THE SR, ND ISOTOPIC COMPOSITION OF LAMPROPHYRES IN LAOWANGZHAI GOLD OREFIELD, YUNNAN PROVINCE

Lamprophyres are widely spread over the Laowangzhai gold orefield in n orthern Mt. Ailao structure zone, Yunnan Province. Lamprophyres in the orefield are temporally and spatially related to gold mineralization. Lamprophyres in the orefield have anomalous Sr and Nd isotopic compos itions, e.g. the Sr-87/Sr-86 ratios (0.706 665-0.707 960, the Sr-87/Sr -86 ratio Of sample YLW-44 is 0.709 041) are higher than the modem val ue oi the original mantle (0.704 5), the Nd-143/Nd-144 ratios (0.512 4 63-0.512 551) are lower than the modern value of the original mantle ( 0.512 638), and the calculated values epsilon(Sr) > 0 (8.1-63.7) and e psilon(Nd) < 0 ( - 1.34- - 3.07). The results of investigation indicat e that lamprophyres in the Laowangzhai gold orefield with anomalous Sr and Nd isotopic compositions would not be produced as a result oi con tamination of the primary magma with the isotopic features of MORE by the crust materials high 87Sr/86Sr ratios and low Nd-143/Nd-144 ratios in the processes of rising or in the magma chamber, but the result of metasomatism of source mantle.