The Cenozoic Alkali-rich Igneous Rocks, Western Yunnan And Their Relation With Metallogenesis Of Gold

In this dissertation, based on the data and fruits of previous studies, we take Beiya gold deposit and associated alkali-rich intrusions as main studying objects, aim to discuss the formation processes of the Cenozoic alkali-rich igneous rocks, the differentiation process of fluids from the alkali-rich magma during its uprising and evolution, and genesis of the Beiya gold deposit, and consequently discover the relationship between the Beiya gold deposit and associated alkali-rich intrusions. By studies of major and trace element (include REE) geochemistry, isotope (e.g. Pb, Sr, Nd, C, O, S and He-Ar) geochemistry and fluid inclusion geochemistry we have acquired the following principal cognitions:1. The Cenozoic alkali-rich intrusive rocks in western Yunnan are derived from enriched metasomatized mantle (EM- II), and the metasomatizing processes of mantle area were related to the subduction of the Palaeo-Tethyan oceanic crust. The metasomatism of hanging mantle wedge by aqueous fluids from subducted slab is accounted for the aqueousness of the alkali-rich intrusive rocks in western Yunnan which are equivalent to A-type granite. Thus the alkali-rich magmatism is considered to be a delayed arc magmatism. The intermediate-felsic alkali-rich magma is evolved from alkaline basaltic magma by assimilation of ca.20% of crustal materials that mainly from metamorphosed basement. The alkali-rich magma has relatively high-oxygen-fugacity (logfO2>FMQ+2, FMQ represents fayalite-magnetite-quartz buffer) as convergent margin magmas and has differentiated large amounts of fluids during its uprising and evolution process, and thus is of Cu-Au high mineralization potential.2. The mineralization age of Beiya gold deposit is dated as 33 ± 1.5Ma, which is consistent with the age of associated alkali-rich intrusions (32~34Ma).3. The primary fluid or early stage ore-forming fluid related to Beiya gold deposit has a Nd and Sr isotopic composition (i.e. Isr 0.722390, Nd(t) -2.8) identical to that of the fluid differentiated from alkali-rich magma within the metamorphosed basement, a Pb isotopic composition (i.e. ~206Pb/~204Pb= 18.590, . ~207pb/~204pb= 15.606 , ~208pb/~204pb =38.770 identical tothe average of alkali-richintrusions within the district or in the periphery of Beiya gold deposit, a mimic chondrite-normalized REE pattern of associated alkali-rich intrusions, and a C\ O isotopic composition (i.e. ~13C -5‰, 18O 11‰) peculiar to magmatic hydrothermal fluids, as well as a crust-mantle mixing He-Ar isotopic composition (i.e. ~3He/~4He= 1.80-1.94 Ra, ~3He/~36Ar =1.19 X 10~-3 -3.93X 10~-3) that also shows remarkableHe-Ar differentiation (~40Ar*/~4He=0.34-0.75). In addition, it is characterized by high-temperature-pressure, high-salty and CO2-rich. The above signatures indicate that primary fluid is principally differentiated from alkali-rich magma within the metamorphosed basement. The mineralizing fluids were gradually evolved from the primary fluid by separation of CO2-rich and thO-NaCl phases with the decrease of temperature and pressure and preferential partition of ore-forming metals into the CO2-rich phases, and released the ore-forming metals by processes of gas loss or mixing with meteoric water.4. About 80% of ore-forming metals are from the alkali-rich magma and the other fractions might be mainly scavenged by the magmatic fluid from metamorphosed basement.Conclusion: the Beiya gold deposit has a close genetic relationship with associated alkali-rich intrusions.