Fingerprinting serpentinization and carbonation of mantle rocks from the Paleo-to-Mesoarchean Holenarsipur Greenstone Belt, Western Dharwar Craton, Southern India: Implications on mantle metasomatism and supergene magnesite mineralization

Exposed slivers of mantle rocks are excellent archives for investigating mantle characteristics, post-magmatic alterations such as serpentinization and carbonation, and their role in the genesis of economically significant mineral deposits. The serpentinized harzburgite and dunite pods from the oldest part of the Dharwar Craton, namely, the Holenarsipur Greenstone Belt (HGB) represent a preserved section of Paleo- to Mesoarchean depleted mantle. In situ major and trace element composition of olivine, serpentine polymorphs (lizardite, antigorite, and chrysotile), and bastite reflecting progressive hydration events, resulting in differential assimilation of Fluid Mobile Elements (FME) during serpentinization. Olivine → lizardite breakdown marks high concentration FME (Ce, Rb, Sr, and Ba), whereas, lizardite → antigorite transition exhibits a lower FME (Rb, Sr, and Ba) budget suggesting higher temperatures. Late-stage chrysotile veins (Rb:~1.58 ppm, Sr:~7.36 ppm, and Ba:~5.41 ppm) and serpentinized orthopyroxene (Rb:~1.60 ppm, Sr:~4.30 ppm, and Ba:~5.47 ppm) act as a sink for FME. The LREE-enriched trace element pattern of bastite/orthopyroxene affirms metasomatic refertilization. Thus, the geochemical systematics suggest that HGB harzburgite represents a depleted mantle that has undergone multiple episodes of serpentinization and metasomatism, primarily driven by subduction-derived fluids, contributing to the geochemical heterogeneities of the Archean mantle. In addition, these findings highlight the replenishment of an Archean depleted mantle reservoir beneath the Western Dharwar Craton (WDC).

The carbon (δ¹³C_VPDB = avg. -2.20 ‰) and oxygen (δ¹⁸O_VSMOW = avg. 28.50 ‰) isotope studies of ultramafic hosted vein-type magnesite mineralization from the HGB suggest late-stage supergene enrichment at shallow to surface level conditions under ambient temperature (~35^oC). Precipitation of magnesite by the interaction of atmospheric carbon-enriched mineralizing fluid with weathered ultramafic rocks indicates the sequestration of atmospheric CO₂ during the carbonation of serpentinized mantle rocks in the HGB.