Diamonds From the Coromandel Area, Minas Gerais, Brazil

Abstract

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 ).

Diamond From the Los Coquitos Area, Bolivar State, Venezuela

“…According to experiments by Badro et al (2003), ferroan periclase in the lowermost mantle (1700-2900 km) must be enriched in Fe because of the transition from high-spin to low-spin state of Fe at 70 GPa. There are observed indications of increasing content of iron in lower mantle mineral phases toward the core-mantle boundary, e.g., ilmenite, chromian spinel and "olivine" in lower mantle associations are characterized by a higher iron index than the same phase in upper mantle (e.g., Harte et al 1999, Kaminsky et al 2001b. Considering this hypothesis, the wüstite in crystal #47 may well belong to the lowermost mantle.…”

“…Ferroan periclase, typical of the superdeep paragenesis, is included in diamond crystals with a "juvenile" carbon isotopic composition, with ␦ 13 C from -3 to -6‰ PDB (e.g., Harte et al 1999, Kaminsky et al 2001b. In contrast to periclase-including diamond, crystal #47 has a "light" carbon isotopic composition, with ␦ 13 C = -12.4‰ PDB (see Table 3).…”

“…Errors on the concentrations determined were less than 20%. One can fi nd the principles that were used for interpretation of IR analysis of diamond in Kaminsky et al (2000Kaminsky et al ( , 2001b, Kaminsky & Khachatryan (2001, and Khachatryan & Kaminsky (2003). Isotope analyses were performed by K. Maltsev (GEOHI, Russia) using samples from 2 to 15 mg in weight with the VARIAN-MAT-230 mass spectrometer; this was done with an accuracy of <0.1‰ PDB ( 13 C/ 12 C PDB = 0.0112372).…”

Brazilian Diamonds: A Historical and Recent Perspective

“…The town was probably named by Portuguese traders for the Coromandel region of India, an important diamond source since antiquity (Legrand, 1980). Also known as the Alto Paranaíba province or the Triângulo Mineiro, this area is the second most important source of diamonds in Minas Gerais, serving as the center of the local diamond trade (figure 14; Leonardos, 1956;Kaminsky et al, 2001;Karfunkel et al, 2014).…”

Origin and composition of the Subcontinental Lithospheric Mantle (SCLM) along the lineament of 125° Azimuth, Brazil

“…Later, several other kimberlite prospects were identified along a corridor (Svisero et al, 1982) that was characterized by Gonzaga and Tompkins (1991) as the lineament of 125° Azimuth (see Chapter 2). absence of mechanical transportation features in the diamonds suggest nearby primary sources (e.g., Kaminsky et al, 2001). According to Benitez (2009) most of the alluvial diamonds are above 2.1 cts and have octahedral and rhombododecahedral shapes;…”