REE residence, behaviour and recovery from a weathering profile related to the Serra Dourada Granite, Goiás/Tocantins States, Brazil

Abstract

The Serra Dourada Granite (SDG) is a roughly ellipsoidal massif with batholith dimensions that crop out in Goiás and Tocantins States, Central Brazil. It has been studied since the 1970′s due to its position in a complex geological framework, and exploited by both artisanal miners and mining companies, targeting mainly cassiterite. Circa 2008–2011, it begun to be systematically surveyed in respect to its REE occurrences, culminating in the Serra Verde Deposit, a REE ion adsorption-type mineralization hosted in a reddish coarse-grained biotite granite, the first of its kind in Brazil. Given that the massif has multiple facies, this paper reports the REE residence and behaviour along a weathering profile beyond the current limits of the Serra Verde Deposit, formed at the expense of a pinkish fine-grained muscovite-biotite granite, in order to verify its potential to host a similar mineralization. It also presents the results of a 2-step sequential leaching, to assess the REE recovery from the weathered material. Monazite-(Ce), xenotime-(Y), zircon, thorite, REE fluorocarbonates and fluorite were the main REE sinks identified at the base of the profile, where the parental granite occurs. In weathered samples from horizons above, monazite-(Ce), Th-rich monazite, xenotime-(Y), thorite, Ti-bearing Fe-oxide, cassiterite, Pb-rich cassiterite, Pb-oxide and zircon were the main accessory minerals. Whole-rock analyses in the parental granite reveals REE contents of 828 ppm, and the highest grade saprolite samples yields 1,075 ppm. The chondrite-normalized REE patterns of all samples shows that the granite is LREE-selective, as well as the patterns obtained after the complete extraction. Positive Ce anomalies characterize the upper samples, whereas negative Ce anomaly characterizes horizons below. The recovery rate of the weakly adsorbed REE ions (step-1) ranges from 16 to 31%, whereas step-2 (REE associated with Fe-oxides), extracts 44–57% relative to whole rock REE. XRD analyses reveals that kaolinite and illite are the most abundant clays in weathered samples, so that those clays are the main reservoirs that adsorb REE ions released from minerals during weathering. The accumulation of the LREE in the uppermost and lower half of the profile coupled with HREE depletion, arises from the different resistance to weathering of LREE- and HREE-enriched minerals in the bedrock and saprolite. The HREE higher affinity with complexants such as OH–, F and CO32– possibly present in percolating water may also play a secondary role in leaching them out contributing the the HREE depletion. The main conclusions of this study is that easily soluble LREE fluorocarbonates provide most of the REE to the weathering profile, and that the overall REE grade in saprock and saprolite is akin to other important ion-adsorption ores, although the REE contents in leachate solutions herein are somewhat lower than others. The exception is Ce, which contents and recovery, specially following step-2, are considerable.