<p>Felsic pyroclastics and sedimentary rocks, which occur in close proximity with the intermediate-felsic lava flows of the Abor volcano-sedimentary sequence (AVS) of the Eastern Himalayan Syntaxis in Northeast India, exhibit varying degrees of hydrothermal alteration. The interaction of hydrothermal solutions with the pyroclastic-sedimentary strata of AVS produced siderite and pyrite in the altered rocks. The hydrothermally altered rocks are melanocratic in nature and consist of abundant siderite-rich concretions. The concretions provide a vivid picture of the texture of the Fe- or siderite-rich rocks, which appear to be fine-grained in nature and composed of numerous leucocratic veins filled with secondary minerals, such as calcite and quartz. The interaction of hydrothermal solutions with pyroclastic protoliths resulted in the enrichment of Fe, Ca, P, Mn, V, Ni, Sr, and certain rare-earth elements, whereas Si, Na, and K abundances were depleted. Based on the findings of this study, we hypothesize that hot seawater interacted with crustal mafic rocks, including basaltic rocks of the AVS, to form a hydrothermal solution rich in Fe, Mn, and Ca compared to the intermediate-felsic volcanic rocks. The solution also contained high concentrations of P, S, and CO₂ as evidenced by whole-rock analysis as well as carbonate and sulfide minerals found throughout the altered rocks. This hydrothermal solution enriched in Fe, Mn, Ca, P, S, and CO₂ reacted with the pyroclastic and sedimentary strata of our study area, resulting in a possible siderite deposit in the AVS of the Eastern Himalaya. We also confirm that the volcanic rocks of the AVS were generated in a continental rift setting associated with the eastern Gondwana breakup as a result of the Kerguelen plume activity, which impacted significant portions of Eastern India, Western Australia, and Antarctica.</p> Graphical abstract <p></p>