Gold & Quartz
Caturama - BA
The area of mining process no. 871.977/2021 occupies 174.90ha and is located in the municipality of Caturama, Bahia, Brazil.
HIGHLIGHTS
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LOCATION AND LOGISTICS
The mining process area No. 870.977/2021 covers approximately 175 hectares and is located on the municipal borders between Rio do Pires and Caturama, in Bahia, Brazil. Access from the nearest town, Rio do Pires, to the visited areas is via an 8 km dirt road in good condition.
Rio do Pires is approximately 720 km from Salvador, the state capital, following the longer but better-quality route, which passes through Feira de Santana, Maracás, Brumado, and finally Rio do Pires.
Rio do Pires has limited infrastructure to support a mining project, as it offers few hotels/inns, lacks specialized workshops, and does not provide heavy equipment rental services, among other factors typical of a small town. However, the city of Caetité-BA, located 150 km south of the visited areas, has better infrastructure to meet such needs
Fig 1. The mapped polygon is accessed through local roads. A notable feature within the area is the “Pedra do Boi”, the largest rock outcrop present in the region.
GEOTECTONIC EVOLUTION AND REGIONAL GEOLOGY
Regionally, the mining processe is set within the geological context of the Espinhaço Supergroup, specifically in the western domain of the Chapada Diamantina, involving primarily the Paraguaçu and Rio dos Remédios Groupsaccording to older studies. However, the book “Geologia da Bahia: Pesquisa e Atualização, 2012” references more recent works, such as “Guimarães et al., 2005” and “Loureiro et al., 2008,” which propose a different compartmentalization for this mapped area by including the Serra da Gameleira Formation at the base of the stratigraphic column. This updated compartmentalization will be used as the reference for the interpretations presented here.
The crystalline basement is composed of rocks from the Paramirim Complex.
The following sections provide key descriptions of these groups, from the base to the top, with the aim of correlating the literature with the field mapping observations.
Fig 2. Regional geology with emphasis on the distribution of the Rio dos Remédios Group and the Serra da Gameleira Formation, the main regional influences in the area.
PARAMIRIM COMPLEX (CRYSTALLINE BASEMENT)
It is located throughout the entire valley of the Paramirim River and is the most prominent lithostratigraphic unit in this valley domain. According to Arcanjo et al. (2000), it is predominantly composed of orthogneisses, which can be classified as augen-mylonitic and migmatitic. However, Sá et al. (1976) describes this complex through four lithological groups: banded gneisses with anatectic features, (hornblende)-biotite orthogneisses tonalitic-granodioritic migmatitic, biotite orthogneisses tonalitic to granitic, migmatitic gneisses, and associated granitic rocks: augen monzogranitic gneisses mylonitized and granodioritic orthogneisses.
The rocks of the Paramirim Complex are typically polydeformed and display a progressive mineralogical association consistent with the high amphibolite facies. The rocks are dated as Archean by Mascarenhas & Garcia (1989).
SERRA DA GAMELEIRA FORMATION
It is interpreted as a sequence of metasedimentary rocks with a total package thickness of up to 200 meters, where a wide variety of lithotypes can be found, including metaquartzarenites ranging from coarse to fine, metargillites, metaconglomerates (both polymictic and oligomictic), metarcoses, etc.
The Serra da Gameleira Formation lies on the crystalline basement rocks of the Paramirim Complex, with a total package thickness ranging from 70 to 200 meters, and is overlain by the rocks of the Rio dos Remédios Group through a regional erosional unconformity.
It is interpreted by most authors as originating from a period of crustal thinning before the opening of a rift-type basin, where sediments were accumulated in desert environments through aeolian processes, followed by debris flows and lacustrine sedimentation, which completed the deposition of this formation.
RIO DOS REMÉDIOS GROUP
This group can have direct contact with the basement rocks, but erosional contacts predominate with the Serra da Gameleira Formation at the base and the Paraguaçu Group at the top. This group is subdivided into three formations: Novo Horizonte, Lagoa de Dentro, and Ouricuri do Ouro.
Of the cited formations, Novo Horizonte, located at the stratigraphic base, holds significant metallogenetic importance, as it contains volcanic rocks (such as metadacites, metarriolites, metaquartz porphyries, and metaphenoandesites) that have been deformed and altered by various hydrothermal processes, resulting in important mineral occurrences, including gold, crystals, barite, and cassiterite. This formation was formed through four magmatic volcanic events.
Just above, the Lagoa de Dentro Formation has a maximum thickness of up to 850 meters and consists of rocks divided into three associations: at the base, metasiltstones, metargillites, impure metarenites with possible levels of oligomictic conglomerates, calcitic metarcoses, and metamuds; above that, there are metarenites with wavy markings in the intermediate zone; and at the top, metapelites, metasiltstones, and metarenites that are metasomatized.
At the top of the Rio dos Remédios Group, is the Ouricuri do Ouro Formation, composed of five associations of siliciclastic rocks deposited during a period of continental terrigenous sedimentation in an era of significant tectonic instability. These include polymictic metaconglomerates, metagravelites, metarcoses, metaquartzarenites, and metarenites with feldspar, among others. The most striking characteristics of this formation are the presence of conglomerates formed by gravitational flows and the high variation of lithofacies that constitute it.
PARAGUAÇU GROUP
The Paraguaçu Group is stratigraphically positioned above the basement and/or above the rocks of the Rio dos Remédios Group, in both cases over erosional unconformities. This group is subdivided into two formations: Mangabeira and Açuruá, and, as noted by Derby (1906), it consists of a thick package of quartzites, metasiltstones, metapelites, and discontinuous layers of metaconglomerates.
This group is defined by some authors as a supersequence of rocks deposited in a wide, shallow basin formed during passive subsidence with little tectonic activity.
Fig 3. An example of the stratigraphic columns that comprise the stacking of the previously mentioned metasedimentary rocks, placing the Serra da Gameleira Formation at the base of the sequence and the Paraguaçu Group at the top.
REGIONAL TECTONIC EVOLUTION
Several authors, such as Alkmin et al. (1993), Rocha & Dominguez (1993), Costa & Inda (1982), etc., describe the regional tectonic evolution influencing the areas mapped here as belonging to the development of the “Paramirim Corridor”, a zone located between the western edge of the Chapada Diamantina Group and the eastern edge of the Serra do Espinhaço Sententrional.
The structural evolution of this deformation corridor was complex, leading to different hypotheses among authors regarding the chronology of the events of its formation. However, in summary, it is understood that the Paramirim Corridor was generated from a marginal zone of an ancient rifting (crustal opening), which, after sedimentary deposits associated with basin formation, advanced to a closing (compressional) phase with fault reactivation and exposure of rocks of different natures intercalated by regional structures.
The “Basic Geological Survey Program of Brazil – Paramirim Valley Project, 2000”, carried out by CPRM in partnership with CBPM, describes the entire evolution of this compartmentalization in 8 distinct stages, with the last stage being the inversion (closure) of the Espinhaço rift and the uplift of the Paramirim block.
Bringing the knowledge of this evolution to the reality of the mapped areas, it is understood that transcurrent and compressive structures are common, primarily characterized by shear zones and the restructuring of rocks, forming mylonites over large territorial extents, especially in transition zones between metasedimentary units and the crystalline basement.
QUARTZITES AND QUARTZ
QUARTZITES
The metasedimentary unit also features some types of quartzites and metarenites. However, in most representative outcrops, certain characteristics are less favorable, such as the colors (highly variable but generally tending toward gray) and the fracture systems, which are always present..
Despite this, a highly interesting rock layer for producing high-value blocks was identified in the area. It consists of sheared white metarenite with quartz veining found at the "Garimpo do Lu," which is also associated with the primary gold source of the central target.
This rock does not exhibit significant thickness but spans approximately 180 meters in outcrop length, with massive and minimally fractured bands. Most importantly, it displays excellent physical and textural quality, significantly increasing its market value.
A diamond wire cutting test was conducted on a sample collected from this rock at point 542. The results revealed a white to translucent quartzite.
QUARTZ
The complex tectonic evolution of the area allowed for the formation of various zones of quartz veins and veinlets of different dimensions. These are associated with almost all types of ductile and brittle structures observed in the field, such as faults, shear zones, and those embedded within the foliation of the rocks.
Despite the widespread spatial distribution of these quartz veins across the area, the metasedimentary unit, particularly associated with quartzites and metarenites, shows the most significant occurrences of veins. Numerous old crystal mining sites ("catas") and large outcrops were identified in these units.
The high economic interest in this material is justified by its apparent chemical quality and purity, likely consisting of rocks with a high silica content and few contaminants. Some veins are so significant that they suggest the potential for evaluation as ornamental stone (block production).
The photographs below illustrate some of the "findings" from quartz crystal mining and depict one of the most promising veins identified during the mapping process. This vein, located at point P464, features large dimensions and very promising physical characteristics.
IDENTIFIED MINERAL POTENTIAL OF GOLD
GOLD ORE
The existing and known gold within the mapped area, as previously explained, is associated with a contact zone between two different units. This zone is characterized by intensely deformed rocks and the presence of quartz veins/veinletsassociated with black/greenish volcanic rock.
During a preliminary visits, samples collected from this contact zone (CZ) revealed positive gold anomalies, further supporting the presented hypothesis.
This zone is understood to be the source of the detrital gold present in the mined colluvium.
Fig 4. Example of gold collected by prospectors in the colluvium of the Central Target.
GEOLOGICAL CONTEXT OF GOLD MINERALIZATION
In simplified terms, the gold in the researched target is associated with quartz vein layers containing sulfides and hydrothermal iron oxides. These zones are correlated with fault or shear structures where the quartz is embedded.
The description above refers to PRIMARY ore, meaning the gold contained in the rock. Over time, through weathering processes, these layers release detrital gold in the form of dust and nuggets into the soil and drainage systems, forming SECONDARY ore.
Due to the existence of both types of deposits in the area (gold in the rock + gold in the soil/drainages), the exploratory research was conducted to investigate both.
Illustrations of gold-mineralized rocks in the Rio do Pires/Caturama project region.
Illustration of secondary gold ore.
DETAILED MAPPING
The layers of rock with high potential for gold presence were identified, along with the definition of the topographic correlation between the mineralization styles (primary and secondary), which allowed for the projection of the quantity and spatial distribution of the trenches.
During the mapping and planning phase, the study area was subdivided into two sectors: "Target NE-A" and "Target NE-B", facilitating the internal data correlation. The results of the subsequent phases to be described will be presented separately.
Fig 4. Detailing of the two sub-targets defined for research within Target NE.
OPENING OF WELLS AND TRENCHES
This stage was carried out with a Caterpillar 416 4x4 backhoe. In total, 4 trenches were opened in Target NE-A and 9 trenches in Target NE-B. The length and maximum depth of all trenches varied according to the type of geological material identified, but the width remained standard, around 90cm.
Illustration of the trenching operation and excavation process.
Fig 5. Location of the opened trenches.
DESCRIPTION OF THE TRENCHES.
The description of the trenches (TRs) was carried out with the collection of various data, such as point-by-point coordinates, altitudes, compass direction, and geology according to the type of existing information. All trenches crossed the soil and gravel layers down to the hard rock below (saprolite).
In the case of the gravels and soils, characteristics of the predominant cobble dimensions, matrix/cobble ratio, cobble types, matrix type, thickness, and the likely level of rework by former mining activities were described.
Regarding the rocks, the main structures were evaluated, such as faults/fractures, shear zones, and foliation, as well as the division of various lithotypes, classifying them according to their importance for gold deposits. An example of this includes the primary ore layers, where channel sampling was carried out.
These pictures illustrate primary ore layers identified during the opening of TR6-B, TR3-B, and TR3-A, respectively.
SAMPLING
The sampling consisted of collecting rock samples (primary ore) and gravel/soil samples (secondary ore).
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Primary ore samples (rock): As mentioned earlier, in some trenches, layers of gold-bearing rock were found. In these locations, detailed descriptions of the material were made, and channel samples were collected.
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A channel sample consists of collecting the gold-rich rock and its surrounding matrix, allowing for a more accurate assessment of the average grade of that zone. A total of 6 channel samples were taken from 6 different trenches.
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Throughout the report, some spreadsheets will provide additional data on this sampling.
Fig 6. Example of channel sampling, in this case from AM-PR-TR3 in TR3-A, where 100 cm by 10 cm of mineralized material and surrounding sterile matrix were collected.
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Secondary ore sampling (soil/gravel): A total of 29 samples were collected in Target A and 39 samples in Target B. Several control steps were taken for each trench, such as:
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Definition of sample spacing;
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Determination of the total length of the sampling area;
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Observation of the material type (mined gravel or virgin gravel);
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Cleaning of collection tools (tarps + tools).
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For each defined sample, collection was carried out as follows:
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In areas with preserved (virgin) gravel, only this material was collected consistently across its thickness for the entire length of the sample;
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In areas with preserved gravel mixed with soil/gravel from previous mining, all of this material was included in the sample.
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In all trenches and for all samples, the collection process followed these steps:
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Material removal from the trench wall;
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Screening with a 0.35mm granulometry;
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Classification (ratio of sand, clay, and gravel);
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Packaging and titling;
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Weighing.
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Note: Only TR7B was sampled in raw material without screening.
DATA TREATMENT
The collected samples were panned for: heavy concentration, gold spotting + description, and finally for chemical characterization. All data obtained in all the stages mentioned were considered for the final interpretation of results.
Maps and geoprocessing products were created and helped define the spatial distribution of the information on the surface and determine the indicated volume of mineable ore.
RESULTS OBTAINED
In summary, both targets NE-A and NE-B showed predominantly positive results for the presence of gold, with target NE-B being the most significant in terms of grade and volume.
MAP OF ISOCHORES
Based on the results of the chemical analyses to be conducted, interpretative maps of gold concentration density and colluvium thickness will be generated, providing information on average grades for the deposit.
INDICATED VOLUME
For the estimation of the existing ore volume, some crucial points must be recalled:
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The calculations to be presented refer ONLY to the SECONDARY ore, i.e., the gold dust deposit in the soil and gravel of the area;
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The calculations considered EXCLUSIVELY the gold identified in the panning count, remembering that:
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A manual process was performed, without the aid of precision tools such as a bench magnifying glass or fine vibrating table, for example;
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The samples were panned, which made it impossible to detect coarser gold, so only grains with a maximum granulation of 0.30mm could be identified.
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The volumes presented will be treated as INDICATED for the NE-A and NE-B targets. When considering the dimensions of the entire mineralized area outside these targets, the volume will be treated as INFERRED (lower degree of information).
CONCLUSIONS
ABOUT THE GEOLOGY AND ECONOMIC POTENTIAL OF THE AREA:
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The existence of a mineral deposit of gold in economic ore conditions has been proven, where it is possible to extract and benefit the ore for profit, considering the secondary ore. however, the primary ore (rock) still needs further evaluation for more precise definitions;
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Only two small polygons within the ne target area (ne-a and ne-b targets) have been well researched so far, which considerably increases the exploratory potential of the project as a whole, which has yet to be fully explored. in addition to the remaining polygon of the ne target area (more than 70%), there are at least two other mapped targets with great potential;
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The ore in the area varies from sandy to clayey, and it also contains a significant amount of very fine gold, all distributed between packages of virgin gravel and soil disturbed by former miners. these characteristics make it difficult to accurately estimate the volume, grade, and waste/ore ratio.
ABOUT THE STRATEGIC CHARACTERISTICS OF THE PROJECT (NE TARGET), IT CAN BE SAID THAT:
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They are positive in terms of surface conditions and access, however, challenges will arise regarding water supply and energy usage;
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The region where the project is located does not present industrial mining operations in progress. therefore, any operation to be initiated in the area must be well planned concerning approvals, monitoring, and environmental issues.
