Abstract:

The Machangqing Mo-Cu-Au polymetal deposit is a typical deposit which is associated with the Cenozoic alkaline porphyry along the Jinshajiang-Ailaoshan fault. Based on the results of CL images, LA-ICP-MS trace elements analysis, U-Pb dating of zircon from ore-bearing porphyritic granite and Re-Os dating of molybdenite, referring to theory of the transmagmatic fluid and former reseach, genetic mechanism of the Machangqing deposit are discussed in detail here. It is suggested that the Machangqing polymetal deposit and the ore-bearing porphyritic granite were formed at the same time. The process of diagenesis and mineralization are controlled by mineralizing fluid effect caused by large scale event of Cenozoic alkali-rich magma and granitic magma from deep crust remelting. The mineralizing fluid is the ore-bearing mantle fluid contained in magma unmixingly. During migration and ascent of the mineralizing fluid, the mineralizing fluid and magma separated in varying degrees, following crystallization diagenesis of magma. It shows three kinds of typical mineralization, porphyry copper deposit of orthomagmatic mineralization in the inner alkali-rich porphyritic granite, skarn copper (molybdenum) deposit of contact metasomatic mineralization at the contact zones between porphyry and surrounding rocks and the tectonoclastic altered rock type gold deposit in strata. In these processes, the property  of mantle fluid was changed from magma→supercritical fluid→hydrothermal fluid following the changes in depth and environment ,and the corresponding changes of physical and chemical conditions, and the moving fluid carried, activated and enriched ore-materials to suitable places; it was the action of fluid that accelerated crust-mantle materials overlapping mineralization, even resulted different commodities and varied types of metallogenetic influence in different localizations.

Key words: Machangqing in western Yunnan, granite, zircon U-Pb dating, Re-Os dating of molybdenite, between mantle-crust interaction, metallogenic series, ore deposit