Petrology and space-time structure of alkali-rich dikes and its constraints on mineralization in the Laojiezi lead-silver deposit, central Yunnan

The Laojiezi Pb-Ag deposit, central Yunnan, is located in Yao'an lead-polymetallic mine district in Jinshajiang-Ailaoshan alkali-rich porphyry metallogenic belt.Alkali-rich dikes, widely developed in the area and closely related to the structure, according to mineral assemblage, are divided into syenite porphyry dikes, trachyte porphyry dikes, lamprophyre dikes and leucite porphyry dikes. Pb-Ag mineralized bodies mostly occur in syenite porphyry and are enriched in the hanging wall and footwall of trachyte porphyry dikes.The alkali-rich dikes are controlled by the fault structure and are mainly distributed in NE and NW direction.Combined with the interspersed relationship of alkali-rich dikes(masses), the sequence of emplacement is inferred as follows: trachyte → syenite → trachyte porphyry→ leucite porphyry→ lamprophyre.Through the petrogeochemical characteristics, it is considered that the four types of alkali rich dikes are belong to the shoshonite, formed in the intraplate extensional environment after collision in the subduction background, mainly derived from the thickened lower crust and mixed with a small amount of enriched mantle components.Different degrees of magma evolution, zonation in the magma chamber in the source area and emplacement layer by layer of magma lead to the the finally formation of several types of alkali-rich dikes(masses) with different mineral assemblages and chemical components.Comprehensively comparing and analyzing the temporal and spatial distribution and source characteristics of the four types of dikes, and exploring the coupling relationship with tectonic and mineralization, constructing a time-space model of alkali-rich dikes, so as to provide a basis for deep and peripheral prospecting predictions.