Decoding imprints of hydrothermal alteration around Imalia polymetallic sulphide deposit, Central Indian Tectonic Zone, and its implications on ore genesis.

Abstract The Imalia polyphase polymetallic deposit, located in the Mahakoshal belt of Central India, is hosted by carbonate rocks. The major part of mineralisation at Imalia results from hydrothermal activity induced by the intrusion of quartz porphyry dykes, which also formed an alteration halo of silicate and oxide minerals during their interaction with the host lithology. The initial silicates to form were hydrothermal Ca-amphibole and a minor amount of garnet, followed by potassic phases including biotite and K-feldspar, as well as rutile, apatite, titanite, pyrite, magnetite and rare calcite and ilmenite. A subsequent, cooler pulse of hydrothermal fluid overprinted the earlier-formed silicates and was responsible for the bulk of the sulphide mineralisation at Imalia. During this transformation phase, propylitic silicates primarily consisting of chlorite and epidote, along with a modest proportion of actinolite, albite, titanite, sericite and calcite, formed, accompanied by iron-rich oxide phases including magnetite, hematite and ilmenite. These silicates mostly formed under high water-to-rock ratios with significant meteoric water influence. Geothermometric and fluid inclusion data indicate that alteration zone minerals formed at temperatures between approximately 150°C and 550°C, at pressures of around 1 kbar and depths of less than 10 km, with a mean oxygen fugacity of log f O 2 -32, closely aligned with the FMQ buffer. The alteration zone minerals record the evolution of hydrothermal fluids in a predominantly brittle structural regime, characterised by episodic decompression due to fluid overpressuring and hydrofracturing, as evidenced by various types of breccias, diverse quartz veins, open space-filling textures and fluid inclusion data.