Fluid and melt inclusion study on mineralized and barren porphyries, Jinshajiang-Red River alkali-rich intrusive belt, and significance to metallogenesis

Alkali-rich Cu (Au, Mo) deposits are of increasing economic significance and are an attractive exploration target. They include some of the world's highest grade and largest porphyry related gold resources as well as some of the largest gold accumulations in epithermal settings. The Jinshajiang–Red River alkali-rich intrusive belt, with many porphyry Cu (Au, Mo) deposits, is a representative magmatic belt associated with mineralization. The Jinshajiang–Red River alkali-rich intrusive belt contains several Cu (Au, Mo) mineralized alkali-rich porphyry rocks including the Yulong quartz monzonite porphyry, the Machangqing granite porphyry, the Tongchang quartz syenite porphyry, and the Beiya quartz syenite porphyry. Additionally, there are also some barren alkali-rich porphyry rocks in the belt, such as the Yanshuiqing quartz syenite porphyry. Fluid inclusion petrography and microthermometry on those porphyry rocks are carried out in detail. The results show that the fluid inclusion assemblages in ore-bearing and barren porphyries are distinct: inclusions from barren porphyry are dominated by primary melt inclusions, and contain rare or no fluid inclusions, whereas inclusions from ore-bearing porphyries are dominated by fluid inclusions, and contain rare melt inclusions. Furthermore, halite, sylvite, calcite daughter minerals and an opaque phase in fluid inclusions from ore-bearing rocks are common, but rare in fluid inclusions from barren rocks. The results suggests that the evolution of ore forming fluids especially the halite, sylvite, calcite and opaque daughter minerals bearing fluid inclusions of quartz phenocrysts could be used to judge the degree of metasomatism and mineralization of a porphyry system.