Chemistry and boron isotope composition of tourmaline as a robust tool to characterize the origin of porphyry molybdenum systems: The case of Donggebi deposit in East Tianshan, China

Tourmaline is a chemically and mechanically stable mineral that commonly occurs in hydrothermal mineral systems, and has been used to trace the mineralization processes of many genetic types of ore deposits. Here we apply tourmaline chemistry and isotopic composition to trace the genesis of a porphyry Mo system for the first time. Tourmaline is widespread in the Donggebi Mo deposit in East Tianshan, NW China, where it occurs in five distinct associations: type PG disseminated in the porphyritic granite, TS as hydrothermal tourmalite in the country rocks (strata), V1 in quartz-molybdenite vein, V2 in quartz-polymetallic sulfide vein, and V3 quartz-calcite-fluorite vein. All the tourmaline types belong to the alkali group, with the V3 being schorl and the other types being dravite and schorl. The PG-type tourmalines partly inherit the features of the porphyritic granite, and are rich in F, Na, Al and Sc, and poor in Fe, Mg, Ca, Sr, Ba, Pb, V and Mo compared to the other types. They have low Fe3+ concentrations and low Fe3+/(Fe3++Fe2+) ratios, suggesting a reducing fluid system. They also display weaker Eu-depletion (with an average (Eu/Eu*)N value of 0.96) than the porphyritic granite, and are considered to be the products of interaction of the reducing fluids and the granites. The average (Eu/Eu*)N values of tourmalines decrease from 6.66 of type V1, 5.65 of type V2, to 4.80 of type V3, indicating that the fluids became progressively less reducing along with sulfide precipitation. The δ11B values of the tourmalines range between −15.9 ‰ and −12.5 ‰, resembling those of S-type granites or clastic sediment-sourced hydrothermal fluids. The tourmaline grains in our study do not display any remarkable δ11B variation from the cores to rims, indicating that the δ11B values of the fluids and their source were relatively constant.