Trace elements and oxygen isotopes of gem spinels in marble from the Luc Yen – An Phu areas, Yen Bai province, North Vietnam

Abstract

Trace elements investigated by electron microprobe analysis (EMPA) have been combined with oxygen isotopic composition of pink, red and other colored spinels (blue, purple, brown, orange, lavender) hosted by marbles and found in placers from Luc Yen and An Phu deposits, Yen Bai province, North Vietnam. The deposits are those from Nuoc Ngap, Cong Troi, Bai Son and different placers from the An Phu area. Trace elements such as Fe-Zn-Cr-V in red and pink gem spinels permit to separate those from Cong Troi and those from the others deposits of the An Phu area. Spinels from Cong Troi have low to extremely low Zn (< 500 ppm) and high Fe content (3,000 to 16,000 ppm) while those from An Phu area are Zn-rich (up to 11,000 ppm). Iron is the dominant element for the other colored spinels whereas Zn, Cr and V contents are extremely variable. The Bai Son blue spinel is Fe-rich (5,000 to 7,200 ppm) with some V (950 to 1,830 ppm), Cr (270 to 480 ppm), Co (240 to 400 ppm) and Ni (550 to 950 ppm). The O-isotope composition of the whole spinels ranges between 12.1 and 24.2‰ (n = 25). Within each deposit, the range of δ18O values for red, pink and colored spinels is usually similar. However, the red and pink spinels from An Phu present two distinct sets of δ18O values, respectively between 13.2 to 17.0‰ (n=7) and 22.5< δ18O< 24.2 (n = 5). Those from Cong Troi are from 14.8 to 17.7 ‰ (n = 3) and their range overlaps that of An Phu. The use of O-isotopes is not useful for distinguishing between the deposits, but the low to extremely low Zn content of the Cong Trois spinels is a discriminant. The variation of δ18O values (12.1< δ18O< 24.2‰) of the whole spinels indicates that the oxygen isotopic compositions of the metamorphic fluids were probably buffered by the local δ18O values of the impure host marbles.