Chemical and boron isotope compositions of tourmaline in the Hadamiao porphyry gold deposit, Inner Mongolia, China

The Hadamiao porphyry gold deposit in Inner Mongolia, China, is located in the northern margin of the North China Craton (NCC). Tourmaline is a common mineral in hydrothermal veins and granitic rocks contemporaneous with the mineralized granite porphyry in this deposit. The tourmaline can be divided into six types, including magmatic tourmaline (Tur-M1) in granite porphyry dikes, magmatic-hydrothermal transitional tourmaline (Tur-M2) in nodular granitic rocks, and hydrothermal tourmalines in early tourmaline veins (Tur-V1), gold-bearing quartz-tourmaline-chalcopyrite veins (Tur-V2), breccia cements (Tur-V3) and late carbonate-quartz veins (Tur-V4). The gold mineralization is closely related to the occurrence of Tur-V2.

Almost all the tourmalines from the Hadamiao deposit belong to the alkali group and show a schorlitic–dravitic composition, except for some Tur-M1 belonging to uvite. The concentrations of most elements in different tourmaline types overlap. Tur-M1 has higher Ca, W, Th, U, Pb, Mn, and Zr contents than other types, and the hydrothermal tourmalines (V1-V4) show slightly higher Ba, Cu and Y contents than Tur-M1 and Tur-M2. The concentration of Au in Tur-M1 and Tur-M2 can reach up to 80 ppb and is higher than that in Tur-V1; the Au content in Tur-V2 is an order of magnitude higher than that in other stages of tourmaline. Systematic contrasts in major and trace element compositions between magmatic and hydrothermal tourmalines are attributed to the assimilation of the country rock by the magmatic-hydrothermal system.

The six types of tourmaline exhibit variable δ¹¹B values ranging from -8.7‰ to +21.2‰. Tur-M1 shows the highest δ¹¹B values of +15.1‰ to +21.2‰, and Tur-V2 yields the lowest values ranging from -8.7‰ to -5.4‰. The boron probably came from two sources with different isotopic compositions, one with high positive δ¹¹B values and the other with negative δ¹¹B values. Magmatic hydrothermal fluids could have provided isotopically light boron for hydrothermal tourmalines, whereas the heavy boron in magmatic tourmaline was probably derived from marine carbonates that are widespread in the study area. The magma related to the mineralization and the one containing magmatic tourmaline may have experienced different degrees of interaction with wall rocks, resulting in the diversity of δ¹¹B values in Hadamiao tourmaline.