Andrew H. Rankin

Composition of natural, volatile-rich, Na-Ca-REE-Sr carbonatitic fluids trapped in fluid inclusions

Abstract A carbonatite-derived fluid trapped under closed-system conditions is preserved as fluid inclusions in quartzitic country rocks of the Kalkfeld carbonatite complex, Namibia. The fluid inclusion population provides a unique opportunity of investigating in situ the composition of a natural, volatile-rich carbonatitic fluid with all components trapped and preserved, and its relationship to the volatile-deficient, parental Kalkfeld carbonatite. Individual fluid inclusions display all intermediate compositions between three end-members: (1) low-density CO 2 ; (2) a Na,K,Ca,Cl − , HCO 3 − -bearing aqueous brine; and (3) an assemblage of solid phases comprising nahcolite, halite, burbankite, sylvite, fluorocarbonate (rouvilleite ?), cryolite, Mn–Fe–calcite, feldspar/feldspathoid, fluorite, base metal sulfides, and phosphate. Cathodoluminescence imaging of the host quartzites features a network of fluid migration pathways. In situ exsolution of invasive fluids and necking-down processes of fluid inclusions, triggered by rapid cooling of interconnected fluid ponds, caused heterogeneous entrapment of the various fluid components. Consequently, synchrotron–XRF analyses of individual inclusions show a broad range of compositions with Th/U = 1 to 45 and Y/Ho = 1 to 28, but consistent rare-earth element (REE) cn patterns. Analysis of the bulk carbonatitic fluid yielded the following element ratios: Na/K = 1.5 to 2.7, Na/Ca = 1.5, Na/ΣREE = 4 to 17, Fe/Mn = 5.2 to 7.2, Th/U = 3 to 13, and Y/Ho = 25 to 65. An estimation of the volatile components in the fluid population at CO 2 = 20 wt% and H 2 O = 20 wt% permits a quantitative assessment of the composition of the Na−Ca−REE−Sr alkali-carbonatitic fluid with 40 wt% H 2 O and CO 2 , 28 to 29 wt% Na 2 O + K 2 O, 13 to 16 wt% CaO, 3.0 to 4.1 wt% FeO tot , up to 3 wt% ΣREE, up to 3 wt% Sr, 1 to 1.7 wt% MgO, 1 wt% TiO 2 , 0.6 wt% MnO, and Th and Ba reaching 1600 and 8000 ppm, respectively. The REE patterns of individual fluid inclusions and of the bulk fluid extend over two orders of magnitude from La to Lu, and in this respect are similar to those of the parental Kalkfeld carbonatite, but are distinguished by a negative (Eu/Eu∗) cn anomaly of 0.5 to 0.6. The data suggest that this fluid is a direct sample of those expelled during a late stage of carbonatite fractionation. A comparison between this alkali–carbonatitic fluid with the volatile-deficient, sovitic Kalkfeld carbonatite suggests that virtually all alkali metals and Cl, and a major proportion of F, Th, U, and Ti were preferentially partitioned into this fluid. This fluid was also able to accommodate significant concentrations of Rb, Cs, Cu, Pb, and Zr in individual samples. The qualitative sequence: Fe = Mn > Sr = REE > Mg = F > Ba = Y > Ti > Th = U > (Zr,Cu,Pb,Rb,Cs) > K > Na = Cl represents an increasing tendency from left to right to partition into the fluid relative to the crystallizing carbonatite melt. As this fluid migrates through and interacts with invaded host rocks, elements will tend to precipitate in the same qualitative sequence from left to right. This selective precipitation of elements from a migrating fluid accounts for observations made in metasomatized crustal and mantle–derived rocks.

The nature of orthomagmatic, carbonatitic fluids precipitating REE,Sr-rich fluorite: fluid-inclusion evidence from the Okorusu fluorite deposit, Namibia

Abstract The Okorusu fluorite deposit occurs in Pan-African carbonaceous rocks of the Damara Sequence in northern Namibia. The epigenetic mineralization formed from fluids expelled from the Cretaceous Okorusu carbonatite, which is part of the Damaraland igneous province related to the South Atlantic oceanic opening. The fluorites contain abundant high-field-strength elements (∑REE=80–285 ppm, Sr=1900–2500 ppm, Y=8–50 ppm). They have 87 Sr/ 86 Sr 127 Ma values of 0.70458–0.70459, which are close to those of the Okorusu carbonatite. Primary and secondary aqueous fluid inclusions in the fluorites contain 3 wt.% NaCl equiv. at most, and homogenization temperatures are around 120 °C. The immediate country rocks of the fluorite mineralization contain a highly varied H 2 O–CO 2 fluid-inclusion population trapped in quartz which we, based on their distribution and composition, consider to represent an orthomagmatic fluid responsible for the fluorite mineralization. Cathodoluminescence imaging of the quartz delineates the pathways for carbonatitic fluid movement and suggests a single fluid infiltration event. The fluid was trapped under conditions of extreme heterogeneity to form an inclusion population consisting of all volumetric proportions between an aqueous brine, a CH 4 -bearing CO 2 phase and solid phases. Based on energy-dispersive X-ray analysis of opened inclusions and laser Raman spectrometry of unopened inclusions, the following solid phases have been identified: halite, sylvite, nahcolite, K-feldspar, a Ba,Ca oxide/hydroxide, possibly zharchikite AlF(OH) 2 , an LREE hydroxide/fluoride (?), galena and cerussite, fluorapatite, cryolite, burbankite, pyrite, fluorite, barite and kyanite (?), amongst others. Crush–leach analysis applied to the inclusion-bearing quartz yielded 87 Rb/ 86 Sr=0.15059 and 87 Sr/ 86 Sr=0.70495 for the fluid leachate, 87 Rb/ 86 Sr=0.65141 and 87 Sr/ 86 Sr=0.70551 for the residual quartz, and 87 Rb/ 86 Sr=0.22752 and 87 Sr/ 86 Sr=0.70501 for the bulk quartz (all values calculated for T =127 Ma). The regression line for these data points represents a mixing line between the Damaran host rock and a Cretaceous fluid. At 87 Rb/ 86 Sr=0, the regression line yields 87 Sr/ 86 Sr 127 Ma =0.70478, which is only slightly more radiogenic than recorded for the fluorites and associated Okorusu carbonatite. Crush analysis of the bulk quartz and normalization of the raw data to 100 wt.% at CO 2 =20 wt.% and H 2 O=20 wt.%, yielded the following chemical composition for the carbonatitic fluid: Na 2 O=21.1 wt.%, K 2 O=8.0 wt.%, CaO=5.5 wt.%, FeO (total)=3.2 wt.%, F=3.0 wt.%, Cl=10.8 wt.%, ∑REE=3 wt.%, Ba=32,754 ppm, Sr=11,484 ppm, Zr=137 ppm, Y=462 ppm, Th=444, U=15.9 ppm and Pb=1260 ppm. It is concluded that the trapped fluid-inclusion population represents a sample of the carbonatitic fluid with only minor crustal contamination, which reacted with crustal carbonate rocks to precipitate fluorite. The characteristic features of this fluid are its high-alkali and comparably low-Ca contents resulting in (Na+K)/Ca=5.7, a high F content (Cl/F=3.6) and high-field-strength element concentrations in the percentage range. As a consequence, we infer that the low-temperature and low-salinity inclusions in fluorite are unlikely to represent the fluids responsible for the primary mineralization for carbonatite-associated fluorite deposits, as exemplified by the Okorusu samples.

Burbankite, a (Sr,REE,Na,Ca)-carbonate in fluid inclusions from carbonatite-derived fluids: Identification and characterization using Laser Raman spectroscopy, SEM-EDX, and synchrotron micro-XRF analysis

Abstract Burbankite, ideally (Na,Ca)3(Sr,REE,Ba)3(CO3)5, is a rare REE carbonate mineral that until now had been encountered only at a few localities including highly alkaline silicate rocks, carbonatites, and lacustrine sediments. It was identified as an abundant solid phase in fluid inclusions that represent fluids derived from the Kalkfeld carbonatite complex (Namibia). Burbankite occurs in association with other solids including nahcolite, halite, sylvite, rouvilleite (?), fluorite, calcite, cryolite, base metal sulfides, and phosphates. The carbonatite-derived fluids were trapped in quartzite country rocks close to the carbonatite contact. The optical and geochemical identification of burbankite has been confirmed by confocal Laser Raman spectrometry. The burbankite crystals show a Raman shift at 1078 cm-1, which is significantly displaced relative to peaks for other common carbonates and is much broader. The elemental composition of burbankite was determined by a combination of SEMEDX on opened inclusions and synchrotron-XRF analysis on unopened wafers. The SEM-EDX analyses of the burbankite crystals yielded a compositional range (in wt%) of Na2O 10.6-17.5, CaO 3.6-17.4, SrO 12.0-26.7, BaO 2.5-5.5, La2O3 3.5-7.0, Ce2O3 4.7-9.0, Nd2O3 0.9-2.1, and CO2 (calc.) 29.8-35.2. The Na/Ca ratios are between 1.0 and 4.3, which is high in comparison with rock-forming burbankite occurrences, and clearly distinguishes the burbankite crystals from carbocernaite. Synchrotron micro-XRF spectra yielded REE patterns decreasing from La to Yb over 2.5 orders of magnitude with small negative Eu anomaly [(Eu/Eu*)cn = 0.5-1.0] in some cases. The Y/Ho ratios range from 1 to 5, and Th/U ratios are between 1 and 10. The fluids trapped are interpreted to represent a highly evolved but pristine, alkali-rich, hydrous, carbonate melt, which had not lost alkalis to the country rocks by fenitization processes. The common occurrence of burbankite crystals in the fluid inclusions shows the high capability of carbonate melts and fluids to transport high-field-strength and large-ion-lithophile elements.

Silicothermal fluid: A novel medium for mass transport in the lithosphere

New experimental data from synthetic fluid-inclusion studies in the system K 2 O-CO 2 -SiO 2 -H 2 O (KCSH) show that a potassic, silica-rich (≈ 90 wt% SiO 2 ) fluid can coexist immiscibly with a supercritical, alkaline, aqueo-carbonic fluid and quartz from temperatures as low as 300 °C to more than 750 °C at relatively low geologic pressures ( 2 -rich fluids, if they form in the lithosphere, are likely to be important in the mobilization and transport of silica and large ion lithophile elements (e.g., K, Cs, Ba) and metals of economic significance (e.g., Au, Ag, U).

Fluid-rock interaction during progressive migration of carbonatitic fluids, derived from small-scale trace element and Sr, Pb isotope distribution in hydrothermal fluorite

Associated with the Cretaceous Okorusu carbonatite complex (Namibia) is a hydrothermal fluorite mineralization hosted in Pan-African country rock marbles, which resulted from fluid–rock reaction between the marbles and orthomagmatic, carbonatitic fluids expelled from the carbonatite. Yellow fluorite I was deposited in veins up to 5 cm away from the wallrock contact, followed by purple and colorless fluorite II, smoky quartz and barite, a Mn-rich crust on early calcite, and pure calcite. This clear-cut sequence of mineral growth allows an investigation into fluid–rock interaction processes between the marble and the migrating carbonatitic fluid, and element fractionation patterns between the fluid and subsequent hydrothermal precipitates. Fluorite I shows a progressive change in color from dark yellow to colorless with purple laminations over time of deposition. Subsequent fluorite I precipitates show an increase in Ca, and a continuous decrease in F, Sr, REE, Y, Th, U and Pb contents. The ratios (Eu/Eu*)cn, Th/Pb and U/Pb increase whereas Y/Ho, Th/U and (La/Yb)cn decrease. The Sr-isotopic composition remains constant at 87Sr/86Sr = 0.70456–0.70459, but with varying, highly radiogenic Pb (206Pb/204Pb = 32–190, 238U/204Pb = 7–63). Fluorite II has 87Sr/86Sr = 0.70454–0.70459, 206Pb/204Pb = 18.349, and 207Pb/204Pb = 15.600, and a chemical composition similar to youngest fluorite I. The Mn-rich crust on early calcite accumulated REE, Ba, Pb, Zr, Cs, Th and U, developing into pure calcite with a prominent negative Ce anomaly and successively more radiogenic Sr. The calculated degrees of fluid–rock interaction, f = weight fraction of fluid/(fluid + marble), decrease from fluorite I and most fluorite II (f = 0.5) to calcite (f = 0.2–0.3) and hydrothermal quartz (f ≪ 0.1). A crush-leach experiment for fluid inclusions in the hydrothermal quartz yielded a Rb-Sr isochron age of 103 ± 12 Ma. Crush-leach analysis for the carbonatitic fluid trapped in the wallrock yielded a trend from the fluid leachate to the host quartz (206Pb/204Pb = 18.224 and 18.602, 207Pb/204Pb = 15.616 and 15.636, respectively) extending from carbonatite towards crustal rocks. Calculated trace element distribution coefficients fluorite/fluid are below unity throughout, and increase from La to Yb. Elements largely excluded from fluorite (Ba, Pb, LREE relative to HREE) were incorporated later into the Mn-rich crust on calcite. The trace element patterns of the hydrothermal minerals are related to changing aCO2 and aF− in the fluid during continued fluid–marble reaction. A predominance of carbonate over fluoride complexing in the fluid as reactions proceeded controlled the Y/Ho, Th/U and REE patterns in the fluid and the crystallizing phases. Deviations from these trends indicate discontinuous processes of fluid–rock reaction.

Aragonite in olivine from Calatrava, Spain—Evidence for mantle carbonatite melts from >100 km depth

Aragonite, as an inclusion in olivine from a leucitite lava flow, provides evidence for high-pressure crystallization and carbonatitic activity beneath the geophysical lithosphere in Calatrava, Spain. The aragonite occurs as a single crystal within olivine (Fo 87 ), interpreted to have crystallized from a carbonated silicate melt at mantle depths. Experimental data constrain the stability of aragonite to depths of >100 km at CO 2 -H 2 O-bearing mantle solidus temperatures. This is the first documented evidence of magmatic aragonite crystallized in the mantle. Entrained as xenocrysts, the olivines have not crystallized from the carrier melts, which must have formed deeper within the mantle. Lead isotope data of the leucitite and carbonate inclusions indicate that the source melts show isotopic enrichment relative to mid-oceanic ridge basalt and most ocean island basalt. Our evidence strengthens the argument for direct and deep mantle-derived volcanic carbonatite in alkaline volcanic provinces containing maar-type volcanism, such as Calatrava.

Retrograde mineral reactions in saline fluid inclusions: The transformation ferropyrosmalite clinopyroxene

Abstract Evidence is presented for retrograde reaction of silicate minerals with saline brine inside fluid inclusions during post-entrapment cooling. Ferropyrosmalite [(Fe,Mn)8Si6O15(OH,Cl)10, where Fe>>Mn] has previously been interpreted as a daughter mineral in saline inclusions in magmatic quartz from altered granodiorite associated with the Vyhne-Klokoč Fe-skarn deposit of Slovakia. Based on combined Raman spectroscopic, microthermometric, and SEM-EDX techniques this phase is shown to react, on heating above 450 °C with a Ca-enriched saline inclusion fluid, to form clinopyroxene. This suggests that clinopyroxene was originally present in the inclusions at high temperature, and then underwent retrograde reaction with the saline fluid, on cooling, to form ferropyrosmalite. In its simplest form, the equilibrium reaction for this transformation is: Fe8Si6O15[(OH)6Cl4] + 3Ca2+(aq) = 3CaFeSi2O6 + 5Fe2+(aq) + 4Cl-(aq) + 3H2O (ferropyrosmalite) (hedenbergite) In practice, the reaction is complicated by the presence of Mn and Mg. The resulting “daughter” mineral assemblage observed at room temperature is actually a low-temperature equilibrium assemblage very different to that originally present at high temperature. Although silicate daughter minerals such as clinopyroxene are rarely, if ever, described in fluid inclusions, they may have originally been present but underwent similar retrograde reactions. Ferropyrosmalite itself is likely to be frequently overlooked, as in the absence of Raman spectroscopic analyses it may be mistaken for one of a range of possible hydrated iron-chloride minerals. Improved Raman spectra for ferropyrosmalite are presented which will make future identification easier. Failure to recognize intra-inclusion retrograde reactions such as this may lead to misinterpretation of fluid inclusion chemistry, including metal and silica solubilities.

Petrogenetic Significance of solid Carbonate Inclusions In Apatite Of The Sukulu Carbonatite, Uganda

Abstract Many solid inclusions occur in apatite of the Sukulu carbonatite, Uganda, of which the most abundant are carbonate, which can be classified into clear (Mg-calcite) and pitted (calcite) inclusions based on their morphology, texture and chemical composition. Although such solid inclusions are ubiquitous in carbonatite apatite and have been described by many workers, this study provides new insight into their genesis and petrogenetic significance. The pitted inclusions commonly have elongate or spherical shapes and are spatially related to microfractures in the apatite host. They probably developed from early primary aqueous or Mg-calcite solid inclusions by infiltration of post-magmatic fluids through the microfractures. The clear inclusions generally have spheroidal shapes and are thought to represent an early magmatic phase and to be typically magmatic in origin. Electron microprobe analysis indicates that the clear inclusions contain > 1.6 wt.% MgO and the pitted ones 1100°C) prior to apatite formation. In contrast, other clear inclusions became dark or brownish and remarkably homogeneous on heating at relatively moderate temperatures (740–912°C) indicating that they may represent true melt inclusions trapped as melts during apatite growth. The present findings clearly illustrate the importance of both magmatic and post-magmatic processes in the genesis of the carbonate of the Sukulu carbonatite complex. They also suggest that extensive post-magmatic processes are likely to have been responsible for development of the low Mg-calcite and associated dolomite which dominate the sovites of this complex.

Rare-earth mobility as a result of multiple phases of fluid activity in fenite around the Chilwa Island Carbonatite, Malawi

Abstract Carbonatites are enriched in critical raw materials such as the rare-earth elements (REE), niobium, fluorspar and phosphate. A better understanding of their fluid regimes will improve our knowledge of how to target and exploit economic deposits. This study shows that multiple fluid phases penetrated the surrounding fenite aureole during carbonatite emplacement at Chilwa Island, Malawi. The first alkaline fluids formed the main fenite assemblage and later microscopic vein networks contain the minerals of potential economic interest such as pyrochlore in high-grade fenite and rare-earth minerals throughout the aureole. Seventeen samples of fenite rock from the metasomatic aureole around the Chilwa Island carbonatite complex were chosen for study. In addition to the main fenite assemblage of feldspar and aegirine ± arfvedsonite, riebeckite and richterite, the fenite contains micro-mineral assemblages including apatite, ilmenite, rutile, magnetite, zircon, rare-earth minerals and pyrochlore in vein networks. Petrography using a scanning electron microscope in energy-dispersive spectroscopy mode showed that the rare-earth minerals (monazite, bastnäsite and parisite) formed later than the fenite feldspar, aegirine and apatite and provide evidence of REE mobility into all grades of fenite. Fenite apatite has a distinct negative Eu anomaly (determined by laser ablation inductively coupled plasma mass spectrometry) that is rare in carbonatite-associated rocks and interpreted as related to pre-crystallization of plagioclase and co-crystallization with K-feldspar in the fenite. The fenite minerals have consistently higher mid REE/light REE ratios (La/Sm ≈ 1.3 monazite, ≈ 1.9 bastnäsite, ≈ 1.2 parisite) than their counterparts in the carbonatites (La/Sm ≈ 2.5 monazite, ≈ 4.2 bastnäsite, ≈ 3.4 parisite). Quartz in the low- and medium-grade fenite hosts fluid inclusions, typically a few micrometres in diameter, secondary and extremely heterogeneous. Single phase, 2- and 3-phase, single solid and multi solid-bearing examples are present, with 2-phase the most abundant. Calcite, nahcolite, burbankite and baryte were found in the inclusions. Decrepitation of inclusions occurred at ∼200°C before homogenization but melting-temperature data indicate that the inclusions contain relatively pure CO2. A minimum salinity of ∼24 wt.% NaCl equivalent was determined. Among the trace elements in whole-rock analyses, enrichment in Ba, Mo, Nb, Pb, Sr, Th and Y and depletion in Co, Hf and V are common to carbonatite and fenite but enrichment in carbonatitic type elements (Ba, Nb, Sr, Th, Yand REE) generally increases towards the inner parts of the aureole. A schematic model contains multiple fluid events, related to first and second boiling of the magma, accompanying intrusion of the carbonatites at Chilwa Island, each contributing to the mineralogy and chemistry of the fenite. The presence of distinct rare-earth mineral microassemblages in fenite at some distance from carbonatite could be developed as an exploration indicator of REE enrichment.