Carbonatites and Carbonatites and Carbonatites

Abstract

Carbonatites are redefined using a mineralogical-genetic classification and divided into two groups: primary carbonatites, and carbothermal residua. Attention is drawn to the fact that carbonatite is both a petrographic term applicable to a particular rocktype as well as a group name applied to a complex of related carbonate and silicate rocks in a magmatic or extrusive complex. Primary carbonatites, in terms of mineralogical-genetic classifications, rather than simple modal classifications, can be divided into a group of bona fide magmatic carbonatites formed from diverse mantle-derived magmas, i.e., carbonatites associated with the melilitite, nephelinite, aillikite and kimberlite clans, with the latter best being termed calcite kimberlites. Each magma type and associated carbonatites are considered to be genetically distinct, and formed at different depths in the upper mantle by different degrees of partial melting. Carbonatites associated with the melilitite and nephelinite clans can have a multiplicity of origins, and may be formed by fractional melting, fractional crystallization or liquid immiscibility. Calcite kimberlites are smallvolume late-forming differentiates that are not related to other carbonatites or their parental magmas. The origin and genetic relationships of the Oldoinyo Lengai natrocarbonatite cannot be unambiguously determined, although these rocks are regarded as a distinct variety of primary carbonatite. Carbonate-rich rocks associated with diverse potassic or sodic peralkaline saturated to undersaturated magmas derived predominantly from metasomatized lithospheric mantle, together with REE-carbonate-rich rocks of undetermined genesis, are best termed carbothermal residua rather than carbonatite. There can be mineralogical (or modal) convergence between these rocks and low-pressure REE-rich derivatives of bona fide primary carbonatites. Carbonate-rich rocks formed by pneumatolytic reactions or anatectic melting of crustal rocks should not be considered to be carbonatites. soMMAIRe Le clan des carbonatites est ici redéfini dans le contexte d'une classification minéralogique et génétique, et subdivisé en deux groupes: carbonatites primaires et résidus carbothermaux. On souligne le fait que "carbonatite" est à la fois un terme pétrographique applicable à un type de roche particulier, et un terme faisant référence à un groupe de roches, et appliqué à des roches carbonatées and silicatées génétiquement liées dans un complexe magmatique ou extrusif. On peut diviser les carbonatites primaires, en termes de classifications minéralogiques et génétiques plutôt que de simples classifications modales, en un groupe de carbonatites véritablement magmatiques formées à partir de divers magmas mantelliques, i.e., carbonatites associées aux clans de mélilitite, néphélinite, aillikite et kimberlite, avec ces dernières manifestations préférablement appelées kimberlites à calcite. Chaque type de magma et les carbonatites associées seraient génétiquement distincts, et formées à des profondeurs différentes du ...

Fluid-rock interaction in the Kangankunde Carbonatite Complex, Malawi: SEM based evidence for late stage pervasive hydrothermal mineralisation

“…Carbonatites are also considered primary mantle melts generated through partial melting of CO 2 -bearing peridotite [15][16][17][18][19]. They are subdivided into primary magmatic carbonatites and carbothermal residual carbonatites formed from low temperature fluids that are rich in CO 2 , H 2 O and fluorine [20]. Although carbonatite magmas have formed from primary melts or as immiscible melt fractions of CO 2 -saturated silicate melt, the associated RE carbonatite are considered to be a late 'magmatic' to 'hydrothermal' event [21][22][23][24][25][26][27][28][29].…”

Rare Earth Elements (REE) Deposits Associated with Great Plain Margin Deposits (Alkaline-Related), Southwestern United States and Eastern Mexico

“…The carbonatites at Laughlin Peak and in Mexico are best described as carbonatites associated with potassic and sodic plutonic rocks [203]. This type of carbonatites typically represents low-temperature, low-pressure, fractional-crystallization derivatives of diverse alkaline parental magmas derived from metasomatized lithospheric mantle [203][204][205]. Thus, both upper mantle and lower to upper crustal source rocks are involved in forming GPM deposits, which is supported by the geochemical and isotopic evidence (Tables 1 and 4).…”

A deposit model for carbonatite and peralkaline intrusion-related rare earth element deposits

“…Most identified carbonatites are intrusive bodies, but a few extrusive examples are known, including the active Oldoinyo Lengai carbonatite volcano in northern Tanzania (Van Straaten, 1989). Carbonatites can be quite diverse and likely originate from multiple processes (Woolley, 2003;Mitchell, 2005). Alkaline intrusive rocks tend to be spatially associated with carbonatites and contain elevated concentrations of REEs; thus, we chose to treat these two subclasses of alkaline rocks together.…”

“…Field data, experimental petrology, stable and radiogenic isotopic systematics, trace element geochemistry, and evaluation of mantle xenoliths provide insight into the nature of the initial melts and their possible evolutionary paths. Mitchell (2005) stressed that not all carbonatites are alike and suggested that the chemical variations of carbonatites could be the result of different mantlesource characteristics. Current models for the generation of carbonatites have been summarized in recent reviews by Le Bas (1987), Gittins (1989), Bailey (1993), andWinter (2001).…”