Estudo geológico e aspectos econômicos de um depósito de esmeralda clássico, Socotó-BA

Abstract

The world trade in gemstones, which today involves values ​​of around 80 billion dollars/year, dates back to millennia, having had a great influence on the development of human civilization. Emerald occupies a prominent place in this trade, and Brazil, with large gemological provinces, is an important source of this mineral. The Brazilian production, with approximately 80 tons/year of raw emerald, is favored by the relative political and social stability of the country, which constitutes an advantage in relation to other producing countries. On the other hand, the export of raw gemstones, the absence of a mineral and research policy in this sector, prevent the generation of greater resources, benefiting only small groups, usually foreigners. The present work aims to show that the Socotó emerald deposit, due to its classical characteristics, can serve as a model in view of prospecting in other areas with a similar geological environment, as well as to allow the clarification of some questions related to the formation of this mineral. The geological framework of Socotó mainly consists of an intensely fractured/failed basal-ultrabasic rock body, corresponding to a "roof pendant" or a mega-enclave embedded in the Campo Formoso granite batholith. The intrusive magma of granitic composition, rich in molybdenum, volatile elements and beryllium, reacted with the meta-ultrabasic rocks; suppliers of the chromophore elements Cr, Fe and V, which resulted, along dikes and veins, in abundant emerald mineralization controlled by geochemical, physical and structural factors. An intense metasomatism allowed the development of at least three concentric zones in the contact aureole, characterized by specific mineral associations, namely: an external zone, an intermediate zone and a more internal zone in the enclave. Through geophysical survey by scintillometry and magnetometry, the shape and the following dimensions were defined for this enclave: 2,150 meters long, 180 to 335 meters wide and a thickness of up to 161 meters. The analysis of the remaining magnetization showed the thermal evolution of the granitoids, from their placement 1973 ± 24 m.a ago (Torquato et al, 1978), to the closing of the mineralizing cycle 1784 ± 47 m.a., age obtained by the Rb/Sr method in phlogopites associated with emerald. Finally, the recognition of the different types of inclusions and the large amount of growth tubes and channels along the C axis of emerald crystals, whether or not filled with minerals, allowed the prediction, with economic objectives, of conventional or complex chemical treatment, involving the application of a laser beam and the displacement of atoms along these natural filaments.