DEPÓSITOS DE ALEXANDRITA DE MALACACHETA, MINAS GERAIS

Résumé

Since 1975, alexandrite [Be(Al2-xCrx)2O4], the chromium-bearing crysoberyl variety, has been exploitedfrom alluvial and paleo-alluvial deposits in the Malacacheta region. The alluvial deposits consist ofresedimented gravel along the present drainage streams. The paleo-alluvial deposits are richer in alexandrite,and show well-developed soil horizons covering the alexandrite-bearing gravel layer. Alexandrite grainsshow angular shapes with very sharp edges, suggesting transport for short distances. The country rocks arequartz-mica schist and peraluminous mica schist (Salinas Formation), covered by alternating mica schistand quartzite layers (Capelinha Formation). Both formations are of Neoproterozoic age. They host tectonicslabs of metaultramafic rocks, and are cut by intrusive granites of Cambrian age. No alexandrite-bearingrock has been found in the area, probably due to the intense tropical weathering. However, some of themapped rocks are sources for Be (granites), Cr (metaultramafics) and Al (peraluminous schists), the essentialelements for alexandrite crystallization. We suggest a metasomatic system of Cambrian age for alexandritegenesis in the area, involving the interaction of granite-related Be-rich fluids with metaultramafic rocksand peraluminous schists.