Geochemistry and Origin of Listwanites in the Sartohay and Luobusa Ophiolites, China

摘要

Gold-bearing listwanites of the Sartohay ophiohte, northwestern China, consist mainly of talc, magnesite, chlorite, quartz, and dolomite, locally accompanied by chromian mica (fuchsite). They formed along NE-trending faults by hydrothermal alteration of serpentinized peridotites. The original protoliths were clinopyroxene-bearing harzburgites and Iherzolites containing relatively aluminous magnesiochromite. Compared to their protoliths, listwanites generally have lower MgO and significantly higher CaO, TiO_2, and total rare-earth elements. Gold mineralization occurs chiefly in large quartz veins with grades up to several tens of g/t. Listwanites in the Luobusa ophiolite, Tibet, form a narrow band along the southern boundary fault of the ophiolite. Some of these listwanites have relict textures, suggesting derivation from serpentinized peridolite; however, adjacent harzburgites and clinopyroxene-bearing harzburgites are very fresh, with less than 1 percent serpentinization. The Luobusa listwanites consist essentially of talc and magnesite with lesser amounts of quartz. Most are indistinguishable geochemically from the unserpentinized protoliths except for having notably higher loss on ignition. None of these rocks are mineralized. The Sartohay listwanites are spatially associated with granitic intrusions dated at 301 Ma. They were altered by relatively high temperature fluids, as indicated by homogenization temperatures of fluid inclusions in quartz ranging from 340 to 360 deg C. Those in the Luobusa ophiolite are not associated with any known igneous activity and are believed to have formed at somewhat lower temperatures. Although both varieties are completely altered and have generally similar mineral assemblages, only those of Sartohay show significant geochemical modification and gold mineralization. Both types have been moderately silicified and are easily recognized by their light brown color and resistant outcrops.