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 world's 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 Abaeté 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 Goiás, Mato Grosso, Rondônia and Piauí 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 Araxá 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 São Luis River in northern Mato Grosso contain, in addition to garnet and pyroxene, periclase, ferripericlase, wüstite, 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 São 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.