The Metallogenic Model Of The Typical Deposits In Jinsha River—Ailaoshan Alkali-Rich Porphyry Belt

The Jinshajiang-ailaoshan alkali porphyry metallogenic belt is one of the most important ore areas studying on the Cenozoic metallogeny in China and also a significant economic ore field yielding Cu, Au, Pb, Zn ores. On this thesis, we specifically selected the Beiya polymetallic Au deposit and Machangqing Cu-Mo-Au deposit as the representative studying objects, and conducted systemic and comprehensive investigation in the aspects of fieldwork, mineralogy and thermodynamics combined with other multidisciplinary approaches, and then we quantified their geochemical variations as to support our assumption. Consequently, we ascertained the petrogenesis of the alkali porphyry and the origin-immigration-deposition processing and mechanism of enrichment, and modeled the alkali porphyry metallogenic processes of the polymetallic ore deposit.The U-Pb dating of zircons from the alkali porphyry-porphyritic granite-quartz syenite porphyry series employing LA-ICP-MS technique yields an age of 36±0.3 Ma, which is 2 Ma younger than the previous age(34.7±0.5 Ma) by molybdenite Re-Os dating from reference. The quartz porphyric syenite yielded an age of 36±0.8 Ma, which is very close to the molybdenite Re-Os age of 36.8±0.5 Ma, indicating the simultaneous magmatic and ore-forming process. Geochemical evidences from major, trace elements and isotopes indicate that the alkali porphyry bears certain adakitic signatures, which possibly is the mixed product of partibal melting of thickened lower crust and certain mantle substances.Based on the investigation of field log of drill core combined with microscopic mineralogy, we studied the ore-forming phases of the two ore deposits. The Machangqing deposit has certain potassium alteration in its early ore phase, little alteration observed within the ore vein in the main phase, and obvious chlorite-sericite alternation in the ore vein from the late phase. The early ore phase has the characteristics of high temperature, medium-high salinity and a high pressure, with the phenomenon of phase separation. For the main ore phase, the ore-forming fluids have the features of medium-high temperature, medium-high salinity and decreased pressure, and precipitation of ore Cu, Mo, etc. due to the fluid boiling caused by depressurization. During the late phase, the ore-forming fluids are featured by with medium-low temperature, low salinity and low pressure. The H-O isotopic pattern shows that the ore-forming fluids predominately derives from the magma in its earlier phase and added by the meteoric water in its later ore phase. In the Beiya ore deposit potassium alteration can be observed within its ore vein in its early phase, and the main ore phase shows the signature of skarn alternation with the presence of chlorite and epidote. The late ore phase signifies by the sericite and chlorite alteration. Sharing similar features to the Machangqing ore deposit, the temperature and salinity of the ore-forming fluids keep declining with the evolution going. The ore Cu and Fe isotopes present that the Cu element may originate from the deep magma. The Fe isotopes indicate that lamprophyre or MME and the dark color xenolith in the alkali porphyry may also provide certain ore-forming substances to the magnetite origin.Summarily we proposed a metallogenic model for the alkali porphyry polymetallic deposit. The Cenozoic Jinshajiang-Red River, etc., large deep strike-slip and pull-apart fracture induced the formation of the alkali porphyry series as granite porphyry-monzonitic granite porphyry. Meanwhile the separation of the melting and liquids during fractional crystallization of the primary magma and the interaction between fluids and the wallrocks leads to the formation of the large scale of metallogenesis.