Diamond from the Guaniamo area, Venezuela

More than 5,000 diamond crystals (or fragments) from kimberlite sills and p lacer deposits in the Guaniamo area of Venezuela have been characterized in terms of morphology, internal structure, carbon isotopic composition, syng enetic mineral inclusions, and the abundance and aggregation state of nitro gen. Ours is the first comprehensive mineralogical study of diamond from th e Guaniamo area. About 50% of the crystals are resorbed dodecahedral forms; octahedra are the next most common form. In most cases, the diamond is col orless; 55-90% show radiation-induced pigmentation. About 20% of the stones have very low N contents (Type II): the remainder belong to the transition al IaAB type, with B > A. Ninety-three mineral inclusions were extracted fr om 77 crystals or fragments of diamond and analyzed by electron microprobe and LAM-ICP-MS to establish their trace-element compositions and the pressu res and temperatures of diamond crystallization. In all, 86% of the diamond samples contain inclusions of the eclogitic paragenesis, represented by ga rnet, omphacite, rutile, ilmenite, pyrrhotite, and probable coesite. Inclus ions indicative of the peridotite paragenesis are pyrope, chromian spinel a nd olivine. One inclusion of ferroan periclase may indicate a lower-mantle origin. The delta C-13 of 108 diamond samples ranges from -3.2% to -28.7%, but most stones have delta C-13 less than or equal to 10%. We contend that in large part, the diamond in placers in the Guaniamo area was derived from the Guaniamo kimberlite sills. P-T estimates on mineral inclusions suggest that most originated near the base of the lithosphere (T 1200-1300 degrees C); this zone may contain a substantial proportion of eclogite formed by su bduction of crustal material. The very high proportion of diamond derived f rom an eclogitic association in the Guaniamo deposits, and several features of the mineral inclusions trapped in diamond, show striking parallels to t he Argyle deposit of Australia. Both deposits occur within cratons that hav e experienced extensive Proterozoic tectonothermal activity.