Tourmaline as a potential mineral for exploring porphyry deposits: a case study of the Bilihe gold deposit in Inner Mongolia, China

Tourmaline is widespread at the Bilihe porphyry gold deposit in Inner Mongolia, China. Five types of tourmaline are recognized based on their occurring locations and microscopic structures: (1) Tur-A in quartz-magnetite +/- K-feldspar veins and related potassic-altered rocks, (2) Tur-B in auriferous quartz veinlets, (3) Tur-C in the intermediate argillic shell, (4) Tur-D in late quartz-carbonate-tourmaline-pyrite veins, and (5) Tur-HB in hydrothermal breccia pipes, consisting of three sub-generations (Tur-HB1, -HB2, and -HB3). Almost all these tourmalines have a schorlitic-dravitic composition and belong to the alkali group. Most of the tourmalines contain high Al contents, which may suggest that the fluids are Al-enriched, and that (Al)(NaFe)(-1) and (AlO)(R2+)(-1)(OH)(-1) exchanges have dominated their elemental substitutions. Tur-A and -B yield similar REE patterns with no strong fractionation of LREE and HREE, whereas LREE-enrichment and HREE-depletion characterize Tur-C and Tur-D; Tur-HB1 and -HB2 have flat REE patterns. Tourmalines in hydrothermal veins always show negative Eu anomalies, whereas the ones replacing plagioclase in the altered rocks yield positive Eu anomalies. The mineral precipitation and assimilation of host rocks probably both contributed to the variable compositions of Bilihe tourmaline. Tur-A-D from the main orebody all yield positive delta B-11 values ranging from +0.7 to +6.3 parts per thousand, whereas Tur-HB1-HB3 from the hydrothermal breccia pipe show negative delta B-11 values of -8.8 to -3.5 parts per thousand. Magmatic hydrothermal fluids enriched in isotopically light boron could have dominated the formation of Tur-HB. In contrast, the heavy boron in Tur-A-D was probably leached from the widespread marine carbonates around the deposit. Systematic contrasts in the trace element compositions of tourmaline from different mineralization types and granitic rocks suggest that (Sn + Li) vs. (Ni + V + Zn), ( n-ary sumation REE + Y + Zr) vs. (Ni + V + Zn) and V vs. Zn are potential elemental groups for distinguishing tourmaline from different environments.