Authigenic tourmaline is a rare and minor component of siliciclastic rocks, however, once it occurs, it can be used to interpret a number of subsurface parameters and processes, such as fluid composition, redox state, salinity, pH, and fluid migration paths. Notwithstanding, the conditions of tourmaline authigenesis still need to be recognize with greater clarity. Carboniferous and Permian siliciclastic rocks in the subsurface of western Poland in the vicinity of the Dolsk Fault show unusually high boron contents, at places surpassing 1000 μg/g, which are associated with the presence of authigenic tourmaline. The mineral forms fine, elongated crystals, of nano- to micrometer width, grouped in radiating or semi-parallel aggregates, which are dispersed in the clay matrix of shales and graywackes, coat grain surfaces and line secondary pores in porous sandstones, replace some grains in volcaniclastic breccia, and overgrow detrital tourmalines in quartzarenites. Regardless of host rock compositions, the tourmaline is Si-deficient, moderately Al-rich, and shows significant X-site vacancies and highly variable Mg/(Mg + Fetot) ratios. In rare zoned tourmaline, the Mg/(Mg + Fetot) ratios distinctly decrease and vacancies slightly increase towards the growth direction. The results suggest that the tourmaline nucleated rapidly under a strong driving force at high boron concentration and high temperature. The fluids which enabled the process were acidic, reducing, B- and Mg-bearing, and of low Na and Ca contents. The tourmaline grew primarily at the expense of kaolinite, possibly of other clay minerals and K-feldspar, in the temperature range ~150–200 °C. Its crystallization was a short episode in the diagenetic history of the host rocks. Tourmaline authigenesis across the Carboniferous/Permian unconformity, in the proximity of a prominent fault zone, is evidence for fluid and heat migration along faults, and for temporarily open geochemical system during late diagenesis. The source of boron has been unproven, yet the circulation of bitterns originated from Upper Permian evaporites was highly probable.