Actual traces of mantle fluid from alkali-rich porphyries in Western Yunnan, and associated implications to metallogenesis

Abstract

The formation and evolution of plentiful alkali-rich porphyries with deep-xenoliths have an inevitable connection to the Cenozoic continental deformations, tectonisms, mantle magmas, anatectic fluids, and associated abundant polymetallic deposits. By the petrographical and petrochemical studies and the analyses using electron microprobe, scanning electron microscopy and energy spectrum, we confirmed that the connection was brought about by the deep geological processes and the action of ore-bearing mantle fluids. Microscopic traces and existing modalities of the deep processes and the mantle fluid action can be directly manifested as the following: Na-rich glass interpenetrated among deep xenoliths in a veined or disseminated manner; Fe-rich glasses interpenetrated in a veined, agglomerated or disseminated manner among the host rocks and various xenoliths; and iron-rich melt xenoliths as independent xenoliths existed in aegirine syenite porphyry. Both the Fe-rich glasses and iron-rich melt xenoliths are of submicroscpic crystalline texture under the electron microscope. Their main mineral compositions are silicate and quartz, as well as moissanite, native iron with chromium, specularite with titanium, ilmenite, apatite, etc. The unmixing texture, which is assumed to be caused by the unmixing of minerals between micro-metal and nonmetal, indicates that the mantle fluid has the properties of magmatic melt and is unmixing with alkali-rich magma. On the basis of synthesis analyses and in combination with the theory of metallogenesis by transmagmatic fluid, we may draw the following conclusions: the mantle fluid's properties were changed from magma→supercritical fluid→liquid by the changes of depth and environment, and corresponding changes of physical and chemical conditions, and the moving fluid carried, activated and enriched ore-material to suitable places; it was the deep process and the action of fluid that accelerated crust-mantle materials overlapping mineralization, facilitated the deep ore-forming and benefited the formation of large and supper-large deposits.