Thermal characteristics of borates and its indication for endogenous borate deposits.

[Display omitted] • The formation temperature of typical borates in skarn-type deposit were between 500 °C and 700 °C. • The formation of Mg(OH) 2 would control the pH value of contact metamorphism. • Contact metamorphism was the main metallogenic mode for skarn-type and metamorphic borate deposits. • Na-Ca borate in the exogenous ores may be converted into insoluble endogenous Mg borate. Borate minerals are important industrial materials that are produced in both exogenous and endogenous borate deposits. The former includes volcanic-sedimentary and salt lake sedimentary borate or boron-rich brine, and the latter contains skarn-type and metamorphic borate deposits. In this paper, XRD and TG-DTG-DSC are used to study the thermal decomposition characteristics of three natural borates, and we find that they are ulexite, mixture of tincalconite and northupite, and mixture of szaibelyite, dolomite, magnesite and lizardite, respectively. The thermal decomposition of borates mainly contain three processes: dehydrate crystal-water under 300 °C, remove hydroxyl accompanied by the amorphous at 300∼550 °C, and crystallization at 550∼700 °C. The ulexite and tincalconite mix with calcite and magnesite respectively and react for two hours at 500∼700 °C to form kotoite and takedite, which indicates that temperature, boron-rich fluid and calcium-magnesium carbonate are the main factors to control contact metamorphism. This also provides a theoretical basis for mineral combinations and wall rock alterations of metamorphic borate deposits, which is similar to that of skarn-type borate deposits. The MgO formed by thermal decomposition of magnesite and dolomite reacts with hydrothermal fluid to form Mg(OH) 2 during skarn mineralization, which may be an important factor in controlling the pH value of hydrothermal fluid, meanwhile it provides reactants for the formation of kotoite. Mixture of szaibelyite, dolomite, magnesite and lizardite react for 2 h at 500∼700 °C to form suanite, indicating it can be produced by dehydration of szaibelyite. Therefore, it is speculated that the Liaodong borate deposits may experience at least two stages: deposition stage of the early hydrous borates (or boron-rich sediment), contact metamorphic stage is similar to skarn-type mineralization, whether there is a high temperature dehydration stage needs further research. Combined the experimental results with the relationship between the endogenous and exogenous borate deposits in ore-forming age, tectonic setting and source link, it is found that the endogenous borate deposits may be another form preserved by early exogenous borate deposits. [ABSTRACT FROM AUTHOR]