Axinite over tourmaline: A new perspective on boron pathfinders in the gold-bearing zones of Dongargarh-Kotri belt, Central India

Dongargarh-Kotri Belt (DKB) is a storehouse for gold occurrences and shows the potential concentration of gold (12 ppm to 37 ppm) with associated arsenopyrite-rich acid volcanics and BIF throughout the DKB. Boron is commonly associated with gold mineralisation, as suggested by the widespread occurrences of tourmaline in gold-bearing quartz veins worldwide. Axinite, a calc-silicate of boron, is rare and instead of tourmaline, at places associated with auriferous zones or base-metal deposits. We have reported the axinite associated with gold-sulphide-rich quartz veins along the interface of volcanic and sedimentary rocks of the DKB. The mineral chemistry shows high B2O3 8.03 avg. wt.%,, FeO 8.99 avg. wt.%, with very low MnO and MgO (<0.5 wt.% and classified it ferro-axinite. The formation of ferro-axinite suggests limited carbonatization, therefore available Ca combines with B, Fe, and Si in ferro-axinite and avoids the generation of tourmaline. The presence of pyrite, chalcopyrite, pyrrhotite, marcasite and the conversion of marcasite from pyrrhotite generally confirm high sulphidation, at 300 °C and low pH (<5). The fluid inclusion and Raman studies revealed the presence of primary aqueous and CO2 phases with low to moderate salinity and two ranges of the temperature of homogenization i.e, 150–200 °C and ~300 °C. The presence of axinite-quartz vein with associated sulphide and gold mineralisation within the boronrich hydrothermal system along the brittle-ductile zones in the DKB suggested their low pressure, low to moderate salinity, low pH and ~300 °C temperature also suitable for precipitation of gold. The present study provides evidence of the spatiotemporal association of ferro-axinite with the sulphide gold mineralisation in the area as well as along the DKB may be used as an indicator mineral or pathfinder.