Diamonds from the Coromandel Area, West Minas Gerais State, Brazil: an update and new data on surface sources and origin

Abstract

Important diamond deposits southeast of Coromandel and the local geology have been studied in an attempt to understand what surface source provided the stones. River gravels of Pleistocene to Recent age from this region have supplied most of Brazil's large diamonds over 100 ct. The upper cretaceous Capacete Formation of the Mata da Corda Group, composed of mafic volcanoclastic, pyroclastic and epiclastic material, has been worked locally for diamonds, nevertheless considered non-economic. The authors present results of their study of a deactivated small mine, representing the first report with description and analyses of two gem diamonds washed from this material. Hundreds of kimberlites, discovered in the last half century in the region, are sterile or non-economic. We propose that the surface source of the diamonds is the Capacete "conglomerado". The volume of this material is enormous representing a potential resource for large-scale mining. The authors suggest detailed studies of the volcanic facies of this unit focusing on the genesis, distribution and diamond content. As to the question concerning the origin of these diamondiferous pyroclastic rocks, the authors exclude the kimberlites and point towards the large Serra Negra and Salitre alkaline complexes which are considered the primary source for the pyroclastic units of the Mata da Corda Group. They propose that early eruptive phases of this alkaline complex brought diamonds from a mantle source to the surface, much as happens with traditional kimberlites, to explain the association of such huge carbonatite complexes and diamonds.

Evidence of young, proximal and primary (YPP) diamond source occurring in alluviums in the Santo Antônio do Bonito, Santo Inácio and Douradinho rivers in Coromandel region, Minas Gerais

“…More recent work has contributed significantly to the knowledge of the post-Gondwana stratigraphy in Alto Paranaíba (Sgarbi et al 2001;Fernandes et al 2014;Karfunkel et al 2014). In addition to confirming the presence of diamond in the basal conglomerate of the Capacete Formation, the description of diamonds recovered in the Canastrel digging, extracted directly from the conglomerate , sheds light on the fact that the components of this formation constitute the secondary source of diamonds extracted in the alluvium of the Santo Antônio do Bonito, Santo Inácio and Douradinho rivers.…”

“…12). According to Karfunkel et al (2014), the last owner recovered 3 stones in the digging, weighing about 2 carats. Analyses of two of the stones indicated white and yellow colored, gem-type diamonds weighing 0.750 and 0.497 carats, respectively, and possibly dodecahedral .…”

“…The main sources of alluvial diamond deposits in the drainage to the south and southeast of Coromandel, especially in the Santo Antônio do Bonito, Santo Inácio and Douradinho rivers, are the basal conglomerates of the Capacete Formation and rocks of the Araxá, Ibiá and Canastra groups, which serve as bedrock for the conglomerates Karfunkel et al 2014). Also, kimberlites of different ages, kamafugites and related rocks contributed to the formation of the alluvium associated to the current drainage.…”

Geochemistry and sedimentary provenance of the Upper Cretaceous Uberaba Formation (Southeastern Triângulo Mineiro, MG, Brazil)

“…According to information from garimpeiros (diggers) a 42 ct diamond has also been recovered from an alluvial deposit. To one of the co-authors (J.K.), several diggers reported the largest recovered stone in the headwaters of the Uberaba River gravel in the 60's ties that weigh 42 ct, and a second one of 34 ct. Karfunkel et al (2014Karfunkel et al ( , 2015 discuss and put forwards some hypotheses to explain the origin of these gems, from local sources, or transported from long distances by extrusive rocks. Jr.…”

“…1906, Karfunkel et al 2014). In this respect, it is interesting that some authors correlate the Uberaba Formation to diamond-producing conglomerates, which crop out 95 km to the northeast, in the Romaria region (Ferreira Jr. 1996, Coelho 2010.…”