Gajdošová M., Huraiová M., Hurai V., Slobodník M. & Siegfried P.R., 2019: Two types of scapolite in Evate carbonatite deposit (Mozambique): implications for magmatic versus metamorphic origins. Acta Geologica Slovaca, 11, 2, 63–74.

Two types of scapolite in Evate carbonatite deposit (Mozambique): implications for magmatic versus metamorphic origins

Michaela Gajdošová¹, Monika Huraiová¹, Vratislav Hurai², Marek Slobodník³ & Pete R. Siegfried⁴

¹Department of Mineralogy and Petrology, Faculty of Natural Sciences, Comenius University in Bratislava, Ilkovičova 6, 842 15 Bratislava, Slovakia; michaela.gajdosova@uniba.sk
²Earth Science Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia
³Institute of Geological Sciences, Masaryk University, Kotlářská 267/2, 611 37 Brno, Czech Republic
⁴Geoafrica Prospecting Services cc, PO Box 24218, Windhoek, Namibia

Abstract

Two types of scapolite occur in the Evate carbonatite deposit – the largest resource of apatite in south-east Africa. Calc-silicate rocks composed of amphiboles (hastingsite, hornblende, actinolite, and pargasite), diopside, Ba-rich phlogopite, allanite, epidote, apatite, K-feldspar, titanite, and minor calcite, also contain scapolite with 62–74 mol. % of meionite – (Me) end-member, XCl = 0.06–0.17 and the crystallochemical formula corresponding to (Ca_2.46–2.99Na_1.00–1.47)_{Σ4.00–4.20}Al_4.62–5.04Si_6.95–7.37O₂₄(Cl_0.06–0.17S_0.18–0.26C_{0.64 –0.70})_Σ1.00. In contrast, carbonatite contains scapolite ranging from Na meionite to Ca marialite with 46–63 mol. % Me, XCl = 0.19–0.47 and the crystallochemical formula (Na_1.43–2.07Ca_1.82–2.50)_{Σ3.97–4.11}Al_4.29–4.73Si_7.27–7.70O₂₄(Cl_0.19–0.47S_{0.00– 0.25}C_0.45–0.74)_Σ1.00. The chemical composition of scapolites mirrors the complex history and multistage evolution of the Evate deposit. The low-Cl, high-S scapolites from calc-silicate rocks are chemically indistinguishable from metamorphic scapolites, whereas the Cl-rich scapolite from carbonatite is interpreted as magmatic in origin. The gradual Cl enrichment reflects an increasing NaCl activity in the scapolite-forming fluid or melt. Both scapolite types are closely associated with amphibole and phlogopite, thus indicating amphibolite-facies conditions during retrograde stages of Late-Proterozoic (Ediacaran) granulite-facies metamorphism and/or Ordovician reactivation.

Key words: scapolite, Evate deposit, Mozambique

Manuskript doručený: 2019-06-24

Manuskript revidovaný: 2019-11-14