论文部分内容阅读
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.