GEOCHEMISTRY OF FELDSPARS AND MUSCOVITE IN GRANITIC PEGMATITE FROM THE CAP DE CREUS FIELD, CATALONIA, SPAIN

Abstract

The Cap de Creus pegmatite field exhibits a well-defined regional zonation of four types of granitic pegmatite: barren K-feldspar-rich pegmatites (type I), beryl–columbite-subtype pegmatites (type II), beryl–columbite–phosphate-subtype pegmatites (type III) and albite-type pegmatites (type IV). There is an increase in the albite content, relative to K-feldspar, with increasing pegmatite evolution. Type-I microcline-rich pegmatites have low contents of albite, whereas in type IV, albite is the only feldspar. The K/Rb value in K-feldspar decreases progressively from type-I to type-III pegmatites. In addition, there is a progressive enrichment in P, Rb, Cs, Y, Sr, Ga, Ce, W and Sn from type-I to type-IV pegmatites. Concentrations of Pb and Zr decrease in K-feldspar from type-I to type-III pegmatites. K-feldspar from the Cap de Creus pegmatites exhibits higher contents in Ba, Rb, Cs, Nb, Y, Sn, V and Cr than albite. Albite is richer in Sr, Zr, Cu, As, Ce and Ga. Pb, Zn, W, Co and Ni have similar affinity for K-feldspar and albite. Muscovite exhibits similar geochemical evolution to K-feldspar. These progressive trends suggest a common origin for all of the Cap de Creus pegmatites by fractionation of the same parental magma. The high P content of feldspars, up to 1.67 wt.% P 2 O 5 in K-feldspar and 1.75 wt.% in albite, indicates buildup in this element from the earliest stages of pegmatite crystallization. Trace-element variation in feldspars and muscovite reflects enrichments in the melt; a higher content correlates with characteristic minerals of these elements. In particular, the phosphorus content of albite is proposed as a useful mineralogical guide to niobium–tantalum-rich oxide mineralization.