Xenotime Grains Research Articles

Age and fluid source constraints of the Haoyaoerhudong orogenic gold deposit, North China: Evidence from geochronology and noble gas isotopes

The Haoyaoerhudong gold deposit, located in the northwestern part of the North China craton (NCC), has produced over 120 metric tonnes (t) of gold since 2007. It has a total reserve of > 240 t at average gold grade of 0.62 g/t, making it one of the largest open pit gold mines in China. The steeply dipping, large-tonnage, low-grade, vein- or veinlet-type gold orebodies are hosted in strongly-deformed Mesoproterozoic carbonaceous schist of the Bayan Obo Group. The laminated/boudinaged veins/veinlets in the sinistral ductile–brittle shear zones are dominated by quartz, biotite, gold-bearing löllingite, pyrrhotite, (arseno)pyrite, with minor native gold, titanite and xenotime. In this paper, we present new in situ U–Pb geochronological data on magmatic zircon from the pre-ore dikes, on metamorphic and hydrothermal xenotime, and on hydrothermal titanite from the hydrothermally altered carbonaceous schist and auriferous quartz–sulfides veins/veinlets, as well as He-Ar isotopic analysis on gold-bearing (arseno)pyrite in the syn-ore stage. The metamorphic xenotime U–Pb age of 426 ± 6.0 Ma (2σ) records a regional metamorphic event, possibly related to the accretion of the Bainaimiao arc onto the NCC. Two pre-ore andesitic dikes yielded similar emplacement ages at ∼ 278 Ma constrained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb zircon data. Hydrothermal xenotime grains from the altered carbonaceous schist and auriferous quartz–sulfides veins yielded U–Pb ages of 256.0 ± 4.1 Ma (2σ) and 254.4 ± 2.1 Ma (2σ), respectively, overlapping with that of the hydrothermal titanite at 255.4 ± 0.8 Ma (2σ) from the laminated quartz–sulfides veinlets. This indicates that the gold mineralization occurred at ca. 255 Ma. The ∼ 255 Ma gold mineralization age is much younger than the previously reported Early–Middle Permian regional magmatic activity (ca. 291 Ma to 268 Ma), and may be associated with the regional sinistral strike-slip event in the late orogenic cycle related to the collision between the Siberian craton and the NCC. The 3He/4He (R/Ra) and 40Ar/36Ar values of the gold-bearing (arseno)pyrite are 0.04 to 0.09 (average = 0.07) and 375.8 to 2023 (average = 1045), which reveal the ore-forming fluids dominantly originated from the crustal rocks, with limited involvement from the mantle. Collectively, our new geochronological data, noble gas isotopic analyses, and geological evidence support a typical orogenic gold deposit model at Haoyaoerhudong.

Diagenetic and Epigenetic Mineralizing Events in the Kalahari Copperbelt, Botswana: Evidence from Re-Os Sulfide Dating and U-Th-Pb Xenotime Geochronology

Abstract The ages of sedimentation and copper-silver mineralization in the late Meso- to Neoproterozoic Kalahari Copperbelt in Botswana, an economically significant copper province, have previously been poorly constrained within an ~600 m.y. period that spans the Neoproterozoic from the assembly and breakup of Rodinia to the assembly of Gondwana. Rhenium-osmium geochronology of molybdenite and copper sulfide minerals and U-Th-Pb laser ablation split-stream inductively coupled plasma-mass spectrometry (LASS ICP-MS) analysis of xenotime grains are utilized to provide absolute and relative age data on the host rocks and mineralizing events within the Ghanzi Ridge region of the Kalahari Copperbelt. The data reveal a prolonged history of events, which is partially comparable with depositional and mineralizing events in the neighboring Central African Copperbelt. Abundant disseminated molybdenite is located within a shale layer near the base of the Proterozoic D’Kar Formation at the Northeast Mango Two deposit. Unusual molybdenite textures suggest organic matter may have been a precursor. Two molybdenite separates from a small calcite-molybdenite stringer in a wall-rock fragment that is enclosed within an epigenetic quartz-calcite-chalcopyrite vein with ill-defined and mismatched margins yielded Re-Os ages of 981 ± 3 and 981 ± 7 Ma. These ages indicate an early hydrothermal mineralizing event in the basin. A xenotime inclusion intergrown with molybdenite and chalcopyrite within the epigenetic vein yielded a younger U-Th-Pb age of 538 ± 8 Ma, suggesting two mineralizing events are preserved in a complex 6-cm-wide vein. Based on vein texture and alteration, the ages represent an ~981 Ma calcite-molybdenite mineralization event overprinted by an ~538 Ma quartz-chalcopyrite-molybdenite mineralization event, perhaps during reopening of the vein. Re-Os and U-Th-Pb geochronology were utilized at the Zone 6 deposit on minerals associated with a hydrothermal quartz-calcite-chalcocite-idaite-bornite vein. Several authigenic xenotime grains that occur along the margin of the vein yielded three concordant U-Th-Pb ages that indicate xenotime growth at ~950 to 925 Ma while other xenotime grains in a similar position yielded mostly discordant data, suggesting disturbance of the isotopic system in the xenotime grains. A coprecipitated chalcocite-idaite mixture within the hydrothermal vein produced an Re-Os age of 549.0 ± 11.2 Ma. Re-Os analysis obtained from a coprecipitated molybdenite-bornite mixture at the Northeast Fold deposit yielded an age of 515.9 ± 2 Ma. Together, the earliest Neoproterozoic Re-Os molybdenite and U-Th-Pb xenotime ages provide both a minimum depositional age constraint for the lowermost D’Kar Formation and clear evidence that diagenetic hydrothermal mineralizing events took place within the Ghanzi basin. The timing of this mineralizing event corresponds with a poorly documented regional thermal event that affected the northern margin of the Kalahari craton during the final stages of the assembly of Rodinia at ~980 Ma. The lower to middle Ghanzi Group of the Kalahari Copperbelt is at least 100 m.y. older than the host rocks within the neighboring Central African Copperbelt, which are associated with the breakup of Rodinia. The latest Neoproterozoic to Cambrian Re-Os and U-Th-Pb ages indicate that hydrothermal copper-silver mineralizing events occurred during the Pan-African (~600–480 Ma) fold-thrust evolution of the Ghanzi-Chobe zone and were broadly synchronous with widespread epigenetic hydrothermal copper-cobalt mineralizing events in the adjacent Central African Copperbelt.

A new analytical protocol for high precision U–Th–Pb chemical dating of xenotime from the TTG gneisses of the Bundelkhand Craton, central India, using CAMECA SXFive Electron Probe Micro Analyzer

Xenotime is a significant accessory mineral which is being extensively used for precise U–Th–Pb geochronology by Electron Microprobe Analysis (EPMA). This paper presents a protocol for high analytical precision (<3% uncertainties on the measured ages) developed for the accurate estimation of U–Th and Pb content in xenotime using SXFive EPMA at the Department of Geology, Banaras Hindu University, by deploying five spectrometers attached with TAP, LIF, LPET, LTAP and PET crystals. The protocol is applied to the xenotime grains of tonalite-trondhjemite-granodiorite-gneiss (TTG) rocks from the geochronologically well-constrained terrain of the Bundelkhand Craton, central India. The obtained xenotime age 2929±23 Ma of TTGs is in agreement with the earlier published Neoarchaen 2697±3 Ma Pb–Pb zircon ages from the same area which validates the authenticity of the analytical method developed at the BHU-EPMA facility.

REE and Y Mineralogy of the Krudum Granite Body (Saxothuringian Zone)

The Krudum granite body comprises highly fractionated granitic rocks ranging from medium-F biotite granites to high-F, high-P2O5 Li-mica granites. This unique assemblage is an ideal site to continue recent efforts in petrology to characterize the role of zircon, monazite, and xenotime as hosts to rare earth elements (REEs). The granitic rocks of the Krudum body analyzed in this study were found to contain variable concentrations of monazite and zircon, while xenotime was only found in the high-F, high-P2O5 Li-mica granites and in the alkali-feldspar syenites of the Vysoký Kámen stock. Intermediate trends between cheralite and huttonite substitutions are characteristic for analyzed monazite grains from all magmatic suites. The highest concentration of cheralite was found in monazite from the alkali-feldspar syenites (up to 69.3 mol %). The proportion of YPO4 in analyzed xenotime grains ranges from 71 to 84 mol %. Xenotime grains are commonly enriched in heavy rare earth elements (HREEs; 9.3–19.5 wt % HREE2O3) and thorite-coffinite and cheralite exchange was observed. Some xenotime analyses return low totals, suggesting their hydration during post-magmatic alterations. Analyzed zircon from granite suites of the Krudum granite body contains moderate Hf concentrations (1.0–4.7 wt % HfO2; 0.010–0.047 apfu Hf). The highest concentrations of HfO2 were found in zircon from the high-F, high-P2O5 Li-mica granites (1.2–4.7 wt % HfO2). Analyzed zircon grains from the high-F, high-P2O5 Li-mica granites and alkali-feldspar syenites are enriched in P (up to 8.29 wt % P2O5; 0.24 apfu P), Al (0.02–2.0 wt % Al2O3; 0.00–0.08 apfu Al), Ca (up to 3.9 wt % CaO; 0.14 apfu Ca), Y (up to 5.5 wt % Y2O3; 0.10 apfu Y), and Sc (up to 1.17 wt % Sc2O3; 0.03 apfu Sc). Zircon grains from the high-F, high-P2O5 Li-mica granites were sometimes hydrated and fluorized. The concentrations of F in zircon from partly greisenised high-F, high-P2O5 Li-mica granites reached up to 1.2 wt % (0.26 apfu F).

Mineralogical and Petrographic Characteristics of Indium and REE-Bearing Accessory Phases in the Kymi Granite Stock, Southern Finland

The Wiborg rapakivi batholith (1.64 Ga) in southeastern Finland with documented occurrences of REE, indium and Zn-Cu-Pb sulphide mineralization was studied. Hydrothermal greisen and quartz vein type Fe-Sn and Zn-Cu-Pb are found in the Kymi granite stock as intrusions. They are enriched with indium and rare earth elements, with roquesite (CuInS2) being a major indium- carrier, whereas monazite (Ce), allanite (Ce), bastnäesite (Ce), xenotime-(Y) and thorite are the main REE carriers. Combination of optical and field emission scanning electron microscopy (FE-SEM) and electron probe microanalysis (EPMA) were used to study the indium and REE-bearing mineral assemblages. EPMA of roquesite found in galena had a composition of 26.16% S, 0.02% Fe, 25.06% Cu, 0.03% Zn, 1.06% As, 0.31% Sb and 47.14% In. Substitution reaction Pb2+S2-Cu+In3+S2- is the cause of the incorporation of indium in the galena structure. The majority of the LREE are carried by monazite, bastnäesite and allanite, and the HREE by xenotime and zircon. There is a partial solid solution between monazite and xenotime with minor or trace amounts of LREE in xenotime grains (6.0 wt%). LREE (>95 mol% LREE) and less than 5 mol% HREE + Y reflects the enrichment of chondrite-normalized REE of the monazite grains of the Kymi granite stock. The xenotime grains (small and irregular) main composition contains 71 - 76 mol% YPO4, 16 - 27 mol% HREE, and 6 - 8 mol% LREE. It is believed that indium and REE-mineralization presence is due to the combination of magmatic and postmagmatic processes, particularly at later stages by fluid fractionation.

Geochemical characterisation of xenotime formation environments using U-Th

Xenotime (YPO4) is a trace component in many metasedimentary and some igneous rocks, altered rocks and many hydrothermal ore assemblages, where it forms in response to a range of different processes from igneous crystallisation to low-temperature early diagenesis. Due to its wide range of formation temperatures, its suitability and isotopic robustness for reliable U-Pb geochronology, dissolution/precipitation characteristics during overprint events, and widespread occurrence, it is one of the most valuable minerals for U-Pb geochronology leading to four-dimensional (4D) studies of many terranes. The formation environment of xenotime can be deduced from careful petrography and reliable U-Pb in-situ geochronology within a 4D framework. Further, xenotime is a physically robust mineral during sediment transport and may carry distinctive geochemical fingerprints, including age, to secondary environments.From published works, we review the U-Th-contents of well-characterised examples of xenotime formation to provide chemical fingerprints, which assist in identifying the xenotime formation environment. Although the U-Th characteristics of xenotime from all formation environments show considerable overlap, we make the following observations which may be distinctive of some ore formation environments: (i) hydrothermal xenotime formed from low salinity ore fluids (e.g. iron ore and orogenic gold deposits) trend to have the lowest U-contents (<100ppm U) and U/Th (<4), in contrast to xenotime formed from higher salinity ore fluids (Sn-W and base metal deposits); (ii) xenotime associated with unconformity-related U-ores have the highest U-contents at U/Th>10; (iii) diagenetic xenotime has the most variable and highest U/Th, whereas (iv) xenotime from Precambrian orogenic gold deposits has the least variable and lowest U/Th.Petrogenetic inferences from these observations for ore deposit research and exploration include: (i) support for the homogenising effect of the crustal scale of the fluid system in Precambrian orogenic gold deposits, and (ii) the potential for distinctive U-Th geochemical fingerprints for U ores and some Au ores. These characteristics may be reflected in detrital xenotime grains recovered from routine exploration sampling programs.

Nanoscale deformation twinning in xenotime, a new shocked mineral, from the Santa Fe impact structure (New Mexico, USA)

Abstract Shock microstructures in refractory accessory minerals such as zircon and monazite provide crucial evidence for deciphering impact-related deformation in a wide variety of planetary materials. Here we describe the first occurrence of shock deformation in xenotime, YPO4, from a shocked quartz–bearing shatter cone in granite at the Santa Fe impact structure (New Mexico, USA). Backscattered electron imaging shows that shocked xenotime grains near the surface of a shatter cone contain multiple orientations of closely spaced planar fractures. High-resolution electron backscatter diffraction mapping reveals that some of the planar microstructures in {112} contain deformation twin lamellae that range from 50 nm to 200 nm in width on the polished surface and occur in up to three crystallographic orientations. Other features attributed to impact, such as planar low-angle boundaries and planar deformation bands, record crystal-plastic deformation. Shatter cone formation and co-existing shocked quartz constrain minimum shock pressure experienced by the xenotime grains to 5–10 GPa. An upper limit of 20 GPa is tentatively assigned based on the absence of YPO4 polymorphs and shock twins in co-existing zircon. We propose that {112} deformation twins in xenotime constitute a diagnostic record of shock metamorphism, similar to {112} twins in zircon; they have not previously been reported in nature and occur in a rock with conspicuous evidence of shock deformation. Documentation of deformation twins in xenotime, a widely applied U-Pb geochronometer, can be used to identify hypervelocity deformation in shocked rocks, detrital grains, and other materials, and may be particularly ideal for recording low-pressure (&amp;lt;20 GPa) impact conditions that do not produce diagnostic shock microstructures in zircon.

Diagenetic xenotime dating to constrain the initial depositional time of the Yan-Liao Rift

In the Mesoproterozoic and Neoproterozoic, the North China Craton (NCC) underwent long-lived sedimentation with minor volcanism in the Yan-Liao Rift forming the Mesoproterozoic Changcheng, Jixian and Qingbaikou Groups in sequence. The basal boundary age of the Lower Changcheng Group indicates the initial time of the NCC as a stable cratonic block and can be used to make correlations among several representative basins worldwide. Recent studies have debated the initial depositional time of the Changcheng System, which is the key to understanding the Proterozoic tectonic evolution of the NCC. This study documented the xenotime and monazite from the lowermost Changzhougou and Chuanlinggou Formations of the Changcheng Group. The monazite grains from the Chuanlinggou Formation yielded U–Pb ages between 1810Ma and 2500Ma, which are comparable to the detrital zircon U–Pb ages in the same formation, indicating their detrital origin. Xenotime mostly occurs as overgrowths on the detrital zircon grains, supporting a post-depositional origin. The low U content of the Changzhougou Formation's xenotimes indicates a hydrothermal origin. Its peak age of 1570±15Ma may indicate a hydrothermal event. The xenotime from the Chuanlinggou Formation is characterized by a high U content and a REE pattern with a nonexistent or weak negative Eu anomaly and a higher Gd/Yb ratio, implying a diagenetic origin. These xenotime overgrowths yielded a wide age variation, even on a single grain, due to prolonged xenotime precipitation. The oldest population of 1716±3Ma is interpreted as the minimum depositional age of the Chuanlinggou Formation. The lowest boundary of the Changchengian System must be older than 1716Ma. This work demonstrates that diagenetic xenotime could better constrain the depositional time; however, extensive systematic research is required, especially with higher spatial resolution and more diagenetic xenotime grains.

SHRIMP U–Pb and REE data pertaining to the origins of xenotime in Belt Supergroup rocks: evidence for ages of deposition, hydrothermal alteration, and metamorphism

The Belt–Purcell Supergroup, northern Idaho, western Montana, and southern British Columbia, is a thick succession of Mesoproterozoic sedimentary rocks with an age range of about 1470–1400 Ma. Stratigraphic layers within several sedimentary units were sampled to apply the new technique of U–Pb dating of xenotime that sometimes forms as rims on detrital zircon during burial diagenesis; xenotime also can form epitaxial overgrowths on zircon during hydrothermal and metamorphic events. Belt Supergroup units sampled are the Prichard and Revett Formations in the lower Belt, and the McNamara and Garnet Range Formations and Pilcher Quartzite in the upper Belt. Additionally, all samples that yielded xenotime were also processed for detrital zircon to provide maximum age constraints for the time of deposition and information about provenances; the sample of Prichard Formation yielded monazite that was also analyzed. Ten xenotime overgrowths from the Prichard Formation yielded a U–Pb age of 1458 ± 4 Ma. However, because scanning electron microscope – backscattered electrons (SEM–BSE) imagery suggests complications due to possible analysis of multiple age zones, we prefer a slightly older age of 1462 ± 6 Ma derived from the three oldest samples, within error of a previous U–Pb zircon age on the syn-sedimentary Plains sill. We interpret the Prichard xenotime as diagenetic in origin. Monazite from the Prichard Formation, originally thought to be detrital, yielded Cretaceous metamorphic ages. Xenotime from the McNamara and Garnet Range Formations and Pilcher Quartzite formed at about 1160–1050 Ma, several hundred million years after deposition, and probably also experienced Early Cretaceous growth. These xenotime overgrowths are interpreted as metamorphic–diagenetic in origin (i.e., derived during greenschist facies metamorphism elsewhere in the basin, but deposited in sub-greenschist facies rocks). Several xenotime grains are older detrital grains of igneous derivation. A previous study on the Revett Formation at the Spar Lake Ag–Cu deposit provides data for xenotime overgrowths in several ore zones formed by hydrothermal processes; herein, those results are compared with data from newly analyzed diagenetic, metamorphic, and magmatic xenotime overgrowths. The origin of a xenotime overgrowth is reflected in its rare-earth element (REE) pattern. Detrital (i.e., igneous) xenotime has a large negative Eu anomaly and is heavy rare-earth element (HREE)-enriched (similar to REE in igneous zircon). Diagenetic xenotime has a small negative Eu anomaly and flat HREE (Tb to Lu). Hydrothermal xenotime is depleted in light rare-earth element (LREE), has a small negative Eu anomaly, and decreasing HREE. Metamorphic xenotime is very LREE-depleted, has a very small negative Eu anomaly, and is strongly depleted in HREE (from Gd to Lu). Because these characteristics seem to be process related, they may be useful for interpretation of xenotime of unknown origin. The occurrence of 1.16–1.05 Ga metamorphic xenotime, in the apparent absence of pervasive deformation structures, suggests that the heating may be related to poorly understood regional heating due to broad regional underplating of mafic magma. These results may be additional evidence (together with published ages from metamorphic titanite, zircon, monazite, and garnet) for an enigmatic, Grenville-age metamorphic event that is more widely recognized in the southwestern and eastern United States.

Detrital zircon without detritus: a result of 496-Ma-old fluid–rock interaction during the gold-lode formation of Passagem, Minas Gerais, Brazil

Zircon and xenotime occur in tourmaline-rich hydrothermal pockets in the auriferous lode of Passagem de Mariana, a world-class gold deposit. Zircon grains show pristine oscillatory zoning, but many of them are altered, exhibiting porous domains filled with graphite. Uranium–Pb dating of zircon, using in-situ laser ablation–inductively coupled plasma–mass spectrometry, yields ages between 3.2 and 2.65 Ga, which match those for detrital zircon of the footwall quartzite of the > 2.65-Ga-old Moeda Formation. Discordant analyses point to zircon-age resetting during the Brasiliano orogeny at ca. 500 Ma. This interpretation is supported by U–Pb dating of euhedral xenotime immediately adjacent to altered zircon within the same tourmaline pocket. The xenotime grains give a Concordia age of 496.3 ± 2.0 Ma, which is identical to that determined for monazite of a quartz–hematite vein-type deposit (i.e., jacutinga lode) in the region (Itabira), another important mineralisation style of gold. The occurrence of relatively abundant inherited detrital zircon, but absence of rock fragments in the tourmaline pocket investigated here, implies that detrital material was completely replaced by tourmaline. The graphite overprint on the altered detrital zircon attests to a reducing fluid, which was likely formed by fluid–rock interaction with carbonaceous phyllite of the Batatal Formation, the host rock of the Passagem lode.

Geochemical prospecting for rare earth elements using termite mound materials

The Blockspruit fluorite prospect, located in North West State of the Republic of South Africa, occurs within an actinolite rock zone that was emplaced into the Kenkelbos-type granite of Proterozoic age. There are a large number of termite mounds in the prospect. For geochemical prospecting for rare earth elements (REEs), in total, 200 samples of termite mound material were collected from actinolite rock and granite zones in the prospect. Geochemical analyses of these termite mound materials were conducted by two methods: portable X-ray fluorescence (XRF) spectrometry and inductively coupled plasma-mass spectrometry (ICP-MS). Comparison of the two methods broadly indicates positive correlations of REEs (La, Ce, Pr, Nd, and Y), in particular Y and La having a strong correlation. As the result of modal abundance analyses, the actinolite rock at surface mainly consists of ferro-actinolite (89.89 wt%) and includes xenotime (0.26 wt%) and monazite (0.21 wt%) grains as REE minerals. Termite mound materials from actinolite rock also contain xenotime (0.27 wt%) and monazite (0.41 wt%) grains. In addition, termite mound materials from the actinolite rock zone have high hematite and Fe silicate contents compared to those from granite zone. These relationships suggest that REE minerals in termite mound materials originate form actinolite rock. Geochemical anomaly maps of Y, La, and Fe concentrations drawn based on the result of the portable XRF analyses show that high concentrations of these elements trend from SW to NE which broadly correspond to occurrences of actinolite body. These results indicate that termite mounds are an effective tool for REE geochemical prospection in the study area for both light REEs and Y, but a more detailed survey is required to establish the distribution of the actinolite rock body.

Alluvial xenotime and heavy minerals assemblage from the northern edge of Nisa-Albuquerque Batholith, eastern Portugal: Provenance and geochemical implications

The xenotime-bearing heavy mineral assemblages present in the alluvial samples from Vila Velha de Ródão, Nisa, Póvoa e Meadas and Sto. António das Areias, localized in the northern edge of Nisa-Albuquerque Batholith, Eastern Portugal, were studied in detail. The insights for mineral provenance came from the analysis of the drainage network and from the composition of the regional alluvial samples and outcropping lithologies. Since xenotime is a mineral with economic interest, the motivation of the current work is its contribution for exploration studies, considering the potential information contained in the alluvial samples. In the study region, the alluvial heavy minerals and their morphology reflect clearly the mineralogy and relative distance to their source, respectively. In alluvial samples with direct provenance from the Nisa granite (late Carboniferous), the increase in xenotime concentration and decrease in monazite, apatite, zircon, ilmenite and iron oxide concentration, from W (Nisa) to E (Sto. António das Areias), matches the increase in P2O5 and decrease in REE, CaO, Zr, TiO2, and Fe2O3 contents in granite rocks.The geochemical signature of xenotime studied reveals an igneous source, characterized by the characteristic strong Eu negative anomaly in REE patterns. Their YPO4 values (72–78mol%) are similar to xenotime from Erzgebirge (Germany) granites. The high values of Y/Ho (50–71) and, in some cases, downward kinks at Ho in REE plots, suggest generation in late stages of crystallization and association with a siliceous evolved magmatic system. The slight decrease in HREE contents in xenotime, from W to E, and Ho anomalies in the normalized REE pattern are similar to those identified in the Nisa granite, which supports the sourcing of xenotime from these granitic rocks. The genetic development of the batholith chemical zonation (more evolved to E) seems to have favoured the generation of xenotime instead of apatite, monazite and zircon, in the eastern end area (Sto. António das Areias).Supporting the geochemical signature, the alluvial zircons also display morphological typologies that are compatible with provenance from peraluminous granites formed at temperatures of 600–700°C, possibly from the Nisa granite. Since all xenotime grains show the same magmatic affinity, this leads to the hypothesis that the xenotime from the Vila Velha de Ródão sample, with provenance from Cenozoic sedimentary rocks, has been transported to this northern area, after been disaggregated from Nisa batholith granites, most probably by braided anastomosing and erratically sandy channels, that explains its actual position in the opposite bank of the Tagus river. This phosphate has been deposited with other sediments in tectonic depressions and subsequently included in the formation of sedimentary rocks. The greater development of Ca, Th and U phosphate inclusions/substitutions in these xenotime grains can be explained by the chemical mobility provided during all the geological processes.

Carboniferous inherited grain and age zoning of monazite and xenotime from leucogranites in far-eastern Nepal: Constraints from electron probe microanalysis

Chemical Th-U-total Pb isochron method (CHIME) of monazite and xenotime from three leucogranites in far-eastern Nepal revealed the presence of Carboniferous inherited grains of monazite and intense intrusion of leucogranites around 18-16 Ma in the High Himalaya. In garnet-bearing sillimanitemuscovite- biotite leucogranite, most monazite grains have inherited cores with the chemical dates of ~504-418 and ~342-272 Ma, which were overgrown by Early Miocene mantles and rims of 17.9 ± 1.5 Ma. Early Ordoviciain and Carboniferous ages are rarely found in the same euhedral-subhedral cores. In addition to the previously recognized Early Paleozoic magmatism, monazite cores with Carboniferous ages in Early Miocene leucogranites provide evidence for two periods of magmatism at the base of the High Himalaya prior to the Cenozoic Himalayan orogeny. In muscovite-biotite leucogranite, no inherited domains were observed in monazite and xenotime grains. They yielded the CHIME monazite and xenotime dates of 16.1 ± 2.0 and 19.8 ± 6.5 Ma, respectively. Monazite grains adjacent to xenotime have significantly lower concentrations of UO2 and Y2O3 compared to those isolated from xenotime. These results imply that xenotime influences Y and U contents in monazite, reflecting local equilibrium system. In aplitic leucogranite, monazite grains yielded the mean of apparent chemical date of 18.0 ± 2.2 Ma. The CHIME monazite ages of ~18-16 Ma in three leucogranites reflect the timing of melt crystallization.

Mineral chemistry of Rare Earth Element (REE) mineralization, Browns Ranges, Western Australia

‘Green energy futures’ are driving unprecedented demand for Rare Earth Elements (REE), underpinning significant exploration activity worldwide. Understanding how economic REE concentrations form is critical for development of exploration models. REE mineralisation in the Browns Ranges, Gordon Downs Region, Western Australia, comprises xenotime-dominant mineralisation hosted within Archaean to Palaeoproterozoic metasedimentary units (Browns Range Metamorphics). Mineralogical, petrographic and mineral–chemical investigation, including trace element analysis by Laser-Ablation Inductively-Coupled Plasma Mass Spectroscopy, gives insights into the mineralogical distribution and partitioning of REE, and also provides evidence for the genetic evolution of the Browns Range REE mineralisation via a succession of hydrothermal processes.Two main REE-bearing minerals are identified: xenotime [(Y,REE)PO4], which is HREE selective; and subordinate florencite [(REEAl3(PO4)2(OH)6] which is LREE selective. Two morphological generations of xenotime are recognised; compositions are however consistent. Xenotime contains Dy (up to 6.5wt.%), Er (up to 4.35wt.%), Gd (up to 7.56wt.%), Yb (up to 4.65wt.%) and Y (up to 43.3wt.%). Laser Ablation ICP–MS element mapping revealed a subtle compositional zoning in some xenotime grains. LREE appear concentrated in the grain cores or closest to the initial point of growth whereas HREE, particularly Tm, Yb and Lu, are highest at the outer margins of the grains. The HREE enrichment at the outer margins is mimicked by As, Sc, V, Sr, U, Th and radiogenic Pb. Florencite is commonly zoned and contains Ce (up to 11.54wt.%), Nd (up to 10.05wt.%) and La (up to 5.40wt.%) and is also notably enriched in Sr (up to 11.63wt.%) and Ca. Zircon (which is not a significant contributor of REEs overall due to its low abundance in the rocks) is also enriched in REE (up to 13wt.% ΣREE) and is the principal host of Sc (up to 0.8wt.%).Early, coarse euhedral xenotime has undergone fracturing, partial breakdown and replacement by florencite. Second generation xenotime occurs as abundant small blades commonly associated with acicular hematite. Mineralization is attributed to percolation of a volatile-rich, acidic fluid, possibly granite-derived, through porous arkose units. Late hematite may suggest mixing with meteoric water and subsequent oxidation. Field observations suggest that faults acted as fluid conduits and that brecciation, possibly associated with release of volatiles from the fluid, occurred along these faults.The data provide valuable constraints on chemical compositional trends in xenotime and coexisting minerals. Given the current surge in exploration for REE, this information will assist in the development of exploration models for comparable terranes.

Phase equilibrium, geothermobarometric and xenotime age dating constraints on the Alpine metamorphism recorded in chloritoid schists from the southern part of the Tisia Mega-Unit (Slavonian Mts., NE Croatia)

The chloritoid schists from the Slavonian Mts., which are attributed to the basal part of Devonian to Permian “Hercynian Semimetamorphic Complex,” represent a very rare lithology, not only in the Tisia Mega-Unit outcrops in Croatia, but also in the wider area. The investigated outcrop in the Kutjevacka Rijeka transect (Mt. Papuk) encompasses chloritoid-bearing metapelitic and metapsammitic lithologies. Both contain K-white mica, chlorite, chloritoid (10–15 vol.%), quartz and minor K-feldspar, plagioclase (albite), opaque minerals and pyrophyllite, together with accessory zircon, rutile, xenotime. The Th–U–Pb age dating on xenotime grains within the K-white mica + chlorite + quartz matrix and on inclusions found inside the chloritoids gave an average age 120 ± 36 Ma. Peak metamorphic conditions during the Alpine chloritoid-forming event reached 3.5–4 kbar and 340–380 °C, based on phengite barometry, chlorite–chloritoid thermometry and intersection of chlorite and chloritoid isopleths in the KFMASH quantitative phase diagram. The post-tectonic character of lath- and rosette-shaped chloritoids with respect to two foliations in the rock, together with the older age of 219 ± 81 Ma obtained on Yb-rich xenotime core domain(s), implies a possible existence of older low-grade metamorphic phase(s). The chemistry of the chloritoid schists bears the signature of upper continental crustal felsic rocks as potential protoliths, probably the felsic rocks of the nearby Papuk Complex of Slavonian Mts. The evidence presented here for the chloritoid-bearing low-grade metamorphic rocks from the Slavonian Mountains clearly show that the prograde Alpine metamorphic event had a more significant influence on the evolution of the southern part of Tisia Mega-Unit than previously considered.

Constraints on the Timing of Co-Cu Au Mineralization in the Blackbird District, Idaho, Using SHRIMP U-Pb Ages of Monazite and Xenotime Plus Zircon Ages of Related Mesoproterozoic Orthogneisses and Metasedimentary Rocks

The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships observed in thin section, xeno-time and cobaltite formed during multiple episodes. The oldest age for xenotime (1370 ± 4 Ma), determined on oscillatory-zoned cores, may date the time of initial cobaltite formation, and provides a minimum age for the host metasedimentary rocks. Additional Proterozoic xenotime growth events occurred at 1315 to 1270 Ma and ca. 1050 Ma. Other xenotime grains and rims grew in conjunction with cobaltite during Cretaceous metamorphism. However, ages of these growth episodes cannot be precisely determined due to matrix effects on 206 Pb/ 238 U data for xenotime. Monazite, some of which encloses cobaltite, uniformly has Cretaceous ages that mainly are 110 ± 3 and 92 ± 5 Ma. These data indicate that xenotime, monazite, and cobaltite were extensively mobilized and precipitated during Middle to Late Cretaceous metamorphic events.

Experimental incorporation of Th into xenotime at middle to lower crustal P-T utilizing alkali-bearing fluids

In this study, a natural Th-absent xenotime [(Y+HREE)PO 4 ], is enriched in specific areas with respect to Th + Si utilizing a series of alkali-bearing fluids that included 2 N NaOH, 2 N KOH, Na 2 Si 2 O 5 + H 2 O, and NaF + H 2 O, in addition to ThO 2 and SiO 2 . Charge and fluid were sealed in Au capsules and placed in the piston-cylinder apparatus (CaF 2 assemblies) at 1000 MPa and 900 °C (8 to 25 days) or in cold-seal autoclaves on a hydrothermal line at 500 MPa and 600 °C (23 days). BSE imaging, EMP analysis, and TEM indicate that a fraction of the xenotime grains in the 2 N KOH, Na 2 Si 2 O 5 + H 2 O, and NaF + H 2 O experiments have altered areas enriched in Th + Si. No reaction was observed in the 2 N NaOH experiments. The altered areas occur as a series of curvilinear intergrowths with sharp compositional boundaries that extend from the edge of the xenotime grain into the interior. Formation of these Th + Si enriched areas is interpreted as a consequence of fluid-mediated coupled dissolution-reprecipitation.

In situ U–Th–Pb geochronology of monazite and xenotime from the Jack Hills belt: Implications for the age of deposition and metamorphism of Hadean zircons

Quartz-pebble conglomerate and quartzite at Eranondoo Hill in the Jack Hills belt in the northwestern Yilgarn Craton, Australia, contain monazite and xenotime grains in heavy mineral layers. In situ SHRIMP U–Th–Pb geochronology of detrital monazite and xenotime, as well as monazite and xenotime inclusions in detrital quartz, gives ages between 3.3 and 3.1 Ga. The conglomerates, which host abundant Hadean zircon grains, contain metamorphic monazite in the muscovite matrix that yields an age of 2653 ± 5 Ma. This date is interpreted to represent the time of low-grade metamorphism and therefore is a direct minimum age for sediment deposition. The conglomerates also contain authigenic xenotime that gives an age of 800 ± 25 Ma, which suggests that part of the succession was affected by a previously unrecognised Neoproterozoic event. Authigenic xenotime in conglomerate immediately to the east of Eranondoo Hill gives an age of 2660 ± 10 Ma. Metamorphic monazite in garnet-bearing quartz-mica schist from the middle of the belt records an age of 1858 ± 6 Ma, which likely reflects an episode of deformation and low-grade metamorphism associated with major shearing. Several monazite grains from the schist indicate that the centre of the belt also experienced metamorphism at ∼2.65 Ga. Massive magnetite–hematite ore from banded iron formation in the northeastern part of the belt contains authigenic xenotime that formed at 3080 ± 20 Ma. Co-existing monazite gives a similar age (3080 ± 35 Ma), indicating that at least part of the belt was deposited before ∼3.08 Ga. A second population of xenotime was identified, indicating a later episode of heating and fluid flow at 1820 ± 25 Ma, possibly related to reactivation during the 1.83–1.78 Ga Capricorn orogeny. The sedimentary rocks dated in this study were deposited before ∼2.65 Ga, and in some cases, before ∼3.08 Ga, indicating that much of the Jack Hills belt is Neoarchean or older. The metasedimentary rocks, and the Hadean zircons they preserve, experienced a protracted history of deformation and heated fluid flow spanning more than two billion years (3080-800 Ma).

Parageneses and Th-U distributions among allanite, monazite, and xenotime in Barrovian-type metapelites, Imjingang belt, central Korea

The paragenetic relationships and Th-U distributions among allanite, monazite, and xenotime were investigated in a progressive sequence of garnet- to kyanite-zone metapelites of the Imjingang belt, Korea. Allanite is predominant in the garnet and staurolite zones, whereas monazite and xenotime predominate in the kyanite zone. Epidote grains in the lower garnet zone are commonly zoned, from allanite (core) to relatively Y-rich, rare-earth-element (REE)-epidote to clinozoisite (rim), although both REE-epidote and clinozoisite disappear in higher-grade metapelites. Moreover, allanite and REE-epidote often contain minute inclusions of thorium silicate. The isogradic distributions and similarity of REE patterns between allanite and monazite suggest that the latter has grown at the expense of the former. In addition, the discontinuous Th zoning in monazite is apparently inherited from heterogeneous Th distribution and thorium silicate inclusions in allanite. Thus, thorium silicate possibly provided the additional Th and U necessary for the monazite formation. Paragenetic relationships of allanite and monazite inclusions within various index minerals suggest that at amphibolite-facies conditions allanite is stable at higher pressures than monazite. Xenotime grains in the staurolite zone are rarely produced by the breakdown of clinozoisite and REE-epidote, whereas those in the kyanite zone are grown primarily at the expense of garnet. Incorporation of Th and U into monazite and xenotime is governed mainly by the brabantite and thorite substitutions, respectively. Taken together, our results suggest that the allanite-to-monazite transformation is primarily responsible for the distributions of REEs, Th, and U among metapelitic phases, and that the xenotime formation was facilitated by the contribution from major silicates, particularly garnet.

In situ U–Pb dating and element mapping of three generations of monazite: Unravelling cryptic tectonothermal events in low-grade terranes

In situ U–Pb dating of monazite and xenotime in sedimentary rocks from the mid-Archean Soanesville Group in the Pilbara Craton, yields ages for provenance, diagenesis and multiple low-grade metamorphic events. Detrital monazite and xenotime grains give dates >3250 Ma, whereas diagenetic xenotime provides a new minimum age of 3190 ± 10 Ma for deposition of the basal Soanesville Group, previously constrained between ∼3235 Ma and ∼2955 Ma. Metamorphic monazite provides evidence for three episodes of growth: at 2.88, 2.16 and 1.65 Ga. Element mapping of monazite for La, Sm, Y and Th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ U–Pb dating by sensitive high-resolution ion microprobe (SHRIMP). Specifically, La and Sm distributions closely correlate with different generations of monazite. The presence of two generations in single monazite crystals highlights the need for characterizing mineral chemistry prior to geochronology. It also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire, potentially multi-aged, crystals. The ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the craton. The 2.88 Ga age coincides with a phase of regional deformation, metamorphism and gold mineralization along a major crustal lineament, whereas the most common monazite age population (at 2.16 Ga) corresponds with the migration of a foreland fold-and-thrust belt across the craton. The youngest age (1.65 Ga) coincides with an episode of tectonic reworking in the Capricorn Orogen along the southern Pilbara margin. The prolonged history of monazite growth may, in part, relate to channelized fluid flow during reactivation of long-lived N- to NE-trending crustal structures that transect the craton. Despite repeated episodes of metamorphism, the isotopic system in each generation of monazite remained unperturbed, yielding precise dates. The ability of monazite to record three separate events, and in some instances two events in a single crystal, distinguishes it from most other low-temperature mineral chronometers, which are readily reset during metamorphic overprinting. Low-temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far-field fluid flow related to tectonic processes. The previously unrecognized history of crustal fluid flow in the Pilbara Craton has implications for chemical, mineralogical and isotopic studies seeking to understand conditions on the early Earth.