In-situ elemental and boron isotopic variations of tourmaline from the Maogongdong deposit in the Dahutang W-Cu ore field of northern Jiangxi Province, South China: Insights into magmatic-hydrothermal evolution

摘要

In this paper, we present a systematic investigation of in-situ major, trace elements and boron isotopic variations of tourmaline from the Maogongdong deposit in the Dahutang W-Cu ore field of northern Jiangxi Province, South China, which demonstrates both the magmatic and hydrothermal origin for the formation of the tourmalines. The tourmaline occurs mainly in granites, pegmatites and quartz veins. Five types of tourmaline are identified: (1) disseminated tourmaline in the pegmatites (Tur-P type); (2) isolated and disseminated tourmaline clots in the granites (Tur-R1 type); (3) typical anhedral quartz-tourmaline nodulars in the granites (Tur-R2 type); (4) tourmaline clots in quartz-tungsten-sulfide veins (Tur-Q1 type); (5) euhedral needle-columnar tourmaline crystals in quartz or quartz-sulfide veins (Tur-Q2 type). These tourmalines mostly fall into the alkali group representing schorl-dravite solid solution series. Petrography and chemical discrimination diagrams suggest that the Tur-P, Tur-R1 and Tur-R2 are of magmatic origin and the Tur-Q1 and Tur-Q2 are of hydrothermal origin. Hydrothermal tourmalines have higher Mg/(Mg + Fe) ratios, higher Li, Be, Sn, Rb, Co, Sr, V, Pb, Zn, Ni contents and lower Na/(Na + Ca) ratios, lower Nb, Ta contents than the magmatic tourmalines. The Tur-P show negative Eu anomalies and their REE patterns are equivalent to the muscovite granite. The Eu anomalies in Tur-R1 and Tur-R2 change from negative to positive. Compared with magmatic tourmalines, Tur-Q1 and Tur-Q2 exhibit more pronounced positive Eu anomalies, and a majority of the Tur-Q1 and Tur-Q2 have higher FREE contents. Tourmalines from the Maogongdong deposit have a B isotopic composition range from -17.9 parts per thousand to - 12.3 parts per thousand. The average delta B-1(1) values of the Tur-P, Tur-R1 and Tur-R2 are -13.8 parts per thousand, -14.8 parts per thousand and -14.5 parts per thousand, respectively. Tourmalines in early hydrothermal veins have similar delta B-1(1) values (average -14.6 parts per thousand), while the tourmalines in later hydrothermal veins have slightly lower delta B-11 values (average -16.5 parts per thousand). Boron isotopic variations of tourmaline are controlled by temperature, Rayleigh fractionation and external fluid.