Trace Element Chemistry Research Articles

Sulfur in Archean komatiite implies early subduction of oceanic lithosphere

Komatiite is a magnesium-rich volcanic rock that erupted almost exclusively during the Archean era, requiring high partial melting of Earth's upper mantle to form. Archean komatiites, therefore, provide the most reliable information on the mantle geochemistry of the early Earth. However, it remains unclear whether komatiite compositions reflect only remnants of an early deep mantle or may provide evidence of recycling of subducted components. Sulfur mass-independent fractionation (S-MIF) could be useful to distinguish the two scenarios because S-MIF is produced only by photochemistry in anoxic atmospheres. We report here multiple sulfur isotope analyses of the 2.7 Ga Belingwe komatiites and related volcanics. Some Belingwe komatiites show clear S-MIF signatures (Δ33S from −0.20‰ to −0.12‰). Petrology, trace element chemistry, and Sr-Nd-Pb isotope compositions demonstrate that the samples contain Ni-poor pristine igneous sulfides and crustal contamination is minimal. The observed S and initial Nd isotope covariation can be reconciled if the komatiite source involved an early depleted shallow mantle which acquired S-MIF from seawater (Δ33S < 0‰) via seafloor alteration. We suggest that the Belingwe komatiites were derived at least partly from a past subducted oceanic lithosphere that stagnated in the mantle from the Archean era to the present. Therefore, the sulfur isotopes in Archean komatiite provide new constraints on the history of subduction and the recycling of crustal material.

Fluid Pressure Changes Recorded by Trace Elements in Quartz

AbstractFluid pressure is a key parameter in earthquake mechanics, controlling seismic failure and plate coupling in convergent zones. Yet fluid pressure is also extremely difficult to quantify at seismogenic depth, which limits our knowledge of the stress state in accretionary prisms. Here, we show that the geochemical record of exhumed hydrothermal quartz veins may be used to place quantitative bounds on fluid pressure variations in subduction zones. The studied veins come from sediments accreted and exhumed by plate convergence in southwestern Japan. Quartz in veins displays growth rims of contrasted bright blue/dark brown cathodoluminescence (CL) colors, high/low Al concentrations, and low/high fluid inclusion abundance. Because Si‐Al substitution (and charge compensation by Li) strongly depends on the rate of quartz precipitation and Si solubility, Al‐Li concentrations must be sensitive to fluid pressure. This is confirmed by fluid inclusions, the density of which, converted into trapping pressures, record fluid pressure drops by up to ∼70 MPa from CL‐brown, Al‐Li‐poor rims to CL‐blue, Al‐Li‐rich quartz rims. CL‐blue rims grow at a fast rate, high Si supersaturation and low fluid pressure whereas CL‐brown rims grow at a slower pace, lower Si supersaturation, and higher fluid pressure. Quartz trace element chemistry thus offers a promising tool to quantify deep fluid pressure variations and their relationships to earthquakes.

Ore forming and reworking processes in the Xitieshan Pb–Zn deposit, Qinghai Province, China: Constraints from in situ trace-element and S isotope compositions of sulfides

Although volcanogenic massive sulfide (VMS) deposits always form in extensional settings, many have been reworked by metamorphism, deformation, and the circulation of hydrothermal fluids during subsequent accretionary events. The genesis of VMS deposits has been well documented, however, the mechanisms that rework the mineralization have been relatively poorly constrained. An understanding of these mechanisms, though, is critical to the development of robust mineral deposit models as they are capable of remobilizing ore-forming metals. The Xitieshan Pb–Zn deposit, located in Qinghai Province, western China, is the third largest Pb–Zn deposit in China, with a total Pb–Zn metal reserve of 6.4 million tons. The occurrence of laminated and non-laminated orebodies in the Xitieshan deposit make it an ideal candidate to i) constrain the effects of metamorphic and hydrothermal processes on remobilization of metals in these base-metal systems, and ii) assess the genetic nature of this deposit. This is accomplished here by integrating new S isotope data and trace-element chemistry of sulfides from laminated ores with previous information on regional and deposit geology, and deformation characteristics preserved by the mineralized system. The S isotope composition of pyrite and sphalerite grains from laminated ores are consistently positive and range from 2.08 ‰ to 5.81 ‰, indicative of mixing of reduced sulfur from three distinct sources — volcanic rocks, seawater and thermochemical reduced sulfate, which is typical of VMS deposits globally. Greigite (Fe3S4) grains in laminated ores exhibit oscillatory zonation with respect to Cu, Co, Mn, Sb, and Tl concentrations; such compositional variations indicate that at least some of the primary VMS mineralization characteristics have been preserved despite subsequent reworking by metamorphism, deformation, and hydrothermal circulation. Combined with the results of previous studies, a two-stage genetic model is proposed for the formation and modification of the Xitieshan Pb–Zn deposit — i) initial formation of the primary Pb–Zn mineralization with minor of Cu, and ii) subsequent metamorphism and hydrothermal reworking of the deposit, which did not completely destroy the textural and chemical characteristics of the primary mineralization.

High-resolution trace element chemistry across the Cretaceous-Tertiary boundary in Denmark

Closely spaced samples on millimeter scale across the Cretaceous-Tertiary boundary clay in Denmark were analyzed by instrumental neutron activation analysis. The results show a strong correlation between Cr and Ir while Co, Zn, As, Sb and Au are correlated with Fe. The elemental peaks for Ir-Cr and Fe are separated by 6 mm. The Fe peak is found in burrow-fillings at the lowermost part of the boundary clay. Chromite grains extracted from boundry clay in Denmark and New Zealand show a compositional common group. The composition points to an ultrabasic (kimberlitic) origin of the chromites. Tandem accelerator mass spectroscopy demonstrated, that there are no Ir carriers in addition to elemental carbon in the boundary clay from two localities in Denmark.

The trace element chemistry of quartz in carbonatite-related REE deposits: Implication for REE exploration

Quartz is a common gangue mineral in many magmatic-hydrothermal ore deposits (e.g., porphyry, epithermal and skarn deposits) and its trace element composition was widely used to constrain ore genesis. Quartz is also a common phase in carbonatite-related rare earth element (REE) deposits, but its geochemical characteristics and significance for ore genesis have not previously been documented in this environment. In this contribution, we report in-situ LA-ICP-MS trace element compositions of quartz from seven carbonatite-related REE deposits, namely the Maoniuping, Lizhuang and Dalucao deposits in the Mianning-Dechang REE metallogenic belt, SW China and the Miaoya, Taipingzhen, Huangshuian and Huanglongpu deposits in the Qinling REE metallogenic belt, Central China. Rare earth element mineralization in these deposits is associated with well-developed hydrothermal systems in which the dominant REE minerals, bastnäsite, parisite, and monazite, are intergrown with aegirine-augite, arfvedsonite, calcite, fluorite, barite, and quartz. The results show that quartz grains from the carbonatite-related REE deposits are characterized by low budgets of trace elements, of which Al is generally less than 50 ppm whereas others are generally less than 10 ppm. In general, the trace element concentrations in quartz from different deposits are similar (the total concentration is commonly <50 ppm), and are distinguishable from those of quartz in other magmatic-hydrothermal systems. The low concentration of trace elements in carbonatite-related REE deposits is controlled by the unusual nature of carbonatitic magmas and their derived fluids, and thus can be used for defining the affinity of hydrothermal REE systems that are of unknown origin. Significantly, the trace element chemistry of quartz samples collected from a cross-section in the open pit of the Maoniuping REE deposit displays a systematic spatial variation such that the concentrations of Al, Ti, Na, and K decrease with increasing distance of the samples from carbonatites. This spatial variation correlates well with the variation of physio-chemical conditions during the evolution of ore-forming fluids. This implies that the geochemistry of quartz can be used as a vector detecting the concealed carbonatite intrusions in carbonatite-related REE deposits.

The retrograde evolution of F-rich skarns: Clues from major and trace element chemistry of garnet, scheelite, and vesuvianite from the Belka Pahar wollastonite deposit, India

Skarn settings host many ore deposits and are ideal for studying the mobility of elements in hydrothermal fluids. Belka Pahar, in the Sirohi district of Rajasthan (NW India), is the largest wollastonite deposit in India (estimated wollastonite reserve of 56 million tonnes). No detailed textural and geochemical study has been conducted on these skarn rocks. In this study, we use petrographic studies, and major-trace element chemistry of skarn minerals from Belka Pahar to deduce the sequence of mineral transformations and to characterize the changes in the composition of the fluid with the progress of fluid-rock interaction during skarn formation. Two major rock types from the exoskarn have been considered for detailed study: prograde skarn rocks comprising pyroxene, garnet, wollastonite, quartz, and calcite, and retrograde skarn rocks containing garnet, scheelite, vesuvianite, fluorite, and sphalerite. Two types of garnet are recognized in the rocks-texturally earlier prograde garnet (Grt 1 ) and texturally late retrograde oscillatory-zoned Grt 2 , which is further sub-divided into three generations Grt 2a , Grt 2b and Grt 2c . The Grt 1 (And 27–47 ) and Grt 2a (And 50–60 ) have high Y-concentrations, and high HREE/LREE ratio, which is in stark contrast to the late Grt 2b (And 40–50 ) and Grt 2c (And 27–40 ), which is characterized by high F, low Y-concentrations, and HREE-depleted REE patterns. The chemistry of different generations of garnet suggests a shift from diffusive to more advective metasomatic conditions and increasing water/rock ratio, accompanied by a shift in pH of the fluid from neutral to acidic to weakly acidic. The crystallization of F-rich garnet, vesuvianite, and fluorite indicates an increase in F-concentration in the later-stage fluid. An increase of W concentration in Grt 2b and co-precipitation of scheelite and fluorite also suggests its possible link with scheelite crystallization. The high grossular component of Grt 2b and Grt 2c suggests transport of Al in the hydrothermal fluid as F-complexes. The hydrothermal fluid composition extrapolated from scheelite chemistry agrees with the LREE enriched patterns exhibited by all skarn minerals. Principal Component Analysis done on a compiled dataset of skarn garnet shows that its trace element composition can be a useful proxy of fluid composition and can be used as an indicator of associated ore type. • Garnet composition is a useful proxy of fluid chemistry in the skarn system. • High F concentration in fluid enhanced Al mobilization, forming grossular-rich garnet. • Scheelite precipitation is linked to interaction of F-rich fluid with calc-silicate rocks. • PCA shows control of fluid composition on garnet chemistry. • Garnet trace element chemistry is a robust proxy of ore-genetic association.

Helium and oxygen isotopic variations in the Iceland plume source controlled by entrainment of recycled oceanic lithosphere

Icelandic basalts have low oxygen isotope (δ18O) values compared to other ocean island localities. While this observation is often ascribed to the assimilation of low-δ18O crust, a low-δ18O mantle beneath Iceland has also been suggested. To discern crustal from mantle-derived signals, high-quality in-situ and bulk crystal δ18O measurements have been obtained from olivine crystals covering 16 Ma of activity at the Iceland hotspot. The results are combined with olivine (ol) major, minor and trace element chemistry. Relationships between δ18Ool and indicators of melt evolution do not support a singular process responsible for lowering of δ18O values. However, correlations are observed between δ18Ool values and indicators of crustal processes. Such patterns are used to filter out data that are likely to reflect effects from crustal assimilation to highlight δ18Ool values indicative of source-derived variability only. Although filtered, the dataset reveals, that δ18Ool values, significantly lower than the canonical depleted upper mantle value, are derived from the Iceland mantle. Coupled δ18Ool and 3He/4Heol measurements done on olivine crystals from the same samples demonstrate that low-δ18O components (down to δ18Oolivine = 4.2‰) are a trait of the modern Iceland plume and that low-δ18O and low-3He/4He components have become more apparent in the hotspot products since 60 Ma. Olivine chemistry characteristics suggest that this low-δ18O component is best sampled in melts that reflect contributions from pyroxenitic mantle lithologies, likely related to the recycling of oceanic lithosphere within the plume. An increase in plume flux, as traced by increasing plume temperatures and plume buoyancy after 35 Ma, led to enhanced entrainment of lower mantle material carrying recycled low-δ18O oceanic lithosphere. Such material has become more apparent with time as is reflected in source-derived low-δ18O and high 3He/4He values in olivine from the modern Iceland plume. Moreover, the coincidence of the Iceland plume-head and the North Atlantic Rift at from ∼25 Ma likely assisted and further promoted enhanced plume-melting. Thus, the combination of changes in mantle upwelling and tectonic reorganisation of the North Atlantic led to the introduction of recycled oceanic lithosphere into the Iceland plume and the formation of the Iceland Plateau ∼25 Ma.

Zircon geochronology and Hf isotopic study from the Leo Pargil Dome, India: implications for the palaeogeographic reconstruction and tectonic evolution of a Himalayan gneiss dome

AbstractU–Pb geochronology, Hf isotopes and trace-element chemistry of zircon grains from migmatite of the upper Sutlej valley (Leo Pargil), Northwest Himalaya, reveal a protracted geological evolution and constrain anatexis and tectonothermal processes in response to Himalayan orogenesis. U–Pb geochronology and ϵHf record separate clusters of ages on the concordia plots in the migmatite (1050–950 Ma, 850–790 Ma and 650–500 Ma). The 1050–950 Ma zircon population supports a provenance from magmatic units related to the assembly of Rodinia. A minor amount of Palaeoproterozoic grains were likely derived from the Indian craton. The potential source rock of the 930–800 Ma detrital zircons may be granitoid present in Greater Himalayan rocks themselves and the Aravalli Range, which has 870–800 Ma granitic rocks. The arc-type basement within the Himalayan–Tibet orogen recorded (900–600 Ma) igneous activity, which may depict a northeasterly extension of juvenile terranes in the Arabian–Nubian Shield. The granitoid of 800 Ma may be a potential source for 790 Ma detrital zircons owing to scatter in 206/238 dates. The 650–500 Ma zircon population suggests their derivation from the East African Orogen and Ross–Delamerian Orogen of Gondwana. The Cambrian–Ordovician magmatism during the Bhimphedian Orogeny and observed late Neoproterozoic to Ordovician detrital zircons have been derived to some extent from Greater Himalayan magmatic sources. We found no detrital zircon grains that cannot be explained as coming from local sources. One sample yielded a discordia lower intercept age of 15.6 ± 2.2 Ma, the age of melt crystallization.

Tectonic control on the spatial distribution of Sn mineralization in the Gejiu Sn district, China

Late Cretaceous granitic rocks occur in the Gejiu ore district to the east and west of the N-S striking Gejiu Fault, whereas major Sn deposits are only known to occur to the east of the Gejiu Fault. Comparison of the whole-rock chemistry, the apatite trace-element chemistry, and zircon Hf and O isotope data of the various granites demonstrates that fertile granites occur to both sides of the Gejiu fault. The results demonstrate that the c. 83 Ma old granitic intrusions (i) have similar magma sources, which are dominated by metasedimentary rocks that had experienced intense chemical weathering, resulting in reduced melts, (ii) had similar melting conditions, i.e., high temperature biotite dehydration melting, and (iii) in part had experienced large extents of fractional crystallization. The most evolved granites to both sides of the Gejiu Fault have the characteristics typical of tin granites. Therefore, the absence of major deposits to the west of the Gejiu Fault is not due to the absence of fertile granites. The areas to the east and the west of the Gejiu Fault, however, have fundamentally different fault pattern, which indicates different orientation of the stress field to both sides of the Gejiu Fault at the time of the emplacement of the Cretaceous granites. Late Cretaceous dextral movement along the Ailaoshan Fault Zone resulted in a (trans)tensional setting in areas to the east of the Gejiu Fault and in a (trans)pressional setting to the west of that fault. We speculate that the tectonic setting influences the potential for mineralization because the Sn bearing fluids need efficient pathways to transporting metals from the roof zone of the batholith into the wall rocks. In contrast to regional compression, hydraulic fracturing in an overall extensional setting has the potential to develop efficient fluid pathways and, thus, may lead to major ore deposits. To the west of the Gejiu Fault, however, granite intrusions in an overall compressional setting are likely to develop no or only small mineralization.

Major and Trace-Element Chemistry of Cr-Spinel in Upper Mantle Xenoliths from East Antarctica

Cr-spinels in the upper mantle peridotite xenoliths from two Late Mesozoic intrusions of alkaline-ultramafic rocks in Jetty Peninsula (East Antarctica) were studied in situ for their major and trace-element compositions by SEM and LA-ICP-MS. The upper mantle xenoliths were collected from the magmatic bodies “sampled” from different upper mantle domains. One domain was represented by mostly lherzolites (Cpx-poor Spl, Cpx-rich Spl and Spl-Grt) and another one by Spl harzburgites and dunites. Spinels occur as grains of different shapes, sizes and origins. Three main textural types of spinel were identified: primary spinel represented by clean homogeneous grains, a rim of recrystallization/resorption surrounding primary spinel grains and irregular interstitial resorbed grains. Primary spinels are characterized by the concentrations of Al2O3 21–51 wt%, MgO 15–20 wt%, FeO 10–24 wt% and Cr2O3 14–37 wt% with the Cr# of 0.16–0.54. Most trace elements are present in spinels in very low amounts. Only Ti, V, Mn, Co, Ni, Zn and Ga display concentrations in the range of tens to hundreds (up to thousands) ppm. Concentrations of other trace elements vary from below the detection limit to &lt;10 ppm. Spinel major oxide and trace element features allowed the suggestion that the studied upper mantle peridotites represent both simple melt residues and residues strongly influenced by the MORB-like and the SSZ-related melts. The MORB-like melts may be related to the beginning of the Lambert–Amery rift system development, whereas SSZ-like melts could be related to reactivation of SSZ material buried during much earlier amalgamation of East Antarctica.

Strontium isotope stratigraphy of late Cenozoic fossiliferous marine deposits in North Borneo (Brunei, and Sarawak, Malaysia)

• Sr-isotope stratigraphy of fossiliferous marine deposits in northern Borneo. • Burdigalian to Tortanian ages revealed for thirteen fossil-rich shallow marine deposits. • Origin of reworked specimens could be traced by the obtained SIS ages. • Trace element and stable isotope chemistry of the well-preserved shells can be linked to habitat-conditions. • Altered and recrystallized specimens helped tracing diagenetic pathways. Neogene marine deposits of North Borneo are locally very rich in fossils that provide glimpses into the past biodiversity. However, dating these onshore sediments with biostratigraphy is often hampered by the lack and/or the poor preservation state of index fossils. Therefore, the fossiliferous sites were targeted with strontium isotope stratigraphy (SIS) to obtain higher precision relative dating. Well-preserved macrofossils were screened using a multidisciplinary approach, and 87 Sr/ 86 Sr ratios of the most pristine remains were used to date the embedding sediments. Most of the measured ages fall in the expected chronostratigraphic framework established by large scale studies for the region. The oldest, Burdigalian (early Miocene) ages were measured for the Sibuti Formation in Sarawak (17.71 ± 0.2My and 16.7 ± 0.2My) followed by a Serravallian (middle Miocene) age within the Belait Fm in Brunei (12.1 + 1.4/-1.2My). Eight localities from the younger units, the Miri and Seria formations in Brunei, gave a range in age from 10.5 ± 1 to 7.0 + 0.9/-0.5My (Tortonian-Messinian). Reworked fossil assemblages from Tutong beach were also investigated and the SIS ages of Late Miocene support an origin from the younger part of the Seria Fm. One locality, in Lumapas where limestone crops out in Brunei, gave an unexpected younger age (Tortonian, late Miocene, 10.6 ± 1My) compared to estimates projected for its assumed stratigraphic position in the lower Belait Formation (late Burdigalian). These challenging data require more research, yet if the young age is accepted, the stratigraphic situation of the limestone needs further revision.

Vectors to ore in replacive volcanogenic massive sulphide deposits of the northern Iberian Pyrite Belt: Major and trace element mineral chemistry

As part of a broader characterization and study of vectors to ore in VMS systems of the Iberian Pyrite Belt (IPB, Spain), we have investigated in detail mineral chemistry vectors in a representative replacive deposit hosted in felsic volcanic rocks. At the Aguas Teñidas deposit (northern IPB) the hydrothermal system affected rocks of an originally homogeneous composition extending hundreds of metres beyond its footprint. The major and trace element chemistry of white micas, chlorite and carbonates have been analysed by electron microprobe (EMP) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to characterize geochemical vectors across the extent of the hydrothermal system at Aguas Teñidas.White micas are dominated by muscovite in both regionally altered lithologies and within the hydrothermal system, with coexisting paragonite occurring in a halo beyond the first disseminated pyrite surrounding the stockwork and in the proximal hanging wall. Systematic variations have been observed in FeO/(FeO + MgO) and Na2O/(Na2O + K2O) across the alteration zone.Chlorite is predominantly clinochlore in composition, with chamosite restricted to the centre of the hydrothermal system. In regionally altered lithologied it is characterized by a constantly low Al and AlIV at variable Fe/(Fe + Mg); in rocks influenced by the hydrothermal system it presents higher AlIV and total Al, and a progressive increase in Al, AlIV and Fe/(Fe + Mg) towards its core.Regional carbonates consist of calcite, with additional dolomite forming in proximal rocks. Iron content in dolomite increases towards the centre of the hydrothermal system, with ankeritic compositions becoming dominant.Systematic variations in Ba, Cs, Li, Pb, Rb, Sn, Sr, Tl and Zn have been observed in white micas consistent with a proximal-distal transition (vector). Trends in chlorite have been less well characterized due to analytical limitations, although variations have been observed in the contents of As, Co, Li and Zn.A schematic model has been proposed which accounts for the observed mineralogical and mineral chemistry trends across the hydrothermal footprint of the Aguas Teñidas VMS deposit. Major element signatures in white micas and chlorite are interpreted as having been controlled by the upwards and outwards flow of hot reducing Fe-rich hydrothermal fluids producing a coupled increase in temperature and fluid Fe/(Fe + Mg), and a decrease in fO2, with increasing fluid/rock ratios towards the centre of the stockwork.The data presented herein are not only applicable to VMS exploration in the IPB, but on a broader scale improve our general understanding of vectors to ore in VMS deposits in general.

Formation of the Banxi Sb deposit in Eastern Yangtze Block: Evidence from individual fluid inclusion analyses, trace element chemistry, and He-Ar-S isotopes

The Banxi Sb deposit (>100,000 t Sb metal with a grade of 0.02%–64.5% Sb (typically 15.3%–25.9% Sb) in the Eastern Yangtze Block is hosted in Neoproterozoic low-grade metamorphic clastic rocks of the Banxi Group. The deposit is characterized by vein-type Sb-only mineralization. Analyses of individual quartz-hosted fluid inclusions obtained by LA-ICP-MS identified Na as the most abundant cation (thousands to tens of thousands of ppm) in the ore-forming fluids of the Banxi Sb deposit, together with variable contents of K, Ca, Al, As, Fe and Sb (tens to thousands of ppm). Multiple chemical criteria, including high Na+/K+ (typically 3–15) and low Mn2+/Na+ (0.003–0.2) and Sr2+/Na+ (0.001–0.01) ratios of individual fluid inclusions, suggest that the fluids responsible for quartz and stibnite precipitation in the Banxi Sb deposit were likely dominated by basinal brines within low-grade metasedimentary rocks. Moreover, the extremely low 3He/4He ratios (0.001–0.048Ra) but relatively high 3He/36Ar (0.5 × 10−5–5.8 × 10−4) and 40Ar/36Ar ratios (323–1111 with an average of 737) of fluid inclusions trapped in ore-stage quartz further support a sedimentary origin of ore-forming fluids in this deposit. The positive and homogeneous sulfur isotopic values (δ34S = 8.0–9.5‰) of stibnite and arsenopyrite are comparable to those of sulfides in Proterozoic metasedimentary rocks (δ34S = 5.6–11.5‰), indicating these metasedimentary rocks may have served as the primary source of S for the Banxi Sb deposit. Combined with geological features and new trace element chemistry data for quartz and stibnite, we envisaged a new formation model for the Banxi Sb deposit that assumes that circulation of weakly alkaline basinal brines through faults under an extensional tectonic setting during the Early Cretaceous (∼130 Ma), mobilization of Sb and S from the underlying and host metasedimentary rocks and ascent of ore- forming fluids along faults, and ultimately precipitation of Sb minerals in or around fault/fracture zones as a result of the decreasing pH of the ore-forming fluids as well as sulfidation during fluid-rock reaction.

Trace elements in pyrite from five different gold ore deposit classes: a review and meta-analysis

Abstract Scientific studies that use pyrite trace element signatures to facilitate interpretations of the geological history of hydrothermal gold deposits are becoming more common. Given this rise in popularity in pyrite trace element chemistry studies, a review of trace element incorporation in pyrite from various gold deposit classes is highly timely. We provide a global compilation of over 20 000 chemical data points resulting from over 5520 spot measurements of pyrites from five separate ore deposit classes, each of which is known to have high gold concentrations. The ore deposit classes considered here include orogenic gold deposits, Carlin-type gold deposits, high- and low-sulfidation gold deposits, and Au-bearing porphyry systems, whereas the pyrite minor- and trace elements that we consider include Au, As, Bi, Co, Cu, Ni, Pb, Sb, Se, Te, Tl and Zn. Meta-analyses of these data suggest that, on a global scale, both existing pyrite trace element chemistry discrimination tools and the newly-established principal component analysis (PCA) approach have a relatively low efficacy in correctly predicting the metallogenic setting in which the pyrite formed. At a deposit scale, however, pyrite trace element chemistry continues to be a useful tool for understanding Au mineralizing processes, especially when coupled with additional analytical techniques or lines of geological evidence. Further meta-analysis of the data tentatively suggests the solubility limits for the various trace elements in pyrite mineral structure and reveals that the correlations between temperature and trace element incorporation (i.e. pyrite geothermometry) are complex. The study highlights that pyrite, by virtue of its mineralogy and propensity for element substitutions, remains an important repository of chemical information related to the evolution of hydrothermal gold mineralizing systems. In as much as this repository is now readily accessed utilizing modern analytical techniques (e.g. routine LA-ICP-MS mapping of trace element distributions at ppb concentrations; mapping element distributions at the nanoscale using atom probe tomography), its full value will only be realized through continued study into the kinetics and mechanisms of trace element incorporation into pyrite structure at conditions relevant to ore-forming processes.

A Nearest-Neighbour Gaussian Process Spatial Factor Model for Censored, Multi-Depth Geochemical Data

Abstract We investigate the relationships between local environmental variables and the geochemical composition of the Earth in a region spanning over 26,000 km2 in the lower South Island of New Zealand. Part of the Southland–South Otago geochemical baseline survey—a pilot study pre-empting roll-out across the country—the data comprise the measurements of 59 chemical trace elements, each at two depth prescriptions, at several hundred spatial sites. We demonstrate construction of a hierarchical spatial factor model that captures inter-depth dependency; handles imputation of left-censored readings in a statistically principled manner; and exploits sparse approximations to Gaussian processes to deliver inference. The voluminous results provide a novel impression of the underlying processes and are presented graphically via simple web-based applications. These both confirm existing knowledge and provide a basis from which new research hypotheses in geochemistry might be formed.

Mica trace-element signatures: Highlighting superimposed W-Sn mineralizations and fluid sources

In the Echassières district of the French Massif Central, occur several outstanding magmatic/hydrothermal systems enriched in strategic metals, such as the Beauvoir rare-metal granite. In this contribution we propose a systematic approach, based on mica trace chemistry, to decipher the different events leading to mineralization. Twelve groups of micas were defined by their specific petrographic features and/or location in the district. Their trace element composition, obtained by LA-ICP-MS, varies widely from one mica group to another, although homogeneous signatures within groups could be distinguished. Some of the trace elements are remarkably enriched, such as W in igneous lepidolite and Sn in greisen muscovite, both of which occur in the Beauvoir granite. A statistical approach based on a set of multivariate analyses highlights that the trace chemistry of micas is inherited from their source, whether hydrothermal or igneous, thus providing a signature for their origin. This approach also shows that differences in major element composition (i.e., different mica species) impact only slightly the trace-element signature. For instance, muscovite and zinnwaldite from one granite have a coherent signature, but they contrast with same mica species in another granite or hydrothermal veins. It is thus possible to genetically link two different mica species from remote locations, or inversely, to recognize different origins for a same mica species in the same sample (e.g., superposed alterations). A second, important implication is that trace-element signatures of micas provide a record of metal remobilization and transport. In the Echassières district, greisen alteration of the Beauvoir granite caused dissolution of W-rich (ca. 290 ppm in average) lepidolite and cassiterite (SnO2). Newly-formed greisen muscovite incorporated most remobilized Sn (ca. 1000 ppm in average), while W precipitated in distal quartz veins as wolframite. As a consequence, Sn is concentrated in the granite, while W occurs outside of it. This is also underlined by the gradual Sn decrease and W increase recorded in micas from distal veins. Finally, wolframite-bearing veins do not contain cassiterite, validating mica trace-element chemistry as a powerful tool to decipher Sn-W ore forming hydrothermal processes.

Arc Crust Formation of Lesser Antilles Revealed by Crustal Xenoliths from Petit St. Vincent

Abstract The Lesser Antilles volcanic arc is known for its magmatic diversity and unusually abundant plutonic xenoliths. Xenoliths from Petit St. Vincent (Grenadines’ archipelago) are particularly interesting because of their textural and petrogenetic range. Here we combine petrographic observations, Electron Backscatter Diffraction (EBSD) analysis, major and trace element chemistry of xenoliths and lavas and geochemical and thermal modelling to explore the construction of arc crust beneath Petit St. Vincent. Petit St. Vincent xenoliths are dominated by calcic plagioclase, clinopyroxene and amphibole, and can be divided into two main categories, igneous and meta-igneous. Igneous xenoliths typically have cumulate textures; meta-igneous xenoliths range texturally from those that preserve vestiges of primary magmatic fabrics to intensely deformed varieties characterised by grain-size reduction and foliation development. Meta-igneous xenoliths also contain the most calcic plagioclase (An98–100). The presence of both meta-igneous and igneous xenoliths provides evidence for reworking of older arc crust and antecedent igneous intrusions. The latter have a protolith composition similar to high-MgO, low-Sr picrites and high-Ca, high-Sr ankaramites from the neighbouring islands of Petite Martinique and Grenada. The meta-igneous xenoliths derive from older, mafic arc crust present at the onset of subduction. Trace element chemistry and EBSD analyses of meta-igneous xenoliths are consistent with a complex history of re-melting and deformation mediated by chlorine-bearing H2O rich fluids (including melts). Thermal modelling supports crustal reworking through repeated magma intrusions and indicates that the observed thermal structure and thickness of crust beneath Petit St. Vincent could have developed on a timescale of approximately 4 million years at rates compatible with the regional arc magma flux. Based on evidence from thermodynamic models and exhumed ancient arc crust sections, Collins et al. (Nature Geoscience, 13, 331–338, 2020) have proposed that water-fluxed melting may be an important aspect of deep arc crust sections world-wide. Textures and mineralogy of xenoliths from Petit St. Vincent, including their characteristic high-An plagioclase, testify to such a process beneath an active, intra-oceanic arc.

Quantifying fish otolith mineralogy for trace-element chemistry studies

Otoliths are frequently used to infer environmental conditions or fish life history events based on trace-element concentrations. However, otoliths can be comprised of any one or combination of the three most common polymorphs of calcium carbonate—aragonite, calcite, and vaterite—which can affect the ecological interpretation of otolith trace-element results. Previous studies have reported heterogeneous calcium carbonate compositions between left and right otoliths but did not provide quantitative assessments of polymorph abundances. In this study, neutron diffraction and Raman spectroscopy were used to identify and quantify mineralogical compositions of Chinook salmon Oncorhynchus tshawytscha otolith pairs. We found mineralogical compositions frequently differed between otoliths in a pair and accurate calcium carbonate polymorph identification was rarely possible by visual inspection alone. The prevalence of multiple polymorphs in otoliths is not well-understood, and future research should focus on identifying otolith compositions and investigate how variations in mineralogy affect trace-element incorporation and potentially bias environmental interpretations.

Characteristics of Mineralization-Forming Fluid and Metallogenic Mechanism for the Mianhuakeng Uranium Deposit in South China: Constraints from In Situ Geochemical Signatures and Sulfur Isotopes of Syn-Mineralization Pyrite and Pitchblende

The Mianhuakeng deposit is the most representative granite-related hydrothermal vein-type uranium deposit in South China; however, the characteristics of the mineralization-forming fluid and metallogenic mechanism are still less constrained. To address the scientific problems above, we investigated the trace element chemistry and sulfur isotope compositions in syn-mineralization pyrite and pitchblende from the Mianhuakeng uranium deposit. The trace element chemistry shows that the mineralization in the Mianhuakeng deposit belongs to an intermediate-to-low temperature hydrothermal system, which is consistent with the homogenization temperature distribution of fluid inclusions. Redox-sensitive elements (such as Co and Se) in syn-mineralization pyrite suggest a reductive nature of the mineralization-forming fluids. The fractionation between light and heavy REE in pitchblende from the Mianhuakeng deposit is most similar to those from the Changjiang pluton. The pronounced negative Eu anomaly is coincident with mineralization-bearing granites. The δ34S values of syn-mineralization pyrite range from −10.2 to −1.4‰, which is higher than those values of pyrite from granites near the studied area and lower than the δ34S values of pyrite from diabase in the ore district. The REE signatures of pitchblende and sulfur isotope composition of syn-mineralization pyrite suggest that the major U source for the Mianhuakeng deposit is most likely the Changjiang pluton, probably accompanied by the incorporation of mantle-derived fluids. The circulations of CO2-rich hydrothermal fluids leached uranium from granite source rocks, especially from the Changjiang pluton. The change of physicochemical conditions of the mineralization-forming fluid resulted in the deposition of the uranium minerals in favorable structural traps to form the hydrothermal vein-type Mianhuakeng uranium deposit.

Geochemical characterisation of offshore New Zealand phosphorites, and mechanisms for their formation

Phosphate deposits can form in a range of environments, and despite similar P contents, their mineralogy, and hence major and trace element chemistry, can vary substantially. Continental margins with high surface-water productivity are recognised as a major environment for the formation of extensive phosphate deposits. The geochemical characterisation of continental-margin-derived phosphorites from Chatham Rise and south-eastern offshore New Zealand, as well as newly discovered seamount phosphorites from Bollons Seamount (first documented here), offer insights into the different mechanisms of phosphorite formation from these two different environments. Though less studied, seamounts are increasingly recognised for their phosphorite deposits as well as Fe-Mn crusts and Mn nodules. We propose the first model for the formation of nodules at Bollons Seamount, and an updated model for Chatham Rise nodules (and south-eastern offshore NZ phosphorites) is presented. Suboxic-to-anoxic enrichments of U and V are observed in Chatham Rise and south-eastern offshore phosphorites that contrast with oxic enrichments of Mn, Ce, Co, and Cr in Bollons Seamount phosphorites. Differences in uptake of Light Rare Earth Elements (LREE) vs. Heavy Rare Earth Elements (HREE) and variations in mineralogy (e.g., presence of glauconite vs. Fe-Mn crusts), allow phosphate deposits that formed in organic-rich, upwelling continental-margin environments to be distinguished from those that formed in oxic-suboxic organic-poor seamount environments.