Mineralogy And Tin Metallogensis Of The Xianghualing Skarn,Hunan Province

The Xianghualing tin deposit is one of the most important tin-polymetallic deposits in Nanling Region,and hosts Li,Be bearing ribbon rocks and abundant Sn-bearing minerals.In this paper,the mineralogical characteristics of rock-forming minerals and Sn-bearing minerals from Xianghualing skarn are systematically analyzed by electron microprobe technology,in combination with microscopic and field observations,its aims is to reveal(1)the mode of occurrence and crystal chemistry of tin,(2)tin metallogenic processes,(3)fluids features,and(4)origin of Sn.The results are as follows:(1).The Xianghualing tin deposit contains both calcareous and magnesian skarn,which occur in the contact zones between F-rich granite and wall rocks(carbonate rocks).The wall rocks are mainly composed of limestone and dolomitic limestone of Middle Upper Devonian period.(2).The Xianghualing skarn is characterized by rhythmically banded structure(ribbon rocks)and is rich in Li,Be minerals.According to mineral paragenesis,the ribbon rocks may be divided into several types:garnet-magnetite skarn,garnet-vesuvianite skarn,spinel-taaffeite ribbon rock,spinel-chrysoberyl ribbon rock,fluoborite-fluorite-phlogopite ribbon rock and cassiterite-sulfide ribbon rock.(3).The results of electron microprobe analysis(EMPA)show that the garnet from the Xianghualing skarn mainly belong to grossularite(Ca3Al2[SiO4]3)and andradite(Ca3Fe3+2[SiO4]3)series;pyroxene is mainly hedenbergite(CaFe2+Si2O6);amphibole is mainly ferropargasite,tremolite and pargasite;chlorite is mainly ripidolite and brunsvigite;humite is mainly norbergite;spinel is rich in Zn,Fe,Sn.(4).Sn-bearing minerals in Xianghualing skarn include Sn minerals(cassiterite,nigerite,mengxianminite etc.)and Sn-rich minerals(spinel,taaffeite,chrysoberyl,magnetite,pargasite,prehnite etc.);three stages of Sn mineralization were distinguished:during the early skarn stages,Sn entered into the structures of spinel,amphibole and other rock-forming minerals,leading to the formation of Sn-rich minerals,Sn minerals(cassiterite,nigerite,mengxianminite etc.)gradually crystallized during the oxidation stages,and during the the hydrothermal stages early Sn-bearing minerals experienced hydrothermal alteration with locally precipitation of cassiterite,and/or the formation of minerals with Sn-rich rims by Sn-rich fluids.(5).The mineralogical paragenesises indicate that hydrothermal fluids in the Xianghualing skarn are enriched in F,CO2 and Li,which controlled the processes of enrichment,transport and crystallization of Sn.Sn in Xianghualing skarn was fundamentally derived from the ambient stratum,and the metallogenic process of Sn reflects a geochemical circulation of Sn in Earth Crust.