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More than 5,000 diamond crystals (or fragments) from kimberlite sills and placer deposits in the Guaniamo area of Venezuela have been characterized in terms of morphology, internal structure, carbon isotopic composition, syngenetic mineral inclusions, and the abundance and aggregation state of nitrogen. Ours is the first comprehensive mineralogical study of diamond from the Guaniamo area. About 50% of the crystals are resorbed dodecahedral forms; octahedra are the next most common form. In most cases, the diamond is colorless; 55-90% show radiation-induced pigmentation. About 20% of the stones have very low N contents (Type II); the remainder belong to the transitional IaAB type, with B > A. Ninety-three mineral inclusions were extracted from 77 crystals or fragments of diamond and analyzed by electron microprobe and LAM-ICP-MS to establish their traceelement compositions and the pressures and temperatures of diamond crystallization. In all, 86% of the diamond samples contain inclusions of the eclogitic paragenesis, represented by garnet, omphacite, rutile, ilmenite, pyrrhotite, and probable coesite. Inclusions indicative of the peridotite paragenesis are pyrope, chromian spinel and olivine. One inclusion of ferroan periclase may indicate a lower-mantle origin. The ␦ 13 C of 108 diamond samples ranges from -3.2‰ to -28.7‰, but most stones have ␦ 13 C ≤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°C); this zone may contain a substantial proportion of eclogite formed by subduction of crustal material. The very high proportion of diamond derived from an eclogitic association in the Guaniamo deposits, and several features of the mineral inclusions trapped in diamond, show striking parallels to the Argyle deposit of Australia. Both deposits occur within cratons that have experienced extensive Proterozoic tectonothermal activity.Keywords: diamond, carbon isotope, nitrogen, mineral inclusions, eclogitic association, Guaniamo, Venezuela. SOMMAIRENous avons caractérisé plus de 5,000 cristaux (ou fragments) de diamant provenant de filons-couches de kimberlite et de dépôts alluvionnaires dans la région de Guaniamo, au Vénézuela, selon leur morphologie, leur structure interne, la composition isotopique du carbone, leurs inclusions minérales syngénétiques, et l'abondance et l'état d'agrégation de l'azote. Ce travail * Publication No. 213 of the ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC). § E-mail address: felixvkaminsky@cs.com 1348 THE CANADIAN MINERALOGIST constitue la première étude minéralogique compréhensive du diamant de la région de Guaniamo. Environ 50% des cristaux ont une forme dodécaédrique résorbée; l'octaèdre est la deuxième forme la plus courante. Dans la plupart des cas, le diamant est incolore; entre 55 et 90% des échantillons font preuve d'une pigmentation due à une i...

ABSTRA CT O ver nne thousand diamonds from the Rio Sao Francisco and Rio Parnnafba basins have been comprehe nsively studied, In addition 10 cbamcrcnzation of diamond morphology and microanalysis of minera l inclusions, ultraviolet . infrared (l R) and mass-spectrometry techniq ues were used to coustm iu clcnjcutn l and isotopic compositions of thesc diamo nds. The regional specificity of diamo nds Ironubc Co ro ma ndc: 1a rea co nsists of the pre domin ance of rounded rhombododccohcdm l crysta ls, the rela tive ly high proporti ons o f crys tals wit h brown pigme ntation spot s. nnd a high proportion of crystals with marks of intensc mec hanical eros ion. Bused on Ik -cluunctcrisrics . Coroma ndc l diamon ds belo ng 10 the most common Group 2b, with C" + C p = 400 to 1000 at.pprn, along with South Afr ican and Sibe rian diamo nds, while their nitrogen aggregatio n propo rtions %N = lOON I (N A + N lI ) indicate th at Cor omnn dc l diam ond s be long to the tran siti ona l Gro up 2 wit h % N o = 35 to 75 %, a long wi th 'rukunn n and \fcnezuela~1 diamo nds. The sci o f'Ccromandc l diamonds show SI 'C value s varying:frum -l .5to · 11. J%c, with a pea k betwee n -3 and -7%0, The overwhelming majority of diamond s appear to he isotopically ' hea vy'. i.c.. s how SI'C > · IO%l'. 95% of the diamonds with mine ra l inclusions contain uttmmnfic (p. type) incl usion s: olivine, enstatite. pyrope. chro mian spinel ami sulfides . O nly two dia monds are Ectyp c : the y co nta in omphac ite inclusions. Th e predomin ance of mine rals of ultram afic ass ociauon a mo ng the nunc rnl inclusio ns in Ccromandcl dia mond s indic ates thnt thc kimberl ite magma origi nated in an ultramafic cuviromrcnt. at approximately 150-2IX) kill de pth. and diamonds arc from a homogeneous mantlesource of carbon. The similar character of all diamonds Iromtbc Rio dn Pnun sys tem may indicate :I W il li lio n provenance, a lternat ively they Illay have or igi nated from sources that arc distinct hut closel y refuted to eac h other. The Non bcm Arc u is situated close r to the inferre d diamo nd pro venance rhnn the Ribcirn do Pc ixc Area. There arc probabl y so me areas in Coromaudcl. where diam onds came to placer deposit s directl y fro m priuuu'y sc urccts ).

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