Paragenetic Sequence Research Articles

Mineralogy, paragenesis, and mineral chemistry of REEs in the Olserum-Djupedal REE-phosphate mineralization, SE Sweden

The rapidly growing use of rare earth elements and yttrium (REE) in modern-day technologies, not least within the fields of green and carbon-free energy applications, requires exploitation of new REE deposits and deposit types. In this perspective, it is vital to develop a fundamental understanding of the behavior of REE in natural hydrothermal systems and the formation of hydrothermal REE deposits. In this study, we establish a mineralogical, textural, and mineral-chemical framework for a new type of deposit, the hydrothermal Olserum-Djupedal REE-phosphate mineralization in SE Sweden. An early, high-temperature REE stage is characterized by abundant monazite-(Ce) and xenotime-(Y) coexisting with fluorapatite and subordinate amounts of (Y,REE,U,Fe)-(Nb,Ta) oxides. During a subsequent stage, allanite-(Ce) and ferriallanite-(Ce) formed locally, partly resulting from the breakdown of primary monazite-(Ce). Alteration of allanite-(Ce) or ferriallanite-(Ce) to bastnäsite-(Ce) and minor synchysite-(Ce) at lower temperatures represents the latest stage of REE mineral formation. The paragenetic sequence and mineral chemistry of the allanites record an increase in Ca content in the fluid. We suggest that this local increase in Ca, in conjunction with changes in oxidation state, were the key factors controlling the stability of monazite-(Ce) in the assemblages of the Olserum-Djupedal deposit. We interpret the alteration and replacement of primary monazite-(Ce), xenotime-(Y), fluorapatite, and minor (Y,REE,U,Fe)-(Nb,Ta) oxide phase(s), to be the consequence of coupled dissolution-reprecipitation processes. These processes mobilized REE, Th, U, and Nb-Ta, which caused the formation of secondary monazite-(Ce), xenotime-(Y), fluorapatite, and minor amounts of allanite-(Ce) and ferriallanite-(Ce). In addition, these alteration processes produced uraninite, thorite, columbite-(Fe), and uncharacterized (Th,U,Y,Ca)-silicates. Textural relations show that the dissolution-reprecipitation processes affecting fluorapatite preceded those affecting monazite-(Ce), xenotime-(Y), and the (Y,REE,U,Fe)-(Nb,Ta) oxide phase(s). The mineralogy of the primary ore mineralization and the subsequently formed alteration assemblages demonstrate the combined mobility of REE and HFSE in a natural F-bearing high-temperature hydrothermal system. The observed coprecipitation of monazite-(Ce), xenotime-(Y), and fluorapatite during the primary REE mineralization stage highlights the need for further research on the potentially important role of the phosphate ligand in hydrothermal REE transporting systems.

Episodic modes of operation in hydrothermal gold systems: Part II. A model for gold deposition

Abstract Coupling between the physical processes intrinsic to a hydrothermal system can lead to episodic and chaotic behaviour. Such behaviour includes variations in both space and time of the temperature, fluid pressure and activity of H 2 S, which result in the deposition of alteration mineral assemblages, zoned pyrite and gold; these variations are multifractal. In particular, the coupling of deformation and simultaneous endothermic and exothermic reactions with fluid flow leads to the highly localized deposition of gold. We discuss the physical and chemical mechanisms for such episodic and localization behaviour and explore the non-linear dynamic reasons why such mechanisms are recorded in the multifractal paragenetic sequence and deformation history. The synchronization of intrinsic episodicity as described here and extrinsic forcing induced by episodic seismicity provides another mechanism for enhancing the yield of gold deposition processes and hence the grade of orogenic gold deposits.

Basin‐scale thermal and fluid flow histories revealed by carbonate clumped isotopes (Δ47) – Middle Jurassic carbonates of the Paris Basin depocentre

AbstractThe reconstruction of past diagenetic conditions in sedimentary basins is often under‐constrained. This results from both the analytical challenge of performing the required analyses on the minute sample amounts available from diagenetic mineral phases and the lack of tracers for some of the diagenetic parameters. The carbonate clumped isotope thermometry (Δ47) opens new perspectives for unravelling the temperatures of diagenetic phases together with the source of their parent fluids, two parameters that are otherwise impossible to constrain in the absence of exploitable fluid inclusions. Here is reported the study of a large number of sedimentary and diagenetic carbonate phases (from Middle Jurassic reservoirs of the Paris Basin depocentre) by combining detailed petrographic observations with a large number of Δ47 data (n > 45) on a well‐documented paragenetic sequence, including calcite and dolomite burial cements. The data reveal carbonate crystallization at temperatures between 29°C and 98°C from fluids with δ18Owater values between −7‰ and +2‰, in response to the progressive burial and uplift of the Paris Basin, throughout 165 Myr of basin evolution. Coupled with the time–temperature evolution previously estimated from thermal maturity modelling, these temperatures allow determining the timing of four successive cementation episodes. The overall data set indicates a history of complex water mixing with a significant contribution of hypersaline waters from the Triassic aquifers migrated upward along faults during the Cretaceous subsidence of the basin. Subsequent large‐scale infiltrations of meteoric waters induced a dilution of these pre‐existing brines in response to the Paris Basin uplift in the Tertiary. Overall, the data presented here allow proposing an integrated approach to characterize the cementation events affecting the studied carbonate reservoir units, based on temperature, oxygen isotope composition and salinity of the parent fluids as well as on petrographic grounds.

Geology and Metallogenesis of the Sediment-Hosted Cu-Ag Deposit of Tizert (Igherm Inlier, Anti-Atlas Copperbelt, Morocco)

The Tizert Cu-Ag deposit is the largest of a series of sediment-hosted copper deposits of the Anti-Atlas copper province in Morocco. Mineralized rocks in the deposit contain disseminated sulphides within a Late Ediacaran, dominantly siliciclastic sedimentary formation named the Basal Series. Isopach map of the Basal Series thickness shows that during the Late Ediacaran the area was composed of large subsiding zones separated by paleohighs. The ore-grade zones are well developed along basin margins adjacent to the basement paleohighs. These mineralized zones display a lateral sulphide zoning with central bornite-chalcocite zones grading outward to intermediate chalcopyrite and external pyrite zones. There is also a vertical sulphide zoning with evolution from bornite and chalcocite dominant mineralized rocks at the bottom to chalcopyrite and pyrite dominant mineralized rocks at the top of the lithostratigraphic succession. A second style of mineralization is represented by sulphide filled fractures and veins present in the Basal Series, as well as in the basement and the overlying dolomites. The similarity of the paragenetic sequences between the disseminated and the vein-hosted mineralization suggests that they may be related to the same mineralizing event, the disseminated style of mineralization being rapidly followed by the onset of the vein-style mineralization.

Micro-scale sulfur isotope and chemical variations in sphalerite from the Bleiberg Pb-Zn deposit, Eastern Alps, Austria

The Bleiberg Pb-Zn deposit in the Drau Range is the type locality of Alpine-type carbonate-hosted Pb-Zn deposits. Its origin has been the subject of on-going controversy with two contrasting genetic models proposed: (1) the SEDEX model, with ore forming contemporaneously with sedimentation of the Triassic host rocks at about 220 Ma vs. (2) the epigenetic MVT model, with ores forming after host rock sedimentation at about 200 Ma or later. Both models assume that, on a deposit or even district scale, a fixed paragenetic sequence of ore minerals can be established. The results of our detailed petrographic, chemical and sulfur isotope study of two key ore-samples from two major ore horizons in the Wetterstein Formation at Bleiberg (EHK02 Erzkalk horizon and Blb17 Maxer Bänke horizon) demonstrate that there is no fixed paragenetic sequence of ore minerals. Small-scale non-systematic variations are recorded in textures, sphalerite chemistry and δ34S. In each sample, texturally different sphalerite types (colloform schalenblende, fine- and coarse-grained crystalline sphalerite) co-occur on a millimeter to centimeter scale. These sphalerites represent multiple mineralization stages/pulses since they differ in their trace element inventory and in their δ34S. Nonetheless, there is some correspondence of sphalerite micro-textures, sulfur isotope and chemical composition between the two samples, with microcrystalline colloform schalenblende being Fe-rich, having high Fe/Cd (15 and 9, respectively) and a light sulfur isotope composition (δ34S −26.0 to −16.2‰). Cadmium-rich and Fe-poor sphalerite in both samples has relatively heavier sulfur isotope composition: in sample EHK02 this sphalerite has Fe/Cd of ∼0.5 and δ34S from −6.6 to −4.6‰; in sample Blb17 Fe/Cd is ∼0.1 and δ34S ranges from −15.0 to −1.5‰. Barite, which is restricted to sample EHK02, has δ34S ≈ 17‰. The large variations in δ34S recorded on the mm to cm-scale is consistent with variable contributions of reduced sulfur from two different sulfur reservoirs. The dominant reservoir with δ34S values <−20‰ likely results from local bacteriogenic sulfate reduction (BSR), whereas the second reservoir, with δ34S about −5‰ suggests a hydrothermal source likely linked with thermochemical sulfate reduction (TSR). Based on this small- to micro-scale study, no simple, deposit-wide paragenetic and sulfur isotope evolution with time can be established. In the Erzkalk ore (sample EHK02) an earlier Pb-Zn-Ba stage, characterized by heavy sulfur isotope values, is succeeded by a light δ34S-dominated Zn-Pb-F stage. In contrast, the several mineralization pulses identified in the stratiform Zn-Pb-F Maxer Bänke ore (sample Blb17) define a broad trend to heavier sulfur isotope values with time. The interaction documented in these samples between two sulfur reservoirs is considered a key mechanism of ore formation.

Shengli Oilfield, China as a natural analogue of CO2 storage: Diagenetic fluid evolution mode and potentials for carbon sequestration

The Shengli Oilfield is discussed to investigate CO2 trapping mechanisms involved in geothermal oil reservoir systems. Diagenetic fluid evolution of the dawsonite‐bearing sandstones from the Dongying Sag, Bohai Bay Basin, China, was studied to understand the CO2 migration and accumulation and the formation of dawsonite, as a possible mechanism for mineral trapping of CO2 in geothermal fields. Information about the petrology, dawsonite‐forming fluid, authigenic mineral associations, and diagenetic fluid evolution mode was analysed using techniques including thin‐section identification, scanning electron microscope observation, X‐ray diffraction analysis, X‐ray fluorescence spectrum analysis, hydrochemistry, and stable isotope analyses. The paragenetic sequence, from early to late, is early calcite (A)–compaction–dissolution–quartz overgrowth–late calcite (B1)–ferrocalcite (A)–dawsonite–late calcite (B2)–ferrocalcite (B)–ankerite–siderite or pyrite. The current in situ formation water type of the dawsonite‐bearing sandstone strata is NaHCO3, with the typical characteristics of weak alkaline, high mineralization, and high Na+ + K+ and HCO3− content. The formation fluid from which the dawsonite forms is mainly sealed underground brine, which has been affected by atmospheric precipitation and magmatic activity, and is thus mixed with surface meteoric water and subsurface hydrothermal fluid. The fluid evolution process sequence is alkaline fluid environment prior to CO2 influx, an early micritic or sparry calcite (A) association; acidic fluid environment after CO2 influx, a quartz overgrowth and kaolinite association; alkalescent fluid environment, an association of late calcite (B1), ferrocalcite (A), and illite; alkaline fluid environment, a dawsonite and late calcite (B2) association; and weak‐alkaline fluid environment, an association of ferrocalcite (B), ankerite, siderite, and pyrite.

Geology and ore genesis of the late Paleozoic Heijianshan Fe oxide–Cu (–Au) deposit in the Eastern Tianshan, NW China

The Heijianshan Fe–Cu (–Au) deposit, located in the Aqishan-Yamansu belt of the Eastern Tianshan (NW China), is hosted in the mafic–intermediate volcanic and mafic–felsic volcaniclastic rocks of the Upper Carboniferous Matoutan Formation. Based on the pervasive alteration, mineral assemblages and crosscutting relationships of veins, six magmatic–hydrothermal stages have been established, including epidote alteration (Stage I), magnetite mineralization (Stage II), pyrite alteration (Stage III), Cu (–Au) mineralization (Stage IV), late veins (Stage V) and supergene alteration (Stage VI). The Stage I epidote–calcite–tourmaline–sericite alteration assemblage indicates a pre-mineralization Ca–Mg alteration event. Stage II Fe and Stage IV Cu (–Au) mineralization stages at Heijianshan can be clearly distinguished from alteration, mineral assemblages, and nature and sources of ore-forming fluids.Homogenization temperatures of primary fluid inclusions in quartz and calcite from Stage I (189–370 °C), II (301–536 °C), III (119–262 °C) and V (46–198 °C) suggest that fluid incursion and mixing probably occurred during Stage I to II and Stage V, respectively. The Stage II magmatic–hydrothermal-derived Fe mineralization fluids were characterized by high temperature (>300 °C), medium–high salinity (21.2–56.0 wt% NaCl equiv.) and being Na–Ca–Mg–Fe-dominated. These fluids were overprinted by the external low temperature (<300 °C), medium–high salinity (19.0–34.7 wt% NaCl equiv.) and Ca–Mg-dominated basinal brines that were responsible for the subsequent pyrite alteration and Cu (–Au) mineralization, as supported by quartz CL images and H–O isotopes. Furthermore, in-situ sulfur isotopes also indicate that the sulfur sources vary in different stages, viz., Stage II (magmatic–hydrothermal), III (basinal brine-related) and IV (magmatic–hydrothermal). Stage II disseminated pyrite has δ34Sfluid values of 1.7–4.3‰, comparable with sulfur from magmatic reservoirs. δ34Sfluid values (24.3–29.3‰) of Stage III Type A pyrite (coexists with hematite) probably indicate external basinal brine involvement, consistent with the analytical results of fluid inclusions. With the basinal brines further interacting with volcanic/volcaniclastic rocks of the Carboniferous Matoutan Formation, Stage III Type B pyrite–chalcopyrite–pyrrhotite assemblage (with low δ34Sfluid values of 4.6–10.0‰) may have formed at low fO2 and temperature (119–262 °C). The continuous basinal brine–volcanic/volcaniclastic rock interactions during the basin inversion (∼325–300 Ma) may have leached sulfur and copper from the rocks, yielding magmatic-like δ34Sfluid values (1.5–4.1‰). Such fluids may have altered pyrite and precipitated chalcopyrite with minor Au in Stage IV. Eventually, the Stage V low temperature (∼160 °C) and low salinity meteoric water may have percolated into the ore-forming fluid system and formed late-hydrothermal veins.The similar alteration and mineralization paragenetic sequences, ore-forming fluid sources and evolution, and tectonic settings of the Heijianshan deposit to the Mesozoic Central Andean IOCG deposits indicate that the former is probably the first identified Paleozoic IOCG-like deposit in the Central Asian Orogenic Belt.

Age and origin of uranium mineralization in the Camie River deposit (Otish Basin, Québec, Canada)

The Camie River uranium deposit is located in the southeastern part of the Paleoproterozoic Otish Basin (Québec). The uranium mineralization consists of disseminated and vein uraninite and brannerite precipitated close to the unconformity between Paleoproterozoic fluviatile, pervasively altered, sandstones and conglomerates of the Matoush Formation and the underlying sulfide-bearing graphitic schists of the Archean Hippocampe greenstone belt. Diagenetic orange/pink feldspathic alteration of the Matoush Formation consists of authigenic albite cement partly replaced by later orthoclase cement, with the Na2O content of clastic rocks increasing with depth. Basin-wide green muscovite alteration affected both the Matoush Formation and the top of the basement Tichegami Group. Uraninite with minor brannerite is mainly hosted by subvertical reverse faults in basement graphitic metapelites±sulfides and overlying sandstones and conglomerates. Uranium mineralization is associated with chlorite veins and alteration with temperatures near 320°C, that are paragenetically late relative to the diagenetic feldspathic and muscovite alterations. Re-Os geochronology of molybdenite intergrown with uraninite yields an age of 1724.0±4.9Ma, whereas uraninite yields an identical, although slightly discordant, 1724±29Ma SIMS U-Pb age. Uraninite has high concentrations in REE with flat REE spectra resembling those of uraninite formed from metamorphic fluids, rather than the bell-shaped patterns typical of unconformity-related uraninite. Paragenesis and geochronology therefore show that the uranium mineralization formed approximately 440 million years after intrusion of the Otish Gabbro dykes and sills at ∼2176Ma, which constrains the minimum age for the sedimentary host rocks. The post-diagenetic stage of uraninite after feldspathic and muscovite alterations, the paragenetic sequence and the brannerite-uraninite assemblage, the relatively high temperature for the mineralizing event (∼320°C) following the diagenetic Na- and K-dominated alteration, lack of evidence for brines typical of unconformity-related U deposits, the older age of the Otish Basin compared to worldwide basins hosting unconformity-related uranium deposits, the large age difference between basin fill and mineralization, the older age of the uranium oxide compared to ages for worldwide unconformity-related U deposits, and the flat REE spectra of uraninite do not support the previous interpretation that the Camie River deposit is an unconformity-associated uranium deposit. Rather, the evidence is more consistent with a PaleoProterozoic, higher-temperature hydrothermal event at 1724Ma, whose origin remains speculative.

Diagenetic evolution and its implication for reservoir quality: a case study from the Eocene Es4 interval, Dongying Depression, Bohai Bay Basin, China

Petrographic analysis combined with thin section observation, fluid inclusions, burial history analysis, and core porosity and permeability, were used to determine diagenetic evolution and its effect on reservoir-quality of beach-bar sandstone reservoirs of Eocene Es4s interval in the Boxing sag, Bohai Bay Basin. Diagenesis is mainly characterized by strong compaction, undercompaction, cementation of carbonate, quartz, pyrite and clay minerals, dissolution of detrital grains and carbonate cements, lime-mud matrix recrystallization and hydrocarbon filling. Paragenetic sequence is determined as “early compaction / precipitation of calcite, dolomite and framboidal pyrite → dissolution of feldspar grains and early-formed carbonate cements/precipitation of quartz → hydrocarbon filling → precipitation of ferroan calcite and ankerite/lime-mud matrix recrystallization → dissolution of ferroan carbonate cements”. In the same diagenetic environment, diagenetic processes for different lithofacies significantly varied and subsequently resulted in great heterogeneity for reservoir quality during progressive burial. Lithofacies for both bar body and beach edge subfacies which are far from sandstone-mudstone contacts, are well sorted and have a strong ability to resist compaction. Abundant primary and secondary porosities are well developed because of modifications by acidic dissolution and hydrocarbon filling. The present physical properties are relatively well and numerous oil beds are developed through shallow to deep burial depth. Lithofacies for both bar body and beach edge subfacies which are adjacent to sandstone-mudstone contacts, are typically characterized by extensive cementation of calcite and dolomite. The present physical properties are bad and dry beds are extensively developed for sandstone reservoirs in both bar body and beach edge subfacies which are adjacent to sandstone-mudstone contacts. Lithofacies for both bar edge and beach mat subfacies were deposited in weak hydrodynamic conditions with high contents of limemud matrix. The reservoir porosity decreases continually due to strong compaction and weak dissolution. The present physical properties are extremely poor and dry beds are commonly developed through shallow to deep burial except for minor oil beds developed in the shallow burial.

Hydrothermal nontronite formation associated with microbes from low‐temperature diffuse hydrothermal vents at the South Mid‐Atlantic Ridge

AbstractOceanic nontronite deposits have been identified to be closely related to low‐temperature hydrothermal activities. However, their formation mechanisms associated with microbes in diffuse hydrothermal vents still remain largely unknown. The friable deposits, collected from the low‐temperature diffuse flow at the Southern Atlantic Ridge, display a layered structure. Scanning electron microscope and transmission electron microscope analyses reveal that abundant filamentous, spherical, and rod‐shaped mineralized forms are preserved in the yellowish‐green layer of the deposits. These mineralized forms primarily consist of Si and Fe. Selected area electron diffraction patterns of the mineralized forms indicate that they are composed of nontronite. High intensities of 12C and 12C14N signals derived from cellular structures determined by nanosecondary ion mass spectrometry suggest the intimate relationship between nontronite and microbes. The results of 454 pyrosequencing analyses provide insights into the microbial communities involved in the biologically induced mineralization in the yellowish‐green layer. We propose an evolutionary model for establishing paragenetic sequences among nontronite, Mn oxide, and Fe oxyhydroxide in the deposits. This paragenetic sequence could be widespread in modern and ancient low‐temperature hydrothermal fields.

Diagenetic evolution of a shallow marine Kimmeridgian carbonate ramp (Jabaloyas, NE Spain): implications for hydrocarbon reservoir quality

The upper Kimmeridgian outcrops in Jabaloyas (NE Spain) studied here have significant similarity in terms of carbonate ramp morphology and its facies architecture with Member D of the Upper Jurassic Arab Formation reservoirs in the Middle East. Geological models from analogue outcrops enhance or challenge the understanding of multi-scale sedimentological and diagenetic heterogeneities in the subsurface hydrocarbon reservoir models by providing morphometric parameters. For this purpose, conventional diagenetic studies (petrographic microscopy analysis and cathodoluminescence) have been performed to assess the impact of diagenetic modifications through burial. More than 50 thin sections have been used to define a full paragenetic sequence. Additional mineralogical mapping (Qemscan®) on a selection of thin sections serves as a calibration tool for semiquantitative analytic studies of every mineral phases and the proposed porosity evolution. This work unravels the whole paragenetic diagenetic processes and focused on those that occurred right after deposition to shallow burial (eogenesis). High-sequence stratigraphy allows linking these events to variations of the environmental domains that, in fact, respond most likely to sea-level fluctuations. In particular, it provides key parameters to frame the diagenetic overprint such as cementation processes plugging primary interparticle porosity and selective dissolution linked to master bounding surfaces: calcite meniscus cements in vadose zones and selective dissolution processes linked to firmgrounds. Ultimately, they offer valuable trends that would be potentially relevant to decipher and characterize reservoir parameters for the upscaling workflow of fabrics from microscopic to inter-well scale in carbonate hydrocarbon reservoirs.

U-Pb zircon and 40Ar/39Ar geochronology of sericite from hydrothermal alteration zones: new constraints for the timing of Ediacaran gold mineralization in the Sukhaybarat area, western Afif terrane, Saudi Arabia

The Sukhaybarat East and Red Hill deposits, in the northeastern part of the Arabian Shield, are mesothermal vein-type gold deposits hosted by late Cryogenian-Ediacaran intrusive rocks of the Idah suites (diorite, tonalite, granodiorite) and, at Sukhaybarat East, also by Ediacaran metasedimentary rocks. Gold mineralization comprises quartz-arsenopyrite veins (Sukhaybarat East), quartz-carbonate-pyrite veins (Red Hill), and subordinate gold-base metal sulfide veins. In the Red Hill deposit, alteration is complicated due to multiple overprinting hydrothermal events and is characteristically affected by pervasive, pink quartz-K-feldspar-hematite alteration which is overprinted by potassic alteration characterized by a quartz-biotite-carbonate-muscovite/sericite-rutile-apatite assemblage. This assemblage is associated with molybdenite veins which appear to form late in the paragenetic sequence and may represent either evolution of the ore fluid composition, or a later, unrelated mineralized fluids. Hydrothermal alteration at the Sukhaybarat East deposit is dominated by quartz-carbonate-sericite-arsenopyrite assemblages. Zircon from ore-hosting tonalite at Sukhaybarat East yields a U-Pb age of 629 ± 6 Ma, and biotite from the same rock gives an 40Ar/39Ar age of 622 ± 23 Ma. The 40Ar/39Ar age is within the uncertainty range for the U-Pb age of the host intrusion and is interpreted as a minimally disturbed cooling age for the tonalite. In the Red Hill area, granodiorite was emplaced at 615 ± 5 Ma, whereas muscovite/sericite separated from a mineralized sample of a quartz-carbonate-pyrite vein, that was overprinted by molybdenite-bearing veinlets, yields an 40Ar/39Ar age of 597 ± 8 Ma. We interpreted this age to represent the maximum age of the molybdenite mineralization and the probable minimum age of gold mineralization in the Red Hill deposit.

Geochemistry of magnetite from theHuanggangskarn iron–tin polymetallic deposit in the southernGreat Xing'an Range, NE China

The Huanggang iron–tin polymetallic skarn deposit is located in the southern Great Xing'an Range. According to the ore types and mineral assemblages, the paragenetic sequence of the Huanggang deposit can be divided into three stages, and these magnetite grains were mainly formed at the retrograde and sulphide skarn stages. The magnetite (sample HG12‐13) from the early retrograde stage is represented by fine‐grained magnetite cutting across coarse‐grained magnetite surrounded by quartz and calcite. The magnetite (sample HG12‐73) from the late retrograde stage is locally replaced by haematite along the margin or interior and is surrounded by calcite. The magnetite (sample HG12‐88) from the early sulphide stage is characterized by obvious core–rim textural features. The magnetite (sample HG‐62) from the late sulphide stage is featured by zone‐like magnetite occurring along the margin and interior of the primary magnetite.Laser ablation inductively coupled plasma mass spectrometry was used to obtain trace element concentrations of magnetite from the different mineralization stages in order to better understand the geochemical variations in the ore‐forming process. Some magnetite grains have abnormally high Mg, Al, K, Cu, Zn, and Sn due to the presence of numerous inclusions (e.g., chlorite, sylvite, chalcopyrite, sphalerite, and cassiterite). In general, magnetite grains from the different mineralization stages demonstrate similar bulk continental crust normalized trace elements patterns, suggesting that they share a similar origin. The increasing Si + Al/Mg + Mn ratios and decreasing Mg + Mn contents for magnetite show an increasing fluid–rock ratio from the retrograde to sulphide stage. Co contents of magnetite decrease abruptly from the retrograde stage to the sulphide stage, whereas Mn contents show the reverse trend, which is affected by minerals coprecipitating with magnetite. A zoned magnetite from the sulphide stage shows decreasing Ti and V contents from core to outer rim; the variation of Ti and V may be related to the temperature or oxygen fugacity. Magnetite compositions from the Huanggang deposit are similar to those from Fe, Cu‐polymetallic, or other skarn deposits, but display more variable compositions than previously presented. Our study demonstrates that the evolution of magnetite in the skarn deposit and trace element of magnetite could be a powerful tool in determining the origin of skarn iron deposit.

Diagenetic apatite character and in situ ion microprobe U–Pb age, Keeseville Formation, Potsdam Group, New York State

An occurrence of diagenetic fluorapatite cement is documented within the Cambro-Ordovician Keeseville Formation, Potsdam Group, near Chateaugay in New York State. The fluorapatite cement occurs as stratiform layers within ephemeral fluvial quartz arenites, which have been reworked by aeolian processes prior to burial. The paragenetic sequence includes the following: compaction of dust-rimmed grains → quartz cementation → minor kaolinite → fluorapatite cementation followed by secondary dissolution porosity and telogenetic hematite cementation. Mesogenetic illitization of kaolinite may have taken place prior to or following fluorapatite cementation. The fluorapatite occurs as elongated bladed crystals that characteristically contain ladder-like, inclusion-rich cores running parallel to crystal length, surrounded by clearer rims, and larger blocky crystals towards the middle of interstices. In situ SHRIMP analyses of blocky fluorapatite crystals yield a U–Pb age of 486 ± 29 Ma, indicating that the cement formed during mesogenetic burial processes and (or) during fluid flow driven by Taconic orogenic events. There is no obvious source of phosphorous for the fluorapatite cement within the Potsdam Group, but phosphorous-rich lithologies are known from the adjacent basement of the Adirondack Dome. Phosphorous-rich fluids may have been derived from these basement lithologies. The occurrence of the rare fluorapatite cement in the Keeseville Formation adjacent to the Chateaugay Lake Fault raises the possibility that alkaline phosphatic fluids were focused within the fault and migrated laterally away from the fault into the host Keeseville Formation to form stratiform fluorapatite cement in the sandstone.

Uranium-lead dating of hydrothermal zircon and monazite from the Sin Quyen Fe-Cu-REE-Au-(U) deposit, northwestern Vietnam

The Sin Quyen deposit in northwestern Vietnam contains economic concentrations of Cu, Au and LREE, and sub-economic concentration of U. In this deposit, massive and banded replacement ores are hosted in Neoproterozoic metapelite. The paragenetic sequence includes sodic alteration (stage I), calcic-potassic alteration and associated Fe-REE-(U) mineralization (stage II), Cu-Au mineralization (stage III), and sulfide-(quartz-carbonate) veins (stage IV). The Sin Quyen deposit experienced an extensive post-ore metamorphic overprint, which makes it difficult to precisely determine the mineralization age. In this study, zircon and monazite U-Pb geochronometers and the Rb-Sr isochron method are used to constrain the timing of mineralization. Zircon grains in the ore are closely intergrown or texturally associated with hydrothermal minerals of stage II (e.g., garnet, allanite, and hedenbergite). They may contain primary fluid inclusions and display irregular zoning in cathodoluminescence (CL) images. Zircon grains are rich in U (688 to 2902 ppm) and poor in Th (0.2 to 2.9 ppm). Their δ18OV-SMOW values range from 11.9 to 14.0‰, higher than those of typical magmatic zircon. These textural and compositional features imply that zircon precipitated from 18O- and U-rich hydrothermal fluids, coeval with the minerals of stage II. Monazite occurs in close association with stage II magnetite and allanite and has low contents of Th (<2700 ppm), indicative of a hydrothermal origin. Hydrothermal zircon and monazite have indistinguishable U-Pb ages of 841 ± 12 and 836 ± 18 Ma, respectively, representing the timing of Fe-REE mineralization. There is no direct isotopic constraint on the timing of the Cu-Au mineralization, but geological observations suggest that the Cu-Au and Fe-REE ores most likely formed within a single evolved hydrothermal process. In the plot of 87Rb/86Sr vs. 87Sr/86Sr, the composition of bulk-ore and biotite separates from ore lie along a reference line for 30 Ma, which is consistent with the timing of metamorphism in the region. The mineralization age of the Sin Quyen deposit falls within the overall age range (740 to 860 Ma) of the regional Neoproterozoic igneous rocks. This temporal linkage, in combination with the magmatic-like sulfur isotopes of sulfide minerals (δ34SV-CDT = −0.8 to 3.1), indicates that the mineralization may have a close genetic association with the Neoproterozoic igneous activity.

Structural, mechanical and vibrational study of uranyl silicate mineral soddyite by DFT calculations

Uranyl silicate mineral soddyite, (UO2)2(SiO4)·2(H2O), is a fundamental component of the paragenetic sequence of secondary phases that arises from the weathering of uraninite ore deposits and corrosion of spent nuclear fuel. In this work, soddyite was studied by first principle calculations based on the density functional theory. As far as we know, this is the first time that soddyite structure is determined theoretically. The computed structure of soddyite reproduces the one determined experimentally by X-Ray diffraction (orthorhombic symmetry, spatial group Fddd O2; lattice parameters a = 8.334Å, b = 11.212Å; c = 18.668Å). Lattice parameters, bond lengths, bond angles and X-Ray powder pattern were found to be in very good agreement with their experimental counterparts. Furthermore, the mechanical properties were obtained and the satisfaction of the Born conditions for mechanical stability of the structure was demonstrated by means of calculations of the elasticity tensor. The equation of state of soddyite was obtained by fitting lattice volumes and pressures to a fourth order Birch-Murnahan equation of state. The Raman spectrum was also computed by means of density functional perturbation theory and compared with the experimental spectrum obtained from a natural soddyite sample. The results were also found in agreement with the experimental data. A normal mode analysis of the theoretical spectra was carried out and used in order to assign the main bands of the Raman spectrum.

Very early diagenesis in a calcareous, organic-rich mudrock from Jordan

The diagenesis in the organic-rich Cretaceous to Eocene Al Hisa Phosphorite Formation (AHP), Muwaqqar Chalk Marl Formation (MCM) and Umm Rijam Chert-Limestone Formation (URC) formations of Jordan can be linked directly to the fluctuating sedimentary environment of this shelf depositional system in the Middle to Late Eocene, and its influence on the composition of the deposited sediment and the early burial diagenetic environment. Most cementation was early, mostly within the first 10 m of burial, perhaps entirely within the first 100 m of burial. We propose that the siliceous cements are derived from biogenic silica, probably of diatoms, deposited in a shelf of enhanced productivity. Volumetrically, the most important processes were the redistribution of biogenic opal-A (diatoms) and calcite to form pervasive, layered and nodular cements. The formation of the silica and carbonate cements is closely linked through the effects their dissolution and precipitation have on pore fluid chemistry and pH. The chert beds have a biogenic silica origin, formed through replacement of diatoms and radiolaria by opal-CT, and subsequently by quartz. Calcite cement has carbonate derived from microbial diagenesis of organic matter and calcium derived from seawater. The Mg for early dolomite may have been generated by replacement of opal-CT by quartz, ore dissolution of unstable high Mg calcite bioclasts. The silica and carbonate diagenetic processes are both linked to microbial diagenesis of organic matter, and are intimately linked in both time and space, with pH possibly influencing whether a silica or a carbonate mineral precipitates. The paucity of metal cations capable of precipitating as sulphides is crucial to the creation of acidic pore water favourable to silica precipitation, either as opal-CT, chalcedony or quartz. The lack of clay minerals as a sink for the Mg required for opal-CT polymerisation is the principal factor responsible for the remarkably early silica cementation. All the diagenetic processes, with the probable exception of the opal-CT to quartz transition are early, almost certainly within the first 10 m of burial, possibly much less. A paragenetic sequence is presented here based on these two cores that should be tested against a wider core distribution to see whether this diagenetic history can be generalised throughout the basin. Warm bottom water temperatures probably led to silica diagenesis at much shallower burial depths than occurs in many other sedimentary basins. Silicified layers, in turn, commonly host fractures, suggesting that mechanical properties of the strata began to differentiate at a very early stage in the burial cycle. This has wide implications for processes linking diagenesis to deformation.

Multiple episodes of mineralization revealed by Re-Os molybdenite geochronology in the Lala Fe-Cu deposit, SW China

The Lala Fe-Cu deposit is one of the largest iron oxide-copper-gold (IOCG) deposits in the Kangdian copper belt, southwest China. The paragenetic sequence of the Lala deposit includes six hydrothermal stages: pre-ore pervasive Na alteration (I); magnetite stage with K-feldspar and apatite (II); polymetallic disseminated/massive magnetite-sulfide stage (III); banded magnetite-sulfide stage (IV); sulfide vein stage (V); and late quartz-carbonate vein stage (VI). Fifteen molybdenite separates from stages III to VI were analyzed for Re-Os dating. Our new Re-Os data, together with previous studies, identify four distinct hydrothermal events at the Lala deposit. Molybdenite from the stage III disseminated to massive chalcopyrite-magnetite ores yielded a weighted average Re-Os age of 1306 ± 8 Ma (MSWD = 1.1, n = 6) which represents the timing of main ore formation. Molybdenite from the stage IV-banded magnetite-chalcopyrite ores yielded a weighted average Re-Os age of 1086 ± 8 Ma (MSWD = 2.2, n = 7), i.e., a second ore-forming event. Molybdenite from the stage V sulfide veins yielded a weighted average Re-Os age of 988 ± 8 Ma (MSWD = 1.3, n = 7) which represents the timing of a third hydrothermal event. Molybdenite from the quartz-carbonate veins (stage VI) yielded a weighted average Re-Os age at 835 ± 4 Ma (MSWD = 0.66, n = 10) and documented the timing of a late hydrothermal event. Our results indicate that the Lala deposit formed during multiple, protracted mineralization events over several hundred million years. The first three Mesoproterozoic mineralization events are coeval with intra-continental rifting (breakup of the supercontinent Nuna) and share a temporal link to other IOCG-style deposits within the Kangdian Copper Belt, and the last Neoproterozoic hydrothermal event is coeval with the Sibao orogeny which culminated with the amalgamation of the Yangtze Block with the Cathaysia Block at 860–815 Ma.

The Tanco Pegmatite At Bernic Lake, Manitoba. XVII. Mineralogy and Geochemistry of Alkali Feldspars

Abstract The alkali-feldspar paragenetic sequence in the Tanco pegmatite spans the entire crystallization and cooling history of the pegmatite and includes several different K-feldspar and albite varieties. Total feldspar modal abundance approaches 50% for the entire pegmatite volume. Blocky K-feldspar, aplitic albite, and albite of the cleavelandite habit together account for most of the total feldspar volume, whereas late, low-temperature feldspars in veins and cavities are mostly contained in inner zones associated with pollucite and petalite. In an inward progression through the concentrically zoned pegmatite, average bulk compositional data for blocky K-feldspar reveal steady increases in Rb 2 O (1.45–3.08 wt.%) and Cs 2 O (0.11–0.29 wt.%), increasing followed by decreasing contents of P 2 O 5 (0.34–0.47 wt.%), Li 2 O (0.014–0.095 wt.%), and Tl (105–239 ppm), decreasing Na 2 O (2.5–1.1 wt.%) and CaO (0.13–0.08 wt.%), and variable BaO (0.004–0.17 wt.%) and Ga 2 O 3 (0.006–0.010 wt.%). In aplitic albite and albite of the cleavelandite habit, CaO decreases (0.08–0.03 wt.%) and overall K 2 O and P 2 O 5 contents are variable. Late adularia feldspars from veins and clusters in pollucite exhibit extreme ranges in contents of Rb 2 O (1.80–24.84 wt.%) and Cs 2 O (0.10–1.43 wt.%) and significant P 2 O 5 (up to 0.51 wt.%). X-ray powder-diffraction data indicate near-maximum structural order of the early feldspar varieties as maximum microcline and high albite. Hydrothermal adularia crystals are essentially endmember (Or 100 ) high sanidine. The large and economically significant Tanco pegmatite represents an extreme in terms of petrologic processes involving fractional and disequilibrium crystallization from granitic melts enriched in volatiles and fluxing components (H 2 O, B, F, P), rare alkalis (Li, Rb, Cs), and high field strength elements (Ta, Nb, Zr, Hf). The alkali feldspars are the most significant carriers of P, Li, Rb, Tl, and Cs prior to the stabilization of large volumes of amblygonite–montebrasite, petalite, lepidolite, and pollucite. The rise and fall of trace-element content in feldspars, in relation to the spatial distribution of different zones and mineral assemblages, has implications in terms of the internal evolution of complex granitic pegmatites, as well as on the types and extents of subsolidus readjustments.

Processes and ore genesis at the Yaochong Mo deposit, Henan Province, China

The Yaochong Mo deposit in Henan Province is located in the Dabie orogen, and its orebodies primarily occur as numerous veinlets hosted in the contact zone of the Yaochong stock with the Dabie Complex. Based on vein crosscutting relationships, mineral assemblages, and paragenetic sequence, the mineralization is divided into four stages: a quartz–K-feldspar–magnetite–hematite stage, a quartz–K-feldspar–molybdenite stage, a quartz–polymetallic sulfide mineral stage, and a quartz–carbonate stage. Three types of fluid inclusions have been recognized in the quartz: CO2-bearing fluid inclusions, H2O-rich fluid inclusions, and solid-bearing fluid inclusions. Microthermometric analyses of the fluid inclusions suggest that the fluids evolved gradually during the ore-forming process. In stages 1, 2, 3, and 4, the homogenization temperatures varied in the ranges 264–400°C, 175–487°C, 171–433°C, and 119–172°C, respectively, and the salinities varied in the ranges 4.34–7.70wt% NaClequiv, 2.77–32.49wt% NaClequiv, 2.20–30.70wt% NaClequiv, and 1.06–7.73wt% NaClequiv, respectively. The calculated δ18OH2O values of stages 1, 2, 3, and 4 ranged from 2.2‰ to 3.9‰, 0.9‰ to 1.7‰, −0.8‰ to −0.1‰, and −1.3‰ to 0.6‰, respectively, and the δ18D values varied from −68‰ to −62‰, −73‰ to −64‰, −75‰ to −65‰, and −68‰ to −65‰, respectively. These results show that the ore-forming fluids of the Yaochong molybdenum deposit were mainly derived from magmatic water, which likely included a small component of meteoric water. The δ34S values of pyrite and molybdenite varied from −2.9‰ to −1.8‰, and −2.7‰ to −1.6‰, respectively, and the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the sulfide minerals ranged from 16.193 to 16.818, 15.105 to 15.288, and 36.698 to 37.307, respectively. The sulfur and lead isotope compositions indicated that the ore-forming constituents were derived from a deep igneous source originating in the lower crust. The Yaochong porphyry Mo deposit may have formed in a post-collision extensional setting. The Mo mineralization in the Yaochong deposit occurred at depths of 0.87–1.68km, and phase separation and immiscibility of the ore-forming fluids were critical for the formation of the deposit.