Origin of listwanite from Baer ophiolite, Yarlung Zangbo suture zone

Abstract: Listwanite is a suite of silica-carbonate alteration products which are formed when CO2-bearing hydrothermal fluids meet and react with serpentinized mafic and ultramafic rocks. This alteration product is of great importance for its economic significance. Gold, mercury, magnesite and base metal deposits are often associated with listwanite. In China, Not enough attention has been paid to the petrogenesis of listwanite and related mineralization. This paper reports the study of listwanite occurring along the northeast edge of Baer ophiolite in the western part of the Yarlung Zangbo?suture (YZSZ). Baer ophiolite lies in Ngari Prefecture of southwest Tibet. The Baer listwanite crops out along the northeast boundary of the ophiolite with a width of ca. 20 meters and stretches a few kilometers in NWW direction. Based on the features of spinel under optical microscope, the authors recognized two stages of listwanite: Stage I listwanite contains spinel relicts while Stage II listwanite rarely contains spinel relicts. This means Stage II listwanite experienced more intense alteration. Geochemically, Baer listwanite is mainly composed of SiO2, MgO and CO2. 8 samples (containing 3 Stage I samples and 5 Stage II samples) show variation of major oxides, and the MgO/SiO2 ratios are not consistent, which implies that the listwanization was not an iso-chemical process. Baer peridotite and two stages of listwanites show similar trace elements distribution patterns. Both stages of listwanites show little enrichment of the LREE related to the protolith. From peridotite to two stages of listwanites, the behavior of HREE might have experienced depletion followed by slight enrichment. All samples show positive Sr anomalies and their values go up stage by stage. The values of Au and Hg in both stages of listwanites show that they have not been subjected to Au and Hg mineralization.