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Use of radiogenic heat for demarcation of hydrothermal alteration zones in the Pernambuco-Brazil
Journal of Applied Geophysics 145 (2017) 111–123
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Use of radiogenic heat for demarcation of hydrothermal alteration zones in the Pernambuco-Brazil Leandro O. Cunha, Alanna C. Dutra ⁎, Alexandre B. Costa Universidade Federal da Bahia, Department of the Earth and Environment Physics, Institute of Physics, Campus Universitário de Ondina, Salvador, BA CEP: 40210-340, Brazil
a r t i c l e
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Article history: Received 6 April 2016 Received in revised form 14 August 2017 Accepted 16 August 2017 Available online 19 August 2017 Keywords: Radiogenic heat Gamma spectrometric Magnetic method Mineral prospecting
a b s t r a c t In this paper, we identify hydrothermal alteration zones that are located at the eastern extreme of Borborema Province, Pernambuco State, using radiogenic heat, in combination with magnetic and radioelement data collected both from the air and on the ground. The use of these methods enabled the mapping of geological lineaments based on total magnetic intensity maps, radioelement concentration maps, ternary maps and the F factor, as well as physical properties provided by ground data (thermal conductivity, density, and magnetic susceptibility). The data integration was based on low values of radiogenic heat and high K concentrations, as well as high F factors, amplitudes of the analytic signal, and K/eTh and eU/eTh values. These characteristics occur within the Pernambuco lineament and within other features to the southeast, a short distance south of the lineaments, which are made up of units including a migmatitic gneiss complex, as well as the Modern, Itaporanga and Sierra Passira Intrusive Suites. In these areas, elongated sources that are consistent with lineaments were identified. These sources were assigned depths of up to 3 km and are 5–10 km long, with the most significant extending SE-NW. These areas are favorable for the mineralization of iron, titanium and nickel. As the results are satisfactory, such areas can be studied in detail in the future. © 2017 Published by Elsevier B.V.
1. Introduction Geological mapping, combined with the interpretation of airborne gamma spectrometry and magnetic data, were carried out to evaluate the depth extent of potential shear zones to identify terrane boundaries, sutures and promising areas for mineral prospecting. Gamma spectrometry provides information on the minerals located at the surface. In the study of the radiometric data, the contents of the elements K, eU and eTh contributed most of the information. We highlight those related to different types of granites and metagranites, typically with values of gamma radiation above the regional background that likely indicate intermediate composition gneisses (Ferreira et al., 2014). Radiometric surveys and maps are applicable for mineral prospecting (Tourlière et al., 2003; Ribeiro and Mantovani, 2016), geochemical mapping and structural geology, and “enable the comparison of geological features over large regions” according to Minty, 1997. The contents of the elements K, eU and eTh are used to calculate the radiogenic heat production of rocks. Radiogenic heat generation within the lithosphere, created by the decay of K, eTh, and eU, accounts for an estimated 30–40% of heat loss through the continents. Accurate estimates of heat generation are necessary for computing lithospheric temperatures and heat flow across both the Moho and the lithosphere– asthenosphere boundary. ⁎ Corresponding author. E-mail addresses: [email protected] (A.C. Dutra), [email protected] (A.B. Costa).
http://dx.doi.org/10.1016/j.jappgeo.2017.08.004 0926-9851/© 2017 Published by Elsevier B.V.
Searching for hydrothermal alteration zones implies investigating areas with fractures in the crust that provide pathways for geothermal or hydrothermal fluids, which have temperatures between 50 °C and 500 °C. These hydrothermal fluids also change the characteristics of the rocks, which often result mainly in increases in K. Thus, the ratios of gamma-ray spectrometry data are used to identify rocks that have been subjected to these enrichment processes. The eTh/K ratio highlights the “antagonism” between these two elements; however, this ratio is not indicated to characterize granitic rocks, since this ratio is usually greater than 1. The parameter F = K × (eU / eTh) simultaneously highlights K abundance and the eU/eTh ratio, and it is widely used to distinguish areas of hydrothermal alteration from intensely weathered areas (Barbuena et al., 2013). Hydrothermal fluids generate new minerals, depending on their salinity and temperature, which can be investigated using magnetic data. The magnetic method is used for intermediate- and shallow-depth sources and is often used in preliminary geological reconnaissance (Araújo et al., 2014; Airo and Mertanen, 2008; Oskooi and Abedi, 2015). The enhancement techniques commonly applied to potential field data include the analytic signal amplitude (ASA) (Nabighian, 1972, 1974; Roest et al., 1992), total horizontal derivative (THDR) (Cordell and Gruch, 1985), Tilt Derivative (TDR) (Miller and Singh, 1994), Horizontal Derivative of the Tilt Derivative (HD-TDR) methods (Verduzco et al., 2004; Cooper and Cowan, 2006), as well as theta maps (Wijns et al., 2005). Nabighian (1972, 1974) applied the concept of analytic signal amplitude to the potential field data derived from two-dimensional models. The
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Fig. 1. Simplified geological map that shows the region examined in this study and identifies the main regional structures.
concept has been extended to 3D bodies by Roest et al. (1992), in order to estimate the depth of the magnetic sources. However, despite ASA is good for anomalies source positioning, it depends on the magnetic inclination and it is attenuated as source deepens (Li, 2006).
Geophysical exploration is quite relevant for recovering metals such as iron, nickel, titanium, copper and others. Given the great abundance and diversity of minerals found in Brazil, geophysical methods provide effective tools to localize mineral deposits (Barbosa et al., 2013). The
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Fig. 2. Concentration maps of the elements K, eU and eTh in the extreme eastern part of Borborema Province, Pernambuco State.
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airborne geophysical signatures of known deposits are compared with analogous signatures and available geological data. The goals of this study are to (1) provide useful mineral information for use in other studies by determining the signatures of host rocks within available magnetic and gamma-ray data and (2) locate target areas for field verification of potentially unmapped ore bodies. The airborne geophysical datasets provide the necessary input for a regional analysis. Furthermore, airborne magnetic and gamma-ray data are available for the extreme eastern portion of Borborema Province. As such, we believe that this study demonstrates a simple approach that can be used to map hydrothermal zones in areas of restricted access, limited outcrop, and thick alluvium, soil, or vegetation cover. The gamma spectrometry and magnetic measurements produced by airborne geophysical surveys carried out by the Mineral Resources Research Company CPRM (in Portuguese, “Companhia de Pesquisa de Recursos Minerais”) are extremely important for studies of mineral prospecting, as well as tectonic and geological mapping within the study area. In this paper, these geophysical data were successfully used to highlight the relationships between principal crustal blocks, as well as their internal structures and boundaries. Enhancement of gamma-ray spectrometry data showed their usefulness in mapping subtle compositional variations within the study area, which are in agreement with and complement available geological data. The most common technique for the identification of mineralization zones is based on locating and visually correlating the results of gamma-spectrometric methods and potential field anomalies. We propose a methodology that uses airborne geophysical measurements to determine the pattern of contribution of various elements to radiogenic heat production to identify hydrothermal alteration zones.
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Serra da Taquaritinga (which is made up of type-A granite) and Passira (which is a gabbro-anorthosite complex) are part of the study area. The supracrustal rocks of this terrane can be divided into two complexes. i) The Vertentes Complex is a metavolcanosedimentary/metaplutonic unit that is composed of paragneiss, mica, acidic and basic metavolcanic rocks, metadacites, metagraywackes, metavolcaniclastic rocks and granite-granodioritic orthogneiss. ii) The Surubim Complex is a metasedimentary unit that is made up of biotite paragneiss with quartzite and marble, and it may have been deposited on the Vertentes Complex during the Neoproterozoic (Gomes, 2001). These Complexes are often cut by several granitic intrusions that were emplaced before the Brazilian tectonic activity, such as the Serra dos Mascarenhas granite, the Caruaru and Arcoverde batholiths, granite-charnockite rocks from the Santa Cruz of Capibaribe, the Toritama syenite and tabular intrusive peralkaline metassyenites of Tamboatá (Barbosa, 1990; Accioly, 2000; Lima, 2011). The Vertentes Belt is composed of a gneissic-migmatitic complex that alternates with belts of supracrustal rocks of the Vertentes Complex. It is affected by a few shear zones with NNW transport and abundant transcurrent shear zones with NE-SW orientations. The contractional tectonic regime of the Vertentes Belt in the southern RCT is important for the evolution of this terrane, due to the strong influence of the transcurrent shear zones. These zones are subsidiaries of the Pernambuco lineament that mask the terrane's preexisting contractional tectonic features. The radiogenic data were used to classify lithologic types by studying different levels of gamma radiation. It should be noted that different lithologies can express the same range of gamma radiation values. Moreover, chemically differentiated portions can be found within the same lithology (Davis, 1986).
2. Geologic context 3. Methodology for correlating multiple digital maps The extreme eastern part of Borborema Province, which is located in the eastern portion of Pernambuco State, is divided into the Pernambuco, Alagoas and Rio Capibaribe tectonic domains, as shown in the geological map (Fig. 1). According to Santos et al. (2000, 2014), the Pernambuco-Alagoas lineament divides the area into two subprovinces, the Transversal Subprovince in the north and the Meridional Subprovince in the south. The Transversal Subprovince is controlled by two important regional structures that are characterized by NE-SW-trending shear zones. These structures are the Patos and Pernambuco lineaments (Santos and Medeiros, 1999; Santos et al., 2000). The Rio Capibaribe Terrain (RCT) is located in the eastern part of the Transversal Subprovince and is bordered by the Pernambuco-Alagoas lineament to the south (Santos and Medeiros, 1999; Medeiros, 2004). The Meridional Subprovince lies south of the Pernambuco-Alagoas lineament, and it is made up exclusively of the Pernambuco-Alagoas terrane (PEAL). This subprovince is located between the Pernambuco lineament and the N-NE margin of the Congo Craton. It is divided into four sub-domains, which include the Rio Preto, Riacho do Pontal, and Sergipe mobile belts and the Pernambuco-Alagoas terrane. The PEAL is made up of gneissic-migmatitic Paleoproterozoic basement (Silva Filho et al., 1997; Accioly et al., 2010), a supracrustal belt and the metaplutonic Cabrobó and Belém do São Francisco Complexes (Santos, 1995; Brito Neves et al., 2000, Brito Neves, 2011). This set of amphibolites facies rocks are intruded by granitic plutons of various batholitic dimensions. Some restricted Archean cores are known from the western part of the terrane. The predominance of transpressional tectonic deformation is the most important characteristic of this area, as shown by Brito Neves et al. (1995) and Santos (1996). Paleoproterozoic units, such as the Pão de Açúcar and Salgadinho Complexes, and anorogenic Mesoproterozoic intrusions, such as that of
For interpreting radiometric data, the contents of the elements K, eU and eTh contributed to the identification of some geological units with features related to low levels of gamma radiation (Figs. 2 and 4). According to Rybach (1998), the radiogenic heat production rates of granites can be calculated using the following equation: A ¼ 10−5 ρ ½2:56 ThðppmÞ þ 9:52 UðppmÞ þ 3:48 Kð%Þ; ð1Þ where A (μW ∗ m−3) is radiogenic heat production and ρ (kg ∗ m−3)is the density. Fresh rock samples were collected from Pernambuco State, and the concentrations of K, eU and eTh within the samples were determined in the laboratory using gamma spectroscopy. The results show that the contributions to heat production of K, eU and eTh in the samples, as well as the rate of heat production, vary significantly with geological location. Potassium is a major element that contributes to heat production, whereas eU and eTh are trace elements (Figs. 2, 3 and 4). Geophysical grids were prepared with cell dimensions compatible with the scale of the work, and were to be combined with geological mapping. The geophysical maps were generated based on the concentrations of selected radio-elements (K, eTh and eU), their ratios (eU/K, eU/eTh, K/eTh), F factor values, measured ground heat production rates and aeromagnetic data (Figs. 3, 4 and 5). The airborne data used in this study was obtained from two aircraft at a constant ground clearance. The airborne data were acquired using flight lines spaced 500 m apart, with orthogonal tie lines flown every 10,000 m at a height of 100 m above the ground surface. The survey was carried out in two different blocks with different flight- and tie-line directions, and data acquisition was performed perpendicular to the main structures of the surveyed area (CPRM, 2008). The magnetic
Fig. 3. Maps showing: (a) K/eTh ratio; b) eU/eTh ratio and c) F factor values calculated from airborne and ground-based geophysical data in the extreme eastern part of Borborema Province, Pernambuco State.
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Fig. 4. Radiogenic heat: (a) calculated from airborne and (b) calculated from ground-based geophysical data. (c) The ternary map shows the abundances of the elements K, eU and eTh.
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Fig. 5. The radio-element (K, eU, eTh) concentrations of samples collected from various geological units, specifically the Belém do São Francisco Complex (PPbf); the Cabrobó Complex (NPcb); the gneissic-migmatitic complex (Ppgm); the Vertentes Complex (NP1ve); the Serra do Mascarenhas Intrusive Suite (NP3ysm); the Itaporanga Intrusive Suite (NP3yit); the Ouro Branco Intrusive Suite (NP3yo); the Passira Intrusive Suite (PP4ᵟp); and the Serra da Passira Intrusive Suite (PP4ysp).
system used was an optically pumped (cesium vapor) magnetometer that was installed in a stinger extension behind the tail of the aircraft. The output from the magnetometer was sampled at 0.1 s to a resolution of 0.001 nT with a noise envelope less than 0.01 nT. The spectrometer employed 256 spectral channels and consisted of two downwardlooking groups of crystals (thallium-doped NaI) with detector volumes of 1024 in.3 each, for a total of 2048 in.3 of detector volume, and two upward-looking crystals of 256 in. 3 each, for a total of 512 in. 3 (CPRM, 2008). In the next processing step, the data were interpolated to a regular grid, using algorithms that maintain data fidelity at the original measurement locations. This step was followed by correction of spurious effects caused by the leveling of the original grids. The fourthorder difference technique was used to track anomalous spikes in the magnetic data and to condition sampling along the flight lines based on the spatial Nyquist frequency. This was performed on the selected interpolated grid, which contained square cells 125 × 125 m. The algorithm was based on linear interpolation along the direction of the flight lines, and on the Akima spline perpendicular to the flight lines. Microleveling and decorrugation techniques were further applied to
the data. This procedure resulted in several geophysical data products, including thematic maps of both individual variables and composite variables, for use in geologic analysis and interpretation. We propose a method that is based upon the optimal correlation between pairs of quantities from the radiometric data within the data set, including the heat production ratio; F factor values; the contents of the elements K, eU and eTh; and the eU/K, eU/eTh, and K/eTh ratios. The cross-correlation coefficients (C) can be evaluated according to the following formula: P xj − x yj − y C ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi ; 2 P P yj − y xj − x
ð2Þ
where j is the index of each quantity within the data set, and x and y are the mean of each quantity, respectively. We indicate the selected areas on the maps of physical properties. The radiometric methods and radiogenic heat were integrated with the magnetic data to identify deep structures (Luyendyk, 1997).
Fig. 6. Radiogenic heat production and F factor values of samples collected from various geological units, specifically the Belém do São Francisco Complex (PPbf); the Cabrobó Complex (NPcb); the gneissic-migmatitic complex (Ppgm); the Vertentes Complex (NP1ve); the Serra do Mascarenhas Intrusive Suite (NP3ysm); the Itaporanga Intrusive Suite (NP3yit); the Ouro Branco Intrusive Suite (NP3yo); the Passira Intrusive Suite (PP4ᵟp); and the Serra da Passira Intrusive Suite (PP4ysp).
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Fig. 7. Cross-correlation graph between all variables assessed from the airborne and ground-based data, specifically the contents of thorium (Th), uranium (U), and potassium (K), as well as radiogenic heat (A), F factor (F) values, and the K/Th and U/Th ratios.
Quantitative magnetic data analysis was performed in the frequency domain using Fast Fourier Transform (FFT) filters. The Analytical Signal Amplitude (ASA) and Tilt Derivative of the calculated total magnetic field anomaly data show the contacts between the sources and their horizontal geometry. To determine the analytical signal and phase filter, we used the concepts described in Roest et al. (1992) and Thurston (2001), respectively. The resulting shape of the ASA is expected to be centered above the magnetic body. This has the effect of transforming the shape of the magnetic anomaly from any magnetic inclination to one positive body-centered anomaly at least in 2D (Nabighian, 1972). Analytic signal has been utilized widely for mapping of structures and for determining the depth of sources (Roest et al., 1992; Debeglia and Corpel, 1997). We applied the 3D An-Euler method (Salem and Ravat, 2003; Salem and Williams, 2007) using the ASA to obtain depths and structural index values. The “AN-EUL” method, which is based on the combination of the Euler deconvolution method with the analytical signal, allows the estimation of the depths and structural index values of anomalous sources. Assuming that each magnetic anomaly is produced by an isolated body, such as a vertical cylinder, the horizontal diameters of the sources can be estimated as varying from X to Y (Salem and Ravat, 2003). The horizontal derivatives emphasize abrupt lateral changes in physical properties that appear where the edges of the magnetic sources are located. Within the study area, magnetic signals can be identified as indicative of faults/fractures, tectonic effects, lithologies rich in ferromagnesian minerals, features related to igneous rocks of acidic to intermediate composition (granitic and/or volcanic), structural lineaments and possibly geotectonically distinct regions. Some magnetic domains could be distinguished in the map showing the total magnetic intensity. 4. Application to the identification of hydrothermal zones We interpret the data based on heat production rates; F factor values; eU, K and eTh concentrations; K/eTh and eU/K ratios; and analytic signal amplitudes because of the elongated structures and Tilt Derivative, which will show geological contact zones if the contacts are vertical.
Based on high concentrations of K and the K/eTh ratios, because eTh is a radioelement with low mobility, we can better define the zones that have experienced hydrothermal alteration. The radiometric data indicating the contents of the elements K, eU and eTh, the amount of radiogenic heat, and the F factor are shown in Figs. 5 and 6. Interpolated values of thermal conductivity, density and susceptibility generated from measurements made using 409 samples are shown in Fig. 8. Analyzing the data from the all of the ground samples, we noted that the radiogenic heat is most highly correlated with thorium (C = 0.94), followed by uranium (C = 0.77), potassium (C = 0.45) and density (C = 0.26), which indicates that the element thorium has the greatest influence on this parameter, as shown in Fig. 7. The F factor shows a mean correlation with the contents of K and eU. The eU/eTh ratio is common in studies that use gamma spectrometry, but the K/eTh ratio is not used. In this paper, we propose a study of this ratio in airborne and ground-based data. The eU/eTh ratio displayed a better correlation with the F factor in the ground-based data (C = 0.93). The K/eTh ratio yielded a better correlation with the F factor in the ground-based and aeromagnetic data, 0.42 and 0.75, respectively. The integration of information generated by the interpretation of the magnetic and gamma-spectrometric data allowed the identification of several litho-structural domains. Each block was analyzed and compared with the available overall geological data, which are obtained at smaller scales than that of the airborne survey. This implies that the information derived from the geophysical data contains much more detail than the existing maps. We can carry out interpretations based on the F factor, K contents, K/eTh and eU/K ratios, density, magnetic susceptibility, analytic signal amplitudes and tilt angles (Figs. 3, 8 and 9). The high values of heat production rate occur along the borders of elongated structures with high values of the F factor and K/eTh ratios and moderate eU/eTh values. In the density and magnetic susceptibility maps (Fig. 8), the areas in question are associated with high relative magnetic susceptibilities and high densities. We observe high concentrations of K, which are due to the strong presence of feldspar granite and other associated lithologies with high heat production rates that make up the Itaporanga and Moderna Intrusive Suites, as well as the Serra da Passira and Ouro Branco
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Fig. 8. The interpolated values of the a) thermal conductivity, b) density and c) susceptibility obtained from measurements of the rock samples.
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Complexes, which are found in the southeastern portion of the map of K contents. The highest average heat production rate values occur within Neoproterozoic magmatic suites, next to areas with high potential for mineral exploration. The high K concentrations and low eTh values are most notable close to the Pernambuco lineament, near the Modern Intrusive Suite. These features occur in the units corresponding to the Itaporanga Intrusive Suite and the gneissic-migmatitic complex, as well as some belts to the north that are mainly in the Serra do Mascarenhas unit and follow lineaments that are oriented NE-SW. The high values of K are due to associations with granodiorite and tonalite, both of which contain large amounts of plagioclase, and the orthogneisses of the Serra de Passira. The migmatitic complexes, such as the granite and biotite-granite of the Itaporanga and Serra do Mascarenhas felsic units, have high K concentrations. These large areas with low concentrations of thorium represent lineaments that cross the Modern and Itaporanga Intrusive Suites, such as the gneissic migmatites that occur in some areas. The low concentrations of uranium occur to the north. The high values of the K/eTh ratio occur due to the fact that eTh is less mobile than K. There is a greater interest in the low values that are concentrated to the west of the Pernambuco lineament in the Serra do Mascarenhas Unit, the gneissic-migmatitic complex and the Buenos Aires Unit. We use eU/eTh ratios to study the degree of lithologic differentiation. We prioritize the low values found exclusively in the northwestern Pernambuco lineament near the Modern Suite and in the western Itaporanga. The values of the F factor are in with ENE-WSW-trending lineaments in the northeastern portion of the map (Fig. 3). We observed high intensities for the entire Modern Suite and portions of the gneissicmigmatitic complex and the Itaporanga Complex. We selected felsic units that occur in an isolated area south of the Pernambuco lineament with high values of the F factor. The best interpretations were achieved from qualitative analysis of the ternary radioelement map (Fig. 2), as well as the analytic signal amplitude and Tilt Derivative map (Fig. 9b). These maps highlight the key crustal blocks and their unique inter-relationships. We recognized several blocks, which reflect distinct periods of crustal evolution and are described below. a) Structure 1 (S1) contains several geological units, including the Moderna Intrusive Suite, which is composed of syenogranites, syenite quartz and syenite with Fe-hastingsite; the Serra da Passira Intrusive Suite, which is composed of garnet-amphibolite-biotite orthogneiss and granite dominated by alkali feldspar; and the Itaporanga Intrusive Suite, which is composed of monzogranites, syenogranites and porphyritic granodiorites, which range from metaluminous with calcium alkalis and large amounts of K to mildly shoshonitic. It also displays a moderate quantity of dioritic enclaves. The Moderna Intrusive Suite displays high abundances of K and low abundances of eTh and eU. Its K/eTh ratios are high and peak at 0.201 in the center; its eU/eTh ratios show considerable variability; its F factor values are high, with values of 0.228 to 0.316; its heat production rates are high along its eastern border, with values of 3 to approximately 4 μW/m3, and moderate to the west; it displays a substantial magnetic anomaly and analytical signal, yet it contains consistent, SW-NE-trending, geometrical bodies. It may have zones of mineralization occurring along its borders, mainly in the east. Given its lithology, its density and magnetic susceptibility values are high to very high. b) Structure 2 (S2) is represented by the gneissic-migmatitic complex geological unit, which is composed of banded orthogneisses having granitic and granodioritic compositions, as well as tonalitic, monzonitic, monzodioritic and dioritic compositions. These rocks are intercalated with metamafic rocks and amphibolites and include migmatitic structures, such as augen and schlieren. It has been shown that this region presents medium to high quantities of K, low to medium
quantities of eTh and eU, low heat production rates that range from 0.7 to 1.6 μW/m3, and high magnetic anomalies and analytical signal values in Fig. 2. These lithologies primarily represent basement rocks with medium to high K/eTh ratios. High U/eTh values are observed to the west and medium to low values of this quantity are seen in the eastern portion, and primarily high F factors plus moderate values occur in the southeastern portion. In this area, density and magnetic susceptibility are low, caused by the geological diversity. c) Structure 3 (S3) contains the Serra da Passira Intrusive Suite geological unit, which is composed of grenade-anphybolium-biotite orthogneisses. Granite dominated by alkali feldspar occurs in the main portion of the Itaporanga Intrusive Suite, which also contains monzogranites, syenogranites and porphyritic granodiorites, which range from metaluminous with calcium alkalis and large amounts of K to mildly shoshonitic. The data indicate high heat production rates along the borders of this structure and low to medium rates in its center. Moreover, large abundances of K occur in the western portion with a peak value of 2.16%, because felsic rocks are the dominant lithology. Moderate concentrations of U occur in the center, whereas large amounts occur along the borders, and moderate concentrations of Th are also observed. High K/eTh ratios occur in almost all of the structure, as well as high eU/eTh ratios and high F factor values because of the presence of felsic rocks. This area has high density and magnetic susceptibility. d) Structure 4 (S4) is represented by several geological units, with the dominant one being the gneissic-migmatitic complex, which is composed of banded orthogneisses having granitic and granodioritic compositions, as well as tonalitic, monzonitic, monzodioritic and dioritic compositions, next to the Pernambuco lineament. The Itaporanga Intrusive Suite is composed of monzogranites, syenogranites and porphyritic granodiorites, which range from metaluminous with calcium alkalis and large amounts of K to mildly shoshonitic. It also displays a moderate quantity of dioritic enclaves. The data show high K concentrations alongside the structure, with values of 1.1 and 2%, as well as medium to low concentrations of eTh and eU. Because of the great quantities of granites and derivative rocks, along with diorites, this structure is associated with low heat production rates in its center but high rates along its borders; high values of magnetic anomaly and analytical signal that correspond to the geometry of the elongated structure; high K/eTh ratios, because of the dominance of rocks with felsic compositions and low eU/eTh ratios; and high F factor values, with low values being located in the center. This may occur because of the presence of some metamafic rocks within the gneissic-migmatitic complex. In this structure, because of the bordering effects, the density is medium to high. The magnetic susceptibility is also high, up to 22 SI. In selected areas, a method called AN-EUL was applied to determine values of the structural index and the source depth at each location point, which can be of great value in the interpretation of the data. Aeromagnetic products contribute to the understanding of structural detail in the northeastern portion of Borborema Province. Using these data, it is possible to better define the boundaries between crustal blocks and/or tectono-stratigraphic terranes, estimating depths of regional features through Euler deconvolution and defining the nature of the NW-SE trend. These results provide new arguments for use in interpretations and discussions of the geodynamic evolution of this region. The AN-EUL method is based on a combination of the analytic signal and the Euler deconvolution methods. Both the location and the approximate geometry of a magnetic source can be deduced. The method is tested using theoretical simulations with different magnetic models placed at different depths with respect to the observation height. In all cases, the method estimated the locations and the approximate geometries of the sources. The depth estimates resulting from the AN-Euler method vary from 1 up to 3 km. Elongated bodies are buried deeply in the western portion. The depths range from 700 m to 1 km, which corresponds to the
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Fig. 9. The aeromagnetic data, represented in terms of a) magnetic anomalies, b) ASA + phase values, and c) radiogenic heat + depth solutions obtained using the AN-Euler method.
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Moderna Intrusive Suite and some occurrences in the northeastern portion, with their borders following the geological contacts. Some of these occurrences show elongated bodies to the southwest that are approximately 10 km in width and have depths that vary between 700 and 3000 m. A deep anomaly located between those two structures may represent the continuation horizontally displaced of one of them or a third non-outcropping but interconnected shear zone. The favored interpretation considers this sector of the Transversal Zone as formed by deeprooted, northeast-southwest trending shear zones, cut by shallower structures, with east-west trends. In this context, the importance and continuity at depth of the shear zone is highlighted, and as such inferred to be the suture between terrenes, in accordance with field geological data. In this paper, we will stake out areas favorable to mineralization of iron, titanium and nickel, located at the eastern end of Borborema Province, Pernambuco State.
5. Discussion and conclusions The interpretation of the aeromagnetic products was correlated with geological structures having NE-SW trends that are known in the region, the concentrations of radionuclides (eU, eTh, and K) and radiogenic heat production. Magnetic lineaments with N-S and NW-SE orientations are well defined in the interpretations, and these lineaments characterize structures that are not yet well known in this region. The magnetic anomalies associated with the different tectonostratigraphic blocks indicate the continuation at depth to the southsouthwest of the Archean nucleus. Also, different magnetic signatures were noted for the Paleoproterozoic rocks, as well as a low magnetic response. The granitic intrusions that occurred prior to Brasiliano are weakly magnetic, whereas the post Brasiliano intrusions are highly magnetic. Radioactive minerals are present in all the rock samples collected. These radionuclides (eU, eTh, and K) make different contributions to the radiogenic heat produced in rocks as a result of their geological location. Analyzing the data obtained from all of the ground-based samples, we noticed that the radiogenic heat is most strongly correlated with eTh, followed by eU and K, showing that the element Th has a substantial influence on this parameter. The F factor shows a mean correlation with potassium and uranium. The integrated data enable the identification of zones with mineralization, based on radiogenic heat production, F factor values, K concentrations, K/eTh and eU/K ratios, density, magnetic susceptibility, and analytic signal amplitude, because of the elongated structures, and the tilt angle data show geological vertical contact zones. Aeromagnetic data contribute to our detailed structural knowledge of the eastern border of Borborema Province. It is possible to better define the boundaries between crustal blocks and stratigraphic terranes by estimating the depths of regional features through An-Euler deconvolution and defining the nature of the NW-SE trend. We highlight those features that are related to different types of granites and metagranitic rocks, which typically show values of gamma radiation above the regional background, as do rocks of intermediate composition, such as gneisses. We note that the high concentrations of potassium are due to the presence of large amounts of feldspar granite and other associated lithologies with high heat production rates. With the geological map and large-scale distributions of various rock types, the selected areas are the Modern Intrusive Suite, which is composed mainly of syenites with Fe-hastingsite and other adjacent areas, such as Passira, the undivided gneissic-migmatitic complex and Itaporanga, which follow the E-W-trending Pernambuco lineament. There is also an occurrence in the southeastern portion south of the Pernambuco lineament, which correspond to the Belém do São
Francisco, Ouro Branco and Itaporanga units, with Itaporanga being the dominant area. The structures associated with the hydrothermal zones (S2, S3 and S4) contain large quantities of K and low quantities of eTh and eU. They are also associated with high K/eTh ratios with a peak value in the center, widely varying eU/eTh ratios, high F factor values and high heat production rate along the borders, as well as high magnetic anomaly and analytical signal. However, they form consistent geometrical bodies. The potential mineralization zones trend SW-NE, and for some lithologic types, the density and magnetic susceptibility values are very high. The use of the methods described in this study allowed the improvement of geological models for a defined area. They give us excellent estimates of potential mineralization zones occurring along the eastern border of Borborema Province. The depths identified at each point could be used to generate a 3D model that can then be correlated with geological features. References Accioly, A.C.A., 2000. Geologia, Geoquímica e significado Tectônico do Complexo Metanortosítico de Passira-Província Borborema-Nordeste Brasileiro. (168 p). Tese Doutorado - Programa de Pós-graduação em Geoquímica e Geotectônica, USP, São Paulo. Accioly, A.C. de A., Santos, C.A. dos, Santos, E.J. dos, Brito Neves, B.B. de, Rodrigues, J.B., McReath, I., 2010. Geochronology and geochemistry of the meta-volcanic rocks from Riacho do Tigre complex, Borborema province - northeastern Brazil. Proceedings (Brasilia). Airo, M.-L., Mertanen, S., 2008. Magnetic signatures related to orogenic gold mineralization, Central Lapland Greenstone Belt, Finland. J. Appl. Geophys. 64 (1), 14–24. Araújo, I.L., Moraes, R.A.V., Dantas, E.L., Vidotti, R.M., Fuck, R.A., 2014. 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Brito Neves, B.B., Van Schmus, W.R., Santos, E.J., Campos Neto, M.C., Kozuch, M., 1995. O Evento Cariris Velhos na Província Borborema: Integração de dados, implicações e perspectivas. Rev. Bras. Geociênc. 25, 279–296. Brito Neves, B.B., Santos, E.J., Schmus, W.R.V., 2000. Tectonic history of the Borborema Province. In: Cordani, U., Milani, E.J., Thomaz Filho, A., Campos, D.A. (Eds.), Tectonic Evolution of South America. Publisher: SBS, pp. 151–182. Cooper, G.R.J., Cowan, D.R., 2006. Enhancing potential field data using filters based on the local phase. Comput. Geosci. 32 (10), 1585–1591. Cordell, L., Gruch, V.J.S., 1985. Mapping basement magnetization zones from aeromagnetic data in the San Juan Basin, New Mexico. The Utility of Regional Gravity and Magnetic Anomalies Maps. Society of Exploration Geophysicists, pp. 181–197. CPRM, 2008. Projeto Aerogeofísico Borda Leste do Planalto da Borborema, CPRM (Programa Geologia do Brasil). Davis, J., 1986. Statistics and Data Analysis in Geology. John Wiley & Sons (656 p). Debeglia, N., Corpel, J., 1997. Automatic 3-D interpretation of potential field data using analytic signal derivatives. Geophysics 62, 87–96. Ferreira, F.J.F., Fornazzari-Neto, L., Szameitat, L.S.A., Guimarães, G.B., Martin, V.M.O., Prazeres-Filho, H., Ulbrich, Horstpeter H., 2014. Gamma-ray spectrometry as a tool for mapping petrographic domains in granitoids: the examples of the cunhaporanga and três córregos granitic complexes, Paraná state, southern Brazil. Rev. Bras. Geofís. 32 (3), 465–479. Gomes, H.A., 2001. (Org.). Geologia e recursos minerais do Estado de Pernambuco, Texto Explicativo. Brasília, CPRM, 1 mapa, col., 1 CD-ROM. Li, X., 2006. Understanding 3-D analytic signal amplitude. Geophysics 71 (2), L13–L16. Lima, H.M., 2011. Mapeamento geológico e análise tectonoestratigráfica da parte central da faixa Feira Nova, Terreno Rio Capibaribe, Província Borborema. (99 p). Monografia (Graduação) Curso de Geologia, Departamento de Geologia, UFPE, Recife. Luyendyk, A., 1997. Processing of airborne magnetic data. J. Aust. Geol. Geophys. 17 (2), 31–38. Medeiros, V.C., 2004. Evolução geodinâmica e condicionamento estrutural dos terrenos Piancó - Alto Brígida e Alto Pajeú, domínio da Zona Transversal, NE do Brasil, 199 f. Tese (Doutorado) - Centro de Ciências Exatas e da Natureza, Universidade Federal do Rio Grande do Norte, Natal. Miller, H.G., Singh, V., 1994. Potential field tilt – a new concept for location of potential field sources. J. Appl. Geophys. 32 (2–3), 213–217.
L.O. Cunha et al. / Journal of Applied Geophysics 145 (2017) 111–123 Minty, B.R.S., 1997. The fundamentals of airborne gamma-ray spectrometry. AGSO J. Aust. Geol. Geophys. 17 (2), 39–50. Nabighian, M.N., 1972. The analytic signal of two-dimensional magnetic bodies with polygonal cross-section: its properties and use for automated anomaly interpretation. Geophysics 37 (3), 507–517. Nabighian, M.N., 1974. Additional comments on the analytic signal of two-dimensional magnetic bodies with polygonal cross-section. Geophysics 39 (1), 85–92. Oskooi, B., Abedi, M., 2015. An airborne magnetometry study across Zagros collision zone along Ahvaz–Isfahan route in Iran. J. Appl. Geophys. 123, 112–122. Ribeiro, V.B., Mantovani, M.S.M., 2016. Gamma spectrometric and magnetic interpretation of Cabaçal copper deposit in Mato Grosso (Brazil): implications for hydrothermal fluids remobilization. J. Appl. Geophys. 135, 223–231. Roest, W.R., Verhoef, V., Pilkington, M., 1992. Magnetic interpretation using the 3-D analytic signal. Geophysics 57, 116–125. Rybach, L., 1998. Determination of heat production rate. In: Hänel, R., Rybach, L., Stegena, L. (Eds.), Handbook of Ground Heat Flow Density Determination. Kluwer, Dordrecht, pp. 125–142. Salem, A., Ravat, D., 2003. A combined analytic signal and Euler method (ANEUL) for automatic interpretation of magnetic data. Geophysics 68 (6), 1952–1961. Salem, A.B., Williams, S.G.C., 2007. Mapping magnetic basement from aeromagnetic data using AN-EUL method. EGM 2007 International Workshop Innovation in EM, Grav and Mag Methods: A new Perspective for Exploration. Santos, E.J., 1995. O complexo granítico Lagoa das Pedras: Acresção e colisão na região de Floresta (Pernambuco), Província Borborema. (PhD thesis). Instituto de Geociências da Universidade de São Paulo, São Paulo, p. 228. Santos, E.J., 1996. Ensaio preliminar sobre terrenos e tectônica acrecionária na Província Borborema. SBG, Congresso Brasileiro de Geologia, 39, Salvador, Proceedings, pp. 47–50.
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Santos, E.J., Medeiros, V.C., 1999. Constraints from granitic plutonism on proterozoic crustal growth of the Transverse Zone, Borborema Province, NE-Brazil. Rev. Bras. Geociênc. 29, 73–84. Santos, E.J., Brito Neves, B.B., Van Schmus, W.R., Oliveira, R.G., Medeiros, V.C., 2000. An overall view on the displaced terrane arrangement of the Borborema Province, NE Brazil. International Geological Congress, 31th, Rio de Janeiro, Brazil, General Symposia, Tectonic Evolution of South American Platform, pp. 5–9. Santos, et al., 2014. Metalogênese das porções norte e central da Província Borborema, no livro “Metalogênese das Províncias Tectônicas Brasileiras. Organizadores: Maria da Glória da Silva, Manoel Barreto da Rocha Neto, Hardy Jost, Raul Minas Kuyumijan. Editado pela CPRM, pp. 343–388. Silva Filho, A.F., Guimaraes, I.P., Brito, M.F.L., Pimentel, M.M., 1997. Geochemical signatures of main Neoproterozoic Late-Tectonic granitoids from the Sergipano Fold Belt, Brazil: significance for the Brasiliano orogeny. Int. Geol. Rev. Estados Unidos 39 (07), 639–659. Thurston, J.B., 2001. Mapping remanent magnetization using the local phase. Geophysics 66, 1082–1089. Tourlière, B., Perrin, J., Le Berre, P., Pasquet, F., J., 2003. Use of airborne gamma-ray spectrometry for kaolin exploration. J. Appl. Geophys. 53 (2), 91–102. Verduzco, B., Fairhead, J.D., Grenn, C.M., Mackenzie, C., 2004. New insights into magnetic derivatives for structural mapping. Lead. Edge 23 (2), 116–119. Wijns, C., Perez, C., Kowalczyk, P., 2005. Theta map: edge detection in magnetic data. Geophysics 70 (4), L39–L43.
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Use of radiogenic heat for demarcation of hydrothermal alteration zones in the Pernambuco-Brazil Leandro O. Cunha, Alanna C. Dutra ⁎, Alexandre B. Costa Universidade Federal da Bahia, Department of the Earth and Environment Physics, Institute of Physics, Campus Universitário de Ondina, Salvador, BA CEP: 40210-340, Brazil
a r t i c l e
i n f o
Article history: Received 6 April 2016 Received in revised form 14 August 2017 Accepted 16 August 2017 Available online 19 August 2017 Keywords: Radiogenic heat Gamma spectrometric Magnetic method Mineral prospecting
a b s t r a c t In this paper, we identify hydrothermal alteration zones that are located at the eastern extreme of Borborema Province, Pernambuco State, using radiogenic heat, in combination with magnetic and radioelement data collected both from the air and on the ground. The use of these methods enabled the mapping of geological lineaments based on total magnetic intensity maps, radioelement concentration maps, ternary maps and the F factor, as well as physical properties provided by ground data (thermal conductivity, density, and magnetic susceptibility). The data integration was based on low values of radiogenic heat and high K concentrations, as well as high F factors, amplitudes of the analytic signal, and K/eTh and eU/eTh values. These characteristics occur within the Pernambuco lineament and within other features to the southeast, a short distance south of the lineaments, which are made up of units including a migmatitic gneiss complex, as well as the Modern, Itaporanga and Sierra Passira Intrusive Suites. In these areas, elongated sources that are consistent with lineaments were identified. These sources were assigned depths of up to 3 km and are 5–10 km long, with the most significant extending SE-NW. These areas are favorable for the mineralization of iron, titanium and nickel. As the results are satisfactory, such areas can be studied in detail in the future. © 2017 Published by Elsevier B.V.
1. Introduction Geological mapping, combined with the interpretation of airborne gamma spectrometry and magnetic data, were carried out to evaluate the depth extent of potential shear zones to identify terrane boundaries, sutures and promising areas for mineral prospecting. Gamma spectrometry provides information on the minerals located at the surface. In the study of the radiometric data, the contents of the elements K, eU and eTh contributed most of the information. We highlight those related to different types of granites and metagranites, typically with values of gamma radiation above the regional background that likely indicate intermediate composition gneisses (Ferreira et al., 2014). Radiometric surveys and maps are applicable for mineral prospecting (Tourlière et al., 2003; Ribeiro and Mantovani, 2016), geochemical mapping and structural geology, and “enable the comparison of geological features over large regions” according to Minty, 1997. The contents of the elements K, eU and eTh are used to calculate the radiogenic heat production of rocks. Radiogenic heat generation within the lithosphere, created by the decay of K, eTh, and eU, accounts for an estimated 30–40% of heat loss through the continents. Accurate estimates of heat generation are necessary for computing lithospheric temperatures and heat flow across both the Moho and the lithosphere– asthenosphere boundary. ⁎ Corresponding author. E-mail addresses: [email protected] (A.C. Dutra), [email protected] (A.B. Costa).
http://dx.doi.org/10.1016/j.jappgeo.2017.08.004 0926-9851/© 2017 Published by Elsevier B.V.
Searching for hydrothermal alteration zones implies investigating areas with fractures in the crust that provide pathways for geothermal or hydrothermal fluids, which have temperatures between 50 °C and 500 °C. These hydrothermal fluids also change the characteristics of the rocks, which often result mainly in increases in K. Thus, the ratios of gamma-ray spectrometry data are used to identify rocks that have been subjected to these enrichment processes. The eTh/K ratio highlights the “antagonism” between these two elements; however, this ratio is not indicated to characterize granitic rocks, since this ratio is usually greater than 1. The parameter F = K × (eU / eTh) simultaneously highlights K abundance and the eU/eTh ratio, and it is widely used to distinguish areas of hydrothermal alteration from intensely weathered areas (Barbuena et al., 2013). Hydrothermal fluids generate new minerals, depending on their salinity and temperature, which can be investigated using magnetic data. The magnetic method is used for intermediate- and shallow-depth sources and is often used in preliminary geological reconnaissance (Araújo et al., 2014; Airo and Mertanen, 2008; Oskooi and Abedi, 2015). The enhancement techniques commonly applied to potential field data include the analytic signal amplitude (ASA) (Nabighian, 1972, 1974; Roest et al., 1992), total horizontal derivative (THDR) (Cordell and Gruch, 1985), Tilt Derivative (TDR) (Miller and Singh, 1994), Horizontal Derivative of the Tilt Derivative (HD-TDR) methods (Verduzco et al., 2004; Cooper and Cowan, 2006), as well as theta maps (Wijns et al., 2005). Nabighian (1972, 1974) applied the concept of analytic signal amplitude to the potential field data derived from two-dimensional models. The
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Fig. 1. Simplified geological map that shows the region examined in this study and identifies the main regional structures.
concept has been extended to 3D bodies by Roest et al. (1992), in order to estimate the depth of the magnetic sources. However, despite ASA is good for anomalies source positioning, it depends on the magnetic inclination and it is attenuated as source deepens (Li, 2006).
Geophysical exploration is quite relevant for recovering metals such as iron, nickel, titanium, copper and others. Given the great abundance and diversity of minerals found in Brazil, geophysical methods provide effective tools to localize mineral deposits (Barbosa et al., 2013). The
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Fig. 2. Concentration maps of the elements K, eU and eTh in the extreme eastern part of Borborema Province, Pernambuco State.
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airborne geophysical signatures of known deposits are compared with analogous signatures and available geological data. The goals of this study are to (1) provide useful mineral information for use in other studies by determining the signatures of host rocks within available magnetic and gamma-ray data and (2) locate target areas for field verification of potentially unmapped ore bodies. The airborne geophysical datasets provide the necessary input for a regional analysis. Furthermore, airborne magnetic and gamma-ray data are available for the extreme eastern portion of Borborema Province. As such, we believe that this study demonstrates a simple approach that can be used to map hydrothermal zones in areas of restricted access, limited outcrop, and thick alluvium, soil, or vegetation cover. The gamma spectrometry and magnetic measurements produced by airborne geophysical surveys carried out by the Mineral Resources Research Company CPRM (in Portuguese, “Companhia de Pesquisa de Recursos Minerais”) are extremely important for studies of mineral prospecting, as well as tectonic and geological mapping within the study area. In this paper, these geophysical data were successfully used to highlight the relationships between principal crustal blocks, as well as their internal structures and boundaries. Enhancement of gamma-ray spectrometry data showed their usefulness in mapping subtle compositional variations within the study area, which are in agreement with and complement available geological data. The most common technique for the identification of mineralization zones is based on locating and visually correlating the results of gamma-spectrometric methods and potential field anomalies. We propose a methodology that uses airborne geophysical measurements to determine the pattern of contribution of various elements to radiogenic heat production to identify hydrothermal alteration zones.
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Serra da Taquaritinga (which is made up of type-A granite) and Passira (which is a gabbro-anorthosite complex) are part of the study area. The supracrustal rocks of this terrane can be divided into two complexes. i) The Vertentes Complex is a metavolcanosedimentary/metaplutonic unit that is composed of paragneiss, mica, acidic and basic metavolcanic rocks, metadacites, metagraywackes, metavolcaniclastic rocks and granite-granodioritic orthogneiss. ii) The Surubim Complex is a metasedimentary unit that is made up of biotite paragneiss with quartzite and marble, and it may have been deposited on the Vertentes Complex during the Neoproterozoic (Gomes, 2001). These Complexes are often cut by several granitic intrusions that were emplaced before the Brazilian tectonic activity, such as the Serra dos Mascarenhas granite, the Caruaru and Arcoverde batholiths, granite-charnockite rocks from the Santa Cruz of Capibaribe, the Toritama syenite and tabular intrusive peralkaline metassyenites of Tamboatá (Barbosa, 1990; Accioly, 2000; Lima, 2011). The Vertentes Belt is composed of a gneissic-migmatitic complex that alternates with belts of supracrustal rocks of the Vertentes Complex. It is affected by a few shear zones with NNW transport and abundant transcurrent shear zones with NE-SW orientations. The contractional tectonic regime of the Vertentes Belt in the southern RCT is important for the evolution of this terrane, due to the strong influence of the transcurrent shear zones. These zones are subsidiaries of the Pernambuco lineament that mask the terrane's preexisting contractional tectonic features. The radiogenic data were used to classify lithologic types by studying different levels of gamma radiation. It should be noted that different lithologies can express the same range of gamma radiation values. Moreover, chemically differentiated portions can be found within the same lithology (Davis, 1986).
2. Geologic context 3. Methodology for correlating multiple digital maps The extreme eastern part of Borborema Province, which is located in the eastern portion of Pernambuco State, is divided into the Pernambuco, Alagoas and Rio Capibaribe tectonic domains, as shown in the geological map (Fig. 1). According to Santos et al. (2000, 2014), the Pernambuco-Alagoas lineament divides the area into two subprovinces, the Transversal Subprovince in the north and the Meridional Subprovince in the south. The Transversal Subprovince is controlled by two important regional structures that are characterized by NE-SW-trending shear zones. These structures are the Patos and Pernambuco lineaments (Santos and Medeiros, 1999; Santos et al., 2000). The Rio Capibaribe Terrain (RCT) is located in the eastern part of the Transversal Subprovince and is bordered by the Pernambuco-Alagoas lineament to the south (Santos and Medeiros, 1999; Medeiros, 2004). The Meridional Subprovince lies south of the Pernambuco-Alagoas lineament, and it is made up exclusively of the Pernambuco-Alagoas terrane (PEAL). This subprovince is located between the Pernambuco lineament and the N-NE margin of the Congo Craton. It is divided into four sub-domains, which include the Rio Preto, Riacho do Pontal, and Sergipe mobile belts and the Pernambuco-Alagoas terrane. The PEAL is made up of gneissic-migmatitic Paleoproterozoic basement (Silva Filho et al., 1997; Accioly et al., 2010), a supracrustal belt and the metaplutonic Cabrobó and Belém do São Francisco Complexes (Santos, 1995; Brito Neves et al., 2000, Brito Neves, 2011). This set of amphibolites facies rocks are intruded by granitic plutons of various batholitic dimensions. Some restricted Archean cores are known from the western part of the terrane. The predominance of transpressional tectonic deformation is the most important characteristic of this area, as shown by Brito Neves et al. (1995) and Santos (1996). Paleoproterozoic units, such as the Pão de Açúcar and Salgadinho Complexes, and anorogenic Mesoproterozoic intrusions, such as that of
For interpreting radiometric data, the contents of the elements K, eU and eTh contributed to the identification of some geological units with features related to low levels of gamma radiation (Figs. 2 and 4). According to Rybach (1998), the radiogenic heat production rates of granites can be calculated using the following equation: A ¼ 10−5 ρ ½2:56 ThðppmÞ þ 9:52 UðppmÞ þ 3:48 Kð%Þ; ð1Þ where A (μW ∗ m−3) is radiogenic heat production and ρ (kg ∗ m−3)is the density. Fresh rock samples were collected from Pernambuco State, and the concentrations of K, eU and eTh within the samples were determined in the laboratory using gamma spectroscopy. The results show that the contributions to heat production of K, eU and eTh in the samples, as well as the rate of heat production, vary significantly with geological location. Potassium is a major element that contributes to heat production, whereas eU and eTh are trace elements (Figs. 2, 3 and 4). Geophysical grids were prepared with cell dimensions compatible with the scale of the work, and were to be combined with geological mapping. The geophysical maps were generated based on the concentrations of selected radio-elements (K, eTh and eU), their ratios (eU/K, eU/eTh, K/eTh), F factor values, measured ground heat production rates and aeromagnetic data (Figs. 3, 4 and 5). The airborne data used in this study was obtained from two aircraft at a constant ground clearance. The airborne data were acquired using flight lines spaced 500 m apart, with orthogonal tie lines flown every 10,000 m at a height of 100 m above the ground surface. The survey was carried out in two different blocks with different flight- and tie-line directions, and data acquisition was performed perpendicular to the main structures of the surveyed area (CPRM, 2008). The magnetic
Fig. 3. Maps showing: (a) K/eTh ratio; b) eU/eTh ratio and c) F factor values calculated from airborne and ground-based geophysical data in the extreme eastern part of Borborema Province, Pernambuco State.
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Fig. 4. Radiogenic heat: (a) calculated from airborne and (b) calculated from ground-based geophysical data. (c) The ternary map shows the abundances of the elements K, eU and eTh.
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Fig. 5. The radio-element (K, eU, eTh) concentrations of samples collected from various geological units, specifically the Belém do São Francisco Complex (PPbf); the Cabrobó Complex (NPcb); the gneissic-migmatitic complex (Ppgm); the Vertentes Complex (NP1ve); the Serra do Mascarenhas Intrusive Suite (NP3ysm); the Itaporanga Intrusive Suite (NP3yit); the Ouro Branco Intrusive Suite (NP3yo); the Passira Intrusive Suite (PP4ᵟp); and the Serra da Passira Intrusive Suite (PP4ysp).
system used was an optically pumped (cesium vapor) magnetometer that was installed in a stinger extension behind the tail of the aircraft. The output from the magnetometer was sampled at 0.1 s to a resolution of 0.001 nT with a noise envelope less than 0.01 nT. The spectrometer employed 256 spectral channels and consisted of two downwardlooking groups of crystals (thallium-doped NaI) with detector volumes of 1024 in.3 each, for a total of 2048 in.3 of detector volume, and two upward-looking crystals of 256 in. 3 each, for a total of 512 in. 3 (CPRM, 2008). In the next processing step, the data were interpolated to a regular grid, using algorithms that maintain data fidelity at the original measurement locations. This step was followed by correction of spurious effects caused by the leveling of the original grids. The fourthorder difference technique was used to track anomalous spikes in the magnetic data and to condition sampling along the flight lines based on the spatial Nyquist frequency. This was performed on the selected interpolated grid, which contained square cells 125 × 125 m. The algorithm was based on linear interpolation along the direction of the flight lines, and on the Akima spline perpendicular to the flight lines. Microleveling and decorrugation techniques were further applied to
the data. This procedure resulted in several geophysical data products, including thematic maps of both individual variables and composite variables, for use in geologic analysis and interpretation. We propose a method that is based upon the optimal correlation between pairs of quantities from the radiometric data within the data set, including the heat production ratio; F factor values; the contents of the elements K, eU and eTh; and the eU/K, eU/eTh, and K/eTh ratios. The cross-correlation coefficients (C) can be evaluated according to the following formula: P xj − x yj − y C ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi ; 2 P P yj − y xj − x
ð2Þ
where j is the index of each quantity within the data set, and x and y are the mean of each quantity, respectively. We indicate the selected areas on the maps of physical properties. The radiometric methods and radiogenic heat were integrated with the magnetic data to identify deep structures (Luyendyk, 1997).
Fig. 6. Radiogenic heat production and F factor values of samples collected from various geological units, specifically the Belém do São Francisco Complex (PPbf); the Cabrobó Complex (NPcb); the gneissic-migmatitic complex (Ppgm); the Vertentes Complex (NP1ve); the Serra do Mascarenhas Intrusive Suite (NP3ysm); the Itaporanga Intrusive Suite (NP3yit); the Ouro Branco Intrusive Suite (NP3yo); the Passira Intrusive Suite (PP4ᵟp); and the Serra da Passira Intrusive Suite (PP4ysp).
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Fig. 7. Cross-correlation graph between all variables assessed from the airborne and ground-based data, specifically the contents of thorium (Th), uranium (U), and potassium (K), as well as radiogenic heat (A), F factor (F) values, and the K/Th and U/Th ratios.
Quantitative magnetic data analysis was performed in the frequency domain using Fast Fourier Transform (FFT) filters. The Analytical Signal Amplitude (ASA) and Tilt Derivative of the calculated total magnetic field anomaly data show the contacts between the sources and their horizontal geometry. To determine the analytical signal and phase filter, we used the concepts described in Roest et al. (1992) and Thurston (2001), respectively. The resulting shape of the ASA is expected to be centered above the magnetic body. This has the effect of transforming the shape of the magnetic anomaly from any magnetic inclination to one positive body-centered anomaly at least in 2D (Nabighian, 1972). Analytic signal has been utilized widely for mapping of structures and for determining the depth of sources (Roest et al., 1992; Debeglia and Corpel, 1997). We applied the 3D An-Euler method (Salem and Ravat, 2003; Salem and Williams, 2007) using the ASA to obtain depths and structural index values. The “AN-EUL” method, which is based on the combination of the Euler deconvolution method with the analytical signal, allows the estimation of the depths and structural index values of anomalous sources. Assuming that each magnetic anomaly is produced by an isolated body, such as a vertical cylinder, the horizontal diameters of the sources can be estimated as varying from X to Y (Salem and Ravat, 2003). The horizontal derivatives emphasize abrupt lateral changes in physical properties that appear where the edges of the magnetic sources are located. Within the study area, magnetic signals can be identified as indicative of faults/fractures, tectonic effects, lithologies rich in ferromagnesian minerals, features related to igneous rocks of acidic to intermediate composition (granitic and/or volcanic), structural lineaments and possibly geotectonically distinct regions. Some magnetic domains could be distinguished in the map showing the total magnetic intensity. 4. Application to the identification of hydrothermal zones We interpret the data based on heat production rates; F factor values; eU, K and eTh concentrations; K/eTh and eU/K ratios; and analytic signal amplitudes because of the elongated structures and Tilt Derivative, which will show geological contact zones if the contacts are vertical.
Based on high concentrations of K and the K/eTh ratios, because eTh is a radioelement with low mobility, we can better define the zones that have experienced hydrothermal alteration. The radiometric data indicating the contents of the elements K, eU and eTh, the amount of radiogenic heat, and the F factor are shown in Figs. 5 and 6. Interpolated values of thermal conductivity, density and susceptibility generated from measurements made using 409 samples are shown in Fig. 8. Analyzing the data from the all of the ground samples, we noted that the radiogenic heat is most highly correlated with thorium (C = 0.94), followed by uranium (C = 0.77), potassium (C = 0.45) and density (C = 0.26), which indicates that the element thorium has the greatest influence on this parameter, as shown in Fig. 7. The F factor shows a mean correlation with the contents of K and eU. The eU/eTh ratio is common in studies that use gamma spectrometry, but the K/eTh ratio is not used. In this paper, we propose a study of this ratio in airborne and ground-based data. The eU/eTh ratio displayed a better correlation with the F factor in the ground-based data (C = 0.93). The K/eTh ratio yielded a better correlation with the F factor in the ground-based and aeromagnetic data, 0.42 and 0.75, respectively. The integration of information generated by the interpretation of the magnetic and gamma-spectrometric data allowed the identification of several litho-structural domains. Each block was analyzed and compared with the available overall geological data, which are obtained at smaller scales than that of the airborne survey. This implies that the information derived from the geophysical data contains much more detail than the existing maps. We can carry out interpretations based on the F factor, K contents, K/eTh and eU/K ratios, density, magnetic susceptibility, analytic signal amplitudes and tilt angles (Figs. 3, 8 and 9). The high values of heat production rate occur along the borders of elongated structures with high values of the F factor and K/eTh ratios and moderate eU/eTh values. In the density and magnetic susceptibility maps (Fig. 8), the areas in question are associated with high relative magnetic susceptibilities and high densities. We observe high concentrations of K, which are due to the strong presence of feldspar granite and other associated lithologies with high heat production rates that make up the Itaporanga and Moderna Intrusive Suites, as well as the Serra da Passira and Ouro Branco
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Fig. 8. The interpolated values of the a) thermal conductivity, b) density and c) susceptibility obtained from measurements of the rock samples.
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Complexes, which are found in the southeastern portion of the map of K contents. The highest average heat production rate values occur within Neoproterozoic magmatic suites, next to areas with high potential for mineral exploration. The high K concentrations and low eTh values are most notable close to the Pernambuco lineament, near the Modern Intrusive Suite. These features occur in the units corresponding to the Itaporanga Intrusive Suite and the gneissic-migmatitic complex, as well as some belts to the north that are mainly in the Serra do Mascarenhas unit and follow lineaments that are oriented NE-SW. The high values of K are due to associations with granodiorite and tonalite, both of which contain large amounts of plagioclase, and the orthogneisses of the Serra de Passira. The migmatitic complexes, such as the granite and biotite-granite of the Itaporanga and Serra do Mascarenhas felsic units, have high K concentrations. These large areas with low concentrations of thorium represent lineaments that cross the Modern and Itaporanga Intrusive Suites, such as the gneissic migmatites that occur in some areas. The low concentrations of uranium occur to the north. The high values of the K/eTh ratio occur due to the fact that eTh is less mobile than K. There is a greater interest in the low values that are concentrated to the west of the Pernambuco lineament in the Serra do Mascarenhas Unit, the gneissic-migmatitic complex and the Buenos Aires Unit. We use eU/eTh ratios to study the degree of lithologic differentiation. We prioritize the low values found exclusively in the northwestern Pernambuco lineament near the Modern Suite and in the western Itaporanga. The values of the F factor are in with ENE-WSW-trending lineaments in the northeastern portion of the map (Fig. 3). We observed high intensities for the entire Modern Suite and portions of the gneissicmigmatitic complex and the Itaporanga Complex. We selected felsic units that occur in an isolated area south of the Pernambuco lineament with high values of the F factor. The best interpretations were achieved from qualitative analysis of the ternary radioelement map (Fig. 2), as well as the analytic signal amplitude and Tilt Derivative map (Fig. 9b). These maps highlight the key crustal blocks and their unique inter-relationships. We recognized several blocks, which reflect distinct periods of crustal evolution and are described below. a) Structure 1 (S1) contains several geological units, including the Moderna Intrusive Suite, which is composed of syenogranites, syenite quartz and syenite with Fe-hastingsite; the Serra da Passira Intrusive Suite, which is composed of garnet-amphibolite-biotite orthogneiss and granite dominated by alkali feldspar; and the Itaporanga Intrusive Suite, which is composed of monzogranites, syenogranites and porphyritic granodiorites, which range from metaluminous with calcium alkalis and large amounts of K to mildly shoshonitic. It also displays a moderate quantity of dioritic enclaves. The Moderna Intrusive Suite displays high abundances of K and low abundances of eTh and eU. Its K/eTh ratios are high and peak at 0.201 in the center; its eU/eTh ratios show considerable variability; its F factor values are high, with values of 0.228 to 0.316; its heat production rates are high along its eastern border, with values of 3 to approximately 4 μW/m3, and moderate to the west; it displays a substantial magnetic anomaly and analytical signal, yet it contains consistent, SW-NE-trending, geometrical bodies. It may have zones of mineralization occurring along its borders, mainly in the east. Given its lithology, its density and magnetic susceptibility values are high to very high. b) Structure 2 (S2) is represented by the gneissic-migmatitic complex geological unit, which is composed of banded orthogneisses having granitic and granodioritic compositions, as well as tonalitic, monzonitic, monzodioritic and dioritic compositions. These rocks are intercalated with metamafic rocks and amphibolites and include migmatitic structures, such as augen and schlieren. It has been shown that this region presents medium to high quantities of K, low to medium
quantities of eTh and eU, low heat production rates that range from 0.7 to 1.6 μW/m3, and high magnetic anomalies and analytical signal values in Fig. 2. These lithologies primarily represent basement rocks with medium to high K/eTh ratios. High U/eTh values are observed to the west and medium to low values of this quantity are seen in the eastern portion, and primarily high F factors plus moderate values occur in the southeastern portion. In this area, density and magnetic susceptibility are low, caused by the geological diversity. c) Structure 3 (S3) contains the Serra da Passira Intrusive Suite geological unit, which is composed of grenade-anphybolium-biotite orthogneisses. Granite dominated by alkali feldspar occurs in the main portion of the Itaporanga Intrusive Suite, which also contains monzogranites, syenogranites and porphyritic granodiorites, which range from metaluminous with calcium alkalis and large amounts of K to mildly shoshonitic. The data indicate high heat production rates along the borders of this structure and low to medium rates in its center. Moreover, large abundances of K occur in the western portion with a peak value of 2.16%, because felsic rocks are the dominant lithology. Moderate concentrations of U occur in the center, whereas large amounts occur along the borders, and moderate concentrations of Th are also observed. High K/eTh ratios occur in almost all of the structure, as well as high eU/eTh ratios and high F factor values because of the presence of felsic rocks. This area has high density and magnetic susceptibility. d) Structure 4 (S4) is represented by several geological units, with the dominant one being the gneissic-migmatitic complex, which is composed of banded orthogneisses having granitic and granodioritic compositions, as well as tonalitic, monzonitic, monzodioritic and dioritic compositions, next to the Pernambuco lineament. The Itaporanga Intrusive Suite is composed of monzogranites, syenogranites and porphyritic granodiorites, which range from metaluminous with calcium alkalis and large amounts of K to mildly shoshonitic. It also displays a moderate quantity of dioritic enclaves. The data show high K concentrations alongside the structure, with values of 1.1 and 2%, as well as medium to low concentrations of eTh and eU. Because of the great quantities of granites and derivative rocks, along with diorites, this structure is associated with low heat production rates in its center but high rates along its borders; high values of magnetic anomaly and analytical signal that correspond to the geometry of the elongated structure; high K/eTh ratios, because of the dominance of rocks with felsic compositions and low eU/eTh ratios; and high F factor values, with low values being located in the center. This may occur because of the presence of some metamafic rocks within the gneissic-migmatitic complex. In this structure, because of the bordering effects, the density is medium to high. The magnetic susceptibility is also high, up to 22 SI. In selected areas, a method called AN-EUL was applied to determine values of the structural index and the source depth at each location point, which can be of great value in the interpretation of the data. Aeromagnetic products contribute to the understanding of structural detail in the northeastern portion of Borborema Province. Using these data, it is possible to better define the boundaries between crustal blocks and/or tectono-stratigraphic terranes, estimating depths of regional features through Euler deconvolution and defining the nature of the NW-SE trend. These results provide new arguments for use in interpretations and discussions of the geodynamic evolution of this region. The AN-EUL method is based on a combination of the analytic signal and the Euler deconvolution methods. Both the location and the approximate geometry of a magnetic source can be deduced. The method is tested using theoretical simulations with different magnetic models placed at different depths with respect to the observation height. In all cases, the method estimated the locations and the approximate geometries of the sources. The depth estimates resulting from the AN-Euler method vary from 1 up to 3 km. Elongated bodies are buried deeply in the western portion. The depths range from 700 m to 1 km, which corresponds to the
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Fig. 9. The aeromagnetic data, represented in terms of a) magnetic anomalies, b) ASA + phase values, and c) radiogenic heat + depth solutions obtained using the AN-Euler method.
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Moderna Intrusive Suite and some occurrences in the northeastern portion, with their borders following the geological contacts. Some of these occurrences show elongated bodies to the southwest that are approximately 10 km in width and have depths that vary between 700 and 3000 m. A deep anomaly located between those two structures may represent the continuation horizontally displaced of one of them or a third non-outcropping but interconnected shear zone. The favored interpretation considers this sector of the Transversal Zone as formed by deeprooted, northeast-southwest trending shear zones, cut by shallower structures, with east-west trends. In this context, the importance and continuity at depth of the shear zone is highlighted, and as such inferred to be the suture between terrenes, in accordance with field geological data. In this paper, we will stake out areas favorable to mineralization of iron, titanium and nickel, located at the eastern end of Borborema Province, Pernambuco State.
5. Discussion and conclusions The interpretation of the aeromagnetic products was correlated with geological structures having NE-SW trends that are known in the region, the concentrations of radionuclides (eU, eTh, and K) and radiogenic heat production. Magnetic lineaments with N-S and NW-SE orientations are well defined in the interpretations, and these lineaments characterize structures that are not yet well known in this region. The magnetic anomalies associated with the different tectonostratigraphic blocks indicate the continuation at depth to the southsouthwest of the Archean nucleus. Also, different magnetic signatures were noted for the Paleoproterozoic rocks, as well as a low magnetic response. The granitic intrusions that occurred prior to Brasiliano are weakly magnetic, whereas the post Brasiliano intrusions are highly magnetic. Radioactive minerals are present in all the rock samples collected. These radionuclides (eU, eTh, and K) make different contributions to the radiogenic heat produced in rocks as a result of their geological location. Analyzing the data obtained from all of the ground-based samples, we noticed that the radiogenic heat is most strongly correlated with eTh, followed by eU and K, showing that the element Th has a substantial influence on this parameter. The F factor shows a mean correlation with potassium and uranium. The integrated data enable the identification of zones with mineralization, based on radiogenic heat production, F factor values, K concentrations, K/eTh and eU/K ratios, density, magnetic susceptibility, and analytic signal amplitude, because of the elongated structures, and the tilt angle data show geological vertical contact zones. Aeromagnetic data contribute to our detailed structural knowledge of the eastern border of Borborema Province. It is possible to better define the boundaries between crustal blocks and stratigraphic terranes by estimating the depths of regional features through An-Euler deconvolution and defining the nature of the NW-SE trend. We highlight those features that are related to different types of granites and metagranitic rocks, which typically show values of gamma radiation above the regional background, as do rocks of intermediate composition, such as gneisses. We note that the high concentrations of potassium are due to the presence of large amounts of feldspar granite and other associated lithologies with high heat production rates. With the geological map and large-scale distributions of various rock types, the selected areas are the Modern Intrusive Suite, which is composed mainly of syenites with Fe-hastingsite and other adjacent areas, such as Passira, the undivided gneissic-migmatitic complex and Itaporanga, which follow the E-W-trending Pernambuco lineament. There is also an occurrence in the southeastern portion south of the Pernambuco lineament, which correspond to the Belém do São
Francisco, Ouro Branco and Itaporanga units, with Itaporanga being the dominant area. The structures associated with the hydrothermal zones (S2, S3 and S4) contain large quantities of K and low quantities of eTh and eU. They are also associated with high K/eTh ratios with a peak value in the center, widely varying eU/eTh ratios, high F factor values and high heat production rate along the borders, as well as high magnetic anomaly and analytical signal. However, they form consistent geometrical bodies. The potential mineralization zones trend SW-NE, and for some lithologic types, the density and magnetic susceptibility values are very high. The use of the methods described in this study allowed the improvement of geological models for a defined area. They give us excellent estimates of potential mineralization zones occurring along the eastern border of Borborema Province. 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