Occurrence and behavior of monazite-(Ce) and xenotime-(Y) in detrital and saprolitic environments related to the Serra Dourada granite, Goiás/Tocantins State, Brazil: Potential for REE deposits

The mode of occurrence, mobility and fractionation of REE has been investigated in the geochemical context of the Serra Dourada granite, an A-type granitic body located in northern Goiás State, Brazil. Several analyses were performed to best characterize each mode of occurrence of REE, from fresh rock to its products of alteration such as the saprolite, placers and clayish horizon. In the fresh rock it was found that the REE budget is related to monazite-(Ce), xenotime-(Y), bastnäsite and allanite. Resistant monazite-(Ce) and xenotime-(Y) withstand fluvial transportation and are concentrated in placers within the massif, whereas bastnäsite and allanite do not. The alluvial sediments contain up to 0.87 wt.% monazite-(Ce) and 0.46 wt.% xenotime-(Y). Considering only the heavy minerals, monazite-(Ce), zircon and ilmenite represent 60–80% of the total variability, being monazite-(Ce) and xenotime-(Y) the most important REE carriers. The same minerals are also present, although in lower amounts, in the saprolite. Both qualitative and quantitative electron probe microanalyses were carried out in detrital monazite-(Ce) and xenotime-(Y) to assess their main features and composition. Monazite-(Ce) REE contents are 60 wt.% in average, while REE contents in detrital xenotime-(Y) are 55 wt.% in average. Low totals are ascribed to the alteration of the minerals caused by the presence of both organic and inorganic ligands present in the system. The results support the idea that REE are mobile under supergene conditions and during granite weathering, tending to be higher and more fractionated towards the products of alteration (saprolite, alluvial sediments and clayish horizon). REE2O3 contents in the fresh granite, saprolite, alluvial sediments and clayish horizon are 0.07, 0.21, 1.2 and 0.11 wt.% respectively, the clayish horizon being HREE-selective. Placer deposits and the clayish horizon tend to occur in low-slope areas within the Massif, where deposition and accumulation are more favorable. The saprolite occurs preferably in high-slope areas formed in situ at the expense of the granite. The placer deposits could be exploited, but their occurrence is more restricted and local. The saprolite has the advantage of covering extensive areas widely distributed over the granite. Currently a private company, the Serra Verde Mining, posses the mining claims in the area to exploit the REE associated with the saprolite, under the model of ion-adsorption type deposits.