Many finds of low quality alexandrite crystals 2-3 cm in length recorded at Caraiba deposit. Some of the smaller crystals, and sections of larger crystals, are gemstone quality, with excellent clarity and strong color change.
The Upper Curaca River Valley, characterized by an unremitting flat, low-relief terrain, is situated in a semi-arid tropical ecosystem known as the Caatinga, which has xerophytic vegetation and very shallow residual soils. The Caraiba deposit is a Paleoproterozoic sill-like intrusion consisting of magnetite-bearing hypersthenites, melanorites and norites mainly mineralized with disseminated-type Cu-ore (chalcopyrite and bornite) exploited since 1978.
The intrusion is the largest of a set of 100's mafic-ultramafic bodies found within the Curaca Valley terrain, a N-S trending strip of high-grade rocks in the northern part of the Sao Francisco Craton. This terrain comprises of supracrustais and gneisses of their probable basement, the mafic-ultramafic rocks emplaced as sills, all intruded by G1-G3 early- to late-tectonic tonalitic-granodioritic orthogneisses and granites related to highly ductile deformation events between 2.25 to 2.05 Ga (U-Pb ages of zircons).
The chrysoberyl crystals show a regular pattern of cavities that likely represent inclusions that have weathered away. Others crystals appear to have grown across the foliation of the schist, as shown by trains of micaceous inclusions, which create parting planes and increase susceptibility to physical weathering.
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