B. Bühn

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.

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.

Insight into the enigma of Neoproterozoic manganese and iron formations from the perspective of supercontinental break-up and glaciation

Abstract The genesis of manganese and iron formations (MnF and IF) is a closely related process during the Proterozoic, in which oceanic crustal venting provided metal-rich solutions to be deposited on adjacent continental shelves. Their deposition in condensed marine sequences occurred when continental break-up initiated a major transgression and provided favourable conditions for precipitation of chemical sediments from seawater. The Neoproterozoic Rodinian supercontinental break-up shows the relationship between this tectonic process, climate, and MnF/IF genesis. Metallogenesis followed particular rift-drift transitions during supercontinental break-up. MnFs/IFs are often associated with glaciomarine sedimentary rocks. Higher O2/CO2 during continental rifting would, through inducing glacial conditions and withdrawing seawater, increase ocean salinity and thereby allow both deposition of glaciomarine sediments and precipitation of evaporites. Salinities would have been particularly enhanced in the restricted oceanic basins provided by early supercontinental rifting, where atmospheric evaporation would also have played a part. Accumulation of Mn and Fe in the Neoproterozoic may be directly related to a probably glacially derived increased ocean salinity that promoted accumulation and storage of Mn and Fe in ocean water through increased Mn, Fe, and Ba(?) solubility. Very similar processes may have been effective in Palaeoproterozoic times, although then a higher amount of Fe and Mn discharge and different ocean chemistry would have superseded the effect of ocean salinity. Chemical Mn and Fe precipitates related to supercontinental break-up in the Proterozoic as well as the Phanerozoic indicate a similar relationship between tectonics and ore accumulation. The resultant close time and space relationship between potent oceanic source and suitable continental-shelf depositional settings optimised accumulation and preservation of ancient Mn-Fe deposits. This scenario is at variance from present-day environments where source and suitable precipitation areas of long-term preservation potential are mostly widely separated in space.

U-Pb and Sm-Nd constraints on the nature of the Campinorte sequence and related Palaeoproterozoic juvenile orthogneisses, Tocantins Province, central Brazil

Abstract The Palaeoproterozoic era was the most important stage of crustal accretion in the South American Platform, being responsible for the development of several magmatic arcs, which represent approximately 35% of the present-day continental crust. The recently mapped Campinorte volcano-sedimentary sequence and associated plutonic rocks represent this Palaeoproterozoic history in the northern Brasilia Belt, central Brazil. The sequence consists of metapsammites and metapelites, with interbedded lenses of gondites and metacherts, as well as rhyolite and pyroclastic deposits. Tonalite, granodiorite and granite crystallized between c. 2.18 and 2.16 Ga, as indicated by U–Pb zircon analyses. Sm–Nd TDM model ages range between c. 2.1 and 2.7 Ga, with ϵNd values ranging from −2.14 to +3.36, indicating the dominantly juvenile nature of the original magmas. A LA-ICPMS provenance study of zircon grains from a quartzite sample reveals a single sediment source with Palaeoproterozoic age. The data presented here provide new information on the Palaeoproterozoic juvenile crust of central Brazil and suggest correlation with other Palaeoproterozoic provinces, especially the Birimian Belt in West African Craton and those of the Guiana Shield, thus contributing to reconstruction of the Columbia supercontinent.

Petrology and age of the Otjisazu Carbonatite Complex, Namibia: implications for the pre- and synorogenic Damaran evolution

Abstract The Otjisazu Carbonatite Complex intruded crustal rocks of the Damara Orogen in central Namibia close to the Okahandja Lineament. The igneous complex consists of a suite of mela-syenites, syenites and alkali feldspar syenites, and a carbonatite suite comprising clinopyroxenites with calcite and calcite-garnet lenses, feldspathic clinopyroxenites and sovites with wollastonite-rich lenses. The two suites are considered independent magmatic sequences. Thermodynamic calculations suggest that the syenite suite represents a coherent fractionation sequence which evolved by predominantly clinopyroxene, and subordinate fluorapatite and magnetite fractionation at 5–6 kbar. The mid-crustal crystallisation level is supported by the supposed cumulative character of most lithologies of the carbonate suite. Clinopyroxenites and calcite ± garnet lenses of the carbonatite suite are considered largely cumulative rocks and represent the lower portion of a carbonatite magma chamber that crystallised clinopyroxene, calcite, andraditic garnet, titanite and fluorapatite. U-Pb dating of titanites suggests an intrusion age of 837 + 60/−49 Ma, and a metamorphic overprint at 558 ± 5 Ma. These age constraints indicate a rift-related setting of the Otjisazu Carbonatite Complex within the Damaran Orogen.

Precambrian to modern manganese mineralization: Changes in ore type and depositional environment

Manganese mineralization is diverse in occurrence, origin, mineralogy and geochemistry. These variations reflect differences in the processes of formation and depositional environments, which in turn are a response to changes in the land-ocean-atmosphere system over geological time. As such, manganese deposits can act as markers of major events in the dynamic evolution of the Earth’s surface. Modern manganese accumulations provide insights into key factors controlling manganese deposition that cannot readily be determined from examination of ancient ores. A knowledge of oceanic currents, ocean chemistry or small-scale variations in physicochemical patterns of recent basins, for example, may extend our understanding of depositional processes in the past. Equally, the study of Precambrian deposits not only elucidates ancient mechanisms of manganese metallogenesis, but also helps to unravel the impact of comprehensive environmental changes on metal deposition on a scale not realized in younger geological times. The papers collected in this volume provide insights into this changing nature of manganese mineralization from Precambrian sedimentary ores to crusts and nodules on the Cenozoic sea bed. The volume is introduced by Supriya Roy with a review of the range of terrestrial manganese deposits and their relative abundance through geological time. The manner in which manganese mineralization reflects changes in planetary environmental chemistry that was noted above and mentioned by Roy is further developed by papers