Genetic Aspects of Albite Deposits From Central Sardinia: Mineralogical and Geochemical Evidence

Exploration & Mining Geology, Vol. 5, No. 1, 1996

R. BORNIOLI, MAFFEI Sarda S.p.A, Ottana (NU), S. FADDA and M. FIORI, Centro Studi Geominerari e Mineralurgici, C.N.R. Cagliari, Italy, S.M. GRILLO, Dipartimento di Geoingegneria e Tecnologie Ambientali, Cagliari University, Italy C. MARINI Dipartimento di Scienze della Terra, Cagliari University, Via Trentino 51, 09127 Cagliari, Italy

The Na-feldspar orebodies in Sardinia occur along structures belonging to a regional ENE-WSW system that developed during the late stages of the Hercynian orogeny, and that was reactived during the Alpine orogeny. The main petrographic facies include granitoids, albitized granitoids, albitites (more or less rich in quartz), and albitites containing Mg minerals. Field observations and petrological and geochemical data point toward a metasomatic origin for the albitite orebodies. Albitization involved depletion of K from K-feldspar and biotite, dissolution of free quartz, and a lowering of Sr/Na2O and CaO/Na2O ratios in the protolith, under conditions of a positive thermal gradient. The absence of later K-metasomatism implies that, during a subsequent decline in temperature, the composition of the albitizing fluids was modified by an influx of meteoric or other waters.

Keywords: Albite deposits, Mineralogical evidence, Geochemical evidence, Na-feldspar orebodies, albitizing fluids