| dc.description.abstract | The mode of occurrence, mobility and fractionation of REE have been investigated in the geochemical
context of the Serra Dourada Granite, an A-type granitic body located in northern Goi´as 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), bastn¨asite and allanite. Resistant
monazite-(Ce) and xenotime-(Y) withstand fluvial transportation and are concentrated in placers whithin
the massif, whereas bastn¨asite and allanite do not. The alluvial sediments contain up to 0.87wt%
monazite-(Ce) and 0.46wt% 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 microanalysis were carried out in detrital monazite-
(Ce) and xenotime-(Y) to assess their main features and composition. Monazite-(Ce) REE contents are
60wt% in average, whilst 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 0.11wt% 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 is 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. | en_GB |
