1 VALE S.A., Department of Exploration and Mineral Projects. 2 Universidade de ... on time domain electromagnetic principles, configurations. prospecting methods ... on the use of induced polarization and electromagnetic methods applied to .... Geonics Limited, 2006, PROTEM - Operating Manual for. 20/30 Gate Mode: ...
Ground and borehole time domain electromagnetic response at disseminated iron oxide– copper–gold deposits, Carajás Mineral Province, Brazil Marcelo Leão-Santos*1,2, Daniel Brake3, João Paulo Souza1, Cantidiano Freitas1, and Fernando Matos1 1 VALE S.A., Department of Exploration and Mineral Projects. 2 Universidade de Brasília. 3 Quantum Pacific Exploration SAP. Summary Time domain electromagnetic surveys are frequently used to interpret conductors of massive sulfides from strong hydrothermal alteration in iron oxide–copper–gold deposits (IOCG). In areas with disseminated sulfides, electromagnetic surveys are not capable of directly detecting the mineralization. However, the presence of massive magnetite matrix can produce a weak response related to the hydrothermal system. With a sufficient thickness of magnetite, the conductance (conductivity*thickness) of the body can result in an anomaly that may be coherent even beneath conductive overburden. Correct survey planning is essential to achieve the electromagnetic coupling while minimizing excitation of conductive overburden. The interpretation of the data can help to understand the behavior of the conductive trend related to the hydrothermal system. This trend can then be explored to maximize the probability of intersecting the associated copper mineralization. Drilling may then be complimented with borehole electromagnetic survey to investigate for the presence of localized areas of higher conductivity, potential copper mineralization. We demonstrate that the application of all these methods in the Furnas IOCG deposit located at Carajás Mineral Province, Brazil, can reliably detect the massive magnetite of hydrothermal alterations associated with the high and low grade ores.
Nabighian edited a book in 1991, with a complete approach on time domain electromagnetic principles, configurations. prospecting methods and case histories. Many case histories applied electromagnetic surveys to mineral deposits discoveries. For massive sulfides and nickel deposits the use is more effective due to the presence of more conductive minerals as pentlandite. However, some case studies deals with the use of electromagnetic methods applied to disseminated sulfides and copper sulfide deposits. Fountain in 1972, presented some case histories on the use of induced polarization and electromagnetic methods applied to disseminated deposits in British Columbia Canada. McInerney et al. (1994) applied moving-loop, fixed-loop and borehole TEM to detect a deeply buried low-grade volcanogenic lead-silver-coppergold deposit in Australia. Reid et al. (2013) applied ground and borehole TEM surveys to detect a copper deposit in Australia. A case history of Cristalino IOCG deposit in Carajás region dealing with overburden and disseminated ore was presented by Silva et al. (2013). Electromagnetic data interpretation can be carried out by analyzing the channels profiles and plate modeling. The main objective of this paper is to apply the above mentioned electromagnetic data analysis procedures to study an IOCG mineralization using a set of ground electromagnetic data at the Furnas copper-gold deposit. Geological Setting
Introduction Fixed loop electromagnetic surveys are often used in the exploration of mineral deposits to explore for the presence of conductive massive sulfides. For disseminated mineralization this approach does not apply. However, the interconnection of sulfides with massive magnetite can result in a conductive response of the system. Also the presence of conductive overburden in tropical regions further complicates the use of electromagnetic methods. The overburden influence has been studied awhile. Palacky and Sena (1979) deal with the conductive overburden problems to identify volcanogenic massive sulfide ore conductor in Brazilian tropical terrains. Interpretation and design of time domain EM surveys were performed in conductive overburden areas in Australia by Spies (1980).
The Furnas Copper Deposit is located in the Carajás Mineral Province, along the regional Cinzento transcurrent shear zone striking WNW-ESE. Metavolcano-sedimentary rocks correlated to the Grão Pará Group, of the 2.76 Ga Itacaiúnas Supergroup (Wirth et al., 1986), and sedimentary rocks correlated to the Águas Claras Formation host the deposit. The shear zone defines a contact between amphibole schist to the north (hanging wall) and aluminous schist to the south (footwall) of the Itacaiúnas Supergroup (Figure 1), interrupted by the intrusion of the 1.8 Ga Cigano Granite to the east. The region is metamorphosed from greenschist to amphibolite facies (Vale, 2012). The deposit is poorly exposed with a wide laterite cover (thickness of about 60 m). The copper-gold mineralization is hosted in biotite-garnet-grunerite-magnetite hydrothermal alteration assemblages, with moderate to strong foliation. The mineralization itself is contained in
TEM response at disseminated IOCG deposits
massive magnetite, silicification, sodic-calcic, and potassic alteration as can be easily seen on the geological cross section through the high and low-grade orebodies (Figure 1). The main mineralization zone is associated with a latestage chalcopyrite and bornite veinlets and breccias. Conductivity measurements were performed along drill cores and do not show any response correlated to mineralization. Lithologies and Hydrothermal alteration zones
Geological Section
88
SW
111
112
NE
227 m
Weathering Drilling High grade Low grade
306 m
Garnet-grunerite-magnetite alteration zone 468 m Biotitic alteration zone 463 m Silicification Amphibolitic schist Aluminous schist -50
0
530 m
50 100 150 m
Figure 1. Geological section of Furnas deposit (Modified from Vale, 2012).
Ground and Borehole Electromagnetic Surveys The main purpose of the survey was to verify the ore body continuation in depth. Nine fixed loops were planned to the electromagnetic survey with 800 x 600 m size. The ground survey was carried out over 38 lines with 200 m spacing and a total of 42.4 line-km. Borehole surveys were performed in 3 drill holes with a total of 1350 m (Figure 2). The survey was performed in 2011, with PROTEM equipment and 30 Hz frequency (Geonics, 2006).
Electromagnetic Interpretation The electromagnetic interpretation was performed with the analyses of the profiles in the late channels, 13-20. Channels 1-12 show strong overburden influence so are of no use in tracing the mineralization. The interpreted results were modeled with Maxwell software from EMIT. The results were correlated with the geological and ore model from extensive drilling in the area. The profiles from lines 2200 SE and 2800 SE, summarize the electromagnetic responses. First we can see that the data contain considerable noise however the conductive trend was detected. Second we can observe the good fit between the measured data in black with the modeled data in red (Figure 3). The noise in the data is probably related to the thick overburden present in equatorial regions, in this area the overburden can have a thickness from 60 to 100 m. Ground electromagnetic survey show anomalies associated with weak conductors, with 3.5 to 10 Siemens. In general, the trend of the conductors has a well-defined NW-SE direction swinging to E-W direction. The depths to the conductors are approximately 200 m. Channels 14 to 20 were given more emphasis as the earlier channels showed considerable overburden effects. The BHEM data contained considerable noise and were collected with a small number of repeat readings. Data from holes DH 119 and DH 114 indicate more regions of higher conductance, around 30 Siemens, near the holes. These smaller (300 by 300 m) plates can be incorporated in to the surface data collected from loop 17 (lines 3000 NW, 3200 NW, 3400 NW and 3600 NW) without affecting the model significantly or convincingly. These conductors sit on the larger, weaker conductive trend. These local zones are interpreted as being regions with higher grade or a greater thickness mineralization, copper sulfides or magnetite. Detecting these areas of higher conductance is very difficult from surface. Knowing the orientation of the hydrothermal system will greatly increase the probability of exploring the host trend. Electromagnetic Modeling
Figure 2. Electromagnetic survey configuration.
For brevity, the loops 17 and 18 were used to describe the elctromagnetic modeling because they have ground and borehole surveys. In general, one plate was enough to fit the four lines of data for each loop. However, for loop 18 the trend had to be adjusted for each survey line. This is due to the large conductor variation along this interval. In this NW portion the plates have a dip varying from 50° 60° to NE with a plunge 5° to NW and conductance around 3.5 Siemens (Figure 4).
TEM response at disseminated IOCG deposits
Figure 4. Plan view of ground electromagnetic modeled plates to loops 17 and 18. The white lines are the survey lines, the magenta polygon is the loops, and the gray plates are the modeled conductors with 3.5 to 5 Siemens. (a)
Figure 5. Plan view of borehole electromagnetic modeled plates to loop 17. The white lines are the survey lines, the magenta polygon are the loops, the blue lines are the drill holes, and the red plates are the modeled conductors with 25 to 30 Siemens. (b) Figure 3. Ground electromagnetic survey X, Y and Z profiles of lines: (a) 2200 SE and (b) 2800 SE. Measured data in black and modeled data in red.
The response of the modeled plates of loop 17 is similar to loop 18 with 50º dip to NE and 5 Siemens conductivity (Figure 4). This loop was also used to the borehole survey. The BHEM modeled plates are the more conductive zones of the ground EM modeled plates. The plate modeled from drill hole 114 has a dip to 67º NE, a plunge to SE and 25 Siemens. This is the same response to modeled plate of drill hole 134 (not included in the figure). The modeled plate from drill hole 119 has a dip to 75º NW, a plunge to NE and 30 Siemens (Figure 5).
Results The success of the electromagnetic survey and interpretation is evaluated not only by the level of correlation with the orebody, but more importantly, by the correspondence between the 3D modeled plates and the known source, which is the mineralized massive magnetite orebody in this case (which was unknown to the interpreter at the time). Figure 6a shows the ground TEM modeled plates correlated with the high-grade copper orebodies in top view. We can observe the strong correlation between the plates and the orebodies along the strike. Figures 6b and 6c show the west and east view of the ground TEM modeled plates, respectively. We can observe the correlation between the
TEM response at disseminated IOCG deposits
modeled plates and the dip of the orebodies. Those results confirm the effectiveness of the method in this complex scenario.
down-dip extent of the modeled plates indicate that the orebody continues much further down-dip than has been intersected to date. New exploration drill holes could confirm this and increase the deposit resources at depth. Also the continuity of the modeled plates to the northwest direction can be tested and extend the ore bodies.
(a) Top view (a) Top view
(b) West view
(c) East view Figure 6. Ground electromagnetic modeled plates correlated with the high-grade copper orebodies. The white lines are the drill holes, the gray plates are the modeled conductors with 3.5 to 5 Siemens, the green plates are the modeled conductors with 10 Siemens, and the yellow bodies are the high-grade ore.
Figure 7 shows the good correlation between the highgrade ore bodies and the BHEM modeled plates in plan and east view, respectively. Both minerals, magnetite and chalcopyrite, in this system are conductive. Given the thickness and extent, both along strike and down-dip, the magnetite is the source of the electromagnetic response. The conductance values (in Siemens - S) have a variation from 3.5 to 10 S in the ground data and from 25 to 30 S to borehole data. These local increases in conductance are likely the result of local thickening of the hydrothermal mineralization or an increase in conductivity. These results emphasize the importance of BHEM surveys in areas with overburden and unknown strike and dip mineralization. The
(b) East view Figure 7. Borehole electromagnetic modeled plates correlated with the high-grade copper orebodies. The blue lines are the drill holes, the red plates are the modeled conductors, and the yellow bodies are the high-grade ore bodies.
Conclusions These results show that given the presence of an appropriate matrix to connect non-massive chalcopyrite mineralization, electromagnetic methods can be utilized to aid the exploration of the deposit. Furthermore, that incorporating BHEM methods can greatly increase the radius of investigation and further increase the probability of intersecting regions of more concentrated mineralization. Acknowledgments The authors would like to thank Vale – Department of Exploration and Mineral Projects – for permission to publish this work and Vale – Furnas Project – for providing electromagnetic and geological data.
TEM response at disseminated IOCG deposits
References Fountain, D.K., 1972, Geophysical case histories of disseminated sulfide deposits in British Columbia, 37, no. 1, 142-159. Geonics Limited, 2006, PROTEM - Operating Manual for 20/30 Gate Mode: Geonics. Palacky, G.J., and F.O. Sena, 1979, Conductor identification in tropical terrains - Case histories from the Itapicuru greenstone belt, Bahia, Brazil: Geophysics, 44, no. 12, 1941-1962. McInerney, P.M., A.J. Mutton, and W.S. Peters, 1994, Copper-lead-zinc: Abra lead-silver-copper-gold deposit, Western Australia: a geophysical case history: Explorarion Geophysics, 25, no. 3, 164-164. Nabighian, M. (eds.), 1991, Electromagnetic Methods in Applied Geophysics: Volume 2, Application, Parts A and B: Investigations in Geophysics, SEG. Reid, J., D. Price, and E. Summerhayes, 2013, Geophysical case history of the Hollandaire copper deposit, Western Australia: 23rd Geophysical Conference, ASEG, Extended Abstracts, 1-4. Silva, A.T.M.C., M.G. Von Huelsen, U.J. Travaglia Filho, and R.A. Fuck, 2013, A comparison between 1 D electromagnetic modeling programs: a case history for Cristalino Iron-Oxide Copper Gold Deposit, Carajás Mineral Province, Brazil: 13th International Congress of the Brazilian Geophysical Society, SBGf, Extended Abstract, 110-115. Spies B.R., 1980, Interpretation and design of time domain EM surveys in areas of conductive overburden: Exploration Geophysics, 11, no. 4, 130-139. Vale S.A., 2012, Projeto Furnas, Relatório Final de Geologia: Internal report: Technical report. Wirth, K.R., A.K. Gibbs, and W.J. Olszewski, 1986, U-Pb ages of zircons from the Grão-Pará group and Serra dos Carajás Granite, Pará, Brazil: Revista Brasileira de Geociências, 16, no. 2, 195–200.
