Mineralogical and geochemical studies of chromite: Implications for the formation of Luobusa ophiolite-type chromite deposit, Tibetan Plateau

• Ore formation is linked to melt-rock reaction and magma mixing processes. • Chromite ores underwent significant hydrothermal alteration after formation. • The Luobusa chromite deposit records a multistage genetic process. The Luobusa deposit is the only large ophiolite-type chromite deposit identified within the Yarlung Zangbo suture zone so far. This study systematically investigates different types of chromite ores (nodular, disseminated, banded, and massive) as well as altered chromite within the chromitite orebodies and host rocks of the Luobusa deposit through detailed field geological observations, scanning electron microscope (SEM) observations, and in-situ analysis, aiming to elucidate the genetic mechanisms of the different ore types and the effects of post-mineralization fluid alteration on chromite composition and orebody preservation. The results indicate that nodular ores within the dunites exhibit the highest Fe 2+ and V contents and Cr # values, but the lowest Mg 2+ , Al 2 O 3 , and Ti contents and Mg # values. Conversely, massive ores within the chromitite orebodies display the lowest Fe 2+ contents and Cr # values but the highest Mg 2+ , Al 2 O 3 contents, and Mg # values. From the massive ores within the orebodies to the disseminated, banded, and nodular ores in the host rocks, MgO, Ga and Ni are depleted, whereas Mn, Co, and Zn are enriched, with a concomitant increase in Cr # values and Fe 2+# values. Furthermore, Cr # values of different chromite ore types show significant negative correlations with V and Ga. Collectively, these features reflect that the formation of chromite involved significant melt-rock reaction and magma mixing processes. Chromite grains with “core-rim” textures (altered chromite) exhibit distinct depletions in Mg and Al but enrichments in Fe, Mn, Co, and Zn in their rims, indicating that chromite underwent a late hydrothermal alteration by high-temperature fluids during regional tectonic evolution and ophiolite emplacement. Therefore, this study proposes that the formation of the different ore types in Luobusa involved a multistage genesis: (1) Different ore types were initially formed in shallow-level magma conduits through melt-rock reaction and magma mixing, a process that also modified some chromite of deep mantle origin; (2) Subsequently, during ophiolite emplacement, they underwent significant fracture and were altered by slab-derived oxidizing fluids, ultimately developing the distinctive “core-rim” texture and undergoing significant compositional modification.