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Hydrothermal veins associated with the HREE mineralization. a Quartz–xenotime vein crosscutting a preexisting quartz vein. b Quartz–xenotime vein being cut by a smoky quartz vein. c Smoky quartz vein cutting a quartz–xenotime vein. d Minor fault dislocations of a quartz–xenotime vein. e Multiple generations of late milky quartz veins. f Late breccia containing milky quartz in the matrix. Q = quartz; X = xenotime
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Metasedimentary rock-hosted heavy rare earth element (HREE) mineralization occurs as numerous orebodies distributed across a large district of the Tanami region of central Australia, close to a regional unconformity between Archean metasedimentary rocks of the Browns Range Metamorphics (BRM) and overlying Proterozoic Birrindudu Group sandstones. Th...
Citations
... Concerns about the security of supply of REE, mainly due to the strong geographic concentration of mining and processing activities (Patrahau et al. 2020), have led to an increase in exploration activities worldwide in an attempt to diversify the source (e.g., Goodenough et al. 2016). While REE exploration and production since the 1960's has been focussed on rocks containing REE-carbonate minerals such as bastnäsite, there is again an increasing interest in REE-phosphates such as monazite and xenotime in both igneous, hydrothermal, and sedimentary rocks (and placers) as REE resources (Santana et al., 2015;Goodenough et al. 2016;Chen et al. 2017;Marien et al. 2018;Anenburg et al. 2018;Nazari-Dehkordi et al. 2020). ...
To support the role of proximal and remote sensing in geological rare earth element (REE) resource exploration, we studied the reflectance spectroscopy of synthetic single- and mixed-REE phosphate phases. Synthesis yielded monazite for the elements La to Gd, and xenotime for Dy to Lu and Y. Visible-to-shortwave infrared (350–2500 nm) reflectance spectra of synthetic single-REE monazites and xenotimes can be used to identify the ions responsible for the absorption features in natural monazites and xenotimes. Nd³⁺, Pr³⁺ and Sm³⁺ produce the main absorption features in monazites. In natural xenotime, Dy³⁺, Er³⁺, Ho³⁺ and Tb³⁺ ions cause the prevalent absorptions. The majority of the REE-related absorption features are due to photons exciting electrons within the 4f subshell of the trivalent lanthanide ions to elevated energy levels resulting from spin-orbit coupling. There are small (< 20 nm) shifts in the wavelengths of these absorptions depending on the nature of the ligands. The energy levels are further split by crystal field effects, manifested in the reflectance spectra as closely spaced (∼ 5–20 nm) multiplets within the larger absorption features. Superimposed on the electronic absorptions are vibrational absorptions in the H2O molecule or within [OH]⁻, [CO3]²⁻ and [PO4]³⁻ functional groups, but so far only the carbonate-related spectral features seem usable as a diagnostic tool in REE-bearing minerals. Altogether, our study creates a strengthened knowledge base for detection of REE using reflectance spectroscopy and provides a starting point for the identification of REE and their host minerals in mineral resources by means of hyperspectral methods.
... Unlike most other REE ore styles, mineralisation is distinctively HREE rich, is definitively of low temperature (i.e., T < 300 • C) hydrothermal origin, and is hosted in metasedimentary rocks with no apparent association with alkaline magmatism (Nazari-Dehkordi et al., 2017;Nazari-Dehkordi et al., 2018;Nazari-Dehkordi et al., 2020). The REEs are derived from basement metasedimentary rocks of the Browns Range Metamorphics (BRM; Nazari-Dehkordi et al., 2017), yet how these elements are mobilised and concentrated to form orebodies remains a mystery, especially considering that REE-rich minerals, and particularly HREE-rich minerals, are known to be highly insoluble in low temperature hydrothermal fluids (Migdisov et al., 2016;Williams-Jones, 2015). ...
... The mineralisation is reported to have formed between 1.65 and 1.60 Ga at a time of tectonic quiescence, and with no links to contemporaneous magmatism or metamorphism (ca. 1.83 to 1.72 Ga local metamorphism; Nazari-Dehkordi et al., 2020). Samples of the BRM are variably depleted in (H)REE compared to the sedimentary protoliths. ...
... The 1st percolation threshold (from Nasdala et al., 2004) is where aperiodic domains in the zircon structure become interconnected, and the threshold for solubility increase is taken from Ewing et al. (2003). The mineralisation age is taken from Nazari-Dehkordi et al. (2020). As represented by the orange stars, the zircons in all three scenarios would be highly amenable to fluid leaching at the timing of mineralisation (1.65 to 1.60 Ga; Nazari-Dehkordi et al., 2020). ...
... Benefiting from further improvements in dating techniques, direct U-Pb dating can be applied to certain REE-rich, U-and Th-bearing minerals like monazite, xenotime, bastnäsite, and allanite Li et al., 2019;Zhang et al., 2019;Nazari-Dehkordi et al., 2020;Feng et al., 2022;Huang et al., 2022a). Being the main economic minerals in REE ore deposits, these minerals can constrain the REE mineralization age, therefore providing new insights into the genesis of REE mineralization (Sal'nikova et al., 2010;Ling et al., 2016;Zhang et al., 2019;Feng et al., 2022). ...
The Huanglongpu deposit is one of the most representative igneous carbonatite-type molybdenum-rare earth element (Mo-REE) deposits in the eastern Qinling metallogenic region. The molybdenum resources exceed 180 kt, whilst the total rare earth resources are about 37.8 kt. However, the age of REE mineralization, sources of ore-forming minerals, and tectonic setting are not well characterized. Therefore, this study carried out detailed mineralogical, geochronological, and geochemical analyses to provide new insights. Monazite, bastnäsite, parisite, and xenotime were found to be the dominant REE-bearing minerals in the Huanglongpu deposit. LA-ICP-MS U-Pb dating of the REE minerals revealed a two-stage REE mineralization: monazite formed at 219.4 Ma, whereas bastnäsite and xenotime formed at 145.9 Ma and 142.6 Ma, respectively. The carbonatites, as the main ore host, were characterized by a strong enrichment of light rare earth element (LREE) and moderately high heavy rare earth element (HREE) contents. Monazite showed different Nd isotopic compositions, with lower εNd(t) values ranging between -7.12 and -4.18, while the εNd(t) values of bastnäsite and xenotime ranged from -9.57 to -6.49 and from -11.13 to -6.28, respectively. These contrasting geochemical and isotopic signatures suggest that the Huanglongpu REE mineralization occurred over multiple magmatic-hydrothermal events and that the ore-forming material was unlikely to have stemmed from a single enriched mantle (EMI) source. The early REE mineralization stage (219.4 Ma) was closely associated with carbonatite-related Mo mineralization, both of which were products of magmatic mineralization during the crystallization of the host carbonatite emplaced in an extensional environment after the Late Triassic collision. The ore-related magmas may have originated from a hybrid source composed of EMI and partial melting of the lower crust. In contrast, due to the extensive Yanshanian magmatism which could provide massive heat, the later REE mineralization stage (145.9–142.6 Ma) in the Huanglongpu deposit was likely driven by the reactivation and recrystallization of early-formed REE minerals within the host carbonatites, hence representing a hydrothermal redistribution of the primary magmatic ore.
... These metals are in increasing demand due to their unique applications (Naumov, 2008;McLemore, 2011;Massari and Ruberti, 2013); their supply is highly concentrated in a few countries, resulting in their classification as critical minerals (Ram et al., 2019;Jowitt et al. 2020). Hydrothermal processes play an important role in the enrichment of REE (Migdisov et al., 2016), either in conjunction with magmatic processes (e.g. the world's largest REE deposit at Bayan Obo, China; Smith et al., 2000) or in purely hydrothermal settings over a wide range in conditions (e.g., the Iron Oxide Copper Gold (IOCG) deposit at Olympic Dam, South Australia; Bastrakov et al., 2007;Xing et al. 2019); or the low temperature (<125°C), basinal-brine related deposits of the Browns Ranges, Northern Australia (Richter et al., 2018;Nazari-Dehkordi et al., 2020). The near universal occurrence of the REE at low, but detectable levels, together with slight, systematic differences in their responses to changing physico-chemical conditions, make them powerful petrologic indicators for a wide variety of geological environments (Brugger et al., 2008;Williams-Jones et al., 2012;Migdisov et al. 2016). ...
Rare Earth elements (REE) are gaining importance due to their increasing industrial applications and usefulness as petrogenetic indicators. REE-sulfate complexes are some of the most stable REE aqueous species in hydrothermal fluids, and may be responsible for REE transport and deposition in a wide variety of geological environments, ranging from sedimentary basins to magmatic hydrothermal settings. However, the thermodynamic properties of most REE-sulfate complexes are derived from extrapolation of ambient temperature data, since direct information on REE-sulfate complexing under hydrothermal conditions is only available for Nd, Sm and Er to 250 °C (Migdisov and William-Jones, 2008, 2016).
We employed ab initio molecular dynamics (MD) simulations to calculate the speciation and thermodynamic properties of yttrium(III) in sulfate and sulfate-chloride solutions at temperatures and pressures up to 500 °C and 800 bar. The MD results were complemented by in situ X-ray absorption spectroscopy (XAS) measurements. Both MD and XAS show that yttrium(III) sulfate complexes form and become increasingly stable with temperature (≥200 °C). The MD results also suggest that mixed yttrium-sulfate-chloride complexes (that cannot be distinguished from mixtures of chloride and sulfate complexes in XAS experiments) form at ≥350 °C. Two structures with two different Y(III)-S distances (monodentate and bidentate) are observed for Y(III)-sulfate bonding. The formation constants, derived via thermodynamic integration, for the Y(III) mono- and di-sulfate complexes parallel the trends for those of Nd, Sm and Er determined experimentally to 250 °C.
The derived formation constants were used to fit revised Helgeson-Kirkham-Flowers equation-of-state parameters that enabled calculation of formation constants for Y(SO4)⁺ and Y(SO4)2⁻ over a wide range of temperatures and pressures. The presence of sulfate increases the solubility of Y(III) under specific conditions. Since the stability of sulfate is redox sensitive, Y(III) solubility becomes highly redox-sensitive, with rapid precipitation of Y minerals upon destabilisation of aqueous sulfate.
... Previous studies of these deposits have focused on the origins of the carbonatite, field observations, petrology, mineralogy, geochemistry (Yuan et al., 1995;Hou et al., 2006;Hou et al., 2009;Hou et al., 2015;Liu and Hou, 2017;Jia and Liu, 2020), and fluid inclusions (Niu and Lin, 1995;Niu et al., 1996;Xie et al., 2009;Xie et al., 2015;Shu and Liu, 2019;Shu et al., 2020). Previous studies have shown that REE mineralization occurred from the magmatic to hydrothermal stages, although the late hydrothermal stage was the main REE mineralization stage (Salvi and Williams-Jones, 1996;Sheard et al., 2012;Liu and Hou, 2017;Liu et al., 2019a;Nazari-Dehkordi et al., 2019). For the magmatic to hydrothermal stages, radiogenic (e.g., Sr, Nd, and Pb; Hou et al., 2015;Liu and Hou, 2017) and traditional (e.g., C, H, O, and S; Hou et al., 2015;Liu and Hou, 2017;Yang et al., 2017; and non-traditional stable isotopes, such as Zn (Zhai, 2019), Fe (Sun et al., 2013;Chikanda et al., 2019), and Li , have been used to constrain the sources of the ore-forming components (e.g., fluids), mineralization temperatures, and evolution of the mineralizing system in the Bayan Obo Fe-Nb-REE deposit and Cenozoic MD REE belt. ...
Carbonatite-related rare earth element (REE) deposits are the most significant source of REEs worldwide. The processes of REE precipitation, enrichment, and mineralization remain controversial. The Cenozoic Mianning–Dechang (MD) REE belt, located in Sichuan Province, southwestern China, comprises one giant (Maoniuping), one large (Dalucao), and two small–medium (Muluozhai and Lizhuang) deposits. These deposits provide a continuous record of fluid evolution, and thus are ideal for investigating the processes of REE mineralization in carbonatite-related REE deposits. Given that sulfate (i.e., barite and celestite) and sulfide (i.e., pyrite and galena) minerals crystallized and precipitated in the pegmatitic to hydrothermal stages, respectively, the REE minerals formed later than the sulfate minerals. However, the formation sequence of the sulfide minerals and bastnäsite is unclear, although both pyrite and bastnäsite formed in the late hydrothermal stage. We used S isotope data for sulfate and sulfide minerals and Fe isotope data for pyrite to investigate the composition and evolution of ore-forming fluids during the magmatic–hydrothermal stages. The sulfate minerals have positive δ³⁴SCDT values (+3.2‰ to +8.3‰), and the sulfide minerals have negative δ³⁴SCDT values (−13.5‰ to − 5.6‰) in the four REE deposits. In the Maoniuping deposit, δ³⁴SCDT values for barite from the pegmatitic stage (+4.7‰ to +5.7‰) are higher than for barite from the hydrothermal stage (+4.1‰ to +4.5‰), which indicate that hydrothermal activity led to relative enrichment in isotopically light S. The δ³⁴SCDT values for barite (+3.2‰ to +5.5‰) are lower than for celestite (+6.2‰ to +7.2‰) from the pegmatitic stage in the Dalucao deposit. The δ³⁴SCDT values for galena (−13.5‰) are also lower than for pyrite (−13.5‰ to −7.2‰) from the hydrothermal stage in the Guangtoushan section. In general, δ³⁴SCDT values change from positive to negative values (+8.3‰ to −16.4‰) as the fluids evolved from the pegmatitic to hydrothermal stages, which can be attributed to a decrease in oxygen fugacity (fO2) and addition of sediment containing isotopically light S. Iron isotopic compositions of pyrite from the hydrothermal stage show significant variations (δ⁵⁶FeIRMM-014 = −0.03‰ to +0.65‰ for the Maoniuping deposit; −0.14‰ to 0.00‰ for the Dalucao deposit; +0.05‰ to +0.35‰ for the Lizhuang deposit), and are higher than those for the carbonatites (δ⁵⁶Fe IRMM-014 = −0.47‰ to −0.17‰). These data indicate there are two sources of Fe in the MD REE belt, which are the carbonatite–nordmarkite magma and ⁵⁶Fe-rich sediment. Paleozoic–Mesozoic volcanic–sedimentary and Mesozoic clastic and carbonate rocks are exposed in the MD REE belt. In general, the S–Fe isotope data, along with geological and petrographic observations, indicate that the REE minerals formed later than the sulfate minerals, and the S–Fe were derived from both carbonatite magma and sediment containing isotopically light S and heavy Fe.
... Global REE resources are mostly associated with carbonatitic and alkaline igneous rocks (e.g., Salvi et al., 2000;Schmitt et al., 2002;Castor, 2008;Yang et al., 2009;Sheard et al., 2012;Ling et al., 2013;Fan et al., 2020), which typically lack minerals that are commonly used for dating (e.g., zircon). With the rapid development of new dating techniques, some REE ore minerals have recently been utilized for U-Pb dating, such as bastnäsite, monazite, allanite, and xenotime (e.g., Cox et al., 2003;Yang YH et al., 2014b;Li et al., 2019;Zhang et al., 2019;Nazari-Dehkordi et al., 2020). However, these REE minerals are generally susceptible to late hydrothermal or tectono-thermal resetting (e.g., Schoneveld et al., 2015;Slezak and Spandler, 2019;Zhang et al., 2019;Spandler et al., 2020). ...
The Gansha Obo deposit is a recently discovered rare earth element (REE) deposit in northwestern China, with REE2O3 reserves of 0.6 Mt at 1.39–1.65 wt.%. It is hosted in the Early Cretaceous Gansha Obo alkaline–carbonatite igneous complex. In this study, we present a detailed investigation of the carbonate mineral assemblage in the economic carbonatite veins in the Gansha Obo REE deposit, including the mineral textures, geochemical compositions, and U–Pb ages. The mineral textures and compositions show that bastnäsite is the primary ore mineral, while parisite and synchysite are both secondary. In situ U–Pb dating of bastnäsite and synchysite yielded ages of 141.8 ± 4.3 Ma (n = 41; MSWD = 0.53) and 53.3 ± 4.4 Ma (n = 85; MSWD = 1.7), respectively. This significant interval of ages cannot be explained by continuous magmatic–hydrothermal activity. The bastnäsite U–Pb age reflects the timing of REE mineralization in the deposit. In contrast, the synchysite U–Pb age records a later tectono-thermal event that might have been related to Eocene collision between the Indian and Eurasian plates. Our results show that REE carbonate U–Pb dating is a powerful tool for understanding the precipitation of REE minerals and, potentially, for identifying regional tectono-thermal events.
... Most of the orebodies occur as stockworks of hydrothermal veins and breccias (up to 400 m in lateral extent and 10 m in width) within steeply-dipping faults and shear zones in the Browns Range Metamorphics. The ore minerals are xenotime and minor florencite, which both occur in several generations, together with quartz, hydrothermal white mica and hematite (Cook et al., 2013;Nazari-Dehkordi et al., 2020). Currently defined resources of 9.19 Mt. of ore grading at 0.66% TREO (Northern Minerals Ltd., 2019) are contained within the seven discrete orebodies, and it is likely that further resources will be defined in the near future. ...
... Unconformity-related deposits have no clear links to magmatism or mantle sources, but nonetheless also formed distal to active orogenesis and have a strong structural control on ore distribution (Nazari-Dehkordi et al., 2018;Nazari-Dehkordi et al., 2020). The broad tectonic setting of skarn and IOCG-related REE deposits remains highly debated (e.g., Reid, 2019), a situation further complicated when considering the essential role of local hydrothermal processes for mineralisation. ...
Australia is host to a diverse range of rare earth element (REE) ore deposits, and therefore is well placed to be a major supplier of REE into the future. This paper presents a review of the geology and tectonic setting of Australia's hard-rock REE resources. The deposits can be classified into four groups: 1. Carbonatite associated; 2. Peralkaline/alkaline volcanic associated; 3. Unconformity related, and; 4. Skarns and iron-oxide‑copper‑gold (IOCG) related. With the exception of the unconformity related deposits, all of these deposit groups are directly or indirectly related to continental alkaline magmatism. Extensive fractional crystallisation and/or igneous accumulation of REE minerals were essential ore-forming processes for carbonatite-associated and peralkaline/alkaline volcanic-associated deposits, while hydrothermal transport and concentration of REE sourced from basement rocks was responsible for producing ore in unconformity-related, skarns and, potentially, IOCG deposits. The economic potential of many deposits has also been enhanced by supergene alteration processes.
All of Australia's REE deposits formed in an intracontinental setting in association with crustal-scale fault zones or structures that acted as transport conduits for ore-forming magmas or fluids. Most deposits formed in the Neoproterozoic under conditions of relative tectonic quiescence. There is little evidence for the involvement of mantle plumes, with the exception of the Cenozoic peralkaline volcanic systems of eastern Australia, and possibly the IOCG deposits. Instead, ore productive magmas were generated by melting of previously-enriched mantle lithosphere in response to disruption of the lithosphere-asthenophere boundary due to fault activation. REE minerals in many deposits also record episodes of recrystallisation/resetting due to far-field effects of orogenic activity that may significantly postdate primary ore formation. Therefore, REE orebodies can be effective recorders of intracontinental deformation events.
In general, Australia's inventory of REE deposits is similar to the global record. Globally, the Mesoproterozoic appears to be a particularly productive time period for forming REE orebodies, due to favourable conditions for generating ore-fertile magmas, and favourable preservation potential due to a general lack of aggressive continental recycling (i.e., active plate tectonics).
... In all cases, the mineralization is characterized by a simple ore mineralogy of xenotime-(Y) and florencite-(Ce), and consists primarily of quartzxenotime-(Y) veins and breccias accommodated in subvertical faults, surrounded by a halo of low grade ore extended into the host rock. The largest deposit, Wolverine, is a steeply (~75°N) dipping planar orebody, up to 5 m wide, that extends over 400 m in strike length and from the surface to at least 550 m in depth (Nazari-Dehkordi et al. 2019). The orebody lies within a WNW-striking fault in association with an intersecting N-E trending fault. ...
... The earliest stages of the HREE mineralization are represented by xenotime-(Y) formed in highly brecciated zones and quartz-xenotime-(Y) veins, herein referred to as "breccia-hosted" (Figs. 2a and b), and "veintype" (Fig. 2c) mineralization, respectively (see also Nazari-Dehkordi et al. 2019). These early stages of xenotime-(Y) (early xenotime) were subsequently overgrown by euhedral dipyramidal-shaped xenotime-(Y) (late xenotime; Fig. 2d). ...
... The early and late florencite-(Ce) from the Wolverine and Area 5 deposits sit within the compositional field defined by the APS minerals near the florencite end-member (Fig. 7a). These compositions are similar to florencite-rich APS minerals from unconformity U deposits (Fig. 7b), which adds to the list of geological similarities between HREE mineralization (e.g., North Australian HREE+Y mineral field) and U mineralization in unconformityrelated settings (see Nazari-Dehkordi et al. 2019). The composition of the APS minerals can be also used as an indicator of ƒO 2 -pH conditions during mineral formation (Kister et al. 2005(Kister et al. , 2006. ...
This study investigates the paragenesis and ore mineral composition of xenotime [(Y,HREE)PO4] and florencite [LREEAl3(PO4)2(OH)6] from heavy rare earth element (HREE) deposits/prospects of the Tanami and Hall Creek regions of Western Australia. Two stages of xenotime-(Y) formation are recognized: (1) early xenotime-(Y) in breccias (breccia-hosted) and in quartz-xenotime-(Y) veins (vein-type); and (2) late xenotime-(Y) that occurs largely as dipyramidal-shaped overgrowths on the pre-existing early xenotime-(Y). Similarly, florencite-(Ce) formed in two stages including: (1) early florencite-(Ce) that coexists with and is enclosed by early xenotime-(Y) within mineralized veins; and (2) late florencite-(Ce) that replaces early xenotime-(Y), or appears as narrow rims on early florencite-(Ce). All xenotime-(Y) types from a number of examined HREE deposits/prospects are characterized by elevated U contents and marked negative Eu anomalies that we interpret to be inherited from the metasedimentary rocks from which REE and P required for the phosphate ore mineralization, were sourced. Compared to the early xenotime-(Y), the late xenotime-(Y) is richer in HREE and depleted in P, owing to the formation of the coexisting late florencite-(Ce). Moreover, early florencite-(Ce) has a near end-member florencite (s.s.) composition similar to those associated with unconformity-related U deposits, whereas late florencite-(Ce) sits on the florencite-svanbergite compositional spectrum. The high U content of xenotime-(Y) and composition of early florencite-(Ce) potentially support a genetic association between the HREE mineralization and the coeval unconformity-related U deposits of northern Australia. Nevertheless, we also urge for caution in using xenotime-(Y) composition in isolation as an indicator of geological setting.
... This study focuses on two of the larger deposits, namely Wolverine and Area 5, from which fluid inclusion samples and the bulk of the oxygen isotope separates were collected. Detailed characteristics of the major HREE deposits/prospects within and around the BRD are provided in Nazari-Dehkordi et al. (2019), and hence only a brief summary is presented here. ...
... There are three major mineral assemblages associated with the mineralisation and the metasedimentary host rocks across the NAHREY mineral field. A detailed ore mineral paragenesis is reported in Nazari-Dehkordi et al. (2019). ...
... 1090 cm −1 (Fig. 5E). This peak position is characteristics of carbonate minerals (see Frezzotti et al. (2012)), although the fluid inclusions do not contain CO 2 , as shown by the Raman analyses, and carbonate minerals are entirely absent from the ore assemblage (see Nazari-Dehkordi et al. (2019)). Therefore, a carbonate composition for the solid phases is deemed unlikely. ...
This study reports on fluid inclusion and oxygen isotope compositions of mineralised and barren hydrothermal quartz veins and hosting metasedimentary rocks associated with the heavy rare earth element (HREE) mineralisation in the Browns Range Dome of the Tanami Region, Western Australia. The HREE mineralisation consists of quartz and xenotime-bearing hydrothermal veins and breccias that occurs along sub-vertical faults within Archean to Paleoproterozoic metasedimentary rocks. Based on analysis of nearly 550 quartz-hosted primary fluid inclusions, three fluid inclusion types were identified in the mineralised samples: type I low-salinity H2O-NaCl (largely <5 wt.% NaCl; consistent with meteoric water), type II medium-salinity H2O-NaCl (12-18 wt.% NaCl), and type III low- to high-salinity H2O-CaCl2-NaCl (1 to ca. 24 wt.% NaCl+CaCl2). Homogenisation temperatures of all fluid inclusion types vary over a relatively wide range from 100 to 250 °C. Barren quartz veins contain only type I low-salinity H2O-NaCl fluid inclusions, with homogenisation temperatures extending from 170 to 350 °C. Raman analyses of all three fluid inclusion types confirmed their aqueous nature with no carbon-bearing fluid species identified. The three fluid inclusion types indicate mixing of three hydrothermal fluids: a low-salinity H2O-NaCl meteoric fluid (< 5 wt.% NaCl), a medium-salinity H2O-NaCl (12-18 wt.% NaCl) fluid, and a high-salinity H2O-CaCl2-NaCl (ca. 24 wt.% NaCl+CaCl2) fluid.
Limited LA-ICP-MS analysis found detectable Y, Ce, U and Cl only in the type III fluid inclusions, which indicates that transport of ore metals was (at least partly) by Cl complexes in the type III fluid. The δ18Ofluid values calculated from quartz from mineralised samples are in the range defined by the Archean metasedimentary host rocks of the Browns Range Metamorphics (δ18Ofluid = +1.8 to +5.2‰) and the unconformably-overlying Paleoproterozoic Birrindudu Group sandstones (δ18Ofluid = +8‰). Collectively, our fluid inclusion and oxygen isotope data, together with other field, mineralogical and geochemical data, support an ore genesis model involving mixing of the three hydrothermal fluids in fault zones and along unconformity surfaces in, and around, the Browns Range Dome. The meteoric low-salinity H2O-NaCl fluid potentially carried P from the Birrindudu Group sandstones, and the high-salinity H2O-CaCl2-NaCl fluid leached HREE+Y from metasedimentary rocks of the Browns Range Metamorphics. Ore deposition occurred following mixing of the P-bearing and HREE+Y-bearing fluids, and was associated with a widespread white mica alteration. The temperature and pressure during the fluid-fluid mixing and mineralisation was between 100 and 250 °C, and 0.4 and 1.6 kbar, respectively.
Rapidly searching for carbonatite-related rare earth element (REE) mineralized systems is of paramount importance to explorers, because these systems provide the world’s most significant supply of REE. Biotite and barite are suitable indicator minerals that can offer reliable elemental geochemical information used for REE exploration. In this study, we provided an elemental geochemical dataset of biotite and barite from Cenozoic REE deposits in the eastern Tibetan Plateau, and compared the obtained data to those of biotite and barite forming in other geological settings. The F (i.e., fluorine) contents in carbonatite-hosted biotites are highly variable, with biotites from mineralized carbonatite complexes having much higher values (1.05–2.37 apfu, where apfu = atoms per formula unit) than their counterparts from barren carbonatite complexes (≤ 1 apfu). It is therefore believed that the presence of high-F biotites can be regarded as a sign for predicting REE reservoirs. Barites in REE mineralized systems contain much higher (even one or to orders of magnitude) total REE contents (ΣREE >100 μg/g) relative to those species forming in carbonate-hosted fluorite-barite deposits and open marine environments. This means that the high ΣREE concentration of barite can serve as an effective index for REE exploration. The above information is integrated into a REE exploration program in which the areas with strong local tectonism along cratonic edges are priority exploration targets, and high-F biotites and REE-rich barites can serve as direct indices for rapidly identifying carbonatite-related REE mineralized systems at a strategic distance from other geological settings.
