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In order to understand the effect of fluid-induced alteration on the Sm-Nd isotope systematic in apatite, a series of fluid/apatite reaction experiments, which involve reacting the well-characterized Durango fluorapatite with CO 2 ± CaCO 3 -, NaF- or HCl ± CaCl 2 -bearing solutions with a known 143Nd/144Nd ratio, were conducted at 800 or 600 °C at 200 MPa. In experiments involving CO 2 -H 2 O ± CaCO 3 , the fluorapatite grains did not react with the solution, such that the Sm-Nd isotopic system was undisturbed. In experiments involving NaF, the fluorapatite grains were partially to completely altered. During the alteration process, REE mobilization was retarded via the coupled substitution Na+ + REE3+ = 2Ca2+ due to the high activity of Na in the fluid. Because the REE were not mobilized, the 147Sm/144Nd ratios remained constant. However, the 143Nd/144Nd ratios were slightly altered due to small degrees of Nd isotopic exchange between the fluid and fluorapatite. In experiments involving HCl ± CaCl 2 , the fluorapatite grains were partially altered, and the REE were variably leached from the altered fluorapatite. Leaching of REE was accompanied by an increase in the 147Sm/144Nd ratio, which is related to the higher compatibility of Sm in the fluorapatite structure and the lower mobility of Sm in Cl-bearing fluids. Although the 147Sm/144Nd ratios were strongly affected, the 143Nd/144Nd ratios experienced only a small change, which is related to the slow rate of transport for Nd between reaction-front fluid and bulk fluid. In general, the experimental results indicate that fluid chemistry is the main factor controlling the response of the Sm-Nd isotopic system to fluid-induced alteration. The 147Sm/144Nd ratio of apatite can be highly modified, and in turn the Sm-Nd isotopic system is disturbed when the fluids are rich in ligands that are able to facilitate REE mobilization and fractionation. Therefore, a thorough evaluation of the fluid-rock history, along with the conjectured fluid chemistry, is necessary before using apatite Sm-Nd isotopes as geological indicators. [ABSTRACT FROM AUTHOR]

Mineral replacement reactions are common in the various environments where rocks have undergone re-equilibration with geologic fluids. Replacement reactions commonly take the form of fluid-aided, coupled dissolution-precipitation and often result in pseudomorph formation. One class of environment that frequently shows significant examples of mineral replacements is hydrothermal ore deposit systems. The goal of this study was to test the simultaneous reactivity of fluorapatite and monazite in Na- and Si-rich hydrothermal fluids, which partially mimic the mineralogy and fluid chemistry of the Llallagua tin deposit in Bolivia. A series of experiments were performed at 300 to 600 °C and 100 MPa, utilizing various combinations of monazite, fluorapatite, and H2O + Na2Si2O5. Reaction products were evaluated using scanning electron microscopy, electron microprobe analysis, and single-crystal X-ray diffraction. The results of this experimental study show that fluorapatite and monazite are differentially reactive under the conditions studied. The reaction products, pathways, and kinetics have a large temperature dependence. The 300 and 400 °C experiments show variable amounts of monazite replacement and only minor, if any, dissolution or reactivity of fluorapatite. The high-temperature 500 and 600 °C experiments are characterized by massive replacement of monazite by vitusite and britholite. Exclusively at 600 °C, monazite alteration takes the form of symplectite development at the reaction front as vermicular intergrowths of vitusite and britholite. The higher-temperature experiments also show substantially more reactivity by fluorapatite, which is partially pseudomorphically altered into britholite. This is an example of regenerative mineral replacement where both fluorapatite and britholite share the same atomic structure and are crystallographically coherent after the partial replacement. The britholite replacement is characterized by the presence of oriented nanochannels, which facilitate fluid-based mass transfer between the bulk solution and the reaction front. The fluorapatite replacement is enhanced by monazite alteration through a self-perpetuating, positive feedback mechanism between these two reactions, which enhance the REE mobility in alkali-bearing fluids and further drives bulk re-equilibration. These results have potential geochronologic implications and may be significant in the evaluation of monazite and fluorapatite as potential solid nuclear waste forms. They also give us deeper insights into the mechanism of mineral replacement reactions and porosity development. [ABSTRACT FROM AUTHOR]

Fluorapatite-monazite-xenotime-allanite mineralogy, petrology, and textures are described for a suite of Kiruna-type apatite-iron oxide ore bodies from the Grängesberg Mining District in the Bergslagen ore province, south central Sweden. Fluorapatite occurs in three main lithological assemblages. These include: (1) the apatite-iron oxide ore bodies, (2) breccias associated with the ore bodies, which contain fragmented fluorapatite crystals, and (3) the variably altered host rocks, which contain sporadic, isolated fluorapatite grains or aggregates that are occasionally associated with magnetite in the silicate mineral matrix. Fluorapatite associated with the ore bodies is often zoned, with the outer rim enriched in Y+REE compared to the inner core. It contains sparse monazite inclusions. In the breccia, fluorapatite is rich in monazite-(Ce) ± xenotime-(Y) inclusions, especially in its cores, along with reworked, larger monazite grains along fluorapatite and other mineral grain rims. In the host rocks, a small subset of the fluorapatite grains contain monazite ± xenotime inclusions, while the large majority are devoid of inclusions. Overall, these monazites are relatively poor in Th and U. Allanite-(Ce) is found as inclusions and crack fillings in the fluorapatite from all three assemblage types as well as in the form of independent grains in the surrounding silicate mineral matrix in the host rocks. The apatite-iron oxide ore bodies are proposed to have an igneous, sub-volcanic origin, potentially accompanied by explosive eruptions, which were responsible for the accompanying fluorapatite-rich breccias. Metasomatic alteration of the ore bodies probably began during the later stages of crystallization from residual, magmatically derived HCl- and H2SO4-bearing fluids present along grain boundaries. This was most likely followed by fluid exchange between the ore and its host rocks, both immediately after emplacement of the apatite-iron oxide body, and during subsequent phases of regional metamorphism and deformation. [ABSTRACT FROM AUTHOR]

The REE enrichment process in fluorapatite and the REE redistribution among fluorapatite, monazite, and allanite were studied in a series of three sets of experimental runs at P-T conditions of 0.5 to 4 GPa and 650 to 900 °C. The first two sets of experimental runs utilized fluorapatite as a P-source, synthetic monazite or allanite as the REE sources, albite, quartz, and NaF-H2O or NaCl-H2O. The third set of runs was carried out with powdered Ca3(PO4)2, allanite, quartz, (±Al2O3), and a NaF-H2O solution. In all runs REE-bearing fluorapatite with up to 28 wt% ∑REE2O3 formed at the expense of monazite or allanite; either as narrow zones at the margin of synthetic fluorapatite in runs 1 and 2 or as discrete grains in run 3. The REE-enrichment of fluorapatite in melt-bearing runs is explained in terms of the high solubility of monazite in the presence of alkali-rich melts together with the high partitioning values for REEs among fluorapatite and alkali-rich melts. The formation of REE-enriched fluorapatite in melt-absent runs implies that the solubility of monazite and the REE-uptake of fluorapatite are similarly high in both alkali-rich melts and fluids and depends foremost on the activity of alkalis in fluids or melts. The results from this study show the importance of fluorapatite as a REE-carrier in rocks whose petrogenesis involved alkali-bearing fluids/melts. In metamorphic rocks, alkali-enriched fluids or melts will likely form under higher-grade conditions, explaining the preferential occurrence of REE-enriched fluorapatite in granulite and eclogite-facies rocks. [ABSTRACT FROM AUTHOR]

Prior research has shown that fluorine and chlorine X-ray count rates vary with exposure to the electron beam during electron probe microanalysis (EPMA) of apatite. Stormer et al. (1993) and Stormer and Pierson (1993) demonstrate that the EPMA-operating conditions affect the halogen intensities in F-rich natural Durango and Wilberforce apatites and in a Cl-rich apatite. Following these studies, we investigated the effects of operating conditions on time-dependent X-ray intensity variations of F and Cl in a broad range of anhydrous fluor-chlorapatites. We tested 7, 10, and 15 kV accelerating voltages; 4, 10, and 15 nA beam currents; 2, 5, and 10 µm diameter fixed spot sizes; and the influence of 2 distinct crystal orientations under the electron beam. We find that the halogen X-ray intensity variations fluctuate strongly with operating conditions and the bulk F and Cl contents of apatite. We determined the optimal EPMA operating conditions for these anhydrous fluor-chlorapatites to be: 10 kV accelerating voltage, 4 nA beam current (measured at the Faraday cup), 10 µm diameter fixed spot, and the apatite crystals oriented with their c-axes perpendicular to the incident electron beam. This EPMA technique was tested on a suite of 19 synthetic anhydrous apatites that covers the fluorapatite-chlorapatite solid-solution series. The results of these analyses are highly accurate; the F and Cl EPMA data agree extremely well with wet-chemical analyses and have an R² value >0.99. [ABSTRACT FROM AUTHOR]

Various styles of ore deposits may form from a single magmatic-hydrothermal system. Identification of a possible genetic link between different ore types in a region is not only of critical importance for a better understanding of the magmatic-hydrothermal processes, but can also help in successful mineral exploration. Both iron oxide-apatite (IOA) and iron skarn deposits are closely associated with intrusive rocks of intermediate to felsic in composition, but whether these two ore types can form from the same magmatic intrusion remains poorly understood. In this study, we present a comparative study between a newly identified subsurface IOA ore body located at the apex of a diorite porphyry and the iron skarn ore bodies located immediately above it in the Jinniu volcanic basin of the Daye district, Middle-Lower Yangtze River metallogenetic belt (MLYRMB), eastern China in order to highlight a genetic link between these two styles of mineralization. The IOA ores are dominated by Ti-rich magnetite with variable amounts of fluorapatite, diopside, and actinolite. This mineralogical assemblage is distinctly different from the iron skarn ores, which consist mainly of Ti-depleted magnetite and subordinate pre-ore garnet and diopside, and post-ore quartz, chlorite, calcite, and pyrite. In addition, magnetite from the IOA ores is characterized by well-developed ilmenite lamellae and has high concentrations of Ni, V, Co, and Ga, consistent with high temperature crystallization, whereas magnetite grains from the iron skarn ores usually exhibit oscillatory growth zones and contain much lower Ni, V, Co, and Ga, indicating their formation under relatively low temperatures. Titanite and fluorapatite from the IOA ores have U-Pb ages of 132.5 ± 2.4 Ma to 128.4 ± 3.0 Ma, which match a titanite U-Pb age for the associated iron skarn ores (132.3 ± 2.0 Ma), and are consistent with zircon U-Pb ages for the ore-hosting diorite porphyry (130.4 ± 0.7 Ma to 130.3 ± 0.5 Ma). This age consistency supports a possible genetic link among the diorite porphyry, IOA ores, and iron skarn ores. We propose that the IOA and skarn ores are the products of two consecutive mineralization stages of the same magmatic-hydrothermal system, involving a high-temperature, hypersaline fluid coexisting with the diorite porphyry magma during emplacement and a subsequent low temperature, diluted hydrothermal fluid. Other IOA and iron skarn deposits of similar ages (130 Ma) are found in a series of volcanic basins in the MLYRMB, which forms one of the world's largest IOA metallogenic belts. The close association of the two ore styles identified at Daye provides a useful exploration guide for IOA and iron skarn deposits both on a local and regional scale. [ABSTRACT FROM AUTHOR]

[Display omitted] • Iron oxide-apatite (IOA) ores discovered on Prins Karls Forland, Svalbard. • The ores reveal various degree of deformation and differ in mineral composition. • The newly discovered ores record multi-stage metasomatism. • They are interpreted to be fractionated from hypersaline fluid associated with gabbros. On Prins Karls Forland, Svalbard Archipelago, a set of small iron oxide-apatite (IOA) ore bodies have been discovered within a crustal shear zone, which deformed the polymetamorphosed Neoproterozoic metasedimentary rocks. The ores have various styles and grades of deformation and distinct mineral assemblages whose compositions record a multi-stage tectonothermal and metasomatic history. These IOA ore bodies can be subdivided into fluorapatite-bearing and predominant low-Th monazite in the upper section of the shear zone and F-Cl apatite-bearing and predominant high Th-monazite in the structurally lower higher-grade deformed part. The first stage of alteration for these ore bodies resulted in metasomatic alteration of the apatite and liberation of REE and P redeposited as monazite and xenotime. The transport of dissolved REE and P was likely enhanced by deformation. The second stage of alteration had a distinct impact on the individual ore bodies, which resulted in the Th-enrichment of a small subset of the monazite grains in the upper section of the shear zone. In the lower section of the shear zone most of the monazite was replaced by high Th monazite. Here the original fluorapatite is enriched in Cl, Mn, and Sr, most probably due to interaction with CaCl 2 -rich fluids enriched in Sr and Mn that was scavenged from the hosting metasediments and altered metagabbros. Contrasting textures, mineral assemblages, and the geochemistry of the ores from distinct localities reflect involvement of compositionally different fluids from the gabbroic rocks and surrounding metasedimentary rocks during the protracted tectonothermal evolution of Prins Karls Forland. Therefore, it is concluded that the IOA ore bodies most likely resulted due to the fractionation of Fe, P, Ca, and REE from hypersaline fluids associated with the gabbros. Once deposited, these IOA ore bodies were subsequently altered during at least one and perhaps two later metamorphic events. [ABSTRACT FROM AUTHOR]

The experimental alteration of monazite to allanite, REE-epidote, fluorapatite, and/or fluorapatite-britholite was investigated at 450 to 610 MPa and 450 to 500 °C. Experiments involved monazite + albite ± K-feldspar + muscovite ± biotite + SiO2 + CaF2 and variety of fluids including H2O, (KCl + H2O), (NaCl + H2O), (CaCl2 + H2O), (Na2Si2O5 + H2O), 1 M HCl, 2 M NaOH, 2 M KOH, 1 M Ca(OH) 2, 2 M Ca(OH) 2, and (CaCO3 + H2O). The reaction products, or lack thereof, clearly show that the stability relations between monazite, fluorapatite, and allanite or REE-epidote are more dependent on the fluid composition and the ratio of silicate minerals than on the P-T conditions. A high Ca content in the fluid promotes monazite dissolution and the formation of fluorapatite and allanite or REE-epidote. Lowering the Ca content and raising the Na content in the fluid decreases the solubility of monazite but promotes the formation of allanite. Replacing Na with K in the same fluid causes fluorapatite, with a britholite component, to form from the monazite. However, allanite and REE-epidote are not formed. Monazite is stable in the presence of NaCl brines. In KCl brine, monazite shows a very limited reaction to fluorapatite. When the fluid is (Na2Si2O5 + H2O), strong dissolution of monazite occurs resulting in the mobilization of REEs, and actinides to form fluorapatite-britholite and turkestanite. These experimental results are consistent with natural observations of the partial to total replacement of monazite by fluorapatite, REE-epidote, and allanite in fluid-aided reactions involving the anorthite component in plagioclase at mid- to high-grade metamorphic conditions. In contrast, an alkali-bearing environment with excess Na prevents the growth of allanite and eventually promotes the precipitation of secondary monazite. The results from this study provide implications for geochronology and for deducing fluid compositions in metamorphic rocks. [ABSTRACT FROM AUTHOR]

Solution calorimetric measurements have been made on 17 synthetic fluorapatite-chlorapatite crystalline solutions at 50 °C in 20.0 wt% HCl under isoperibolic conditions. Analysis of the calorimetric data indicates that heats of mixing across the series may reach values as high as 8.3 kJ/mol. Normally such a high degree of thermodynamic nonideality would be associated with immiscibility, yet no such miscibility gap is indicated by either synthetic or natural fluor-chlorapatite specimens. Based on full chemical analyses, all Cl-rich samples (XCl > 0.65) of this study have halogen deficiencies that imply the presence of 4-11 mol% vacancies in the anion sites, which are interpreted to be associated with oxyapatite substitution. Separate analysis of data for the vacancy-free samples produces a linear fit for enthalpy of solution vs. composition, which yields an alternative interpretation of thermodynamic ideality. Together these models define the limits of enthalpy behavior for the fluor-chlorapatite system. [ABSTRACT FROM AUTHOR]

Llallagua is one of the world's biggest tin deposits and part of the metallogenic Bolivian Tin Belt, which occurs in the inner arc of the Central Andes. The Llallagua deposit formed through emplacement of a subvolcanic porphyry stock, of intermediate dacitic to rhyodacitic composition, metasomatism, and hydrothermal mineralization. The deposit is the subject of a well-established geochronologic controversy that includes contradictory ages (≈40 and 20 Ma) from multiple geochronometers. Geochronological characterization of phosphate minerals from the metasomatized igneous porphyry and hydrothermal vein assemblages, along with detailed petrography and chemical analyses, are used to reconcile the age controversy. The new interpretation is further supported by existing textural and geochemical data. The U Pb ages of unaltered igneous fluorapatite (e.g. 21.2 ± 2.9 Ma), monazite (e.g. 21.22 + 0.80/−0.66 Ma), and zircon (e.g. 21.15 ± 0.39 Ma) from the Llallagua porphyry are all equivalent within error at ≈20 Ma, and are interpreted to represent the age of porphyry stock emplacement. Ages determined from altered portions of these minerals (e.g. apatite, 18.8 ± 8.0 Ma) are within error the same as unaltered portions, suggesting alteration of the porphyry soon after emplacement. U Pb ages from unaltered fluorapatite (e.g. 21.4 ± 6.7 Ma), altered and unaltered monazite (e.g. 19.4 ± 1.4 Ma and 20.29 ± 0.30 Ma respectively), and unaltered xenotime (e.g. 19.32 ± 0.67 Ma) from the hydrothermal veins are also ≈20 Ma, indicating a very short hiatus or temporal continuity between stock emplacement and hydrothermal vein formation. The early Miocene age for tin mineralization at Llallagua is consistent with neighboring mineralization centers at the Morococala, Colquechaca, Japo, and Santa Fe mines, and with regional trends observed across the Bolivian Tin Belt, including stratigraphy, magmatic, and tectonic history of the Andean Eastern Cordillera. • Igneous phosphates and zircon date the Llallagua porphyry emplacement at ≈ 20 Ma. • Alteration of the porphyry occurred soon after emplacement. • Hydrothermal vein formation also dates to ≈ 20 Ma at Llallagua. • Selective metasomatism of igneous and hydrothermal phosphates has occurred. [ABSTRACT FROM AUTHOR]