Inhandjara topaz leucogranite: A late rare metal-mineralized stock within the A-type Itu batholith, SE Brazil

Abstract

The Inhandjara leucogranite is a highly differentiated Ediacaran stock with crystallization age of ca. 570 Ma (SHRIMP zircon U–Pb dating). It is a satellite intrusion of the Itu batholith, a large post-orogenic A-type granite in south-eastern Brazil. The Inhandjara stock consists of two leucogranitic facies: (1) an early inequigranular to porphyritic biotite-bearing monzogranite (BG), with potassium feldspar megacrysts in a medium to coarse-grained matrix; and (2) a more evolved equigranular medium to fine-grained topaz-bearing alkali feldspar granite (TAG) with subhedral albite, anhedral quartz and potassium feldspar, and accessory Li-bearing siderophyllite, topaz, Y-bearing fluorite, zircon, ilmenite, and columbite-group minerals. Both facies display signs of autometasomatism, mainly as pervasive albitization and fissure to pervasive greisenization. The Inhandjara leucogranite is reduced, metaluminous to weakly peraluminous and shows A-type affinity. There is a progressive increase of SiO2, Al2O3, Na2O, F, Cs, Rb, Nb, Ta and Y from the early biotite granite to the late topaz granite, parallel to a decrease in TiO2, Fe2O3, MgO, CaO, Sr, Ba and Zr. The seagull-wing-shaped REE pattern of the topaz granite, with a strong negative Eu anomaly (Eu/Eu* = 0.003), is unique in the Itu batholith. Both facies have very negative εNd(570Ma) values, ranging from −11.7 to −13.2, with the lowest values within the Itu batholith. Zircon crystals from biotite granite show oscillatory zoning and chemical evolution from core to rim; whereas in the topaz granite, zircon is unzoned and strongly enriched in Y, Hf, Nb, Th, and U, suggesting crystallization from a residual melt in the presence of a fluid phase. The Inhandjara stock crystallized from a highly evolved magma derived from a lower crustal source. The strong enrichment in incompatible lithophile elements mostly results from protracted magmatic fractionation, enhanced by the interaction with magmatic-hydrothermal fluids. These characteristics reveal that the leucogranite is the most differentiated occurrence within the Itu batholith and support its relation with regional rare metal mineralization (Nb–Sn–W).