macla nº 16. junio ‘12 218 revista de la sociedad española de mineralogía

Subsolidus Processes in the Longonjo and Bailundo Carbonatites (Angola) / AMAIA CASTELLANO (1), JOAN-CARLES MELGAREJO (1), PURA ALFONSO (2) AURORA C. J. M. BAMBI (), ANTONIO-OLIMPIO GONALVES ()

(1) Departament de Cristallografia, Mineralogia i Dipsits Minerals, Universitat de Barcelona, Martí i Franqus, 002 Barcelona. (2) Departament d’Enginyeria Minera i Recursos Naturals. Universitat Politcnica de Catalunya, Avda. Bases de Manresa, Manresa. () Departamento de Geologia, Universidade Agostinho Neto, Avda. de fevereiro, 11 Luanda, Angola.

INTRODUCTION strong silicification event on the apatite crystals (Fig. 1), accompanied carbonatites. by accessory minerals as magnetite, The Bailundo and Longonjo carbonatites primary pyrochlore, baddeleyite and are extremely affected by subsolidus pyrite. processes leading to extensive development of iron oxide deposits. Secondary minerals fully replace the Hence, these carbonatites were early mineral associations. A general considered as a target for Fe and, in less silicification produces milky quartz extension, P. In addition, these filling veins; at the same time fine- carbonatites are a good laboratory to grained anhedral quartz replaces the study the behavior of Nb and the REE hosting carbonates along grain rims. during the subsolidus processes. The Hydrothermal quartz may be aim of this contribution is to provide accompanied with fine-grained information about the textures and euhedral pyrochlore, ilmenorutile, chemistry of some secondary minerals. barite, fluorite and abundant fine- grained secondary apatite (Fig. 1). GEOLOGICAL SETTING Sr-rich dolomite and different Bailundo and Longonjo carbonatite fig. 1. SEM-BSE image of a sample of an altered generations of calcite are the main complexes are located in the Huambo carbonatite from the center of the Bailundo component of the replaced carbonatite complex, showing primary and province, and are among the largest secondary phases. Mineral abbreviations: Pcl: carbonatites. Cathodoluminescence carbonatite bodies in Angola. These pseudomorphs of euhedral primary pyrochlore, Ap: (CL) has been used to distinguish intrusives belong to the Parana-Namibia- primary apatite, Cal: calcite, MnOx: secondary Mn- among different generations of oxide, FeOx: secondary Fe-hydroxides Angola early Cretaceous alkaline- pseudomorphic after magnetite crystals. Cal-Ap: fine carbonates (Fig. 2) and apatite. The carbonatitic province; wich is related to intergrowths of secondary carbonates with EPMA analyses of these calcite crystals the break-up of Pangea (Ernst and Bell, secondary apatite. PREE: Late REE phosphates. show that small differences in the 2010). Bailundo and Longonjo massifs proportions of Mn, Mg and Fe produce are associated with the Lucapa The external parts of the complexes are an important difference in CL behavior structure, a tectonic corridor that characterized by ring dikes, made of (Table 1). crosses Angola trending NE-SW. Most of carbonatite breccias with a strong the carbonatites and kimberlites in fenitization of the host rock. Calcite wt wt wt CL Angola occur into this tectonic structure Mn Mg Fe All the carbonatites have been (more details in Castellano et al. 2011). 1 0.20 0.38 0.61 none extremely weathered and residual clay 6 0.81 0.09 0.05 yellow STRUCTURE deposits unconformably cover part of 0.59 0.07 0.07 yellow the outcrops. 0.01 0.62 0.02 none The carbonatites of Bailundo and Longonjo are hosted by Archean granitic PETROGRAPHY Table 1: Example of changes in CL among different generations of calcite, as related with changes in rocks exhibiting a deep fenitization. Both major chemical composition. ring complexes have concentric dikes The primary mineral assemblages have around the carbonatitic core, been strongly overprinted by new In absence of significative contents of corresponding with a plug intrusion in mineral associations produced during REE elements, Mn should be the CL subvolcanic context. hydrothermal and supergenic processes. activator and Fe the CL quencher in Therefore, it is difficult to reconstruct the these samples. However, the CL The core of the structures consists of primary mineral associations. Textural behavior of calcite could be influenced carbonatites and carbonatite breccias. relationships of samples of NW and by small amounts of REE elements, Both units are crosscut by a dense center of Bailundo and Longonjo which need more precise analysis to be network of quartz veins. These veins are massifs, suggest a primary association detected. The low proportions of Sr and 1 cm to 1 m in width and produced a comprising an intergrowth of calcite and REEs in the analyzed samples, in any

palabras clave: Carbonatita, Pirocloro, Carbonatos de REE key words: Carbonatite, Pyrochlore, REE carbonates

resumen SEM 2012 * corresponding author: [email protected]

macla nº 16. junio ‘12 revista de la sociedad española de mineralogía 219

case, suggest that all of these calcite complex and has important differences However, these elements could also be generations are secondary. with the well established alteration present in the hosting granitic rocks, and sequences in pyrochlores from other be enriched in the fluids during the carbonatites in Angola and in the world fenitization processes. In this case, (i.e., Wylliams et al., 1997; Torró et al., differences in composition between 2012). secondary pyrochlores can be explained by different degrees of interaction with Worldwide pyrochlore crystals are the host rock. Stable and radiogenic generally complex, and the existence of isotope data are necessary to solve this several generations has been attributed problem. to modifications of the primary compositions, by subsolidus processes, ACNOWLEDGEMENTS due to the action of hydrothermal and supergenic fluids. This type of This research has been financially replacements produces changes mainly supported by the Spanish project in the A position, and at the same time, CGL2009-13758 and the SGR444 of the the original F is replaced by OH. In a first Generalitat de Catalunya. A. Castellano stage, Na can be extracted from the received a grant from the Spanish structure. In a second stage, Ca can be MICINN and A. Bambi a grant from AECI. partly or completely removed. Charge The Departamento de Geologia of the fig 2. Complex internal texture of a pyrochlore balance is achieved by introducting Ba, Agostinho Neto University provided the crystal from Bailundo. A Pb-rich secondary Pb or Sr. necessary field assistance. pyrochlore (1b) replaced completely the crystal along the primary zoning, and a late generation with Ca-Na-F rich pyrochlore (1) replaces all the above In general terms, the Bailundo and REFERENCES generations along the grain rims and cracks. SEM- Longonjo pyrochlores follow these BSE image. chemical trends of alteration. However, Alberti A, Castorina F, Censi P, Comin- the pyrochlore crystals from Longonjo Chiaramoni P, Gomes CB (1): and Bailundo have some clearly Geochemical characteristics of Cretaceous secondary pyrochlores having F-, Ca- and carbonatites from Angola. J. Afr. Earth Sci. Na- rich compositions, close to those 2: -. from the typical primary pyrochlores (fig. Castellano Calvo A., Melgarejo J.C. , Alfonso 2). On its turn, these “primary” Abella P., Joao Mateus A., Gonalves A. Castillo M. (2011): Minerales de Nb y REE pyrochlores are again replaced by the en las carbonatitas de Longonjo y normal sequence of secondary Bailundo, Angola. Macla, 1: -6. pyrochlores. De Araujo AG, Perevalov OV (1): Carta de recursos minerais - Mineral Resources Secondary pyrochlores are enriched in Map. Ministério de Geologia e Minas. Ba in Bailundo, whereas those from Instituto Geologico de Angola, Luanda. Longonjo are devoid of Ba. , , Jukov RA (1): Carta geológica de Angola - Geological Map of Angola. Ministrio da Indústria. Instituto Nacional fig . Nb-bearing rutile partly replaced along grain Nb-bearing rutile is more common in de Geologia, Luanda. rims by goethite. Slighter areas in the rutile core Longonjo. It occurs in discrete small De Carvalho H, Tassinari C, Alves PH (2000): correspond to Nb-rich domains. SEM-BSE image. anhedral grains scattered in the matrix Geochronological review of the of secondary carbonates or replacing Precambrian in western Angola: links with Fe-Mn botryoidal hydroxides are very magnetite. They are heterogeneous Brazil. J. Afr. Earth Sci. 1, -02. common and can produce large because of slight changes in the Nb Egorov N, Roman’ko EF, Podvysotsky VT, accumulations. Most of them are content (Fig. 3). They are replaced by Sablukov SM, Garanin V, D’yakonov DB produced by pseudomorphic iron hydroxides. (200): New data on kimberlite replacement of the Fe- and Mn-rich magmatism in southwestern Angola. Russian Geology and. Geophysics. , 2- carbonates, magnetite and pyrite, and DISCUSSION AND CONCLUSIONS 6. can be accompanied with a late Ernst RE, Bell (2010): Large Igneous generation of Mn-rich calcite. Large The complex sequence of alteration of Provinces (LIPs) and carbonatites. amounts of fine-grained REE phosphates pyrochlores can be explained by the Mineralogy and Petrology : -6. and REE carbonates overgrowth these repetition of subsolidus processes, Lapido-Loureiro FEV (1: Carbonatitos de late oxides. Finally, a late generation of perhaps because the existence of new Angola. Memorias e trabalhos Instituto de thin calcite veinlets cuts all the pre- hydrothermal events related with Investigaoes Científicas de Angola, 22 p. Torró, L, Villanova, C., Castillo, M., Campeny, existing minerals. different pulses of carbonatite M., Gonalves, A.O., Melgarejo, J.C. (2012): intrusions. Niobium and rare earth minerals from the Nb- MINERALS Virulundo carbonatite, Namibe, Angola. The source for the elements found in the Mineral. Mag. 6, -0. Pyrochlore from Longonjo is found as secondary pyrochlores (Ba, Sr, Pb) is Williams, C.T., Wall, F., Woolley, A.R., Phillipo, small euhedral octahedral crystals, less controversial. Large amounts of Ba and S. (1): Compositional variation in than 250 µm, but in Bailundo they can Sr could be produced during pyrochlore from the Bingo carbonatite, reach up to 1 mm in diameter. Internal replacement of primary magmatic Zaire. J. African Earth Sci. 2, 1-1. texture of all of these grains is extremely carbonates by hydrothermal fluids.