Mineralogical and geochemical characterization of rare-earth occurrences in the Serra do Mendes massif, Goiás, Brazil

Abstract

The Serra do Mendes granitic massif (SMGM) is located in the Goiás Tin Province (GTP), which is characterized by Sn, REE-bearing A-type granites. The Serra do Mendes granite is the largest massif of the Paranã Subprovince, with three main facies related to the Pedra Branca Granite Suite (PB). Previous studies on this suite have indicated ages of 1.77 to 1.74 Ga, in addition to subalkaline to alkaline signatures and high Zr, Y and REE contents.Samples of fresh rock, saprolite and alluvial sediments were collected from the most common facies, biotite monzogranite (PB1a) and leucomonzogranite (PB1c), in order to characterize the REE-bearing minerals and their abundance. The analytical techniques included petrography, scanning electron microscopy (SEM), electron probe micro analyzer (EPMA) and total rock geochemistry.The most evolved facies, PB1c contains the highest quantities of REE-bearing minerals, where fluocerite-(Ce), REE oxyfluorides, bastnaesite, monazite, allanite, thorite-zircon solid solution, fluorite and apatite are the most important constituents.REE contents in facies PB1a range from 363.8–985.9 ppm, and in its respective saprolite from 60 to 1283.9 ppm with LaN/YbN ratios between 4 and 17.7%. Facies PB1c contains REEs in concentrations between 460.9 and 958.6 ppm, and its saprolite exhibits concentrations between 495.7 and 902.1 ppm. The LaN/YbN ratio in fresh rocks and saprolite is between 2 and 12.3, respectively. Along with the REE minerals, it is noteworthy the occurrence of zircon and thorite with up to 20% REE2O3 + Y2O3.These results, in addition to other factors such as the local climate and geomorphology, make the Serra do Mendes granite massif an interesting target for prospecting and exploration of REE.