Darcy P. Svisero

Distribution and origin of diamonds in Brazil: An overview

Abstract Brazil was the first western country to produce diamonds from the washing of alluvial deposits found in central Minas Gerais in the early 1700s. For a century the country remained the worlds greatest producer, losing its position only after the discovery of the Kimberley field in South Africa. Currently there are placer deposits (garimpos) scattered throughout the majority of the states with official production averaging 1,000,000 ct/y. Mechanized exploitation using modern dredges has succeeded in only two distinct localities: along the Jequitinhonha River (Diamantina) and at the Fazenda Camargo (Mato Grosso). Large diamonds of several hundred carats have been found periodically in the area of the municipalities of Abaete and Coromandel in western Minas Gerais State. Carbonado, a polycrystalline variety of diamond, was intensively mined in several localities of the Chapada Diamantina in central Bahia State, mainly in the second half of the last century. Kimberlite-type rocks, on the other hand, were discovered only in the late 1960s, first in the Coromandel area in Minas Gerais and later in Goias, Mato Grosso, Rondonia and Piaui States. Little is yet known about these intrusions, mainly because the discoveries have been made by foreign companies operating in the country. Detailed studies reported during the Kimberlite Conference of Araxa in 1991 revealed that some intrusions of the Coromandel area have mineralogical and petrographical characteristics, as well as major chemical element compositions, similar to worldwide kimberlites. However, their isotopic signatures in terms of Sr and Nd are intermediate between Groups I and II kimberlites of South Africa. As to mineral inclusions, Brazilian diamonds contain the common phases of olivine, garnets, pyroxenes, sulphides and oxides as observed in diamonds elsewhere. Furthermore, diamonds from the Sao Luis River in northern Mato Grosso contain, in addition to garnet and pyroxene, periclase, ferripericlase, wustite, nickel, iron-nickel alloy and moissanite. This high-pressure assemblage resembles the mineralogy predicted for depths of ~650 km, thus suggesting an asthenospheric origin for the Sao Luis diamonds. This paper presents a review of general aspects of the most representative deposits of diamonds in Brazil. Field relationships point to the existence of at least three distinct ages for the secondary source rocks as illustrated by the Upper Proterozoic metaconglomerates of Diamantina, the Permo-Carboniferous diamictites of Tibagi and the Upper Cretaceous conglomerates of the Romaria Mine. The primary sources remain unknown in all localities. Additional comments are provided on the findings of great diamonds, diamond inclusions and on the study of kimberlite-type rocks carried out in the country in the past two decades.

A Note on the Geology of Some Brazilian Kimberlites

Recently several possible kimberlites have been discovered in Brazil, particularly in the states of Minas Gerais, Mato Grosso, Piaui, Santa Catarina, and Rondonia. The presence of alluvial diamonds and kimberlites in the Alto Paranaiba region of western Minas Gerais has resulted in this area being the most investigated. The magmatism here is related to the Alto Paranaiba uplift that developed in the late Cretaceous following reactivation of ancient regional fractures. Kimberlites in other Brazilian localities may be approximately synchronous with the activity in Alto Paranaiba. Associated with the kimberlitic diatremes, especially in the Minas Gerais region, are lamprophyric and other alkalic intrusions.

Mathiasite-loveringite and priderite in mantle xenoliths from the Alto Paranaíba Igneous Province, Brazil: genesis and constraints on mantle metasomatism

Alkali-bearing Ti oxides were identified in mantle xenoliths enclosed in kimberlite-like rocks from Limeira 1 alkaline intrusion from the Alto Paranaíba Igneous Province, southeastern Brazil. The metasomatic mineral assemblages include mathiasite-loveringite and priderite associated with clinopyroxene, phlogopite, ilmenite and rutile. Mathiasite-loveringite (55–60 wt.% TiO2; 5.2–6.7 wt.% ZrO2) occurs in peridotite xenoliths rimming chromite (∼50 wt.% Cr2O3) and subordinate ilmenite (12–13.4 wt.% MgO) in double reaction rim coronas. Priderite (Ba/(K+Ba)< 0.05) occurs in phlogopite-rich xenoliths as lamellae within Mg-ilmenite (8.4–9.8 wt.% MgO) or as intergrowths in rutile crystals that may be included in sagenitic phlogopite. Mathiasite-loveringite was formed by reaction of peridotite primary minerals with alkaline melts. The priderite was formed by reaction of peridotite minerals with ultrapotassic melts. Disequilibrium textures and chemical zoning of associated minerals suggest that the metasomatic reactions responsible for the formation of the alkali-bearing Ti oxides took place shortly prior the entrainment of the xenoliths in the host magma, and is not connected to old (Proterozoic) mantle enrichment events.

Morfologia cristalina de diamantes provenientes das regiões de Chapada dos Guimarães, Poxoréu, Paranatinga, Diamantino e Alto Paraguai, Mato Grosso

This paper presents the main results of a mineralogical investigation carried out on detritic diamonds from the municipalities of Chapada dos Guimaraes, Poxoreu, Paranatinga, Diamantino and Alto Paraguai, in the state of Mato Grosso, Brazil. The main features observed were the dominance of rounded dodecahedral crystals, followed by cleavage fragments, irregular forms and aggregates. Less frequent morphological types included octahedral, transitional forms between the flat octahedral and the rounded dodecahedral, flat twins, cube, pseudohexatetraedral, as well as some pollycrystaline diamonds (carbonado and ballas).

Diamonds from Minas Gerais, Brazil: An Update on Sources, Origin, and Production

The State of Minas Gerais (MG) was the main diamond producer worldwide for almost 150 years. Today, Brazil contributes about 1.5% to the world diamond production, with 220,000 carats (ct) coming from MG; the two principal source areas are located in the Diamantina region of Espinhaco province and in western MG. Although many theories have been advanced to explain the primary source of diamonds in MG, all commercial production is from secondary deposits. This paper reviews the geology of these deposits and emphasizes the importance of the redistribution of diamonds in Cretaceous and younger sediments. The Espinhaco diamond province, with distinct diamond districts and fields, varies in average grade from 0.008 to over 0.2 ct/m3; however, immense reserves, such as the Jequitinhonha River gravels, locally can reach 400,000,000 m3 or more. However, the largest stones weigh only several tens of carats, unlike in Western MG (which supplied practically all of Brazils large stones), where many weigh over 100 ct....