minerals

Article
Application of Geophysical Methods in the Identification of
Mineralized Structures and Ranking of Areas for Drilling as
Exemplified by Alto Guaporé Orogenic Gold Province
Jorge Echague 1, * , Marcelo Leão-Santos 1,2 , Rodrigo Melo 3 , Thiago Mendes 1 and Welitom Borges 1

1 Institute of Geosciences, University of Brasília, Brasília 70910-900, DF, Brazil; marcelo.leao@unb.br (M.L.-S.);
thiagomendes.geof@gmail.com (T.M.); welitom@unb.br (W.B.)
2 Faculty of Planaltina, University of Brasília, Brasília 73345-010, DF, Brazil
3 Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, Rio Claro 13506-900, SP, Brazil;
rp.melo@unesp.br
* Correspondence: jorgetrovo15@gmail.com

Abstract: Mineral exploration works conducted in the Alto Guaporé Gold Province (AGGP), sit-
uated in the southwest region of the Amazon Craton in Brazil, faces the challenges of many gold
provinces around the world, i.e., declines in the discoveries of new economic deposits and increases
in exploration costs. Ground geophysical methods, combined with structural analyses and geolog-
ical mapping, are valuable tools that have potential to improve accuracy in selecting exploration
targets and in determining drilling locations. AGGP deposits are primarily associated with regional
N20◦ –W50◦ inverse faulting and sheared geologic contacts between Meso-Neoproterozoic siliciclastic
metasedimentary rocks and Mesoproterozoic basement (granite and volcano–sedimentary sequences).
Mining currently occurring in the central portion of the province drives exploration works towards
the many existing targets at the area. Among them, the ABP target is one of the most promising
for being located few kilometers north of the Pau-a-Pique mine. At the ABP target, gold is associ-
ated with hydrothermal alteration located in the sheared contacts and in the hinge zone of folded
Citation: Echague, J.; Leão-Santos, M.; metasedimentary sequence. Hydrothermal phases include Fe-oxides, sulfide (py), muscovite and
Melo, R.; Mendes, T.; Borges, W. quartz veins. In this study, we use magnetic and geoelectric (induced polarization) surveys coupled
Application of Geophysical Methods
with structural and geological mapping to identify potential footprints within the ABP target. The
in the Identification of Mineralized
results from induced polarization (IP) profiles successfully mapped the shape and orientation of
Structures and Ranking of Areas for
the main structures down to approximately 350 m at the ABP target, indicating potential locations
Drilling as Exemplified by Alto
for hydrothermal alteration hosting gold. Additionally, 3D magnetic data inversions illustrated
Guaporé Orogenic Gold Province.
Minerals 2024, 14, 788. https://
the distribution of magnetic susceptibilities and magnetization vectors associated with shear zone
doi.org/10.3390/min14080788 structures and isolated magnetic bodies. Magnetic data highlighted fault zones along the contacts
between metamorphic rocks and granites, while IP data identified areas with high chargeability,
Academic Editors: Stanisław Mazur
correlating with sulfidation zones mineralized with gold. These findings suggest a metallogenic
and Amin Beiranvand Pour
model where gold deposits are transported through deep structures connected to regional faults,
Received: 24 May 2024 implying significant tectonic and structural control over gold deposition. The results underscore the
Revised: 4 July 2024 potential of multiparameter geophysics in identifying and characterizing deposits in both deep and
Accepted: 12 July 2024 strike, thereby advancing our understanding of mineral occurrences in the region and enhancing the
Published: 31 July 2024
search for new mineralized zones.

Keywords: Alto Guaporé Gold Province (AGGP); gold exploration; magnetometry; induced polarization
(IP); fault zones; geological-geophysical prospecting model
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons 1. Introduction
Attribution (CC BY) license (https:// Although the discovery of new metallic mineral deposits, including gold, has declined
creativecommons.org/licenses/by/ on a global scale, costs have risen significantly in recent years, placing pressure on mineral
4.0/).

Minerals 2024, 14, 788. https://doi.org/10.3390/min14080788 https://www.mdpi.com/journal/minerals

Minerals 2024, 14, 788 2 of 23

exploration teams to enhance their precision through more meticulous target selection [1]
and a reduction in geological drilling expenses.
When entering an immature province, greenfield exploration teams typically deal
with limited information. Therefore, area selection must be based on the use of conceptual
models that are based on the mineral system concept and applied at the province scale. In
addition to this, the models must consider parameters of fertility, architecture, geodynamics
and preservation (e.g., [2]). An important parameter in this analysis is the matter of scale.
Gold deposits of different types (e.g., orogenic gold, Carlin type, iron oxide copper gold
ore deposits (IOCG), intrusion-related gold deposits (IRGD)) exhibit distinct characteristics
when analyzed individually. However, at the lithospheric scale, they may have been formed
in the same geodynamic context. Consequently, the individual analysis and ranking of
targets should be accompanied by an analysis of the tectonic context where these targets
are embedded. (e.g., [1]).
In more mature provinces, the challenge for brownfield exploration teams is to cor-
relate and interpret large volumes of data and generate new targets from existing data in
order to optimize work and reduce exploration costs. Roshanravan et al. [3] put forth a
methodology comprising the following steps: (a) correlation of available data; (b) identifi-
cation of key expressions and ingredients of mappable ore-forming processes, using spatial
data modeling (particularly inversion and modeling together with advanced geophysical
data filtering); (c) development of a conceptual model based on the mineral system concept;
(d) translation of the conceptual model into an effective tool in target generation.
In the western portion of the Brazilian state of Mato Grosso, situated in close proximity
to the international border with Bolivia, a series of Au + base metal deposits have been
identified. These include (1) gold deposits of the Alto Guaporé Gold Province in Mato
Grosso (e.g., [4–10]); (2) gold deposits of the San Ramon Gold Province in Bolivia; (3) the
Cu-Au-Ag polymetallic Don Mario deposit, Bolivia [11]; and (4) the Cu-Au polymetallic
Cabaçal deposit [12,13] considered a VMS by some authors (Figure 1). Although the
deposits in question are relatively small to medium in size, their occurrence in a geotectonic
context (i.e., near the Neoproterozoic margin of the Amazon Craton) serves to highlight the
region as an important exploratory frontier in South America. This is because the region has
large areas that remain unexplored alongside more developed provinces. The Alto Guaporé
Gold Province (AGGP) is the most extensively studied of the aforementioned deposits. The
province encompasses a series of gold deposits situated along a fold belt spanning over
500 km, designated the Aguapeí Belt [14–17]. The total resources are estimated at >1.8 Moz,
based on production, resources and reserves from exploration and mining activities over
the past 40 years [9]. However, these deposits have been exploited since the colonial period
(e.g., [18]).
These deposits, which were formed in the early Neoproterozoic (~920 Ma), are
interpreted as orogenic gold deposits generated by low-salinity aquo-carbonic fluids
(e.g., [4–7,9,18]). The mineralizing fluids originated from metamorphic processes involving
siliciclastic sedimentary sequences (Aguapeí Group) deposited in a rift basin during the
late Mesoproterozoic (e.g., [17,19–23]). AGGP deposits are hosted in second- or third-
order structures near the suture zone between the Paraguá Block and the Amazon Craton
(e.g., [24]). Mineralization styles include quartz veins with sulfides (e.g., pyrite, chalcopy-
rite, pyrrhotite) found in regional folds and shear zones at the contact between Aguapeí
metasediments and basement rocks ([9]).
The ABP target, situated 6 km to the north of the Pau-a-Pique mine, represents a strate-
gic exploratory site. Within the target, gold occurrences have been found within the contact
zone between the granites of the Pindaituba Intrusive Suite and the metasediments of the
Aguapeí Group, as well as within hydrothermally altered folded Aguapei metasediments.
The contact between the igneous basement and metasediments is marked by faulting
and shearing associated with the corridor shear zone ([25,26]). The ore zone consists of
quartz veins with hydrothermal muscovite, which host high-grade mineralization. The
Minerals 2024, 14, 788 3 of 23

hydrothermally altered folded Aguapeí metasediments are situated within the context of
the Caldeirão syncline hinge zone (e.g., [7–9,18,27]).
Despite being considered a mature province in terms of exploration, the Alto Guaporé
Gold Province (AGGP) lacks significant geophysical research at the district or deposit scale
compared to major orogenic gold provinces worldwide. Regionally, the existing literature
consists of the following: (1) seismology that investigates the deep crust in the region
(e.g., [28]); (2) regional studies in the neighboring Cabaçal deposit that employ potential
methods that utilize the contrast between magnetic and radiometric properties to identify
dioritic/porphyritic intrusions hosting gold, copper and zinc mineralization [29]; (3) studies
that apply inversion of regional aeromagnetic data to guide exploration activities [30].
None of aforementioned studies are focused on identifying structurally controlled gold
deposits at the deposit or district scale. The effectiveness of using geophysical–geological
tools to search for orogenic gold mineralization has been reported in the literature. At
the regional scale, integrating aeromagnetic and geoelectric data with geological and
structural information facilitates the identification of prospective areas, which can then be
validated by using robust geological and structural interpretation (i.e., shear zones) and
their correlation with known deposits (e.g., [31]). At both the district and deposit scales,
detailed terrestrial potential and electromagnetic surveys conducted throughout various
stages of exploratory research, integrated with geological (geochemical and structural) data,
enable the development of a prospective (favorability) model for mineralization, which
is validated using robust mathematical tools to establish associations between known
deposits, occurrences and geophysical signatures (e.g., [32]). In deeper portions of the crust,
integrating potential, seismic and electrical data with good quality geological information
can enhance our understanding of the genesis of orogenic gold mineralization, as validated
by drilling data (e.g., [33,34]).
Previous works have utilized induced polarization to investigate electrical conduc-
tivity and chargeability (disseminated) metallic minerals associated with orogenic gold
deposits (e.g., [35–38]). Examples demonstrating the effective application of this method
in gold exploration are provided, all of which rely on accurate geological knowledge of
the area and optimized survey scales (i.e., mineralization thickness; alterations; host/host
rocks and type of sulfidation) [35–38].
The ABP target was chosen as a case study for ground geophysical surveys (magne-
tometry and IP) due to its strategic location near the Pau-a-Pique mine and its geological
and structural context. The objective of this study was to assess the applicability of these
methods at a district/deposit scale in brownfield exploration of orogenic gold deposits.
The geological environment where the target is situated is characterized by low magnetic
gradients [39] and potential for polarization (e.g., presence of disseminated sulfides). The
findings of this study illustrate the potential of ground geophysics as a tool for guiding the
exploration of orogenic gold deposits in mature provinces.

2. Geological Context
2.1. Geological Settings
The study area is situated in the southwestern portion of the Amazon Craton (Figure 1)
within the geo-tectonic context of the Sunsás–Aguapeí orogeny (1.25–1.00 Ga) (e.g., [26,40,41]).
The AGGP deposits are hosted along the Aguapeí Belt, a narrow (~30 km wide) belt
of low-grade Neoproterozoic metamorphic folds (~0.92 Ga) [18]. The Western Amazon
Belt developed during the final stages of reactivation, transpression and closure of rift-
type basins, resulting from the convergence between the Paraguá Block and the Amazon
Craton [42].
Minerals 2024,
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2024, 788x FOR PEER REVIEW 4 of 424
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Figure 1. Regional geologic settings (extracted and modified from [9]). (A) Major geochronological
Figure 1. Regional geologic settings (extracted and modified from [9]). (A) Major geochronological
provinces of the Amazon Craton according to [40] and its location in South America. (B) Major ge-
provinces of the Amazon Craton according to [40] and its location in South America. (B) Major
ologic and tectonic domains of the southwest Amazon Craton (modified from [24,27], after [42–45]).
geologic
The dashed and blue
tectonic
line domains ofthe
represents thelimits
southwest Amazon
of early Craton (modified
Neoproterozoic belts thatfrom [24,27],the
constitute after [42–45]).
Western
The dashed
Amazon blue
Belt line represents
[42].The the the
Aguapeí Belt, limits of early
youngest Neoproterozoic
fold–thrust belts during
belt formed that constitute the Western
the Sunsás–Agua-
peí orogeny
Amazon ([14,15,17]),
Belt [42].The beganBelt,
Aguapeí withthe
theyoungest
deposition of siliciclastic
fold–thrust sediments
belt formed of thethe
during Aguapeí Group
Sunsás–Aguapeí
in rift-type basins (1265–1150 Ma) [19,20,41], later deformed and metamorphosed
orogeny ([14,15,17]), began with the deposition of siliciclastic sediments of the Aguapeí Group at a low grade in
during the final stages of compression and transpression along the Mesoproterozoic
rift-type basins (1265–1150 Ma) [19,20,41], later deformed and metamorphosed at a low grade margin ofduring
the
Amazon Craton (e.g., [9,42]).
the final stages of compression and transpression along the Mesoproterozoic margin of the Amazon
Craton (e.g., [9,42]).
Minerals 2024, 14, 788 5 of 23

The Aguapeí Group encompasses a thick sequence of siliciclastic metasedimentary rocks

deposited in an aulacogen-type basin ([21,23,46]) between 1265 and 1149 ± 7 Ma ([19,47]). The
group is divided into three formations: the Fortuna Formation (conglomerates and quartz
arenites), the Vale da Promissão Formation (metapsammites and metapelites) and the
Morro Cristalina Formation (fluvial sandstones and siltstones) [21–23]. The aforementioned
rocks exhibit green schist facies low-grade metamorphism ([18,23,27,48]).
The Aguapeí Group’s basement comprises rocks originating from the Rio Alegre and
Jauru terrains. The Rio Alegre Terrain is composed of meta-volcano–sedimentary rocks
(Minouro, Santa Izabel and São Fabiano Formations) [49], which have been interpreted as
a suture zone between the Paraguá Block and the Amazon Craton (e.g., [24]). The Jauru
Terrain (1780 Ma–1420 Ma) encompasses Paleoproterozoic and Mesoproterozoic igneous
and metamorphic rocks, including the Alto Jauru Group, Alto Guaporé Metamorphic
Complex and Figueira Branca Intrusive Suite [26].
In the study area, the basement of the Aguapeí Group is represented by granites and
granitoids of the Pindaituba Intrusive Suite (Maraboa, Guaporé, Santa Elina granites and
Lavrinha Tonalite) [25,26]. These rocks are interpreted as continental magmatic arc grani-
toids, with crystallization ages estimated between 1465 ± 4 Ma [50] and 1461.6 ± 4 Ma [9].

2.2. ABP Target

The ABP target is located in the central part of the Alto Guaporé Gold Province (AGGP),
between the municipalities of Pontes e Lacerda and Porto Esperidião, approximately 6 km
north of the Pau-a-Pique Mine (Figure 2). In this area, Mesoproterozoic granites are exposed
in the relatively flat foothills of the NW-oriented Pau-a-Pique ridge, while metasedimentary
rocks of the Aguapeí Group form outcrops on the mid-slope and hilltop of the ridge. The
shaded relief in the background of the geological map (Figure 3B) illustrates the topography
and elevation differences, which range from 325 m to 575 m, in the target area.
The ABP target comprises three gold occurrences known as Cunha, Serrinha and
Ferruginous (Figure 3B). The Cunha occurrence, situated in the eastern part of the target, lies
at the eastern contact of a wedge-shaped layer of metaconglomerate, tectonically emplaced
within the surrounding basement granite. This metasedimentary wedge is approximately
100 m thick and extends over 400 m in length (Figure 3B). In cross-section, it exhibits a
triangular shape (Figure 3A) and is interpreted as tectonically intercalated metasediments
within the granite basement. Above-background gold grades mark a gold anomaly at
the surface, which is associated with a zone rich in iron oxide minerals (e.g., hematite,
specularite and minor magnetite). The hydrothermal fluids circulating throughout the
shear zone altered the mylonite into layers of hydrothermal muscovite and biotite and
precipitated quartz veins (e.g., [9]). The shear zone is estimated to be approximately 200 m
thick and over 3.5 km in length (Figure 3B).
The Serrinha occurrence resembles the Cunha occurrence as it is also located along the
sheared contact between the granite and metasediments (i.e., metarenites and lithic metaren-
ites and conglomerates). The hydrothermal alteration observed at Serrinha mimics that of
the Cunha occurrence, characterized by swarms of centimeter-scale quartz veins associated
with hydrothermal muscovite-biotite “schists”, iron oxide (magnetite) (Figure 4iv,v), dis-
seminated sulfides (pyrite)and pervasive silicification. The Ferruginous occurrence features
a zone with strong presence of Fe-oxides (hematite + magnetite) (Figure 4iii) and sulfides
(cubic to octahedral pyrite), disseminated throughout the folded metasedimentary rocks
(metarenite and metaconglomerate) of the Aguapeí Group. This zone, characterized by
Fe-oxides and sulfides, is associated with higher-than-background gold grades and has a
lenticular shape, approximately 50 m thick and over 3 km in length, arranged as lenses
along the strike.
In both Serrinha and Cunha, the quartz veins are white and translucent, occur-ring at
centimeter-scale, sometimes with calcite (Figure 5A) and albite (Figure 5B) and surrounded
by hydrothermal muscovite and biotite (Figure 5). Other hydrothermal phases present,
Minerals 2024, 14, 788 6 of 23

Minerals 2024, 14, x FOR PEER REVIEW

include 6 of 24
magnetite (Figure 5B), calcite (Figure 5C) and sulfides (pyrite and lesser amounts
of chalcopyrite) (Figure 5A,D–F).

Figure 2.
Figure 2. Geological
Geological map
map of
of the
the central
central portion
portion of
ofAguapeí
AguapeíGold
GoldProvince
Province(AGGP)
(AGGP)indicating
indicating the
the
approximate location of the studied deposit (ABP target) as well as commercial mines and known
approximate location of the studied deposit (ABP target) as well as commercial mines and known
gold occurrences. BN = Bananal; CL = Caldeirão; ER = Ernesto; JP = Japonês; LV = Lavrinha; MB =
gold occurrences. BN = Bananal; CL = Caldeirão; ER = Ernesto; JP = Japonês; LV = Lavrinha;
Maraboa; ND = Nosde; NN = Nene; PB = Pombinhas; PQ = Pau-a-Pique. Modified from [9].
MB = Maraboa; ND = Nosde; NN = Nene; PB = Pombinhas; PQ = Pau-a-Pique. Modified from [9].
Minerals 2024, 14, 788
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of 23

Figure 3. Geological map and profile section of APB Target. (A) Interpreted geological section of the
Figure 3. Geological map and profile section of APB Target. (A) Interpreted geological section of the
studied area. (B) Detailed geological map with mapped hydrothermal alteration of the three gold
studied area. (B) Detailed geological map with mapped hydrothermal alteration of the three gold
occurrences of ABP target. PPQ = Pau-a-Pique and ABP = BP Anomaly.
occurrences of ABP target. PPQ = Pau-a-Pique and ABP = BP Anomaly.
Minerals 2024, 14, x FOR PEER REVIEW 9 of 24

Minerals 2024, 14, 788 8 of 23

compressional to transpressional regimes with reactivation of pre-existing structures. The
most prominent plane of deformation (Sn + 1) is consistently parallel to the bedding (S0).

Figure 4. Photos showing the aspects of weathered surface exposures of the ABP target’s hydrother-
Figure 4. Photos showing the aspects of weathered surface exposures of the ABP target’s hydrother-
mal alteration. (i) Intensely deformed metasedimentary rocks cropping on hangwall of Serrinha oc-
mal alteration. (i) Intensely deformed metasedimentary rocks cropping on hangwall of Serrinha
currence, shown by vertically displayed layers of metaconglomerate interbedded with medium- to
occurrence,
coarse-grainedshown by vertically
metarenite. displayed
(ii) Detail layers
of fine of metaconglomerate
to medium interbedded
grained metarenite present with
on medium-
hangwall
to
Serrinha occurrence showing moderate deformation and stratification (S0) parallel to the hangwall
coarse-grained metarenite. (ii) Detail of fine to medium grained metarenite present on main foli-
Serrinha
ation (Snoccurrence showing moderate
+ 1); (iii) ferruginous metarenitedeformation and stratification
from ferruginous occurrence(Scharacterized
0 ) parallel to the main folia-
by limonitized
tion (Sn + 1); (iii)
hydrothermal ferruginous metarenite
hematite/specularite from ferruginous
and oxidized pyrite; (iv)occurrence
sample of characterized by limonitized
silicified metarenite with dis-
seminated magnetite from Serrinha occurrence; and (v) weathered sample of hydrothermal
hydrothermal hematite/specularite and oxidized pyrite; (iv) sample of silicified metarenite with altera-
dis-
tion of Serrinha occurrence characterized by sigmoidal shape quartz veins associated
seminated magnetite from Serrinha occurrence; and (v) weathered sample of hydrothermal alteration with musco-
vite,
of oxidized
Serrinha pyrite, limonite
occurrence and boxby
characterized work texture.shape
sigmoidal Az = Azimute; Mgt associated
quartz veins = magnetite;with
BW muscovite,
= boxwork;
PyOx = oxidized sulfide; Ms = muscovite; Ser = sericite; Lm = limonite and Qtz = quartz.
oxidized pyrite, limonite and box work texture. Az = Azimute; Mgt = magnetite; BW = boxwork;
PyOx = oxidized sulfide; Ms = muscovite; Ser = sericite; Lm = limonite and Qtz = quartz.
Minerals 2024, 14, 788
Minerals 2024, 14, x FOR PEER REVIEW 10 of 924
of 23

Figure 5. Drill core photos showing the aspects of hydrothermal alteration of Cunha and Serrinha
Figure 5. Drill core photos showing the aspects of hydrothermal alteration of Cunha and Serrinha oc-
occurrence in ABP target. Photos shows that Cunha hydrothermal alteration is given by quartz+cal-
currence
cite veinsin(C)
ABPin target. Photos
association shows
with that Cunha
hydrothermal hydrothermal alteration
muscovite—biotite “schist”isformed
given byat quartz ±calcite
the sheared
veins (C)(A)
contact in association with hydrothermal
between metasediments of the muscovite—biotite
Aguapeí Group and “schist” formed
the granitic at the sheared
basement. contact
The other
(A) between metasediments
hydrothermal phases includeofmagnetite
the Aguapeí(B);Group
albite and
(B,F);the granitic
calcite basement.
(C,E); The other
pyrite (A,D–F) andhydrothermal
chalcopy-
rite (E).include
phases The characteristics
magnetite (B);of albite
mineralization in the
(B,F); calcite context
(C,E); of (A,D–F)
pyrite the Serrinha
and system, with (E).
chalcopyrite gradation
The char-
between the
acteristics granite basement
of mineralization andcontext
in the venulated deformed
of the Serrinhazones (hydrothermal
system, schist
with gradation muscovite
between lay-
the granite
ers) have paragenesis defined by Qtz + Cal + Al + Py > Cpy + Ms-Ser and Mgt and Bt trace. Mgt =
basement and venulated deformed zones (hydrothermal schist muscovite layers) have paragenesis
magnetite; Py = pyrite; Cpy = calcopirite; Ms = muscovite; Ser = sericite; Cal = calcite; Bt = biotite and
defined by Qtz + Cal + Al + Py > Cpy + Ms-Ser and Mgt and Bt trace. Mgt = magnetite; Py = pyrite;
Qtz = quartz.
Cpy = calcopirite; Ms = muscovite; Ser = sericite; Cal = calcite; Bt = biotite and Qtz = quartz.
Minerals 2024, 14, 788 10 of 23

Similarly to the Pau-a-Pique deposit, the mineralized zones of the Serrinha and Cunha
are hosted within a hydrothermal muscovite layer associated with quartz veins and sulfide
(Figure 4), developed at the contact between the granite in the footwall and the metasedi-
ments in the hangwall. These mineralized zones, formed along subvertical, NNW-oriented
shear zones, dipping approximately 75◦ , with an average thickness of 7 m. The hydrother-
mal muscovite layer exhibits well-developed schistosity and contains abundant laminated
quartz veins that show sigmoidal shapes and are parallel to the foliation. The footwall of the
mineralized zones consists of medium- to coarse-grained leucocratic biotite granites with
frequent porphyritic texture. These granites are associated with the Pindaituba Intrusive
Suite. Occasionally, these rocks exhibit penetrative foliation, sometimes accompanied by
mylonitic deformation. The hangwall is composed of intercalations (i.e., fining-upward
or coarsening upward sequences) between oligomictic metaconglomerates with quartz
and feldspar pebbles up to 5 cm in size. Additionally, it includes metarenites and lithic
metarenites as well as conglomerates of the Fortuna Formation, which has quartz and
feldspar clasts elongated in the direction of the foliation. These layers show stratification
(S0 ) parallel to the main foliation (Sn + 1).
The structural framework of the area where the Target is located is characterized by
a complex association of regional-scale structures, resulting from the confluence between
the corridor shear zone and the hinge zone of the Caldeirão Syncline (e.g., [5,6,8,9,48,49])
(Figure 3). This is the reason why the target was selected as a case study for this re-
search project. In the region of the ABP target, structures associated with two phases of
compressional deformation (D1 and D2) and one phase of transpression (D3) have been
identified ([7,8,49,51–53]). According to [9], the shear zone was formed during the D2
compression event, which led to the development of mylonitic foliations. However, it is
believed that the gold deposition occurred during the D3 phase, during the transition from
compressional to transpressional regimes with reactivation of pre-existing structures. The
most prominent plane of deformation (Sn + 1) is consistently parallel to the bedding (S0 ).

3. Methods
In this study, magnetic and induced polarization (IP) surveys were conducted along
lines intersecting the three gold occurrences of the ABP target.

3.1. Ground Magnetic Survey

The ground magnetic data were acquired by the author with the support of the Aura
Minerals Inc. (Road Town, UK) team. The acquisition occurred along lines of approximately
1.5 km in length, oriented NE–SW. A total of 71 lines were surveyed, covering a distance of
107.37 km in total (Figure 6A).
The ground magnetic survey was conducted using two different line spacings
(Figure 6A). In the northern portion of the area, where the main gold occurrences of
the ABP target are located, the survey grid had a line spacing of 50 m, with readings
taken every 25 m along acquisition lines. Conversely, in the southern part, the survey was
conducted with a line spacing of 150 m between lines, with readings taken every 25 m
along the lines.
The survey configuration was defined based on previously mapped structures in the
field, using 1:2500 geological mapping. This approach was important for locating the main
structure (corridor shear zone), in the areas of low magnetic anomalies, particularly in the
northern portion where a higher level of detail (50 × 25 m) was applied.
The equipment used included of two magnetometers, GSM-19TW and GSM-19W
models, manufactured by GEM Systems. To monitor the diurnal variation of the mag-
netic field, recordings were taken at 30-s intervals at a base station located in a magnetic
interference-free area. During the data preprocessing stage, diurnal correction and the
removal of the International Geomagnetic Reference Field (IGRF) were performed. The data
were processed using Oasis Montaj software (Educational Version 2022.1) [54]. Furthermore,
data modelling was conducted to determine the distribution of magnetic susceptibility
 3. Methods
In this study, magnetic and induced polarization (IP) surveys were conducted along
lines intersecting the three gold occurrences of the ABP target.
Minerals 2024, 14, 788 11 of 23

3.1. Ground Magnetic Survey

The ground magnetic data were acquired by the author with the support of the Aura
values and the direction of the magnetization vector (MVI), which were extracted using
Minerals Inc. team.
VOXI software Thecell
2.1. The acquisition
size used occurred
was 30 m along
× 30 mlines
× 10ofmapproximately
and the default1.5 km in
inversion
length, oriented NE–SW. A total of 71 lines were surveyed, covering
parameters of Oasis Montaj were applied for the inversion process. a distance of 107.37
km in total (Figure 6A).

Figure
Figure 6.
6. Location
Locationmap
map of
of topographic
topographic conditions:
conditions: (A)
(A)magnetic
magnetic data
data acquisition
acquisition lines,
lines, (B)
(B) induced
induced
polarization IP acquisition lines and geochemical anomalies of the area. Digital elevation model
polarization IP acquisition lines and geochemical anomalies of the area. Digital elevation model
(DEM). Source: ALOS PALSAR—radiometric terrain correction. Municipal access points corre-
(DEM). Source: ALOS PALSAR—radiometric terrain correction. Municipal access points correspond
spond to the red lines. PPQ = Pau-a-Pique and ABP = BP Anomaly.
to the red lines. PPQ = Pau-a-Pique and ABP = BP Anomaly.
The ground
3.2. Induced magnetic
Polarization survey was conducted using two different line spacings (Fig-
Survey
ure 6A). In the northern portion of the area, where the main gold occurrences of the ABP
The induced polarization data were acquired along six lines in the northern portion
of the ABP target. These IP survey lines were approximately 1.5 km long (Figure 6B) and
oriented NE–SW, perpendicular to the strike of the gold occurrences. The survey grid
had a line spacing of 100 m (Figure 6B). Electrodes and dipole lengths (MN and AB) were
installed with a spacing of 50 m, except for line 6 (L6), where data acquisition involved two
spacings of 25 m and 50 m, respectively. Line 6 was chosen for deploying dipole-dipole
 Minerals 2024, 14, 788 12 of 23

(DD) and pole-dipole (PD) arrays with 10 levels of investigation depth, aiming to select the
most suitable for the study area. Line 6 was selected to test different acquisition spacings
and arrays, because it covers all the mapped occurrences in the ABP target and exhibits the
Minerals 2024, 14, x FOR PEER REVIEWhighest gold grades in both rock geochemical samples and the best intervals in drill13 ofholes
24
(Figure 7).

Figure 7. 7.
Figure Inverted sections
Inverted covering
sections the
covering two
the configurations
two tested
configurations inin
tested line 6, 6,
line dipole–dipole (DD)
dipole–dipole and
(DD) and
pole–dipole (PD) and spacings of 25 and 50m between the electrodes. In (A–D) sections for line 6
pole–dipole (PD) and spacings of 25 and 50 m between the electrodes. In (A–D) sections for line 6 with
with a spacing of 25 m between the electrodes, with both dipole–dipole and pole–dipole arrange-
a spacing of 25 m between the electrodes, with both dipole–dipole and pole–dipole arrangements.
ments. (E–H) show sections of the two arrays at 50 m spacing.
(E–H) show sections of the two arrays at 50 m spacing.
The results obtained from the dipole–dipole (DD) array were limited in the study
In order to facilitate the acquisition of IP data, the following equipment was used: (i) a
area, at both spacings (25 and 50 m), due to the limited depth of investigation and its low
10-channel ELREC-PRO receiver and (ii) a VIP 4000 transmitter, both manufactured by
signal-to-noise ratio compared to the pole–dipole (PD) array. Conversely, the PD array
Iris Instruments.
offers the benefit of reaching greater depth of investigation. Consequently, the pole–di-
pole array with 50 m spacing was deemed optimal for continuing the study on the remain-
ing lines, despite the aforementioned limitations. Additionally, when compared to the 25
m spacing, the gains in depth of investigation were significantly greater, considering the
larger dimensions of the anomalies observed with the 50 m spacing.
Minerals 2024, 14, 788 13 of 23

The data collection parameters included a current injection and recording cycle of
4000 ms (e.g., 2 s on and 2 s off) and time windows (sampling) of the Cole–Cole potential
curve [40]. The electrical current injected by the transmitter ranged from 1 to 3 A, depending
on the electrical contact resistance between the current electrode and the ground (e.g., lower
currents in areas of outcropping silicified metasediments).
The topography along the IP lines was obtained using a Geodetic GPS model GNSS
Topcon HiPer VR manufactured by Topcon Positioning Systems, Inc. (Livermore, CA, USA).
The data filtering was conducted using the Prosys III software, version V2.11 (Iris In-
struments Inc., Orléans, France), based on the following principles: extraction of resistivity
records and chargeability outside the log-normal distribution and the identification of
chargeability with decay curves that deviate from decreasing exponential functions.
The IP data modeling was performed using the Res2dinv software, version 2024.1.1
(Seequent). In the inverse modeling, the smoothness constrained routine [55] of this program
was employed to achieve smoother transitions (e.g., geological materials) and a vertical
filter to enhance vertical structures (e.g., shear zones). Subsequently, points with higher
RMS errors (spikes) were removed.
The results obtained from the dipole–dipole (DD) array were limited in the study
area, at both spacings (25 and 50 m), due to the limited depth of investigation and its low
signal-to-noise ratio compared to the pole–dipole (PD) array. Conversely, the PD array
offers the benefit of reaching greater depth of investigation. Consequently, the pole–dipole
array with 50 m spacing was deemed optimal for continuing the study on the remaining
lines, despite the aforementioned limitations. Additionally, when compared to the 25 m
spacing, the gains in depth of investigation were significantly greater, considering the larger
dimensions of the anomalies observed with the 50 m spacing.

4. Results
4.1. Ground Magnetic Data
The results of the processing of ground magnetic data are presented in maps of residual
magnetic field (RMF) and total gradient (GT), Figure 8A,B, respectively, as well as in 3D
inversion models of the magnetic data with the recovery of amplitude value of the magnetic
vector inversion, which was realized in VOXI (Oasis Montaj).
The analysis of magnetic anomalies on the map, particularly in the total gradient
(GT) map (Figure 8B), reveals a linear pattern intertwined with high-frequency magnetic
anomalies (0.594–2.591 nT) oriented along a NNW trend. The highest values, both in the
total gradient (GT) (Figure 8B) and in the residual magnetic field (RMF) (Figure 8A), are
generally associated with exposures of metasedimentary rocks from the Aguapeí Group
(metarenites and metaconglomerates) containing iron oxides (magnetite, hematite and
ilmenite), while the porphyritic granite of the Pindaituba Intrusive Suite exhibits low
magnetic intensity, which contrasts with the metasedimentary rocks amidst a scenario of
field variations.
The three-dimensional inversion models (50 × 50 m mesh) of the recovered magnetic
bodies (Figure 9) demonstrate that the higher-intensity magnetic anomalies manifest as
linear bodies with a NNW trend, which are sub-vertical with a high-angle dip towards the
WSW (Figure 9B,C). This pattern is most prominently observed in two regions of the study
area: one in the NNW portion, where the most pronounced anomalies are situated, and
another in the southeast portion, where a smaller anomaly in terms of area and volume is
identified (Figure 9A,B).
In the NNW portion of the study area, a series of open antiforms and synforms
with hinge zones oriented in a NNW–SSE direction, folding metasedimentary rocks of
the Aguapeí Group are mapped (Figure 5). Regional aeromagnetic surveys ([56,57]) are
consistently associated with this orientation pattern and the intensity of total gradient
anomalies with the presence of folded Aguapeí Group metasedimentary rocks, especially
at the sheared contact between metasediments and basement rocks, typically characterized
by subvertical shear zones. In the central–southern part of the belt, these subvertical
 4. Results
Minerals 2024, 14, 788 4.1. Ground Magnetic Data 14 of 23
The results of the processing of ground magnetic data are presented in maps of re-
sidual magnetic field (RMF) and total gradient (GT), Figure 8A,B, respectively, as well as
shear
in 3D zones hostmodels
inversion some ofofthe
theknown gold
magnetic deposits,
data such
with the as the Pau-a-Pique
recovery of amplitudemine,
valuewhere
of the
mineralized
magnetic zones
vector are oriented
inversion, along
which wasNNW trends
realized with (Oasis
in VOXI a dip toMontaj).
the SW.

Figure 8. Ground
Figure 8. magnetic data:
Ground magnetic data: (A)
(A) residual and (B)
residual and (B) total
total gradient.
gradient. Black
Black dashed
dashed lines
lines highlight
highlight
the continuity of the corridor shear zone demarcated by a low magnetic anomaly with orientation
the continuity of the corridor shear zone demarcated by a low magnetic anomaly with orientation
NW–SE. Digital elevation model (DEM) Source: ALOS PALSAR—radiometric terrain correction.
NW–SE. Digital elevation model (DEM) Source: ALOS PALSAR—radiometric terrain correction.
PPQ = Pau-a-Pique and ABP = BP Anomaly.
PPQ = Pau-a-Pique and ABP = BP Anomaly.
The analysis of magnetic anomalies on the map, particularly in the total gradient (GT)
It is also probable that high RMF and GT values are also observed in some of the gold
map (Figure 8B), reveals a linear pattern intertwined with high-frequency magnetic anom-
deposits along the belt due to the presence of some ferromagnetic minerals (magnetite,
alies (0.594–2.591
hematite, ilmenite nT)
and oriented along a NNW
minor pyrrhotite), trend. The
particularly thosehighest values,
situated in the both in the
contact total
between
gradient (GT) (Figure
the metasediments and8B)
theand in the
granite residual the
or between magnetic field (RMF)
metasediments and(Figure 8A), are gen-
the metavolcanosed-
erally associated with exposures of metasedimentary rocks
imentary basement, which typically exhibits GT values < 0.594 nT. from the Aguapeí Group (me-
tarenites and metaconglomerates) containing iron oxides (magnetite, hematite
The magnetic anomaly in the southeast corner of the map occurs in an area and ilmen-
where
ite),
intrusive rocks from Pindaituba Suite are mapped (Figure 8). These rocks are magnetic
while the porphyritic granite of the Pindaituba Intrusive Suite exhibits low typically
characterized by low GT and RMF values. The anomaly is interpreted as a lithological unit
oriented approximately NW–SE, which is not identified in surface exposures and has a
magnetic signature distinct from that of the porphyritic granites forming the basement of
the area.
 bodies (Figure 9) demonstrate that the higher-intensity magnetic anomalies manifest as
linear bodies with a NNW trend, which are sub-vertical with a high-angle dip towards
the WSW (Figure 9B,C). This pattern is most prominently observed in two regions of the
study area: one in the NNW portion, where the most pronounced anomalies are situated,
Minerals 2024, 14, 788
and another in the southeast portion, where a smaller anomaly in terms of area and vol-
15 of 23
ume is identified (Figure 9A,B).

Figure 9. Three-dimensional
Figure 9. Three-dimensional mosaic with the shape of the the magnetic
magnetic bodies
bodies in
in depth
depth recovered
recovered byby
inversion
inversion of magnetic
magnetic data
dataanomalies,
anomalies,with
withdashed
dashed lines
lines delimiting
delimiting thethe area
area of greater
of greater detail
detail (C)
(C) and
DEM
and topographic
DEM layout
topographic of the
layout of area. (A) Residual
the area. magnetic
(A) Residual fieldfield
magnetic (RMF) relief
(RMF) (view
relief to NW).
(view (B)
to NW).
Recovered
(B) Recoveredmagnetic model
magnetic modeloverlaid
overlaidbybyDEM
DEMtopography
topography(view
(view to
to NW), same results
NW), same resultswith
with5050×× 50
50
and 150 × 150 m mesh. (C) Representation of the northernmost portion of the detail with
and 150 × 150 m mesh. (C) Representation of the northernmost portion of the detail with a 50 × 50 m a 50 × 50
m mesh, from a different viewing angle (from N to S).
mesh, from a different viewing angle (from N to S).

In the NNW
Another linearportion
anomalyof the study
with a NNWarea,direction,
a series ofclearly
open antiforms
highlightedandonsynforms
the map with
(GT
hinge
and zones
RMF) oriented
(Figure in aisNNW–SSE
8A,B), observed in direction, folding metasedimentary
the northeastern part of the studyrocksarea. ofThis
the
Aguapeí Group are mapped (Figure 5). Regional aeromagnetic surveys
anomaly coincides with the presence of oxidized metasediment lenses, tectonically wedged ([56,57]) are con-
sistently
into associated granite
the porphyritic with thisandorientation pattern
whose contact and the by
is marked intensity of totalofgradient
the presence anom-
hydrothermal
alies with the presence of folded Aguapeí Group metasedimentary rocks,
alteration hosting mineralized zones with grades of up to 8.7 g/t Au (Serrinha occurrence).especially at the
sheared
In contact
this case, bothbetween metasediments
the geological context and andthe
basement
pattern rocks, typically
of magnetic characterized
anomalies (GT and by
subvertical
RMF) shear
are very zones.
similar toIn the central–southern
those part of the belt,
observed at the Pau-a-Pique minethese
withsubvertical
the presence shear
of
zones host ilmenite,
magnetite, some of the knownand
hematite gold deposits,locally.
pyrrhotite such as the Pau-a-Pique mine, where miner-
alized zones
Some are oriented alongdipole
shorter-wavelength NNW anomalies
trends withare a dip to the
visible in SW.
the southwestern portion
of theItRMF
is also probable
map (Figurethat
8A)high
andRMF and GT to
correspond values are also
exposures ofobserved in some of the gold
meta volcano–sedimentary
deposits along
sequences fromthethebelt
Rio due to the
Alegre presence
Terrane. Theofabsence
some ferromagnetic minerals in
of reversed polarities (magnetite,
the area
indicates
hematite,that residual
ilmenite andeffects
minororpyrrhotite),
demagnetization have had
particularly a lesser
those influence.
situated in the contact be-
tweenThethe3D magnetic amplitude
metasediments and thedata inversion
granite withthe
or between recovery of magnetic
metasediments andsusceptibility
the metavol-
bodies demonstrates
canosedimentary zones ofwhich
basement, contrast between
typically positive
exhibits GTand negative
values anomalies,
< 0.594 nT. character-
ized by protrusions and depressions in RMF (Figure 9A), which mark the contact between
metasediments and granitic basement. These zones are linked to low magnetic shear zones.
It can also be observed that the recovered values of magnetic susceptibility from
positive anomalies often occur at the edges of shear zones related to magnetic hydrothermal
alteration in the context and the correspondence of structural attitudes with a southwest
dip, as well as the presence of folds associated with the SW–NE thrust.

4.2. Geoelectrical Survey

The cross-sectional resistivity (Figure 10) and chargeability (Figure 11) profiles were
generated from the induced polarization (IP) surveys performed in the northern part of the
area, reaching depths of ~350 m.
 dip, as well as the presence of folds associated with the SW–NE thrust.

4.2. Geoelectrical Survey

The cross-sectional resistivity (Figure 10) and chargeability (Figure 11) profiles were
Minerals 2024, 14, 788 16 of 23
generated from the induced polarization (IP) surveys performed in the northern part of
the area, reaching depths of ~350 m.

Minerals 2024, 14, x FOR PEER REVIEW 17 of 24

Figure 10. 3D view of geoelectric resistivity profiles, correlated with surface geochemical anomalies.
Figure 10. 3D view of geoelectric resistivity profiles, correlated with surface geochemical anomalies.
White dashed lines highlight the contacts of units mapped in the field. FMAR = feldspatic metarenite;
White dashed lines highlight the contacts of units mapped in the field. FMAR = feldspatic me-
PBSG = porphyritic
tarenite; biotite sieno-granite;
PBSG = porphyritic FERM = ferruginous
biotite sieno-granite; metarenite;metarenite;
FERM = ferruginous QZV (SS) =QZV
quartz vein
(SS) =
(Serrinha
quartz veinsystem) andsystem)
(Serrinha QZV (CS)and=QZVquartz vein
(CS) (Cunha
= quartz system).
vein (Cunha system).

Figure 11. 3D view of geoelectric chargeability acquisition lines, associated with surface geochemical
Figure 11. 3D view of geoelectric chargeability acquisition lines, associated with surface geochemi-
anomalies.
cal White
anomalies. dashed
White lines
dashed highlight
lines the the
highlight contacts of units
contacts mapped
of units in the
mapped in field. FMAR
the field. = feldspatic
FMAR = feld-
metarenite;
spatic PBSG =PBSG
metarenite; porphyritic biotite sieno-granite;
= porphyritic FERM = ferruginous
biotite sieno-granite; metarenite; QZV
FERM = ferruginous (SS) = quartz
metarenite; QZV
vein=(Serrinha
(SS) system)
quartz vein and QZV
(Serrinha (CS)and
system) = quartz
QZV vein
(CS) (Cunha
= quartzSystem).
vein (Cunha System).

4.2.1. Resistivity
4.2.1. Resistivity
As illustrated in Figure 10, the resistivity exhibits a range of values between 178 and
As illustrated in Figure 10, the resistivity exhibits a range of values between 178 and
64,426 ohm.m, encompassing both high and low electrical resistivity regions. The bound-
64,426 ohm.m, encompassing both high and low electrical resistivity regions. The bound-
aries between these regions are abrupt and sub-vertical, with a NNW–SSE orientation.
Correlating the resistivity sections with the geological map (Figure 3) reveals that the re-
gions with lower apparent resistivity background (<16,240 ohm.m) are mainly associated
with the occurrence of the porphyritic granite basement or hydrothermally altered
Minerals 2024, 14, 788 17 of 23

aries between these regions are abrupt and sub-vertical, with a NNW–SSE orientation.
Correlating the resistivity sections with the geological map (Figure 3) reveals that the
regions with lower apparent resistivity background (<16,240 ohm.m) are mainly associ-
ated with the occurrence of the porphyritic granite basement or hydrothermally altered
metasediments, specifically where hydrothermal alteration is marked primarily by the
frequent presence of iron oxides (ferruginous metarenite with magnetite and hematite).
Zones of higher electrical resistivity (>16,240 ohm.m) are associated with exposures of
metasediments from the Aguapeí Group represented by quartz–feldspar metarenites. These
zones also exhibit strong hydrothermal alteration, characterized by the presence of swarms
of quartz veins. These regions exhibit the highest gold concentrations within the area.
In the sections of Figures 10 and 12B, a zone of high resistivity with a thickness of
approximately 100 m is observed in the eastern portion of the area, with resistivity values
ranging from 16,240 to 24,271 ohm.m (Figure 12B). This zone of high resistivity is associated
with the presence of metarenites and mylonites (contact zone with basement) with quartz
veins + oxidized sulfides + specularite and dissemination of magnetite in the sericitization
matrix of the Cunha occurrence. The region between 800 and 1100 m, with an average
elevation of 350 m, exhibits elevated resistivity values up to 64,426 ohm.m. These values
may represent potential continuations and/or down-dip influences for the metasediment
layer of the Cunha System, as well as a portion of the granitic basement with hydrothermal
alteration (i.e., silicification).

Figure 12. Geophysical–geological interpretative model (geoelectric–magnetic) for gold deposits

in AGGP. (A) Chargeability and (B) resistivity inverted sections of line 5. (C) Vertical section in 3D
model of the MVI (magnetic vector inversion) along line 5 (IP). (D,E) 3D voxel products of IP data
(chargeability and resistivity respectively). (F) 3D geological model. In thinner dashed lines (A–C),
geological interpretation of the sections. DDH = diamond drill hole; L1 = line 1; L2 = line 2; L3 = line
3; L4 = line 4; L5 = line 5 and L6 = line 6.
Minerals 2024, 14, 788 18 of 23

In the region between 550 and 800 m and average elevations of up to 400 m, the values
vary within the range of 16,240–64,426 ohm.m, associated with the Serrinha System, which is
defined by metasediments (metarenites/metaconglomerates), with sulfide disseminated in
quartz veins and oxidized, sometimes as boxworks associated with specularite + magnetite.
It can be observed that fault structures and shear zones are present between 500 to
600 m and 1250 m (Figure 12), respectively. These are not apparent at the surface, but
can be correlated with zones of low resistivity in proximity to intermediate resistivity (i.e.,
porphyritic granites) and high resistivity (metasediments).

4.2.2. Chargeability
The high chargeability values (>18 mV/V) obtained with the induced polarization
method were found to be restricted and only localized in certain sectors (Figures 11 and 12A).
This is supported by geochemical rock anomalies with Au grades at the surface (i.e., along
the lines) and drill holes.
The maximum values did not exceed 24 mV/V, with the majority located in two areas
along the sections: near the Serrinha occurrence (metasediment adjacent to the contact with
the basement) and along the “conduit” of the possible extension of the corridor shear zone.
The response is situated between 350 to 900 m, initiating its anomaly from the surface to
480 m (Figure 12A). Additionally, between 400 and 250 m (Z-axis) in a tapered aspect, the
anomaly appears to follow the morphology of subvertical structures. On the X-axis, at
approximately 1200 m, a chargeability of approximately 10 mV/V was observed, with an
estimated depth of 100 m. This observation corroborates with the mapped area, where
polarizable responses of metasediments with associated pyrite, arranged in the Cunha
occurrence, are observed.
Furthermore, in the eastern region adjacent to the Cunha occurrence, approximately
1250 m (X-axis), a structural feature of a fault mapped within the context of the granitic
basement is observed. All lines executed, as well as L5 in question, were acquired orthogo-
nally to the target systems in question (Figures 10 and 11) and the polarizable anomalies
were restricted to systems of quartz veins with disseminated sulfides. Moreover, anomalies
have been identified in the potential extension of the shear zone context. Although the
host rock also holds trace Au mineralization (<0.2 g/t) and indications of hydrothermal
alterations, including sulfidation, no anomalous signs were detected in these sectors.

5. Discussion
The metallogenic characteristics associated with the deposits of the ABP target and part
of the Alto Guaporé Gold Province indicate that these deposits predominantly occur within
fault zones that develop along the contacts between metamorphic rocks and granites (as
depicted in Figures 8, 9 and 12C). The regions most conductive to gold deposits encompass
the extension of the corridor shear zone, specifically during the D3 phase (transpressional)
on NE fault planes and intersections of faults with different orientations.
From the inversion of the 3D magnetic data with recovery of the magnetization vector,
contrasting zones between positive and negative anomalies can be observed, characterized
by protrusions and depressions that mark the contact between metasediments and granitic
basement, associated with low magnetic zones. These zones are highly favorable for the
identification of deep shear structures that possibly favored the percolation of mineralizing
fluids in the area (see Figure 12C). The granite of the Pindaituba Intrusive Suite exhibits
low magnetic intensity. In contrast, the metamorphic rocks (metasediments) show high
magnetic anomalies (presence of iron oxides such as magnetite and hematite) amidst a field
of varying intensity. In addition, it is noteworthy that the values of magnetic susceptibility
recovered from positive anomalies often occur at the edges of shear zones related to
the magnetic hydrothermal alteration of the context (hematitization and sericitization),
corresponding to structural attitudes dipping southwestward, as well as the presence of
folds associated with SW–NE thrusting.
Minerals 2024, 14, x FOR PEER REVIEW 20 of 24

Minerals 2024, 14, 788 19 of 23

discussion, the prospective geological-geophysical parameters are summarized in (Fig-

ures 12 and 13).Geological-Geophysical Model for Gold Deposits in the Alto Guaporé
5.1. Prospective
Gold The results of the MVI in the region (Figures 12C and 13) indicate a high degree of
Province
relevance. Through model
The proposed correlation
for thewith the electrical
orogenic (resistivity
gold deposits andthe
within chargeability)
AGGP context andrefers
geo-
logical data, it was determined
to the tectono-structural to be an important
configurations proposedprospective
by [2], where indicator in AGGP.
mineralization isFirstly,
hosted
itincan be observed
large-scale thatstructural
crustal the low magnetic
corridorssignature of the silicification
and/or faults/shear zones hydrothermal
cut by accommoda- zone
(i.e.,
tion faults; thrust-related anticlines; irregular contacts on faulted granite margins; and
between 500 to 600 m in the 2D section) corresponds to the corridor shear zone and
behaves similarly
architectures to vectors in a magnetic
of triple-quadruple points inbar, with magnetic
granitic intrusions.induction
In termsvectors in oppo-
of geophysics,
site
the directions emanating from
lithological-structural the characteristics
and ore low magnetic zone,were as detected
utilized as aby [58].information
priori Secondly, the to
electrical resistivity with
enhance correlation valuesthedelineate
structuralthe bodiesofofthe
features metasediment enclosed the
context. Furthermore, in the granite
interaction
with
betweenremarkable
magneticprecision, thereby
and geoelectric facilitating
methods servedtheas identification
a mechanism to ofpredict
potentialthe milonitic
positions
contact
and/or zones. Furthermore,
mineralized regions. the chargeability isosurface generated (Figure 10) indicates
that the mineralizing
Subsequently, an system is continuous,
interpretative extendingmodel
gold prospecting both was
along the shear based
constructed zone and at
on the
the edges of magnetic bodies associated with the contact of metasediments
geophysical and geochemical signatures of the area with the objective of identifying the ore (i.e., oxidized)
with
bodies.theIngranite basement.
consideration This
of the is corroborated
geological by the Au
and geophysical grades of
attributes evidenced
exploration in line
Figure
“5”
12A–C.
of target ABP, which serves as a representative case within the context under discussion,
the prospective geological-geophysical parameters are summarized in (Figures 12 and 13).

Figure
Figure 13.
13. Correlation of magnetic
Correlation of magneticandandgeoelectric
geoelectricresults
resultsininline
line
5, 5, with
with thethe magnetic
magnetic vector
vector inver-
inversion
sion and amplitude inversion image section, superimposed by the chargeability isosurface
and amplitude inversion image section, superimposed by the chargeability isosurface (>18 mV/V). (>18
mV/V). MS = magnetic susceptibility and MVI = magnetic vector
MS = magnetic susceptibility and MVI = magnetic vector inversion. inversion.

In
Thesummary,
results ofthe results
the MVI of in magnetometric
the region (Figuresand geoelectric
12C and 13) inversion
indicatesuggest
a high adegree
good
correlation between polarizable, resistive and low magnetic values with
of relevance. Through correlation with the electrical (resistivity and chargeability) and the potential for
mineralization of rocks in the AGGP.
geological data, it was determined to be an important prospective indicator in AGGP.
Firstly, it can be observed that the low magnetic signature of the silicification hydrothermal
5.2.
zone3D(i.e.,
Geological
between Features
500 to 600 m in the 2D section) corresponds to the corridor shear zone
and As
behaves similarly to vectors
illustrated in (Figure 12F), intheathree
magnetic bar, withsystems
mineralizing magnetic induction
previously vectors in
described in
opposite directions emanating from the low magnetic zone, as detected by
Section 2.1 are evident in the 3D visualization. Additionally, the arrangement of fault sys- [58]. Secondly,
the electrical
tems betweenresistivity values
the contacts delineate
of the the bodieswith
metasediment of metasediment enclosed in
the granitic basement is the granite
observed,
with remarkable
where the faults that precision,
control thereby facilitating
the ore exhibit the identification
characteristics of potential
of sub-vertical surfacesmilonitic
along
contact
their zones. Furthermore,
NNW–SSE strike. In thethe chargeability
northwest isosurface
portion, a systemgenerated
of folds in(Figure 10) indicates
the metasediments
that
is the mineralizing
displayed, resemblingsystem is continuous,
a “chicken’s foot”extending both along
pattern, which the shearwith
is associated zonethe and at the
closure
edges of magnetic bodies associated with the contact of metasediments
of the Caldeirão syncline converging with the corridor shear zone (i.e., the main feeder(i.e., oxidized) with
of
the granite basement. This is corroborated by the Au grades evidenced
mineralization throughout the context). The model was generated using surface structural in Figure 12A–C.
Minerals 2024, 14, 788 20 of 23

In summary, the results of magnetometric and geoelectric inversion suggest a good

correlation between polarizable, resistive and low magnetic values with the potential for
mineralization of rocks in the AGGP.

5.2. 3D Geological Features

As illustrated in (Figure 12F), the three mineralizing systems previously described
in Section 2.1 are evident in the 3D visualization. Additionally, the arrangement of fault
systems between the contacts of the metasediment with the granitic basement is observed,
where the faults that control the ore exhibit characteristics of sub-vertical surfaces along
their NNW–SSE strike. In the northwest portion, a system of folds in the metasediments
is displayed, resembling a “chicken’s foot” pattern, which is associated with the closure
of the Caldeirão syncline converging with the corridor shear zone (i.e., the main feeder of
mineralization throughout the context). The model was generated using surface structural
data as well as geological-hydrothermal mapping data (Aura Minerals Inc., internal report
2022) within the Leapfrog Geo software, version 2023.1.1, from Seequent (Bentley Systems).

6. Conclusions
The advancements observed in this study at the district scale in the AGGP were the
result of a convergence between technical and scientific developments and previous studies.
In this context, potential prospects can be selected based on long-term practical experience
and at more regional scales, thereby supporting the application of exploratory tools in
the context. Given the structural architecture of the known deposits and mines in the
province, a target for detailed geophysical application was selected for the purpose of
investigating possible continuities (e.g., strike and depth) of mineralization in shear zones
hosted near contacts between the granitic basement of the Pindaituba Intrusive Suite and
metasediments of the Aguapeí Group. The establishment and application of a prospective
geological–geophysical model can contribute as a guide for future research regarding
exploration in the Alto Guaporé Gold Province. The following conclusions were drawn
from this study:
The methods and prospective ideals developed are part of traditional superficial and
relatively deep prospecting methods. In contrast to gold prospecting in the Alto Guaporé
Gold Province, which primarily employs techniques for identifying magnetic anomalies
and mapping mineralized structures, the district-scale scope of this study enabled us to
delineate the footprints of polarizable and resistive anomalies, as well as their distinctions
in magnetic properties. These differences are determined by low magnetic anomalies
(e.g., silicified zones), in comparison to mineralized gold zones with the presence of
hydrothermal magnetite, as is the case with other targets and/or deposits in the AGGP
region. Furthermore, this approach enabled the mapping of potential continuities in
ore-controlling fault zone systems in both deep and near-surface areas.
The gold deposits in the Alto Guaporé Gold Province (AGGP) are controlled by
large-scale regional faults and shear zones that are associated with the contact between
metasedimentary rocks and a granitic basement. The ore shoots in the ABP target area
are primarily attributed to deformed zones associated with mylonites and/or schists in
structures secondary to the main feeder. The faults that host the ore developed along the
contact interfaces between Mesoproterozoic metasedimentary rocks and Mesoproterozoic
granites. These exhibit a subvertical metallogenic model that is parallel to the regional
structure and has a preferential N20-50W direction. This model offers a promising technical
premise, particularly for targets in the southern region of the province.
The indicative footprints in the integrated geological–geophysical prospecting model
include a subvertical metallogenic model, low magnetic zones (main conduits), low resis-
tivity and high chargeability in metasediments and contact zones.
The geophysical exploration of gold deposits in the target area demonstrated the
feasibility of characterizing and identifying metallogenic footprints and mineralized zones
Minerals 2024, 14, 788 21 of 23

at varying depths between 100 and 350 m, a finding that was subsequently validated
through drilling.

Author Contributions: J.E., R.M. and M.L.-S. conceived and designed the research ideas; J.E., T.M.,
R.M. and M.L.-S. participated in field activities; J.E., T.M. and M.L.-S. conducted data analysis; J.E.,
R.M., T.M., M.L.-S. and W.B. reviewed and edited the drafts. All authors have read and agreed to the
published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement: The geophysical and geochemical data are confidential.
Acknowledgments: The authors would like to express their gratitude to the “Graduate Program
in Applied Geosciences and Geodynamics” for providing the necessary infrastructure (laboratories,
study environment and equipment) to conduct this research. Additionally, they extend their appreci-
ation to the Institute of Geosciences at the University of Brasília. Furthermore, the authors would like
to express their gratitude to the company “Aura Minerals Inc. (Unit Aura Apoena)” for providing
the private data that was acquired and collected by them and for granting the publication of this
information as an article.
Conflicts of Interest: The authors declare no conflicts of interest.

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