an indicator methodology based on detrital gold to beyond the search for gold itself. Here we highlight how distinctive gold compositional signatures derived from alloy composition and deposit- specific suites of mineral inclusions could contribute to exploration for Cu–Au porphyries, redox-controlled uranium mineralization and ultramafic-hosted PGE mineralization. Future refinement this approach will focus on establishing the spatial distribution of elements at trace levels within gold particle sections using ToF-LA-ICP-MS and application of Exploratory Data Analysis to the resulting datasets. This approach is in its infancy, but aims to develop a classification algorithm useful to researchers irrespective of their previous experience. A pilot study has shown that random forests provide the best approach to establishing gold particle origins.

Composition of the Sub‐Cratonic Mantle of the Guiana Shield Inferred From Diamond‐Hosted Inclusions

AbstractThe composition of diamond‐hosted inclusions provides insight into the character of the sub‐cratonic lithosphere of the Guiana Shield. Guyana's Paleoproterozoic diamonds preserve an inclusion suite comprised of forsterite (Fo ∼89.3–91.8), enstatite, chromite, and Cr‐pyrope. Raman thermobarometry of entrapped olivine and pyrope inclusions indicate entrapment pressures of ∼5.3–7.0 GPa. Unpolarized Fourier transform infrared spectroscopic measurements of forsterite and enstatite inclusions produce low absorbances from OH. Using established calibrations, those absorbances indicate forsterite and enstatite contain median values of 26 ± 14 and 14 ± 18 ppm H2O, respectively, and suggest a high effective viscosity of 1023.7 ± 2.1 Pa∙s for the lithospheric mantle. When combined with inclusion thermobarometry, diamond residence temperatures suggest paleo‐geotherms ranging from 35 to 40 mW m−2. Low‐to‐moderate Fe2O3 content (1.9 ± 0.8 wt.%) and low oxygen fugacity (log ƒO2 (ΔFMQ) −1.6 ± 1.1) determined from chromite inclusions indicate crystallization in reducing conditions. Forsterite and chromite inclusions retain evidence for metasomatic alteration, including Mn‐enrichment in forsterite and chromite rich in Zn. These characteristics indicate that the sub‐cratonic lithosphere of the Guiana Shield experienced episodes of partial melting and fluid‐driven metasomatism of dry, strongly viscous, and moderately depleted garnet‐spinel harzburgite. The Guiana Shield has been relatively stable since the Paleoproterozoic, meaning diamond inclusions may also provide the best means for understanding current conditions in the region's lithospheric mantle.

Mesozoic to Paleoproterozoic diamond growth beneath Botswana recorded by Re-Os ages from individual eclogitic and websteritic inclusions

Re-Os isotope systematics are reported from a suite of eclogitic and websteritic sulphide inclusions extracted from well-characterised diamond growth zones from the Orapa and Jwaneng kimberlite clusters. Re-Os ages (786 ± 250 Ma) are within uncertainty of previously determined Sm-Nd ages (853 ± 55 Ma), demonstrating isotopic equilibrium, at varying levels of completeness, across multiple isotopic systems in different minerals at the time of diamond formation and inclusion encapsulation. These data confirm the concept that inclusion isochron ages, when used with detailed textural/ growth zone control, reflect the timing of diamond crystallisation. Our data substantiate previous Re-Os and Sm-Nd inclusion ages of diamonds from Orapa and Jwaneng, indicating that major tectono-magmatic events formed discrete diamond populations of Paleo- (~ 2.0 to 1.7 Ga), Meso- (~ 1.2 to 1.1 Ga) and Neoproterozoic (~ 0.9 to 0.75 Ga) age. Some of these processes occurred simultaneously across the Kalahari Craton and can be traced over 100's of km illustrating the significance of diamond inclusions for monitoring continental tectonics.Inclusion ages indicating diamond formation that are younger than 300 Ma appear to be more common than previously recognised, consistent with evidence of relatively abundant, young, fluid-rich “fibrous” and polycrystalline diamonds at Jwaneng and Orapa. The increasingly widespread evidence for Mesozoic diamond-forming events in southern Africa and elsewhere appears closely linked with the kimberlite-related magmatism that affected these regions and subsequently transported diamonds to the surface. The inclusion isochron ages emphasise that diamond formation is a multi-stage and episodic process that can occur contemporaneously in disparate substrates and produce multiple diamond populations in the sub-continental lithospheric mantle.

Chemical and physical heterogeneity within native gold: implications for the design of gold particle studies

Studies of populations of gold particles are becoming increasingly common; however, interpretation of compositional data may not be straightforward. Natural gold is rarely homogenous. Alloy heterogeneity is present as microfabrics formed either during primary mineralization or by modification of pre-existing alloys by chemical and physical drivers during subsequent residence in either hypogene or surficial environments. In electron-probe-microanalysis (EPMA)-based studies, the combination of Cu, Hg, and Pd values and mineral inclusion suites may be diagnostic for source style of mineralization, but Ag alone is rarely sufficient. Gold characterization studies by laser-ablation-ICP mass spectrometry linked to both quadrupole and Time-of-Flight (ToF-MS) systems show that only Ag, Cu, and Hg form homogenous alloys with Au sufficiently often to act as generic discriminants. Where present, other elements are commonly distributed highly heterogeneously at the micron or submicron scale, either as mineral inclusions or in highly localized, but low concentrations. Drawing upon our own data derived from individual inspection and analyses of approximately 40,000 gold particles from 526 placer and in situ localities worldwide, we show that adequate characterization of gold from a specific locality normally requires study of a minimum of 150 particles via a two-stage approach comprising spatial characterization of compositional heterogeneity, plus crystallographic orientation mapping, that informs subsequent targeted acquisition of quantitative compositional data by EPMA and/or laser-ablation ICP-MS methods. Such data provide the platform to review current understanding of the genesis of gold particle characteristics, elevating future compositional studies from empirical descriptions to process-focused interpretations.

Garnet sand reveals rock recycling processes in the youngest exhumed high- and ultrahigh-pressure terrane on Earth

Rock recycling within the forearcs of subduction zones involves subduction of sediments and hydrated lithosphere into the upper mantle, exhumation of rocks to the surface, and erosion to form new sediment. The compositions of, and inclusions within detrital minerals revealed by electron microprobe analysis and Raman spectroscopy preserve petrogenetic clues that can be related to transit through the rock cycle. We report the discovery of the ultrahigh-pressure (UHP) indicator mineral coesite as inclusions in detrital garnet from a modern placer deposit in the actively exhuming Late Miocene-Recent high- and ultrahigh-pressure ((U)HP) metamorphic terrane of eastern Papua New Guinea. Garnet compositions indicate the coesite-bearing detrital garnets are sourced from felsic protoliths. Carbonate, graphite, and CO2 inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO2 (cristobalite) and K-feldspar (kokchetavite) that we interpret as rapidly cooled former melt inclusions. Application of elastic thermobarometry on coexisting quartz and zircon inclusions in six detrital garnets indicates elastic equilibration during exhumation at granulite and amphibolite facies conditions. The garnet placer deposit preserves a record of the complete rock cycle, operative on <10-My geologic timescales, including subduction of sedimentary protoliths to UHP conditions, rapid exhumation, surface uplift, and erosion. Detrital garnet geochemistry and inclusion suites from both modern sediments and stratigraphic sections can be used to decipher the petrologic evolution of plate boundary zones and reveal recycling processes throughout Earth's history.

U–Pb ages of rare rutile inclusions in diamond indicate entrapment synchronous with kimberlite formation

The timing of diamond crystallization is generally inferred from radiometric dating of individual or multiple mineral inclusions in diamond, where ages are derived from isochrons or isotopic evolution models. Resulting ages often significantly predate the emplacement ages of host kimberlites, but age information from both approaches can be ambiguous due to long-lived mantle heterogeneities where mixing can mimic isochrons or in-situ aging. Direct dating of accessory mineral inclusions in diamonds, by contrast, has rarely been attempted because of the scarcity of such inclusions, requiring careful high grading of large amounts of diamond concentrates. Here, we report secondary ionization mass spectrometry (SIMS) U–Pb ages from a suite of rare rutile inclusions which were extracted from diamond in an eclogitic paragenesis from the Mir kimberlite pipe, Siberia. One population of rutile inclusions shows diamond crystal-shapes and was petrographically identified as completely enclosed in diamond prior to host burning for inclusion extraction. These rutile inclusions were screened for U abundances (0.24–23 μg/g U) and those with the highest U contents were dated, yielding a uniform U–Pb concordia age of 375.5 ± 7.0 Ma (error 95% confidence; mean square of weighted deviates MSWD = 0.74; number of spots n = 19). This age is nearly coeval with, but significantly (at the 95% confidence level) older than combined U–Pb rutile ages for rutile intergrown with polycrystalline fibrous diamond (362.9 ± 9.5 Ma; MSWD = 0.96; n = 7) and rutile from the eclogitic xenolith matrix (369 ± 16 Ma; MSWD = 0.30; n = 5). Rutile ages are consistent with published ages for kimberlite emplacement (ca. 360 Ma), but much younger than Re–Os sulfide isochron ages between ca. 913 Ma and 2.1 Ga for groups of individual inclusions in diamond from the same location. When assuming the oldest Re–Os sulfide age as the timing of rutile entrapment, diffusive Pb-loss from rutile inclusions over ca. 2 Ga would require diffusivities of Pb in diamond of >10−22 m2/s, as well as Pb partitioning coefficients between diamond and rutile at near or greater unity. Alternatively, if Pb is immobile in diamond, ancient rutile ages could have been reset due to processes other than volume diffusion through diamond (e.g., due to fluid migration along cracks). However, in this scenario it remains difficult to explain why all rutile inclusions dated here would have been located along cracks, whereas sulfide inclusions investigated in other studies were not. Another explanation is that rutile inclusions were entrapped at the time of diamond formation, and that this event is recorded by U–Pb rutile ages, whereas sulfides predate diamond crystallization and their Re–Os systematics were incompletely reset. Trace element heterogeneity in rutile suggests that they originated in chemically different environments, and thus rutile likely predates diamond formation. However, due to rapid diffusion of Pb in rutile, radiogenic Pb accumulated only after entrapment in diamond (for inclusions) or eruptive quenching (for intergrown or matrix rutile). Overall, rutile geochronology indicates that at least some diamond from the Mir pipe formed briefly (within the ca. 20 Ma age difference between U–Pb rutile ages for inclusions and intergrown/matrix rutile) before kimberlite eruption, supporting models that link diamond formation with carbon-rich precursors of kimberlite magmas.

Ferropericlase crystallization under upper mantle conditions

Ferropericlase is a common lower mantle phase that can occur as inclusions in “superdeep” diamonds. However, it has been proposed that some of these inclusions could be formed in the upper mantle (Brey et al. 2004). To explore this hypothesis, two types of experiments on ferropericlase crystallization were conducted at 5–12 GPa and 1500–1700 °C: (1) equilibrium experiments with Ca–Mg–Fe carbonate + MgO + olivine mixtures and (2) “sandwich” experiments containing a layer of metallic iron overlain by olivine, with a carbonate layer on top. The equilibrium experiments produced a series of carbonate–silicate melts saturated in olivine and ferropericlase. These melts contain 2–12 wt% SiO2, which correlates negatively with CaO and CO2 contents and positively with temperature. A comparison of the composition of the carbonate–silicate melts with literature data for melts saturated in olivine and low-Ca pyroxene indicate that melts saturated in ferropericlase cannot be produced by crystallization of melts derived by melting carbonated harzburgite or lherzolite at upper mantle conditions. On the other hand, the sandwich experiments reveal that ferropericlase and diamond (or metastable graphite) can crystallize simultaneously during the reduction of carbonate–silicate melt. The resulting ferropericlase crystallizing in equilibrium with olivine will be richer in Fe compared with lower mantle ferropericlase in equilibrium with bridgmanite. Thus, the considerable variation in Mg# values observed for some suites of ferropericlase inclusions in diamond could in part be attributable to ferropericlase formation in the upper mantle.

Ore fluid, geochronology and tectonic setting of mesothermal gold metallogeny in southeastern Jilin Province, Northeast China: A case study of the Shajingou gold deposit

The Shajingou gold deposit is a recently discovered medium-scale gold deposit in southeastern Jilin Province, Northeast China. The deposit is located in the easternmost segment of the northern margin of the North China Craton (NCC). This study investigated the mineralization age and tectonic setting of the Shajingou gold deposit based on geology, microthermometry and H–O isotopic analyses of fluid inclusions (FIs), pyrite Re–Os and zircon U–Pb dating, petro-geochemistry and zircon Hf isotopic analyses. The findings are placed in the wider context of mesothermal gold metallogeny in Northeast China.Metallogenesis of the gold deposit can be divided into three stages based on the ore mineral assemblage: quartz–K-feldspar–pyrite (stage I), quartz–polymetallic sulfide–native gold (stage II) and quartz–carbonate (stage III). Four types of primary FI were identified in quartz and calcite minerals: liquid-rich aqueous (L-type), CO2-bearing (C1-type), CO2-rich (C2-type) and CO2-pure (C3-type). From stage I to stage III, the fluid inclusion suite changes from L-type and C2-type in stage I, all four types in stage II and only L-type in stage III. Homogenization temperatures of stages I to III are 318–394 °C, 264–356 °C and 202–276 °C, respectively. Salinities of stages I–III are 7.31–12.42, 2.02–8.94 and 0.53–3.53 wt% NaCl equiv., respectively. The ore-forming fluid system evolved from a H2O–NaCl–CO2 system in stages I and II to a H2O–NaCl system in stage III. During the stage II, the addition of meteoric water led to decreases in temperature and pressure, resulting in an immiscible H2O–NaCl–CO2 fluid system. This immiscibility in turn led to the escape of CO2 and other volatiles, the increase of pH and the decrease in solubility and stability of Au–S clathrates, thus facilitating the deposition of gold and associated polymetallic sulfides.Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating of the Shajingou diorite yielded a weighted mean 206Pb/238U age of 171.7 ± 0.9 Ma (MSWD = 3.0). Re–Os dating of the gold-bearing pyrite yielded an isochron age of 166 ± 29 Ma (MSWD = 5.2). The Shajingou diorite has high concentrations of SiO2, Al2O3, Na2O, Mg#, Cr and Ni, weakly positive Eu anomalies, and is LREE- and LILE-enriched, HREE- and HFSE-depleted, as well as has positive εHf (t) values (+3.0 to +7.1). This indicates the diorite was sourced from the partial melting of a depleted-mantle wedge that had been modified by the subducted- slab-derived fluids. The deposit geology, ore genesis, geochronology and tectonic setting of the Shajingou gold deposit differ from those of other gold deposits in the eastern segment of the XMOB, but are similar to those of gold deposits in the Jiapigou–Haigou gold belt (JHGB) of southeastern Jilin Province at the northern margin of the NCC. These findings suggest the JHGB extends southeastward to the Helong area of Yanbian, which indicates the need for further research on the gold mineralization potential of the southeastern JHGB and further gold prospecting in the region. Comparative studies of Shajingou and other gold deposits in the JHGB have confirmed that a significant tectonic–magmatic–hydrothermal event occurred during the Middle Jurassic due to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.

Microchemical characterization of placer gold grains from the Meyos-Essabikoula area, Ntem complex, southern Cameroon

Gold occurs as a native metal, usually containing silver, and in some cases mercury, copper, and palladium. It may also occur as inclusions within sulfur-rich minerals, such as pyrite and arsenopyrite. The style and variety of gold mineralization is influenced by the geological setting, chemistry of the ore fluids, and the nature of their interactions with rocks. Gold grains liberated from bedrock into surficial sediments during weathering and erosion are chemically stable and may be characterized according to their mineralogy: i.e the alloy composition and suite of mineral inclusions revealed within polished sections, characteristics faithful to gold from the hypogene source. This approach has been applied to placer gold grains from the Meyos-Essabikoula area, Cameroon, where the source of gold is not yet confirmed due to poor outcrop exposure.A total of 221 alluvial gold grains from 10 sites, tributaries of Sing and Bivele River over the Ntem Complex have been studied using Electron Probe Micro-Analysis (EMPA) to determine the concentration of minor alloying metals, (notably Au, Ag, Cu, and Hg) and Scanning Electron Microscopy (SEM) in order to evaluate the assemblage of mineral inclusions within the gold. Most of the grains are sub-rounded with pitted surfaces and inclusions of pyrrhotite, acanthite, and chalcopyrite were observed. The grains are AuAg alloys ranging from 54.4 to 99.8 wt% Au, 0.1–48.4 wt% Ag, 0.1–0.8 wt% Hg and 0–0.3 wt% Cu. The presence of Fe oxide (magnetite) inclusions containing Cr and V (to around 5 wt %) has not been reported elsewhere and suggests a strong interaction between hot reducing ore fluids and local mafic lithologies.

An unusual suite of eclogitic, websteritic and transitional websteritic-lherzolitic diamonds from the Voorspoed kimberlite in South Africa: Mineral inclusions and infrared characteristics

A study of the morphology, mineral inclusions, nitrogen content, and nitrogen aggregation state of diamonds from the Voorspoed kimberlite, was conducted. The diamonds are characterised by a highly unusual inclusion mineral paragenesis dominated by eclogitic and transitional websteritic-lherzolitic inclusions along with related websteritic inclusions, while a comparatively minor harzburgitic diamond inclusion suite account for the remainder. This differs substantially from many of the diamond populations in kimberlites on the Kaapvaal Craton where a harzburgitic inclusion paragenesis predominates. Only in the case of the Orapa kimberlite in Botswana has a similar diamond inclusion suite been encountered before, although in that instance an eclogitic and websteritic inclusion suite predominate.Calculated garnet-clinopyroxene equilibration temperatures, at an assumed pressure of 50 kbar, range from 1040 °C to 1296 °C. Temperatures of 1114 °C to 1348 °C, at 50 kbar, are calculated for lamellar orthopyroxene-clinopyroxene intergrowths. ‘Reconstituted’ pyroxenes mathematically reconstructed from the lamellar orthopyroxene-clinopyroxene intergrowths produce temperatures of 1238 °C to 1416 °C, suggesting that the lithosphere cooled by at least 100 °C after diamond crystallisation.Nitrogen contents and nitrogen aggregation states of whole diamonds range from below the detection limit (~10 ppm) to 1442 atomic ppm (average 167 atomic ppm), and up to 97% of the highly aggregated ‘B' component (average 65%), respectively. The elevated aggregation state of the nitrogen in the diamonds from Voorspoed, coupled with a high level of platelet degradation in many of the diamonds analysed, relative to a world-wide database, is unusual, but broadly comparable to diamonds from kimberlites occurring in craton margin settings, such as Argyle and Venetia.Diamond inclusion thermobarometry, as well as the elevated nitrogen aggregation states of the diamonds (coupled with the evidence for platelet degradation in the diamonds) are indicative of a diamond crystallisation event associated with a transient thermal pulse, as well as associated deformation of the mantle containing the diamonds.The websteritic and the transitional websteritic-lherzolitic mantle source rocks of the Voorspoed diamonds likely formed through melt infiltration into mantle peridotite, possibly in the reaction envelope surrounding an ascending mantle plume. In order to account for the unusually low abundance of harzburgitic diamonds at Voorspoed, it is postulated that a primary, pre-existing suite of harzburgitic diamonds which have crystallised in the Mesoarchaean, was destroyed through heating and oxidation during the passage of magmas associated with the Ventersdorp large igneous province at 2.72 Ga, and which was subsequently followed by the crystallisation of a younger eclogitic and transitional websteritic-lherzolitic suite of diamonds.

Mineral inclusions in diamonds from Karowe Mine, Botswana: super-deep sources for super-sized diamonds?

Mineral inclusions in diamonds play a critical role in constraining the relationship between diamonds and mantle lithologies. Here we report the first major and trace element study of mineral inclusions in diamonds from the Karowe Mine in north-east Botswana, along the western edge of the Zimbabwe Craton. From a total of 107 diamonds, 134 silicate, 15 oxide, and 22 sulphide inclusions were recovered. The results reveal that 53% of Karowe inclusion-bearing diamonds derived from eclogitic sources, 44% are peridotitic, 2% have a sublithospheric origin, and 1% are websteritic. The dominant eclogitic diamond substrates sampled at Karowe are compositionally heterogeneous, as reflected in wide ranges in the CaO contents (4–16 wt%) of garnets and the Mg# (69–92) and jadeite contents (14–48 mol%) of clinopyroxenes. Calculated bulk rock REEN patterns indicate that both shallow and deep levels of the subducted slab(s) were sampled, including cumulate-like protoliths. Peridotitic garnet compositions largely derive from harzburgite/dunite substrates (~90%), with almost half the garnets having CaO contents <1.8 wt%, consistent with pyroxene-free (dunitic) sources. The highly depleted character of the peridotitic diamond substrates is further documented by the high mean and median Mg# (93.1) of olivine inclusions. One low-Ca garnet records a very high Cr2O3 content (14.7 wt%), implying that highly depleted cratonic lithosphere at the time of diamond formation extended to at least 220 km depth. Inclusion geothermobarometry indicates that the formation of peridotitic diamonds occurred along a 39–40 mW/m2 model geotherm. A sublithospheric inclusion suite is established by three eclogitic garnets containing a majorite component, a feature so far unique within the Orapa cluster. These low- and high-Ca majoritic garnets follow pyroxenitic and eclogitic trends of majoritic substitution, respectively. The origin of the majorite-bearing diamonds is estimated to be between 330 to 420 km depth, straddling the asthenosphere–transition zone boundary. This new observation of superdeep mineral inclusions in Karowe diamonds is consistent with a sublithospheric origin for the exceptionally large diamonds from this mine.