Styles of Alteration of Ti Oxides of the Kimberlite Groundmass: Implications on the Petrogenesis and Classification of Kimberlites and Similar Rocks

The sequence of replacement in groundmass perovskite and spinel from SK-1 and SK-2 kimberlites (Eastern Dharwar craton, India) has been established. Two types of perovskite occur in the studied Indian kimberlites. Type 1 perovskite is found in the groundmass, crystallized directly from the kimberlite magma, it is light rare-earth elements (LREE)-rich and Fe-poor and its ΔNNO calculated value is from −3.82 to −0.73. The second generation of perovskite (type 2 perovskite) is found replacing groundmass atoll spinel, it was formed from hydrothermal fluids, it is LREE-free and Fe-rich and has very high ΔNNO value (from 1.03 to 10.52). Type 1 groundmass perovskite may be either replaced by anatase or kassite along with aeschynite-(Ce). These differences in the alteration are related to different f(CO2) and f(H2O) conditions. Furthermore, primary perovskite may be strongly altered to secondary minerals, resulting in redistribution of rare-earth elements (REE) and, potentially, U, Pb and Th. Therefore, accurate petrographic and chemical analyses are necessary in order to demonstrate that perovskite is magmatic before proceeding to sort geochronological data by using perovskite. Ti-rich hydrogarnets (12.9 wt %-26.3 wt % TiO2) were produced during spinel replacement by late hydrothermal processes. Therefore, attention must be paid to the position of Ca-Ti-garnets in the mineral sequence and their water content before using them to classify the rock based on their occurrence

Magma mingling in kimberlites: evidence from the groundmass cocrystallization of two spinel-group minerals

We present the results of a detailed petrographic study of fresh coherent samples of the Menominee kimberlite sampled at site 73, located in Menominee County, MI, USA. Our objective is to account for its unusual and complex paragenetic sequence. Several generations of olivine, ilmenite, and spinel-group minerals are described. Early olivine and ilmenite are xenocrystic and were replaced or overgrown by primary minerals. Zoned microcrysts of olivine have a xenocrystic core mantled by a first rim in which rutile, geikielite, and spinel s.s. (spinel sensu stricto) cocrystallized. The in situ U-Pb dating of a microcryst of primary rutile yielded 168.9 ± 4.4 Ma, which was interpreted as the age of emplacement. The groundmass consists of olivine, spinel s.s., a magnesian ulvöspinel-ulvöspinel-magnetite (MUM) spinel, calcite, and dolomite. An extremely low activity of Si is suggested by the crystallization of spinel s.s. instead of phlogopite in the groundmass. The presence of djerfisherite microcrysts indicates high activities of Cl and S during the late stages of melt crystallization. The occurrence of two distinct spinel-group minerals (spinel s.s. and qandilite-rich MUM) in the groundmass is interpreted as clear evidence of the mingling of a magnesiocarbonatitic melt with a dominant kimberlitic melt

Minerales de Nb y REE en las carbonatitas de Longonjo y Bailundo (Angola)

Bailundo y Longonjo son dos de los mayores macizos carbonatíticos de Angola, en los que Lapido-Lourelio (1973) y Alberti et al. (1999) mencionaron la presencia de minerales de Nb, pero las condiciones políticas impidieron durante más de 40 años el desarrollo de estudios de detalle. El objetivo de este trabajo consiste en aportar nuevos datos referentes a la mineralogía de los elementos raros, así como de los procesos subsolidus implicados en el enriquecimiento en Nby REE de las carbonatitas

Nb and REE minerals from the Virulundo carbonatite (Namibe, Angola)

The Virulundo carbonatite is located in SW Angola, in the Namibe province, and is one of the biggest carbonatite outcrops in the world. It is a Cretaceous subvolcanic plug, which belongs to the Parana-Namibia-Angola alkaline-carbonatitic province. It is found intruding Early Archean granitoids, which are fenitized on the contact

Minerales de Nb y REE en las carbonatitas de Longonjo y Bailundo, Angola

Bailundo y Longonjo son dos de los mayores macizos carbonatíticos de Angola, en los que Lapido-Lourelio (1973) y Alberti et al. (1999) mencionaron la presencia de minerales de Nb, pero las condiciones políticas impidieron durante más de 40 años el desarrollo de estudios de detalle. El objetivo de este trabajo consiste en aportar nuevos datos referentes a la mineralogía de los elementos raros, así como de los procesos subsolidus implicados en el enriquecimiento en Nby REE de las carbonatitas.Peer ReviewedPostprint (published version

Cu-U-V stratabound deposits in red beds in the Catalan Pyrenees: Structure and Mineralogy

The Eureka mining group (La Plana de Mont-Ros, Pyrenees, Catalonia) has one of the best outcrops of Cu-U-V stratabound deposits in red-beds of Western Europe. The deposit was mined in surface trenches and underground galleries. This has allowed an accurate mineralogical and textural study of this deposit, which can be used as a model to explain the relationships between Cu-U-V minerals during precipitation of primary mineralization and supergenic stage

Mineralogy and distribution of critical elements in the Sn–W–Pb–Ag–Zn Huanuni deposit, Bolivia

The polymetallic Huanuni deposit, a world-class tin deposit, is part of the Bolivian tin belt. As a likely case for a “mesothermal” or transitional deposit between epithermal and porphyry Sn types (or shallow porphyry Sn), it represents a case that contributes significantly to the systematic study of the distribution of critical elements within the “family” of Bolivian tin deposits. In addition to Sn, Zn and Ag, further economic interest in the area resides in its potential in critical elements such as In, Ga and Ge. This paper provides the first systematic characterisation of the complex mineralogy and mineral chemistry of the Huanuni deposit with the twofold aim of identifying the mineral carriers of critical elements and endeavouring plausible metallogenic processes for the formation of this deposit, by means of a multi-methodological approach. With In concentrations consistently over 2000 ppm, the highest potential for relevant concentrations in this metal resides in widespread tin minerals (cassiterite and stannite) and sphalerite. Hypogene alteration assemblages are hardly developed due to the metasedimentary nature of host rocks, but the occurrence of potassium feldspar, schorl, pyrophyllite and dickite as vein material stand for potassic to phyllic or advanced argillic alteration assemblages and relatively high-temperature (and low pH) mineralising fluids. District-scale mineralogical zonation suggests a thermal zonation with decreasing temperatures from the central to the peripheral areas. A district-scale zonation has been also determined for d34SVCDT values, which range -7.2‰ to 0.2‰ (mostly -7‰ to -5‰) in the central area and -4.2‰ to 1.0‰ (mainly constrained between -2‰ and 1‰) in peripheral areas. Such values stand for magmatic and metasedimentary sources for sulfur, and their spatial zoning may be related to differential reactivity between mineralising fluids and host rocks, outwardly decreasing from the central to the peripheral areasPeer ReviewedPostprint (published version

Ti-V oxide deposits in the Kunene Anorthositic Complex (SW Angola)

The Kunene Complex, outcropping between SW Angola and NW Namibia, is one of the biggest Proterozoic anorthositic complexes in the world [1]. It is cut by a set of Ti-Fe oxide dykes or lenticular bodies, that are hectometric to kilometric in length and metric to decametric in width

Ti-rich Cr-spinel and Ni-Fe-Cu sulphides from the Hamutenha basic-ultrabasic intrusion (Cunene anorthosite complex, Angola)

The Cunene complex is a large group of gabbro-anorthosite intrusions that covers an area of 15.000 km2 between southern part of Angola and northern Namibia. Although ultrabasic rocks have not been found to data in the main anorthositic intrusion of the Cunene complex, a great number of small basic-ultrabasic bodies appear close to the west margin of the main intrusion

Mineralizaciones de Nb y tierras raras en la carbonatita de Virulundo (Namibe, Angola)

Los complejos carbonatíticos de Angola forman parte de la estructura de Lucapa, que se extiende a través de Angola en dirección NE-SW. A su vez, estas estructuras forman parte de la provincia carbonatítica de ParanáAngola, extendiéndose hasta el Brasil. Estos cuerpos, de edad Cretácico inferior, a menudo están asociados con intrusivos alcalinos. En Angola se reconocen cuerpos carbonatíticos de dimensiones y estructura muy variables