Secondary Apatite Research Articles

Trace geochemistry of the Early to Late Cretaceous deposits of the Grajcarek thrust-sheets – a palaeoenviromental approach (Małe Pieniny Mts., Pieniny Klippen Belt, Poland): reply

The chemical composition of the Cretaceous deposits of the Grajcarek thrust-sheets (Pieniny Klippen Belt, Poland) has been investigated to provide information on palaeoenvironment and provenance of pelagic and turbiditic particles. The material studied shows large variations in terrigenous and biogenic content. Phyllosilicates (mirrored in amounts of Al2O3, average 15 wt.%) and carbonates (6 wt.% of CaO) are common mineral components of the deposits excluding the Cenomanian radiolarian shales (CRS) that are enriched in silica (mean content of SiO2 is 64 wt.%). “Immobile” elements may be accommodated by phyllosilicates and accessory minerals (i.e. zircon, xenotime, apatite and Ti-oxides). Heavy minerals are significant within the Szlachtowa Fm. High field strength elements (HFSE) in the Malinowa Fm. are housed in secondary apatite and Fe-oxides. Lithophile trace elements (LILE) concentrations in the material studied are lower/comparable to Post-Archean Australian Shale (PAAS). Ba concentration in the CRS probably reflects enhanced bioproductivity. Interaction between major oxides, distributions of “immobile” and lithophile elements suggest that variation in trace elements through the succession was mainly controlled by the terrigenous input. The material studied was sourced from intermediate to felsic rocks of the Czorsztyn (Oravic) Ridge. The Szlachtowa Fm. and CRS are more mature than others due to low contents of clay minerals. The Szlachtowa Fm. also contains recycled material. The CRS correspond to the oceanic anoxic event 2 (OAE 2) whereas the “Black Flysch” of the Szlachtowa and Opaleniec formations may be related to the Early Cretaceous OAE 1

DIAGENETIC HISTORY OF WOOLLY MAMMOTH (MAMMUTHUS PRIMIGENIUS) SKELETAL REMAINS FROM THE ARCHAEOLOGICAL SITE CRACOW SPADZISTA STREET (B), SOUTHERN POLAND

Skeletal remains of woolly mammoths have been studied using polarizing microscopy, SEM, XRD, and FTIR to characterize their diagenetic history. Formation of different secondary minerals in the bones is related to changing conditions of chemical diagenesis, both in the sediment and in the bone itself. Bone voids are commonly infilled with calcite and/or carbonate sediment, and dentinal tubules are coated or infilled with secondary apatite. The latter may have formed during the life of the organism. Some osteocyte lacunae were observed to be coated with Fe-Mn (hydroxy)oxides. The average hydroxylapatite Ca/P ratios are higher (1.78–2.10) than in stoichiometric hydroxylapatite. Hydroxylapatite crystallinity indices are generally low at 0.06–0.12, as expected for the young bones. Some of the bones are partly altered by microbial attack. Different postdepositional events affecting the bones (recrystallization of apatite, bacterial alterations, mineral and sediment infillings and cracking) were distinguished and their succession proposed.

Overgrowths of large authigenic apatite crystals on the surface of conodonts from Cantabrian limestones (Spain)

Conodonts from the middle to upper Paleozoic limestones of the Cantabrian zone commonly show apatite overgrowths. A large crystal microtexture observed under the SEM corresponds to local rims of euhedral to subhedral apatite crystals, which were preceded by the neoformation of smaller crystals. Four types of this microtexture (blocky, columnar, fan, and denticular) are described on different areas of the oral surface of conodonts, whereas dissolution features may be present in the basal cavity area. The distribution of these types of microtexture in different areas of conodont morphology suggests a general trend to neocrystallization, where crystal size increases towards the top of the conodont ornamentation and a chemical gradient controls the crystalline growth. This arrangement is widely related to the surface morphology and to the general conodont histology. The large crystal microtexture grows during early diagenesis from near surface to moderate burial and is linked to the known secondary apatite cement present in natural fused clusters of conodonts. The features described here are also compared to microtextures developed on conodonts during low- to medium-grade metamorphic conditions, where phosphate in solution is available.

Fluid-mineral interactions and constraints on monazite alteration during metamorphism

AbstractClasts of metamorphosed Cadomian granites from the ∼50—60 Ma Carpathian flysch in Gródek near the Rożnowskie Lake (Silesian Unit, SE Poland) are studied. They are considered to represent the Silesian Ridge, one of the hypothetical, currently unexposed source areas that supplied Carpathian sedimentary basins with clastic material. The gneisses preserve several examples of corona textures that include cores of primary monazite surrounded by polygonal grains of secondary apatite with thorianite inclusions, with intermediate zones of lamellar grains of secondary monazite and outermost rims of clay minerals, or various combinations thereof. Preservation of the complete textures is rare with polygonal apatite with thorianite inclusions, lamellar grains of monazite and clay minerals being particularly prevalent. Locally, polygonal apatite with thorianite inclusions surrounded by allanite and REE-epidote corona with a bastnasite-synchysite phase occurs also. The textures observed developed during primary monazite breakdown and replacement by secondary minerals. The variation in reaction products indicates that alteration was strictly dependent on the local chemical system.

The geochemistry of phosphorus in different granite suites of the Western Carpathians, Slovakia: the role of apatite and P-bearing feldspar

The geochemical behaviour of phosphorus in granites, coupled with compositional variations in apatite, discriminate the different genetic suites of Variscan granitic rocks in the West-Carpathian orogenic belt. Meso-Variscan granites, mostly of S-type affinity, show a good correlation between SiO 2 and P 2O 5, with decreasing amounts of P in the poorly and moderated fractionated granites, and increasing phosphorus in the most evolved and late differentiation products. The aluminium saturation index (ASI) correlates negatively with bulk P contents in I-type granites, and positively with P in S-type granites. The phosphorus content of alkali feldspar (mainly Kfs) is usually very low (typically <0.05 wt.% P 2O 5), or not detected in the slightly differentiated granites. In the lateVariscan I/S-type granitoids, phosphorus is hosted mainly in the early-magmatic apatite, but is present also in the late K-feldspar (up to 0.15 wt.% of P 2O 5) that formed from the residual melts. In the post-orogenic S-type granites, the bulk of the phosphorus occurs in alkali feldspar (∼0.3 wt.% P 2O 5), with a minor proportion present in primary early-magmatic apatite. Clusters of small secondary apatite grains with a distinctive composition are distributed in alkali feldspar grains of the post-orogenic S-type granites. This apatite has formed post-magmatically from P released from P-rich alkali feldspar during decreasing temperature, by reaction with a fluid rich in volatiles (including F and B), and alkali and alkali earth metals. A-type granites have the lowest bulk phosphorus contents. Consequently, phosphate minerals are relatively rare in this granite type, and the alkali feldspar has negligible P 2O 5 contents. Except for Sr, the minor element contents in apatite partly correlate with the bulk composition of their host rocks and partly reflect some thermodynamic properties of melt. The Mn contents of apatite increase with the peraluminous character of the melt and with decreasing fO 2 and so apatite from the S-type granites has significantly higher Mn contents than apatite from the I-type granites. Apatite from A-type granites is enriched in Fe and HREE, reflecting the higher bulk Fe and HREE content of this granite type. Apatite from specialized S-type granites is enriched in Y and HREE. In general, the Cl and S content of apatite is higher in the basic members of the I-type affinity granitic rocks.

Apatite in early Archean Isua supracrustal rocks, southern West Greenland: its origin, association with graphite and potential as a biomarker

Rare earth element (REE) abundances in individual apatite crystals in banded iron formations (BIFs), metacherts, metacarbonates and mafic dykes in the Isua supracrustal belt (ISB) have been determined by laser ablation inductively coupled plasma mass spectrometry. The results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential, proposed in earlier studies, for use of the apatite-graphite association as a biomarker. The chondrite-normalized distribution patterns of apatite in metasedimentary BIFs and metacherts fall into three groups. Relatively flat profiles with distinct positive Eu anomaly are interpreted as characterizing sedimentary (diagenetic) apatite that carry the REE signature of the Archean ocean. Secondary apatite in Isua metasdiments with either middle REE enriched profiles or with light REE depleted profiles is interpreted to have crystallized from percolating carbonate-rich metasomatic fluids or from fluids derived from cross-cutting mafic dykes, respectively. The occurrence together of these different genetic types of apatite with distinct REE signatures within cm-scale samples shows the immobility of REE in preexisting apatite during metamorphic episodes. Apatite crystals in Isua rocks of uncontested chemical sedimentary origin (BIF and metachert samples) do not have graphite inclusions or coatings. Graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks. In these rocks graphite is produced by thermal-metamorphic reduction of carbonate ion, derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor, oxidizing to form magnetite. In view of the non-sedimentary, metasomatic origin of Isua metacarbonates and the abiogenic source of graphite, the apatite–graphite assemblage can not be considered as a biomarker and does not provide information on early Archean life in the ISB.

Mineralogy and geochemistry of trace elements in bauxites: the Devonian Schugorsk deposit, Russia

AbstractProcesses of mineral alteration involving the mobilization and deposition of more than 30 chemical elements during bauxite formation and epigenesis have been studied on specimens from the Devonian Schugorsk bauxite deposit, Timan, Russia. Chemical analyses of the minerals were obtained by electron microprobe and element distribution in the minerals was studied by element mapping. Interpretation of these data also utilized high-resolution BSE and SE images.The main rock-forming minerals of the Vendian parent rock are calcite, dolomite, feldspar, aegirine, riebeckite, mica, chlorite and quartz; accessory minerals are pyrite, galena, apatite, ilmenite, monazite, xenotime, zircon, columbite, pyrochlore, chromite, bastnaesite and some others. Typically, the grainsize of the accessory minerals in both parent rock and bauxite is from 1 to 40 µm. However, even within these rather small grains, the processes of crystal growth and alteration during weathering can be determined from the zonal distribution of the elements. The most widespread processes observed are: (1) Decomposition of Ti-bearing minerals such as ilmenite, aegirine and riebeckite with the formation of ‘leucoxene’, which is the main concentrator of Nb, Cr, V and W. Crystal growth can be traced from the zonal distribution of Nb (up to 16 wt.%). Vein-like ‘leucoxene’ is also observed in association with organics. (2) Weathering of columbite and pyrochlore: the source of Nb in ‘leucoxene’ is now strongly weathered columbite, while the alteration of pyrochlore is expressed in the growth of plumbopyrochlore rims around Ca-rich cores. (3) Dissolution of sulphide minerals and apatite and the formation of crandallite group minerals: ‘crandallite’ crystals of up to 40 µm size show a very clear zonation. From the core to the rim of a crystal, the following sequence of elements is observed: Ca → Ba → Ce → Pb → Sr → Nd. Sulphur also shows a zoned but more complicated distribution, while the distribution of Fe is rather variable. A possible source of REE is bastnaesite from the parent rock. More than twelve crandallite type cells can be identified in a single ‘crandallite’ grain. (4) Alteration of stoichiometric zircon and xenotime with the formation of metamict solid solution of zircon and xenotime: altered zircon rims also bear large amounts of Sc (up to 3.5 wt.%), Fe, Ca and Al in the form of as yet unidentified inclusions of 1–2 µm. Monazite seems to be the least altered mineral of the profile.In the parent rock, an unknown mineral of the composition (wt.%): ThO2 – 54.8; FeO – 14.6; Y2O5 – 2.3; CaO – 2.0; REE – 1.8; SiO2 12.2; P2O5 – 2.8; total – 94.2 (average from ten analyses) was determined. In bauxite, another mineral was found, which has the composition (wt.%): ThO2 – 24.9; FeO – 20.5; Y2O5 – 6.7; CaO 2.0; – ZrO – 17.6; SiO2 – 8.8; P2O5 – 5.4; total – 89.3 (F was not analysed; average from nine analyses). Presumably, the second mineral is the result of weathering of the first one. Although the Th content is very high, the mineral is almost free of Pb. However, intergrowths of galena and pyrite are observed around the partially decomposed crystals of the mineral. Another generation of galena is enriched in chalcophile elements such as Cu, Cd, Bi etc., and is related to epigenetic alteration of the profile, as are secondary apatite and muscovite.

Phosphorus distribution in BIF-derived iron ores of Hamersley Province, Western Australia

The Brockman Iron Formation is one of the most important sources of iron ore in Australia. The usable reserves are limited, however, by high phosphorus levels within some of the orebodies. Understanding of the nature and distribution of this phosphorus is critical when techniques are being devised to upgrade the ores for ironmaking. Although some of the phosphorus is present as secondary apatite and other less common phosphates, it has been known for some time that the majority of the phosphorus in Brockman iron ores is associated with goethite, in amounts often exceeding 1%. It has now been shown that phosphorus levels are dependent not only on the amount of goethite but also on the microtextural types of goethite in the ore. Extensive microanalytical studies of a wide range of ore types have established that the highest phosphorus levels are consistently associated with late-formed colloform goethite in cracks and pore networks. Preliminary studies by scanning transmision electron microscopy reveal that high phosphorus levels are limited to goethite composed of ultrafine crystallites. Where dehydration or recrystallization of the goethite has occurred it typically results in expulsion of the contained phosphorus. The implications of these findings for phosphorus removal are discussed.

Possible secondary apatite fission track age standard from altered volcanic ash beds in the middle Jurassic Carmel Formation, Southwestern Utah

Secondary age standards are valuable in intra- and interlaboratory calibration. At present very few such standards are available for fission track dating that is older than Tertiary. Several altered volcanic ash beds occur in the Middle Jurassic Carmel Formation in southwestern Utah. The formation was deposited in a shallow marine/sabhka environment. Near Gunlock, Utah, eight ash beds have been identified. Sanidines from one of the ash beds (GUN-F) give a single-crystal laser-probe 40Ar/ 39Ar age of 166.3±0.8 Ma (2 σ). Apatite and zircon fission track ages range from 152–185 Ma with typically 15–20 Ma errors (2σ). Track densities in zircons are high and most grains are not countable. Apatites are fairly common in most of the ash beds and have reasonable track densities ranging between 1.2–1.5 × 10 6 tracks/cm 2. Track length distributions in apatites are unimodal, have standard deviations <1 μm, and mean track lengths of about 14–14.5 μm. High Cl apatites (F:Cl:OH ratio of 39:33:28) are particularly abundant and large in ash GUN-F, and are fairly easy to concentrate, but the concentrates contain some siderite, most of which can be removed by sieving. GUN-F shows evidence of some reworking and detriaal contamination based on older single grain 40Ar/ 39Ar analyses and some rounding of grains, but the apatite population appears to be largely uncontaminated. At present BJK has approximately 12 of apatite separate from GUN-F.

Evolução supérgena do carbonatito de Juquiá (SP)

O intemperismo sobre as rochas carbonatíticas do complexo de Juquiá (SP) gerou uma jazida residual de fosfato apatítico com teores médios de 16% em P2O5 A rocha original é um carbonatito do tipo beforsito, que ocorre de maneira contínua, expondo, no atual nível de erosão, uma área de cerca de 2km² de material carbonatítico, o que não é comum nos maciços alcalinos com carbonatitos, onde estas rochas costumam ocorrer apenas como diques e veios. A rocha primária é formada basicamente por dolomita rica em ferro e manganês e apatita (fluoridroxiapatita). Magnetita, barita e flogopita são acessórios, podendo chegar a 5% do volume da rocha. Barita hidrotermal é facilmente encontrada em veios de até 1m de espessura. A apatita primária pode chegar a 50% do volume da rocha original, sendo este o fator responsável pela manutenção das estruturas primárias em grande parte do manto de alteração e ainda pelos altos teores do minério residual. As fácies de alteração provenientes do carbonatito representam um evento de dissolução dos carbonatos, seguido por reciclagem da apatita primária, com produção de apatita secundária. As composições mineralógica e química dos materiais estudados integradas às suas relações morfológicas permitiram considerações sobre o comportamento dinâmico das principais espécies químicas envolvidas, ressaltando: - lixiviação intensa de CO3(2-), Mg2+, H4SiO4, SO4(2-) e K+, - reciclagem no perfil de PO4(3-), Ca2+, Ba2+ e subordinadamente CO3(2-)e - fixação preferencial e lixiviação eventual de Fe3+, Ti4+, Al3+ e Mn4+.

Phosphorites of Rocinha Mine; Patos de Minas (Minas Gerais, Brazil); genesis and evolution of a middle Proterozoic deposit tectonized by the Brasiliano Orogeny

In central Brazil (the states of Minas Gerais and Goias) the Brasiliano orogeny ([asymp]600 Ma) delineates the Sao Francisco craton and, to the west, the Brasilia fold belt. Rocks of the fold belt are thrust to the east and display increasing deformation and metamorphism westward.Almost all the phosphate deposits are found in the Brasilia fold belt, either along its boundary with the craton or in external structural units showing polycyclic deformation and greenschist metamorphism. They occur in the metasedimentary rocks of the middle Proterozoic Paranoa Group and in the upper Proterozoic Sao Francisco Group and its equivalents. Middle Proterozoic phosphorites are economically the more important. Most are spatially and temporally associated with a stromatolite reef barrier. However, the Rocinha-Patos de Minas deposit, the only presently mined, is located south of the reef barrier. It has developed on a shallow to deep ramp between a wide internal basin with reduced oxygen conditions and limited detritus and a more oxygenated marine environment to the west.In the Rocinha mine, phosphate forms grains, angular fragments, lenses, or centimeterthick beds in schists and carbonate or glauconitic schists. The P 2 O 5 content of phosphatic rocks ranges between 10 and 15 percent. Annual production is around 200,000 tons of ore containing 24 percent P 2 O 5 . The mine area comprises four tectonic slices made up of schists, quartzoschists, and phosphatic schists, separated by subhorizontal shear planes. These are defined by tectonic breccias in which secondary apatite has recrystallized. The mine is exposed in a 50- to 75-m phosphatic sequence which is repeated in a lower slice about 100 m below mine level.The phosphatic ore is a homogeneous crypto- to microcrystalline apatite in fairly continuous centimeter-thick beds which were stretched, fragmented, and boudinaged during Brasiliano isoclinal folding and subsequent subhorizontal shearing.X-ray diffraction, IR spectrometry, and chemical analysis indicate that the metasedimentary phosphate is a weakly carbonated fiuorapatite with low F and CO 2 contents. The influence of the Brasiliano orogenesis is marked by partial substitution of F by OH. Secondary prismatic apatite, kaolinite coatings, and wavellite sheaflike structures are products of weathering and are found in mine sections and bore holes to a depth of 10 to 15 m. Secondary apatites are enriched in CO 2 compared to primary metasedimentary apatites.

Genesis and significance of the silico-aluminous nodules in the Ordovician of the Montagne Noire and the Massif Armoricain (France)

Siliceous nodules of the Arenigian of the Montagne Noire (southern France) and the Massif Armoricain (western France) are made up of quartz, illite and chlorite with secondary apatite and, less frequently, dolomite. Diffractometrical and geochemical data indicate that the chlorite is a chamosite (trioctahedral 2:1 chlorite), whose development may be accompanied by that of apatite and diminution of illite. Three types of nodules are defined: Type 1, siliceous nodules, with more than 85% of SiO 2, very frequent in the Montagne Noire; Type II, nodules made of a micaceous siltstone with chlorite matrix, abundant in the Massif Armoricain but present also in the Montagne Noire; Type III, chloritic nodules with a finely globular structure, especially developed in the Massif armoricain but observed in the Montagne Noire as well. The nodules are always poorer in illite than their host sediments and often show traces of the initial sediments. Their formation occurs after the deposition and prior to the development of foliation. The frequent presence of undistorted fossil remains suggests an early formation linked to the physico-chemical modifications occurring in an unconsolidated sediment around a decaying organism. The post-depositional mineralogical evolution should have started with an illite/dolomite assemblage and end up with a chamosite-dominant one, under cold climatic conditions. The simultaneous presence of the three types of nodules in the same formation, or even in the same beds, shows that this mineralogical differentiation is not strictly related to the diagenetic and metamorphic transformation of this material.

COMPORTAMENTO GEOQUÍMICO DOS ELEMENTOS TERRAS RARAS NOS ALTERITOS DA JAZIDA DE FOSFATO E TITÂNIO DE TAPIRA (MINAS GERAIS, BRASIL): A IMPORTÂNCIA DOS FOSFATOS

The geochemical behaviour of the rare earth elements (REE) during the supergenic evolution of the rocks is not yet well known. This paper deals with the REE behaviour in ferralitic alteration profiles of the residual phosphate and titanium deposit of Tapira, originated from pyroxenites and peridotites containing apatite and perovskite. This study was carried out on a rock constituted essentially of apatite, perovskite and titaniferous magnetite, with diopside and phlogopite as accessory minerals. The analysis of a microfractured and silicified fresh rock has shown perovskite as the main bearing REE primary mineral (1.5 to 2.0% RE 2 O 3 ). The alteration of this rock occuring on a first stage with preservation of the volume (isovolume) leads to: 1. total removal of silica (opal and quartz); 2. partial dissolution of primary apatite along the fractures and neoformation of secondary apatite, enriched in fluor and depleted in REE; and 3. total transformation (pseudomorphose) of perovskite into microbothroidal anatase in the voids of which a crystal association (geodes) of rare earth and Ca phosphate of rhabdophane group is neoformed. On a second alteration stage it is observed that: 1. all the apatite is dissolved leaving behind important assembly of voids; 2. the phosphate of the rhabdophane group is partially dissolved and replaced by a phosphate of the crandallite group rich in REE; 3. a certain Ti mobilization occurs in the interior of the anatase (corrosion gulf; titaneous cutans) followed by its partial epigenization by ferruginous products and an initial alteration of the titaniferous magnetite. The study seems to indicate that in Tapira: 1. the phosphorous has a fundamental role on trapping the REE released by hydrolysis of the bearing primary minerals; 2. the REE remain in the alterites and are fixed in mineralogical phases of a sequence of REE phosphates richer in aluminium; 3. the REE and the Ti remain closely associated in the studied weathering phases.

Magmatic and hydrothermal R.E.E. fractionation in the Xihuashan granites (SE China)

The Xihuashan stock (South Jiangxi, China) is composed of cogenetic granitic units (granites Xe, γa, γc, γd and γb) and emplaced during the Yanshanian orogeny (153±0.2 Ma). They are two feldspars, Fe-rich biotite±garnet and slightly peraluminous granites. Primary accessory minerals are apatite 1, monazite, zircon, uranothorite±xenotime in granites Xe and γa, zircon, uranothorite, uraninite, betafite, xenotime 1; hydrothermal minerals are monazite altered into parisite and apatite 2, Y-rich parisite, yttroparisite, Y-rich fluorite and xenotime 2 in granites γc and γb. Petrographic observations, major element, REE, Y and Rb−Sr isotropic data point to a magmatic suite (granites Xe and γa → granites γc and γd → granite γb) distinct from hydrothermal Na-or K-alteration of γb. From granite Xe to granite γb, LREE, Eu, Th and Zr content are strongly depleted, while HREE, Y and U content increase. During K-alteration of γb, these variations are of minor importance. Major and accessory mineral evidences, geochemical and fluid inclusion results indicate two successive alteration fluids interacting with γb, (1) a late-magmatic F− and CO2−rich fluid and (2) a post-magmatic, aqueous and slightly saline fluid. The depletion of LREE and Th content and the increase in HREE, Y and U content correspond, in the magmatic suite to the early fractionation of monazite in the granites where there is no hydrothermal alteration (granites Xe and γe) and to the hydrothermal alteration of monazite into parisite and secondary apatite, intense new formation of yttroparisite, Y enrichment and U loss in the uranothorite and late crystallization of uraninite in the granites γc and γb. Moreover, simulated crystallization of monazite and temperature of monazite saturation show early fractionation of monazite from the magma in the less evolved granites (Xe and γe) and prevailing hydrothermal leaching of monazite in the most evolved granites (γc-γd and γb) related to a late-magmetic event. The slight variations of REE, Y, Th and U content in the K-altered granites compared to granite γb emphazes the distinct chemical nature of the successive hydrothermal fluids. Rb−Sr and Sm−Nd isotopic results point to a 30 Ma period of time between the late-magmatic and the post-magmatic fluid circulation.

Secondary phosphate mineralization in a karstic environment in Central Sri Lanka

At Eppawala in central Sri Lanka secondary phosphate mineralization is intimately associated with laminated fabrics within depressions (sinkholes and smaller cavities) formed in the thick weathering profiles of a hilly terrain underlain by a Precambrian apatite-bearing formation. The lowermost levels of the profile show extensive zones of leaching where derived apatite crystals occur within fine-grained, laminated stromatolite sequences. The stromatolitic groundmass, which diagenetically formed by percolating oxygenated phosphate and carbonate-rich groundwaters, is impregnated by the phosphate minerals francolite and collophane. Scanning electron microscopy (SEM) reveals that fine filaments, characteristic of microorganisms, are associated with the secondary phosphate mineralization. Continuous degradation and fragmentation of the stromatolitic mat has produced pellets, peloids, and intraclasts all enriched in secondary apatite. Degrading recrystallization around the edges of the primary apatite crystals has developed coated grains. The widespread occurrence of phosphate-enriched allochems in stromatolitic groundmasses is a unique development of a modern phosphorite in a karstic environment.

A modern terrestrial phosphorite-an example from Sri Lanka

At Eppawala in Central Sri Lanka, a thick weathering profile is mined for use as phosphate fertilizer. Recent tropical subaerial weathering processes have produced a phosphate-enriched horizon in the weathering profile which has developed on a Precambrian apatite marble formation. The enriched horizon with P 2O 5 compositions ranging from 10 to 40% is formed essentially of laminar phoscretes occurring in a network of sinkholes. Detailed examination of the laminar phoscretes reveals that primary metamorphic apatite crystals occur in association with phosphatic allochems such as coated grains, ooids, pisoids and intraclasts in a finely layered groundmass. It has been observed that the primary apatite grains released from the parent apatite marble due to tropical weathering processes have been deposited in sinkholes where nutrient-rich percolating meteoric waters were precipitating secondary apatite. Subsequent diagenetic processes active in such sinkhole sedimentary environments have produced the phosphatic allochems characteristic of secondary phosphate mineralization. The allochems together with their mineralized groundmass and the associated primary apatite grains have given rise to the Eppawala phosphorite deposit.

Dispersion of niobium and phosphorus in soil overlying the Qaqarssuk Carbonatite Complex, Southern West Greenland

Rocks and soils overlying the Qaqarssuk Carbonatite Complex, southern West Greenland, were analyzed for major and trace elements by various analytical methods. The carbonatitic rocks contain up to 5% P 2O 5 and up to 0.2% Nb. The soils are relatively depleted in Ca due probably to dissolution of calcite by CO 2-rich waters. Apatite is not affected by this mechanism so that P 2O 5 tends to be enriched in the soils. A CaO/MgO ratio of less than about 1.5 is indicative of apatite accumulation in the soils. The dominant carbonate mineral in soils is dolomite and secondary apatite was not observed. Soil grains are angular and not seriously affected by mechanical transport. The investigations show that soil sampling for geochemical exploration in the arctic environment is suitable for mapping and prospecting for carbonatites.

CONTRIBUIÇÃO DA GEOQUÍMICA DAS TERRAS RARAS À CARACTERIZAÇÃO DOS CARBONATITOS DA FAZENDA VARELA, LAGES, SC

As a part of a petrological and geochemical study the distribution of the rare earth elements in Fazenda Varela carbonatites was discussed. These carbonatites were the first described in Lages Dome. Santa Catarina State, Brazil. They intruded and fenitized sedimentites of the Tubarão Group; the fenitizing fluids are dominantly potassic. The main constituents of the carbonatites are ankerite, barite, K feldspar, and synchisite; secondary apatite is rare. High Ba/Sr, Nb/Ta, and Th/U ratios are conspicuous. The concentration of the rare earth elements is very high relative to the concentration in chondrites, the mean in the earth crust and even in most carbonatites; normalized distribution curves show higher concentration of the light lanthanides (Ce group) in comparison with the heavy lanthanides (Y group). In addition to other petrological and geochemical features, high absolute and relative concentrations of the light lanthanides indicate that these carbonatites represent the last phases of carbonatitic fractionation and reinforce, the possibility of occurence of carbonatites from the early phases of crystallization at the Lages alkaline district.

CONTRIBUIÇÃO DA GEOQUÍMICA DAS TERRAS RARAS À CARACTERIZAÇÃO DOS CARBONATITOS DA FAZENDA VARELA, LAGES, SC

As a part of a petrological and geochemical study the distribution of the rare earth elements in Fazenda Varela carbonatites was discussed. These carbonatites were the first described in Lages Dome. Santa Catarina State, Brazil. They intruded and fenitized sedimentites of the Tubarao Group; the fenitizing fluids are dominantly potassic. The main constituents of the carbonatites are ankerite, barite, K feldspar, and synchisite; secondary apatite is rare. High Ba/Sr, Nb/Ta, and Th/U ratios are conspicuous. The concentration of the rare earth elements is very high relative to the concentration in chondrites, the mean in the earth crust and even in most carbonatites; normalized distribution curves show higher concentration of the light lanthanides (Ce group) in comparison with the heavy lanthanides (Y group). In addition to other petrological and geochemical features, high absolute and relative concentrations of the light lanthanides indicate that these carbonatites represent the last phases of carbonatitic fractionation and reinforce, the possibility of occurence of carbonatites from the early phases of crystallization at the Lages alkaline district.