Summary A swarm of subparallel steeply dipping carbonate dikes is exposed on numerous small islands in the central part of Paint Lake in the Superior Boundary Zone in central Manitoba. The swarm has been traced over a distance of 21 km and is generally conformable to the regional tectonic structure and gneissosity. The principal constituent of all dikes is calcite enriched in Sr, Y and rare-earth elements (REE) and showing evidence of plastic deformation and cataclasis, but the modal composition and texture of individual bodies vary from anchimonomineralic zones of coarse-grained calcite to fine-grained saccharoidal rocks with phlogopite-rich stringers to inequigranular foliated varieties containing a large proportion of calcic amphiboles, apatite, diopside, scapolite and xenocrysts. Regardless of these textural variations, the rocks are consistently enriched in Sr, light REE and show 18 OSMOW, 13 CPDB, Y/Ho, Zr/Hf, Th/U and Nb/Ta ratios similar to the primitive-mantle values. The contents of chalcophile and high-field-strength elements are systematically low. On the basis of the available structural, petrographic and geochemical data, the examined rocks are interpreted as calcite carbonatites of postorogenic affinity. The Paint Lake carbonatites host a variety of REE minerals, including (in order of decreasing abundance): allanite, titanite, monazite and bastnasite.
Marianoite, a new member of the cuspidine group of minerals, occurs in phlogopite–calcite silicocarbonatite in the western part of the Mesoproterozoic Prairie Lake intrusive complex, in northwestern Ontario, Canada. The mineral forms flattened prismatic crystals with resorbed faces, up to 0.3 mm in length, and is associated with uranoan pyrochlore, titanite and natrolite–muscovite pseudomorphs after xenocrystic nepheline. The crystals are translucent, very pale yellow macroscopically and colorless in plane-polarized light. The mineral is biaxial negative (α 1.700, β 1.715, γ 1.725, 2 V meas 80°, 2 V calc 78°) and shows a weak optic-axis dispersion ( r v ). Marianoite is relatively homogeneous in composition; its average empirical formula is Na 1.93 (Ca 4.00 Mn 0.04 ) ∑4.04 (Nb 0.97 Zr 0.90 Ti 0.09 Fe 0.08 Mg 0.03 Hf 0.01 ) ∑2.08 (Si 3.97 O 14 )O 2.93 F 1.07 . By analogy with wohlerite, the simplified formula of marianoite should be written Na 2 Ca 4 (Nb,Zr) 2 (Si 2 O 7 ) 2 (O,F) 4 . The structure of the new mineral species, refined by single-crystal methods to an R 1 of 4.65% (for | F o | > 4σ F ), is monoclinic, space group P 2 1 ; the unit-cell parameters are: a 10.8459(15) A, b 10.2260(14) A, c 7.2727(10) A, β 109.332(3)°, V 761.1(3) A 3 ( D calc 3.45 g/cm 3 ). The mineral is isostructural with wohlerite [Na 2 Ca 4 (Zr,Nb) 2 (Si 2 O 7 ) 2 (O,F) 4 ], but shows the preponderance of Nb in the smallest octahedrally coordinated cation sites in its crystal structure. The occupancy of these sites cannot be determined accurately because of the similar X-ray scattering characteristics and ionic radii of Nb 5+ and Zr 4+ . The mineral is named in honor of Anthony Nicola Mariano (b. 1930), in recognition of his contributions to the study of alkaline rocks and carbonatites.
Titanite is a relatively rare Ti silicate in carbonatitic rocks. It is a primary phase in alkali rich carbonat ites, and may also occur in silicocarbonatit es whose composition was modified by assimilation of wallrock silicate material. More typical is late-stage titanite that forms by reaction of a precursor Ti mineral with deuteric fluids. Both genetic types show significant variations in chemical composition arising mostly from the substitution of Ti with AI, Fe, Nb and Zr. Cationic substitutions at the Ca site are limited to several atomic per cent. Zoning in primary titanite typically involves a decrease in the proportion of Nb and Zr toward the rim, whereas deuteric crystals show the reverse zoning pattern.
Perovskite (CaTiO3) is a common early crystallizing accessory phase in a variety of alkaline rocks, and has been shown to contain enough U and Th for U-Pb dating. U and Pb analysis of perovskite has been primarily carried out using the SHRIMP or ID-TIMS techniques, and the resulting U-Pb dates commonly yield the emplacement age of the host rock. To our knowledge, only one U-Pb study of perovskite has been done using the LA-ICP-MS (Cox and Wilton, 2006). Some of the advantages of this method over the SHRIMP and ID-TIMS techniques include greater speed and lower cost of analysis.
We examined the mode of occurrence, pattern of zoning and composition of magnetite and associated spinel-group minerals in three types of calciocarbonatite from the Kerimasi volcano, in Tanzania. In all samples, magnetite is one of the earliest phases to have crystallized, and shows an appreciable compositional variation. The majority of compositions correspond to magnetite with low to moderate proportions of magnesioferrite and ulvospinel components (10–28 and 2–28 mol.%, respectively) and 2 O 4 ). The two trace elements consistently present in appreciable amounts are V (400–2000 ppm) and Zn (700–3300 ppm); the abundances of other trace elements are much lower and very variable (≤15 ppm Cr, 170 ppm Ni, 220 ppm Co, 490 ppm Zr, 14 ppm Hf, 95 ppm Nb, 3 ppm Ta, and 80 ppm Ga). Magnetite is thus a minor host of Zr, Hf, Nb and Ta in carbonatites. The composition of magnetite crystallizing from carbonatitic magma evolves by becoming depleted in Mg and Ti, whereas its Al content inversely correlates with the V content and, thus, is sensitive to variations in f (O 2 ). The compatibility of V is interpreted to decrease, and that of Mn to increase, with increasing f (O 2 ). Covariation between the Mn and Zn contents suggests that the partitioning behavior of Zn is controlled by the coupled substitution Zn 2+ Mn 3+ Fe 2+ −1 Fe 3+ −1 . The Mg–Ti depletion trend is accompanied by a decrease in Zr and Ta contents at constant or decreasing levels of Nb and Hf, which has implications for the partitioning behavior of high-field-strength elements in carbonate melts. In addition to the magmatic evolutionary trend, the Kerimasi magnetite exhibits a previously unrecognized trend arising from a reaction of the magnetite with the carbonatitic magma. This trend involves enrichment of the peripheral parts of magnetite crystals in Mg, Al, Mn, Zr and Nb, and their mantling by Fe-rich spinel. This trend requires that a (Mg 2+ ) and a (Al 3+ ) in the magma increase with evolution, whereas a (SiO 2 ) remains low to impede the precipitation of Mg–Al silicates.
Abstract Carbonatite complexes are globally significant sources of rare earth elements (REEs); however, mechanisms governing REE deposition in various tectono-lithologic settings, encompassing host rocks, wall rocks, ore-controlling structures, and metasomatism, remain inadequately understood. The Zhengjialiangzi mining camp, situated within the extensive Muluozhai deposit (containing 0.45 million metric tons [Mt] at 4.0 wt % REE2O3) in the northern segment of the Mianning-Dechang belt, Sichuan (southwestern China), is characterized by a complex vein system that evolved within metamorphosed supracrustal rocks of the Yangxin and Mount Emei Formations. The mineralization is coeval with Oligocene intrusions of carbonatite and nordmarkite at ~27 Ma. The major gangue minerals include fluorite, barite (transitional to celestine), and calcite, with bastnäsite serving as the primary host for REEs in all analyzed orebodies. Several other accessory to minor minerals were identified in the ore veins, including some that had not previously been known to occur in the Muluozhai deposits (e.g., thorite and pyrochlore). The stable isotopic (C-O-Ca) and trace element compositions of calcite, along with whole-rock data, suggest that carbonate material was derived from the mantle and subsequently reequilibrated with the Yangxin marbles. The radiogenic isotope (Sr-Nd-Pb) compositions of vein material remained unaffected by wall-rock contamination and suggest a mantle source influenced by crustal recycling, consistent with other REE deposits hosted by carbonatite and nordmarkite in the region. The combined petrographic and geochemical evidence suggests derivation of Muluozhai mineralization from a carbonatitic source and interaction of carbonatite-derived fluids with wall rocks, xenoliths, and early-crystallizing mineral phases, particularly barite.
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