Abstract:

R/Ra values of fluid inclusions from pyrite and quartz veins from alkali-rich porphyry molymetallic deposits in western Yunnan are mainly in the range of 0.160 8-3.470 0, being much higher than the corresponding values of the crust, but a little lower than those of the mantle; the average ²⁰Ne/²²Ne values and ²¹Ne/²²Ne values are averaged at 11.271 0 and 0.032 2, being close to those of the mantle;⁴⁰Ar/³⁶Ar values and ³⁸Ar/³⁶Ar values are averaged at 395.510 0 and 0.197 60 respectively, being higher than the air ratio but are lower than the MORB ratio; compared with air, ¹²⁸Xe/¹³⁰Xe，¹²⁹Xe/¹³⁰Xe，¹³¹Xe/¹³⁰Xe，¹³²Xe/¹³⁰Xe，¹³⁴Xe/¹³⁰Xe，¹³⁶Xe/¹³⁰Xe values show a surplus characteristics; δ³⁰Si values of the silicon-rich ore-forming fluids vary from -2.4‰ to -0.1‰, showing a characteristic feature of the original mantle fluids almost without  isotopic kinetic fractionation, while δ³⁰Si values of the alkali-rich magma range from 0.0‰ to 0.4‰,showing a feature of the metasomatic enriched mantle fluids with drastic isotopic kinetic fractionation. So noble gas and silicon isotopic compositions reveal that ore-bearing fluids have both mantle-derived and the crust-modified characteristics in western Yunnan. The ore-forming materials mainly derived from the differentiation products of the metasomatized enriched mantle. These alkali-rich magma and related silicon-rich ore-forming fluids carried the ore-forming materials upwards into the crust through the deep faults. The replacement and alteration of the alkali-rich porphyry and their host strata by the silica-rich ore-forming fluids during the crystallization processes of the alkali-rich magmas led to the overprinting and contamination of the mantle and crust materials. It is such a process of fluids that makes up the key controlling factors for the Cenozoic alkali-rich porphyry polymetallic deposits in western Yunan.

Key words: noble gas isotope, silicon isotope, crust-mantle mixing, mantle-fluid metasomatism, western Yunnan