Halogen-rich Scapolite-Biotite Rocks from the Tongmugou Pb-Zn Deposit, Qinling, Northwestern China: Implications for the Oreforming Processes

Shao-Yong Jiang, M. R. Palmer, Chun-Ji Xue and Yan-He Li
Department of Geology, University of Bristol, Bristol, BS8 1 RJ, U.K.
Xi’an College of Geology, Xi’an, 710054, China
Institute of Mineral Deposits, Beijing, 100037, China

Abstract: Coexisting scapolite, biotite and hornblende in scapolite-biotite rocks from the Tongmugou Pb-Zn deposit, Qinling, northwestern China are characterized by high levels of chlorine. Scapolite composition varies from EqAn27 to EqAn47 with 47–80 wt.% Cl. The scapolite composition is a sensitive indicator of the NaCl activity in coexisting hydrothermal fluid. Biotite contains 0.3–1.2 wt. % Cl and also has high F contents (0.2–0.7 wt.%). The hornblende is a Cl-rich hastingsite with Cl>3.5 wt.% and high (Na2O + K2O) contents (3.2–3.9 wt. %), high Xk [= K/(K + Na)] values (0.45–0.55) and high XFc [= Fe/(Fe + Mg)] values (0.76–0.79). The chlorine within these minerals is thought to be derived from evolved seawater. The scapolite-biotite rocks are products of Cl-rich alteration of volcanoclastic sedimentary rocks during submarine hydrothermal processes. Multiple-stage hydrothermal activity culminated with the circulation of a high-NaCl fluid that was also responsible for the formation of the massive sulphide deposits.

Keywords: halogens • scapolite-biotite rock • hydrothermal fluids • China • Pb-Zn deposits

Mineralogical Magazine; December 1994 v. 58; no. 393; p. 543-552; DOI: 10.1180/minmag.1994.058.393.02
© 1994, The Mineralogical Society
Mineralogical Society (www.minersoc.org)