Orthopyroxene-bearing alteration at the roots of the Carajas IOCG mineral system, Brazil: an example from GT-34 prospect

The GT-34 prospect, from the Carajas Mineral Province, northern Brazil, represents an unusual nickel occurrence associated with the Carajas IOCG district. It occurs at the southern copper belt associated with regional NE-SW shear zones. Similarly to other IOCG deposits worldwide, alkali-iron alteration zones were individualized taking in consideration textural relationships and mineral chemistry. The orthopyroxene-bearing alteration is associated with the initial scapolite rich sodic metasomatism, with the first orthopyroxene occurrence related to IOCG alteration zones. The scapolite is widespread, chemically homogenous and of marialite composition, with 2.9-3.5 Cl wt%. The orthopyroxene is laden with scapolite, with composition of En68-76 and extremely low amounts of Ti, Cr, Ca and Al, similar to charnockite chemistry. Following the sodic alteration is the calcic alteration, with hornblende, oligoclase and clinopyroxene formation and subsequent brecciated nickel mineralization. Metasomatic orthopyroxene typically occurs in charnockites, under high temperature (>700 degrees C) with a high-CO2 activity fluid. On the other hand, scapolite chemistry suggest a highly saline fluid. Therefore, for scapolite-orthopyroxene formation, a mixture between those fluids is necessary. Due to CO2 known immiscibility with saline fluids, an immiscible, high temperature saline-CO2 fluid is suggested for this atypical parageneses formation.