Oxygen, hydrogen, and lead isotope studies of the Taolin lead-zinc ore deposit, China

Abstract

The Taolin Pb-Zn deposit consists of a series of hydrothermal veins emplaced in a fault zone between the 136-m.y.-old Mofushan granitic batholith and the Proterozoic metasediments of the Lengjiaxi formation. Two stages of ore deposits were distinguished: I, sulfides-quartz-fluorite and II, barite-sulfides-quartz-calcite. The delta 18 O values of the vein quartz have been used, together with temperatures based on sulfur isotope fractionation between cogenetic sulfides (Ding and Rees, 1984), to show that delta 18 O values of water of the hydrothermal fluid tended to decrease throughout the cooling and progressive precipitation of the ore deposit. The D/H ratios of water in fluid inclusions vary erratically through this sequence but show no overall trend between stages I (x = -46 + or - 10ppm) and II (x = -48 + or - 13ppm). A stage II quartz-calcite assemblage was precipitated at the lowest observed temperature, from essentially pure meteoric water (delta D = -46ppm). The isotopic evolution of the hydrothermal fluid can be accounted for by circulation of meteoric water through a convection system heated by the granite, causing exchange of oxygen isotopes with the granite and country rock, and possibly involving some admixture of magmatic water. Lead isotopes in both the ore and granite give a model age of 418 m.y.; this, combined with the high delta 18 O values of least-altered granite, suggests that the granite formed by anatexis of lower crustal, U-depleted rock. The lead of the ore deposit was probably derived from the granite.