Composition Of Accessory Minerals Research Articles

Utilizing compositions of zircon and apatite for prospecting of Cu-Mo-Auporphyry mineralization in the Pekinsky and Tessemsky granitoid massifs of the Taimyr-Severozemelskaya folded area

Research subject. The Tessemsky granite massif is located in the North Taimyr tectonic zone, surrounded by Cambrian rocks. The Pekinsky granite massif is located within the Central Taimyr zone, surrounded by metamorphosed Proterozoic rocks. Aim. To develop a methodology for using the composition of accessory granitoid minerals when prospecting Cu-Mo-Au-porphyry mineralization on the example of the Pekinsky and Tessemsky granitoid massifs of the Taimyr Peninsula. Materials and methods. Accessory zircon and apatite contained in two granitoid samples from the Pekinsky massif (P1, P2) and two granitoid samples from the Tessemsky massif (T2, T3) were studied. Their mineral composition was examined using an EPMA Cameca SX100 instrument. The element content in minerals was determined by LA-ICPMS using an NexION 300S instrument equipped with an NWR 213 attachment. Results. Most of the zircons from the Pekinsky and Tessemsky massifs were formed at T < 738°C in oxidized magma with ΔFMQ of 0.6–2.6, which is a favorable sign for the identification of Cu-Mo-Au-porphyry mineralization. Zircons are characterized by elevated (Eu/Eu*)Y and (Ce/Nd)n/Y ratios, which is also a favorable, though not a strongly reliable, sign for identifying porphyry mineralization. The Eu/Eu* and Sr/Y ratios in the apatites from the Tessemsky massif are higher than those in the apatites from the Pekinsky massif. The rock compositions of both massifs fall within the fields of adakites on the classification diagrams. The estimates of oxygen fugacity (logfO2) calculated from Mn in apatites for four samples agree well within the error limits. Conclusion. Specific features of using the composition of accessory minerals (zircon and apatite) for prospecting the Cu-Mo-Au-porphyry mineralization associated with granitoids were considered. Accessory indicator minerals can be used to rank granitoid massifs in order to assess their ore content. The example of two granite intrusions of the Taimyr Peninsula made it was possible to show that the Tessemsky massif is more promising for the discovery of associated Cu-Mo-Auporphyry mineralization than the Pekinsky massif.

Insights from apatite and zircon geochemistry into peraluminous I-type granitoid: A case study of granodiorite porphyry and lamprophyre in Baoshan, China

Granitoids are the most important component of continental crust, yet there has been debate regarding the classification and petrogenesis of peraluminous I- and S-type. As a result of fractional crystallizing and crustal contamination, whole-rock geochemistry sometimes fails to accurately reflect the type of primitive magma. Recent studies, however, suggest that the accessory mineral compositions can shed light on the character and petrogenesis of their primitive magma. In this contribution, we use apatite and zircon as indicators to explore the distinctions between peraluminous I- and S-type granitoids, and the petrogenesis of typical peraluminous I-type granitoids (Baoshan granitoids). Apatite trace elements indicate that their initial magma was mafic I-type, even though whole-rock compositions appear to be the hybrids of I- and S-type granitoids. Additionally, we propose that the assimilation and fractional crystallization processes are responsible for the decoupling between the compositions of whole-rock and accessory minerals. The compositions and isotopes of zircon can also reveal the components of the magma source region. The zircons εHf(t) values of the Baoshan granodiorite porphyry and lamprophyre have comparable Hf(t) values (−9.5 to −6.2 and −12.5 to −6.2, respectively). Based on the spotting of ~900 Ma inherited zircons and enriched εHf(t) values, we propose that the granitoids were formed by the partial melting of felsic Paleoproterozoic crust and a little of Neoproterozoic mafic juvenile crust, while lamprophyre was generated by the cooling of upwelling magma from the same source region as granitoids. According to the apatite trace element ratios (Sr/Th vs. La/Sm), the source region of the Baoshan intrusion is identified to been metasomatized by slab-derived fluid. Our data, in conjunction with previous studies, suggest that the paleo-Pacific slab roll-back triggered the high-temperature asthenosphere mantle upwelling, while the assimilation and fractional crystallization occurring along with the rising melts in route to the surface.

Mantle contribution to the generation of the giant Jinduicheng porphyry Mo deposit, Central China: New insights from combined in-situ element and isotope compositions of zircon and apatite

Identifying crust–mantle interaction is important for understanding the genesis of granite-hosted mineral deposits in post-collisional or intracontinental settings. We have used combined zircon-apatite trace element compositions, zircon HfO isotopes and apatite SrNd isotopes, which were all acquired by in-situ techniques, to evaluate the mantle contribution to the formation of a giant porphyry Mo deposit, the Jinduicheng deposit in the East Qinling–Dabie orogenic belt, Central China. Zircons from the host granites yielded a UPb age of ∼139 Ma, suggesting a post-collisional setting for the ore-related intrusion. Most magmatic zircons from the granites have Ti-in-zircon temperatures lower than 830 °C, moderate Zr/Hf ratios (25–55), and mantle-like δ18O values (peaking at ∼5.6‰), similar to typical I-type granites worldwide. The magmatic zircons have variable εHf(t) values (−26.7 to −7.2) and Ti-in-zircon temperatures (646–894 °C, peaking at ∼740 °C). The coexisting igneous apatites have initial 87Sr/86Sr ratios of 0.70694–0.71027, and variable εNd(t) values (−16.0 to −9.3), Sr contents (132–663 ppm), and Eu/Eu* values (0.12–0.47). The Sr–Nd–Hf isotopes of the apatites and zircons, along with the positive correlations between apatite Sr contents, Eu/Eu* values, and εNd(t) values, a positive correlation between Ti-in-zircon temperatures and zircon εHf(t) values, as well as mixing modeling using SrNd isotopes, suggest that at least two end-member melts were involved in the formation of the host granites: one is the ancient lower crust-derived melt, and the other is the metasomatized lithospheric mantle-derived melt, which have relatively enriched and depleted Sr–Nd–Hf isotopic compositions, respectively. We suggest that the post-collisional lithospheric thinning, asthenospheric upwelling and underplating of the mantle-derived mafic magmas may have triggered the crustal melting, magma mixing, and generation of the host granites of the Jinduicheng deposit. We postulate that, besides the heat, the metasomatized lithospheric mantle-derived magmas likely supplied additional volatiles (e.g., Cl, S), facilitating the porphyry Mo formation. Our study highlights that, combined in-situ element and isotope compositions of accessory minerals can provide multidimensional insights into petrogenetic tracing, and enhance the genetic interpretations of granites and related mineral deposits.

АКЦЕССОРНЫЕ НИОБИЙ-ТАНТАЛОВЫЕ МИНЕРАЛЫ АЛЕЙСКОГО ГРАНИТНОГО МАССИВА (СЗ РУДНОГО АЛТАЯ)

Актуальность. Ниобий-танталовая акцессорная минерализация гранитоидов представляется важным дополнительным индикаторным признаком выявления потенциальной редкометальной рудоносности лейкогранитов – производных заключительной фазы становления полифазного Алейского гранитоидного массива (D2–3). Цель исследования включает установление форм нахождения ниобия и тантала, а также сопутствующих им других редких элементов, изучение распространенности и возможностей использования их в качестве минералов-индикаторов условий формирования лейкогранитов. Методы исследования включают традиционные минералого-петрографические, изучение видового состава акцессорных минералов в искусственных протолочках, начальный вес которых составлял 5…8 кг; аналитические исследования минералов выполнены в ИГМ СО РАН (г. Новосибирск) на электронном сканирующем микроскопе JSM–6510LV (Jeol Ltd) с энергодисперсионным спектрометром INCA Energy 350+ (аналитик мл. науч. сотр. М. В. Хлестов) и на рентгено-флюоресцентном микроскопе (отделение геологии Инженерной школы природных ресурсов ТПУ, аналитик канд. геол.-минер. наук Рудмин М.А.). Результаты. Установлены две формы нахождения ниобия и тантала в лейкограните Алейского массива: 1) структурная примесь в титаните, цирконе, апатите; 2) самостоятельные минералы ниобия и тантала – ильменорутил, колумбит. Концентрации ниобия обычно резко доминируют над концентрациями тантала. Специфическая особенность химизма акцессорного колумбита – высокая железистость, и повышенные концентрации примесного скандия соответствует редкой разновидности – скандистому ферроколумбиту. Выделение минералов ниобия – ильменорутила и колумбита – происходило на постмагматическом этапе становления лейкогранитного интрузива в щелочную стадию автометасоматоза, причем ильменорутил по отношению к колумбиту является более ранним минералом.

Effects of bentonite heating on U(VI) adsorption

Engineered barrier systems designed to isolate high-level radioactive waste utilize bentonite, a montmorillonite-rich material, to restrict contaminant transport due to its low hydraulic conductivity and high adsorption capacity. High temperatures (100–200 °C) near waste canisters resulting from radioactive decay may alter the clay's ability to adsorb contaminants. In this study, we examine U(VI) adsorption onto two different bentonite samples subjected to (1) 18 years of in situ heating during an underground experiment and (2) short-term (7-week), high temperature (300 °C) heating in the laboratory. Results show that U(VI) adsorption was lower for field-heated bentonite located closest to the heater, which experienced temperatures of approximately 95 °C, compared to a control sample, which experienced temperatures of approximately 20 °C, over a range of aqueous chemical conditions. On average, Kd values for U(VI) adsorption were 31% lower for 95 °C heated samples. By contrast, U(VI) adsorption onto intermediate-heated bentonite (50 °C) was indistinguishable from the adsorption onto the cold-zone (20 °C) sample. U(VI) adsorption onto lab-heated bentonite was also lower than onto control bentonite over the pH range 4.5–8.0, with an average decrease in Kd values of 50% after heating. Lower U(VI) adsorption to field-heated bentonite persisted after bentonite was purified to isolate the clay fraction. This allows us to rule out changes in pore-water chemistry or accessory mineral composition as causes of the lower adsorption. No evidence of montmorillonite illitization was observed in the heated samples. While some of the lower U(VI) adsorption in the lab-heated bentonite can be explained by changes in aqueous U(VI) speciation, we propose that lower U(VI) adsorption to field-heated bentonite may be primarily due to changes in the montmorillonite edge structure. The observed changes in U(VI) adsorption to bentonite after heating have implications for U(VI) diffusive transport through engineered barriers and must be considered when designing radioactive waste disposal repositories.

Altered rocks of the Onguren carbonatite complex in the Western Tansbaikal Region: Geochemistry and composition of accessory minerals

The paper discusses the mineralogy and geochemistry of altered rocks associated with calcite and dolomite–ankerite carbonatites of the Onguren dyke–vein complex in the Western Transbaikal Region. The alteration processes in the Early Proterozoic metamorphic complex and synmetamorphic granite hosting carbonatite are areal microclinization and riebeckitization; carbonates, phlogopite, apatite, and aegirine occur in the near-contact zones of the dolomite–ankerite carbonatite veins; and silicification is displayed within separated zones adjacent to the veins. In aluminosilicate rocks, microclinization was accompanied by an increasing content of K, Fe3+, Ti, Nb (up to 460 ppm), Th, Cu, and REE; Na, Ti, Fe3+, Mg, Nb (up to 1500 ppm), Zr (up to 2800 ppm), Ta, Th, Hf, and REE accumulated in the inner zone of the riebeckitization column. High contents of Ln Ce (up to 11200 ppm), U (23 ppm), Sr (up to 7000 ppm), Li (up to 400 ppm), Zn (up to 600 ppm), and Th (up to 700 ppm) are typical of apatite–phlogopite–riebeckite altered rock; silicified rock contains up to (ppm): 2000 Th, 20 U, 13000 Ln Ce, and 5000 Ва. Ilmenite and later rutile are the major Nb carriers in alkali altered rocks. These minerals contain up to 2 and 7 wt % Nb2O5, respectively. In addition, ferrocolumbite and aeschynite-(Ce) occur in microcline and riebeckite altered rocks. Fluorapatite containing up to 2.7 wt % (Ln Ce)2O3, monazite-(Ce), cerite-(Ce), ferriallanite-(Ce), and aeschynite-(Ce) are the REE carriers in riebeckite altered rock. Bastnasite-(Ce), rhabdophane-group minerals, and xenotime-(Y) are typical of silicified rock. Thorite, monazite-(Ce), and rhabdophane-group minerals are the Th carriers.

Secondary Ionization Mass Spectrometry Analysis in Petrochronology

The goal of petrochronology is to extract information about the rates and conditions at which rocks and magmas are transported through the Earth’s crust. Garnering this information from the rock record greatly benefits from integrating textural and compositional data with radiometric dating of accessory minerals. Length scales of crystal growth and diffusive transport in accessory minerals under realistic geologic conditions are typically in the range of 1–10’s of μm, and in some cases even substantially smaller, with zircon having among the lowest diffusion coefficients at a given temperature (e.g., Cherniak and Watson 2003). Intrinsic to the compartmentalization of geochemical and geochronologic information from intra-crystal domains is the requirement to determine accessory mineral compositions using techniques that sample at commensurate spatial scales so as to not convolute the geologic signals that are recorded within crystals, as may be the case with single grain or large grain fragment analysis by isotope dilution thermal ionization mass spectrometry (ID-TIMS; e.g., Schaltegger and Davies 2017, this volume; Schoene and Baxter 2017, this volume). Small crystals can also be difficult to extract by mineral separation techniques traditionally used in geochronology, which also lead to a loss of petrographic context. Secondary Ionization Mass Spectrometry, that is SIMS performed with an ion microprobe, is an analytical technique ideally suited to meet the high spatial resolution analysis requirements that are critical for petrochronology (Table 1). View this table: Table 1 Advantages and limitations of in-situ SIMS analysis for petrochronology in comparison with other isotope selective methods In SIMS, bombardment of solid targets with an energetic ion beam removes atoms from the sample where primary ions are implanted into the target material to a depth of < 5–10 nm. Lateral resolution is controlled by primary ion beam dimensions (sub-μm to few 10’s of μm) with an upper limit set by the acceptance …

Study of the minerogenetic mechanism and origin of Qinghai nephrite from Golmud, Qinghai, Northwest China

Electronic microprobe analysis showed that all QN samples are mainly composed of tremolite and minor accessory minerals, such as diopside, calcite, serpentinite, and magnetite. According to the cation coefficients, the crystallo-chemistrygenesis illustration demonstrates that all QN deposits are contact metasomatic. Depending on the mole percent of Fe2+(3+)/(Mg2++Fe2+(3+)) and the content of Cr, Co, and Ni in all QN samples measured by X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), green and azure-green QNs are characterized as serpentinite-related contact metasomatic deposit (S-type), whereas white, green-white, brown, blue-violet, yellow, and viridis QNs are dolomite- related contact metasomatic deposit (D-type). The assemblages and chemical composition of accessory minerals of the eight-color QN samples show evident characteristics, which reveal four possible ore-forming processes. We also measured trace and rare earth elements (REEs) in these samples through ICP-MS to deduce the origin of and the changes in metallogenic conditions. The chondrite-normalized REE patterns of D-type QN exhibit moderately negative Eu anomalies with moderate light REE enrichment, flat heavy REE (HREE), and low ƩREE concentrations, similar to dolomitic marble. Green QN samples of S-type show enrichment in HREE and moderately negative Eu anomalies, which is consistent with characteristics of dunite. Whereas azure-green QN samples of S-type exhibit a right-dipping V-type curve with severe depletion in Eu (δEu=0.36–0.47), in accordance with the characteristics of gabbro from Yushigou ophiolite in North Qilian mountains. Furthermore, this finding is consistent with the content of trace elements and the petrographic analysis results. On the basis of several significant differences in the characteristic elements, which may have been affected by the metallogenic environment, we inferred the differences in oxygen fugacity and basicity of mineralization environments in different-colored QNs.

Hf isotope analysis of small zircon and baddeleyite grains by conventional Multi Collector-Inductively Coupled Plasma-Mass Spectrometry

Protocols for solution nebulization (SN) multi collector (MC) ICP-MS analysis of the Hf isotopic composition of accessory minerals (zircon and baddeleyite) selected for high-precision ID-TIMS UPb geochronology have improved significantly during the last two decades. Precise crystallization age and Hf isotopic composition from the exact same volume of mineral can be retrieved, provided that time-consuming sample preparation and purification is possible. The present work focuses on the analytical capabilities of SN-MC-ICP-MS for Hf isotope analysis on non-purified reference materials. Analytical issues arise for (i) igneous and metamorphic zircons, often enriched in trace elements and REEs, which increase the amount of isobaric interferences. (ii) Small minerals that challenge the limits of quantification of the apparatus. JMC475 Hf standard, Plešovice and Temora-2 zircons and Phalaborwa baddeleyite were used to characterize the sources of analytical bias and to set up an optimized protocol for Hf isotope analysis of small accessory minerals with accurate correction of the isobaric interferences and accounting for the matrix effects. Doping of JMC475 with major (Zr, Si), trace (U, Th, Y, Ti) and rare earth elements (Ce, Nd, Gd, Dy, Ho, Er) shows that oxide species produced in the ICP induce an offset of the measured 176Hf/177Hf ratios that is corrected by normalization to the reference value. The 176Yb isobaric interference is efficiently removed for samples with Yb/Hf ratios <0.6 using in-run determined 176Yb/173Yb reference ratios from Yb doped reference material. This isotopic ratio varies (i) with time and (ii) sample matrix. Plasma loading by Zr is not the source of these variations. We propose that Yb oxidation in the plasma torch is responsible for variations in the 176Yb/173Yb ratio. Our dataset shows that matrix matched standards should be preferred for data reduction, and that Plešovice is a good zircon standard because of its availability, quantifiable REEs composition and potential for wide range of Yb doping. Consequently, low Hf concentration solutions from Temora-2 zircons were accurately measured in a small sample volume (<120μl), using a short acquisition protocol (50×1s cycles), and sample-standard bracketing for mass bias coefficient determination in a [Hf]~10ng/g Plešovice solution measured through 80×4s cycles. The repeatability of the measurement (2SD) is improved by factors of up to ~4.4 and the limit of quantification is brought down to <2ng/g Hf, corresponding to zircon minerals of <24ng.

Accessory mineralization of rocks from Late Cretaceous intrusive series with Li–F granites in the Far East

Accessory mineralization of the Late Cretaceous intrusive series in the Far East was investigated on the basis of published data and the author’s original evidence. The composition of accessory minerals from leucogranite, monzonitoid rocks, and Li–F granites has been established. The trend in the evolution of Late Cretaceous granitoids is characterized by an increase in the mineral-forming role of iron and rare elements. Diverse accessory minerals and their typomorphic assemblages have been identified for Li–F granites and ongonites. The regional specificity of accessory mineralization in rare-metal granites consists in the leading role of the minerals W, Ta, Nb, Bi, Y, REE, and As. The uniformity of mineral species and mineral assemblages and the typomorphism and evolution of accessory minerals are inherent to the Far East belt of Li–F granites.

Timing and conditions of high-pressure metamorphism in the western Grenville Province: Constraints from accessory mineral composition and phase equilibrium modeling

Previous geochronological analyses of high pressure (HP) metamorphic rocks in the western Grenville Province, Ontario, Canada have yielded precise U–Pb zircon ages; however, uncertainty has remained as to whether these ages represent the timing of HP metamorphism or the granulite/amphibolite facies overprint accompanying exhumation to a hot middle orogenic crust. Detailed study of these HP rocks, involving garnet, rutile, and zircon trace element analysis, phase equilibrium modeling, and zircon U–Pb geochronology, has yielded much improved constraints on the timing and conditions of HP metamorphism. Zircon from five of the six HP samples yield anchored discordia upper intercept and 207Pb/206Pb weighted average ages between 1097 and 1085 Ma, and typically have trace element compositions consistent with growth in a garnet-rich, plagioclase-poor eclogite-type assemblage (i.e. no negative Eu anomaly and flat HREE trends). Titanium-in-zircon and Zr-in-rutile thermometry indicates that the range of zircon crystallization temperatures for most samples (643–767 °C) is close to that of rutile inclusions in garnet (668–753 °C) and matrix rutile (690–772 °C). Phase relations in a pseudosection calculated for the sample that best preserves the HP assemblage indicate that: (1) the stability field for the inclusions observed in garnet and kyanite is between 11.5 < P < 14 kbar and 600 < T < 700 °C, and (2) zircon and rutile crystallization temperatures intersect the inferred HP assemblage field (Grt + Cpx + Ky + Rt + Hbl + Qtz) and garnet and kyanite modal isopleths at P > ~ 15 kbar, indicating that the ca. 1090 Ma zircon ages date metamorphism at eclogite facies conditions. Thus, the deep burial of mafic lower crust that resulted in HP metamorphism in the western CGB occurred just prior to the main “Ottawan” phase of continental collision in the western Grenville Province (ca. 1080–1040 Ma).

Consistent patterns of rare earth element distribution in accessory minerals from rocks of mafic-ultramafic complexes

AbstractThis paper summarizes analytical data accumulated in the world literature and other materials about the regularities of the REE distribution in minerals contained in ultramafic and mafic rocks as accessory phases. These minerals are tentatively divided into two groups. The first includes garnets, zircons, apatites and perovskites, which can accumulate increased amounts of REE in their structure. The second consists of minerals whose structure can accumulate only limited contents of these trace elements. These are chrome-spinels, ilmenites, and micas. These minerals, in respect of REE geochemistry, are studied to a varying degree because of the different levels of accumulations of these elements, different degrees of occurrence in rocks, tiny sizes of their grains and other reasons. The analytical database formed on their basis includes about 600 original analyses. The overwhelming majority of presently available data on REE geochemistry in accessory minerals from ultramafic and mafic rocks have been published only in the recent 15 years. The studies became possible due to the development and introduction of new highly sensible microprobe analyses allowing detection of REE and many other trace elements in minerals grains directly in thin sections. The greatest numbers of these analyses were performed for garnets and zircons, fewer for apatites, and the fewest for chrome-spinels, ilmenites, micas, and perovskites. In general, the regularities of REE distribution in these minerals from ultramafic and mafic rocks are less studied compared to the rock-forming minerals from ultramafic and mafic rocks. Among the analytical methods, which were used to study the REE composition of accessory minerals, the most efficient was the mass-spectrometry with inductively coupled plasma (ICP-MS).

Minute-sized crystals of HFSE minerals in evolved, fluorite-bearing granite, an example from the Sörvik granite, central Sweden

Scanning Electron Microscopy and Energy Dispersive Analysis are used to study minute-sized crystals of high-field-strength-element minerals in an evolved batch of fluorine-rich granite. The batch separated due to buoyancy from the bulk magma before fluid separation was reached. Fluorine acted as the main complexing agent in the magma, facilitating the transport of many elements. Enhanced concentrations of elements such as Nb, Y, rare earth elements, Th and possibly Be gave the granite an accessory mineral composition similar to that of NYF pegmatite. Biotite was chloritized and often no remaining biotite can be identified. Frequent are two minerals from the aeschynite—euxenite—polycrase group. One is blocky and formed together with chlorite. It was replaced by an acicular variety, which continued to crystallize into the late hydrothermal stage. Bastna¨site and synchysite are also common in the granite. Both started crystallizing together with chlorite, but only synchysite can be followed into the late hydrothermal stage. Hingganite and gadolinite occur as inclusions in fluorite and chlorite. Galgenbergite formed as fracture-fillings. Some other rare minerals have merely been identified as accidental inclusions, but their identification is less certain. They include brenkite, fluocerite, fergusonite and possibly håleniusite. Chemical heterogeneities in the minerals point to incomplete equilibration.

The Omolon pallasite: Chemical composition, mineralogy, and genetic implications

The chemical composition of mineral components of the Omolon pallasite was determined by neutron-activation. Six types of olivines were distinguished. Four types differ in the abundance of Co relative to Ni of CI chondrites. The fifth and sixth types were distinguished on the basis of REE distribution in them. Both last types are variably enriched in LREE relative to CI chondrites. In terms of Ca content relative to CI chondrite, these six types are subdivided into two groups: low-calcium and high-calcium. The difference in Ca contents can be caused by different cooling rate of the precursor of these olivines. The distribution pattern of siderophile elements in the pallasite metal indicates that a metallic phase experienced chemical transformations since the time of its formation. The analysis of chemical composition of accessory minerals showed that: (1) HREE are accumulated in tridymite; (2) troilite and daubreelite were formed under different temperature conditions; (3) magnetite is the mineral of the outer zone of melting crust. Four fragments with anomalous contents of lithophile elements were found in the pallasites and studied. The unusual chemical composition of phases and high degree of HREE fractionation in the fragments suggest their formation at high temperatures at the early stage of the Solar system evolution. It is assumed that the Omolon pallasite was formed as impact-brecciated mixture of the asteroid core (with composition close to IIIAB group of iron meteorites) and mantle olivine from incompletely differentiated parent body of chondrite composition.

Accessory minerals from the Primorsky rapakivi granite complex, West Baikal Region

The composition of accessory minerals from granites of the second phase, quartz-muscovite (+fluorite), and quartz-muscovite-topaz greisens from the Primorsky rapakivi granite complex, West Baikal region, were studied using backscattered scanning electron microscopy. Ilmenite from granites contains inclusions of cassiterite, titanocolumbite, fergusonite-(Y), polycrase-(Y), and betafite. Allanite-(Ce), bastnaesite-(Ce), xenotime-(Y), Y- and Zr-thorite, zircon, and cyrtolite have been identified in granites. Greisens contain cassiterite, ferrocolumbite (Ta/Nb = 0.02−0.06), pyrochlore-group minerals, ilmenorutile, rutile, wolframite, polycrase-(Y), monazite-(Ce), fluocerite-(Ce), bastnaesite-(Ce), cerphosphorhuttonite, thorite, and other minerals. The ferrocolumbite + ilmenorutile assemblage is typical of quartz-muscovite greisen, whereas the rutile + ilmenorutile + wolframite + W-columbite assemblage is contained in the quartzmuscovite-topaz greisen as a result of an increase in Eh and decrease in pH and potassium activity of solution in the back zone. The compositions of Th- and REE-bearing minerals indicate the important role of phosphate and fluorine complexes in the transport of these elements.

Quartz-albite-microcline metasomatic rocks in the Main Sayan Fault Zone: Evolution of metasomatism and composition of accessory minerals

Quartz-albite-microcline metasomatic rocks (qualmites) localized in the Main Sayan Fault Zone at the boundary of Irkutsk oblast and Buryatia (Eastern Sayan, middle reaches of the Kitoi River) were formed after dynamometamorphic biotite and biotite-amphibole granite gneisses. The zone of alkaline metasomatism up to 250–300 m thick extends for about 12 km along the Main Sayan Fault. Riebeckite-aegirine and hornblende-clinopyroxene qualmites were formed during the early alkaline stage of metasomatism. Biotite-magnetite qualmites, which are occasionally superimposed on riebeckite-aegirine rocks, are products of the following stage of increasing acidity. The 40Ar/39Ar age of amphibole from metasomatic rocks is 321 ± 5 Ma. The metasomatic rocks are enriched by 2–4 times in Zr, Nb, Y, REE, Be, Th, and U relative to unaltered granite gneisses and contain rare-metal mineralization (fergusonite-(Y), betafite, Nb- and Y- bearing titanites, gadolinite-(Y), zircon, thorite, allanite-(Ce), chevkinite-(Ce), etc.). The composition of accessory minerals was studied on a LEO-1430VP SEM. Their composition and mineral assemblages show that meta-somatic alteration and Ti-Nb-Zr-REE mineralization were formed synchronously. The stage of acid solution neutralization was characterized by crystallization of epidote and andradite and by replacement of chevkinite with allanite and titanite in metasomatic rocks. Hydro- and fluorcarbonates of LREE, phosphates (monazite-(Ce)), and fluorides (F-bearing thorianite?) were formed during the final low-temperature stage.

Rare earth element release from phosphate minerals in the presence of organic acids

The primary objective of this research was to investigate the effects of aliphatic and aromatic low molecular weight organic acids (LMWOAs) on rare earth element and yttrium (REY) release from the phosphate minerals apatite and monazite. Since prior studies have shown that redox status can affect REY partitioning during incongruent dissolution, a secondary objective was to assess the influence of dissolved O 2 concentration. Increasing LMWOA concentrations from 0 to 10 mM resulted in enhanced REY release. In general, REY release increased in the order: no ligand ≈ salicylate < phthalate ≈ oxalate < citrate. REY–ligand stability constants were only useful for predicting REY release for oxalate reacted with apatite and phthalate reacted with monazite. The role of dissolved oxygen in dissolution of the phosphate minerals was mixed and inconsistent. Mineral type was observed to significantly affect REY pattern development. REY release patterns for apatite range from nearly flat to those exhibiting the lanthanide contraction effect (radius-dependent fractionation); whereas, monazite REY release patterns are best described as exhibiting an M-type lanthanide tetrad effect (radius-independent fractionation). Weathering of apatite in the presence of aliphatic LMWOAs resulted in development of the lanthanide contraction effect fractionation pattern, and the aliphatic LMWOAs further developed MREE and radius-independent fractionation during monazite dissolution. Geochemical and mineral-specific REY signatures may, therefore, have utility for distinguishing the impacts of biota on soil weathering processes on early Earth. The development of such signatures may be mitigated, in part, by accessory mineral composition, the types and concentration of LMWOAs present, and precipitation of secondary minerals.

Rutile geochemistry as a guide to porphyry Cu–Au mineralization, Northparkes, New South Wales, Australia

The textures and compositions of accessory minerals formed during hydrothermal alteration in porphyry Cu–Au deposits may reflect those processes. Furthermore, if the accessory minerals are stable during weathering processes, they should be able to be used as a guide to mineralization even when the sulphides have been destroyed by weathering. In the Northparkes district, central western NSW, Australia, several Cu–Au ore deposits occur as disseminated or veined sulphides associated with a number of quartz monzonite porphyries intruded into lower Palaeozoic K-rich trachyandesites. This paper reports results from 2000 analyses of rutile grains from 80 samples of rock and soil from around one of these deposits, the E26N porphyry Cu–Au deposit. Within 100 m of ore, rutile grains are larger (commonly with length × breadth >4000 μm 2 ) than the grains further from ore (length × breadth 2 ). The rutile grains closer to ore also are zoned and contain V-rich portions in which V contents are >0.2% (commonly >0.4%) whereas barren rutile generally contains 0.15% occur in rutile in the E26N ore zone and in an altered zone within an adjacent earlier intrusion. Thus, Nb in rutile is not as consistently related to mineralization as V in rutile. Vanadium- (and Nb-) rich rutile is present in both the saprolite and soils derived from mineralized rocks at the deposit. Thus, the V contents in rutile can be utilized as a guide to mineralization even in highly weathered material.

Accessory mineral behavior during differentiation of a granite suite: monazite, xenotime and zircon in the Sweetwater Wash pluton, southeastern California, U.S.A.

Compositional and textural characteristics of the accessory minerals monazite, xenotime and zircon in the Sweetwater Wash granites and related aplites indicate that these phases not only controlled much of the trace-element geochemistry of the suite, but that they also record the melt compositional changes that occurred during magmatic differentiation. Fractionation of monazite due to decreasing saturation levels of its essential structural constituents with falling temperature was likely responsible for an ongoing trend of light rare-earth element (REE) depletion. This was accompanied by increasing heavy-REE concentrations until xenotime joined the crystallizing assemblage; subsequently, combined monazite-xenotime fractionation resulted in lowering of the entire REE budget. Zircon, which contains inherited cores that were apparently resorbed and rounded during initial anatexis, was saturated throughout the differentiation history of the Sweetwater Wash suite. Accessory phases in granite and aplite exhibit strong compositional differences at a variety of scales. The differences are best displayed by monazite: on average, crystals in more differentiated rocks (aplites) are relatively depleted in light REE's and contain higher concentrations of the substituting elements U and Th. Zircon displays complimentary increases in Hf and Y, while xenotime shows a slight increase in Th and in Gd/Ho ratios; both phases also exhibit higher U concentrations in aplites than in granites. These differences in accessory mineral compositions are observed not only between granites and the more differentiated aplites, but also within individual thin sections, due to in situ fractionation. On an even smaller scale, strong compositional variations are present within single crystals, possibly due to diffusion-controlled melt-compositional gradients in the regions (a) adjacent to growing major phases, and (b) adjacent to the growing accessory crystal itself. Our observations indicate that compositional variations among accessory minerals are potentially useful for tracking of magmatic processes, but that the scale of observed variations must be carefully considered.

Trace element mobility during early diagenesis of volcanic ash: Applications to stratigraphic correlation

We have analyzed the relative mobility of trace elements in five rhyolitic and two basaltic tephra layers preserved in both altered and unaltered states, as a means of developing a rapid, whole-rock method for correlating altered tephra layers. The tephra layers were altered to zeolites or clay minerals in either saline-alkaline pore fluids, characteristic of small ephemeral lakes, or slightly more dilute pore fluids associated with perennial lakes and brackish coastal groundwaters. Comparison of whole rock trace elements analyses from the unaltered and altered portions of each tephra layer indicates that immobile elements, such as Ti, Zr, Hf, and Ta, retain their initial concentrations. Relatively more reactive alkaline and alkaline-earth elements, such as Rb, Sr, and Cs, are generally mobile. The lanthanide elements display variable mobility. The Light Rare Earth Elements (LREE) generally retain their initial concentrations, whereas the Heavy Rare Earth Elements (HREE) are generally depleted in alkaline solutions but immobile in more dilute pore fluids. We suggest that differences between LREE and HREE mobility result from the combined effects of LREE partitioning into stable accessory minerals and HREE transport as solution complexes, although our data set was not designed for quantitative analysis of these proposed mechanisms. Where both the diagenetic environment, and the accessory mineral compositions are well-constrained, the trace element signatures of the unaltered and altered tephra layers can be used to correlate an altered tephra layer to its unaltered equivalent, and to distinguish between different altered tephra layers. Four of the seven altered tephra layers can̈be distinguished on the basis of ratios of immobile elements, e.g. Zr Ti vs. Hf Ti . The remaining three, all W-type rhyolitic ashes, can be largely differentiated using cluster analysis on a combination of immobile elements and elements with similar mobilities.