Characteristics and Geological Significance of Late Miocene Skarn-Type Tungsten Mineralization in Ramba,Southern Tibet
The Ramba leucogranite,located in the eastern part of the Northern Himalaya,is mainly composed of two-mica granite.The amphibolite in contact with it is commonly skarnized and accompanied by scheelite mineralization.To qualify the diagenetic and metallogenic ages and the ore-forming fluid characters,an integrated approach involving zircon U-Pb and mica Ar-Ar dating,and mineral geochemistry was carried out.The Ar-Ar age(6.3 Ma)of muscovite from the scheelite-bearing quartz vein yielded is comparable to the zircon U-Pb age(7.3 Ma)of two-mica granite,indicating that the granite emplacement and associated tungsten mineralization both occurred in the Late Miocene,which is the youngest known rare-metal mineralization event in the region.The iron ratios of garnet and pyroxene mineral pairs within the scheelite-bearing skarn(3.12 to 3.74),and the relatively low Mo content(12.0×10-6-56.8×10-6)of scheelite,collectively indicate that the ore-forming fluid is reduced,which is consistent with the magmatic zircon oxygen fugacity calculated results(ΔFMQ=-5.78 to-2.08).The rare-earth element characteristics of scheelite are inherited from the leucogranite granite,further indicating that the W-rich ore-forming fluid was dissolved from the evolved granitic melt and subsequently reacted with the Ca-rich wall rocks to induce the precipitation of scheelite.This study reveals that the contact metamorphic zone between Himalayan leucogranite and surrounding wall rock is a favorable site for rare-metals,and deserves attention in future prospecting.
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