The formation process and Mo (W) mineralizaton of the skarn in the Nannihu-Sandaozhuang Mo (W) deposit

Based on field observation and microscopic and backscattered electron (BSE) images, the authors recognized a two-stage formation process of the skarn in the Nannihu-Sandaozhuang Mo (W) deposit. At the first stage, the fluid reacted with marble to form wollastonite, andradite, grossularite, hedenbergite and diopside. As the fluid migrated outward, the dark limestone was replaced by diopside skarn or wrigglite diopside skarn. At the second stage, the fluid spread into the wall rock along fractures to form new skarn which was superimposed on the earlier skarn. According to these observations, the skarn minerals such as garnet and pyroxene could be divided into two generations. The first generation of garnet Grt_I) (Gro_3-82And_15-96) is spot-like in wollastonite marble or skarn, and the first generation of pyroxene (Px_I) (Di_18-86Hd_13-70Jo_0-13) could intergrow with Grt_I in wollastonite marble or skarn and with plagioclase (Ab_55-70An_30-44) in wrigglite diopside skarn. The second generation of garnet (Grt_II) (Gro_23-58And_37-74) and the second generation of pyroxene (Px_II) (Di_0-68Hd_28-84Jo_3-16) were superimposed on GrtI and Px_I with smaller space area. The mineralization of molybdenite and scheelite commenced at the very beginning of skarnization and covered the whole skarn formation process. The local reducing environment, induced by the reaction between the fluid with adequate concentration of Mo and W elements and the wall rocks might have triggered metal precipitation.