Garnierites and garnierites: Textures,mineralogy and geochemistry of garnierites in the Falcondo Ni-laterite deposit,Dominican Republic

Garnierites (Ni–Mg-bearing phyllosilicates) are significant ore minerals in Ni-laterites of the hydrous silicate-type. In the Falcondo Ni-laterite deposit (Dominican Republic), garnierites are found within the saprolite horizon mainly as fracture-fillings and thin coatings on joints. Field observations indicate an important role of active brittle tectonics during garnierite precipitation. Different greenish colours and textures can be distinguished, which correspond to different mineral phases, defined according to X-ray diffraction (XRD) and electron microprobe (EMP) analyses: a) talc-like (10 Å-type), b) serpentine-like (7 Å-type), c) a mixture of talc- and serpentine-like, and d) sepiolite-like types. Compositional data indicate continuous Mg–Ni solid solution along the joins lizardite–népouite (serpentine-like), kerolite–pimelite (talc-like) and sepiolite–falcondoite (sepiolite-like). In general, talc-like garnierite is dominant in Falcondo Ni-laterite and displays higher Ni contents than serpentine-like garnierites. EMP analyses showing deviations from the stoichiometric Mg–Ni solid solutions of serpentine and talc are best explained by talc- and serpentine-like mixing at the nanoscale. A detailed textural study by means of quantified X-ray element imaging provides a wealth of new information about the relationships between textural position, sequence of crystallization and mineral composition of the studied garnierite samples. These results indicate several stages of growth with variable Ni content, pointing to recurrent changes in the physical–chemical conditions during garnierite precipitation. In addition, our detailed mineralogical study of the Falcondo garnierites revealed that the different types identified have characteristic H₂O content and SiO₂/MgO ratios, which play important roles during the pyrometallurgy process.     

The first occurrence of platinum group minerals (PGM) in a chromite deposit in the Eastern Desert, Egypt

 The platinum-group mineralogy (PGM) of the chromitite from Gebel Lawi, in the southeastern desert has been investigated. The most abundant base metal sulfides (BMS) associated with the Lawi chromite are pentlandite, millerite and heazlewoodite. The major platinum-group minerals identified were as follows: laurite (IrOsRu)S2, osmian iridium (OsIr), hollingworthite (RhAsS), tellurian arsenopalladinite (PdTeSbAs), potarite (PdHg) besides cuprian palladian gold (CuPdAu), a Pd-Sb-Hg and HgTe phases. Laurite and osmian iridium occur preferentially in chromite. Os-Ir commonly forms composite PGM with laurite. Hollingworthite and tellurian arsenopalladinite are included within serpentine and, close to the base-metal sulfides, the cuprian palladian gold shares boundaries with chromite. Potarite together with the Pd-Sb-Hg and HgTe phases are embedded in serpentine. Palladium is the most abundant PGE in the Gebel Lawi chromite. A paragenetic sequence of PGM formation is described. Textural evidence indicates that Os-, Ir- and Ru-bearing PGM formed early and were followed by Rh- and Pd-bearing PGM. The concentration of all five PGE could be magmatic, but much of the PGE mineralogy except for laurite and osmian iridium in the center of chromite grains, has been modified by subsequent processes. At later stages, the environment became Te-, Sb-, As- and Hg-rich, which finally led to the formation of low-temperature alteration minerals. Received: 24 April 1995 / Accepted: 28 March 1996     

Geological,geochemical and mineralogical characteristics of REE-bearing Las Mercedes bauxite deposit,Dominican Republic

Bauxite deposits, traditionally the main source of aluminum, have been recently targeted for their remarkable contents in rare earth elements (REE). With ∑REE (lanthanoids + Sc + Y) concentrations systematically higher than ∼1400 ppm (av. = 1530 ppm), the Las Mercedes karstic bauxites in the Dominican Republic rank as one of the REE-richest deposits of its style.The bauxitic ore in the Las Mercedes deposit is mostly unlithified and has a homogeneous-massive lithostructure, with only local cross-stratification and graded bedding. The dominant arenaceous and round-grained texture is composed of bauxite particles and subordinate ooids, pisoids and carbonate clasts. Mineralogically, the bauxite ore is composed mostly of gibbsite and lesser amounts of kaolinite, hematite, boehmite, anatase, goethite, chromian spinel and zircon. Identified REE-minerals include cerianite and monazite-Ce, whose composition accounts for the steady enrichment in light- relative to medium- and heavy-REE of the studied bauxites.Considering the paleo-geomorphology of the study area, we propose that bauxites in the Las Mercedes deposit are the product of the erosion and deposition of lithified bauxites located at higher elevations in the Bahoruco ranges. Based on the available data, we suggest a mixed lithological source for the bauxite deposits at the district scale: bedrock carbonates and an igneous source of likely mafic composition.     

Platinum group minerals in podiform chromitites of the Bou Azzer ophiolite, Anti Atlas, Central Morocco

Summary The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite. Metallurgical-grade high-Mg and high-Cr spinels (cores with 48–62 wt% Cr₂O₃) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios [(Mg/(Mg + Fe²⁺)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel. Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides, arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides, sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As ores. Author’s address: Dr. Frank Melcher, Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany     

Platinum group minerals in ophiolitic chromitites from Tehuitzingo (Acatlán complex, southern Mexico): implications for post-magmatic modification

Summary Podiform chromitite bodies occur in serpentinites at Tehuitzingo (Acatlán complex, southern Mexico). Serpentinite and chromitite are believed to represent a fragment of Paleozoic ophiolitic mantle formed in a supra-subduction zone setting. The ophiolitic mantle sequence is associated with eclogitic rocks, enclosed in a metasedimentary sequence. This association suggests that serpentinites, chromitites and eclogitic rocks underwent a common metamorphic evolution, starting from high pressure (eclogite facies) followed by retrogression (epidote-amphibolite and greenschist facies). The chromitites are strongly altered so that chromite grains are transformed to ferrian chromite; no primary silicates (i.e. of magmatic origin) have been preserved. The chromitites are Al-rich, and contain up to 303 ppb platinum group elements (PGE), with a marked predominance of Os + Ir + Ru over Rh + Pd + Pt, resulting in a characteristic negative-slope of the chondrite-normalized PGE pattern. Consistent with the geochemical data the platinum group minerals (PGM) assemblage is dominated by Ru–Os–Ir minerals, occurring both as single-phase or as composite grains generally less than 10 μm in size. The PGM mineralogy includes laurite, osmium, irarsite and Ru–Fe oxide or hydroxide. Based on textural relations, paragenesis and composition, it was possible to establish that Os-rich laurite and irarsite were early liquidus phases, which now occur as inclusions in unaltered chromite. However, most of the PGM are found in the alteration assemblages of the chromitites in close association with ferrian chromite, chlorite, and heazlewoodite. Laurite from the secondary assemblage is Os-poor and commonly shows overgrowths of Os–Ir alloys. Internal zoning of some laurite grains indicates that Os-poor laurite formed from a Os-rich laurite by release of Os and some Ir, that are readily incorporated in the Os–Ir alloys. Such process requires a decrease of sulfur fugacity with decreasing temperature; this is not consistent with the fS₂-T trend in magmatic systems. It is proposed, therefore, that the magmatic PGM assemblage underwent mineralogical reworking starting from relatively high temperature during metamorphism. Temperatures, estimated from chlorite geothermometry (399–210 °C), possibly reflect effects of low-grade metamorphism. After that the PGM and the associated sulfides started to be oxidized. Although it is difficult to determine the extent of PGE mobilization on the basis of mineralogical observations, our data suggest that the metamorphism affecting the Tehuitzingo chromitites caused only re-distribution of PGE on a small scale. Thus, we conclude that metamorphism modified the primary PGM assemblage without having changed the whole-rock PGE concentration.     

云南哀牢山金矿带墨江金镍矿床铂族元素(PGE)地球化学及其对矿床成因的制约

文章系统地测定了墨江金镍矿床中的金镍矿和镍矿矿石中的PGE含量,结果均较低,∑PGE为(2.58～109.66)×10-9,与超基性岩的∑PGE〔(14.58～50.48)×10-9〕相差不大,且矿石与超基性围岩PGE Au的球粒陨石标准化模式基本一致,均为Pt和Ir相对亏损和Ru及Rh相对富集的M型,显示墨江金镍矿PGE来源较为一致,主要来自超基性岩体;墨江金镍矿Pd/Ir比值为0.18～10.0,远低于典型热液型镍矿的Pd/Ir值(>100),而与岩浆型镍矿相近,说明其中的镍主要为岩浆成因,后期热液改造并不是Ni成矿的主导因素,因此墨江应为早期岩浆型Ni矿和晚期热液型Au(Ni)矿叠加形成的复合矿床。墨江金镍矿的超基性围岩直接来自地幔,是由经历了基性岩浆抽提和交代作用形成的亏损地幔发生程度不同的部分熔融形成的,其原始岩浆中硫已达到饱和。     

PGM in chromite ore of the Sopcheozero deposit, Kola Peninsula

The Sopcheozero chromite deposit is hosted in dunite of the Monchegorsk layered intrusion as a sheetlike body of disseminated ore with a chromite grade varying from 20 to 60%. The total PGM content in the ore attains 0.5–0.8 g/t. The composition of host rocks varies from plagioclase peridotite to dunite, but PGM were found only in chromite-bearing dunite. PGM inclusions were detected in the interstices of chromite and olivine grains and within grains themselves. The data obtained confirm the known tendency toward variation in PGM composition with increasing sulfur and light PGE contents in the residual magmatic melt. The first particles of refractory Ir, Os, and Ru intermetallides appeared at the final stage of olivine crystallization, whereas laurite (Ru,Os,Ir)S₂ and pentlandite (Fe,Ni)₉S₈ were formed at the final stage of chromite crystallization, when the sulfur concentration in the residual melt became sufficient.     

Platinum-group minerals in the Santo Tomas II (Philex) porphyry copper-gold deposit,Luzon Island,Philippines

Mineralized quartz diorites of the Santo Tomas II porphyry copper-gold deposit, carry high Au contents (average: 1.8 ppm) as well as 160 ppb Pd and 38 ppb Pt. Values of other platinum-group elements (PGE) and rhenium are below the analytical detection limits. There is a significant positive correlation between Au and Cu. The highest Pd values were detected in the most Au- and Cu-rich rocks. Platinum-group minerals (PGM) occur exclusively as inclusions in chalcopyrite and bornite. Potential Pd and Pt contents in sulphide concentrates are estimated at 1.5 g/t and 0.4 g/t, respectively. The precious metal assemblages consist of merenskyite (main PGM), kotulskite, moncheite, native gold, electrum, hessite and petzite. Polyphase fluid inclusions in quartz veinlets, associated with a PGM-bearing bornite-chalcopyrite-magnetite assemblage, are characterized by high salinity (35 to > 60 eq. wt% NaCl) and high trapping temperatures (between 380 and 520 °C). They may represent primary magmatic-hydrothermal fluids, which have been responsible for the transport of Pd, Pt and Au as chloride complexes.     

Primary fluids in low-temperature eclogites: evidence from two subduction complexes (Dominican Republic,and California,USA)

Eclogites occur as isolated blocks in melanges of both the Samana Peninsula, Dominican Republic, and the Franciscan Complex, California, USA. In some of these eclogites, fluid inclusions were found in omphacite and sodic-calcic amphibole grains. Textures show that non-planar populations of fluid inclusions formed during growth of clinopyroxene and amphibole. In addition, planar arrays of secondary fluid inclusions are found along healed cracks. Homogenization temperatures to liquid were used to calculate isochores for the fluid inclusions. These data were compared with petrologic geothermobarometry. Temperature conditions of 500–700° C were estimated from garnetclinopyroxene geothermometry. The jadeite contents of omphacite indicate minimum pressures of 8–11 kbar in this temperature range. The P-T estimates agree well with calculated isochores for primary fluid inclusions from the Samana Peninsula, and show some overlap for both primary and secondary fluid inclusions from the Franciscan Complex. Salinities of 1.2–5.3 wt% NaCl equiv. were estimated for both primary and secondary fluid inclusions from Samana and Franciscan eclogites. These data suggest that low-salinity aqueous fluids attended eclogite-facies metamorphism and perhaps retrograde metamorphism in both subduction complexes. The salinities and densities of fluid inclusions in eclogites from the Samana Peninsula and the Franciscan Complex resemble those of counterparts from garnet amphibolites of the Catalina Schist, southern California. An external source for such fluids is suggested by their homogeneous populations coupled with their low salinities. Geologic evidence suggests that the Samana and Franciscan eclogites may have been derived from a Catalina-like source terrane. The Catalina rocks are inferred to have interacted with large volumes of sediment-derived fluid during subduction zone metamorphism at similar P but higher T conditions than those determined for Samana and Franciscan eclogite blocks. These results contrast with data for fluid inclusions from eclogites of the Monviso area, western Alps. The Monviso eclogites yield similar estimates for metamorphic P-T to those obtained in this study, but contain fluid inclusions of brine and of other saline aqueous fluids, all of which are less dense than expected for incorporation at the reported eclogite-facies conditions. The differences between the properties of fluid inclusions from the ecologites and garnet amphibolites of the Samana-Franciscan-Catalina subduction complexes and those of Monviso probably reflect differences between fluid-flow regimes during metamorphism.     

Pandaïte,baddeleyite and associated minerals from the Bingo niobium deposit,Kivu, Democratic Republic of Congo

Pandaïte, cut by numerous veinlets of a crandallite containing Sr, Ba and RE was found during an investigation of the pyrochlore mineralization of the important niobium deposit of Bingo. The alkaline rocks of the Bingo area also contain locally a particular mineral association of baddeleyite and cassiterite in veinlets.

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Résumé De la pandaïte, recoupée par de nombreuses veinules de crandallite riche en Sr, Ba et TR, a été décelée durant une étude de la minéralisation à pyrochlore de l'important gisement de niobium de Bingo. Les roches alcalines de la région de Bingo contiennent localement une association minérale particulière constituée par de la baddeleyite et de la cassitérite en veinules.

     

Volcanic domes and gold mineralization in the Pueblo Viejo district, Dominican Republic

Gold mineralization in the Pueblo Viejo district, Dominican Republic, is spatially and temporally related to a series of Early Cretaceous volcanic domes. Separate but overlapping hydrothermal cells, centered on the domes, together deposited more than 40 million oz. of gold, 240 million oz. of silver, 3 million tonnes of zinc, and 0.4 million tonnes of copper. Two principal deposits (Moore and Monte Negro) and a number of smaller deposits (Cumba, Mejita, Upper Mejita, Banco V, Arroyo Hondo I and II) have contributed ore since mining commenced in 1975. New geologic mapping has identified a series of previously unrecognized volcanic domes that vary from andesite to dacite in composition. A dacite porphyry dome intrudes epiclastic sediments in the Moore deposit and is surrounded by a baked contact metamorphic aureole. Crumble breccias of mixed epiclastic and pyroclastic origin mantle andesite domes in the Monte Negro, Cumba, and Mejita deposits. Epiclastic volcanic sediments surrounding each of the domes reflect the composition of the local source rock. Andesite domes of the Monte Negro deposit are surrounded by andesitic volcaniclastic sediments. Epiclastic sediments surrounding a dacite porphyry dome in the Moore deposit contain detrital quartz eyes and debris flows of dacite porphyry. A series of at least seven volcanic centers interfinger, overlap, and are interbedded with locally derived epiclastic sediments. Field relations indicate that volcanic dome emplacement, epiclastic sediment accumulation, hydrothermal alteration, and gold mineralization were coeval events. Domes were emplaced in a shallow subaqueous environment on the flanks of an emergent volcanic edifice. Hydrothermal cells responsible for gold mineralization are controlled by high-angle faults. These same faults influenced the emplacement of volcanic domes, an essential step in the development of gold ore in the Pueblo Viejo district. Received: 15 March 1999 / Accepted: 29 September 1999     

云南牟定安益铂钯钛磁铁矿床铂族矿物特征及成因初探

云南牟定安益矿床为一处铂族金属与钛磁铁矿共同产出的大型钛磁铁矿铂族金属矿床。目前对该矿床中铂族元素的赋存状态研究甚少。结合野外宏观地质特征和室内岩矿鉴定,笔者利用TIMA和LA-ICP-MS-Mapping分析方法,对安益矿床中铂族金属矿物学特征进行研究,发现安益矿床中的铂族元素（PGEs）主要以独立矿物的形式存在。铂族矿物（PGMs）多为铂和钯的砷化物、碲化物,如砷铂矿、砷钯矿、黄碲钯矿、碲钯矿等;主要分布于硅酸盐矿物中,其次为硫化物边缘,部分分布于磁铁矿边缘;铂族矿物成因主要有岩浆成因和热液成因2种。岩浆作用形成的铂族矿物分布于硅酸盐矿物中或硫化物边缘,硅酸盐中的铂族矿物是早期PGE与半金属元素形成的纳米团簇颗粒随岩浆演化形成矿物颗粒,被结晶的硅酸盐矿物包裹;分布于硫化物边缘的铂族矿物是残余熔浆结晶的结果。热液作用将PGE以类质同象的形式富集于钛磁铁矿单辉岩的部分矿物中,如热液蚀变较强烈的黄铜矿中含有较高的Rh,这也与铂族矿物集中分布在钛磁铁矿单辉岩中一致。     

Palladium and gold minerals from the Baronskoe-Kluevsky ore deposit (Volkovsky complex, Central Urals, Russia)

Summary Drill cores from the newly discovered Baronskoe-Kluevsky Pd–Au deposit (Volkovsky massif, Central Urals) have been investigated by reflected-light and electron microscopy, and the ore minerals were analyzed by electron microprobe. The most abundant Platinum-group mineral (PGM) is vysotskite, ideally PdS, characterized by an unusual Pt,Ni-poor composition. Palladium also occurs in kotulskite (PdTe), stillwaterite (Pd₈As₃), and unknown Pd–As–Te compounds with vincentite-type Pd₃(As,Te), stillwaterite-type Pd₈(As,Te)₃, and Pd₇(As,Te)₂ stoichiometries. The main carrier of Au is Pd-rich electrum, approaching the composition Au₇₅Ag₁₅Pd₁₀, with minor Fe, Cu, Ni and Pt. The precious minerals are closely associated with minute blebs of chalcopyrite+magnetite disseminated throughout serpentinized olivine-apatite host rock. Paragenetic relationships among the ore minerals define a succession of crystallization events in the order: 1) Cu–Pd sulfides+electrum, 2) replacement by Pd–Te–As and late Pd–As PGM, 3) final replacement by magnetite. The paragenesis is tentatively related with cooling of a fluid phase in the late- to post-magmatic stage.     

Platinum group elements in Permo-Triassic volcanics in West Siberia (the first data)

The first data on PGE contents in the volcanic rocks of the West Siberian Plate are presented. Analysis has shown that most of the studied rocks have clarke contents of these elements. Rocks from the central areas of paleorift valleys are enriched in ΣPGE (2.0–32.0 ppb), particularly in Pt (0.1–24.2 ppb) and Pd (0.3–8.0 ppb), which might be related to the action of plume. The magmatic PGE pattern confirms the earlier conclusions about the mantle genesis of the studied rocks.     

Platinum group element and nickel sulphide ore tenors of the Mount Keith nickel deposit, Yilgarn Craton, Australia

A set of platinum group element (PGE) analyses of about 120 samples from a 250-m continuous drill core through the Mount Keith komatiite-hosted nickel orebody, combined with Ni, Cu, Co, S, and major elements, reveals a complex trend of covariance between the original cumulus components of a thick sequence of nearly pure olivine–sulphide liquid adcumulates. The intersection is divided into informal chemostratigraphic zones, defined primarily by combinations of fine-scale cyclicity in original olivine composition, defined by Mg#, and sulphide composition, defined by Pt/S and Ni/S. Contents of Ni and PGE in 100% sulphides (tenors) were determined from linear regressions of the Ni–S and PGE–S covariance for each zone. Inferred olivine compositions range from about Fo₉₂ to Fo_94.6 and show a broad decrease from bottom to top of the sequence complicated by numerous reversals, revealing crystallisation in an open conduit system. Ni and PGE tenors of Mount Keith sulphide ores have typical values similar to the type I deposits of the Kambalda Dome. Mobility of S, at least on the scale of 2-m sample composites, is evidently relatively minor. Tenors for the various zones range 12–22% Ni, 370–1540?ppb Pt, 970–3670?ppb Pd, 100–460?ppb Ir, 170–460?ppb Rh, and 710–1260?ppb Ru. Pt, Pd, and Rh tenors are very strongly correlated, but the iridium group of platinum group elements (IPGEs; Ir and Ru) less so. Tenor variations are predominantly controlled by variations in magma/sulphide ratio R (100–350), with a minor component of variance from equilibrium crystallisation trends in the parent magma. PGE depletion in the silicate melt due to sulphide liquid extraction is limited by entrainment of sulphide liquid droplets and continuous equilibration with the transporting silicate magma. Ratios of the PGEs to one another are similar to those in the host komatiite magma, with the exception of Pt, which is systematically depleted in ores, relative to Rh and Pd and relative to host magma, by a consistent factor of about 2 to 2.5. This anomalous Pt depletion relative to PGE element ratios in unmineralized komatiitic rocks matches that observed in bulk compositions of many komatiite-hosted orebodies. The highly consistent nature of this depletion, and particularly the very strong correlation between Pt, Pd, and Rh in the Mount Keith deposit, argue that this depletion is a primary magmatic signal and not an artefact of alteration. Differential diffusion rates between Pt and the other PGEs, giving rise to a low effective partition coefficient for Pt into sulphide liquid, is advanced as a possible but not definitive explanation.     

Gold and silver minerals in low-sulfidaton ores of the Julietta deposit (northeastern Russia)

Gold- and silver-containing pyrites of the Tikhii area at the Julietta deposit (Engteri ore cluster, Magadan Region) were studied by optical and scanning electron microscopy and electron probe microanalysis. One- or two-phase rounded microinclusions consisting of electrum (450-680%c) and/or galena or of petrovskaite and/or uytenbogaardtite, galena, and sphalerite have been found in early pyrites. Later As-pyrites (up to 2.6 wt.% As) contain multiphase xenomorphic microinclusions of acanthite, uytenbogaardtite, freibergite, argentotetrahedrite-tennantite, naumannite, petzite, selenopolybasite-selenostephanite, tellurocanfieldite, and other ore minerals localized in pores, cracks, and interstices. Pyrites that underwent hypergene alterations have rims and veinlets formed by acanthite, goethite, anglesite, plattnerite, and native silver. The presence of rounded ore mineral microinclusions and large pores in the early pyrites suggests the participation of volatiles in the mineral formation and the uptake of large amounts of impurities by pyrite under high-gradient crystallization conditions. The thermobarogeochemical studies of fluid inclusions in quartz have shown that the ore zone formed under boiling-up of hydrothermal medium-concentration NaCl solutions at 230-105 °C. The results of thermodynamic calculations evidence that Ag-Au-S-Se minerals formed under decrease in temperature and fugacity of sulfur (log₁Q/S₂ = -22 to -9) and selenium (log₁Q/S_e2 = -27 to -14) and change of reducing conditions by oxidizing ones in weakly acidic to near-neutral solutions.     

Main minerals of abnormally high-grade ores of the Tomtor deposit (Arctic Siberia)

The Tomtor massif of Paleozoic ultramafic alkaline rocks and carbonatites is located in the northern part of the Sakha Republic (Yakutia). The massif (its total area is ~ 250 km²) is ~ 20 km in diameter, with a rounded shape and a concentrically zoned structure. The core of the massif consists of carbonatites surrounded by a discontinuous ring of ultramafic rocks and foidolites. The outer part is composed of alkali and nepheline syenites. All rocks are weathered and covered with eluvium, which is the thickest after carbonatites enriched in phosphates and REE. The weathering profile consists of four layers, from the top: kaolinite-crandallite, siderite, goethite, and francolite. The highest-grade ores are observed in the bedded deposit which fills depressions in “sagging” eluvium. The ores are laminated and cryptogranular, with high Nb, Y, Sc, and REE contents (on average, 4.5% Nb₂O₅, 7-10% REE₂O₃, 0.75% Y₂O₃, and 0.06% Sc₂O₃). The highest-grade ores are natural Nb and REE concentrates. The total REE content in some layers is > 10%. The morphologic features of the highest-grade phosphate ores from the northern part of the Burannyi site were studied. The ore-forming minerals belong to the pyrochlore group, crandallite group (goyazite), and monazite-Ce. The pyrochlore group minerals occur mainly as crystals that were completely replaced by barium-strontium pyrochlore and/or plumbopyrochlore but retained the original faces; also, they occur as numerous conchoidal fragments. The grains of the pyrochlore group minerals sometimes have a zonal structure, with an unaltered pyrochlore core and a reaction rim. Goyazite occurs predominantly as colloform grains. According to SEM and TEM data, monazite occurs in the ores as ~ 50 nm particles, which cover the outer part of halloysite tubes (800–3000 nm long and 300 nm in diameter) as a dense layer and make up peculiar biomorphic aggregates. The mineralogical data, the occurrence of biomorphic aggregates, and the close association of organic remains with ore minerals suggest that the high-grade ores of the Tomtor deposit, including the Burannyi site, resulted from a hydrothermal-sedimentary process with a presumably important role of bioaccumulation of REE phosphates.