An airborne electromagnetic (EM) and magnetic survey was conducted to prospect for massive sulfide ore bodies in the Itapicuru greenstone belt in the state of Bahia, Brazil. Because tropical weathering is intense in this region, most of the recorded airborne EM anomalies were caused by a conductive overburden. Standard procedures to select targets for ground investigation (follow‐up) were found only partially satisfactory because such procedures resulted in anomalies in conductive environment being largely ignored. Because of this difficulty, a new scheme of target selection was developed which is based on a statistical analysis of conductance within a given lithologic unit. Four case histories of ground geophysical surveys, which were carried out during the follow‐up of airborne anomalies are described. The case histories focus on problems likely to be encountered in environments of intense tropical weathering. In areas covered by a conductive overburden, the EM response is sensitive to a proper choice of field equipment, frequency, and coil separation. As a result of the geophysical survey, bodies of volcanogenic massive sulfides were found in the Itapicuru greenstone belt, but so far none is of economic importance.
This paper aims at analysing the application of the gravimetric method in the search for copper ore in the Valley of Curaçá River in northern Bahia, Brazil. The area where this study was carried out is known as Angico Farm, one of the claims of Caraíba S.A., a copper producer in the northern Bahia, Brazil. There are 18 drill holes at the Angico Farm target, drilled in order to investigate the mineralizations in depth. We have obtained information such as geographic coordinates and chemical results from the company in order to test the geophysical response and the correlation with geology. A 3D inverse gravimetric model was generated in order to verify the validity of the method in exploring for copper ore associated with hydrothermally altered mafic and ultramafic. Both mafic/ultramafic rocks and copper ore present high density, therefore the gravity method may not be effective for identification. We have shown, however, that copper ore from the Curaçá Valley presents a fairly good gravity response, and 3D inverse mathematical model pointed out a well‐delimited copper orebody in the regions where drill holes intersect the ore and coincide with the positive gravity anomaly. The ore contents were overlapped on cross‐sections of density extracted from the inverse model and such information helped us to check out the consistency of the gravimetric method in mapping and modelling mineralized bodies associated with mineral occurrence. Additionally, magnetic susceptibility and gammaspectrometric data were acquired along 18 drillcores to investigate their possible correlation with orebodies.
The Serra das Éguas Complex, in Brumado, state of Bahia, located in the northeastern region of Brazil, is a geological structure comprising volcanochemical, siliciclastic and carbonate units that contain layers and lenses of magnesite, important as a refractory in steel, cement, glass and copper and other applications. Considering the importance of such sort of deposit, this work aims to (i) study the physical properties of magnesite deposits via direct measurements of radiometric spectrometry and magnetic susceptibility associated with chemical analysis of the available borehole core samples; (ii) analyse the magnetic and radiometric data acquired by the combined airborne survey over the area with known magnesite deposits; (iii) estimate the magnetic response of the magnesite deposits and surrounding rock formations via 2D forward and 3D inverse magnetic modelling; (iv) evaluate and, if necessary, reconsider the stratigraphic sequences of the identified geological units of the Complex using the results of the 2D and 3D magnetic modelling. Through forward/inverse models, we noted an expressive magnetic signature likely related to the iron formations and metamafic and meta‐ultramafic rocks. The radiometric data analysis on the drill cores showed that the reddish magnesite has a well‐defined U/Th signature. Furthermore, the results show that the carbonate unit, which contains the magnesite deposits, has the lowest amplitude of magnetization values, and both signatures might be helpful for their identification. Additionally, the magnetic models show that the carbonate unit lies in the upper part of the stratigraphic sequence of the Serra das Éguas Complex. This conclusion drives us to propose a new stratigraphy sequence, where the volcanochemical unit that includes the iron formations lies at the bottom of this stratigraphic sequence.
Seu conteúdo foi revisado pelo ComitêTécnico do IV SimBGf, mas não necessariamente representa a opinião da SBGf ou de seus associados. É proibida a reprodução total ou parcial deste material para propósitos comerciais sem prévia autorização da SBGf.
This paper proposes to analyze the application of the gravimetric method in copper ore prospecting in the Valley of Curaçá River in northern Bahia, Brazil. Was generated 3D inverse gravimetric model in order to verify the validity of the method in copper ore mining associated with mafic and ultramafic rocks. The inverse mathematical model was generated through the automatic iterative method with setting limits . Finally , ore levels earned in drill core were faced with vertical profiles of the inverse model in order to verify the applicability and consistency of the gravimetric method in mapping and modeling bodies associated with mineral occurrence.
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