Boron isotope geochemistry of hydrothermal ore deposits in China: a preliminary study

Abstract

A preliminary boron isotope study of tourmaline was carried out for a number of sediment-hosted massive sulfide deposits and granite-related hydrothermal Sn deposits in China. Tourmalines from the Yindongzi-Tongmugou (Devonian) and the Qingchengzi (Proterozoic) Pb-Zn deposits show δ¹¹B values of −7.6 to −13.7‰, which are within the typical δ¹¹B ranges of massive sulfide deposits worldwide. At Dachang Sn-polymetallic deposit, the hydrothermal fluids responsible for the formation of banded tourmalinites have remarkable different δ¹¹B values (−1 to −5‰) from those of quartz-tourmaline veins near the granite contacts (∼ −10‰), which indicate their different origins. Together with other geological and geochemical evidence, the banded tourmalinites at Dachang are of submarine hydrothermal exhalative origin. Overall, the boron isotopic compositions of tourmaline from massive sulfide deposits are mainly controlled by source rock compositions, and non-marine evaporites may have played an important role in the massive sulfide ore formation in same cases. Other controlling factors include temperatures, water/rock ratios, and seawater entrainments.

Tourmalines from the granite-related Yidong Sn deposit display a gradual increase in δ¹¹B value from within the granite (−13.9‰), to the contact (−12.7 to −11.9‰) and the country rocks (−11.4 to −11.3‰). It is suggested that these variations reflect the boron isotope fractionation during magmatic degassing and magmatic-hydrothermal ore-forming processes with ¹¹B preferentially partitioned into hydrothermal solutions. The similar and limited δ¹¹B variations (−19.5 to −13.9‰) in granitic tourmalines from the granite-related Sn deposits in South China suggest that these Sn-rich granites may have similar source rocks from the basement of metasedimentary rocks and volcanic rocks in this region.