Resumo APLICAÇÃO DE DADOS AEROGEOFÍSICOS À EXPLORAÇÃO MINERAL EM TERRENOS IRREGULARMENTE EXPOSTOS DO GREENSTONE BELT RIO DAS VELHAS A abordagem geofísica utilizada neste trabalho consiste no processamento e realce de dados geofísicos, os quais puderam ser interpretados com ênfase no contexto geológico. O raciocínio usado é que, em exploração, não se deve observar apenas as anomalias geofísicas, mas também as respostas relacionadas à mineralização, à litologia e às estruturas que podem ter importância econômica.A interpretação de dados magnéticos de alta resolução forneceu tanto uma visão geral da arquitetura regional, bem como uma melhor compreensão dos controles estruturais dos depósitos auríferos hospedados no Greenstone Belt Rio das Velhas. Os quatro mapas radiométricos (contagem total, potássio, urânio e tório) foram realçados individualmente pela manipulação de seus histogramas e, posteriormente, apresentados em falsa cor, na forma de imagens ternárias (R:G:B = K:U:Th). Unidades litológicas individuais podem ser traçadas, uma vez que possuem resposta radiométrica distinta. Uma transformação logarítmica foi aplicada aos dados de condutividade, no intuito de distribuir melhor o intervalo de valores. Isso também permitiu uma melhor interpretação visual dos dados e de sua relação com as fontes geológicas. As rochas hospedeiras mais importantes, ou domínios mineralizados, mostram alta resposta de condutividade. Existe uma correlação forte entre estes domínios e as formações mapeadas como Morro Vermelho, Quebra Osso e Córrego Ouro Fino. Estes domínios abrigam as mineralizações auríferas conhecidas, ilustrando a utilidade do procedimento adotado na melhora do mapeamento geológico e na compreensão da mineralização.Palavras-chaves: Projeto Rio das Velhas, Magnetometria, gamaespectrometria, eletromagnetometria no domínio do tempo, mineralização aurífera.Abstract The highly prospective nature of the Rio das Velhas Greenstone Belt is known since the end of the last century. Archean greenstone-hosted gold deposits represent a distinctive group with many common characteristics. The geophysics approach employed concentrated on processing and enhancing the geophysical data to accurate positioning of geological boundaries. The rationale was that in exploration, we should not be looking for geophysical anomalies, but the responses related to mineralization, lithology, and structure that may have economic importance. Aeromagnetics identifies magnetic greenstone units, important structures related to mineralization and allowed a better understanding of sctrutural geophysical pattern. The radiometric data was an excellent tool for mapping and trace individual lithological units in the in areas of outcrop. The most important host rocks or mineralized domains show high conductivity response. There is a strong correlation of these domains with the mapped Morro Vermelho, Quebra Osso and Ouro Fino Formations. Such domains host the known gold mineralization, illustrating the utility of the EM data to improve the geologic map.
Strong hydrothermal alteration modifies rock physical properties in iron oxide-copper-gold deposits (IOCGs) and may result in characteristic signatures detectable in geophysical surveys. Magnetic data are commonly used in characterizing orebodies, and 3D inversions are often used to assist in interpretations. In areas with strong remanence and self-demagnetization, the total magnetization can have directions different from the inducing field direction. This deviation precludes the use of traditional inversion methods. Magnetic amplitude inversion offers one solution to this challenge because the amplitude data are weakly dependent on the magnetization direction. In addition, the low magnetic latitude also imposes difficulty in amplitude data calculation due to the instability in the component conversion in the wavenumber domain. To formulate a practical approach, we present a case study on applying the magnetic amplitude inversion to the Furnas southeast IOCG deposit at the low magnetic latitude in Carajás Mineral Province, Brazil, and demonstrate that the approach can reliably recover an interpretable distribution of effective magnetic susceptibility and identify massive magnetite from hydrothermal alterations associated with the high-grade ore.
SOI LSURVI:Y BY DC ELECfRICAL RESISnVITYAND GAM MA •RAYS PECTROMF:lRY Tbe purpose ofthis work is 10 pre sent proced ures fo r reconn aissance o f so ils. based 0 11 DC elec trica l resistivity measurement s and gamm a-my spectrometry, T ile study area has 5 km 2 and is located in the Jurdim Bctflnico de Brasilia (Bra zil), :111 environmental co nservation area. In this urea. resistivity measurements were ob tained nt 190 d iffe rent points. and radiom etric measure ments (uranium, thorium, potassium. ami total co unt) at 9 differe nt poin ts. Apparent resis tivity values below 15000 Olnu .m we re observed in d ay so ils and between 15000 and ] O{}OOOluu .m in sandy so ils. It was also observ ed that radioa ctivity front c lay so ils is between HOOOand 14000 cps at total co unt. while in sandy so ils. this value is be low HO(X)cps. For the study area. although Ihe cl ay soils arc more radioact ive. the K/ U and Kr r h ratios arc higher in sandy so ils. This fact can be related with d ifferen ces in mineralogical co mpos ition of these materials. The process ing o f data prod uced an app are nt resistivity map for the study area and its integration with rad io metric data using a statistical procedure. the cluster analys is. T he products o f proc essing were compared with the so il map for 1() correlate the geo physica l units with different kinds o f so ils. Keywonls : geo physics. resisti vity. gumma-ray spectrometry. cluster analysis. trop ical so ils. RCSlI lIIO 0 objctivo dc stc unbulho C aprc scn tar procedimcntos para reco nhccim en to de so los. bnscndos em medir i >es de sua rcsistividudc clctricu c de sua mdioat ividndc natural. A area de cstudo possui 5 kill! c taz par te do Jnrdim Botflnico de Brasilia. lima unidndc de conscrvncno nmbicntal. Neste local ob tivcrnm-sc mcdidas de rcslstlvldudc e m 190 pont es disti ntos e 'linda mcdidas rudiomdtric as (urflnio. torte. ponissio C co mngcm tota l) em 9 pomo s distin tos. Ob scrvaram-sc vulorcs de rcsistividadc aparcnte infcriorcs a 15(XX) O hm.m para so los argilo sos ce ntre 15(X)() e ] (X}(}() Ohm.m para solos arcnosos. Ob scrvou-sc tambcm que a radioativida dc dos so los argilosos emais clc vuda, entre SOlX) e 14000 cps na contngcm total. <10 passe qm: nos arcnusos. cste valor sitna-sc abaixu de H()()() cps. Na area de cstudo. cm lxuu os so los argilosos sejum os mais rudioativos, verificou-sc que as razocs Kl U c Kfr h sat) mais elc vada s nos solos arenosos. fato cs te que (lode csrnr relacionado CO Ill d iferencas na co mpos icao rniucralogica destes materials. 0 proc essam cnto dos dados incluiu a confeccao de um mapa de resistividadc aparc ntc para a area de estudo c sua integrucao com os dados rudiometricos Ix)r meio de urn proccdi mento estatfstico. a ana lise de gl1JIX)S. Os produt os o btidos forum com parados co m 0 mupa de so los dispon fvcl. de modo a cstabelc ccr corrclac oc s entre os d ifcrentcs so los c os pudrfcs obscrvndos nos mapas gcoffsicos. P t l! a l'/'(/,\'-C-!UII '£,: gcoffsicu. rc slstivktad c. gumacspcctromctrin. untilise de a...
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