HALOGEN-RICH SCAPOLITE-BIOTITE ROCKS FROM THE TONGMUGOU PB-ZN DEPOSIT, QINLING, NORTHWESTERN CHINA - IMPLICATIONS FOR THE ORE-FORMING PROCESSES

Coexisting scapolite, biotite and hornblende in scapolite-biotite rock s from the Tongmugou Pb-Zn deposit, Qinling, northwestern China are ch aracterized by high levels of chlorine. Scapolite composition varies f rom 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 X(K) [= K/(K + N a)] values (0.45-0.55) and high X(Fe)[= Fe/(Fe + Mg)] values (0.76-0.7 9). The chlorine within these minerals is thought to be derived from e volved seawater. The scapolite-biotite rocks are products of Cl-rich a lteration of volcanoclastic sedimentary rocks during submarine hydroth ermal 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.