Petrogenesis of the massive chromitite layer from the Jacurici Complex, Brazil: evidence from inclusions in chromite

The Jacurici Complex hosts the largest chromite deposit in Brazil in an up to 8-m-thick chromitite layer within a tectonically segmented 300-m-thick intrusion. The ore has been interpreted as the result of crustal contamination-driven crystallization in a magma conduit. This study addresses the stratigraphy, mineralogical and textural relationships, and mineral chemistry of the Monte Alegre Sul segment focusing on chromite-hosted inclusions from the Main Chromitite Layer to understand the role of volatiles in the genesis of the massive chromitite. Silicate inclusions (enstatite, phlogopite, magnesiohornblende, diopside and olivine) are commonly monomineralic and sub- to euhedral, and crystallized prior to, or coeval with, the chromite crystallization. Carbonate inclusions (dolomite and magnesite) are irregular or have negative crystal shapes, suggesting entrapment as melt droplets. Sulfides (pentlandite, millerite, heazlewoodite, polydymite, pyrite, and chalcopyrite) are often polymineralic, irregular, or hexagonal-shaped, indicating entrapment as sulfide melt and as monosulfide solid solution. The inclusions indicate an H2O- and S-saturated resident magma with immiscible droplets of carbonate melt during chromite crystallization. Inclusion-rich and inclusion-free chromites that occur together have similar compositions and are considered to have formed from the same magma in response to variations in the degree of Cr saturation. Hot primitive magma might have heated and mobilized CO2 and probably water from devolatized and assimilated carbonate-rich wall rocks, increasing fO2 and triggering chromite crystallization. We propose that the formation of the chromitite layer started as in situ crystallization with additional material added by slumping of locally remobilized chromite slurries, facilitated by the presence of volatiles.