Crystallization and destabilization of eudialyte-group minerals in peralkaline granite and pegmatite: a case study from the Ambohimirahavavy complex, Madagascar

Abstract

Eudialyte-group minerals (EGM) are very common in highly evolved SiO2-undersaturated syenites and are characteristic minerals of agpaitic rocks. Conversely, they are extremely rare in peralkaline granites, with only a handful of EGM occurrences reported worldwide. Here, we study two new examples of EGM occurrence in two types of peralkaline pegmatitic granites from the Cenozoic Ambohimirahavavy complex, and assess the magmatic conditions required to crystallize EGM in peralkaline SiO2-oversaturated rocks. In the transitional granite (contains EGM as accessory minerals) EGM occur as late phases and are the only agpaitic and major rare-earth element (REE) bearing minerals. In the agpaitic granite (contains EGM as rock-forming minerals) EGM are early-magmatic phases occurring together with two other agpaitic minerals, nacareniobsite-(Ce) and turkestanite. In these granites, EGM are partly-to-completely altered and replaced by secondary assemblages consisting of zircon and quartz in the transitional granite and an unidentified Ca-Na zirconosilicate in the agpaitic granite. Ambohimirahavavy EGM, as well as those from other peralkaline granites and pegmatites, are richer in REE and poorer in Ca than EGM in nepheline syenites. We infer that magmatic EGM are rare in SiO2-oversaturated rocks because of low Cl concentrations in these melts. At Ambohimirahavavy, contamination of the parental magma of the agpaitic granite with Ca-rich material increased the solubility of Cl in the melt promoting EGM crystallization. In both granite types, EGM were destabilized by the late exsolution of a fluid and by interaction with an external Ca-bearing fluid.

Adsorption of rare earth elements in regolith-hosted clay deposits

“…The country rocks (Mesozoic marl, limestone and mudstone) and the main syenites have low abundances of primary REE-bearing minerals, whereas the peralkaline granite, nepheline syenite and pegmatite dykes contain a wide range of REE-rich minerals, including REE fluorcarbonates, zirconosilicates, silicates, oxides and minor phosphates. Many of these comprise a secondary paragenesis formed through late-magmatic hydrothermal alteration of primary igneous phases, such as eudialyte-group minerals (EGMs) (Na-Ca-zirconosilicates) 35 , alongside Nb minerals, such as members of the pyrochlore group and aeschynite-(Y) 28,34 . The Malagasy regolith profile is up to 30 m thick and comprises, from the bottom up: unaltered bedrock, saprock, saprolite and a pedolith made up of a mottled zone, a ferruginous zone and a topsoil (Fig.…”

“…The Malagasy regolith profiles developed on highly heterogeneous bedrock comprising silica-oversaturated and -undersaturated syenites, peralkaline granites and pegmatites, as well as volcanic lithologies belonging to the Cenozoic Ambohimirahavavy subvolcanic ring complex (c. 23 Ma) 27 – 29 , 33 , 34 . The country rocks (Mesozoic marl, limestone and mudstone) and the main syenites have low abundances of primary REE-bearing minerals, whereas the peralkaline granite, nepheline syenite and pegmatite dykes contain a wide range of REE-rich minerals, including REE fluorcarbonates, zirconosilicates, silicates, oxides and minor phosphates.…”

Alkali pyroxenes and amphiboles: a window on rare earth elements and other high field strength elements behavior through the magmatic-hydrothermal transition of peralkaline granitic systems

“…at Ambohimirahavavy, unknown at Manongarivo) by an assemblage of secondary HFSE-bearing minerals (Lacroix 1923;Rakotovao et al 2009;Estrade et al 2018).…”

“…All pegmatites and granites are dominated by quartz, alkali feldspar (perthites, albite, orthoclase, and/or microcline) that are commonly strongly albitized, plus alkali amphiboles and, mostly in pegmatites, aegirine Ca and Na zirconosilicates, which are characteristic minerals of agpaitic rocks, are mostly eudialyte-group minerals (EGM) at Ambohimirahavavy (Lacroix 1923;Estrade et al 2018), and elpidite at Evisa, Khan Bogd and Strange Lake (Bonin 1988;Salvi and Williams-Jones 1990;Grigor'eva et al 2011). These zirconosilicates were partially to totally replaced by secondary miaskitic mineral assemblages, which commonly form pseudomorphs (Poitrasson et al 1998;Estrade et al 2014b;Gysi et al 2016).…”

Hydrothermal Alteration of Eudialyte-Hosted Critical Metal Deposits: Fluid Source and Implications for Deposit Grade

“…Late-stage hydrothermal alteration of eudialyte is inextricably linked to agpaitic complexes and is often attributed to the fluid-rich nature of the melts [22][23][24][25][26][27][28]. In Ilímaussaq, eudialyte alteration ranges from localised breakdown along crystal edges and cracks to complete pseudomorphic replacement.…”