Astrophyllite-group minerals from the Ilímaussaq complex, South Greenland (contribution to the mineralogy of Ilímaussaq no. 123)

Abstract

Electron microprobe analyses are presented for astrophyllite-group minerals from hydrothermal veins and pegmatites of the Ilimaussaq complex, South Greenland. The analyses fall mainly into two groups: (1) niobophyllites with the highest Nb/(Nb+Ti) ratios yet recorded (∼0.9), occurring only in the veins, and (2) an essentially continuous sequence from astrophyllite to niobophyllite with Nb/(Nb+Ti) up to 0.6, found in veins and pegmatites. It is highly likely that there is complete solid solution between astrophyllite and niobophyllite. More limited substitution of Mn for Fe has resulted in the formation of kupletskite in some rocks. Altered zones in certain astrophyllites and niobophyllites have compositional features similar to the type ‘hydroastrophyllite’. The astrophyllite-group minerals in the hydrothermal veins crystallized at temperatures below 400°C at 1 kbar and under high pH and low oxygen fugacity, whereas those in the pegmatites were formed from water-rich melts which were hotter (≥450°C), less basic and more oxidized.

Main mafic and accessory mineralogy in granites and greisens from selected occurrences in the A-type Graciosa Province, S-SE Brazil, and petrological implications

“…Of importance, they have significant higher Mn contents (up to 0.91 apfu) and, thus, lower Fe values (up to 6.18 apfu). These compositions are akin to either magmatic or hydrothermal/metasomatic astrophyllite from St. Peters Dome and Kûgnat Fjeld Complex (Macdonald and Saunders, 1973), the Mount Gharib Complex (Abdel-Fattah and Abdel-Rahman, 1992) and to those from Luskfjell, Norway, the Oslo Rift Valley and the Illimaussaq Complex (Piilonen et al, 2003a;Macdonald et al, 2007).…”

“…The main compositional variations observed in the studied astrophyllite generations are depicted in Figure 3 (Macdonald et al, 2007); KAN: Kangerdlugssuaq (Layne et al, 1982); KGC: Kûngnat Fjeld Complex (Macdonald, 1973) (Piilonen et al, 2003a); MGC: Mount Gharib Complex (Abdel-Fattah and Abdel-Rahman, 1992); MON: Mongolian Altai (Žáček et al, 2016); OBM: Oktyabrsk Massif (Kryvdik and Sharygin, 2014); ORV2: Oslo Rift Valley (Andersen et al, 2014); SLC: Strange Lake Complex (Birkett et al, 1996); SPD: St. Peters Dome (Macdonald and Saunders, 1973).…”

“…Among the agpaitic minerals, astrophyllite [K2NaFe7Ti2(Si4O12)2O2(OH)4F] is one important HFSE-bearing accessory minerals in peralkaline systems. Most occurrences are described in nepheline syenites (Macdonald, 1973;Piilonen et al, 2003b;Andersen et al, 2014), peralkaline granites (Abdel-Fattah and Abdel-Rahman, 1992;Birkett et al, 1996;Marks et al, 2003;Siegel et al, 2018), and related pegmatites (Macdonald and Saunders, 1973;Layne et al, 1982;Macdonald et al, 2007); it is less abundant in rhyolitic volcanic rocks (Žáček et al, 2016). In these systems, astrophyllite occur either as a primary, in general late-magmatic, or as post-magmatic phase, with significant compositional variability, which monitor and estimate extensive and intensive parameters (e.g., fluid compositions, T, fO2, aH2O) acting during its crystallization (Abdel-Fattah and Abdel-Rahman, 1992;Piilonen et al, 2003bPiilonen et al, , 2004.…”

“…Figure 3-5: Nb-Zr-Ti ternary classification diagram showing (A) the measured astrophyllite compositions for both Mandira Massif (MAN) and Papanduva pluton (PAP) and (B) astrophyllite compositions for interesting localities worldwide. ILL: Illimaussaq Complex(Macdonald et al, 2007); KAN: Kangerdlugssuaq(Layne et al, 1982);…”