Formation Of Xenotime Research Articles

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.

Rare-earth-element minerals in martian breccia meteorites NWA 7034 and 7533: Implications for fluid–rock interaction in the martian crust

Paired martian breccia meteorites, Northwest Africa (NWA) 7034 and 7533, are the first martian rocks found to contain rare-earth-element (REE) phosphates and silicates. The most common occurrence is as clusters of anhedral monazite-(Ce) inclusions in apatite. Occasionally, zoned, irregular merrillite inclusions are also present in apatite. Monazite-bearing apatite is sometimes associated with alkali-feldspar and Fe-oxide. Apatite near merrillite and monazite generally contains more F and OH (F-rich region) than the main chlorapatite host and forms irregular boundaries with the main host. Locally, the composition of F-rich regions can reach pure fluorapatite. The chlorapatite hosts are similar in composition to isolated apatite without monazite inclusions, and to euhedral apatite in lithic clasts. The U–Th-total Pb ages of monazite in three apatite are 1.0±0.4Ga (2σ), 1.1±0.5Ga (2σ), and 2.8±0.7Ga (2σ), confirming a martian origin. The texture and composition of monazite inclusions are mostly consistent with their formation by the dissolution of apatite and/or merrillite by fluid at elevated temperatures (>100 °C).In NWA 7034, we observed a monazite-chevkinite-perrierite-bearing benmoreite or trachyandesite clast. Anhedral monazite and chevkinite-perrierite grains occur in a matrix of sub-micrometer REE-phases and silicates inside the clast. Monazite-(Ce) and -(Nd) and chevkinite-perrierite-(Ce) and -(Nd) display unusual La and Ce depletion relative to Sm and Nd. In addition, one xenotime-(Y)-bearing pyrite-ilmenite-zircon clast with small amounts of feldspar and augite occurs in NWA 7034. One xenotime crystal was observed at the edge of an altered zircon grain, and a cluster of xenotime crystals resides in a mixture of alteration materials. Pyrite, ilmenite, and zircon in this clast are all highly altered, zircon being the most likely source of Y and HREE now present in xenotime. The association of xenotime with zircon, low U and Th contents, and the low Yb content relative to Gd and Dy in xenotime suggest the possible formation of xenotime as a byproduct of fluid–zircon reactions.On the basis of relatively fresh apatite grains and lithic clasts in the same samples, we propose that the fluid–rock/mineral reactions occurred in the source rocks before their inclusion in NWA 7034 and 7533. Additionally, monazite-bearing apatite and REE-mineral-bearing clasts are possibly derived from different crustal origins. Thus, our results imply the wide-occurrence of hydrothermal fluids in the martian crust at 1 Ga or older, which were probably induced by impacts or large igneous intrusions.

SHRIMP U-Pb Ages of Xenotime and Monazite from the Spar Lake Red Bed-Associated Cu-Ag Deposit, Western Montana: Implications for Ore Genesis

Xenotime occurs as epitaxial overgrowths on detrital zircons in the Mesoproterozoic Revett Formation (Belt Supergroup) at the Spar Lake red bed-associated Cu-Ag deposit, western Montana. The deposit formed during diagenesis of Revett strata, where oxidizing metal-bearing hydrothermal fluids encountered a reducing zone. Samples for geochronology were collected from several mineral zones. Xenotime overgrowths (1–30 μm wide) were found in polished thin sections from five ore and near-ore zones (chalcocite-chlorite, bornite-calcite, galena-calcite, chalcopyrite-ankerite, and pyrite-calcite), but not in more distant zones across the region. Thirty-two in situ SHRIMP U-Pb analyses on xenotime overgrowths yield a weighted average of 207 Pb/ 206 Pb ages of 1409 ± 8 Ma, interpreted as the time of mineralization. This age is about 40 to 60 m.y. after deposition of the Revett Formation. Six other xenotime overgrowths formed during a younger event at 1304 ± 19 Ma. Several isolated grains of xenotime have 207 Pb/ 206 Pb ages in the range of 1.67 to 1.51 Ga, and thus are considered detrital in origin. Trace element data can distinguish Spar Lake xenotimes of different origins. Based on in situ SHRIMP analysis, detrital xenotime has heavy rare earth elements-enriched patterns similar to those of igneous xenotime, whereas xenotime overgrowths of inferred hydrothermal origin have hump-shaped (i.e., middle rare earth elements-enriched) patterns. The two ages of hydrothermal xenotime can be distinguished by slightly different rare earth elements patterns. In addition, 1409 Ma xenotime overgrowths have higher Eu and Gd contents than the 1304 Ma overgrowths. Most xenotime overgrowths from the Spar Lake deposit have elevated As concentrations, further suggesting a genetic relationship between the xenotime formation and Cu-Ag mineralization.

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.

XENOTIME-(Y) FROM CARBONATITE DYKES AT LOFDAL, NAMIBIA: UNUSUALLY LOW LREE:HREE RATIO IN CARBONATITE, AND THE FIRST DATING OF XENOTIME OVERGROWTHS ON ZIRCON

Xenotime-(Y) is a common accessory mineral in many igneous and high-grade metamorphic rocks, but it is very rare in carbonatite. Uniquely, at Lofdal, Namibia, xenotime-(Y) occurs in many carbonatite dykes. It mantles and replaces zircon in calcite carbonatite and also occurs as aggregates in ankerite carbonatite, aggregates associated with hematite, and crystals associated with monazite-(Ce) and synchysite-(Ce) in highly oxidized iron-rich calcite carbonatites. The paragenetic sequence places the xenotime-(Y) at the end of magmatic activity and certainly into the hydrothermal stage. A U-Pb date of 765 +/- 16 Ma (2 sigma) for xenotime-(Y) overgrowths on zircon obtained by LA-ICP-MS, the first dating of fine overgrowths of xenotime on zircon by this technique, confirms that the formation of xenotime-( Y) is directly related to the crystallization of the carbonatite and provides a date consistent with published dates for Lofdal and Oas syenites. The xenotime-(Y) is heavy-REE-enriched (chondrite-normalized graphs peak at Lu) but can be distinguished from xenotime-(Y) in granitic rocks by the lack of Eu anomaly, higher Gd (reaching > 6 wt%) and lower Yb (below 4 wt%). A monazite-(Ce) - xenotime-(Y) geothermometer developed for metamorphic rocks gives possible but relatively high temperatures of > 450 degrees C for the formation of xenotime-(Y). Overall, the whole-rock compositions are light-REE-enriched, in common with most carbonatites, but the degree of light REE enrichment is less than almost all published datasets (La/Yb-n at Lofdal ranges from 1 to 70), and at 0.5-0.8 wt%, the total REE content at Lofdal is also higher than in many carbonatites. These features are most important in controlling the production of xenotime-( Y) at Lofdal. Exploration for Y in carbonatites should therefore concentrate on rocks that have REE concentrations above 2000 ppm and La/Yb values lower than 70, similar to Lofdal, as well as weathered carbonatite regoliths and carbonatites subjected to extreme hydrothermal conditions, where Y can be concentrated.

A LA-ICP-MS EVALUATION OF Zr RESERVOIRS IN COMMON CRUSTAL ROCKS: IMPLICATIONS FOR Zr AND Hf GEOCHEMISTRY, AND ZIRCON-FORMING PROCESSES

The results of ~4000 LA–ICP–MS analyses in 152 thin sections from common crustal rocks reveal that several rock-forming minerals contain tens to a few thousand ppm of Zr. The highest concentrations of Zr are found in xenotime, followed by titanite, ilmenite, rutile, allanite, amphibole, clinopyroxene, garnet, magnetite and, less commonly, plagioclase, K-feldspar and orthopyroxene. Olivine, cordierite, biotite, muscovite, apatite, epidote and monazite have low levels of Zr (<5 ppm, generally <1 ppm). The minerals with the highest K D Hf/ K D Zr are titanite (2.5), orthopyroxene (2.0), amphibole and clinopyroxene (1.8), and epidote and rutile (1.6–1.7). Ilmenite, magnetite, the feldspars and apatite have K D Hf/ K D Zr ≈1, and values less than one were found in xenotime and zircon (0.8), garnet (0.7), and allanite (0.6). The most important implications of these results follow. First, the growth of a Zr-bearing phase during partial melting does not influence the Zr concentration of the melt, but increases the fraction of zircon that can be dissolved at a given temperature. This accelerates the disappearance of zircon from the protolith or, in melts already segregated, the dissolution of inherited zircon. The effect will be more marked in metaluminous magmas precipitating amphibole and titanite than in any other type of magma. Second, the presence of Zr-bearing phases has little influence of the zircon-saturation “geothermometer”. It may cause somewhat higher (20–30°C) results in metaluminous rocks, but has no effect on peraluminous rocks. Third, the uptake of Zr by major minerals in crystallizing magmas may reduce both the concentration of Zr in the melt available to form zircon and the temperature at which zircon begins to precipitate. Mineral–melt reactions involving Zr-bearing phases may lead to zircon grains with complicated patterns of zoning and texturally discordant zones, apparently diachronous. Fourth, higher-than-chondrite Zr/Hf fractionates may arise from titanite, amphibole or clinopyroxene fractionation, but this requires very little or no crystallization of zircon. Significantly lower-than-chondrite Zr/Hf magmas only result from zircon fractionation. Lastly, two new examples of mineral reactions that involve the formation of a mass-balancing accessory phase, useful for high-resolution geochronology, are described: the formation of xenotime as a product of the breakdown of garnet in amphibolite-grade metapelites, and the subsolidus growth of a new rim on zircon included in Zr-bearing feldspars.

Uraniferous diagenetic xenotime in northern Australia and its relationship to unconformity-associated uranium mineralisation

Stratabound, uraniferous diagenetic xenotime cements provide a minimum depositional age of 1,632±3 Ma for the sedimentary Birrindudu Group in the Killi Killi Hills, Tanami Region in northern Australia. The age of xenotime formation is broadly coeval with that recently proposed (1,650–1,600 Ma) for uranium mineralisation in the unconformity-associated deposits of the Pine Creek Inlier, northern Australia, and Athabasca Basin, Canada. The geological setting and formation model for the uraniferous xenotime crystals are similar to those widely proposed for unconformity-associated uranium deposits, suggesting a genetic link between the two. However, xenotime formation in the Birrindudu Group occurred during an apparently earlier stage of diagenesis, compared to late diagenetic formation of unconformity-associated uranium deposits. This could be explained by variations in the thickness of sediment cover and diachronous diagenesis across the basin, at the time of the basin-wide uranium mobilisation event, herein dated at ca. 1,630 Ma. In such a scenario, stratabound uraniferous xenotime cements could represent the remote distal zones of a more deeply buried, uranium mineralising system. Alternatively, the xenotime layer represents a precursor to, or a source for, later unconformity-associated ore deposition. In this case, the presence of diagenetic uraniferous xenotime in an area prospective for unconformity-associated uranium mineralisation would be an indication of, and still provide an approximate age for, uranium mobilisation within the cover sequence. Xenotime is a far more robust mineral than uraninite for U–Pb geochronology and can potentially provide a more reliable and precise timeframe for uranium mineralisation and subsequent recrystallisation events if present in the immediate uranium-ore environment.

Zircon growth in very low grade metasedimentary rocks: evidence for zirconium mobility at ~250°C

Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimentary rocks worldwide, ranging in age from mid-Archaean to Palaeozoic. The outgrowths comprise minute (typically <3 μm) crystals that form an irregular fringe on detrital zircon grains, and in a few cases, on diagenetic xenotime outgrowths. Textural relationships indicate that while zircon growth postdates diagenetic xenotime precipitation, it precedes or is synchronous with metamorphic xenotime formation. Unlike xenotime, zircon outgrowths are absent in unmetamorphosed sedimentary rocks, and only appear in prehnite-pumpellyite facies rocks, suggesting that zircon growth commences at temperatures of ∼250°C. The greater abundance of zircon outgrowths in shales than in to other sedimentary rocks may relate to higher halogen concentrations, which have been linked to enhanced zirconium mobility in hydrothermal systems. The growth of zircon in metasedimentary rocks indicates that zirconium was transported in aqueous fluids, possibly as fluorine complexes, during very low-grade metamorphism.

The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany; Part II, Xenotime

Other| December 01, 1998 The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany; Part II, Xenotime Hans-Juergen Foerster Hans-Juergen Foerster GeoForschungsZentrum Potsdam, Potsdam, Federal Republic of Germany Search for other works by this author on: GSW Google Scholar American Mineralogist (1998) 83 (11-12_Part_1): 1302–1315. https://doi.org/10.2138/am-1998-11-1219 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Hans-Juergen Foerster; The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany; Part II, Xenotime. American Mineralogist 1998;; 83 (11-12_Part_1): 1302–1315. doi: https://doi.org/10.2138/am-1998-11-1219 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Xenotime, from a geochemically heterogeneous series of mildly to strongly peraluminous granites of the Erzgebirge, Germany, displays extended compositional variability with respect to abundances of HREE, Y, U, and Th. With few exceptions, the maximum and minimum concentrations for the lanthanides and actinides exceed those noted for this mineral from other geologic environments. Xenotime chemistry is dominated by the theoretical end-members HREE-PO 4 and YPO 4 (>90 mol% in total). Typical xenotime grains from the Erzgebirge granites and from other granites worldwide contain 70-80 mol% YPO 4 , and 16-25 mol% HREE-PO 4 . This study documents the occurrence of xenotime, abnormally rich in HREE in place of Y, with up to 45 mol% HREE-PO 4 . Substitutions of thorite-coffinite, (Th,U,Pb)SiO 4 , brabantite, (Ca,Th,U)(PO 4 ) 2 , and monazite, (La-Sm)PO 4 , are of minor importance, reaching maximum levels of 5-6 mol% each with a total contribution typically <<10 mol%. Typically, the incorporation of actinides in xenotime from these and other granites, as well as from other rocks, is dominated by thorite-coffinite substitutions. However, in some xenotime grains, the U and Th concentrations are largely accounted for by the substitution mechanism 2 (REE,Y) (super 3+) <-->(Th,U) (super 4+) +Ca (super 2+) . The total lanthanide and actinide contents in xenotime and host granite are not strongly correlated. However, formation of xenotime unusually rich in HREE occurs only in the A-type Limica granites that are strongly enriched in these elements. The shapes of HREE patterns of xenotime and host rock are similar which, combined with mass-balance calculations, indicate the importance of this mineral in affecting the HREE as well as Y evolution of late-stage granitic melts. Late xenotime may show strong Y/Ho fractionation, thus recording the decoupling of Y and the HREE, which can occur in the latest stages of crystallization of evolved granites and which may be associated with deuteric alteration. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Rare earth element mobility in the Roffna Gneiss, Switzerland

The Roffna Gneiss, a deformed Hercynian granite porphyry within the Penninic nappes of eastern Switzerland, underwent extreme cataclasis with the progressive development of phengite towards the margins of the nappe under conditions of the glaucophane schist to greenschist facies. This resulted in the selective mobilization of major and trace elements over distances of 10's to 100's of meters and the resetting of the Rb — Sr whole rock isotopic systems some 100 my ago. The component ratios and compositionvolume relationships of progressively deformed gneiss samples studied here suggest that this process was essentially isovolumetric. The mineralogy of the deformation sequence appears to have been controlled by a reaction involving the breakdown of microcline, albite and biotite and the formation of phengite and quartz. The fluids introduced Mg and H2O, promoting the development of phengite, and removed the Na being released by the breakdown of albite. The fluids were most probably derived from the surrounding Triassic carbonates and quartzites. These relatively high fO2 and carbonate rich fluids also introduced rare earth elements (REE) into the gneiss. The gneiss was progressively enriched in Eu up to 60%, Y up to 40%, and Yb up to 100%. These enrichments are associated with the development of epitaxial xenotime around zircon in the most phengite-rich sample. While the REE were mobile, uranium and thorium were essentially immobile. The formation of xenotime was suggested to explain the observed heavy REE enrichment when large differences in the REE contents were found for replicate analyses using HF and then lithium metaborate for dissolution. These differences arose because xenotime, like monazite, can be difficult (if not impossible) to dissolve in hydrofluoric acid. Due to the possibility of incomplete sample dissolution, we now recommend fusion with lithium metaborate for all REE, Lu — Hf or Sm — Nd studies.