Two Paragenetic Types of Cookeite from the DolnÍ Bory-hatĚ Pegmatites, Moldanubian Zone, Czech Republic: Proximal and Distal Alteration Products of Li-bearing Sekaninaite

Cookeite occurs in pseudomorphs after sekaninaite associated with muscovite, quartz, tourmaline, albite, nontronite, and chlorite in pegmatite No. 1 (proximal cookeite type), and in pockets lined with albite and quartz in the Oldřich pegmatite (distal cookeite type), Dolní Bory-Hatě, Bory pegmatite district, Moldanubian Zone, Czech Republic. The mineral assemblages of the Bory pegmatites with common Al,Fe-rich minerals (annite, schorl, sekaninaite, andalusite, diaspore, ilmenite, and almandine) suggest their primitive, peraluminous, LCT geochemical signature. Proximal cookeite forms aggregates of sheets, up to ~500 μm in diameter, intergrown with prevailing muscovite and quartz. Beige to grayish, porous aggregates of distal cookeite, up to 15 cm in size, consist of thin sheets, up to ~70 μm in diameter, locally associated with small grains of pyrite and pale brown siderite as a matrix. Electron microprobe data of cookeite from the No. 1 and Oldřich pegmatites yielded very similar results: Si (3.23–3.27, 3.21–3.32 apfu) and Al (4.59–4.65, 4.48–4.65 apfu), respectively, and low concentrations of Fe (Fe_tot ⩽ 0.08 apfu), Mg (⩽ 0.07 apfu), and K (⩽ 0.15 apfu). Concentrations of Li, B, and Be obtained by LA-ICP-MS (and wet chemical analysis—Li, Oldřich) of cookeite from both pegmatite dikes yielded an average of 2.22 wt.% Li₂O (0.78 apfu) and 2.38 wt.% Li₂O (0.84 apfu), low contents of B = 370–780 ppm and 250–430 ppm, and very low Be ⩽16 ppm and ⩽1.2 ppm for pegmatite No. 1 and the Oldřich pegmatite, respectively. Strong hydrothermal alteration of common Li-bearing sekaninaite (0.11–0.55 wt.% Li₂O) is evident and a potential source of Li for both the proximal and distal cookeites. Cookeite precipitated from hydrothermal fluids at T ~350–270 °C and P < ~2 kbar. These data are consistent with the cookeite formation in granitic pegmatites and the distal cookeite demonstrates the high mobility of Li in hydrothermal conditions. The crystallization of cookeite at the No. 1 and Oldřich pegmatites was followed by crystallization of an Fe-rich mineral—nontronite and siderite, respectively.