ELECTRON BACKSCATTER DIFFRACTION-BASED IDENTIFICATION AND QUANTIFICATION OF DIAMONDS FROM THE RIF GNEISSES (SPAIN AND MOROCCO): ECONOMIC IMPLICATIONS

Microdiamonds from granulite facies gneisses of the Northern Rif (Spain and Morocco) have been studied by electron backscatter diffraction (EBSD). Diamond identification using other techniques, such as Raman spectroscopy, is hindered by microdiamond size and by the presence of graphite bands. In contrast, EBSD enabled the following: (1) the rapid identification of diamonds and their distribution, (2) the estimate of the diamond concentration, and (3) the analysis of the crystallographic relationships between diamonds and the surrounding phases. EBSD study has revealed that (1) diamonds are concentrated mainly in large mesoperthite and quartz + coesite grains, (2) the diamond concentration in the rocks is notably higher than that estimated on the basis of diamond concentration in garnet, and (3) clear orientation relationships exist among diamonds, coesite, and quartz. These data would not have been revealed using classical methods, since most diamonds can be recognized by optical microscopy only when EBSD maps have previously revealed their location. The high diamond grades determined in this preliminary study suggest that these deposits could be of economic interest.