RARE-EARTH ELEMENTS MOBILITY DURING HYDROTHERMAL ALTERATION: THE EXAMPLE OF THE SERRA BRANCA GRANITE -CENTRAL BRAZIL

Abstract

Among the Goiás Tin Province batholiths, the Serra Branca Granite (SBG) is characterized by the high level and the wide extension of post-magmatic metasomatic and hydrotermal processes, chiefly greisenization related to Sn-concentrations. The following discussion is about the REE behaviour during these processes, and the evaluation of relative contribution of magmatic and hydrothermal sources for REE concentrations. The similarities between all the chondrite-normalized REE patterns within the SBG rocks, the less altered granites as well as the most completely transformed rocks (greisens and micaceous veins), support the hypothesis of a unique magmatic signature, the effect of greisen-forming fluid circulation being restricted to a simple dilution of the initial REE stock, without important partitioning. All the REE patterns for separated minerals (micas, fluorite, cassiterite) show similarities with the REE profils in the SBG rocks. The behaviour of this elements seems to be consistent with a first stage of primary concentration during the magmatic differentiation, and a later stage of partial leaching related to post-magmatic transformations (especially greisenization) due to important fluid circulations. So, these fluids probably did not transport significant REE amounts from a distant unknown source, but they more likely mobilized part of the REE stock previously concentrated in the granitic rocks. The REE patterns support the affinity of the SBG with A-type granites and they can be compared to REE profiles in other tin-granites from the Province of Goiás and from Alaska.