FLUID EVOLUTION IN THE ZONED RARE-ELEMENT PEGMATITE FIELD AT CAP DE CREUS, CATALONIA, SPAIN

Abstract

The Cap de Creus LCT granitic pegmatites, in Catalonia, northeastern Spain, belong to the beryl – columbite and beryl – columbite – phosphate subtypes and to the albite type. Fluid inclusions have been studied in all these types of pegmatite to determine the nature of the fluids involved in their formation. Beryl – columbite pegmatites are zoned; border, wall, intermediate zones and a quartz core and albite-dominant replacement bodies are distinguished. Beryl – columbite – phosphate pegmatites, in addition, feature veins of albite and quartz–muscovite. In the albite pegmatites, phosphate and albite–chlorite veins also occur. Five types of fluid inclusions have been identified: hypersaline aqueous–carbonic fluid inclusions (type A), low-salinity aqueous– carbonic inclusions (type B), carbonic inclusions (type C), multiphase aqueous fluid inclusions (type D), and late multiphase aqueous–carbonic fluid inclusions (type E). In the early stages of crystallization, a hypersaline fluid exsolved from the melt. It is CO 2 -rich, 41–46 wt.% NaCl eq., in beryl – columbite – phosphate pegmatites, and the aqueous fluid contains 23–38 wt.% NaCl eq. in albite pegmatites. A second immiscibility process occurred in the last stages of the crystallization of the intermediate zones; the fluid unmixed into two fluids: one CO 2 -rich, salt-poor (3–5% NaCl eq.) and the other hypersaline (up to 35% NaCl eq.) and CO 2 -poor. The CO 2 -rich fluid is related to the formation of the quartz core, and the second fluid formed albite veins and circulated throughout the pegmatite, owing to the development of a network of fractures, which produced a Na-metasomatic alteration of the previously formed units. These processes took place at 415–450°C and 2.2–2.7 kbar. Later, quartz–muscovite veins developed at 2.4 kbar and 400°C. The fluid that formed these veins (36–39 wt.% NaCl eq.) produced a K-rich alteration that affected the previously formed pegmatite units. In addition, in albite pegmatites, late albite–chlorite veins formed at pressures lower than 700 bars and more than 340°C.