Fenites associated with carbonatite complexes: A review

Abstract

A B S T R A C TCarbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization. Although many attempts have been made in the literature to categorize and name fenites, it is recommended that the IUGS metamorphic nomenclature be used to describe predominant mineralogy and textures. Complexing anions greatly enhance the solubility of REE and Nb in these fenitizing fluids, mobilizing them into the surrounding country rock, and precipitating REE-and Nb-enriched micro-mineral assemblages. As such, fenites have significant potential to be used as an exploration tool to find mineralized intrusions in a similar way alteration patterns are used in other ore systems, such as porphyry copper deposits. Strong trends have been identified between the presence of more complex veining textures, mineralogy and brecciation in fenites with intermediate stage Nb-enriched and later stage REE-enriched magmas. However, compiling this evidence has also highlighted large gaps in the literature relating to fenitization. These need to be addressed before fenite can be used as a comprehensive and effective exploration tool.

Light rare earth element redistribution during hydrothermal alteration at the Okorusu carbonatite complex, Namibia

“…High-temperature hydrothermal activity commonly affects the rocks around carbonatite complexes, notably through the formation of fenites (Zharikov et al, 2007, Elliott et al 2018, and so it is reasonable to ask if such magmatic fluids played a role in the hydrothermal alteration described here. Carbonatite melts are low viscosity (Jones et al, 2013) and rich in chloride salts and volatiles, predominantly CO 2 and H 2 O (Mitchell, 2005;Rankin 2005).…”

The common origin and alteration history of the hypabyssal and volcanic phases of the Wadi Tarr albitite complex, southern Sinai, Egypt

“…Fenites are metasomatic products demonstrating pervasive modification of precursor wall rocks that interacted with emanations from alkaline silicate or carbonatite magmas (e.g. Bates and Jackson, 1987;Zharikov et al, 2007;Le Bas, 2008;Elliott et al, 2018). However, fenitization is rather a generic process; in the current study area there is evidence that the alteration products are variable in character, which may reflect variations in the protoliths or in fluid composition, permeability, structure, temperature and pressure.…”

Geology and Mineralogy of Rare Earth Elements Deposits and Occurrences in Finland

“…There is a strong connection between the complexity of fenite textures and associated mineralization due to the enrichment of REE in the intermediate and late stage carbonatite magma generations which produced multiple pulses of fenitizing fluids [21,25]. The presence of brecciated zones within the carbonatite and alkaline complex is the result of an explosive release of fluids and volatiles, thus brecciation indicates development of more evolved magma generations and increased potential of Nb and REE enrichments in the source intrusion [25,26].…”