Serpentinization and potential Ni‐Cr mineralization of the Andong ultramafic block in South Korea

Small‐ to medium‐sized serpentinized ultramafic blocks have been found in several locations in South Korea. Ultramafic‐hosted serpentinization, described as a major hydrothermal process that affects the lithospheric mantle, is known to play a key role in mass fluxes enhanced by multiple fluid–rock interactions that ultimately lead to the formation of hydrothermal mineralization and natural hydrogen production. In this study, we attempted to interpret the hydrothermal alteration and associated Ni‐Cr mineralization of the Andong serpentinized ultramafic block (ASUB). Based on the mineralogical and geochemical studies, the serpentinites and serpentinized ultramafic rocks of the ASUB are interpreted to be derived from mantle peridotite. It shows the dunite, lherzolite, and wehrlite compositions which are mainly composed of olivine and pyroxene. Hydrothermal alteration transformed the ultramafic rock into serpentinitic rock, resulting in changes in rock textures from the parent rock (plutonic texture) to moderately altered rock (pseudomorphic porphyritic texture) and strongly altered rock (pseudomorphic fine grained texture) with mineralogical changes. Serpentinization and Ni‐Cr mineralization do not show any relationship. Considering the reported crystallization age of the parental rock (222 Ma) and measured age (214–187 Ma) of the granites intruded into the ASUB, the source magma responsible for the final hydrothermal event including serpentinization is inferred to be the Late Triassic to Early Jurassic granitic pluton. The ASUB might have formed due to the ascent of a mantle plume along the extensional regime (i.e., rift) in the supra‐subduction zone (SSZ), rather than in ophiolite‐related mid‐oceanic ridges or abyssal environments, which are identified in many places. Ni‐Cr mineralization, with the close association of pentlandite, pyrrhotite, and Cr‐spinel, likely resulted from co‐precipitation and dissemination during the crystallization of the parent peridotitic magma derived from the mantle.