GEOLOGIA, GEOQUÍMICA E MINERALOGIA COMPARATIVA ENTRE AS INTRUSÕES DIAMANTÍFERAS CANASTRA-

GEOLOGIA, GEOQUÍMICA E MINERALOGIA COMPARATIVA ENTRE AS INTRUSÕES DIAMANTÍFERAS CANASTRA-1 E ABEL RÉGIS (MINAS GERAIS).
Hundreds of alkaline-ultrabasic intrusions occur in the south-southwestern region of Minas Gerais state. Two of these intrusions, both diamondiferous, were studied in detail evolving their geological, geochemical and mineralogical aspects. The second was still a subject of LA-MC-ICP-MS dating in zircon. In the geologic aspect, such intrusions present significant morphological differences: the Canastra-1 surface is distorted and of little dimension (150 m in length), while the Abel Régis has large circular diameter (1.4 km); the first presenting root facies, and the other crater facies. The rock chemistry of these intrusions shows peculiar characteristics, mainly related to the alkaline elements: the Canastra-1 pipe is depleted in Na₂O and K₂O, oxides that have significant contents in the Abel Régis pipe. This pipe has been dated in 86.1 ±4.5My. Mineralogical aspects of the diamond indicator phases also were distinct: abundance of Cr-spinel, besides the presence of sanidine, and the mineral chemistry of ilmenite and diopside, indicate a possible lamproitic “tendency” to the Abel Régis pipe, while the Canastra-1 has geochemical and mineralogical features of a typical kimberlite.

MINERAIS INDICADORES KIMBERLÍTICOS E PROSPECTIVIDADE DIAMANTÍFERA

MINERAIS INDICADORES KIMBERLÍTICOS E PROSPECTIVIDADE DIAMANTÍFERA DA INTRUSÃO ALFEU-01 (CANGUÇU, RS).
The Alfeu-01 kimberlite intrusion was discovered in 1994, and is hosted by granitoid rocks of the Pinheiro Machado Complex, of Neoproterozoic age, in the Sul-Riograndense shield. This kimberlite is melanocratic, and has an afanitic matrix where xenoliths of the host rocks as well as macrocrysts of phlogopite, rutil, garnet and, possibly diopside can be observed. The petrographic descriptions (basically phlogopite + olivine + talc + serpentine) indicate that it is an explosive breccia, probably of diatreme facies. In geochemical terms, the high contents of TiO₂, MnO and K₂O are characteristics, in contrast with diamondiferous intrusions of Minas Gerais. The garnets were divided into three different colors, a strong association of colors with the Fe₂O₃, MgO and Cr₂O₃ contents having been observed. All garnets, identified as the pyrope type, are located in the mineral fields known as G4, G5 and G9, generally not associated to fertile intrusions. In the same way, the high oxidation grade in the Mg-ilmenite is not favorable for diamond preservation. Chromite data indicates that the mineral is out of the diamond inclusion field. Diopside of the intrusion is Cr-poor, and recognized as associated to an alkaline basalt suite. All mineralogical parameters characteristically diverge from the observed in fertile kimberlites of Minas Gerais; in this way, they strongly not encourage another prospecting works on the intrusion.

Recursos minerais não-metálicos do Estado de Mato Grosso do Sul, Brasil.

Recursos minerais não-metálicos do Estado de Mato Grosso do Sul, Brasil.
The Pantanal Plain, the largest flooded area of South America, is bordered by three plateaus that concentrate non-metallic mineral resources in the State of Mato Grosso do Sul, Brazil. The Bodoquena Plateau is formed by Precambrian carbonate rocks of the Corumbá Group, whereas the others, namely the Maracaju - Campo Grande and the Taquari - Itiquira plateaus, are supported by sedimentary and volcanic rocks of the Paraná Basin. Carbonate rocks are exploited for cement, while powdery tufa limestone is mined in Bonito for agricultural use. Clay, another important mineral resource in the state, used solely for ceramics, is concentrated in the Rio Verde de Mato Grosso-Coxim and Bela Vista-Jardim regions. Dimension stones are restricted to marls in Bonito and granites around Porto Murtinho. There are problems with building materiais such as sand and gravei, because of their poor quality. Exploitation of the sands also causes environrnental problems, especially in rivers near the cities. Small-scale diamond mining by rudimentary processes leads to similar environrnental problems in the Aquidauana and Coxim regions. Phosphatic rocks occur in the Corumbá Group in the eastern Bodoquena Plateau. The Botucatu Formation, west ofthe Campo Grande, the state capital, presents good potential for sand, mainly for industrial purposes. Detailed geologic studies are required, however, for better economic evaluation of these deposits. The exploitation of industrial minerais in Mato Grosso do Sul State is still not intensive. Nevertheless, new economic perspectives together with the improving transportation system to the neighboring State of São Paulo, mainly through Tietê-Paraná riverway, favour future economic growth. Thus, it is imperative that urgent planning measures be taken in arder to reduce negative environmental impact.

U-Pb ages in zircon of the Grão Mogol diamond-bearing conglomerate (Espinhaço Supergroup)

U-Pb ages in zircon of the Grão Mogol diamond-bearing conglomerate (Espinhaço Supergroup): implications for the diamond origin in the Espinhaço Range in Minas Gerais.
The Espinhaço Range in the Grão Mogol region, center-north of Minas Gerais state, is composed by fine grained quartzites with large cross stratifications (Resplandecente Formation), which are covered with erosional unconformity by monomictic conglomerates, and medium to coarse grained quartzites (Grão Mogol Formation), both units belonging to the Espinhaço Supergroup, of Proterozoic age. At the locality known as "Pedra Rica" (signify Rich Rock, an old diamond digging), rocks of these formations were sampled and separated detrital zircons to acquire U-Pb by Laser Ablation Inductively LA-ICPMS) ages. The analyzed grains are rounded to slightly rounded and show oscillatory zoning. The obtained results indicate a maximum depositional age of 1,595 ± 20 Ma for the Resplandecente Formation, and 1,052 ± 50 Ma for the Grão Mogol Formation. The comparison between the obtained data and the available ages for the Diamantina region and proximities, in the same diamond province, indicates a strong evidence for the existence of at least two primary mineralizing events in the basin, in the age range of 1.35 to 1.05 Ga.

The Basal Conglomerate of the Capacete Formation

The Basal Conglomerate of the Capacete Formation (Mata da Corda Group) and its Relation to Diamond Distributions in Coromandel, Minas Gerais State, Brazil. Braz. J. Geol. [online]. 2014, vol.44, n.1, pp. 91-103. ISSN 2317-4889.  http://dx.doi.org/10.5327/Z2317-4889201400010008.
The diamond bearing district of Coromandel is located in the northwestern part of Minas Gerais, within the Alto Paranaíba Arch, famous for the discovery of most of Brazil's large diamonds above 100 ct. Detailed mapping, aimed at characterizing the Mata da Corda Group of Upper Cretaceous age of Coromandel, has been carried out. This Group was divided into the Patos Formation, composed of kimberlitic and kamafugitic rocks, and the Capacete Formation, presented by conglomerates, pyroclastic rocks, arenite and tuffs. Exposures of the latter Formation have been studied in detail at the small abandoned mine called Canastrel, as well as in the headwater of Santo Antônio do Bonito River. The results have been compared to studies of the kimberlite bodies in the nearby Douradinho River. Kimberlite indicator minerals from these localities show the same compositional trend. Moreover, in the basal conglomerate of the Garimpo Canastrel two diamonds diamonds have been recovered and described. The Garimpo Wilson, situated in the headwater of the river Santo Antônio do Bonito in paleo-alluvium, is composed of material exclusively derived from the erosion of the Capacete Formation and Precambrian (sterile) Canastra quartzites and schists. These detailed investigations suggest that the basal conglomerates of the Capacete Formation represent the main source rock of the alluvial diamond deposits in the Coromandel region.