Interpretation of Geophysical Anomalies for Mineral Resource This Work Is Distributed Under the Creative Commons Attribution 4.0 License

Boletín Geológico, 46, 5–22, 2020 https://doi.org/10.32685/0120-1425/ boletingeo.46.2020.514 Interpretation of geophysical anomalies for mineral resource This work is distributed under the Creative Commons Attribution 4.0 License. potential evaluation in Colombia: Received: December 10, 2019 Accepted: March 6, 2020 Examples from the northern Published online: June 30, 2020 Andes and Amazonian regions

Interpretación de anomalías geofísicas para la evaluación del potencial de recursos minerales en Colombia: ejemplos del norte de los Andes y Amazonía

Ismael Enrique Moyano Nieto1, Renato Cordani2, Lorena Paola Cárdenas Espinosa1, Norma Marcela Lara Martínez1, Oscar Eduardo Rojas Sarmiento1, Manuel Fernando Puentes Torres1, Diana Lorena Ospina Montes1, Andrés Felipe Salamanca Saavedra1, Gloria Prieto Rincón1

1 Servicio Geológico Colombiano, Mineral Resources Direction. 2 Reconsult Geofisica, São Paulo – Brazil. Email: [email protected]

Abstract This paper focuses on presentation of the methodology used by geophysicists at the Servicio Geológico Colombiano (SGC) for the processing, anomaly selection and interpretation of airborne magnetometry and gamma spectrometry data. Three (3) selected magnetic anomalies from different geological settings (Andes Cordillera, San Lucas Range and Ama- zon region) are presented as examples. 3D magnetic vector inversion (MVI) modeling of each of the selected magnetic anomalies shows magnetic sources less than 100 m deep or exposed with sizes from 2.5 to 6 km. The magnetic data interpretation also allows the identification of linear features that could represent structural control for fluid migration and/ or ore emplacement. Additionally, the integration of the geophysical data with other geoscientific information (geologic, metallogenic and geochemical data) leads to the proposition of an exploration model for each anomaly: intrusion-related/ VMS deposits for the Andes, porphyry/intrusion-related/epithermal deposits for San Lucas and carbonatite/kimberlite for Amazonas. The methodology used and examples presented illustrate the potential of SGC airborne geophysical data for mineral resource evaluation and as input for the design of fieldwork for geological, geophysical, geochemical and metallogenic characterization of an area of interest. Key words: Geophysical data processing, 3D modeling, magnetic anomalies, mineral resources.

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Resumen Este documento se centra en la presentación de la metodología usada por los geofísicos del Servi- cio Geológico Colombiano (SGC) para el procesamiento, selección de anomalías e interpretación de datos de aeromagnetometría y gamma-espectrometría. Se presentan como ejemplo tres anoma- lías localizadas en diferentes ambientes geológicos (cordillera de los Andes, serranía de San Lucas y Amazonia). La modelación 3D a partir de la inversión del vector magnético (MVI) de cada una de las anomalías seleccionadas muestra fuentes magnéticas a menos de 100 m de profundidad, o aflorantes, y con tamaños de 2.5 a 6 km. La interpretación de los datos magnetométricos también permitió identificar rasgos lineales que pueden representar controles estructurales para el ascenso o emplazamiento de mineralizaciones. Adicionalmente, la integración de los datos geofísicos con otra información geocientífica (geología, metalogenia y datos geoquímicos) permitió proponer posibles modelos de exploración de cada anomalía: depósitos asociados a intrusivos/sulfuros ma- sivos vulcanogénicos (VMS), en el caso de la anomalía de los Andes, pórfido/asociado a intrusivo/ epitermal para la de San Lucas, y carbonatita/kimberlita para la del Amazonas. La metodología em- pleada y los ejemplos presentados ilustran el potencial de los datos geofísicos del SGC para evaluar el potencial de recursos minerales y como un insumo para la definición del trabajo de campo en- focado en la caracterización geológica, geofísica, geoquímica y metalogénica de un área de interés. Palabras clave: Procesamiento de datos geofísicos, modelado 3D, anomalías magnéticas, recursos minerales.

1. Introduction Colombiano (SGC), separated in blocks that cover broad areas of the Andean and eastern parts of Colombia, in- Airborne geophysics is the easiest and most economical cluding more than 900,000 linear km of high-resolution way to improve geological knowledge of large areas and airborne magnetometric and gamma spectrometric data to detect direct anomalies for further interpretation and (Moyano et al., 2018). Each block is surveyed with a line subsequent drilling. The presence of an isolated magne- separation of 500 or 1,000 m and data sampling of 10 Hz tic anomaly attracts the attention of a mineral explorer. (magnetometry) and 1 Hz (gamma spectrometry), which Once found, the next step is to estimate the physical and yield spatial resolutions of 7-9 m and 70-90 m, respec- geometric parameters of the magnetic rock that produces tively. These sampling rates and flight line separations the anomaly. Magnetic interpretation techniques such as allow the interpolation of grids with spatial resolutions detailed processing, 2D filters and 3D modeling can be of 125 x 125 m and 250 x 250 m, providing details not applied to better understand the anomaly and suggest previously available in Colombia, considering the broad new exploration methods or target drilling. Magnetic coverage of the surveys. anomalies can be the expression of several types of mine- This document presents the interpretation of three ral deposits, such as iron oxide-gold-copper (IOCG) de- magnetic anomalies located in three different geologic posits, porphyries (Au and Cu), kimberlites (diamonds), environments in Colombia: one in the Andes Cordille- alkaline complexes (phosphate and niobium), nickel ra, with some surface expression and a gold geochemical deposits (both sulfide and laterite), volcanogenic massi- anomaly; one in the San Lucas Range area without any ve sulfide deposits (VMS), and banded iron formations surface expression; and one in the Amazonian craton wi- (BIFs). In this context, the absence of a radiometric ano- thout surface expression or rock outcrops. Each area has maly coincident with the magnetic anomaly is a strong different levels of previous geologic and mineral potential clue that we will not find any evidence of the rock at the knowledge, from a lack of data other than the magnetic surface. anomaly in the Amazon region to good geological con- The geophysical data used for the present work co- trol, geochemistry and metallogenic characterization for rrespond to surveys carried by the Servicio Geológico the Andes anomaly. These three anomalies illustrate the

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potential of the geophysical data acquired by the SGC for ronment is supported by the significant production of the assessment of mineral resource potential in Colombia. gold, silver, emeralds, platinum group elements (PGE), ferronickel and other commodities such as copper, lead 2. G eological setting and zinc as principal or secondary products at a modest scale. All of these elements are found in a wide variety Colombia is located northwestern South America, an of geological environments, combined with hundreds of area with specific tectonic features and a physiographic manifestations, occurrences of active or abandoned mi- landscape modeled by the complex interactions among nes and showings (Au, Ag, Pb, Zn, Cd, Cu, Mo, Sb, Hg, the Nazca, South American and Caribbean plates (Figu- Cr, Ni, Pt, Pd, Ti, Mn and Fe); the majority of them have re 1). The Colombian Andes are considered a mixture of no historical exploration, and minimal academic studies many fragments of parautochthonous and allochthonous are available (Shaw et al., 2019). crustal fragments and tectonic wedges of continental, pe- The convergent margin regime that was present ri-cratonic and oceanic affinities. These fragments were throughout the Phanerozoic and the multiple occurren- accreted by strike-slip faults and subduction zones along ces of mineral deposits related to this tectonic regime led the NW margin of the Guyana shield during many pe- to the identification of at least three “copper belts” (Si- riods, with some of the major periods occurring in the llitoe et al., 1982), delimited in the metallogenic map of Permo-Triassic, Jurassic and Late Cretaceous (Bustaman- Colombia (López et al., 2018) as different “metallogenic te et al., 2017). This highly fertile metallotectonic envi- belts” with Jurassic, Miocene and Eocene ages (Figure 1).

Panama 14°0'0"N

Nicaragua Caribbean Plate

Costa Rica 7°10'0"N 10°0'0"N

Panama Venezuela 5°20'0"N 6°0'0"N

Cocos Plate 3°30'0"N 2°0'0"N

Copper Nazca Plate Brazil Belts 1°40'0"N

Ecuador 0 130 260 Km 2°0'0"S Peru 77°20'0"W 75°30'0"W 73°40'0"W 0 135 270 540 Tectonic Fault System Frame Km Eocene Jurassic Miocene

84°0'0"W 80°0'0"W 76°0'0"W 72°0'0"W 68°0'0"W

Figure 1. Tectonic setting of NW South America; the inset shows the copper belts and the main geological faults (dashed black lines) Modified from López et al. (2018)

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Additionally, recent data have defined at least seven mag- more than one million linear kilometers of high-quality matic/mineralization events with a NNE trend and distri- magnetometry and gamma spectrometry within speci- buted all across the country (Leal, 2011), which occurred fied polygons, herein referred to as blocks (Figure 2). between the early Paleozoic and the Pleistocene and were Data acquisition was performed with instruments related to magmatic belts, island arcs of variable compo- mounted on “fixed wing” aircraft, with line spacings of sition and intracratonic environments. 500 m (Andes and Bolivar regions) and 1,000 m (Ama- zon) and a flight height of 100 m above terrain over flat 3. G eophysical methods used areas. In the mountainous areas, a “drape” survey design was used with heights between 100 and 300 m or more, Airborne geophysics is a very useful method to increase securing aircraft safety and good data quality. The on- geological knowledge of large and remote areas such as board geophysical equipment included high-sensitivity the Amazonian region of Colombia, not only for geologi- magnetometers that recorded the variations in the Earth´s cal mapping but also for potential assessment of mineral magnetic field, 512- to 1,024-channel gamma spectrome- resources. For this purpose, the SGC began an extensi- ters with 2,056 in3 of downward-detection crystals and ve airborne geophysical survey that, by the end of 2019, high-definition GPS and radar altimeters to ensure the had accumulated a coverage of nearly 550,000 km2 in the quality of the raw data. The sampling rates were 10 Hz for Andean, Caribbean and Amazonian regions, collecting magnetometry and 1 Hz for gamma spectrometry, measu-

1 Magdalena

10°0'0"N Sucre Cesar Panama Venezuela Córdoba 5

3 8 10 4 Santander Arauca

6°0'0"N Boyacá Casanare 15 Chocó 2 17 Cundinamarca

6 13 Tolima Meta 14 Huila 9

Cauca Guaviare 11 2°0'0"N Nariño

Putumayo Caquetá

Amazonas Ecuador 12 Airborne 0 100 200 Brazil Geophysics Blocks Km 2°0'0"S Caribbean Zone: 1) Cesar - Perija Andean Zona: 2) Andes_N, 3), Antioquia_E, 3) Antioquia_W,Peru 4) Bolivar, 5) Buenaventura, 6) Cordoba, 7) Darien, 8) Garzon, 9) Uraba, 10) Uraba Eastern Zone: 11) Amazonas_N, 12) Amazonas_S, 13) Guainia, 14) Guiania_E, 15) Vichada, 16) Vichada_N, 17) Vichada_W

80°0'0"W 76°0'0"W 72°0'0"W 68°0'0"W

Figure 2. Blocks selected for airborne geophysics surveys

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ring geophysical data every 7 to 9 m and 70 to 90 m, res- 1972). Additionally, other transformations, such as the tilt pectively. The 500 m flight line blocks were processed into derivative (Miller and Singh, 1994) and composite ternary 125 x 125 m grids, and the 1,000 m blocks were processed images of different order vertical derivatives (1, 1.25, and into 250 x 250 m grids. The processed grids included to- 1.5 order), are used to locate anomalies and magnetic do- tal magnetic field anomaly (TFA) and concentrations of mains from a consistent interpretation of the data and to potassium (K, %), uranium (U, ppm) and thorium (Th, relate these anomalies to a geological framework, such as ppm). All postprocessing and interpretation were perfor- intrusions, dikes, shear zones, kimberlites, carbonatites, med by geophysicists and external advisors of the SGC. etc., that may host mineral resources. The magnetometry method, used as an exploration Gamma spectrometry data are useful to identify and tool, evaluates the lateral variations in the Earth’s mag- correlate outcropping lithologies and anomalies in ra- netic field that are assumed to be a response to the va- dioactive element distributions that may be related to riations in the amounts of magnetic minerals contained secondary geological processes of mineral enrichment in the rocks. These changes in magnetic mineral content or depletion, such as hydrothermal alteration zones and could mark variations in the rock type or internal changes weathering zones. The use of gamma spectrometry ima- due to other geological processes, such as hydrothermal ges (potassium, uranium or thorium) and ternary red, alteration or metamorphism. Therefore, magnetometry green, blue (RGB) images (red = K, green = Th, blue = data are useful to identify magnetic bodies covered by U) with lithological information and magnetic anomalies nonmagnetic lithologies, to delineate geological structu- enhances the geological interpretation and the possible res and to target exploration on the increase/decrease in types of related mineralization. magnetic mineral content related to the most common For the data processing, interpretation and target se- geological mechanisms for ore formation: magma intru- lection from geophysical airborne data, a procedure to sion, faulting and hydrothermal alteration processes. generate standardized information for the entire area co- Gamma spectrometry data reflect the relative concen- vered is established. The suggested steps are as follows: trations of the common radioactive elements uranium (U), thorium (Th) and potassium (K) that are naturally »» Calculate the RTP and/or the analytical signal of the concentrated through magmatic differentiation. This TFA process generates the highest concentrations of these ele- »» Calculate the tilt angle derivative. ments in acidic rocks as well as in its metamorphic and »» Calculate vertical derivatives of the RTP and display sedimentary-related lithologies. Because of the relation them on a ternary RGB image (R = lowest order, G = between natural radioactivity and the mineralogy of li- middle order and B = highest order) thological units, gamma spectrometry helps in the “li- »» Make a ternary image (RGB) of the relative distribu- thogeophysical” characterization of surface materials and tions of the concentrations of radioactive elements allows the determination of secondary alteration proces- potassium (R), thorium (G) and uranium (B). ses; such processes change the geochemical composition »» Select magnetic anomalies and create a regular win- of the rocks because of the emplacement of magma bo- dow (polygon) around each anomaly. dies, which triggers processes that result in ore deposits. »» Run inverse 3D modeling of each selected anomaly. For this case, the algorithms used are those inclu- 4. Data processing and anomaly selection methodology ded in Oasis Montaj “magnetization vector inversion (MVI, Ellis et al., 2012)” included with the VOXI ex- The analysis of the magnetometry information aims to de- tension using iterative reweighting inversion (IRI) fo- tect and describe data attributes such as geometry, intensity cusing with 2 passes to sharpen positives. and orientation that may represent anomalies reasonably related to geological features. For this purpose, it is useful Considering this information, every 3D model is des- to reduce the natural complexity of the magnetic anomalies cribed (geometry of the magnetic source, depth to top, by reduction to pole transformation (RTP; Baranov 1957) etc.); then, radiometric data and topography are checked or by computation of the analytical signal (AS, Nabighian to identify whether the source crops out or whether a

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feature related to the source is modeled. In addition, the also to generate multiple exploration targets to aid in the available geology is checked to see if the calculated source development of the mineral potential of Colombia. has already been mapped, and other mining (deposits, occurrences) and geochemical (soil, sediment, rock) data 5.1 Andes anomaly are integrated to refine the interpretation. The Andes anomaly is located on the eastern flank of the Central Cordillera of Colombia, where the Triassic core 5. Examples of anomalies targeting and interpretation of the Andes Cordillera and the Cretaceous Antioquia Batholith form a block limited by the nearly N-S trending The procedure presented above was applied to three ano- Palestina Fault System to the east and the Romeral Fault malies located in different tectonic and geological settings System to the west (Gómez et al., 2015). (Figure 3): the first one in the Central Andes Cordillera, the second one in the western foothills of the San Lucas 5.1.1 Geology Range and the last one in the Guyana shield. These ano- Locally, the southern part of the area corresponds to the malies were used not only to demonstrate the potential northernmost outcrop of the Antioquia Batholith (Kgd, of airborne magnetometry and gamma spectrometry for Figure 4), with some mafic bodies to the north (Kg), ca- increasing the geoscientific knowledge of wide areas but taclastic granites (Kgn), altered lava flows (Krv) and the

SUCRE2 2 ALTOS 8°48'0"N PINILLOS DEL ROSARIO La Guajira Magdalena

Cesar 8°40'0"N 10°0'0"N Panama 22 Venezuela Sucre TIQUISIO MAJAGUAL (Puerto Córdoba Bolívar ACHÍ Rico) 0 6,25 12,5

8°32'0"N Km

Antioquia 74°32'0"W 74°24'0"W 74°16'0"W 1 Santander Arauca YARUMAL 11 ANGOSTURA GUADALUPE 6°53'30"N

6°0'0"N Boyacá Chocó Casanare AMALFI

Cundinamarca Vichada

Tolima 6°48'0"N Valle Del CAROLINA Cauca Meta Huila Guainía YOLOMBÓ SANTA ROSA Cauca DE OSOS GÓMEZ PLATA 6°42'30"N 0 105 Guaviare Km 2°0'0"N Nariño 75°24'30"W 75°19'0"W 75°13'30"W 75°8'0"W Vaupés Putumayo Caquetá 3

1°52'0"S 3 LA PEDRERA

Brazil Ecuador Amazonas 2°0'0"S

33 2°0'0"S

Peru 2°8'0"S

PUERTO ARICA 0 100 200 TARAPACÁ Areas of Interest Km 0 9,25 18,5

2°16'0"S Km

76°0'0"W 72°0'0"W 68°0'0"W 70°40'0"W 70°32'0"W 70°24'0"W 70°16'0"W 70°8'0"W 70°0'0"W

Figure 3. Locations of the selected anomalies: Andes (1), San Lucas (2) and Guyana shield (3)

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75°19'30"W 75°18'0"W 75°16'30"W 75°15'0"W 75°13'30"W 75°12'0"W 75°10'30"W nonmagnetic San Pablo Formation (Kap), a Cretaceous Kgn sedimentary unit frequently intruded by the batholith. Kg Kqd es Qal Krv Specifically, the anomaly is related to a quartz-rich dacitic 6°51'0"N 6°51'0"N porphyry (Kda), strongly linked to the edge facies of the es Kap 6°49'30"N Kap 6°49'30"N batholith (Hall et al., 1972). Guadalupe

Kap Kqd

6°48'0"N Kda 6°48'0"N Kqd 5.1.2 Anomaly description and 3D modeling Kap The Andes anomaly is detected as a strong (326 nT peak- Qal 6°46'30"N 6°46'30"N to-peak amplitude in the TFA) and large (6 km diameter) Kap magnetic anomaly located in an area containing several Qal mineral deposits (López et al., 2018). The gold anomaly 6°45'0"N Qal 6°45'0"N Qal

occurrence (yellow dot, Figures 5a and 5b) in the center 0 2 4 Carolina Km 6°43'30"N 6°43'30"N of the anomaly is important to prioritize its study. Fur- 75°19'30"W 75°18'0"W 75°16'30"W 75°15'0"W 75°13'30"W 75°12'0"W 75°10'30"W thermore, on the tilt derivative image (Figure 5c), domi- Figure 4. Geology of the area containing the Andes anomaly: nant nearly NE-SW lineaments are identified, which may Antioquia Batholith (Kgd), mafic bodies (Kg), cataclastic granites represent a structural control on possible fluid migration (Kgn), altered lava flows (Krv) and the nonmagnetic San Pablo Formation (Kap) and quartz-rich dacitic porphyry (Kda) and/or emplacement. Modified from Hall et al. (1972)

a b

Figure 5. Magnetometry images of the Andes anomaly: a) Total field anomaly (TFA), b) analytical signal (AS) of the TFA, c) tilt derivative image with magnetic lineaments. The yellow circle represents the location of the sample with a high gold content

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The ternary radiometric image (Figure 6) shows that 5.1.3 Mineral resource potential the magnetic anomaly is coincident with a rock outcrop The Andes anomaly is located within the Guadalupe Au- that presents high grades of Th, U and K, generating a (Ag) metallogenic district and east of the Amalfi-Anorí white color in the image. metallogenic district (López et al., 2018). The Guadalupe The 0.03 SI cutoff isosurface from the 3D magnetic district is characterized by the presence of intrusive-related susceptibility MVI model depicts a large (7 x 3.5 km) deposits and includes a copper volcanogenic massive sulfide E-W elongated magnetic body (Figure 7). The model (VMS) deposit known as the Guadalupe project (Figure 8). shows some portions close to the surface or cropping out, For this area, geochemical data report 144,000 ppb of gold but clearly, a large part of the body is below the surface. from a vein inside the most magnetic portion of the anomaly The follow-up procedure for this anomaly could in- and 58.2 ppm of silver within a neighboring dike. Due to the clude fieldwork for rock sampling and a regular sampling metallogenic context and known deposits/occurrences in grid for geochemical analysis. The data obtained can gui- the area of the Andes anomaly, the exploration target could de the next steps in the evaluation of the anomaly. be an intrusion-related/VMS and/or epithermal deposit.

233 6°52'0"N 112 231 b) K U Th Combination Code 223 222 111 333 KLow ULow ThLow COMBINATIONKl-Ul-Thl CODE111

111 LowLow LowLow LowMedium Kl-Ul-ThlKl-Ul-Thm 111112 Low Medium Medium Kl-Um-Thm 122 233 222 Low Low Medium Kl-Ul-Thm 112 123 Low Medium High Kl-Um-Thh 123 6°50'0"N 122 Low Medium Medium Kl-Um-Thm 122 231 Medium Low Medium Km-Ul-Thm 212 333 Low Medium High Kl-Um-Thh 123 223 Guadalupe Medium Medium Medium Km-Um-Thm 222 222 Medium Low Medium Km-Ul-Thm 212 233 Medium Medium High Km-Um-Thh 223 222 Medium Medium Medium Km-Um-Thm 222 Medium High Low Km-Uh-Thl 231 332 MediumMedium MediumHigh HighHigh Km-Um-ThhKm-Uh-Thh 223233 6°48'0"N Medium High Low Km-Uh-Thl 231 223 322 High Low Medium Kh-Ul-Thm 312 122 112 MediumHigh HighMedium HighMedium Km-Uh-ThhKh-Um-Thm 233322 122 222 HighHigh LowHigh MediumMedium Kh-Ul-ThmKh-Uh-Thm 312332 333 HighHigh MediumHigh MediumHigh Kh-Um-ThmKh-Uh-Thh 322333 122 111 222

6°46'0"N 111 High High Medium Kh-Uh-Thm 332 High High High Kh-Uh-Thh 333 233 123 112 112 333 123 222 222 0 3,35 122 6°44'0"N Carolina 212 Km 333 122

75°20'0"W 75°18'0"W 75°16'0"W 75°14'0"W 75°12'0"W 75°10'0"W

Figure 6. Ternary radiometric image of the Andes anomaly with contoured radiometric domains. The yellow circle represents the location of the 144.000 ppb Au sample

12 BOLETÍN GEOLÓGICO 46

Boletin Geologico 46.indd 12 7/07/20 2:13 p. m. Interpretation of geophysical anomalies for mineral resource potential evaluation in Colombia: Examples from the northern Andes and Amazonian regions 1252000 a)a b)b 1248000

Guadalupe 1244000 1240000

Magna-Sirgas Bogota Zone 0 3,35 Km Carolina 1236000

860000 864000 868000 872000 876000 880000

Figure 7. 3D model of the Andes anomaly: a) Analytical signal from the TFA with 0.3 SI MVI model, b) 0.3 SI isosurface of the 3D magnetic susceptibility model from MVI inversion

2 3 4 5 4 1 4 10°0'0"N 4 Legend 5 Venezuela 6 Municipalities 7°7'30"N Fault System 6°0'0"N Segovia 8 Anori Area of Interest 4 5 5 Remedios Mining District: 3 2°0'0"N 2 Guadalupe 3 1 Other Mining Districts Campamento Ecuador 5 Brazil 8 2°0'0"S 8 Peru 6°57'0"N Amalfi 76°0'0"W 72°0'0"W 68°0'0"W

3 2 MINERAL DEPOSITS AND OCCURRENCES

1 EPITHERMAL PLACER Guadalupe Unclassified epithermal Superficial Placer Au 5 Sb (Zn-Cu-Pb-Fe) RESIDUAL DEPOSITS Vegachi Unclassified epithermal Au Laterites Ni INTRUSION-RELATED 6°46'30"N Carolina 1 5 Unclassified intrusion-related ROCKS AND INDUSTRIAL MINERALS Au Ultramafic Asbestos OROGENIC Gomez plata UNCLASSIFIED DEPOSITS Yali Orogenic veins Au 1 VOLCANOGENIC Gold - Au 0 10 6 Unclassified Volcanogenic Cu 2Km Volcanogenic "Beshi" Cu

75°10'30"W 75°0'0"W 74°49'30"W 74°39'0"W

Figure 8. Mineral deposits and metallogenic districts near the Andes anomaly (yellow boxes); the numbers represent the local geology: 1) Felsic igneous rocks (Cretaceous), 2) felsic igneous rocks (Jurassic), 3) mafic and ultramafic igneous rocks (Cretaceous), 4) gneiss and intrusive rocks (Permian-Triassic), 5) low-grade metamorphic rocks (Triassic), 6) high-grade metamorphic rocks (Proterozoic), 7) sedimentary rocks (Cenozoic), 8) volcanic and volcanic-sedimentary mafic rocks (Cretaceous) Modified from López et al. (2018)

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5.2 San Lucas Range Anomaly area of low magnetic contrast related to the sedimentary This anomaly is located in the northernmost part of the cover; the anomaly is limited to the east by a strong gra- Central Cordillera in the western foothills of the San Lu- dient area related to the Palestina fault-controlled transi- cas Range at the confluence of the Magdalena and Cauca tion to the San Lucas Range crystalline rocks (Figures 10a rivers. and 10b). The tilt derivative image (Figure 10c) shows predominantly NNE-SSW-oriented lineaments, one of 5.2.1 Geology which delineates the Palestina Fault located east of the Nonmagnetic floodplain and lacustrine deposits bounded anomaly and other of which is located west of the ano- to the south by the last outcrops of the Central Cordillera maly. This structural trend interpreted from the geophy- and some Cenozoic deposits cover the San Lucas Range sical data suggests that the magnetic source “emplace- anomaly. To the east of the Palestina Fault is mapped the ment” was probably controlled between these structures. gneiss of the San Lucas Formation (MPsl), a gneiss often There is no relation between the gamma spectrome- intruded by igneous bodies and covered by Quaternary try data and the magnetic anomaly, which suggests that deposits and the Sudan Formation (T3s), which consists there could not be a surface expression of the causative of very thick beds of conglomerates intercalated with in- magnetic body (Figure 11). However, gamma spectrome- tervals of clay limestone and beds of tuffaceous sandsto- try data themselves reflect changes in the surface mate- nes (Figure 9). rials that are useful for lithogeophysical mapping, such as changes in the radioactive element contents of the se- 5.2.2 Anomaly description and 3D modeling dimentary deposits, delineation of crystalline rocks cro- The magnetic anomaly corresponds to an isolated dipole pping out to the east of the Palestina Fault and even fault with a 1,000 nT peak-to-peak amplitude (TFA) within an delineation itself.

74°30'15"W 74°28'30"W 74°26'45"W 74°25'0"W 74°23'15"W 74°21'30"W 74°19'45"W 74°18'0"W

Qfal Qfal Qfl Qfl 8°43'45"N 8°43'45"N

Qfl Qfal

Qfal Qfl

Qfal

8°42'0"N Qfc 8°42'0"N T3s

Qfal

8°40'15"N Qfl 8°40'15"N Qfl Qfl Qfal Qfal

Qfal

Qfl 8°38'30"N 8°38'30"N

Qfc Qfc MPsl T3s Qfl Qcal Colorado Fault

T3s Qcal Qfal 8°36'45"N Qfl MPsl 8°36'45"N Palestina Fault (Geomagnetic lineament) Qfl Qco Qcal 0 2,2 4,4 Km MPsl Qfal

74°30'15"W 74°28'30"W 74°26'45"W 74°25'0"W 74°23'15"W 74°21'30"W 74°19'45"W 74°18'0"W

Figure 9. Geology of the San Lucas anomaly: Floodplain (Qfal) and fluvio-lacustrine (Qfl) deposits, fluvial channel (Qfc) deposits, fan and terraces (Qcal) deposits, San Lucas gneiss (MPsl), Sudán Formation (T3s) Modified from Ingeominas (2006)

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a b

Figure 10. Magnetometry images of the San Lucas anomaly: a) Total field anomaly (TFA), b) analytical signal of the TFA, c) tilt derivative image with magnetic lineaments (black lines) and Palestina Fault (yellow line)

K U Th Combination Code K U TH COMBINATION CODE Low Low Low Kl-Ul-Thl 111 8°45'0"N Low Low Low Kl-Ul-Thl 111 323 233 232 Low Low Medium Kl-Ul-Thm 112 111 232 222 333 111 Low Low Medium Kl-Ul-Thm 112 212 122 Low Medium Low Kl-Um-Thl 121 222 233 LowLowMedium MediumMedium LowKl-Um-Thm Kl-Um-Thl122 121 112 LowLow MediumHigh MediumLow Kl-Uh-Thl Kl-Um-Thm131 122 121 MediumLowLow HighLow LowKm-Ul-Thl Kl-Uh-Thl211 131 8°42'30"N 111 111 111 MediumMediumLow LowMedium LowKm-Ul-Thm Km-Ul-Thl212 211 122 MediumMediumMedium Low MediumKm-Um-Thm Km-Ul-Thm222 212 111 112 333 233 MediumMediumMedium Medium MediumHigh Km-Um-Thh Km-Um-Thm223 222 111 111 232 121 111 MediumMedium MediumHigh Medium HighKm-Uh-Thm Km-Um-Thh232 223

8°40'0"N 223 222 131 MediumMediumHigh High MediumHigh Km-Uh-Thh Km-Uh-Thm233 232 222 MediumHigh Medium HighHigh HighKh-Um-Thh Km-Uh-Thh323 233 111 High High High Kh-Uh-Thh 333 212 High Medium High Kh-Um-Thh 323 323 Palestina Fault121 (Geomagnetic lineament) 323 222 111 111 High High High Kh-Uh-Thh 333 122 Colorado Fault 8°37'30"N 333 232 131 121 111 111 0 4,5 212 211Km 323 333 111 111 111 8°35'0"N

74°30'0"W 74°27'30"W 74°25'0"W 74°22'30"W 74°20'0"W 74°17'30"W

Figure 11. Ternary radiometric image of the San Lucas anomaly with contoured radiometric domains

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Since there are no outcrops and no topographic or ra- flows into the Amazon River and is covered by a dense diometric expressions, magnetic 3D modeling is the only tropical rainforest. choice to estimate the parameters of a possible buried body of interest. The MVI model with a cutoff magnetic 5.3.1 Geology susceptibility of 0.03 SI shows a “pipe-like” body of 2 x 3 The surface geology corresponds to nonmagnetic Neo- km (Figures 12a and 12b), located at 100 m depth. gene rocks with ferruginous matrixes (conglomerates, sandstones, claystone, and in some cases coal) that are 5.2.3 Mineral resource potential poorly consolidated and dissected by the principal dra- The San Lucas anomaly is located 25 km to the west of inages of the area (Figure 14). The magnetic basement the Au-(Ag) metallogenic district of Barranco de Loba rock can be related to the Mitú Complex (Rodríguez et (López et al., 2018). This district is characterized by the al., 2011), composed of subalkaline high-potassium roc- presence of Au/Cu porphyries such as the San Carlos ks, in some places with enrichments in rare earth element Project and intrusion-related and epithermal deposits (REE) minerals. (Figure 13). The metallogenic context of the area and the semicircular pipe-like shape of the magnetic source 5.3.2 Anomaly description and 3D modeling allows us to estimate that the corresponding target of this This single magnetic anomaly has a very strong (2,000 nT) anomaly is more likely to be a porphyry system than a peak-to-peak amplitude (TFA) and lacks surface expres- kimberlite/carbonatite-related body. Therefore, addi- sion (Figures 15a and 15b). Through a flight with 1,000 m tional exploration programs for this anomaly should be line spacing, it appears in four flight lines. Because such airborne gradiometric gravity or ground gravity and air- anomalies are not expected in the sedimentary rocks that borne or ground EM. crop out and because there is neither radiometric signature nor geological evidence of a causative body coincident 5.3 Amazonas anomaly with the magnetic anomaly (Figure 16), 3D modeling of This anomaly is found in the southeastern part of Colom- the magnetometry is used to estimate the shape, depth and bia, in an area of low relief with a drainage system that other parameters of the subsurface body. 1459000 a)a b) b 1453500 1448000

Magna-Sirgas Colorado Fault Bogota Zone Palestina Fault (Geomagnetic lineament) 0 4,5 Km 1442500

951500 957000 962500 968000 973500 979000

Figure 12. 3D model of the San Lucas anomaly: a) TFA superimposed with the 3D framework of the susceptibility model with a cutoff of 0.3 SI, b) cross section of the magnetic susceptibility model from 3D MVI inversion with the 3D image of the RTP

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Hatillo de loba

8°57'30"N 4 Pinillos

6 4 Legend 10°0'0"N Venezuela 4 BR A Municipalities 8 Z O 6 Fault System 6°0'0"N Sucre D ELOB Altos del A A rosario Drainage C U 6 A 8°48'0"N 4 C Area of Interest 2°0'0"N

O 4 1 Í R Mining District: Barranco de Loba Ecuador 1 Brazil 2°0'0"S 1 Peru

4 6 9 6 76°0'0"W 72°0'0"W 68°0'0"W

8°38'30"N 7 9 MINERAL DEPOSITS AND OCCURRENCES Achi 4 Puerto rico 1 Majagual 5 EPITHERMAL PLACER 6 Unclassified epithermal Au Superficial Placer Au 5 Low to intermediate sulfidation 6 epithermal Au INTRUSION-RELATED Guaranda 6 Unclassified intrusion-related 8°29'0"N PORPHYRIES Au Calc-alkaline porphyry Au Associated to intrusives veins/alkalin 5 breccia (Au) 9 4 0 10 4 Km6 1 4 74°42'30"W 74°33'0"W 74°23'30"W 74°14'0"W 74°4'30"W

Figure 13. Mineral deposits and occurrences near the San Lucas anomaly (red box); the numbers represent the local geology: 1) Surface deposits (Quaternary), 2) Cenozoic sedimentary rocks (Paleocene-Pliocene), 3) Cretaceous sedimentary rocks (Berriasian-Albian), 4) felsic igneous rocks (Jurassic), 5) subvolcanic rocks (Jurassic), 6) volcanic and volcanic-sedimentary rocks (Permian–Upper Jurassic), 7) continental siliciclastic rocks (Triassic?), 8) metamorphic rocks (Triassic), 9) high-grade metamorphic rocks (Proterozoic) Modified from López et al. (2018)

70°30'30"W 70°27'0"W 70°23'30"W 70°20'0"W 70°16'30"W 70°13'0"W 70°9'30"W

Q-al N1-Sc 1°58'30"S 1°58'30"S 2°2'0"S 2°2'0"S

N1-Sc

í R 2°5'30"S 2°5'30"S

Qu eb rad a A 2°9'0"S gu 2°9'0"S a B lan ca Q-al

0 3,5 7 Km

70°30'30"W 70°27'0"W 70°23'30"W 70°20'0"W 70°16'30"W 70°13'0"W 70°9'30"W Figure 14. Geology of the Amazonas anomaly: Neogene deposits with ferruginous matrix (N1-Sc), alluvial and alluvial plain deposits (Q-al) Modified from Gómez et al. (2015)

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1°57'15"S a)a 1°57'15"S b)b 2°1'30"S 2°1'30"S

2°5'45"S Quebrada Agua Blanca 2°5'45"S Quebrada Agua Blanca

Río Puré Río Puré

2°10'0"S 0 6,5 2°10'0"S 0 6,5 Km Km

70°29'30"W 70°25'15"W 70°21'0"W 70°16'45"W 70°12'30"W 70°8'15"W 70°29'30"W 70°25'15"W 70°21'0"W 70°16'45"W 70°12'30"W 70°8'15"W

Figure 15. Magnetometry images of the Amazonas anomaly: a) Total field anomaly (TFA), b) reduction to magnetic pole (RTP) of the TFA

211 1°57'15"S 233 111 323 111 K U Th Combination Code 232 323 KLow ULow ThLow COMBINATIONKl-Ul-Thl CODE111 232 211 R LowLow LowMedium LowLow Kl-Ul-ThlKl-Um-Thl 111121

í o 322 LowMedium MediumLow LowLow Kl-Um-ThlKm-Ul-Thl 121211 P 222 u MediumMedium LowLow LowMedium Km-Ul-ThlKm-Ul-Thm 211212 r 333 é 2°1'30"S 211 212 MediumMedium LowMedium MediumLow Km-Ul-ThmKm-Um-Thl 212221 111 211 MediumMedium MediumMedium LowMedium Km-Um-ThlKm-Um-Thm 221222 323 221 MediumMedium MediumHigh MediumMedium Km-Um-ThmKm-Uh-Thm 222232 MediumMedium HighHigh MediumHigh Km-Uh-ThmKm-Uh-Thh 232233 Medium High High Km-Uh-Thh 233 232 323 High Medium Medium Kh-Um-Thm 322 233 111 111 HighHigh MediumMedium MediumHigh Kh-Um-ThmKh-Um-Thh 322323 2°5'45"S 211 Qu 222 High Medium High Kh-Um-Thh 323 eb 322 High High High Kh-Uh-Thh 333 ra 212 da High High High Kh-Uh-Thh 333 A 121 g 333 ua Bl an 211 ca 111 111 211 2°10'0"S 2330 6,5 Km 211 212

70°29'30"W 70°25'15"W 70°21'0"W 70°16'45"W 70°12'30"W 70°8'15"W

Figure 16. Ternary radiometric image of the Amazonas anomaly with contoured radiometric domains

Figures 17a and 17b show the results of the 3D MVI dipole in the TFA anomaly (Figures 15a and 17b) could inversion with a magnetic susceptibility cutoff of 0.003 SI. indicate that the magnetic body has a complex magneti- The isosurface shows a 4 km x 3.5 km magnetic source zation (remanence). located close to the surface. Additionally, the shape of the

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260000 Quebrada Agua Blanca

Río Puré

254500 Magna-Sirgas East Zone 0 6,5 Km 249000

1061500 1067000 1072500 1078000 1083500 1089000 1094500 1100000

Figure 17. 3D model of the Amazonas anomaly. a) TFA superimposed with the 3D framework of the susceptibility model, b) cross section of the magnetic susceptibility model from 3D MVI inversion and the 3D image of the RTP

5.3.3 Mineral resource potential kind of geological setting, magnetometry has advantages The average depth of the modeled source (top of the over other geophysical methods due to the strong correla- body) is 40 m, and it is not possible to determine either tion between magnetic mineral contents and ore-deposit the source petrology of the source or the economic value. generation. In this work, the processing and interpreta- Additionally, the anomaly is in an area with poor geolo- tion of the magnetometric data focus on going beyond the gical knowledge due to the dense rainforest coverage and anomaly detection itself to more specialized modeling of lack of infrastructure. the magnetic source geometrical parameters and struc- However, isolated magnetic bodies located within the tural framework, integrated with gamma spectrometry sedimentary cover in the Amazonian Craton have the and available geological, geochemical and metallogenic potential to be alkaline rocks. Kimberlites (diamonds) data. This integration methodology helps in the identi- and carbonatites (phosphate, niobium, and REEs) are the fication, characterization and fieldwork prioritization of main economic targets of anomalies. In this case, roc- the magnetic anomalies found in the blocks covered by ks of the Mitú Complex have REEs, niobium (Nb), and the airborne surveys. tantalum (Ta) minerals associated principally with gra- The Andes anomaly is located at the northern edge of nitic pegmatites, dikes and acidic stocks, which intrude the Antioquia Batholith. The MVI magnetic susceptibility the complex and could also be the source of tin (Sn) and model shows a nearly 7 km E-W elongated body located tungsten (W) (López and Cramer, 2012). mostly at depth but with some possible outcrops related A mineral exploration program should apply other to a radiometric signature. The local geology identifies a methods and then drill the anomaly to find the source. Qz-rich dacitic porphyry (Kda, Figure 4) in the area of The additional methods suggested for this anomaly are i) the anomaly, so it is possible that the magnetic source co- ground magnetics and ii) airborne gradiometric gravity rresponds to the subsurface extent of this intrusive rock. or ground gravity. This indication should increase the potential for mineral resource exploration beyond the outcropping portion of 6. Discussion the rock. The presence of 144,000 ppb gold value in a vein sample that is located at the center of the magnetic source As explained above, the metallotectonic environment and also suggests important potential but demands more de- known mineral deposits in the NW Andes and Colombia tailed work. The location of intrusion-related/VMS Au/ reflect a subduction-related affinity with magma genera- Cu deposits close to the anomaly and the delimitation of tion and emplacement. For exploration purposes in this a metallogenic district in the area are key facts to priori-

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tize the study of this anomaly. The data processing also sence of detailed geological, geochemical or metallogenic allows the identification of NE-SW lineaments that re- information, it is possible to extrapolate the evidence and present a structural control on a possible fluid migration deposits discovered in other cratonic areas (Brazil and and/or emplacement of a potential ore. Venezuela) and expect that this kind of nearly cylindrical The San Lucas Range anomaly is covered by non- and isolated magnetic anomaly is related to alkaline roc- magnetic fluvial and lacustrine deposits of recent age. ks, kimberlites or carbonatites. To the east of the anomaly, separated by the Palestina It is important to remember that geophysical data, Fault (NNE-SSW), metamorphic rocks of the San Lucas for mineral exploration purposes, imply the estimation gneiss (MPsl, Figure 9) crop out. There is no surface or of physical properties of the rocks and materials of the gamma spectrometric expression related to the anomaly, subsurface by indirect measurement of the perturbation which supports the idea that the magnetic source is bu- of a natural field or an induced signal due to contrasts in ried under recent sediments. Magnetometry interpreta- these properties. Furthermore, the use of mathematical tion marks predominantly NNE-SSW lineaments such as modeling to quantitatively estimate the distribution of the Palestina Fault trend and suggests possible structu- the selected physical property that corresponds to the ob- ral control on the emplacement of the magnetic source, served perturbation has almost infinitely many different which appears to be limited by these lineaments to the solutions (the nonuniqueness principle) that can be redu- west and by the Palestina Fault to the east. The MVI mag- ced to more geologically related models when integrated netic susceptibility model shows a 2 km x 3 km “pipe-li- with field data. ke” body located close to the surface, which could indica- In this context, the magnetic susceptibility cutoff used te that the magnetic source is not related to the magnetic to represent the dimension and geometry of each of the igneous-metamorphic basement and probably is a mag- magnetic sources (0.03 SI for the Andes and San Lucas matic body intruded within less magnetic rocks. There is anomalies and 0.003 SI for the Amazonas anomaly) from no direct evidence of fertile rocks or mineralization over the MVI are estimated with the analytical signal (AS) of the anomaly, but 25 km to the east, the Barranco de Loba the TFA. The AS is a mathematical transform that redu- metallogenic district is delimited, characterized by the ces the dipolar signature of a magnetic anomaly and has presence of several Au (Cu) porphyry, intrusion-related the effect of strengthening at the edges of the magnetic and epithermal deposits and occurrences. The size and source (Nabighian, 1972). Using this attribute, the mag- shape of the MVI model of the magnetic source and the netic susceptibility from the model is chosen so that the regional metallogenic context allow us to estimate that surface expression of the isosurface is close to the borders this anomaly could be related to an intrusive body with of the AS of the anomaly. For this reason, the magnetic exploration potential for porphyry, intrusion-related susceptibility used for this work must be considered only and/or epithermal deposits. a reference parameter estimated by mathematical criteria The Amazonas anomaly is located in a very remote and has to be compared with petrophysical data. Howe- area of the Amazon rainforest where geological and me- ver, it is still a reliable and useful approximation in the tallogenic knowledge is restricted to regional extrapola- absence of petrophysical data collected from rocks in the tion from sparse observation points. There is no surface field. or gamma spectrometry evidence that can be related to For the anomalies and models presented in this work, the anomaly. The magnetic anomaly shows a complex the Andes anomaly has strong geological, geochemical dipole with a very strong (2,000 nT) amplitude that sug- and metallogenic controls that allow us to demonstrate gests that the magnetic source could have strong remnant the mineral potential in this area, and the 3D modeling magnetization and be located within almost nonmagne- of the magnetic source reinforces this potential. For the tic rocks. Additionally, the dipole shows sharp lobes, sug- San Lucas and Amazonas anomalies, the lack of other gesting that the source is buried very close to the surface. geoscientific data must encourage exploration in these The MVI magnetic susceptibility model shows a 2.5 km x areas because geophysical modeling shows that there are 3 km source located close to the surface. Even in the ab- magnetic sources that have to be explained, and given our

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actual knowledge of the area, there is no evidence of po- emplacement of the source; one lineament is correlated tential other than the geological setting and, in the case with the Palestina Fault in the east. The presence of the of the San Lucas anomaly, the known mineral deposits in Barranco de Loba Au (Cu) metallogenic district 25 km to the surroundings. the east and the regional metallogenic context mark this The results presented in this work illustrate the me- anomaly as a target for porphyry/intrusion-related and thodology used to manage geophysical data (magneto- epithermal deposits. metry and gamma spectrometry) for the evaluation of the The Amazonas anomaly MVI model (0.003 SI) shows mineral resource potential in Colombia. The three ano- a nearly cylindrical 2.5 km x 3 km body, surrounded by al- malies presented are located in different geological con- most nonmagnetic rocks and located close to the surface texts within the country and represent strong evidence but with no topographic or gamma spectrometric corre- that the integration of available geological, geochemical lation. Geological knowledge about the area is minimal, and metallogenic data with advanced geophysical mo- but evidence from deposits identified in other areas of the deling will provide a better understanding of the nature, Amazonian Craton and the potential estimated for rocks geometry and subsurface distribution of potential targets of the Mitú complex allow us to consider this anomaly a and hence optimize efforts and resources in the fieldwork potential target for carbonatite/kimberlite deposits. phase. Considering that there must be very few outcrops of mineral deposits yet to be found, the supply for the mine- 7. C onclusions ral market must be obtained from underground deposits. This fact, added to the global requirements of environ- The acquisition of high-resolution airborne magnetome- mental protection, means that geophysical methods are try and gamma spectrometry generates valuable informa- increasingly fundamental in mineral prospecting. tion to increase the geoscientific knowledge of Colombia and allows target identification and interpretation for mi- Acknowledgments neral resource exploration. For this purpose, three magnetic anomalies were selected to illustrate the data pro- The authors would like to acknowledge the Servicio Geo- cessing methodology used for the identification of areas logico Colombiano (SGC) for allowing the use of airbor- of interest for future investigation. ne geophysical data from ongoing surveys and the inter- The Andes anomaly is interpreted as a magnetic (0.03 pretations presented in this document. SI) source 3.5 km x 7 km wide and located at depth with some surface expression correlated with gamma spectro- References metry. The geological data correlate this anomaly at the surface with a Qz diorite porphyry, suggesting that the Baranov, V. (1957). A new method for interpretation of aero- subsurface extent of the intrusion is wider. This anomaly magnetic maps: pseudo‐gravimetric anomalies. Geophys- is located within the Guadalupe (Au/Ag) metallogenic ics, 22 (2), 359-382. https://doi.org/10.1190/1.1438369. district and is a potential target for intrusion-related, Bustamante, C., Archanjo, C. J., Cardona, A., Bustaman- epithermal or VMS deposits. The evidence of magnetic te, A., and Valencia, V. A. (2017). U-Pb Ages and Hf lineaments and a vein sample with gold suggest structural Isotopes in Zircons from Parautochthonous Mesozoic control for fluid migration and ore deposition. Terranes in the Western Margin of Pangea: Implica- The San Lucas anomaly is modeled as a magnetic tions for the Terrane Configurations in the North- (0.03 SI) “pipe like” source 2 km x 3 km wide, located ern Andes. The Journal of Geology, 125 (5), 487-500. close to the surface and surrounded by almost nonmag- https://doi.org/10.1086/693014. netic rocks. There is no surface expression related to the Ellis, R. G., Wet, B., and Macleod, I. N. (2012). Inversion anomaly, and the geological data indicate the presence of of Magnetic Data from Remanent and Induced Sour- recent fluvio-lacustrine deposits. The magnetometry su- ces. 22nd International Geophysical Conference and ggests a possible control by NNE-SSW lineaments on the Exhibition, Australia.

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