Diamondiferous placer deposits were discovered in the Guaniamo area in 1968, and since then approximately 20-25 million carats have been produced from this area.Stones up to 40 and 60 carats in weight have been reported here.Recently diamondiferous kimberlite sills were discovered in the Quebrada Grande, Guaniamo River right tributary valley which are the primary sources of the diamonds.We have studied over 4,000 diamonds from both, kimberlites and placer deposits.Morphology.Diamonds from the Guaniamo area include dodecahedroids, octahedra, octahedron + dodecahedroid (O-D) combination-type crystals and their twins and aggregates.In all sills and placer deposits, dodecahedroids are predominant, accounting for about 45-50% of the respective samples.Combination-type O-D crystals are also present in rather high proportions, sometimes up to 23% of the samples.All dodecahedral diamond crystals are rounded dodecahedroids.Eight groups of them were distinguished based on the nature of the dominant crystal surface.Dodecahedroids with concentric striation are predominant.They compose 60-70% of all dodecahedrical diamonds.Octahedral diamonds are subdivided into the following three groups: (1) Octahedra with stepwise lamellar development of trigonal faces, (2) Octahedra with clearly concentric development of crystal faces and (3) Octahedra with bitrigonal faces.Diamonds of the last type are commonly predominant.Single cubic-habit crystals were found.They are represented by tetrahexaedroids and combination-type crystals.There are many varieties of twinned and aggregated diamonds.Their proportion in the larger diamonds increases.Macles, or octahedral spinel-type twins are most common.Only a small number of diamonds from the Quebrada Grande placer show post-magmatic mechanical erosion features and, may therefore have been transported over some distance.
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 trace-element 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 δ 13 C of 108 diamond samples ranges from −3.2‰ to −28.7‰, but most stones have δ 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.
A set of 77 diamond crystals from the Los Coquitos placer on the Guaniamo River in Bolivar State, Venezuela, has been comprehensively studied and compared to previously studied diamond from the Quebrada Grande kimberlite sills and placer deposits, both located 50 km to the southeast. The diamond crystals in the Los Coquitos placer are generally similar to those of the Quebrada Grande area, but with signifi cant morphological differences. Diamond from the Los Coquitos placer has a higher total nitrogen content (N tot = 719 at.ppm versus 614-706 at.ppm in diamond from Quebrada Grande) and a lower level of nitrogen aggregation (% N B = 65 versus 68 in diamond from Quebrada Grande). Carbon isotope values, ␦ 13 C, of the Los Coquitos diamond vary from +0.4 to-20.5‰, compared with-3.2 to-28.7‰ in the Quebrada Grande diamond. In the Los Coquitos suite, mineral inclusions in diamond are characterized by a lower Ca content in garnet, wider compositional variations in pyroxene (mg in the range 0.683-0.849), and lower Fo component in olivine (mg in the range 0.926-0.932), compared to inclusions in diamond in the Quebrada Grande suite. Los Coquitos diamond shows greater depletion in LREE compared to Quebrada Grande diamond. Diamond crystals at Los Coquitos were derived, at least in part, from a new, undiscovered kimberlite source in the Guaniamo River basin. These rocks are similar in type to the Quebrada Grande kimberlites, but were sampled from a different section of the subcontinental lithospheric mantle. The existence of two separate populations of diamond, each with its own primary source, suggests that other localities of placer diamond in Venezuela may have their own local sources.
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 ). Keywords. diam ond ,
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