Th-Rich Zircon from a Pegmatite Vein Hosted in the Wiborg Rapakivi Granite Massif

Abstract

Zircon from a pegmatite vein hosted in the rapakivi granites occurring in the Gubanov Intrusion of the Wiborg Massif was shown by the EPMA method to contain an anomalously high Th of 18.3 wt% ThO2 (about 161,000 ppm Th). The SIMS method obtained a close value for thorium content—146,000 ppm. U increases to 6800 ppm. Zircon contains high values of other non-formula elements: Y (up to 51,500 ppm), Ti (up to 12,500 ppm), P (up to 9000 ppm), Nb (up to 3500 ppm), Ca (up to 2800 ppm) and Ba (up to 600 ppm). The average Hf value is 10,900 ppm. Total REE ranges from 25,500 to 49,400 ppm, and the average REE is 33,300 ppm. The REE patterns show poorly HREE fractionation over the LREE (LuN/LaN ratio average is 6.34) with well-marked negative Eu anomalies (Eu/Eu* averages 0.10) and slight positive Ce anomalies (Ce/Ce* averages 1.16). The formation of Th-rich zircon indicates that residual fluid-saturated pegmatite melt is saturated with Th and other trace elements (REE, Y, U, Ti and Nb).