The Tanco Pegmatite At Bernic Lake, Manitoba. Xvii. Mineralogy and Geochemistry of Alkali Feldspars

Abstract The alkali-feldspar paragenetic sequence in the Tanco pegmatite spans the entire crystallization and cooling history of the pegmatite and includes several different K-feldspar and albite varieties. Total feldspar modal abundance approaches 50% for the entire pegmatite volume. Blocky K-feldspar, aplitic albite, and albite of the cleavelandite habit together account for most of the total feldspar volume, whereas late, low-temperature feldspars in veins and cavities are mostly contained in inner zones associated with pollucite and petalite. In an inward progression through the concentrically zoned pegmatite, average bulk compositional data for blocky K-feldspar reveal steady increases in Rb 2 O (1.45–3.08 wt.%) and Cs 2 O (0.11–0.29 wt.%), increasing followed by decreasing contents of P 2 O 5 (0.34–0.47 wt.%), Li 2 O (0.014–0.095 wt.%), and Tl (105–239 ppm), decreasing Na 2 O (2.5–1.1 wt.%) and CaO (0.13–0.08 wt.%), and variable BaO (0.004–0.17 wt.%) and Ga 2 O 3 (0.006–0.010 wt.%). In aplitic albite and albite of the cleavelandite habit, CaO decreases (0.08–0.03 wt.%) and overall K 2 O and P 2 O 5 contents are variable. Late adularia feldspars from veins and clusters in pollucite exhibit extreme ranges in contents of Rb 2 O (1.80–24.84 wt.%) and Cs 2 O (0.10–1.43 wt.%) and significant P 2 O 5 (up to 0.51 wt.%). X-ray powder-diffraction data indicate near-maximum structural order of the early feldspar varieties as maximum microcline and high albite. Hydrothermal adularia crystals are essentially endmember (Or 100 ) high sanidine. The large and economically significant Tanco pegmatite represents an extreme in terms of petrologic processes involving fractional and disequilibrium crystallization from granitic melts enriched in volatiles and fluxing components (H 2 O, B, F, P), rare alkalis (Li, Rb, Cs), and high field strength elements (Ta, Nb, Zr, Hf). The alkali feldspars are the most significant carriers of P, Li, Rb, Tl, and Cs prior to the stabilization of large volumes of amblygonite–montebrasite, petalite, lepidolite, and pollucite. The rise and fall of trace-element content in feldspars, in relation to the spatial distribution of different zones and mineral assemblages, has implications in terms of the internal evolution of complex granitic pegmatites, as well as on the types and extents of subsolidus readjustments.