Characterization of the Hegenshan podiform chromitites (Inner Mongolia, China): Sub-solidus cooling and hydrothermal alteration

Abstract

Podiform chromitites in the Hegenshan ophiolitic complex from the Inner Mongolia of China display massive, semi-massive, and disseminated textures. The extent of Fe2+–Mg redistribution between high-Al chromite and olivine during sub-solidus cooling is affected by the modal abundances of chromite. High-Al chromite in semi-massive and disseminated chromitites becomes richer in Fe2+ than its primary composition, while the Fe2+ and Mg contents in high-Al chromite in massive chromitite are almost unchanged. After the cation redistribution process, these chromitites undergo hydrothermal alteration, forming partly altered chromite and porous chromite with following assemblages: uvarovite–chlorite, type-1 high-Cr chromite (Chr-1)–brucite–chlorite–metal alloys (awaruite, native copper), and type-2 high-Cr chromite (Chr-2)–brucite–chlorite–apatite–calcite–sulfides. Fe and Mn are added into chromite to form the Chr-1 and Chr-2, while Al, Mg, and Ni are transported out of the Chr-1 and Chr-2 during hydrothermal alteration. Ti is released into fluid from the Chr-1 and enters the Chr-2. Ni, mainly from olivine in podiform chromitites, participates in the formation of the observed metal alloys and sulfides. Thermodynamic calculations suggest that heazlewoodite–chalcocite assemblage in massive chromitite crystallizes under conditions with higher fO2 and fS2 values compared with awaruite–native copper assemblage. Increases in HS− activities and fO2 values and a decrease in pH values facilitate the transformation from metal alloys into sulfides during hydrothermal alteration of podiform chromitites.