Igneous Origin Research Articles

Temperatures and duration of crystallization within gem-bearing cavities of granitic pegmatites

Crystallization within miarolitic cavities in granitic pegmatites begins with the primary igneous minerals – feldspars, quartz, micas, and lesser others – along the margins of pockets and ends with the formation of zeolites, carbonates, and finally dense clay. The euhedral and gem-quality crystals for which many pegmatites are mined form within this interval, and they are commonly suspended in the dense clay with no points of attachment to a substrate. The sequence of mineral assemblages reflects a history of crystallization with cooling that continues well after and to temperatures below the initial magmatic stage. Low-temperature assemblages of zeolites and clays are normally restricted to the miarolitic cavities themselves in otherwise fresh and unaltered pegmatite of igneous origin. This study utilizes feldspar solvus thermometry to delineate the temperature interval over which the euhedral and gem-quality crystals form. Additional data include a survey of Al/Si order in K-feldspar and the mineralogy of the clay-size fraction from miarolitic cavities as further constraints on the conditions of crystallization. The primary feldspars at pocket margins record an average crystallization temperature of 431°C (1σ = 68°C), approximately the same temperature at which crystallization takes place across the massive portions of thin miarolitic pegmatite dikes. The average calculated temperature of closure of perthite exsolution occurred at 338°C (1σ = 60°C). Pairings of glassy, non-perthitic rims on K-feldspar with radial albite of cleavelandite habit on the K-feldspar surfaces record the final stage of crystallization of the primary silicates in pockets at an average temperature of 359° (1σ = 64°C). The glassy rims are not perthitic because they form close enough to the closure temperature of perthite that exsolution does not occur. The zeolites, which include stilbite-Ca, laumontite, and heulandite, are restricted in their stability to less than 300°C, and they follow the precipitation of the gem minerals in all known cases. Thus, the gem-forming stage likely falls between ~ 435°-355°C based on the feldspar solvus temperatures. Cookeite, kaolinite, montmorillonite, and illite enclose zeolites and formed after them. In a thermal model, a dike 1 meter thick emplaced as hydrous granitic liquid at 700°C at a depth of 7 km (ambient temperature of 160°C) cools to 435°C at its center in 12 days, and from 435°C to 355°C – the interval over which the primary silicates crystallize in pockets – in another 14 days. Increasing the host rock temperature to 300°C only extends the crystallization interval of the principal pocket silicates by another 5.2 months. The dike center reaches 275°C, at which temperature laumontite could crystallize, approximately 55 days after emplacement. Clay minerals likely form and perhaps reform below ~200°C, the upper thermal stability limit of stilbite-Ca, as the devitrified products of a remnant and flux-depleted hydrosilicate glass that partially fills the miarolitic cavities. The pegmatite achieves thermal equilibrium with the host rocks at 160°C more than 200,000 years later.

The “Lower Kaimur Porcellanite” (Vindhyan Supergroup) is of Sedimentary Origin and not Tuff

Abstract The ‘Lower Kaimur Porcellanite’ from the Proterozoic Vindhyan Supergroup (~1700-900? Ma) is not only a chronostratigraphic marker but also an indicator of the tectonic setting of the basin. A few other silicified shaly units (porcellanites) from the upper strata have been thought to be tuff. New petrographic (optical microscopic; SEM-BSE), chemical, and U-Pb zircon geochronological studies of the lowermost of these suspected tuff units, however, do not support an igneous origin for these beds. The rocks do not contain phenocrysts or glass shards, but contain remains of mineralized microbial spheres, mudclasts, and other detrital grains that include one datable zircon grain (~1715 Ma). Their chemical compositions are not diagnostic of tuff. Despite this result, investigations of other porcellanites from Upper Vindhyan strata is recommended, because they have the potential of identifying crucially important tuff beds.

Textural and chemical variations of magnetite from porphyry Cu-Au and Cu skarn deposits in the Zhongdian region, northwestern Yunnan, SW China

The Zhongdian region in northwestern Yunnan (SW China) hosts a number of important porphyry and skarn deposits. In this study, we integrate petrographic observations, SEM-EDS elemental mapping, EPMA and LA-ICP-MS geochemical analyses on magnetite from several key deposits in the region. Magnetite grains from the Pulang porphyry Cu-Au, Lannitang porphyry Au-Cu, Langdu Cu skarn, Disuga porphyry, and andesite of Tumugou Formation were analyzed to unravel the magnetite formation mechanism, and the relative oxygen fugacity of the porphyry/skarn ore-related hydrothermal fluids in this region.Two types of magnetites (Magnetite I and II) have been identified. Magnetite I is commonly distributed in the chlorite and/or sericite alteration zone(s). Magnetite I grains are characterized by being textually inhomogeneous. The Magnetite I core (with local ilmenite exsolution lamellae) is Ti-Cr rich, indicative of a dominantly igneous origin. The Magnetite I core and its surrounding primary igneous minerals (biotite, amphibole and plagioclase) underwent late alteration. Magnetite I grains are characterized by core-rim texture, sharp core-rim contact, and irregular grain boundaries. These textural features indicate that Magnetite I may have undergone dissolution-reprecipitation processes, during which the Magnetite I core was chemically purified via losing Ti and gaining Fe. Magnetite II (in potassic and sericite alteration zones) is textually homogeneous with relatively low contents of Ti, Mg, Al, V, Ni, and Mn. We suggest that Magnetite II was formed by hydrothermal alteration. Compared to Magnetite II grains from the Pulang and Langdu deposits, those from the Lannitang deposit have lower Mn and V but higher Sn contents, indicative of higher oxygen fugacity of the hydrothermal fluids. We propose that binary plots such as Ti vs. Sc, V vs. Sn and Ga vs. Sn can distinguish the different origins of magnetites.From an exploration perspective, Magnetite II from the altered Disuga porphyries has similar texture and composition to its counterpart from the potassic-altered Pulang porphyries. This, together with the presence of Cu-sulfide inclusions in the Disuga Magnetite I, suggests significant mineralization potential in these porphyries.

A Review of Boron-Bearing Minerals (Excluding Tourmaline) in the Adirondack Region of New York State

Boron is a biologically important element, but its distribution in the natural environment and its behavior during many geological processes is not fully understood. In most metamorphic and igneous environments, boron is incorporated into minerals of the tourmaline supergroup. In high-grade metamorphic terranes like that of the Adirondack region of northern New York State, uncommon rock compositions combined with unusual and variable geologic conditions resulted in the formation of many additional boron-bearing minerals. This paper reviews the occurrences and geological settings of twelve relatively uncommon boron-bearing minerals in the southern Grenville Province of upstate New York and provides new chemical and Raman spectral data for seven of these minerals. The boron minerals range from relatively simple metal borates (e.g., vonsenite), to chemically complex borosilicates (e.g., prismatine), to a relatively rare borosilicate-carbonate (e.g., harkerite). Some are of primary igneous origin, while others are formed by a variety of prograde and retrograde metamorphic processes or by metasomatic/hydrothermal processes. Most of the boron minerals are formed within, or adjacent to, metasedimentary lithologies that surround the anorthositic massifs of the central Adirondacks. The metasedimentary rocks are thought to be the source of most of the boron, although additional boron isotope studies are needed to confirm this and to constrain the mechanisms of the formation of these unusual minerals.

A new view on the genesis of the Chiweigou gold deposit, Jilin, China: A carbonatite-type deposit

The Chiweigou gold deposit is located in the eastern Jilin and Heilongjiang provinces at the junction of the Paleo-Asian Ocean and Circum-Pacific metallogenic domains, where was affected by the events that marked the transition between these two domains. The gold orebodies in the Chiweigou deposit are hosted in three calcite veins controlled by compressional-shearing faults, which cut across Early Cretaceous intermediate–acidic volcaniclastic rocks. The Au-bearing calcite veins have low metal sulfide contents (<1%), and the main ore deposits display disseminated, brecciated, comb, and miarolitic structures. The minerals vary from euhedral to anhedral. In addition to the main carriers of the Au and Ag are native gold, calaverite, hessite, and aurotellurite, and the gold-carrying minerals coexist closely with other common minerals in the carbonatite veins such as calcite, quartz, apatite, and magnetite, implying genetic relationships among them. The main type of wall-rock alteration is carbonatization, and alkali metasomatism, including fenitization, is widespread. The SiO2 contents of the Au-bearing calcite veins show very large variations (0.87–53.9 wt%), and the veins have high contents of CaO (24.4–54.9 wt%) and low contents of MgO (0.06–0.13 wt%) and (Na2O + K2O) (0.10–0.39 wt%). The main oxide contents show good linear relationships with SiO2 on Harker diagrams, and this indicates that differentiation took place between the siliceous and carbonate components during the formation of the calcite veins. The LREE contents of the calcite veins are higher than the HREE contents (LREE/HREE ratios range from 10.7 to 15.8), and the normalized distribution patterns slope towards the HREE end. Eu and Ce show positive anomalies. The calcite veins are enriched in incompatible elements (Rb, Sr, Hf, and Ta) and depleted in K. The trace element ratios of the samples plot in the CHARAC field, indicating an igneous origin. In the εNd(t) vs. (87Sr/86Sr)i diagram the Chiweigou carbonatite plots close to Mesozoic carbonatites from the adjacent NCC, and the evolutionary trend is from primary mantle to an enriched mantle I-type. Based on our analytical results, and taking into account previous research, we conclude that the Chiweigou calcite veins are carbonatites, that were derived from an end-member EMI-type mantle source that had been metasomatized by highly differentiated C–H–O supercritical fluids. The Au was probably dispersed by and precipitated from colloidal Au–Si complexes (AuH3SiO4). The geochemical characteristics of the contemporary andesitic tuffs and diorite porphyrite indicate that the formation of the Chiweigou Au-bearing calcite veins resulted from extensional tectonics related to Early Cretaceous subduction of the Pacific Plate. During the early stages of intense extension (140 Ma), the carbonatite magmas ascended rapidly along extensional faults, and when they arrived in the shallow crust they mixed with meteoric water to produce the alkali metasomatism (fenitization) and the precipitation of Au as temperatures and pressures fell, thus forming the Au-bearing calcite veins and related mineralization of the Chiweigou deposit.

Tectonic evolution of north‐eastern Tethyan Himalaya: Evidence from U–Pb geochronology and Hf isotopic geochemistry of detrital zircons

It is well established that the Himalayan Orogen was formed by successive amalgamation of continental slices to the Eurasian continent, with the final collision of the Indian continent. The Upper Triassic Langjiexue Group on the north‐eastern margin of the Tethyan Himalaya has been central to debates on the provenance with diverse models linking it with the northern India, Lhasa terrane, or multiple sources from surrounding terranes including Australia. In order to address this debate, here, we present U–Pb ages, trace element characteristics, and Hf isotope data of detrital zircons. The trace element data suggest that analysed zircons are mostly of igneous origin, with a broad affinity to mafic source rock, and some of the grains showing evidence for hydrothermal alteration. The zircon age spectra show distinct Permian to Triassic age peaks at 200–280 Ma with εHf(t) between −6.1 and 13.4, Neoproterozoic to Cambrian ages at 480–750 Ma with εHf(t) ranging from −24.7 to 8.5, and a broad Meso‐ to Neoproterozoic age range of 850–1,150 Ma with εHf(t) values of −8.1 to 10.1. The age spectra from the Langjiexue Group are in contrast with that of the Lhasa terrane which has pronounced age peaks of 300–325, 550–600, 1,150–1,350, and 1,750–1,900 Ma, suggesting that Lhasa might not be the source of Langjiexue Group detritus. In a similar way, north‐western Australia and the Banda Arc are excluded as sources of the Langjiexue Group. The Hf model age spectra show distinct peaks of 750–800 and 1,200–1,300 Ma, and the second peak agrees with that of the Tethyan Himalaya. We infer that the basin of Langjiexue Group formed within the basement of Tethyan Himalaya in the Triassic and conclude that the sediments were deposited at the passive margin of northern Indian continent. Based on the results, we also propose the palaeogeographic evolution of the Himalayan Orogen.

Textures and Chemical Compositions of Magnetite from Iron Oxide Copper-Gold (IOCG) and Kiruna-Type Iron Oxide-Apatite (IOA) Deposits and Their Implications for Ore Genesis and Magnetite Classification Schemes

AbstractTextural and compositional data of magnetite from Igarapé Bahia, Alemao, Sossego, Salobo, and Candelaria iron oxide copper-gold (IOCG) and El Romeral Kiruna-type iron oxide-apatite (IOA) deposits show that some magnetite grains display oscillatory zoning or have been reequilibrated by oxy-exsolution, coupled dissolution and reprecipitation (CDR) reactions, and/or recrystallization. Textures formed via CDR are most widespread in the studied samples. The original oscillatory zoning was likely derived from the crystal growth during fluctuating fluid compositions rather than from variation in temperature and oxygen fugacity. The oxy-exsolution of ilmenite in magnetite is attributed to increasing oxygen fugacity and decreasing temperature with alteration and mineralization, resulting in product magnetite with lower Ti and higher V contents. Recrystallization of some magnetite grains is commonly due to high-temperature annealing that retained primary compositions. Two different types of CDR processes are defined according to textures and chemical compositions of different generations of magnetite. The first generation of magnetite (Mag-1) is an inclusion-rich and trace element-rich core, which was replaced by an inclusion-poor and trace element-poor rim (Mag-2). The third generation of magnetite (Mag-3), inclusion poor but trace element rich, occurs as veins replacing Mag-2 along fractures or grain margins. Type 1 CDR process transforming Mag-1 to Mag-2 is more extensive and is similar to processes reported in skarn deposits, whereas type 2 CDR process is local, transforming Mag-2 to Mag-3. During type 1 CDR process, minor and trace elements Si, K, Ca, Mg, Al, and Mn in magnetite are excluded, and Fe contents increase to various extents, in contrast to type 2 CDR process, which is characterized by increased contents of Si, K, Ca, Mg, Al, and Mn. Type 1 CDR process is possibly induced by the changing fluid composition and/or decreasing temperature during progressive alteration and ore formation, whereas type 2 CDR process can be interpreted as post-ore replacement due to a new pulse of magmatic-hydrothermal fluids.The identification of magnetite core (Mag-1) with igneous origin and rim (Mag-2) with magmatic-hydrothermal origin in the Sossego IOCG and El Romeral IOA deposits supports a fluid changing from magmatic to magmatic-hydrothermal during IOCG and IOA formation and indicates a genetic link between these two deposit types. The large data set here further demonstrates that magnetite is susceptible to textural and compositional reequilibration during high-temperature magmatic and magmatic-hydrothermal processes. Reequilibrated magnetite, particularly that formed by CDR processes, has a chemical composition that can be different from that of primary magnetite. Modified magnetite, therefore, cannot be used to discriminate its primary origin or to interpret its provenance in overburden sediments. Therefore, in situ chemical analysis of magnetite combined with textural characterization is necessary to understand the origin of magnetite in IOCG and IOA deposits.

Petrography and mineral chemistry of the Varena Iron Ore deposit, southeastern Lithuania: implications for the evolution of carbonate and silicate rocks and ore mineralization

The large Varena Iron Ore deposit of southeast Lithuania lies beneath 200–400 m thick sedimentary cover in the East European Craton. Several drilling projects have revealed that it contains ca. 70–200 million tons of iron ore. A metasomatic origin has been proposed by several investigators, with an igneous (mafic, ultramafic carbonatitic) origin also considered. Detailed micropetrography on a variety of ore-bearing and skarnitised samples through backscattered electron imaging, along with mineral chemical analysis and monazite dating by electron microprobe, provide support for a metasomatic origin. High-grade temperature (ca. 700–750° C) skarns containing forsterite, enstatite, spinel and diopside were formed preferentially in carbonate-rich rocks and were subsequently overprinted by amphibole-bearing (tremolite, actinolite, anthophyllite and hornblende) skarns. A low-temperature hydrous influx and changing redox conditions not only caused the serpentinization of the earlier skarns, but also deposited a considerable amount of iron ore in the form of magnetite. Iron was derived from various sources and was accompanied by sulphide, apatite, REE and other mineralizations. The ca. 1.78 Ga monazite grew during the regional metamorphism, while ca. 1.54 Ga is attributed to the last mineralization phases.

Sediment chemistry and detrital zircon record in the Bosque and Paseo del Mar coastal areas from the southwestern Gulf of Mexico

Abstract In this study, we analyzed the mineralogy and bulk chemical composition of 20 sediment samples, as well as trace, rare earth element (REE) concentrations and U–Pb ages of 400 detrital zircons retrieved from the Bosque (PB) and Paseo del Mar (PM) coastal sediments, southwestern Gulf of Mexico, to investigate the provenance. The XRD study revealed that the PB sediments are associated with quartz, muscovite, cordierite, albite, and anorthite. The PM sediments are associated with calcite, aragonite, and biogenic materials, which derived probably within the coastal environment. The weathering indices like Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA) revealed a moderate intensity of weathering for the PB sediments. The chondrite normalized REE patterns of the PB and PM sediments are with negative and positive europium anomalies (Eu/Eu*), suggesting that the sediments were derived by the combination of felsic and mafic source rocks. The Th/U ratios (>0.3) of the PB and PM zircons indicated an igneous origin. The chondrite normalized REE patterns of the PB and PM zircons are depleted in low REE (LREE) and enriched in heavy REE (HREE) with positive cerium and negative europium anomalies. A wide variation in chondrite normalized REE patterns of zircons with respect to ages was observed. The PB is dominated by Proterozoic (~551–2500 Ma) and Paleozoic zircons (~445–480 Ma; Ordovician), whereas PM is dominated by Cenozoic zircons (~5–21 Ma; Miocene), indicating that the zircons were derived from different source terranes. The Chiapas Massif (CMC) and Chuacus complexes are identified as the potential source areas, which contributed Proterozoic zircons to PB. On the other hand, Cuicateco terrane (CUI) and Tuxtla Volcanic Field (TVF) are considered as the potential source areas, which supplied significant number of Cenozoic zircons to the PM coastal area. The zircon U–Pb ages and the bulk sediment geochemistry together indicate that the source terranes, which supplied sediments to the PB and PM coastal areas, consist of mafic and felsic rocks. The results of this study further illustrated the importance of littoral currents in distributing sediments, although rivers are considered as a carrier and agent of distributing sediments along the coastal areas.

Voxel solution and 3D sub-surface imaging in the Douala sedimentary sub-basin (Cameroon)

Gravity data consisting of a total of 128 data points located between latitudes 3° 21′N and 4° 9′ N and longitudes 8° 27′ E and 9° 16′ E have been computed for Bouguer and residual anomaly maps in this study. From the plots of these maps which are characterized by very high positive ring shaped contours towards the centre, which are suggestive of a subsurface intrusive body, three anomaly zones as well as two minor peaks were identified. The maps were also characterized by W-E alignment of contours to the N and S as well as approximately NNW-SSE alignments to the E and to some extent, the W of the study area. A layered earth model which constitutes a depth model built using the geosoft montaj extension, GM-SYS 3D, was used to provide a surface-oriented model for this intrusive body. Using constant layers added to a 3D model at depths of 5 km, 10 km, 15 km, 20 km, 25 km, 30 km, 33 km, 35 km and 39 km; each in the absence of the others, forward calculations were employed to calculate the density contrast of each layer while their densities were recalculated using constant density inversion. Structural inversion of each of these layers then generated their surface-oriented models. The optimum inverse model for the intrusive body was obtained at a depth of about 33 km. The model has a surface and surroundings with a density of about 2.5 g/cm3 which corresponds to that of sedimentary basins. It also presents an intrusive body with density of about 2.77 g/cm3 located at a depth of about 6.1 km with its bottom located at about 33 km giving an average vertical height of about 26.9 km. This 3D relief sub-surface model was used to define the geometry of the starting model for 3D data voxels to generate the block structure of the intrusive body. Applying the basic statistical kriging algorithm, the 3D voxel solution of the intrusive body underneath this study area as well as its corresponding calculated maps were generated. This solution is similar in features to that of the surface-oriented solution and presents a block structure of the intrusive body with a major peak at a depth of about 6.2 km and having a relatively sharp sloping boundary towards its NE edge and gradual sloping edges towards its NW, W and E limits. Its density of 2.77 g/cm3 is further confirmation to the fact that this body is of igneous origin. This model however shows evidence that this block extends further south beyond this study area and consequently requires more data in this direction to image it fully. These results from the 3D surface-oriented solution and the 3D voxel solution are in good agreement with previous results from 2.5D studies in this very area of study.

Combined boron, radiogenic (Nd, Pb, Sr), stable (C, O) isotopic and geochemical investigations of carbonatites from the Blue River Region, British Columbia (Canada): Implications for mantle sources and recycling of crustal carbon

This study reports the combined major, minor and trace element compositions, and stable (C, O), radiogenic (Nd, Pb, and Sr) isotopic compositions, and first δ11B isotopic data for the Fir, Felix, Gum, and Howard Creek carbonatites from the Blue River Region, British Columbia (Canada). These sill-like occurrences were intruded into Late Proterozoic strata during rifting and extensional episodes during the Late Cambrian and Devonian -Mississippian, and subsequently deformed and metamorphosed to amphibolite grade in relation to a collisional-type tectonic environment. The carbonatites at Fir, Gum, and Felix contain both calcite and dolomite, whereas the carbonatite at Howard Creek contains only calcite. The dolomite compositions reported here are consistent with those experimentally determined by direct partial melting of metasomatized peridotitic mantle. The combined major and trace element compositions and δ13CPDB (−5.37 to −4.85‰) and δ18OSMOW (9.14 to 9.62‰) values for all the samples investigated are consistent with those for primary igneous carbonate and support their mantle origin. However, these signatures cannot be attributed to closed system melt differentiation from a single parental melt. The initial Nd, Pb, and Sr isotopic ratios are highly variable and suggest generation from multiple, small degree parental melts derived from a heterogeneous mantle source. The δ11B values for carbonates from Felix, Gum, and Howard Creek vary between −8.67 and −6.36‰, and overlap the range for asthenospheric mantle (−7.1±0.9‰), whereas two samples from Fir yield heavier values of −3.98 and −2.47‰. The latter indicate the presence of recycled crustal carbon in their mantle source region, which is consistent with those for young (<300Ma) carbonatites worldwide. The radiogenic and B isotope results for the Blue River carbonatites are compared to those from contrasting, anorogenic tectonic settings at Chipman Lake, Fen, and Jacupiranga, and indicate that similar upper mantle sources are being tapped for carbonatite melt generation. The pristine, mantle-like δ11B values reported here for the Blue River carbonatites clearly demonstrate that this isotope system is robust and was not perturbed by post-solidification tectono-metamorphic events. This observation indicates that B isotope signatures are a valuable tool for deciphering the nature of the upper mantle sources for carbonates of igneous origin.

A geochemical and mineralogical update on two major tonsteins in the UK Carboniferous Coal Measures

Samples of two prominent tonsteins in the UK Carboniferous coal-bearing strata, representing kaolinised mafic ash and felsic ash have been comprehensively analysed by ICP-MS, laser ablation ICP-MS, SEM/EDX, XRD and XRF. Compositions have been compared with possible parent ashes in order to establish the extent to which elements are lost or retained during the alteration process. This is more readily achieved for the mafic tonsteins because they can be directly linked to the local, contemporaneous volcanics. The volcanic source of the felsic tonsteins is not known with certainty and the geochemical comparisons are made with modern volcanics from what is thought to be a similar tectonic/magmatic setting. Evidence of element migration during the tonstein formation has also been obtained by analysing the adjacent coals. Some mobility of U, Th, Nb, Y, Zr Hf and Ta is demonstrated, which imposes some constraints on the use of discrimination diagrams for the interpretation of original ash composition and tectonic setting. Notwithstanding some mobility of the incompatible elements, the discrimination diagrams are of use in providing broad igneous groupings. The TiO2/Al2O3 ratio is useful in that these two elements show the least mobility and the ratio is diagnostic of ash composition and sediment inclusion in the ash if the quartz content is also taken into account. The sediments associated with the felsic tonstein do not have a constant value for this ratio, which increases with grain size and quartz content and enables ash with included sediment to be recognized. Based on SEM/EDX and LA ICP-MS element locations have been identified. Zircon is a diagnostic volcanogenic mineral which makes an important contribution to the trace element budget, but there are other sources, notably a range of phosphate minerals, some of igneous origin and others diagenetic. This is also demonstrated from the whole rock geochemistry which shows there is an excess of U and Th over that which can be accommodated within the calculated zircon content.The REE profiles for the two types of tonstein differ significantly to each other. One is comparable with the profiles recorded from the associated basalts and partial melting of the upper mantle is thought to be responsible. The felsic tonstein, on the other hand, shows a profile common to many rhyolites with evidence of significant fractional crystallization. Again, in spite of some mobility during the alteration, the REE profiles appear to reflect the original ash composition, which is consistent with other aspects of the geochemistry.

The formation of the ore-bearing dolomite marble from the giant Bayan Obo REE-Nb-Fe deposit, Inner Mongolia: insights from micron-scale geochemical data

The genesis of Earth’s largest rare earth element (REE) deposit, Bayan Obo (China), has been intensely debated, in particular whether the host dolomite marble is of sedimentary or igneous origin. The protracted (Mesoproterozoic to Paleozoic) and intricate (magmatic to metasomatic) geological processes complicate geochemical interpretations. In this study, we present a comprehensive petrographic and in situ, high-spatial resolution Sr-Pb isotopic and geochemical investigation of the host dolomite from the Bayan Obo marble. Based on petrographic evidence, the dolomite marble is divided into three facies including coarse-grained (CM), fine-grained (FM), and heterogeneous marble (HM). All carbonates are ferroan dolomite with high SrO and MnO contents (> 0.15 wt.%), consistent with an igneous origin. Trace element compositions of these dolomites are highly variable both among and within individual samples, with CM dolomite displaying the strongest LREE enrichment. In situ 206Pb/204Pb and 207Pb/204Pb ratios of the dolomite are generally consistent with mantle values. However, initial 208Pb/204Pb ratios define a large range from 35.45 to 39.75, which may result from the incorporation of radiogenic Pb released from decomposition of monazite and/or bastnasite during Early Paleozoic metasomatism. Moreover, in situ Sr isotope compositions of dolomite indicate a large range (87Sr/86Sr = 0.70292–0.71363). CM dolomite is characterized by a relatively consistent, unradiogenic Sr isotope composition (87Sr/86Sr = 0.70295–0.70314), which is typical for Mesoproterozoic mantle. The variation of 87Sr/86Sr ratios together with radiogenic 206Pb/204Pb signatures for dolomite within FM and HM possibly represents recrystallization during Early Paleozoic metasomatism with the contribution of radiogenic Sr and Pb from surrounding host rocks. Therefore, our in situ geochemical data support a Mesoproterozoic igneous origin for the ore-bearing dolomite marble in the Bayan Obo deposit, which subsequently underwent intensive metasomatism during the Early Paleozoic.

Phosphate supply security for importing countries: Developments and the current situation

Phosphorus's essentiality for all life on Earth is beyond doubt. As one of the three major macronutrients, it is a crucial building block of modern-day mineral fertilizers and, thus, an essential production factor for global food security. Its almost-exclusive primary sources are phosphate-rock deposits of either sedimentary or igneous origin. The currently known deposits are widely scattered over the globe in large numbers but not necessarily in commercially feasible phosphate-rock quantities. Europe, in particular, possesses negligible considerable reserves; currently, only one Finnish igneous mine is in operation. Thus, phosphate imports are inevitable. Countermeasures like to foster phosphorus recycling from wastewater systems, will reduce rather than substitute imports. Phosphate trade of any form relies on bilateral commercial agreements as it is not traded on commodity exchanges. Therefore, questions regarding supply security and, particularly, the safeness of supply for import-dependent countries, must also address the issues of market concentration, the dynamic reserve–resource situation, and respectively their development over time. We provide a state-of-the-art fundamental analysis all along the supply chain by applying the Hirschman-Herfindahl Index (HHI) to quantify market concentrations as well as reserve-to-production ratios to evaluate developments over recent decades. Thereby, we overcome the unfavorable nature of these static measures. Results suggest medium to highly concentrated markets for the production of phosphate rock, phosphoric acid, and phosphate fertilizers (MAP, DAP) with increasing trends for the overall supply chain. However, these findings should not be interpreted as alarming developments for import-dependent regions, given that the export-market concentrations for phosphate fertilizers have shown significant decreases since the early 1980s.

Genesis of the Koka Gold Deposit in Northwest Eritrea, NE Africa: Constraints from Fluid Inclusions and C–H–O–S Isotopes

The Koka gold deposit is located in the Elababu shear zone between the Nakfa terrane and the Adobha Abiy terrane, NW Eritrea. Based on a paragenetic study, two main stages of gold mineralization were identified in the Koka gold deposit: (1) an early stage of pyrite–chalcopyrite–sphalerite–galena–gold–quartz vein; and (2) a second stage of pyrite–quartz veins. NaCl-aqueous inclusions, CO2-rich inclusions, and three-phase CO2–H2O inclusions occur in the quartz veins at Koka. The ore-bearing quartz veins formed at 268 °C from NaCl–CO2–H2O(–CH4) fluids averaging 5 wt% NaCl eq. The ore-forming mechanisms include fluid immiscibility during stage I, and mixing with meteoric water during stage II. Oxygen, hydrogen, and carbon isotopes suggest that the ore-forming fluids originated as mixtures of metamorphic water and magmatic water, whereas the sulfur isotope suggests an igneous origin. The features of geology and ore-forming fluid at the Koka deposit are similar to those of orogenic gold deposits, suggesting that the Koka deposit might be an orogenic gold deposit related to granite.

Magmatic evolution of the Cerro Maricunga gold porphyry-epithermal system, Maricunga belt, N-Chile

The Cerro Maricunga gold project is located in the Maricunga gold belt in northern Chile, within the remnant edifice of the Tertiary Ojo de Maricunga volcano. The gold mineralization is hosted within a 23‒15 Ma old complex system of interwoven porphyry intrusions, flows, dikes and diverse breccia types, all forming part of a large diatreme system. In particular, phreatomagmatic and phreatic processes contributed to the formation of the synmineral diatreme complex. The rocks are calc-alkaline (daci)andesites and very rare dacites of the medium-K series with a high-K tendency and a conspicuous “adakitic” signature. Rare-earth element patterns reflect formation during a period of distinctly thickened continental crust with retention of heavy rare-earth elements by a garnet-bearing residue at a depth of ≥45 km. U-Pb zircon ages show a wide spectrum from Proterozoic, over Permian and Triassic ages to Paleocene, Oligocene, early and mid-Miocene ages. The Phanerozoic zircon ages indicate the assimilation of country rocks, whereas the Proterozoic age reflects the presence of a lower crustal Neoproterozoic basement that is also suggested by εNd values of −3.9 to −3.1 for bulk rock, i.e. mantle depletion model ages around 1 Ga. The last magmatic activity at the Ojo de Maricunga volcano was at ∼14.9 Ma. Stable isotope data on hydrothermal barite and supergene acanthite and gypsum (δ34Sacanthite: −0.6 to −0.4; δ34Ssulfate: 0.2 to 4.6; δ18Osulfate: 9.2 to 13.8) indicate an igneous origin of sulfur and arid conditions during mineral formation. The emplacement of the diatreme complex took place during the adjustment of the Chilean flat slab setting in a time of increased crustal thickness. Although the rocks at Ojo de Maricunga show a pronounced adakite-like signature, they formed in a common MASH zone or a deep hot zone within the lower crust. Gold mineralization took place at ∼17.9 Ma (Re-Os on molybdenite), syngenetic with diatreme formation and emplacement of hydrous magma. The thickening of continental crust was more advanced in the northern part of the Maricunga belt in the early to mid-Miocene than previously assumed, and then shifted south where several younger gold porphyries formed.

Archaean phosphates: a case study of transformation processes in apatite from the Barberton greenstone belt

Multiple tools have been used to determine the sensitivity of phosphates from the early Archaean Barberton greenstone belt to transformation. The assessment of the degree of transformation is crucial for verifying data about the parameters of the paleo-environment. From the obtained results, three generations of phosphates can be distinguished. Group A is observed in cherts and banded iron formation BIF early-generation fluor-hydroxyapatite that precipitated from seawater. It is characterized by flat rare earth element (REE) patterns with a positive Eu anomaly and high Y/Ho ratio in the range of 54–70. Apatites in this group lack any visible indicators of secondary alterations at the micro- and nanoscales. Fourier transform infrared spectra indicate that these apatites are relatively rich in water, and, due to cationic substitution, their OH-stretching regions exhibit complex ordering and numerous component bands. The characteristics observed in the cherts and silicified felsic volcaniclastics of group B imply advanced metasomatic alteration. They exhibit light and heavy REE depletion and an absence of water in the halogen site. Nanoscale investigations reveal cracks, pores, nanofluid inclusions and nanochannel-like structures, as well as inclusions. Group C is represented by igneous-derived apatites that partially reflect their igneous origin. The phosphates are predominantly fluorapatite with typical magmatic apatite REE distribution patterns. Imaging at the micro- and nanoscales indicates that they partially preserve the signature of igneous origin. It seems that some of the analyzed apatite partially preserved their primordial features; therefore, they might be used for the reconstruction of Archaean abiotic systems.

Benefits of a Multiproxy Approach to Detrital Mineral Provenance Analysis: An Example from the Merrimack River, New England, USA

AbstractThe advantages in provenance research of U‐Pb dating different detrital minerals along with simultaneously analyzing trace elements is demonstrated in a study of sand from the mouth of the Merrimack River in New England, USA. Zircon ages record episodes of magmatism in the Early Paleozoic, peaking in the Early Devonian, followed by quiescence through the remainder of the Paleozoic and additional magmatic episodes in the Jurassic and Cretaceous. Simultaneous measurement of trace elements in zircons reveals a shift from arc magmatism to crustal melting associated with terrane collision in the Early Devonian, while many Jurassic grains are clearly derived from A‐type granites. Detrital monazites and rutiles have Devonian and Permian ages. Many of the older monazites have trace element characteristics suggestive of igneous origin, while Permian monazites are clearly metamorphic and record orogenesis that is absent from the detrital zircon record. Rutile grains have trace element chemistry indicative of mostly metasedimentary source rocks, and Zr thermometry indicates growth under amphibolite facies conditions. Age offsets between monazite and rutile populations provide information about the region's cooling history. Titanite grains have trace element chemistry mostly consistent with igneous origin and U‐Pb ages lining up with minor zircon age populations in the Ordovician‐Silurian and the Middle Devonian, suggesting that these magmatic episodes produced metaluminous compositions. These results show that combining trace element fingerprinting with dating and analyzing multiple detrital mineral species provide a more complete portrait of the geologic history of the sediment source region than U‐Pb dating of zircon alone.

The polymetallic magmatic-hydrothermal Xiangdong and Dalong systems in the W–Sn–Cu–Pb–Zn–Ag Dengfuxian orefield, SE China: constraints from geology, fluid inclusions, H–O–S–Pb isotopes, and sphalerite Rb–Sr geochronology

Vein-type deposits, including the Xiangdong W–Sn and Dalong Pb–Zn deposits, occur in or near the Dengfuxian composite granite pluton, comprising predominantly Triassic and minor Jurassic intrusions. Liquid-rich NaCl-aqueous inclusions, vapor-rich NaCl-aqueous inclusions, liquid-rich CaCl2-NaCl-aqueous inclusions, two-phase CH4-rich inclusions, three-phase CO2-H2O inclusions, and three-phase calcite-bearing inclusions occur in the quartz veins at Xiangdong, whereas only liquid-rich NaCl-aqueous inclusions occur at Dalong. The Xiangdong veins formed at temperatures near 241 °C, from NaCl-CaCl2-H2O-CH4-CO2 fluids averaging 11.4 wt% NaCl eq. The Dalong deposit formed at temperatures near 186 °C from NaCl-H2O(-CH4) fluids averaging 6.2 wt% NaCl eq. The ore-forming mechanisms at Xiangdong include fluid immiscibility during stage I, fluid mixing during stage II, and mixing with meteoric water accompanied by cooling during stage III. The ore-forming mechanisms at Dalong include cooling and mixing with meteoric water. Oxygen and hydrogen isotopes suggest that the ore-forming fluids from both deposits originated as mixtures of magmatic water with various amounts of meteoric water. Sulfur and strontium isotopes suggest an igneous origin for both deposits and possibly mixing with S and Sr from sedimentary rock for Dalong. Lead isotopes indicate that ore metals originated mainly from the upper crust with minor mantle contributions. Sphalerite from Dalong gives a Rb-Sr isochron age of 151.6 ± 7.1 Ma, consistent with the mineralization age of Xiangdong. Both the W–Sn and Pb–Zn ore-forming events are closely related to Late Jurassic magmatism, which occurred in an environment of lithospheric extension and thinning.

Preliminary review of the geology of the hornblende biotite gneisses of Obudu Plateau Southeastern Nigeria

The Obudu Plateau, southeastern Nigeria is dominantly underlain by gneissose rocks with migmatitic characteristics, one of which is the hornblende-biotite gneiss. The gneisses are banded with quartz and plagioclase as the major minerals in the leucosome while hornblende and biotite occur in the palaeosome. Geochemically, the rocks are enriched in silica and alumina with an average of 71.6wt% and 14wt% respectively. Geochemical plots indicate that they are calc-alkaline to tholeiitic in character and derived from protholiths of igneous origin with minor sedimentary input.KEYWORDS: Obudu Plateau, migmatitic gneiss, calc-alkaline