Study Of The Cretaceous Fe And Sn Mineralization In Tengchong–Linghe District,Sanjiang Region,Southwestern China

Tengchong–Lianghe ore district is one of the important metallogenic districts in southwest Sanjiang metallogenic domain.The early Cretaceous magmatism led to Fe-polymetallic mineralization in skarn,while the late Cretaceous magmatism led to Sn mineralization in greisen.Mineralization difference between the two magmatism can not be explained because the deficiency studies about typical deposits and controversy to the properties of the two kinds of granitoids.In this thesis,the Diantan magnesian skarn Fe deposit?¹23 Ma?and the Xiaolonghe greisen Sn deposit?⁷2 Ma?were detailedly studied in terms of geological characteristics and ore-forming fluid evolution,the no-metallogenic granitoids were compared with the ore-forming granitoids,and the ore-forming granitoids in Fe deposits were contrast to those in Sn deposits.The source,enrichment and precipitation mechanism of Fe,Sn in deposits were revealed during the magma-hydrothermal fluid process.Based on the analysis of minerals,major and trace elements,and zircon?_Hf?t?,it indicates that both the early Cretaceous no-metallogenic granitoids and the ore-forming granitoids have high oxygen fugacity and experienced lower fractional crystallization.Their sources were manily derived from the mature ancient crust,but the source of ore-forming granitoids has more mantle-derived materials.On the contrary,both the late Cretaceous no-metallogenic granitoids and ore-forming have low oxygen fugacity and their sources were manily derived from the mature ancient crust.But the ore-forming granitoids are highly fractionated.Therefore,the source of metallogenic element Fe?Cu-Zn-Pb?was probably mantle-derived materials,and the element Sn was likely drived from the mature ancient crust.The low fractionation and high oxygen fugacity are benefit for the enrichment of Fe in the early Cretaceous ore-forming granitoids,and the enrichment of Sn in the late Cretaceous ore-forming granitoids is controlled by high fractionation and low oxygen fugacity.According to the microthermometry results of fluid inclusions and the mineral?¹⁸O values,in the Diantan Fe deposit,the supercritical fluid?⁹.3 wt%NaCl equiv?exsolved directly from magma was at 636°C,¹200 bars.Prograde minerals including garnet,pyroxene and forsterite formed during fluid-rock interation.Under such high pressure,hidden explosion occurred and led to pyroxene breccia and fluid boiling.In retrograde stage,fluid boiling continued and magnetite precipitated at 527°C,⁴00 bars.the pyroxene and forsterite were retrograde into chondrodite and phlogopite.After hidden explosion,increasing meteoric water involved in to the system and the mixed fluid?5.1 wt%NaCl equiv?led to precipitation of calcite–sulfide viens at 267°C.In the Xiaolonghe deposit,the supercritical fluid with low salinity?3.5–6.5 wt%NaCl equiv?exsolved directly from a magma with 80%crystallinity at>600°C,770 bars.The pre-ore stage greisenization,silicification and potassic alteration of the biotite monzogranite occurred at 570–600°C,0.70–0.78 kbars;high pressure led to hydrofractures and fluid boiled for abrupt reduced pressure.Cassiterite precipitated in these fractures as topaz–cassiterite–quartz veins at 423–450°C,0.32–0.45 kbars.In the post-ore stage,topaz was replaced by sericite and quartz?268°C,0.05 kbars?,and fluorite–quartz veins subsequently formed at 199°C,<0.02 kbars.