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We present results of U-Pb dating (by MC-ICP-MS) of zircons from samples that cover all of the known lithotypes in the Seis Lagos Carbonatite Complex and associated lateritic mineralization (the Morro dos Seis Lagos Nb deposit). The host rock (gneiss) yielded an age of 1828 ± 09 Ma interpreted as the crystallization time of this unit. The altered feldspar vein in the same gneiss yielded an age of 1839 ± 29 Ma. Carbonatite samples provided 3 groups of ages. The first group comprises inherited zircons with ages compatible with the gneissic host rock: 1819 ± 10 Ma (superior intercept), 1826 ± 5 Ma (concordant age), and 1812 ± 27 Ma (superior intercept), all from the Orosirian. The second and the third group of ages are from the same carbonatite sample: the superior intercept age of 1525 ± 21 Ma (MSWD = 0.77) and the superior intercept age of 1328 ± 58 Ma (MSWD = 1.4). The mineralogical study indicates that the ∼1.3 Ga zircons have affinity with carbonatite. It is, however, a tendence rather than a well-defined result. The data allow state that the age of 1328 ± 58 Ma represents the maximum age of the carbonatite. Without the same certainty, we consider that the data suggest that this age may be the carbonatite age, whose emplacement would have been related to the evolution of the K'Mudku belt. The best age obtained in laterite samples (a superior intercept age of 1828 ± 12 Ma) is considered the age of the main source for the inherited zircons related to the gneissic host rock. [ABSTRACT FROM AUTHOR]

The Morro dos Seis Lagos niobium rare earth element, Ti-bearing lateritic deposit (Amazonas, Brazil) is derived from a primary siderite carbonatite. The complex is the only example of a Nb deposit in which Nb-rich rutile is the main Nb ore mineral. Apart from the laterites, at the current level of exposure the complex consists only of siderite carbonatite; silicate rocks are absent. Three types of siderite carbonatite are recognized: (1) a brecciated and oxidized core siderite carbonatite consisting of up to 95 vol% siderite together with: hematite; pyrochlore; Nb-brookite; Ti-maghemite; and thorobastnäsite; (2) a REE- and P-rich variety of the core siderite carbonatite consisting of siderite (up to 95 vol%), hematite, minor pyrochlore, monazite and bastnäsite; (3) a border hydrothermal siderite carbonatite with ~70 vol% siderite, barite (~15 vol%), gorceixite (~7 vol%) and minor rhabdophane and pyrochlore. The country rock gneiss in which the carbonatite was emplaced was affected by potassic fenitization, with the formation of phlogopite and orthoclase together with monazite, fluorapatite and bastnäsite. The siderite carbonatites exhibit a wide variation of δ13C (−5.39‰ to −1.40‰), accompanied by a significant variation in δ18O (17.13‰ to 31.33‰), especially in the REE-rich core siderite carbonatite, and are explained as due to the presence of both H 2 O and CO 2 in the magma. The core siderite carbonatite is the richest in Fe (48.64–70.85 wt% Fe 2 O 3) and the poorest in Ca (up 0.82 wt% CaO) example of a siderite carbonatite yet recognized The ferrocarbonatite has significant contents of Mn, Ba, Th, Pb and LREE, and a very high Nb (up to 7667 ppm) content due to the presence of Nb-brookite. The substitution 3Ti4+ = Fe2+ + 2Nb5+ recognized in Nb-rich brookite explains enrichment of Nb in the core siderite carbonatite and indicates formation in a reducing environment. The high Nb/Ta ratio (1408–11,459) of the carbonatite is compatible with residual liquids derived by fractional crystallization. The 87Sr/86Sr (0.70411–0.70573) and 144Nd/143Nd (0.512663–0.512715) isotopic data suggest the carbonatite is mantle-derived with essentially no crustal contamination and is younger than the maximum age of 1328 ± 58 Ma (U Pb in zircon). We suggest that the Morro dos Seis Lagos carbonatite complex represents the upper-most parts of a differentiated carbonatite magmatic system, and that the siderite carbonatite is related to late-magmatic-to-carbo-hydrothermal processes. • Portrait evolution of the Morro dos Seis Lagos carbonatite complex. • The siderite carbonatite is the primary rock of the world's largest Nb-deposit. • The complex represents the upper-most parts of a carbonatite magmatic system. • The siderite carbonatite is related to late-magmatic-to-hydrothermal stage. • First isotopic data on the Morro dos Seis Lagos carbonatite. [ABSTRACT FROM AUTHOR]

Abstract: The geochemistry of zircon from the granites that host the Sn-Rare-Metal–cryolite deposit of Pitinga Mine in northern Brazil, Amazonia, is discussed based on data obtained by LA–ICP–MS. The ore deposit is one of the largest in the world and is related with F-rich A-type granite intrusions of 1822 ± 2 Ma. REE, Y, U, Th, Nb, Ta, Pb, and Hf contents were determined in zircon grains from the albite-bearing facies that contains the ore deposit and from less evolved facies composed of amphibole–biotite and biotite granites. The trace-element contents of zircon were compared to those of their host rocks and the calculated zircon/rock ratios are like the values of zircon/melt partition coefficients for natural granitic compositions. The concentrations found for all analysed elements are highly variable, even for determinations made in the same grain. However, the average contents and patterns are like those of typical magmatic zircon and can indicate the composition of the melts from which they were crystallized. The interpretation of trace element contents in the zircon grains suggests that: (i) in the albite-bearing facies, zircon crystallized after the volatile phase exsolution and shows typical geochemical features such as: Th/U ratios from 1 to 10, Y/Ho is lower than 20, Sm/Nd ratios are generally higher than 0.5, Nb/Y is higher than 0.08, and Hf is over 2 wt%; (ii) M-type tetrad effects were produced in the REE patterns of most differentiated melts by F-complex stabilization, and were preserved in some zircon grains; (iii) ore deposition in the Pitinga mine initiated in the late stages of magmatic crystallization mainly following resurgent boiling. The trace element contents of zircon are particularly relevant for provenance studies if mineral/melt partition coefficients are taken into account, so that the approximate trace element pattern of their igneous source can be estimated. The geochemistry of trace elements in zircon, in spite of the wide range of contents, can produce a very significant set of information useful for metallogenetic, petrogenetic, exploration, geochronological, and provenance studies. [Copyright &y& Elsevier]