Chemical and Rb–Sr, Sm–Nd isotopic systematics of tourmaline from the Dachang Sn-polymetallic ore deposit, Guangxi Province, P.R. China

Abstract

The Dachang Sn-polymetallic ore deposit (Guangxi, China) is one of the largest Sn deposits in the world, which occurs in Devonian siliceous rocks and limestones as stratiform, massive and vein orebodies. Various models for the ore genesis of the deposit have been proposed, and the major debate focuses on whether the stratiform Sn–Pb–Zn orebodies are syn-sedimentary in origin or they were products of the Yanshanian magmatic-hydrothermal event (∼100 Ma) like the vein-type ores. In this paper, we present a chemical and Rb–Sr, Sm–Nd isotopic study of tourmaline, which occurs both in the stratiform orebodies and in veins associated with the granite in the Dachang deposit, and use these data to constrain the origin of the tourmaline and associated mineralisation. Two types of tourmaline occur in the Dachang deposit: (I) tourmaline from the stratiform Sn–Pb–Zn ores and their host siliceous rocks; (II) quartz-tourmaline veins in or near the granite. Group I tourmalines are Mg-rich dravites with low Fe/(Fe+Mg) ratios (0.01–0.25) and variable Na/(Na+Ca) ratios (0.62–1.0); group II tourmalines are Fe-rich schorls with high Fe/(Fe+Mg) and Na/(Na+Ca) ratios (0.81–0.88 and 0.92–0.97, respectively). Group II tourmalines have low REE contents and LREE-enriched patterns with negative Eu anomalies (Eu/Eu*=0.06 to 0.35). In contrast, group I tourmalines have higher and more variable REE contents with HREE-enriched patterns and negative to positive Eu anomalies (Eu/Eu*=0.18 to 3.64). They also have different contents for other trace elements such as U, Th, Zr, Hf, and Sn, which may reflect the nature of the hydrothermal fluids that circulated deep into the footwall lithologies. Group I tourmalines have 87 Sr/ 86 Sr ratios of 0.71339–0.71818 and 143 Nd/ 144 Nd ratios of 0.51201–0.51210; while the group II tourmalines have more variable 87 Sr/ 86 Sr ratios (0.71187–0.72735) and slightly higher 143 Nd/ 144 Nd ratios (0.51210–0.51224). The Sr and Nd isotopic data display different mixing trends, suggesting different origins for the two types of tourmalines. Group I tourmalines are thought to have formed during the Devonian from deeply circulating submarine hydrothermal fluids. The fluids leached Sr and Nd from the footwall lithologies (with high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd ) and mixed Sr and Nd derived from the overlying Devonian host sedimentary rocks (with low 87 Sr/ 86 Sr and intermediate 143 Nd/ 144 Nd ). Group II tourmalines formed from Yanshanian magmatic-hydrothermal fluids, with their Sr and Nd derived from a granitic source (remelted crustal rocks with high 87 Sr/ 86 Sr and relatively high 143 Nd/ 144 Nd ) and again mixing with Sr and Nd derived from host Devonian sedimentary rocks.