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Economic Geology
Number of Followers: 8  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 0361-0128 - ISSN (Online) 1554-0774
Published by Society of Economic Geologists Homepage  [1 journal]
  • REVIEWERS OF BOOKS

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      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • REVIEWERS OF MANUSCRIPTS

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  • Formation of Gold Deposits (Neil Phillips)

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  • INSTRUCTIONS TO AUTHORS, 2023

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  • INTERESTING PAPERS IN OTHER JOURNALS

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  • A RECENTLY DISCOVERED TRACHYTE-HOSTED RARE EARTH ELEMENT-NIOBIUM-ZIRCONIUM
           OCCURRENCE IN NORTHERN MAINE, USA

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      Abstract: AbstractReported here are geological, geophysical, mineralogical, and geochemical data on a previously unknown trachyte-hosted rare earth element (REE)-Nb-Zr occurrence at Pennington Mountain in northern Maine, USA. This occurrence was newly discovered by a regional multiparameter, airborne radiometric survey that revealed anomalously high equivalent Th (eTh) and U (eU), confirmed by a detailed ground radiometric survey and by portable X-Ray fluorescence (pXRF) and whole-rock analyses of representative rock samples. The mineralized area occurs within an elongate trachyte body (~1.2 km2) that intrudes Ordovician volcanic rocks. Geologic constraints suggest that the trachyte is also Ordovician in age. The eastern lobe (~900 × ~400 m) of the trachyte is pervasively brecciated with a matrix containing seams, lenses, and veinlets composed mainly of potassium feldspar, albite, and fine-grained zircon and monazite. Barite is locally abundant. Minor minerals within the matrix include columbite, bastnäsite, euxenite, chlorite, pyrite, sphalerite, and magnetite. The pXRF analyses of 22 samples (App. Table A1) collected from the eastern lobe demonstrate that this entire part of the trachyte is highly mineralized. Whole-rock geochemical analyses for samples from the eastern lobe document high average contents of Zr (1.17 wt %), Nb (1,656 ppm), Ba (3,132 ppm), Y (1,140 ppm), Hf (324 ppm), Ta (122 ppm), Th (124 ppm), U (36.5 ppm), Zn (689 ppm), and Sn (106 ppm). Among light REE, the highest average concentrations are shown by La (763 ppm) and Ce (1,479 ppm). For heavy REE (HREE), Dy and Er are the most abundant on average (167 and 114 ppm, respectively). No HREE-rich minerals such as xenotime have been identified; the HREE may reside chiefly in monazite and bastnäsite, and within the fine-grained zircon. Very strong positive correlations (R2) of 0.92 to 0.98 exist between Th and Zr, Nb, Y, Ce, Yb, and Sn, indicating that the radiometric data for eTh are valid proxies for concentrations of these metals in the mineralized rocks.Trachyte-hosted REE-Nb-Zr deposits like the occurrence at Pennington Mountain also are known in eastern Australia and in the south Qinling belt of Central China. Based on comparisons with these deposits, and the lack of detailed geologic mapping in the Pennington Mountain region, we suggest that other rare-metal occurrences contained in trachyte may exist elsewhere in northern Maine, and more widely in the Appalachian-Caledonian orogen where volcanic and subvolcanic trachytes have been recognized.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • Timing of Rhyolite Intrusion and Carlin-Type Gold Mineralization at the
           Cortez Hills Carlin-Type Deposit, Nevada, USA

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      Abstract: AbstractCarlin-type gold deposits (CTDs) of Nevada are the largest producers of gold in the United States, a leader in world gold production. Although much has been resolved about the characteristics and origin of CTDs in Nevada, major questions remain, especially about (1) the role of magmatism, whether only a source of heat or also metals, (2) whether CTDs only formed in the Eocene, and (3) whether pre-Eocene metal concentrations contributed to Eocene deposits. These issues are exemplified by the CTDs of the Cortez region, the second largest concentration of these deposits after the Carlin trend.Carlin-type deposits are notoriously difficult to date because they rarely generate dateable minerals. An age can be inferred from crosscutting relationships with dated dikes and other intrusions, which we have done for the giant Cortez Hills CTD. What we term “Cortez rhyolites” consist of two petrographic-geochemical groups of siliceous dikes: (1) quartz-sanidine-plagioclase-biotite-phyric, high-SiO2 rhyolites emplaced at 35.7 Ma based on numerous 40Ar/39Ar dates and (2) plagioclase-biotite-quartz ± hornblende-phyric, low-SiO2 rhyolites, which probably were emplaced at the same time but possibly as early as ~36.2 Ma. The dikes form a NNW-trending belt that is ~6 to 10 km wide × 40 km long and centered on the Cortez Hills deposit, and they require an underlying felsic pluton that fed the dikes. Whether these dikes pre- or postdated mineralization has been long debated. We show that dike emplacement spanned the time of mineralization. Many of both high- and low-SiO2 dikes are altered and mineralized, although none constitute ore. In altered-mineralized dikes, plagioclase has been replaced by kaolinite and calcite, and biotite by smectite, calcite, and marcasite. Sanidine is unaltered except in a few samples that are completely altered to quartz and kaolinite. Sulfides present in mineralized dikes are marcasite, pyrite, arsenopyrite, and As-Sb–bearing pyrite. Mineralized dikes are moderately enriched in characteristic Carlin-type elements (Au, Hg, Sb, Tl, As, and S), as well as elements found in some CTDs (Ag, Bi, Cu, Mo), and variably depleted in MgO, CaO, Na2O, K2O, MnO, Rb, Sr, and Ba. In contrast, some high-SiO2 rhyolites are unaltered and cut high-grade ore, which shows that they are post-ore. Both mineralized and post-ore dikes have indistinguishable sanidine 40Ar/39Ar dates. These characteristics, along with published interpretations that other giant CTDs formed in a few tens of thousands of years, indicate the Cortez Hills CTD formed at 35.7 Ma. All Cortez-area CTDs are in or adjacent to the Cortez rhyolite dike swarm, which suggests that the felsic pluton that fed the dikes was the hydrothermal heat source. Minor differences in alteration and geochemistry between dikes and typical Paleozoic sedimentary rock-hosted ore probably reflect low permeability and low reactivity of the predominantly quartzofeldspathic dikes.Despite widespread pre-35.7 Ma mineralization in the Cortez region, including deposits near several CTDs, we find no evidence that older deposits or Paleozoic basinal rocks contributed metals to Cortez-area CTDs. Combining our new information about the age of Cortez Hills with published and our dates on other CTDs demonstrates that CTD formation coincided with the southwestern migration of magmatism across Nevada, supporting a genetic relationship to Eocene magmatism. CTDs are best developed where deep-seated (~6–8 km), probably granitic plutons, expressed in deposits only as dikes, established large, convective hydrothermal systems.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • Formation of HFSE-Rich Glimmerites by Silicate Liquid-Liquid
           Immiscibility, Suzhou Pluton, Eastern China

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      Abstract: AbstractUnderstanding the formation of high field strength element (HFSE) mineralization remains a challenge. Processes ranging from melt generation to final crystallization have been invoked to play a key role in the enrichment of HFSEs in magmatic-hydrothermal systems, yet the importance of crystal-liquid fractionation, liquid immiscibility, and/or fluid unmixing remains debated. Here, we present results of a study of granites and HFSE-mineralized glimmerites from the granitic Suzhou pluton, eastern China. We provide whole-rock major and trace element compositions, a description of mineral assemblages, and a detailed description of zircon textures, zircon Raman spectra, zircon major and trace element and δ18O compositions, and U-Pb ages. The granites and glimmerites have been dated at ~126 to 124 Ma and are thus coeval. The granites range from biotite and zircon poor to biotite and zircon rich and are in contact with glimmerites. The glimmerites form lenses and layers close to and along the margin of the Suzhou granite, comprising up to ~37 vol % biotite and up to ~2 vol % zircon and other HFSE-rich accessory minerals. The biotite-poor granites contain a single type of zircon (type-A: single crystals, oscillatory zoned, fully crystallized structure, relatively poor in trace elements, and relatively high δ18O), whereas the biotite-rich granites and the glimmerites contain two zircon types (type-A crystals: same features as in the biotite-poor granites; type-B crystals: clustered, unzoned, partially metamict structure, rich in trace elements, and relatively low δ18O). Both granite types are Si, Al, and Na rich, whereas the glimmerites are Fe, Ti, Mn, Mg, Ca, P, F, and HFSE rich and, compositionally, fall off simple fractionation trends. We interpret the textural, mineralogical, and compositional relationships to indicate that the glimmerites are the products of Fe-, F-, and HFSE-rich immiscible melts that unmixed from an alkali-rich, moderately reducing (~QFM + 0.5 and ~QFM + 1.0; QFM = quartz-fayalite-magnetite buffer) Suzhou magma system at low crustal pressure. In addition, we note that the zircon textures and compositions are important recorders of the processes and conditions that led to the HFSE mineralization.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • IN SITU U-Pb MONAZITE GEOCHRONOLOGY RECORDS MULTIPLE EVENTS AT THE MOUNT
           ISA Cu (± Zn-Pb-Ag) DEPOSIT, NORTHERN AUSTRALIA

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      Abstract: AbstractThe Mount Isa Cu (± Zn-Pb-Ag) deposit is the largest Cu deposit in the Western fold belt of the Mount Isa inlier. Previous geochronological studies on the deposit have produced a large range (>150 m.y.) in ages for Cu mineralization and associated hydrothermal alteration. This study combines detailed petrology with in situ monazite U-Pb geochronology on four monazite-bearing samples in order to constrain the age of hydrothermal and tectonic events experienced by the Mount Isa Cu (± Zn-Pb-Ag) deposit and enclosing host shale. Samples EY108402 and EX102476 contain singular subangular monazite grains included in dolomite and siderite, which are associated with premineralization silica-dolomite alteration. Monazite from these samples yields mean weighted 207Pb/206Pb ages of 1587 ± 43 (mean square of weighted deviates [MSWD] = 0.57) and 1623 ± 25 Ma (MSWD = 0.61), respectively. These ages constrain the maximum age of silica-dolomite alteration and Cu mineralization, reflecting monazite growth during periods of peak metamorphism and early basin inversion, respectively. A sample from the 1100 Cu orebody (DDR012-2) contains two clusters of fine-grained monazite that replace siderite associated with silica-dolomite alteration, envelop chalcopyrite, and are crosscut by chlorite-quartz-orthoclase microveins. Monazite from these clusters produces 207Pb/206Pb ages ranging from ca. 1620 to ca. 1360 Ma. The large variation in ages is attributed to variable radiogenic Pb loss from a precursor monazite due to (1) continuous coupled dissolution-reprecipitation reactions over ca. 260 m.y. or (2) partial recrystallization by a ca. 1360 Ma fluid event. As monazite from this sample envelops chalcopyrite, the ca. 1360 Ma age can be used to infer the minimum age of Cu mineralization. Sample 1758-1 is from a highly silicified and fractured section of the Eastern Creek Volcanics located adjacent the deposit. The sampled fracture plane bears a chlorite-illite-rutile infill assemblage with fine-grained irregular-shaped monazite. Monazite from this sample produces a lower intercept age of 1376 ± 32 Ma (MSWD = 1.3) and is interpreted to represent the age of a major fluid flow event coeval with uplift along the Mount Isa fault. The monazite U-Pb geochronology presented in this study brackets the age of Cu mineralization and records the presence of multiple tectonic/hydrothermal events over the history of the deposit and enclosing host rocks.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • AN UNUSUAL EARLY EOCENE, SYNCOLLISIONAL CARBONATITE COMPLEX AND RELATED
           RARE EARTH ELEMENT DEPOSIT IN THE INDIA-ASIA COLLISION ZONE, NORTHWESTERN
           VIETNAM

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      Abstract: AbstractCarbonatite-related rare earth element (REE) deposits, the most significant source of REEs globally, are normally generated in extensional settings, such as intracontinental rifts, mantle plume-related environments, or postcollisional orogens. Syncollisional orogens represent overall compressional regimes, so carbonatites and related REE deposits are rarely identified in such a setting. However, this study reports an anomalous syncollisional carbonatite-related REE deposit, Dong Pao, in the India-Asia collision zone in northwestern Vietnam. The Dong Pao deposit is dated at ca. 52 to 51 Ma through zircon and bastnäsite Th-U-Pb chronometers. The ore-hosting carbonatites were emplaced as stocks with associated syenite. The carbonatite-syenite complex is significantly enriched in light REEs, Ba, and Sr and depleted in high-field strength elements, and has high (87Sr/86Sr)i ratios (>0.707) and low εNd(t) values (–6.5 to –5.6). These geochemical signatures imply that the carbonatite-syenite complex was derived from partial melting of subcontinental lithospheric mantle previously metasomatized and fertilized by REE- and CO2-bearing fluids. Timing of the REE-rich carbonatite-syenite complex indicates that it was related to a far-field stress within the early Eocene main-collision stage at 52 to 51 Ma rather than the late-collision stage at 42 to 35 Ma as previously thought. Collisional tectonism involving block rotation and fault activation are interpreted to have induced disturbance of the lithosphere mantle and created localized, transtensional/extensional environments oblique to the trend of the orogen that facilitated emplacement of the REE-rich carbonatitic magmas. Dong Pao appears to be the first identified, high-tonnage REE deposit that formed in the syncollisional geodynamic setting. Such a finding highlights that tectonic disturbance of an REE-rich lithosphere mantle distal to collision sutures has the potential to generate REE deposits, even during prominent convergence and collision of continents. As such, it defines additional search spaces for exploration of other REE orebodies of this style in complex collisional orogens.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • A Plea for More Skepticism Toward Fluid Inclusions: Part I. Postentrapment
           Changes in Fluid Density and Fluid Salinity Are Very Common

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      Abstract: AbstractThis contribution focuses on fluid inclusions in quartz, which is the most commonly investigated host mineral for fluid inclusions. We demonstrate based on petrographic relationships, cathodoluminescence (CL) images, and microthermometric data that postentrapment migration and/or dismemberment of fluid inclusions in quartz is a very common phenomenon that leads to major changes in the recorded fluid properties. The phenomenon is not restricted to samples that experienced shear stress but is common also in freely grown, euhedral quartz crystals that grew within open-space veins and miarolitic cavities. Generally, both migration and dismemberment lead to substantial increases in fluid density and, thus, in decreased fluid inclusion homogenization temperatures by up to several hundred degrees Celsius. Fluid salinities were commonly less affected, although some high-temperature brine inclusions record salinity increases by up to 30% relative.A good fluid inclusion preservation state is indicated (but not proven) if the inclusions occur on well-defined trails or growth zones, show constant phase proportions, and provide tightly clustered microthermometric data. In contrast, well-developed negative crystal shapes cannot be taken as evidence for a good preservation state. Randomly distributed, regular-shaped fluid inclusions in high-temperature (>300°C) quartz samples are in most cases not of primary origin but rather represent migrated pseudosecondary fluid inclusions. Evidence for fluid inclusion migration and dismemberment includes the occurrence of tiny solid inclusions (mostly mica and rutile) within the host quartz, scattered microthermometric data, and the presence of irregular or c-axis-parallel, dully to nonluminescent quartz patches that depart from healed fracture planes visible in cathodoluminescence images. In summary, pressure-temperature information obtained from fluid inclusions should generally be treated with great caution, unless they meet the above-mentioned criteria for good inclusion preservation.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • A Plea for More Skepticism Toward Fluid Inclusions: Part II.
           Homogenization via Halite Dissolution in Brine Inclusions from
           Magmatic-Hydrothermal Systems Is Commonly the Result of Postentrapment
           Modifications

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      Abstract: AbstractQuartz-hosted brine inclusions that homogenize via halite dissolution occur in many magmatic-hydrothermal systems. In many cases their calculated minimum entrapment pressures are geologically unrealistically high, which has been interpreted to reflect accidental entrapment of halite crystals, and hence, to indicate halite saturation. Here it is demonstrated based on microthermometric trends and laser ablation-inductively coupled plasma-mass spectrometry analyses of fluid inclusion assemblages from eight different ore deposits that the phenomenon of inclusion homogenization via halite dissolution is commonly caused by postentrapment changes in fluid inclusion volume rather than by cotrapping of halite. Many of the investigated assemblages show highly variable liquid-vapor homogenization temperatures at relatively constant fluid salinity—a trend that cannot be produced by cotrapping of halite or by cotrapping of vapor. Furthermore, in those assemblages that do display variable fluid salinities, the elemental Cl/X concentration ratios (where X denotes any element other than Na) do not increase with increasing fluid salinity. The observed microthermometric and compositional trends are best explained by variable extents of postentrapment reduction in inclusion volume, which increased fluid density and in some cases was accompanied by H2O loss and migration of the inclusions within the host quartz. The fact that such trends can be observed even in seemingly well preserved fluid inclusion assemblages suggests that postentrapment modifications in fluid inclusion volume and H2O contents may be widespread and thus that temperature and pressure estimates from quartz-hosted fluid inclusions should generally be treated with caution. This is particularly true for samples that may have experienced deformation, such as massive quartz veins and rock-forming quartz.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • Evaluating Geochemical Discriminants in Archean Gold Deposits: A Superior
           Province Perspective with an Emphasis on the Abitibi Greenstone Belt

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      Abstract: AbstractDiscriminating Archean Au deposit types and related ore-forming processes is challenging but paramount for increasing Au exploration success. This study tests the validity of applying geochemical data generated from conventional bulk versus modern in situ methods as discriminants for classifying Au deposits in the Archean Swayze greenstone belt with further comparison to other deposits in the contiguous Abitibi greenstone belt and Red Lake area (Superior Province, Canada). The study used five well-characterized Au settings, based on new mapping, as a basis for evaluating in situ (δ18Oquartz, δ33, 34Ssulfide, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of pyrite, electron microprobe analysis of gold), and whole-rock geochemical datasets to resolve whether intrusion-related Au deposits can be discriminated from orogenic-type Au deposits. Results show that the in situ methods provide insight into processes related to Au mineralization, both primary and subsequent remobilization and upgrading, and define elemental and isotopic correlations that cannot be resolved using conventional bulk methods. For example, when comparing the whole-rock to laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace metal data, only Ag, Te, and Hg show a consistent positive correlation with Au across deposit types in both data sets. Furthermore, the wholerock datasets combined with in situ isotopic analysis suggest the Archean sanukitoid-associated Au deposits represent a distinct group of intrusion-related deposits with mineralization characterized by low δ34Spyrite (<–5 to –25‰), inferred high fO2, an Hg-Te signature, and hosted in intrusions of <2690 Ma that predate shearing. The data and interpretations presented herein provide a baseline that can be widely utilized in future studies of Au deposits.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • The Kupol Epithermal Au-Ag Vein District, Chukotka, Far Eastern Russia

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      Abstract: AbstractThe Kupol epithermal Au-Ag vein district is located in the northern part of the Okhotsk-Chukotka volcanic belt, a Late Cretaceous subduction-related continental volcanic arc exposed for >3,000 km along the eastern coast of Russia. High-grade veins are hosted in the Kupol andesite sequence, a 300- to 1,000-m-thick, subhorizontal, layered sequence of andesite flows, sills, and ash tuffs, dated at 97 to 96 Ma (Cenomanian). The Kupol andesite sequence is underlain by mixed mafic-felsic volcanic units plus sedimentary rocks (“older volcanics”) and overlain by a >1-km-thick “upper felsic” sequence of dacitic-rhyolitic tuffs and associated dikes and flow domes, dated at 95 to 85 Ma, with local sequences of fluvio-lacustrine sedimentary rocks.The epithermal veins occupy N-striking, steeply dipping normal faults that cut thick coherent andesite flows and sills in the central-upper part of the Kupol andesite sequence. The district is dominated by the large Kupol vein (180.7 tonnes (t) Au and 1,986 t Ag produced to 2020), hosted by the 5.5-km-long Kupol fault, which accommodates normal, east-side-down displacement of up to 190 m. The Moroshka and Providence veins, 5 km east-southeast of Kupol, occupy shorter faults (1- to 2-km strike) with smaller vertical displacements (to 70 m). The Moroshka vein is dated at 93.5 ± 1.5 Ma (Turonian; 40Ar/39Ar method on adularia), and the timing of vein mineralization here and at Kupol overlaps with the early stage of upper felsic sequence magmatism. Veins contain subhorizontal ore shoots, controlled by the intersection of the steep faults with flat-lying Kupol andesite sequence stratigraphy and by steepening of the faults to a more dilational orientation as the inferred paleosurface is approached. Local structural controls are also evident, reflecting a component of oblique slip on the Kupol fault, with the thickest vein segments at steeply pitching jogs and relays. Main-stage veins grew via repeated encrustation by quartz-chalcedony ± amethyst ± lattice bladed calcite (replaced by quartz), with Au-Ag–bearing crustiform adularia ± clays ± sulfides/sulfosalts/electrum ± chlorite ± hematite bands. The main controls on Au grade are inferred to have been boiling, resulting in sharp vertical limits to high metal grades typical of epithermal veins, coupled with optimal dilation of the vein system where the hosting normal fault steepens near surface with decreasing differential stress. Although much of the displacement on the controlling faults is pre-mineralization in timing, lithified cataclastic breccia, coeval with some vein stages, and vein geometry patterns indicate that some vein development occurred contemporaneously during late normal displacement along the fault system. Waning of the hydrothermal system is marked by late carbonate fill, initially Fe dolomite, then coarse calcite as veins, matrix to vein breccia, and central vein cavity fill.The Kupol district veins have proximal adularia-quartz alteration (haloes meters wide), within an extensive (hundreds of meters in scale) clay alteration halo. Clays are zoned both vertically and laterally with respect to veins, with inner illite-chlorite that was magnetite-destructive (at highest paleotemperature; >220°C), grading outward and upward to illite/interlayered illite-smectite with kaolinite, then to an outer zone (or upper blanket) of smectite, at lowest paleo-temperature (<150°C). The boundary between the illite and smectite zones is interpreted to mark the interaction limit of paleo-hydrothermal systems with cooler groundwater. District-scale pathfinder element zonation correlates with clays, with S-Te-Bi-As in the illite-chlorite core and Sb-Cs-Tl(-As-Li) in the smectite blanket. Pathfinder zonation patterns at Kupol point to a magmatic source at depth or, more likely given the scale of the anomalies, multiple magmatic sources, with the surface clay zonation indicating the extent of coalesced paleo-hydrothermal systems associated with upflow plumes. This is the best-defined alteration record with geochemical signature for a complete district hosting a large, high-grade vein deposit. Early definition of clay and pathfinder element patterns across an entire epithermal district can be carried out at low cost to provide useful constraints on vein targeting.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • An Experimental Study of Pyrochlore Solubility in Peralkaline Granitic
           Melts

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      Abstract: AbstractPeralkaline rocks (defined by molar (Na + K)/Al > 1) are typically enriched in Nb and halogens (such as F and Cl). They can further be subdivided into silica-saturated (e.g., alkali granites) and silica-undersaturated (e.g., nepheline syenites). The current study investigates the solubility product (Ksp) of pyrochlore, the most important ore mineral for Nb in peralkaline granites. The Ksp of pyrochlore increases strongly with increasing temperature and with decreasing A/CNK (molar Al2O3/CaO+Na2O+K2O). By contrast, the Ksp of pyrochlore is only weakly dependent on the F content of the melt, if F concentrations are greater than 1 wt %. The Ksp values of pyrochlore from this study are compared to those of columbite from both this study and the literature to evaluate the controls on the crystallization of these two Nb minerals for granites in variable composition. In peralkaline granitic melts with A/CNK < 1, the Ksp values of pyrochlore are lower than those of columbite, but in peraluminous melts with A/CNK > 1, the Ksp values of pyrochlore are higher than those of columbite, and in subaluminous melts, the Ksp values of pyrochlore and columbite are almost the same. Thus, for melts with similar concentrations of essential structural constituents (Ca-Na in the case of pyrochlore and Mn in the case of columbite), the solubility experiments explain why pyrochlore is more common in peralkaline granitic systems, whereas columbite is the main Nb-bearing mineral in peraluminous systems.An expression that describes the dependence of logKsp on temperature and A/CNK was obtained using the experimental results from the F-enriched granitic melts:logKsp=(−5.22±0.50)×(1000⁄T)−(1.91±0.16)×A/CNK+(3.60±0.61)R2=0.97 where temperature (T) is in Kelvin (K). Using this expression, the saturation solubility or the crystallization temperature of pyrochlore can be calculated for the differentiation of peralkaline granitic magmas. This equation was used in conjunction with data from natural melt inclusions to evaluate whether these melts could have been pyrochlore-saturated. In some cases, the melts could not have been pyrochlore-saturated at reasonable temperatures, but in other cases, notably the pegmatite melts at Strange Lake, the concentrations of the essential structural constituents of pyrochlore (i.e., Nb, Ca, Na, F) in the melt inclusions are consistent with magmatic pyrochlore saturation.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
  • Alkaline-Silicate REE-HFSE Systems

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      Abstract: AbstractDevelopment of renewable energy infrastructure requires critical raw materials, such as the rare earth elements (REEs, including scandium) and niobium, and is driving expansion and diversification in their supply chains. Although alternative sources are being explored, the majority of the world’s resources of these elements are found in alkaline-silicate rocks and carbonatites. These magmatic systems also represent major sources of fluorine and phosphorus. Exploration models for critical raw materials are comparatively less well developed than those for major and precious metals, such as iron, copper, and gold, where most of the mineral exploration industry continues to focus. The diversity of lithologic relationships and a complex nomenclature for many alkaline rock types represent further barriers to the exploration and exploitation of REE-high field strength element (HFSE) resources that will facilitate the green revolution. We used a global review of maps, cross sections, and geophysical, geochemical, and petrological observations from alkaline systems to inform our description of the alkaline-silicate REE + HFSE mineral system from continental scale (1,000s km) down to deposit scale (~1 km lateral). Continental-scale targeting criteria include a geodynamic trigger for low-degree mantle melting at high pressure and a mantle source enriched in REEs, volatile elements, and alkalies. At the province and district scales, targeting criteria relate to magmatic-system longevity and the conditions required for extensive fractional crystallization and the residual enrichment of the REEs and HFSEs. A compilation of maps and geophysical data were used to construct an interactive 3-D geologic model (25-km cube) that places mineralization within a depth and horizontal reference frame. It shows typical lithologic relationships surrounding orthomagmatic REE-Nb-Ta-Zr-Hf mineralization in layered agpaitic syenites, roof zone REE-Nb-Ta mineralization, and mineralization of REE-Nb-Zr associated with peralkaline granites and pegmatites. The resulting geologic model is presented together with recommended geophysical and geochemical approaches for exploration targeting, as well as mineral processing and environmental factors pertinent for the development of mineral resources hosted by alkaline-silicate magmatic systems.
      PubDate: Wed, 01 Feb 2023 00:00:00 GMT
       
 
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