Geophysical characterization and modeling of a Neoproterozoic intrusive volcaniclastic lamprophyre in southernmost Brazil

Abstract

Neoproterozoic spessartitic lamprophyres are linked to the Lavras do Sul Shoshonitic Association in the Sul-Riograndense Shield, southernmost Brazil. Bodies of intrusive spessartitic lamprophyres occur in trachyandesites of this association in the Cerro Tupanci region. There, Neoproterozoic igneous and metamorphic complexes are covered by volcanic-sedimentary Ediacaran sequences of the Camaquã Basin that include trachyandesites, conglomerates, sandstones, and spessartitic lamprophyre dikes of which a spessartitic lapilli tuff N-NE oriented is the main occurrence. Gamma spectrometric and gravimetric data for the lapilli tuff allowed definition of its occurrence as a dike elongated N-S, bearing low values for gamma spectrometry and a strong negative gravimetric anomaly approximately 450 m long and 70 m wide. Two-dimensional modeling of this anomaly suggests that the dike is subvertical to vertical. Magnetometry did not provide significant data regarding the lamprophyre but did show a normal dipolar anomaly to the west, suggesting the presence of a non-outcropping cylindrical intrusion in the area.

KEYWORDS:
gamma spectrometry; gravimetry; magnetometry; geophysical modeling

INTRODUCTION

The occurrence of mineralizations associated to lamprophyric rocks has been documented by several authors (Rock 1991, Kerrich and Wyman 1994, Armstrong and Barnett 2003, Ayer and Wyman 2003, Štemprok and Seifert 2011, Zuo et al. 2011, Ibrahim et al. 2015) in spatial and temporal associations, such as in porphyry deposits (Seedorff et al. 2005), diamond in the primary source (Lefebvre et al. 2005), and orogenic gold (mesothermal) (Taylor et al. 1994, Craw et al. 2006). Lamprophyric rocks, according to Rock (1991), are separated in to five groups as follows: alkaline, calc-alkaline and ultramafic lamprophyres, lamproites, and kimberlites, although the last two are not considered to be lamprophyric rocks anymore (Le Maitre 2002), since they are best considered independently. Calc-alkaline lamprophyres, namely, minettes, kersantites, spessartites, and vogesites, are usually found in convergent settings (Rock 1991, Krmíek and Rao 2022), associated with calc-alkaline granitoid plutons, where they are the mafic members of dyke suites and with shoshonitic suites. The more typical association is with late- to post-orogenic bodies, including ring complexes (Rock 1991). Alkaline and ultramafic lamprophyres are commonly related to divergent environments (Krmíek and Rao 2022), with alkaline lamprophyres occurrence associated with alkaline syenite-gabbro plutons or unrelated to other igneous activity, and ultramafic lamprophyres associated with alkaline complexes (Rock 1991).

From a petrological point of view and in terms of classification, the lamprophyres are a group of meso to melanocratic rocks, usually hypabyssal, exhibiting a remarkable panidiomorphic texture, with abundance in mafic phenocrysts of biotite/phlogopite and/or amphibole, with or without pyroxene and olivine, surrounded by a matrix made up of the same mineral phases, with the addition of feldspars and/or feldspathoids (Streckeisen 1979, Le Maitre 1989). In the case of spessartite lamprophyres, they would be characterized by a felsic mineralogy (predominantly in the matrix) constituted by plagioclase >> K-fedlspars and a mafic mineralogy (phenocrysts and matrix) represented mainly by hornblende >> augite.

Studies concerning the physical properties of lamprophyric rocks (sensu Le Maitre 2002) in the literature are rare (e.g., Rao 2008, Nishad et al. 2013, Hrouda et al. 2016), while studies applying geophysical techniques are almost non-existent (e.g., Machek et al. 2014). Considering the relationship between lamprophyres and some types of ore deposits and the frequent use of gamma spectrometric, magnetometric, and gravimetric surveys in the early stages of prospection, this work provides results that can be useful in exploration.

In the Sul-Riograndense Shield (SRGS), spessartitic (calc-alkaline) lamprophyres can be found among the lithotypes that compose the Camaquã Basin (CB) and their occurrence is linked to the Neoproterozoic Hilário Formation (Ribeiro and Fantinel 1978) of the Lavras do Sul Shoshonitic Association (LSSA) (Nardi and Lima 1985). Au and Cu mineralizations were recorded in LSSA rocks (Carvalho 1932, Ribeiro 1966, Nardi 1984, Müller et al. 2012, Lopes et al. 2014), and gold mining has developed sporadically in the district of Lavras do Sul since the early 19th century (Grazia and Pestana 2005). Lamprophyric rocks (calc-alkaline) related to the LSSA were reported as host rocks of Cu mineralization on an abandoned mine by Lopes et al. (2014). Müller et al. (2012) suggest that a possible mixture between latitic and lamprophyric magmas may have generated the ideal conditions for Au transport and precipitation in the latitic dikes of LSSA. However, the geophysical studies in south Brazil have been developed in regional scale (e.g., Costa et al. 1995, Fernandes et al. 1995, Hartmann et al. 2016a, 2016b), while to delimitate the bodies of this important region is less investigated, and its mineral potential under-estimated.

Geophysical techniques have been widely used in the study and mapping of igneous bodies and lavas (e.g., Blum 1945, Bott and Smithson 1967, Davis and Guilbert 1973, Grasty et al. 1979, Graham and Bonham-Carter 1993, Segev et al. 1993, Dickson and Scott 1997, De Castro 2005, Calvín et al. 2014, Nardy et al. 2014, Abedi et al. 2018). Gamma spectrometry surveys are a powerful tool for rock characterization because it is possible to map the radiation distribution emitted by the natural decay of K, U, and Th that can be used in the investigation of outcropping igneous rocks and related alteration (e.g., Shives et al. 2000, Zuzana et al. 2009). The gamma ray spectrometry surveys map the relative abundance or concentration of K, U, and Th in rocks and soil materials up to approximately 50 cm deep (e.g., Dickson and Scott 1997, Minty 1997, Wilford et al. 1997, Wilford 2002). Important information is also obtained from the element ratios (e.g., eU/eTh, eU/K, and eTh/K) and ternary K-eU-eTh radioelement maps. Magnetometry commonly shows good response to igneous bodies and has been widely used in its mapping, particularly in the prospection of primary source diamonds (kimberlitic and lamproitic bodies), especially in airborne surveys for its low cost and high sensitivity (e.g., Macnae 1995, Kono 2007). Igneous rocks, in particular, basic igneous rocks, have the highest magnetic susceptibilities due to their relatively high magnetite content and can generate significative magnetic anomalies (e.g., Geldart et al. 1990, Lino et al. 2018). Gravimetry has been used in the search and characterization of any targets that have significant density contrasts (Nabighian et al. 2005), which frequently is the case in igneous bodies. The technique is ideal for studying shallow subsurface geological structures related to magmatic chambers in volcanic environments (Rymer and Brown 1986, Battaglia et al. 2008) and can be applied to small-scale features (e.g., Breytenbach and Bosch 2011).

Detailed geophysical surveys are used to make considerations regarding the size and geometry of kimberlitic and lamproitic bodies and to identify faciologic variations and different intrusive phases (Macnae 1995). These studies allow accurate estimates of location and depth of subsurface bodies. The application of geophysical techniques to detect small igneous bodies is predominantly described for the study of kimberlitic and lamproitic pipes and diatremes, due to their economic importance, but it has also been applied to other small bodies such as dikes, plugs, and sills (Horsman et al. 2009).

In the northern portion of the SRGS, in the Cerro Tupanci region, an intrusive body of spessartitic lamprophyre (calc-alkaline) occurs, with fragmented texture, composed of fragments of trachyandesites and espessartites, mafic/ultramafic xenoliths, crystal fragments, mainly of amphiboles, pyroxene and plagioclasium, and vitreous material. Secondary minerals are carbonate, chlorite, and epidote. In the matrix Fe-Cu sulphides, gold and platinum were identified using scanning electronic microprobe (Buckowski 2011). According to the author, this body is 300 m long and 40 m wide, with N-NE direction. Descriptively, this rock is classified as a lapilli tuff following the classification proposed by Gillespie and Styles (1999). In this study, we propose to accomplish a geophysical characterization using magnetometry, gamma spectrometry, and gravimetry and model this intrusive body in order to better define its dimensions and form. Magnetic susceptibility and density data are also described.

GEOLOGICAL SETTING

The Neoproterozoic to Ordovician CB is a molassic basin associated with the late to post-collisional phases of the Dom Feliciano Belt of Brasiliano Cycle (Fragoso-Cesar et al. 1984, Paim et al. 2000, Hartmann et al. 2008). According to Paim et al. (2014), the CB keeps the record of four basins overlap: Maricá foreland basin, Bom Jardim west and east strike-slip basins, Santa Bárbara west and east rifts, and Guaritas rift. Volcanic events occurred concomitantly with sediment deposition at least in three of these basins: Hilário volcanism in the Bom Jardim west and east strike-slip basins, Acampamento Velho volcanism in the Santa Bárbara west and east rifts basins, and Rodeio Velho volcanism in the Guaritas rift basin (Lima et al. 2007, Paim et al. 2014).

Hilário volcanism represents the first major magmatic episode of CB (Wildner et al. 2002, Lima et al. 2007, Sommer et al. 2011). The volcanic rocks of Hilário Fm. predominantly have intermediate composition, but vary from basic to acidic, with shoshonitic affinity (Nardi and Lima 1985, Lima 1995, Nardi and Lima 2000, Almeida et al. 2012). The effusive rocks are mainly represented by trachybasalts and trachyandesites, associated with monzonitic, quartz-monzonitic, and lamprophyric hypabissal rocks (Lima et al. 2007), in addition to the occurrence of volcaniclastic deposits such as breccias and tuffs. Janikian et al. (2008) obtained U-Pb and Ar-Ar ages from 585 to 590 My in andesites. U-Pb dating in SHRIMP from andesite zircon grains indicated ages of 580 ± 3.6 My (Janikian et al. 2012), and dating performed on lamprophyres by Almeida et al. (2012) using the U-Pb method resulted in ages of 591.8 ± 3 My. The Hilário Fm. rocks are genetically linked to the LSSA magmatism and represent its volcanic manifestation (Nardi and Lima 1985, Lima et al. 2007), while the plutonic phase is represented by the nucleus of the Lavras do Sul Granitic Complex (LSGC; Nardi 1984) and consists of monzogranitic and granodioritic intrusives.

The geology of the Cerro Tupanci region is represented by trachyandesites, lamprophyres, and volcaniclastic deposits of Hilário Formation, as well as rhyolites and ignimbrites of Acampamento Velho Formation (Fig. 1). The basement is represented by granites, gneisses, and migmatites of the Cambaí Complex (Menegotto and Medeiros 1976, Buckowski 2011, Leitzke 2011, 2013, Barrios 2015). These units are covered by sedimentary rocks of Rio Bonito Formation of the Paraná Basin.

Figure 1
Location of the study area and geological map of the Cerro Tupanci region, showing the Precambrian basement, the sequences of the Camaquã Basin, and the sedimentary rocks of the Paraná Basin. The ground survey area is indicated by the red square.

The study dike of lamprophyre is intrusive in the trachyandesitic rocks and has an average thickness of 40 m, elongated in the N-NE direction (Fig. 1). It presents a brecciated texture, with andesite and lamprophyre fragments and macrocryst fragments of clinopyroxene and amphibole surrounded by a matrix containing plagioclase, clinopyroxene, and amphibole fragments, being classified as a lapilli tuff with spessartitic composition (Buckowski 2011). The matrix is quite altered, mainly to carbonates, chlorite, and epidote. In the region, there is also a massive lamprophyric dike, intrusive in the Cambaí Complex granitoids.

MATERIAL AND METHODS

The geophysical data used in this study were obtained in a field campaign in study areas where the lamprophyric lapilli tuff outcrops (Fig. 2). Regular measurements were taken in 60 geophysical stations spaced 50 m, arranged in four N-S profiles (Fig. 3A). Gamma spectrometry, gravimetry, and magnetometry data were measured in all geophysical stations. A TopCon Hyper Lite model geodetic GPS with 3 mm horizontal and 5 mm vertical accuracy was used to determine the geographic coordinates and altitude of each point, which are fundamental in potential geophysical methods.

Figure 2
(A and B) Photographs of two areas showing the lapilli tuff outcrops. (C) An accidental fragment of andesite immersed in the lapilli tuff. (D) A sample used to obtain the density and magnetic susceptibility data.

Figure 3
(A) Location of the measurement places for the geophysical data (black dots) on the geological map of Barrios (2015). (B) Ternary map (red-green-blue colors) of the study area, area of the outcropping lamprophyre, and radiometric domains. (C–F) Total counts, potassium, uranium, and thorium maps for the area, respectively. All maps indicate an anomaly with low values in the approximate location of the lamprophyre.

Gamma spectrometry measurements (K, eU, eTh) were performed at each point using a Radiation Solutions RS 125 Super Spec gamma spectrometer, where the rock was not or less weathered, in order to reduce the effects of alteration in the gamma spectrometric data. Magnetic data were collected using two proton precession magnetometers (GEM System Inc. Canada), GST 19 model with a resolution of 0.01 nT. The International Geomagnetic Reference Field (IGRF) calculated for the study area at the time of the survey showed an inclination of -38.34°, a declination of -14.22°, and an intensity of 21,096 nT. The gravimetric data were obtained using a Scintrex CG-3 gravimeter with a resolution of 0.005 mGal.

The geophysical data were processed using the Oasis Montaj® (GeoSoft) software, version 7.1.1, of Geosoft Incorporated company. Grids were interpolated using a 15-m spaced data by the minimum curvature method. For gamma spectrometry and magnetometry, we used the mean of the three measures obtained in each station. The magnetometric data were corrected for diurnal variation, and the IGRF was removed to generate a map of the local magnetic field, on which we applied filters of reduction to the magnetic pole, analytic signal, and upward continuation (UC). Reduction to the magnetic pole is a transformation of the magnetic data to the magnetic pole, intended to remove the effects of induced magnetization and strike on the shape of the anomaly (Baranov and Naudy 1964); the analytic signal is function that shift the anomaly peak to coincide either with the center or with the edges of the magnetic anomaly (Nabighian 1972); and the UC is a filter used to minimize or remove the effects of shallow sources or noise, better constraining the deeper sources. To help in the interpretation of the magnetic data, we measured the magnetic susceptibility of the outcropping rocks in the area using a MFK1-FA Multifunction Kappabridge magnetic susceptibility analyzer. The measurements were taken at the Palaeomagnetism Laboratory (USPmag), University of São Paulo, in low (976 Hz) and high (15,616 Hz) frequencies.

For gravity data, we corrected the variations caused by instrumental drift and terrain and applied free air and Bouguer reductions. To generate the Bouguer Anomaly Map, we considered a 2.67 g/cm³ density for the crust. The terrain correction applied to the gravity was made by an algorithm available on Geosoft that combines the algorithms of Kane (1962) and Nagy (1966). We collected density data from samples of different lithologies that occur in the area to better understand the sources of the gravity anomalies. The measurements were taken using liquid pycnometers, where the mass weight of the specimen divided by the volume (mass) of water displaced determines the density of the sample.

The modeling of a subsurface body from geophysical data can be carried out by the direct problem, where the physical problem is generated to calculate the response from the physical properties of rocks inside, or the inverse problem, where the physical parameters are calculated from the observed data (Barbosa and Pereira 2013). Part of the magnetometric data was selected to generate a 3D model by inversion using a 3D voxel-based algorithm available on Geosoft. A linear trend surface was removed from these data, and an Iterative Reweighting Inversion (IRT) link was used, which improves the geometry of the model focusing on the body, making it less dispersed (Barbosa and Pereira 2013). Using the gravimetric data, we generated a model by the direct (or forward) method in a profile of orientation E-W, crosscutting the body, using the GM-SYS integrated with the OMG platform.

RESULTS AND DISCUSSION

The lapilli tuff is constituted by accidental fragments of andesites and cognate fragments of a lamprophyric rock engulfed by a tufaceous matrix (Buckowski 2011). In the field, a massive rock was observed (Figs. 2A and 2B), with a particulate texture (Figs. 2C and 2D), locally occurring spherical exfoliation (Fig. 2A). The lapilli tuff outcrops in the bottom of a small stream and in the surrounding areas.

Petrophysical data

Table 1 shows the density and magnetic susceptibility data obtained for different rocks from the study area. The measured density of the sampled rocks varies between 2.26 g/cm³ for the arenite and 2.91 g/cm³ for the lamprophyre. The obtained density for the lamprophyre is consistent with data published on New Zealand lamprophyres by Tenzer et al. (2011), with density values between 2.65 and 3.39 g/cm³. The magnetic susceptibility obtained from the lamprophyre sample showed positive values (1.28 × 10⁻⁶ S.I.), indicating a paramagnetic behavior (Clark 1999).

Table 1
Density and magnetic susceptibility data for some rocks of the Cerro Tupanci region^*.

Sample ID	Lithology	Field (A/m)	Frequency (Hz)	Mass (g)	Susceptibility (S.I. units)	Density (g/cm3)
CTG092	Siltite	200	15,616	20.069	1.75 × 10−7	2.68
CTG039	Conglomerate	200	15,616	23.371	3.39 × 10−8	2.62
CTG072	Arenite	200	15616	20.243	-5.62 × 10−9	2.26
CTG079	Ignimbrite	200	15,616	22.545	3.40 × 10−8	2.46
CT74	Lamprophyre	200	15,616	21.032	1.28 × 10−6	2.91
CTG086	Riolite	200	15,616	29.6	1.50 × 10−6	2.50
CT09F	Trachyandesite	200	15,616	22.351	1.19 × 10−5	2.74
CTG092	Siltite	200	976	20.069	2.03 × 10−7
CTG039	Conglomerate	200	976	23.371	3.88 × 10−8
CTG072	Arenite	200	976	20.243	-3.47 × 10−9
CTG079	Ignimbrite	200	976	22.545	4.05 × 10−8
CT74	Lamprophyre	200	976	21.032	1.30 × 10−6
CTG086	Riolite	200	976	29.6	1.52 × 10−6
CT09F	Trachyandesite	200	976	22.351	1.20 × 10−5

• ^*
  The magnetic susceptibility for the igneous rocks is similar, except for the ignimbrites.

Gamma spectrometry

The radiometric ternary map of RGB (red, green, blue) composition of potassium (K), equivalent thorium (eTh), and equivalent uranium (eU) elements was generated (Fig. 3). It was possible to divide the study area into five radiometric domains: A, B1, B2, C, and D. Radiometric domain A covers an area approximately coincident with the outcropping area of the lapilli tuff. It has low total count, K, eU, and eTh, respectively, 30–54.7 counts per second (cps), 0.8–2%, 0.4–1.9 ppm, and 4–7 ppm. The radiometric domain B1 and B2 corresponds approximately to the areas where trachyandesites outcrop. It has intermediate to high values for total count (50–150 cps) and widely varying values for K (1.3–4%), eU (1.6–5.3 ppm), and eTh (6–18 ppm). The domain C is located in the southern part of the study area, where conglomerates and sandstones outcrop. The total count, K, and eTh values are high (80–130 cps, 2.8–4.4%, and 12–30 ppm, respectively). Equivalent uranium values are moderate (1.9–2.8 ppm). The radiometric domain D is in the northwestern portion of the study area and also corresponds to trachyandesites. These have high values for total count (90–260 cps), eU (3–6.7 ppm), and eTh (20–70 ppm), and moderate K values (2–2.8 ppm).

The contrast of K, eU, and eTh between domains A and B reflects the compositional variation between lamprophyre and trachyandesites. However, at two points, east of the intrusive body, occur sandstones whose values of these elements do not differ from the lapilli tuff so that these two lithologies are grouped into the same domain (i.e., domain A).

The domain D, although occurring in an area where the outcropping lithology is trachyandesite, has very high eU and eTh values, totally different from the values obtained in other areas of the same lithology. A macroscopic analysis of the domain D trachyandesites showed that the feldspar phenocrysts are strongly altered for epidote and clay minerals. The cause of these higher values can be related to the passage of hydrothermal fluids altering the rock and causing uranium and, particularly, thorium enrichment. It is also important to note that this domain’s area is unlikely to be true to the field occurrence of hydrothermalized trachyandesites, since the measurement stations with these values are at the edge of the map, and when interpolated by minimal curvature, surface smoothing must have extrapolated these high values to a larger area than the actual one. In fact, these anomalous values were obtained at only three measurement stations.

Thorium enrichment in hydrothermal environments can generate one of the four main types of deposits of this element, which are deposits in veins (Barthel and Tulsidas 2014). According to the authors, vein deposits occur in or close to intrusive or extrusive igneous rocks. The enrichment observed in hydrothermalized trachyandesites may have a similar origin, although less intense. In addition, studies in the lamprophyric rocks of this region (Buckowski 2011) show a concentration of K₂O, U, and Th varying from 0.76 to 1.39%, 1.1 to 2.0 ppm, and 3.52 to 4.9 ppm, respectively, while the trachyandesitic host rocks show concentrations of K₂O from 2.7 to 6.4%, U from 1.2 to 4.8 ppm, and Th from 5.1 to 15.2 ppm (Barrios 2015).

Magnetometry

Figure 4 shows the total magnetic field map reduced from IGRF to the area. The reduction to pole (RTP), analytic signal amplitude (ASA), and UC filters were applied to these data and are shown in the same figure.

Figure 4
(A) Total magnetic field map reduced from IGRF showing a dipole in the northern portion. (B) Total magnetic field reduced to the magnetic pole showing the dipole and domains A and B. (C and D) The analytical signal and upward continuation (50 m) maps, respectively.

In the area, the magnetic field has a magnitude of about 190 nT, with higher values predominantly in the south and lower values in the east and north portions, with an anomaly characterized by a dipole in the north-central part of the study area. This anomaly is not centered over the lapilli tuff, but slightly to the west. From the RTP map (Fig. 4), two magnetic domains and the anomaly were separated. Magnetic domain A presents intermediate to low values, ranging from -34 to 73 nT, and occupies the north and east edges of the map, practically surrounding the magnetic dipole. Magnetic domain B is located in the southwest part of the map and covers a large area, with a northwest direction boundary with dipole and domain A. It has magnetic field values higher than domain A, ranging from 73 to 155 nT.

The dipole is located in the central and northern parts of the map (Fig. 4A). The source area is estimated from a circle whose edges run over the center of the two poles, resulting in an area of about 4 ha. It presents normal polarization for the southern hemisphere, with minimum values of -43 nT and maximum values of 129 nT. The magnetic data inversion in the dipole area was performed to obtain a model of its causative body. Thus, a three-dimensional magnetic susceptibility distribution model was created (Fig. 5A). Figures 5B and 5C shows sections on the Y- and X-axes, respectively, and Fig. 5D shows a section on magnetic susceptibility values. From the generated model, it is observed that this anomaly presents its upper limit at approximately 105 m over the sea level, i.e., about 70 m deep, with a cylindrical shape.

Figure 5
Magnetic susceptibility model of the subsurface generated from the magnetic data. (A) Area of the magnetic anomaly and the model. (B and C) Sections on the Y- and X-axes, respectively. (D) A cut in the lowest magnetic susceptibility values.

The absence of an anomaly linked to the lamprophyric body is coherent with the magnetic susceptibility data obtained for this rock type (1.28 × 10⁻⁶ S.I.), which is not much different than the values obtained for the other igneous rocks present in the area (1.50 × 10⁻⁶ S.I. for rhyolites and 1.19 × 10⁻⁵ for trachyandesites).

Gravimetry

Figure 6 shows the Bouguer Anomaly Map of the area. Four gravimetric domains were divided into domains A, B, C, and D. The gravimetric domain A corresponds approximately to the area where the lapilli tuff emerges. It is characterized by a negative anomaly, elongated in the N-S direction, with values between -20 and -18 mGal. Gravimetric domain B occupies a small area west of domain A, with low Bouguer anomaly values between -19 and -18 mGal. The gravimetric domain C occupies the entire southwest area of the map, with moderate to high Bouguer anomaly values between -15 and -18 mGal. The gravimetric domain D occupies the eastern edge of the study area and has high values of gravity, predominantly higher than -16.5 mGal, reaching the highest values obtained in the area, up to -14 mGal. For the gamma spectrometric, magnetic, and gravimetric field data, refer to Supplementary material.

Figure 6
Bouguer Anomaly Map for the study area and gravimetric domains. Profile I – I' corresponds to the 2D gravity model.

Using the direct problem, the subsurface geology was modeled in an east-west profile, perpendicular to the dike. The modeled profile is shown in Fig. 7. For the construction of the model, the density of the trachyandesites was obtained from field samples and their surface boundaries were also determined in the field. The basement was assumed to be the Cambaí Complex, which occurs adjacent to the area and is formed by gneisses in this region. Densities of this rock type range from 2.6 to 2.9 g/cm³ (Sharma 1997), and we have chosen the value that, within this range, best approximated the calculated gravity to that observed. The density of lapilli tuff was chosen from values presented for densities of pyroclastic and epiclastic rocks of kimberlitic composition, whose lowest values reach 1.8 g/cm³ (Arnott and Kostlin 2003). The non-outcropping body geometry was copied from the model generated by the inversion of the magnetic data. Its density, as well as the boundary between trachyandesites and basement, was chosen to best fit the calculated gravity with the observed gravity.

Figure 7
2D model generated from the gravimetric data. The not outcropping intrusion is the possible intrusive body suggested by the magnetometric data.

The Bouguer Anomaly Map has a clear negative anomaly coincident with the outcropping area of the lapilli tuff. According to Parasnis (1973), deep bodies cause large anomalies with low amplitudes, while shallow bodies generate restricted and clear anomalies. Thus, apparently, the lapilli tuff generates a negative anomaly where it is found. However, analyses of samples of lapilli tuff resulted in a density of 2.93 g/cm³, higher than the density of the host rocks (trachyandesites: 2.73 g/cm³; conglomerates: 2.6 g/cm³). Considering that the lapilli tuff is a rock with fragmented texture from which were chosen the less altered samples, and is located a few kilometers from where the basement of the CB emerges, the following possibilities were raised:

• Density measurements do not correspond to reality since samples were chosen from the less fractured and hydrothermalized parts, and the actual density of lamprophyres is lower than that of the host rocks. Densities of the crater facies of kimberlites (pyroclastic and epiclastic, most similar to the texture of the lapilli tuff) are low, reaching 1.8 g/cm³ (Arnott and Kostlin 2003). Densities of pyroclastic rocks of New Zealand vary between 1.13 and 2.35 g/cm³ (Tenzer et al. 2011);

• In the area where occurs the lamprophyre, the CB exposition is thin, and the basement (Cambaí Complex) is just a few meters below. If the basement is denser than the lapilli tuff, it could explain the negative anomaly.

It is also possible that both hypotheses are true, with lamprophyre density lower than that measured in the laboratory and a shallow basement, denser than the lapilli tuff. The model generated from the gravimetric data allowed a reasonable fit between the measured and observed gravity. The near-surface dike boundaries were established according to the observed field geology. The largest error in the fit between the calculated and observed data occurs slightly west of the dike, where, to satisfy the gravimetric data, it would have a larger extent, with nearly twice the observed surface width. This difference between geology and geophysics may be caused by interpolation of the field data, which may have made the anomaly larger than it really is in the east-west direction.

Another remarkable fact is that there is no gravity anomaly corresponding to the magnetic anomaly. Assuming the existence of the intrusive body mentioned in the discussion of magnetic data, its density should be higher than that of the lapilli tuff and very similar to that of the surrounding rocks, i.e., about 2.6–2.8 g/cm³. To better characterize the subsurface of the area, other evaluation methods (e.g., drilling) are necessary.

CONCLUSIONS

The gamma spectrometric and gravimetric data obtained in the field have shown a clear response to the lamprophyre body. An anomaly with low values of total counts, K, eU, and eTh occurs in the area where the lamprophyre outcrops, as well as a negative Bouguer anomaly. These anomalies have approximately 450 m length and 70 m width in the N-S direction. The magnetic data showed no response to the lamprophyre dike but presented a dipolar anomaly, suggesting the existence of a small intrusive with distinct magnetic susceptibility in the area.

ACKNOWLEDGMENTS

This study was partial financed by CNPq (C. A. Sommer — 304036/2018-8, 406925/2018-6) and Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Anderson Baesso thanks the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior — Brasil (CAPES) for research grant. We also thank the Companhia de Pesquisa de Recursos Minerais (CPRM) for lending us the magnetometers. At last, we thank the Laboratório de Paleomagnetismo of the Universidade de São Paulo.

Supplementary data

Supplementary data associated with this article can be found in the online version (http://sfbjg.siteoficial.ws/Sf/2022/4889202220210051.pdf).

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  Manuscript ID: 20210051.

• 2
  How to cite this article: Baesso A., Sommer C.A., Savian J.F., Gambeta J.H., Aquino R.D.S. Neoproterozoic intrusive volcaniclastic lamprophyre in the southernmost Brazil: geophysical characterization and modeling. Brazilian Journal of Geology, 52(3):e20210051, 2022. http://sfbjg.siteoficial.ws/Sf/2022/4889202220210051.pdf.

Publication Dates

• Publication in this collection
  03 Oct 2022
• Date of issue
  2022

History

• Received
  29 June 2021
• Accepted
  16 May 2022