Geochemical Composition and Boron Isotope Characteristics of Tourmalines in the Gejiu Tin Polymetallic Mining Area,Yunnan Province:Constraints on the Properties and Evolution of Ore-forming Fluids
The Gejiu tin deposit is a world-class Sn-Cu polymetallic deposit related to highly differentiated granite in the Late Cretaceous.Its mineral origin and fluid evolution process have always been the focus of research.Tourmaline is widely developed in carbonate rock,granite,and skarn in the Gejiu area;however,the relationship between tourmaline and mineralization is unclear.In this study,systematic petrographic observation and electron Microanalysis(EPMA),and LA-ICP-MS analysis of trace elements and boron isotopes from different types of tourmaline were carried out.The results show that all tourmalines belong to basic tourmaline;TurⅠ,TurⅡ,and TurⅣ are of hydrothermal origin,whereas TurⅢ is of magmatic origin.Compared with hydrothermal tourmaline,magmatic tourmaline has lower Mg,Ca,and other elements and the high Sr content of TurⅠ tourmaline indicates that it was contaminated by the surrounding rock.The variation in trace elements in the tourmaline reveals that Sn was gradually enriched in the magma and precipitated in the porphyritic granite and contact skarn,with an increasing degree of magmatic differentiation from porphyritic granite to equite-grained granite.The different types of tourmalines had uniform δ11B values(-15.2‰--12.8‰),which are consistent with the composition range of granite,indicating that the ore-forming materials were derived from granitic magma.
tourmalineB isotopesmagma hydrothermal tin polymetallic depositsGejiu tin mine cluster area
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