Late-stage Veins Research Articles

Genesis and fluid evolution of the Hatu orogenic gold deposit in the West Junggar, Western China

The West Junggar in Xinjiang, western China, represents a significant gold mineralization belt hosting over 200 gold deposits, with the Hatu gold deposit being the largest among them. In this study, ore geology and fluid inclusion assemblages of quartz samples from the Hatu gold deposit were investigated in an effort to clarify the mineralization process and its relationship with the geodynamic settings. The mineralization process is delineated into early (pyrite-albite-quartz veins), middle-a (quartz-ankerite veins), middle-b (quartz-ankerite-sulfide-native gold veins), and late (quartz-calcite veins) stages. The early stage veins suggest a compressional setting while the middle-a and middle-b stage veins suggest a tensional shear setting. The late stage veins are typically filled fissures cutting through earlier veins. Scanning electron microscope-cathodoluminescence (SEM-CL) reveals that early stage quartz (Q1) exhibits concentric CL-oscillatory growth zoning, while middle-a stage quartz (Q2a) displays unidirectional zoning and ranges from CL-bright to CL-dark, middle-b stage quartz (Q2b) is CL-bright and weakly zoned, and late-stage euhedral quartz (Q3) shows sector zoning transitioning from CL-gray to CL-dark. Quartz that formed in these stages developed three types of fluid inclusions: pure CO2 (PC-type), CO2–H2O (C-type), and H2O–NaCl (W-type). The early stage quartz contains C- and W-type fluid inclusions homogenizing at 309–345 °C, while the late stage quartz contains only W-type fluid inclusions with homogenization temperatures of 164–229 °C. Microthermometry of fluid inclusion indicated the evolutionary of the metallogenic fluid from a high-temperature, CO2-rich, and minor CH4 metamorphic fluid to a low-temperature, CO2-poor meteoric fluid. From the early to middle-a stages, fluid boiling caused the unloading of ore-forming elements whereas fluid mixing led to the precipitation of polymetallic sulfides and gold in the middle-b stage. Trapping pressures in the middle-b stage were estimated using C-type inclusions, illustrating that gold mineralization took placed at depth of 8.0 km. We propose classifying the Hatu gold deposit as an orogenic deposit, originating from the collisional orogenesis between the Yili-Kazakhstan and Siberian continents in the Late Carboniferous.

Syn‐mineralization Uplifting and Exhumation of Porphyry Systems in China: Evidence from Fluid Inclusion Data

AbstractIn order to understand how the metallogenic process of porphyry deposit specifically and directly respond to regional uplifting and exhumation, we compiled previous fluid inclusion data of 32 porphyry deposits in China by recalculating the fluid trapping depths and trapping depth reduction magnitude from early to late mineralization stage veins. The data reveal that the average trapping pressure ratio (Ave TPE/TPL) between early‐ and late‐stage veins of the these deposits are 1.2–18.4, mainly in the range of 1.35–5.83, with average trapping pressure reduction (1–Ave TPL/TPE) from early‐ to late‐ stage veins are 17%–95%, and mainly in the range of 25%–83%. The fluid trapping pressure based mineralization depths most of the porphyry deposits in China had decreased from early to late vein stages by at least 450 m (900–5800 m predominant), or greater than 950 m when take the average depth reduction value, which is greater than the current gap between early‐ and late‐ stage veins of each deposit. We propose that the apparently greater mineralization depth reduction magnitude than the current elevation gaps between early and late veins are likely a consequence of syn‐mineralization uplifting and exhumation process that often occurs in porphyry systems.

Paleozoic Magmatism and Mineralization Potential of the Sanchakou Copper Deposit, Eastern Tianshan, Northwest China: Insights from Geochronology, Mineral Chemistry, and Isotopes

Abstract The Sanchakou Cu deposit is located in the eastern section of the Dananhu magmatic arc in the Eastern Tianshan orogenic belt, northwest China. Sanchakou is hosted by quartz diorite and granodiorite intrusions. Chalcopyrite and bornite are the dominant ore minerals and occur as disseminations, patches, veins, and veinlets. Secondary ion mass spectrometry (SIMS) U-Pb dating of zircons shows that the ore-bearing intrusions were emplaced at ca. 435–432 Ma, recording the early subduction of the Paleo-Tianshan oceanic plate. The enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), and moderate Mg# values, together with mantle-like bulk Sr-Nd and zircon Hf-O isotope signatures (δ18O = 4.0–5.3‰), suggest that they were generated from partial melting of metasomatized mantle materials by subducted slab fluids. In situ S and whole-rock Pb isotope results imply that the Sanchakou diorite magmas provided ore-forming components (S and metals), with additional minor metals (e.g., Cu and Pb) sourced from crustal components beneath the Dananhu arc. The redox state of diorite magmas ranges from initial high fO2 (>FMQ + 2, where FMQ is the fayalite-magnetite-quartz buffer) to relatively low fO2 (<FMQ + 2) upon magma ascent and cooling. The late-stage less oxidized magma compositions are consistent with the presence of magmatic sulfides in primary plagioclase and magnetite. Estimates of water-sulfur-chlorine contents in magma using plagioclase, amphibole, and apatite compositions reveal that the diorite magmas had high water (>7 wt %), normal S (8–393 ppm), and systematically low Cl (38–1,100 ppm) contents. A constant and favorable elevated magma oxidation state appears critical for generating an economic porphyry Cu deposit. Additionally, Cl melt concentrations may be a key factor that controlled metal fertility of the deposits in the Eastern Tianshan, although the mineralization potential may also relate to depth of emplacement of the hydrothermal system. The anomalous presence of stellerite with chalcopyrite in late-stage veins indicates that original porphyry-style mineralization at Sanchakou underwent deformation-related modification after its formation.

Trace element and isotopic zoning of garnetite veins in amphibolitized eclogite, Franciscan Complex, California, USA

Here we present major element, trace element, and oxygen isotope data for garnet from an amphibolitized eclogite block from Ring Mountain, Franciscan Complex, California, USA. Garnetite veins 1–5 cm thick are laterally continuous up to 10 m within an Mg-rich blackwall zone of the eclogite block. Complex major and trace element zoning patterns reveal multiple stages of garnet growth in both the matrix and garnetite veins. Similarities in major and trace element zoning between matrix and vein garnet suggest that crystallization of the garnetite veins began toward the end of matrix garnet core growth, and continued throughout the garnet growth history of the rock. Oscillatory zoning in rare-earth elements suggests garnet growth in pulses, with matrix-diffusion-limited growth in between pulses. Oxygen isotope analyses of matrix and vein garnet have a range in δ18O values of 5.3–11.1 ‰. Differences in δ18O values of up to ~ 4 ‰ between garnet core and rim are observed in both the matrix and vein; garnet cores range from 9.8 to 11.1 ‰ (median 10.4‰), garnet mantles range from 8.3 to 10.0 ‰ (median 9.7 ‰), and garnet rims range from 5.8 to 7.8 ‰ (median 6.7 ‰). Late-stage vein crystallization appears as a garnet “cement” that fills in a network of small (typically 5–50 µm) garnet cores, and likely crystallized from an amorphous phase. The low δ18O values of this latest stage of garnet growth are consistent with interaction with serpentinites, and likely represent the physical incorporation of the eclogite block into the serpentinite matrix melange.

Extreme sulfur isotope fractionation of hydrothermal auriferous pyrites from the SW fringe of the Taupo Volcanic Zone, New Zealand: Implications for epithermal gold exploration

The Taupo Volcanic Zone and its environs, New Zealand, represent an ideal place for epithermal gold exploration. We have documented structurally-bound gold in low temperature hydrothermal pyrite veins from the Erua Basin area on the outside fringe of the Taupo Volcanic Zone. Extreme 34S/32S fractionation of 122.2‰ from auriferous pyrite is documented in hydrothermal veins hosted in alluvial deposits from the Erua Basin. We document three generations of pyrite based on petrographic and Nano-SIMS microstructural analysis. First generation pyrite (Py assemblage -1) has extremely high δ34S values of +70.3 to +100.0‰ probably resulting from sequential-cyclic bacterial reduction and Rayleigh fractionation of limited sulfate. Second generation pyrite (Py assemblage -2) has relatively lower δ34S values of -39.8 to -3.3‰, interpreted to reflect the influx of magmatic-related oxidized fluids mixed with remobilized, reduced sedimentary sulfides that then formed the host calcite-pyrite veins. Third generation pyrite (Py assemblage -3) incorporated much heavier 34S resulting in positive δ34S values ranging from +6.8 to +72.2‰, indicating greater reduction of sulfate to H2S in an anoxic (reducing) environment. One single composite ~ 10 um pyrite grain encompassing all three types of subgrains has a large δ34S range from -22.2‰ along its rim and +100.0‰ from its core (122.2‰ for a single grain, and 139.8‰ for the system). This is the largest known variation in δ34S ever reported in a single pyrite grain from any system on Earth, but is comparable to δ34S ranges in pyrite systems of up to 186‰ reported from microbial sulfate reduction in deep bedrock fracture systems in Sweden. The growth stages from Py-2 to Py-3 indicate a transformation of the environment from an open to a closed (highly reducing) system, and it is these late-stage vein pyrites that are slightly auriferous. This process is interpreted to be induced by the precipitation of sulfides perhaps near the margin of the hydrothermal system or closed growth of pyrite in hydrothermal veins, which both restrict circulation and convection of sulfate within the veins. Our results suggest that the Erua Basin may sit on the fringe of a low-temperature epithermal gold system; other “low-temperature” areas around the TVZ may also preserve similar, potentially enriched zones on the edge of an epithermal Au belt.

Paragenetic relationships between low- and high-grade gold mineralization in the Cripple Creek Au-Te deposit, Colorado: Trace element studies of pyrite

The Cripple Creek alkaline igneous rock-related low-sulfidation epithermal gold telluride deposit is hosted in a 10 km wide Oligocene alkaline volcanic diatreme in Proterozoic rocks. Periods of phreatic eruptions and progressively more mafic alkaline magma emplacement created the diatreme and breccias that host the gold ore, followed by breccia pipe creation, and later by the emplacement of phonolite and later lamprophyre dikes throughout the diatreme. Pervasive potassic alteration affected much of the diatreme, and is spatially related to the late-stage veins containing native gold and precious metal tellurides. Previous studies have mainly focused on the high-grade gold veins primarily located toward the center of the diatreme. The current study evaluates the mineralogy and paragenesis of low-grade mineralization in the currently mined Wild Horse Extension (WHEX), Globe Hill, and Schist Island pits, and the Galena Embayment along the margin of the diatreme next to the WHEX pit, as well as high-grade mineralization in the WHEX and Cresson pits. Given the variable paragenesis of mineralizing stages in different locations in the Cripple Creek deposit, a simplified three stage paragenesis (stages 1, 2, and 3) is recognized here involving low- and high-grade gold mineralization.Scanning electron microscopy, electron microprobe, and laser ablation-inductively coupled plasma-mass spectrometry analyses are used to compare the trace element compositions of pyrite in low- and high-grade gold mineralization. These analyses reveal a complex trace element pattern with variability in Ag, As, Au, Co, Ni, Sb, and Te in the rims, cores, and zones (area between the core and the rim) of stages 2 (Py2) and 3 pyrite (Py3) and spatially between the pits currently being mined. Trace element concentrations in stage 1 pyrite (Py1) are lower than those in Py2 and Py3 and the patterns are uniform. Trace elements reside within the lattice structure of pyrite and as micro- and nano-inclusions (e.g., up to 48,220 ppm As, 6983 ppm Co, 12,250 ppm Ni, and 10,860 ppm Sb). The trace element patterns from the final stage of pyrite growth in high-grade gold telluride veins indicate enrichment in Ag, As, Au, Cu, Pb, Sb, and Te.The paragenesis and compositional zoning of pyrite associated with high-grade ore contrast with those associated with pyrite from low-grade ore, which preceded the formation of high-grade gold telluride veins. This suggests that metallic mineralization occurred in multiple episodes and that low-grade ore did not form from the same hydrothermal fluids that precipitated the high-grade ore. Early pyrite formed under passive, non-boiling conditions, followed by a transition to vigorous boiling in Py2 and Py3, ending with non-boiling conditions recorded in the Au and As-rich rims of Py3. This enrichment in Au and As on the rims of pyrite is consistent with other alkaline igneous rock-related epithermal Au deposits, as are the concentrations of trace elements throughout the pyrite grains. The presence of As, Ag, Co, Cu, Mo, Ni, Sb, Te, and Tl in pyrite are considered vectors to high-grade Au ore in the Cripple Creek deposit.

Sulfide melts in ore deposits from low-grade metamorphic settings: Insights from fluid and Tl-rich sulfosalt microinclusions from the Monte Arsiccio mine (Apuan Alps, Tuscany, Italy)

Sulfide melting is increasingly recognized as an efficient ore remobilization process in several ore bodies deformed and metamorphosed under amphibolite to granulite facies conditions. Actually, sulfide melts may also occur at lower metamorphic conditions, provided the abundance of the so-called Low-Melting point Chalcophile Elements (LMCE). The Monte Arsiccio ore body (Apuan Alps, northern Tuscany, Italy) is strongly enriched in several LMCE, i.e., Ag, As, Hg, Pb, Sb, and Tl, and experienced greenschist facies metamorphism (T = 350–450 °C; P = 0.3–0.4 GPa) during the Alpine orogeny. Trails of polyphase sulfosalt inclusions (up to five phases), coexisting with fluid inclusions, were observed within quartz and baryte crystals found in late-stage veins. Sulfosalt assemblages are formed by LMCE-rich phases, the main being ferdowsiite, Ag8As3Sb5S16, chabournéite, Tl2Pb(Sb,As)10S17, and intermediate members of the arsiccioite/routhierite isotypic pair, (Ag1−xCux)Hg2TlAs2S6, along with minor realgar. Sulfosalt inclusions range between 200 nm and more than 100 µm in size, whereas larger inclusions, likely formed through the coalescence of smaller melt droplets, are up to 500 µm. Sulfosalt inclusions are associated with primary two-phase liquid-rich fluid inclusions (Type 1a), containing CO2 and N2, as revealed by microthermometric investigations and micro-Raman spectroscopy. Secondary two-phase liquid-rich fluid inclusions (Type 1b) were also identified. Microthermometric measurements carried out on Type 1a fluid inclusions gave homogenization temperatures (Th) in the range 265–296 °C, whereas Type 1b fluid inclusions showed lower Th, between 222 and 246 °C. As microthermometric heating experiments on sulfosalt inclusions revealed homogenization temperatures around 270 °C, it is very likely that sulfosalt inclusions were in the liquid state during their entrapment, in agreement with many Tl-bearing systems. These results are fully consistent with the occurrence of LMCE-rich sulfide melt during the formation of the sulfosalt assemblages observed at the Monte Arsiccio mine. In addition, the polyphase nature of most of the observed sulfosalt inclusions may be related to the unquenchable nature of sulfide melts. The formation of LMCE-rich sulfosalt assemblages could be related to the release of these elements from pyrite during its metamorphic recrystallization. Consequently, pyrite ores may produce sulfide melts, with potential implications in the change of speciation of LMCE, from trace to major elements.

Carbonatites of India

Based on the field relations, associated rock types and age, the carbonatite-alkaline rock complexes of India, that are spatially related to deep main faults, rifts and shear zones, have been classified in to two major groups, namely: 1. Middle — late Cretaceous, subvolcanic -volcanic complexes (Amba Dongar, Siriwasan, Swangkre, Mer-Mundwara, Sarnu-Dandali-Kamthai) and 2. Paleo-Neoproterozoic plutonic complexes (Newania, Sevathur, Samalpatti, Hogenakal, Kollegal, Pakkanadu, Udaiyapatti, Munnar, and Khambamettu). The middle Cretaceous Sung Valley and Samchampi complexes also belong to this plutonic group. Three minor associations, belonging to these two age groups include, the Neoproterzoic, late stage veins of carbonatites in peralkaline syenite complexes (e.g., Kunavaram, Elchuru), the diamond-bearing carbonatite and kimberlite at Khaderpet and the lamprophyre-lamproite association (e.g., Pachcham Is. Upper Cretaceous, Deccan Volcanic Province, and the Proterozoic Chitrangi Group). Petrological associations include carbonatite-nephelinite-phonolite (e.g. Amba Dongar, Sarnu-Dandali-Kamthai), dunite-peridotite-pyroxenite-ijolitemelilitite (e.g. Sung Valley), miaskitic syenite-pyroxenite ± dunite (e.g. Sevathur, Samalpatti, Pakkanadu), carbonatite alone with fenites (e.g. Newania), besides those minor associations mentioned above. Sovites (calico-carbonatites) occur as the most dominant type in some ten (10) complexes. Beforsite (magnesio-carbonatite) is the dominant type at Newania and ankeritic-sideritic types are mainly found at Amba Dongar, Siriwasan and Newania. The rare benstonite-bearing carbonatites are found at Jokkipatti and Udaiyapatti in Tamil Nadu. Mineralogically and chemically the carbonatites show considerable diversity. Fenitised zones and types of fenites (Na, K and mixed) vary widely since the carbonatites are emplaced in a variety of hostrocks ranging from granitic, mafic, ultramafic, charnockitic types besides basalts and sandstones. Stable (δ13C and δ18O) and radiogenic (Sr, Nd and Pb) isotopes clearly indicate their mantle origin and also the diverse types of sources (both depleted HIMU and enriched EM 1 and 2). Petrogenetic considerations reveal three types of carbonatites, namely direct partial melts from metasomatised mantle (e.g. Newania), liquid immiscibility from carbonatite-nephelinite association (e.g. Amba Dongar) and through fractionation of ultra-alkaline ultramafic and mafic association (e.g. Sung Valley). Carbonatites of India that host significant resources include Amba Dongar (Fluorite, REE, Nb, P, Ba, Sr), Kamthai (REE), Sevathur (Nb, P, vermiculite), Beldih (P, Fe), Sung Valley (P, Nb, REE, Fe) and Samchampi (P, Nb, Fe, REE).

Mineralogy, Geochemistry and Raman Spectroscopy of Multi-Genesis Serpentine Polymorphs of Darepahn Ophiolites

The present article pays to some Alpine-Himalayan ophiolites of Late Cretaceous age from Darepahn area in the southern part of Nain-Baft Ophiolite belt. Whole rock processed data clearly shows that the metaperidotite rocks are silica-poor with low CaO, Al2O3, and TiO2 that reflect the predominance of olivine over calcic pyroxene in the protolithes. The considerable content of Co (85.6–124.6 ppm) and very low contents of Sc (4.3–15.3 ppm) and Sr (0.7-14 ppm) provided more evidence for olivine enrichment in the rock parentages. Calculating numbers of cation per formula unites based on EMPA results for distinguished Serpentine polymorphs (classified structurally via Raman spectroscopy) display that Tschermak substitution of trivalent cations for lizardite is tetrahedrally, and for chrysotile and antigorite, it is octahedrally. These chemical differentiations, lead to identifying chemical field plots in separator diagrams between SiO2, MgO, H2O, and Al2O3. Lizardite polymorphs can consider as primarily metasomatism results thru an iron-rich fluid occurrence. It has a considerable occupation of trivalent cations (especially Fe3+) and simultaneous less than 4 Si atoms in formula structure that reflected tetrahedral Tschermak substitutions. Antigorites have some deformed recrystallized textures along with high Σoct (Total Octahedral Cations) and low H2O content that could be considered as thermal originated polymorph. Chrysotiles have bimodal MgO and FeO contents that return to magnetite formations in groundmass and octahedral Tschermak substitutions on late-stage veins. The results exemplify compositional variations of Darepahn Serpentine polymorphs as a function of textural behaviors, structural position and thermodynamic condition of the formation process.

In situ LA–ICP–MS trace element analyses of magnetite: genetic implications for the Zhonggu orefield, Ningwu volcanic basin, Anhui Province, China

The Zhonggu orefield is located within the southern Ningwu volcanic basin and is one of the largest iron ore districts within the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) of eastern China. The area hosts the Gushan iron oxide–apatite (IOA) deposit and the Baixiangshan, Longshan, Hemushan, Zhongjiu, and Taipingshan skarn-type iron deposits. Here, we employ laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) to determine trace element concentrations in magnetite from these deposits. Combining these new data with geological information from these deposits indicates that the iron ore within the Gushan deposit has V and Ti compositions that are strongly suggestive of a Kiruna-type IOA origin. Specifically, the V and Ti chemistry of magnetite in iron ore breccias from the Gushan deposit suggests that this style of mineralization formed at a high temperature and as a result of magmatic magnetite precipitation. This was followed by precipitation of lower temperature magmatic–hydrothermal massive magnetite. Both types of magnetite host exsolved ilmenite. Elemental mapping also indicates that Gushan breccia-hosted magnetite records hydrothermal fluid activity that formed late-stage vein mineralization. In comparison, other deposits within the Zhonggu orefield all contain magnetite with compositions that are indicative of skarn mineralization. This implies that these deposits formed as a result of magmatic–hydrothermal rather than purely magmatic or purely hydrothermal activity, contrasting with the Gushan deposit. Finally, the geochemistry of magnetite within thick anhydrite units in the Longshan deposit indicates the formation by low-temperature sedimentary processes, and this magnetite was subsequently overprinted as a result of hydrothermal activity during the formation of the main Longshan deposit. Overall, this study indicates that the IOA, skarn-type, and sedimentary anhydrite-type iron mineralization in the Zhonggu iron ore field record evolving metallogenic processes from initially orthomagmatic mineralizing systems to high- to moderate-temperature magmatic–hydrothermal systems and finally to low-temperature hydrothermal mineralization.

Repeated syn- and post-orogenic gold mineralization events between 1.92 and 1.76 Ga along the Kiistala Shear Zone in the Central Lapland Greenstone Belt, northern Finland

The Central Lapland Greenstone Belt (CLGB) is well endowed with orogenic gold deposits along major shear zones and structural lineaments. In the northern part of the belt, the Kiistala Shear Zone (KiSZ) hosts the Suurikuusikko gold deposit (Kittilä Mine), currently the largest gold producer in Europe. The Iso-Kuotko deposit is located 10 km to the north of Suurikuusikko along the same shear zone. Gold mineralization at Iso-Kuotko formed in two major stages: an early stage refractory gold mineralization with auriferous arsenopyrite with similarities to the ore at Suurikuusikko and a late, main stage mineralization with free gold in carbonate-quartz veins with abundant pyrrhotite, native bismuth and other sulphide minerals. This latter ore is absent at Suurikuusikko. Based on U-Pb dating, as well as Re-Os and Pb-isotope systematics of rock-forming and hydrothermal minerals, we evaluate the relationships between tectonic evolution and formation of gold deposits throughout the late Palaeoproterozoic in the northern part of the CLGB.Results from in situ U-Pb dating of zircon and petrographically well constrained hydrothermal monazite and xenotime by LA-ICPMS, and model age calculations using Re-Os isotopic data for arsenopyrite and in situ Pb-isotope data for galena from the Iso-Kuotko deposit provide evidence for multiple episodes of hydrothermal activity along the KiSZ. Data suggest coincident felsic magmatism and structurally controlled fluid flow events during wrench fault-dominated N-S shearing associated with early, micro-continent accretion ushering in the Svecofennian orogeny. This includes the formation of refractory gold ores at Suurikuusikko (1916 ± 19 Ma) and Iso-Kuotko (>1868 ± 15 Ma). Subordinate barren veining took place at ∼1830 Ma during transient extension between the accretion and final collision stage. Xenotime in late stage veins with visible gold at Iso-Kuotko yield U-Pb ages between 1761 ± 9 and 1770 ± 7 Ma. This time-interval is concurrent with widespread granitoid emplacement at the close of the Svecofennian orogeny. Disturbance of the Re-Os system in the early auriferous arsenopyrite can be connected to the late stage mineralizing processes. Pb isotope data suggest a mixed mantle and lower crust origin for fluids in the post-orogenic hydrothermal system.In situ U-Pb dating in polished thin sections in tandem with electron microprobe analyses of mineral compositions and detailed textural observations, highlights the utility of xenotime and monazite as robust geochronometers capable of recording repeated hydrothermal events in Paleoproterozoic orogenic systems. Results provide precise temporal constraints for orogenic gold mineralization and vital information to refine the tectonic evolution of the Central Lapland Greenstone Belt.

Characterisation of primary and secondary carbonates in hypabyssal kimberlites: an integrated compositional and Sr-isotopic approach

Carbonates in fresh hypabyssal kimberlites worldwide have been studied to understand their origin [i.e. primary magmatic (high T) versus deuteric (‘low T’) versus hydrothermal/alteration (‘low T’)] and identify optimal strategies for petrogenetic studies of kimberlitic carbonates. The approach presented here integrates detailed textural characterisation, cathodoluminescence (CL) imaging, in situ major- and trace-element analysis, as well as in situ Sr-isotope analysis. The results reveal a wide textural diversity. Calcite occurs as fine-grained groundmass, larger laths, segregations, veins or as a late crystallising phase, replacing olivine or early carbonates. Different generations of carbonates commonly coexist in the same kimberlite, each one defined by a characteristic texture, CL response and composition (e.g., variable Sr and Ba concentrations). In situ Sr isotope analysis revealed a magmatic signature for most of the carbonates, based on comparable 87Sr/86Sr values between these carbonates and the coexisting perovskite, a robust magmatic phase. However, this study also shows that in situ Sr isotope analysis not always allow distinction between primary (i.e., magmatic) and texturally secondary carbonates within the same sample. Carbonates with a clear secondary origin (e.g., late-stage veins) occasionally show the same moderately depleted 87Sr/86Sr ratios of primary carbonates and coexisting perovskite (e.g., calcite laths-shaped crystals with 87Sr/86Sr values identical within uncertainty to those of vein calcite in the De Beers kimberlite). This complexity emphasises the necessity of integrating detailed petrography, geochemical and in situ Sr isotopic analyses for an accurate interpretation of carbonate petrogenesis in kimberlites. Therefore, the complex petrogenesis of carbonates demonstrated here not only highlights the compositional variability of kimberlites, but also raises concerns about the use of bulk-carbonate C-O isotope studies to characterise the parental melt compositions. Conversely, our integrated textural and in situ study successfully identifies the most appropriate (i.e. primary) carbonates for providing constraints on the isotopic parameters of parental kimberlite magmas.

Controls on the chemistry of minerals in late-stage veins and implications for exploration vectoring tools for mineral deposits: An example from the Marathon Cu-Pd deposit, Ontario, Canada

Exploration for mineral deposits is becoming increasingly difficult, requiring the use of novel approaches that are reliable and cost-effective. One such approach is the use of vein-hosted mineral chemistry. A number of studies have described the use of vein-hosted mineral chemistry as an exploration tool, but few have assessed a variety of factors that may have controlled the composition of these minerals. An understanding of why and how the composition of minerals varies, however, is critical to the development of robust exploration tools. This contribution combines detailed mineralogy using energy- and wavelength-dispersive spectroscopy, Raman spectroscopy, and μXRD with high-resolution laser ablation ICP-MS trace element analysis to characterize the controls on the chemistry of vein-hosted minerals, and the implications of these controls for the applicability of these minerals for mineral exploration.In and around the Marathon Cu-Pd deposit, veins consist of a complex mixture of phyllo and chain silicates, including chlorite, serpentine, saponite, and Ca-rich amphibole, whereas disseminated alteration consists predominantly of Ca-rich amphibole with lesser chlorite. The abundance of these veins is inversely correlated with the presence of mineralization. Minerals hosted in veins and patchy alteration crystallized from different fluids during temporally distinct events; the veins formed later than the disseminated alteration and from multiple stages of lower-temperature fluids. There is no correlation between the rock types that the veins are hosted by and the composition of the vein minerals, but on a very local scale, the mineral that a vein is hosted by does exhibit a compositional control. Vein minerals that are hosted by magnetite, and to a lesser extent, pyroxene, are enriched in Ti, V, and Cr compared to those hosted by plagioclase. In vein minerals that are hosted by plagioclase, the concentration of Co, Ni, and Zn increase systematically with distance from mineralization. The concentrations of Co, Ni, and Zn in vein minerals hosted by plagioclase also, however, increase systematically with distance from the contact between the mineralized pluton and the underlying Archean country rocks (i.e., the footwall contact). This suggests that the Co, Ni, and Zn in the fluids were likely derived by progressive leaching from pyroxene, olivine, and magnetite as the fluid migrated upwards through the pluton, rather than from the mineralization. Consequently, the correlation between vein-hosted mineral chemistry and sulfide mineralization are misleading. The only vein characteristic that can be used as an exploration tool at Marathon is the inverse correlation between vein density and zones of sulfide mineralization. This study demonstrates that the application of vein and mineral chemistry to mineral exploration needs to be carried out in the context of a careful assessment of the variations and controls on mineral chemistry from the scale of the deposit and its surrounding rocks down to the grain-scale.

Late-stage anhydrite-gypsum-siderite-dolomite-calcite assemblages record the transition from a deep to a shallow hydrothermal system in the Schwarzwald mining district, SW Germany

The majority of hydrothermal vein systems of economic interest occur at relatively shallow crustal levels, although many of them formed at significantly greater depths. Their present position is a consequence of uplift and erosion. Although, many aspects of their formation are well constrained, the temporal chemical evolution of such systems during uplift and erosion is still poorly understood. These vein minerals comprise calcite, dolomite-ankerite, siderite-magnesite, anhydrite and gypsum forming the last gangue assemblages in Jurassic and Tertiary sulphide-fluorite-quartz-barite veins of the Schwarzwald mining district, SW Germany. Mineral textures of samples from nine localities reveal that in these sequences, mineral precipitation follows a recurring pattern: early calcite is followed by anhydrite or gypsum, siderite and/or dolomite. This succession may repeat up to three times.In-situ (LA-ICP-MS) U-Pb age dating of 15 carbonates from three subsequent generations of the late-stage vein assemblage yield robust ages between 20 and 0.6 Ma. Each mineral sequence forms in a distinctive period of about 2–5 Ma. These ages clearly relate these late-stage mineral phases to the youngest geological episode of the Schwarzwald, which is associated with the Cenozoic Rhine Graben rifting and basement uplift.Based on thermodynamic modelling, the formation of the observed mineral assemblages required an deeply sourced Mg-, Fe- and SO4-rich fluid (b), which was episodically mixed with a shallow crustal HCO3-rich fluid (a). As a consequence of fluid mixing, concentrations of Mg, Fe and SO4 temporarily increased and initiated the formation of the observed sulphate-carbonate mineral sequences.This discontinuous large-scale vertical fluid mixing was presumably directly related to episodes of active tectonics associated with the Cenozoic strike-slip regime of the Upper Rhine Graben. Analogously, episodic fluid mixing is a major key in the formation of older (Jurassic to early Tertiary) Pb-Zn-fluorite-quartz-barite assemblages in the same specific vein systems, albeit involving different fluid compositions.Late-stage hydrothermal (∼20–70 °C) vein assemblages reported in this study record the transition from deep (>2 km) to very shallow (0–1 km) crustal conditions. As a consequence of successive uplift, increasing proportions of shallower and cooler (∼50–70 °C) fluids could take part in such mixing processes. Associated changes in the fluid composition caused the vein mineralogy to change from sulphide-quartz-fluorite-barite to calcite-anhydrite/gypsum-siderite-dolomite, as the system passively ascended closer to the surface.

Fluid inclusion and stable isotope geochemistry of the orogenic–type Zinvinjian Cu–Pb–Zn–Au deposit in the Sanandaj–Sirjan metamorphic belt, Northwest Iran

The Zinvinjian polymetallic deposit occurs as veins controlled by a NW–SE trending–structure within the Cretaceous metamorphosed limestone and dolomite, schist, and metavolcanic rocks, northwest of Iran. The retrograde greenschist facies metamorphism was accompanied by large–scale transpressional faulting, crack–seal veins, infiltration of large volumes of hydrous fluid with high XCO2, and is largely overlapped by the main hydrothermal events. The metamorphism has resulted in two stages of mineralization in the Zinvinjian deposit. These are early–stage polymetallic sulfides–quartz and late–stage pyrite–quartz veins. The early–stage veins filled fractures and are undeformed, suggesting a tensional shear setting. The late–stage veins are also mainly open–space fissure–fillings that cut or replace earlier veins. Three types of fluid inclusions (FIs), including aqueous (type–I), mixed carbonic–aqueous (type–II), and carbonic (type–III), were identified in ore–related quartz veins. The early–stage quartz contained all three types of primary FIs homogenized at temperatures of range 197–300°C and salinities of 2.5–15.2wt% NaCl equivalent. In contrast, the late–stage quartz veins contained only type–I FIs with homogenization temperatures ranging between 192 and 210°C, and salinities of 0.2–2.7wt% NaCl equivalent. This indicates that the metallogenic system evolved from a carbonic–rich, metamorphic fluid to a carbonic–poor, one through input of meteoric fluids. All three types of FIs in the early–stage minerals displayed evidence of vein formation during an episode of fluid immiscibility. Quartz δ18O (+15.3 to +19.0‰) and sulfide δ34S (−9.4 to +11.6‰) indicate isotopic equilibrium with host metasediments (rock buffering) and a metasedimentary source of sulfur during early–stage. It is believed that ore mineralization is the result of a decrease in base–metal solubility during an episode of the fluid immiscibility. This study suggests that mineralization at the Zinvinjian deposit is metamorphogenic in style, probably related to a deep–seated orogenic system.

Constraining the thermal history of the North American Midcontinent Rift System using carbonate clumped isotopes and organic thermal maturity indices

The Midcontinent Rift System (MRS) is a Late Mesoproterozoic (∼1.1Ga) sequence of volcanic and sedimentary rocks exposed in the Lake Superior Region of North America. The MRS continues to be the focus of much research due to its economic mineral deposits as well as its archive of Precambrian life and tectonic processes. In order to constrain the post-depositional thermal history of the MRS, samples were analyzed for carbonate clumped isotope composition and organic thermal maturity. Clumped isotope values from sedimentary/early-diagenetic samples were partially reset during burial to temperatures between 68 and 75°C. Solid-state reordering models indicate that maximum burial temperatures of 125–155°C would reset the clumped isotope values to the observed temperature range prior to the onset of regional cooling and uplift. Clumped isotope results from late-stage veins in the White Pine Mine encompass a greater temperature range (49–116°C), indicative of spatially variable hydrothermal activity and vein emplacement after burial temperatures fell below 100°C during regional cooling and uplift. Clumped isotope and organic thermal maturity data do not indicate significant spatial differences in thermal history along the MRS. Observed variability in bulk organic matter composition and biomarker indices are therefore more likely a result of shifts in primary productivity or early-degradation processes. These results demonstrate that the MRS experienced a spatially consistent, relatively mild thermal history (125–155°C) and is therefore a valuable archive for understanding the Late Mesoproterozoic environment.

Mantle hornblendites of Naein ophiolite (Central Iran): Evidence of deep high temperature hydrothermal metasomatism in an upper mantle section

The Naein ophiolite is the most complete ophiolitic exposure in Cental Iran and considered as a remnant of the Mesozoic Central East Iranian microcontinent (CEIM) confining oceanic crust. In the northeastern part of this ophiolite (Darreh Deh area) within the mantle peridotites, a few hundred meters below the top of the Moho transition zone (MTZ), the hornblendites are present as dykes (former cracks and joints) from a few millimeters to nearly 50 cm wide. They have sharp boundaries with the surrounding mantle harzburgites and dunites. These hornblendites are pale green and coarse-grained in hand specimen and composed of magnesio-hornblende (Mg# = 0.93), chlorite (penninite and clinochlore, Mg# = 0.95), Cr-spinel (chromite, Cr# = 0.67 and Mg# = 0.55), tremolite, calcite and dolomite. Tremolites were formed by retrograde metamorphism of hornblendes. Calcite and dolomite occur as late-stage veins. Very high amount of primary hydrous phases (~94 vol % hornblende and chlorite), as well as peculiar mineralogical and chemical characteristics of the Naein ophiolite mantle hornblendites, do not match a magmatic origin. They are possibly products of the reaction between mantle peridotites and seawater-originated supercritical fluids, rich in silicate components. The presence of primary hydrous phases (hornblende and chlorite) may reveal high activity of H2O in the involved solution. The chemical composition of chromite in the hornblendites is near to the average chromite composition from the surrounding harzburgite and dunite. This suggests that the main source of Cr should be chromites of nearby peridotites, which were totally or partly dissolved by hydrothermal fluids. The positive anomaly of Eu in the chondrite-normalized REE patterns of hornblendes, high modal abundance of Ca-rich hornblende, as well as presence of calcite and dolomite, point to seawater ingression through the gabbros in to the uppermost mantle peridotites. The higher value of MgO than CaO, presence of high-Cr chromite and Cr-enrichment of hornblendes and chlorites indicate a higher contribution of peridotites rather than gabbros to the chemical characteristics of the involved fluids. This study shows that circulation of possibly seawater-derived high temperature hydrous fluids in the upper mantle can leach and provide necessary elements to form hornblendite in joints and cracks of the uppermost mantle.

Hydrothermal metasomatism of a peralkaline granite pegmatite, Khaldzan Buragtag massif, Mongolian Altai; complex evolution of REE-Nb minerals

Abstract The low-temperature hydrothermal alteration of certain rare-metal minerals is recorded in a quartz-epidote metasomatite from the Tsakhirin Khuduk occurrence in the Khaldzan-Buragtag Nb-REE-Zr deposit, Mongolian Altai. A peralkaline granitic pegmatite was metasomatized by hydrothermal fluids released from associated intrusions, with the formation of, inter alia, chevkinite-(Ce), fergusonite-(Nd) and minerals of the epidote group. The textural pattern indicates recrystallization and coarsening of these phases. Later, low-temperature alteration by fluids resulted in the chevkinite-(Ce) being replaced by complex titanite-TiO2 -cerite-(Ce)-hingganite-hydroxylbastnasite-( Ce) assemblages. Calcite formed late-stage veins and patches. The hydrous fluids were poor in F and CO2 but had high Ca contents.

Mass transfer and fluid evolution in late-metamorphic veins, Rhenish Massif (Germany): insight from alteration geochemistry and fluid-mineral equilibria modeling

Element mobility and fluid-rock interaction related to the formation of late-metamorphic quartz veins have been studied by combination of mineral chemistry, whole-rock geochemistry, mass balance analysis and fluid-mineral equilibria modeling. The quartz veins are hosted by very low-grade metasedimentary rocks of the fold-and-thrust belt of the Rhenish Massif (Germany). The veins record two stages of evolution, a massive vein filling assemblage with elongate-blocky quartz, chlorite, apatite and albite, and a later open space filling assemblage with euhedral crystals of quartz, ankerite-dolomite and minor calcite and sulfides. Detailed mass balance analysis of an alteration profile adjacent to a representative quartz vein demonstrates that element mobility is restricted to the proximal zone. The most important element changes are gain of Ca, Fe, Mg, Mn, P and CO2, and loss of Si, K and Na. The data demonstrate that wall-rock carbonation is one of the main alteration features, whereas mobility of Si, K and Na are related to dissolution of quartz and destruction of detrital feldspar and muscovite. The whole-rock geochemical data, in conjunction with fluid composition data and pressure-temperature estimates, were used as input for fluid-mineral equilibria modeling in the system Si-Al-Fe-Mg-Ca-Na-K-C-S-O-H-B-F-Cl. Modeling involved calculation of rock-buffered fluid compositions over the temperature interval 100–500 °C, and reaction-path simulations where a rock-buffered high-temperature fluid reacts with fresh host-rocks at temperatures of 400, 300 and 200 °C. Calculated rock-buffered fluid compositions demonstrate that retrograde silica solubility is a strong driving force for quartz leaching in the temperature-pressure window of 380–450 °C and 0.5 kbar. These conditions overlap with the estimated temperatures for the initial stage of vein formation. Reaction-path models show that high-temperature alteration can produce the observed silica leaching, suggesting that fast advection of external hot fluids from deeper crustal levels was essential for the early stage of vein formation. Fluid advection must have occurred as multiple pulses, which allowed for periods of influx of fluids that leached quartz, alternating with periods of cooling and quartz precipitation in the veins. Reaction-path models at high temperatures (300–400 °C) do not produce carbonate alteration, whereas fluid-rock reaction at 200 °C produces carbonate alteration, consistent with the temperature estimates for the late-stage vein carbonate assemblage. Comparison between modeling results and geochemical data suggests that the observed alteration features are the product of fluid-rock reaction under conditions where the external fluid gradually cooled down and evolved with time. The results of this study highlight the importance of late-orogenic fluid migration for the formation of quartz vein arrays in fold-and-thrust belts.

The giant Bayan Obo REE-Nb-Fe deposit, China: Controversy and ore genesis

Bayan Obo ore deposit is the largest rare-earth element (REE) resource, and the second largest niobium (Nb) resource in the world. Due to the complicated element/mineral compositions and involving several geological events, the REE enrichment mechanism and genesis of this giant deposit still remains intense debated. The deposit is hosted in the massive dolomite, and nearly one hundred carbonatite dykes occur in the vicinity of the deposit. The carbonatite dykes can be divided into three types from early to late: dolomite, co-existing dolomite-calcite and calcite type, corresponding to different evolutionary stages of carbonatite magmatism based on the REE and trace element data. The latter always has higher REE content. The origin of the ore-hosting dolomite at Bayan Obo has been addressed in various models, ranging from a normal sedimentary carbonate rocks to volcano-sedimentary sequence, and a large carbonatitic intrusion. More geochemical evidences show that the coarse-grained dolomite represents a Mesoproterozoic carbonatite pluton and the fine-grained dolomite resulted from the extensive REE mineralization and modification of the coarse-grained variety. The ore bodies, distributed along an E–W striking belt, occur as large lenses and underwent more intense fluoritization and fenitization. The first episode mineralization is characterized by disseminated mineralization in the dolomite. The second or main-episode is banded and/or massive mineralization, cut by the third episode consisting of aegirine-rich veins. Various dating methods gave different mineralization ages at Bayan Obo, resulting in long and hot debates. Compilation of available data suggests that the mineralization is rather variable with two peaks at ∼1400 and 440 Ma. The early mineralization peak closes in time to the intrusion of the carbonatite dykes. A significant thermal event at ca. 440 Ma resulted in the formation of late-stage veins with coarse crystals of REE minerals. Fluids involving in the REE-Nb-Fe mineralization at Bayan Obo might be REE-F-CO2-NaCl-H2O system. The presence of REE-carbonates as an abundant solid in the ores shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo. The Bayan Obo deposit is a product of mantle-derived carbonatitic magmatism at ca. 1400 Ma, which was likely related to the breakup of Columbia. Some remobilization of REE occurred due to subduction of the Palaeo-Asian oceanic plate during the Silurian, forming weak vein-like mineralization.