CARLOS MARTIN MEDINA
HYDROTHERMAL ALTERATION, MINERALIZATION AND SPECTRAL FOOTPRINTS AT THE LAVRA VELHA GOLD DEPOSIT, BAHIA, BRAZIL
ALTERAÇÃO HIDROTERMAL, MINERALIZAÇÃO E FOOTPRINTS ESPECTRAIS NO DEPÓSITO AURÍFERO LAVRA VELHA, BAHIA, BRASIL
CAMPINAS 2020
HYDROTHERMAL ALTERATION, MINERALIZATION AND SPECTRAL FOOTPRINTS AT THE LAVRA VELHA GOLD DEPOSIT, BAHÍA, BRAZIL
ALTERAÇÃO HIDROTERMAL, MINERALIZAÇÃO E FOOTPRINTS ESPECTRAIS NO DEPÓSITO AURÍFERO LAVRA VELHA, BAHIA, BRASIL
DISSERTATION PRESENTED TO THE INSTITUTE OF GEOSCIENCES OF THE UNIVERSITY OF CAMPINAS TO OBTAIN THE DEGREE OF MASTER IN GEOSCIENCES IN THE AREA OF GEOLOGY AND NATURAL RESOURCES
DISSERTAÇÃO APRESENTADA AO INSTITUTO DE GEOCIÊNCIAS DA UNIVERSIDADE ESTADUAL DE CAMPINAS PARA OBTENÇÃO DO TÍTULO DE MESTRE EM GEOCIÊNCIAS NA ÁREA DE GEOLOGIA E RECURSOS NATURAIS
ORIENTADOR: PROF. DR. DIEGO FERNANDO DUCART
ESTE EXEMPLAR CORRESPONDE À VERSÃO FINAL DA DISSERTAÇÃO DEFENDIDA PELO ALUNO CARLOS MARTIN MEDINA E ORIENTADA PELO PROF. DR. DIEGO FERNANDO DUCART
CAMPINAS 2020
Marta dos Santos - CRB 8/5892
Medina, Carlos Martín,
M468h MedHydrothermal alteration, mineralization and spectral footprints at the Lavra Velha gold deposit, Bahia, Brazil / Carlos Martín Medina. – Campinas, SP : [s.n.], 2020.
MedOrientador: Diego Fernando Ducart.
MedDissertação (mestrado) – Universidade Estadual de Campinas, Instituto de Geociências.
Med1. Minas e recursos minerais - Exploração. 2. Ouro. 3. Alteração
hidrotermal. 4. Espectroscopia de reflectância. 5. Vetorização. I. Ducart, Diego Fernando, 1974-. II. Universidade Estadual de Campinas. Instituto de
Geociências. III. Título.
Informações para Biblioteca Digital
Título em outro idioma: Alteração hidrotermal, mineralização e footprints espectrais no depósito aurífero Lavra Velha, Bahia, Brasil
Palavras-chave em inglês:
Mines and mineral resources - Exploration Gold
Hydrothermal alteration Reflectance Spectroscopy Vectoring
Área de concentração: Geologia e Recursos Naturais Titulação: Mestre em Geociências
Banca examinadora:
Diego Fernando Ducart [Orientador] Lena Virgínia Soares Monteiro Carlos Roberto de Souza Filho Data de defesa: 17-02-2020
Programa de Pós-Graduação: Geociências
Identificação e informações acadêmicas do(a) aluno(a)
- ORCID do autor: https://orcid.org/0000-0001-9097-464X - Currículo Lattes do autor: http://lattes.cnpq.br/2863300925207764
INSTITUTO DE GEOCIÊNCIAS
AUTOR: Carlos Martin Medina
HYDROTHERMAL ALTERATION, MINERALIZATION AND SPECTRAL FOOTPRINTS AT THE LAVRA VELHA GOLD DEPOSIT, BAHIA, BRAZIL
ALTERAÇÃO HIDROTERMAL, MINERALIZAÇÃO E FOOTPRINTS ESPECTRAIS NO DEPÓSITO AURÍFERO LAVRA VELHA, BAHIA, BRASIL
ORIENTADOR: Prof. Dr. Diego Fernando Ducart
Aprovado em: 17 / 02 / 2020
EXAMINADORES:
Prof. Dr. Diego Fernando Ducart - Presidente
Profa. Dra. Lena Virgínia Soares Monteiro
Prof. Dr. Carlos Roberto de Souza Filho
A Ata de Defesa assinada pelos membros da Comissão Examinadora consta no processo de vida acadêmica do aluno.
Carlos Martín Medina é graduado com honras (Cum Laude) em Geologia pelo Departamento de Geologia e Faculdade de Ciências Exatas da Universidad Nacional de Río Cuarto, Córdoba, Argentina, em 2016.
Entre 2017 e 2018 trabalhou no Ministerio de Agua, Ambiente y Servicios Públicos de la Provincia de Córdoba (Argentina), como inspector ambiental focado principalmente em tarefas relacionadas ao controle do recurso hídrico e extração de areia ao longo do canal ativo do rio Cuarto.
Em 2018 ingressou no programa de pós-graduação da Universidade Estadual de Campinas, com bolsa da CAPES. Atualmente finalizousua dissertação de mestrado em Geociências através do programa de pós-graduação do Instituto de Geociências da UNICAMP.O trabalho é focado em exploração mineral através de estudos petrográficos, MEV e espectroscopia de reflectância em diferentes escalas de observação do depósito aurífero Lavra Velha, Bahia, Brasil.
needed it most (some of them, in a heavenly way).
I want to especially thank “Galera do Pará” to accept me as one of them. They are: Josué and Simmon (members of Cartel da Espectroscopia), Bruno, Cleberson, Laura, Sanny and “Panda” + Thaissa! The purpose of these lines is to express my infinite thanks for having included me.You have all given me some great times, and you guys make up all my best memories of this fantastic and unforgettable journey. I will never, ever forget you and will surely come back…
I would like to thank my advisor Diego for giving me the opportunity to work on the thesis subject, supervising me during these years and for cracking the whip when I needed it. In this sense, I would also like to thank Beto for their guidance and corrections which were invaluable and greatly enhanced the end result of this thesis. Also, I thank Prof. Lena and Prof. Maria Jose for providing helpful advices and detailed revisions on my thesis. To MJ, also for your friendship.
Special thanks Lucho for the immeasurable help in every respect. If it were not for that, my life at Campinas, without doubt, it would have been more difficult. Watching Slam Dunk and Dragon Ball wouldn't have been the same without you haha.
More thanks to all my Brazilian friends I have made since coming to Brazil and Unicamp: Igor, João M., Wellington, Everton, Poli, Robert, Douglas, Eduardo, Raphinha, Luciano, Halina, Raísa, Jussara, Priscila, Jessi, João P., Cesar, Antonio, Keyla, Kamila et al., who have really made my time here fun and worthwhile. Besides the Brazilian ones, I am grateful Hans, Marco and his wife Nedy, you are amazingly caring and kind people. Thank to my countrymen Nacho, Franco, Big Meli and Little Meli for every precious moment shared.
To Amanda, for his kindness, for his trust and for showing me who is the only champion of the nation's interior, o Bugrão! Never change! Hope to see you soon!
Yamana Gold is thanked for financial-logistical support in the fieldwork and analytical studies. Particularly, I thank Leandro for trusting meand allowing this work to have been carried out at Lavra Velha deposit. Also, for understanding that River Plate is bigger than o Tricolor haha. To Yamana Gold staff for the discussions around LavraVelha and for the predisposition and help in field.
Thanks Brazil, UNICAMP and the Institute of Geosciences for the opportunity to be part of their rich history. This research and my life at Brazil was made possible through
“Que la gente crea, porque tiene con que creer…” (09-12-18)
O depósito de ouro Lavra Velha (DLV) faz parte de uma recente descoberta mineral localizada no estado da Bahia, na porção central do Cráton São Francisco, no Brasil. Recentemente, o DLV foi inserido no modelo de depósitos tipo iron-oxide-copper-gold (IOCG) apresentando altos teores de Au-Cu hospedados nas rochas do Granitoide Ibitiara de idade Riaciana (2.1 Ga). A mineralização, alteração hidrotermal e a vetorização em direção aos corpos mineralizados foram investigados via petrografia, microscopia eletrônica de varredura (MEV) e espectroscopia de reflectância associadas à geoquímica de rocha total. O minério ocorre como corpos maciços veios, brechas, dominados por óxidos de ferro (hematita>magnetita), pirita, ouro, tennantita, digenita, bismutinita com abundância de quartzo e turmalina como minerais de ganga. O ouro encontra-se incluso, principalmente, em hematita, pirita e quartzo e raramente em cassiterita. Além disso, o metal pode ocorrer em contato com tennantita e turmalina, bem como associado a minerais secundários, tais quais jarosita e goethita. Definiram-se os diferentes estágios de alteração hidrotermal, a saber: alteração sódica inicial, seguida por um estágio de ampla alteração propilítica com cloritização local e prolongado metassomatismo férrico, formação de turmalina e um estágio tardio marcado por alteração hidrolítica. O Lavra Velha também foi afetado por processos supergênicos que modificaram a prévia assembleia mineral hidrotermal e favoreceram a maiores concentrações da mineralização. A maior parte dos corpos mineralizados ocorre confinada a zonas fortemente deformadas e dominadas tanto por domínios cisalhados quanto por estilos em stockwork. Os horizontes mineralizados apresentam enriquecimento em Au-Cu-Bi-Fe-As-U-V-(Ag-Sb-Sn-W-Pb), enquanto que concentrações anômalas de Cr, Ni e Co ocorrem no footwall e hanging wall próximo. A anomalia geoquímica desses elementos de transição (Cr, Ni, Co) junto ao reconhecimento da associação talco-nontronita por meio da análise espectral sugerem a existência de rochas ultramáficas, que estão estreitamente associadas à mineralização. No depósito Lavra Velha, micas brancas mostraram ser os melhores indicadores para definir vetores às zonas mineralizadas. Os resultados espectrais mostram que a mineralização em Lavra Velha tem uma forte associação com mica branca, hematita e turmalina-Fe, apontando esses minerais como proximais aos orebodies. Particularmente, a mica branca torna-se progressivamente mais fengítica conforme os teors de ouro aumentam, e tende a ser paragonita quando o teor de ouro é diminuído. A cristalinidade da mica branca refletia um forte zonação cujos limites podem ser associados a potenciais locais de deposição de minério. Micas de alta cristalinidade ocorrem relacionadas com zonas de alta deformação e marcam os possíveis caminhos percolados pelos fluidos que transportaram os metais, enquanto rochas ultramáficas foram condicionadores químicos essenciais à precipitação dos metais. Assim, os resultados aqui apresentados indicam que a medida de SWIR hiperespectral da composição e cristalinidade da mica branca pode ser uma ferramenta útil para mapeamento relacionado à alteração, bem como à vetorização no depósito Lavra Velha. Técnicas espectrais assistidas com geoquímica e dados de microscopia fornecem uma abordagem para a caracterização mineral de um depósito complexo como Lavra Velha, ao mesmo tempo em que fornece vetores úteis para direcionar zonas de interesse em um contexto exploratório.
Palavras-chave: Lavra Velha; Ouro; Alteração hidrotermal; IOCG; Espectroscopia de Refletância;
LavraVelha Deposit (LVD) is a newly discovery located at Bahia State, in the frame of the São Francisco Craton, Brazil. LVD was recently inserted within the IOCG class, and is characterized by high Au-Cu grades hosted in the Rhyacian (2.1 Ga) IbitiaraGrainitoid. Ore, hydrothermal alteration mineralogy and vectoring towards ore were constrained by petrography, SEM, reflectance spectroscopy and whole-rock geochemistry. The orebodies take place as massive replacement lenses, veins and locally as breccias, consisting of iron oxides (hematite>magnetite), pyrite, gold, tennantite, digenite, bismuthinite with quartz and tourmaline as main gangue. Gold is found as inclusion in hematite, pyrite, quartz; in contact with tennantite, white mica; and in association with secondary jarosite and goethite. Different types of alteration affect the host rock at LVD: an early sodic stage, followed by a widespread propylitic stage with local chloritization, which were accompanied by a protracted and recurrent iron-metasomatism, tourmaline-quartz formation and a later hydrolytic stage. LavraVelha was also affected by a supergene stage that transformed much of previous hydrothermal mineral assemblages and led to a local upgrading of gold and copper concentration. The paragenetic evolution of the LavraVelha deposit indicate significant varying conditions of the hydrothermal fluids including changes in temperature, pH and oxygen and sulfur fugacity. These conditions led to formation of at least two distinct ore types: (i) gold-pyrite-tennantite-bismuthinite(-magnetite), and (ii) gold-hematite. The orebodies are confined to high strain zones, associated with sheared rocks but also, with stockworks domains. Mineralized horizons have enrichment in Au-Cu-Bi-Fe-As-U-V-(Ag-Sb-Sn-W-Pb) in relation to wall rocks. Anomalous concentrations of Cr, Ni and Co occurs in both in the immediate footwall and hanging wall. This anomaly in transitions elements in addition to spectral association of talc-nontronite, is inferred to represent ultramafic units, which are spatially associated with gold. Spectral results show that mineralization at LavraVelha has a strong association with white mica, hematite, and Fe-tourmaline pointing these minerals as proximal to orebodies. The white mica at LavraVelha is the most helpful proxy mineral to define vectors to orebodies. Particularly, white mica becomes progressively more phengitic (~2212 nm) as higher Au grades are approached, and it tend to be paragonite (~2197 nm) when gold grade is waned. The crystallinity of white mica reflected a strong top-to-bottom zonation illustrated by muscovite → illite → smectite, which temperature-related limits can be associated with potential sites of ore deposition. Thus, the results presented here indicate that hyperspectral SWIR measurement of white mica composition and crystallinity could be a useful tool for alteration-related mapping as well as vectoring on LVD. Also, point spectroscopy demarcates the extent of the Au-bearing oxidized cap and can assist by identifying potential sites of supergene Au-Cu enrichment through presence of jarosite, and increased iron intensity associated to goethite and hematite. Spectral techniques assisted with geochemistry, and thin section data, provide an approach for mineral characterizating of a complex deposit like LavraVelha, at the same time it provides handy vectors for targeting interest zones in exploratory frameworks.
STRUCTURE OF THE MANUSCRIPT ... 15
1. INTRODUCTION ... 16
2. OBJECTIVES ... 19
3. MATERIALS AND METHODS ... 20
3.1 Equipment ... 20
3.2 Literature review ... 20
3.3 Field work and sampling ... 20
3.4 Point spectral data measurement and processing... 21
3.5 SWIR hyperspectral imaging system... 21
3.6 Hyperspectral image processing ... 22
3.6.1 “Spectral Hourglass” approach ... 22
3.7 Petrography ... 24
3.8 Scanning electron microscopy ... 24
3.9 Whole-rock geochemistry ... 25
4. LOCATION OF STUDY AREA AND ACCESS ... 26
5. REGIONAL GEOLOGY AND TECTONIC EVOLUTION ... 27
6. GEOLOGICAL SETTING OF THE IBITIARA REGION ... 31
6.1 Basement... 31
6.1.1 Paramirim Complex ... 31
6.1.2 Ibitiara Granitoid ... 32
6.2 Espinhaço Supergroup ... 32
6.2.1 Serra da Gameleira Depositional Sequence (Pre-Rift) ... 32
6.2.2 Rio dos Remédios Group (Syn-Rift) ... 32
6.2.3 Paraguaçú Group (Pós-rift) ... 34
6.3 Mafic intrusive rocks ... 34
BIBLIOGRAPHY ... 40
APPENDIX I ... 47
Hydrothermal Alteration, Mineralization and Spectral Footprints at the LavraVelha Gold Deposit, Bahía, Brazil ... 47
1. Introduction ... 48
2. Geological setting ... 51
3. Materials and Methods ... 56
4. Spectral parameters and nomenclature ... 58
5. Results ... 60
5.1 Geology of the cross-section ... 60
5.2 Hydrothermal alteration ... 62
Sodic facies ... 62
Propylitic facies ... 63
Chlorite facies ... 66
Tourmaline-quartz veining and alteration... 68
Magnetite-rich facies ... 69
Hydrolytic facies ... 71
Supergene overprint... 72
5.3 Gold (-copper) mineralization ... 76
Sulphide-rich orebody ... 76
Ironstones ... 77
Late sulphide-bearing carbonate(-quartz) veining ... 78
5.4 Geochemical-spectral integration for single mineralized drillholes ... 80
Drillhole FLV-45 ... 80
Evolution of the LavraVelha system ... 91
Considerations about golddeposition and geochemical patterns ... 95
Spectral footprints ... 96
Implications for exploration ... 99
7. Conclusions ... 102
8. References... 104
STRUCTURE OF THE MANUSCRIPT
The present manuscript is articulated in two main parts: a first comprising an introductory section, objectives and methods carried out in this thesis, along with a background from regional and local geology of the study area, in a broader sense. The second part is presented as an APPENDIX in scientific manuscript format titled “Hydrothermal Alteration,
Mineralization and Spectral Footprints at the LavraVelha Gold Deposit, Bahía, Brazil”,
1. INTRODUCTION
The LavraVelha deposit (LVD), located in the regional framework of the São Francisco Craton (SFC) (Almeida, 1977), Bahia state (Brazil), has been interpreted by Campos (2013) as an IOCG-type deposit, with high gold and copper grades (Au>>Cu). The deposit is characterized by hydrothermal veins and breccias, associated with sub-volcanic and granitic rocks of Riacian age (2.1 Ga). LVD renders the main target within a regional exploration program developed by Yamana Gold company in this area of the SFC. The host rocks of the mineralization comprise mainly meta-tonalite and meta-quartz diorite, which are hydrothermally altered showing an intense sericitization and iron oxide formation, as well as, carbonate and epidote zones (Carlin et al., 2018). Recent work carried out by Yamana Gold suggests that the footprint of alteration and mineralization has potential to be much larger than originally thought. Thus, further research is needed for LavraVelha. This study pretends to be a contribution in terms of defining the alteration-ore-system and, vectoring the mineralization.
Iron oxide copper-gold (IOCG) deposits are a relatively new class of ore deposits defined firstly by Hitzman et al. (1992), that encompass a broad spectrum of sulfides deficient, low-Ti magnetite and/or hematite orebodies. Breccias, veins, disseminations and massive lenses with polymetallic enrichments (Cu, Au, Ag, U, REE, Bi, Co, P, LREE) occur associated with an extensive and zoned Na-Ca-K(-Fe-H) metasomatism (Barton, 2014). IOCG´s span a wide range in age from Archean to Cenozoic, and varying tectonic settings from extensional, anorogenic to orogenic frameworks (Barton, 2014; and references therein). Their lithological hosts, ages and tectonic setting are non-diagnostic, but their alteration zones tend to follow a temporal/spatial pattern, with an early calcic-sodic regional alteration at deeper levels, superimposed by a potassic alteration toward intermediate (-shallow) depths, and a shallowest hydrolytic (acid) stage developed late in time (Hitzman, 2000). The host rocks in IOCG deposits can vary from felsic to mafic igneous rocks, from clastic to chemical sedimentary rocks, or even be the metamorphic product of the preceding (Barton, 2014).
In the majority of hydrothermal systems, the types of alteration exhibit gradual variation with increasing distance from mineralization reflecting changes of fluid properties (e.g. composition, temperature). The alteration halos form around the orebodies as a product of hydrothermal fluids flowand often display distinct features when compared with their surroundings (Ni et al., 2018). Therefore, mapping of alteration zoning and physicochemical variations is indispensable, as well as an efficacious manner of defining proxies for mineralized zones. In this sense, the knowledge about the mineralogy of hydrothermal
alteration zones and their timing relationships is fundamentally important for understanding the formation of ore deposits and for devising strategies in pursuing for new mineral resources. The mineral exploration industry is increasingly aware of the importance of the integration of mineralogical data and other sources of information (e.g. geochemical, geophysical data), from which the economic geology is nourished. Nonetheless, achieving such aims with a high level of detail and accuracy involves considerable expenses and time-consuming and hence, major investments. These issues can be addressed using reflectance spectroscopy by means of hand-held spectrometers that allow a rapid, unbiased and cost-effective analysis of minerals including subtle variations of crystallinity and composition, that may be difficult to achieve using traditional logging methods (Thompson et al., 1999).
Diverse terrestrial materials such as minerals, rocks and hydrocarbons exhibit key absorption features along the electromagnetic spectrum, including the visible-near infrared (VNIR) (0.4-1.3 μm), short-wave infrared (SWIR) (1.3-2.5 μm), mid-wave infrared (MWIR) (2.5-6 μm), and long-wave infrared (LWIR) (6-14 μm) regions, in response to electronic and vibrational processes, as well as overtones (Adams, 1975; Burns, 1993; Clark, 1999; Farmer, 1974; Hook et al., 1999; Hunt, 1982; Feng et al., 2011). These spectral data have been commonly used to identify mineralogy of interest on the diverse application fields, with particular emphasis for mapping of hydrothermal systems-related minerals (e.g. micas, clays and carbonates) in the short-wave infrared range (e.g. Thompson et al., 1999; Yang et al., 2005; Crósta et al., 2009; Swayze et al., 2014). The visible-near infrared region can be properly used for detecting minerals such as hematite, goethite, jarosite and copper oxides/carbonates that usually comprise the oxidation zone in many deposits (Pontual et al., 2008a; Kruse, 2012).
Spectroscopic methods using portable field instruments for rock samples have their beginnings in the late 1990s with the works of Kruse (1996) and Taylor et al., (1997). Studies carried out by Sun et al., (2001), Sonntag et al., (2012) and Calvin and Pace (2016) among others, have showed the effectiveness of spectral measurements on core samples for mineral exploration. These authors demonstrated the consistency between results from spectral analysis and other methods, including petrography, geochemistry and XRD. The extraction of quantitative information from spectral curves is a method that is in full development, mainly from the arising of the hyperspectral imaging systems in recent years (Clark et al., 2006; Travers & Wilson, 2015; Asadzadeh& Souza Filho, 2016). The advent and deployment of hyperspectral drill-core scanners, allows that the information can be acquired
rapidly and at high spatial resolutions (e.g., Calvin et al., 2005) while providing help to verify the spatial and temporal relationships of minerals (Kruse et al., 2012; Tusa et al., 2019).
Spectroscopy has been widely used to study alteration mineralogy through multiple mineralized systems worldwide, including epithermal (Ducart et al., 2006; Carrino et al., 2015a), porphyry (Harraden et al., 2013; Neal et al., 2018), orogenic (Arne et al., 2016; Naleto et al., 2019), skarns (Tian et al., 2019), volcanic-hosted massive sulfide (VHMS) (Herrmann et al., 2001; Thompson et al., 2009; Laakso et al, 2016), Iron Oxide Copper Gold (IOCG) (Mauger et al., 2016; Tapper et al., 2011) and gold deposits (Cudahy et al. 2009; Travers and Wilson, 2015). Despite the large alteration zones having key minerals that can be used as pathfinders in IOCG deposits, spectroscopy-based investigations have been limited to regions of Australia (Fabris et al., 2013; Laukamp et al. 2011, Tapper et al., 2013). LavraVelha provides an attractive scenario for VNIR-SWIR-spectroscopy because prior studies (e.g. Campos, 2013) showed that alteration minerals at LavraVelha include sheet-silicates (e.g. white mica, chlorite), along with hematite, magnetite, tourmaline, epidote and carbonate, which (excluding magnetite) are infrared-active minerals over these portions of the electromagnetic spectrum.
The proposal of this master’s thesis focuses on the study of hydrothermal alteration of the LavraVelha Au-Cu deposit. The notion is to define the mineral footprints and vectors towards mineralization that may assist in future prospectivity strategies. The study was carried out using imaging and point spectral technologies on drill cores samples, encompassing diverse scales of scope, and covering the VNIR-SWIR spectral region. Optical petrography, scanning electron microscopy and whole-rock geochemistry were coupled to the study with the goal of achieving a holistic approach of the mineralizing system. Thus, these aims are directed towards gaining a further understanding of the alteration and ore-forming processes involved in the LavraVelha IOCG system, contributing to enhance the knowledge of a relatively barely explored area in Brazil.
2. OBJECTIVES
The main goal of this study is to define the alteration mineral assemblages of the LavraVelha Au-Cu deposit and associated mineralization, aiming to achieve a better understanding of the ore-forming process that took place during the evolution of the system.
Specific aims to meet this major objective are:
✔ To define the alteration mineral assemblages, its spatial distribution in vertical cross-sections, as well as its spatial and temporal relationships with mineralization. For this aim, we implement a multi-methodapproach that includes VNIR-SWIR reflectance spectroscopy (in drill cores samples), petrography, scanning electron microscopy, whole-rock geochemistry and magnetic susceptibility data.
✔ To document the main ore mineralogy and mineral textures aiming to interpret the possible nature of the mineralizing fluids and conditions of formation.
✔ To establish working methods for extraction and processing of quantitative data from different spectral techniques, aiming to obtain information about abundance, composition and degree of structural order of the alteration minerals.
✔ To evaluate physical-chemical properties of white mica and chlorite throughout the drillholes with a focus on vectoring to mineralization.
✔ From investigated spectral-mineralogical features, to propose vectoring tools useful not only in LavraVelha, but for similar deposits elsewhere.
3. MATERIALS AND METHODS 3.1 Equipment
For this thesis, the following instruments and software were used:
● PANalyticalTerraSpec® 4 Hi-Res spectrometer (Yamana Gold Inc). ● Sisu-CHEMA/SWIR hyperspectral camera (Laboratory of the Institute of
Chemistry / Unicamp).
● High-resolution scanning electron microscopy (SEM) (IG/Unicamp). ● TerraplusModelo: KT 10 Plus Susceptibilímeter (Yamana Gold)
● LEICA DM750P petrographic microscope (Microscopy Laboratory - MICRO, IG / Unicamp).
● ENVI® and TSG-8™ software were used for spectral data analysis, image processing and data integration. ArcGIS and Leapfrog software were applied for generating of a local map and to create a cross-section respectively.
3.2 Literature review
For this thesis, bibliographic surveys were carried out on pertinent and particular topics of study, most of them are cited in this manuscript. To date, Carlin et al. (2018) is the only work published on LavraVelha deposit, being the main scientific literature reviewed along with Campos (2013) Master’s thesis and an internal report from Geological Survey of Brazil (Guimarães et al., 2005). In addition, Yamana Gold’s internal briefing were also consulted during the course of the whole study, in order to generate knowledge feedback between the company and the author.
3.3 Field work and sampling
A fieldwork campaign, comprising 18-days, was performed and aimed at making the description of drill cores (logging), spectral measurements and collecting drill core samples. During the field visit, suitable samples were selected for the production of thin sections. One transversal section comprising five drill holes was analyzed, trying to map the shallow, central and deep portions of the mineral system, as well as to document spatial and temporal relationships in hydrothermal alteration and ore. The boreholes used for making up
the cross section involved mineralized three (FLV-45, FLV-97 and FLV-57), and barren two (FLV-038 and FLV-40).
3.4 Point spectral data measurement and processing
Spectral measurements on each drill core by means of a TerraSpec™ 4 Hi-Res spectrometer were carried out on an average of 2m distance apart, resulting in a total of 732 spectra. The TerraSpec spectrometer collects data in the 350-2500 nm wavelength range with a spectral resolution of 6 nm and sampling interval of 1 nm in the SWIR wavelength region. The device performed 70 individual measurements in a row for each spectrum acquired. These spectral measurements were attained using a contact probe (wired to the spectrometer) that hold a steady internal illumination source, thus providing the suitable lighting conditions for measuring. The raw radiance values were converted to reflectance values via the Spectralon™ panel, while instrument calibration was carried out every 15 min during data acquisition.
Spectral data collected in this stage were processed, analyzed and interpreted in order to identify the main alteration minerals, their abundance and chemical composition, as well as their temporal relationships. The processing and analysis was performed through the TSG-8™ software. The spectra were interpreted by comparing the unknown spectra to references spectral libraries from USGS (Clark et al., 2007) and GMEX guides (Pontual et al., 2008a, 2008b). The mineral identification was taken on the basis of geometry and diagnostic absorption features in the spectra, such as wavelength position, width and depth.
Scalars or spectral parameters for the extraction of semi-quantitative data (e.g. abundance, composition) were based on the analysis of the characteristic features aforementioned and following baseline literature such as Cudahy et al. (2008), Haest et al. (2012) and Sonntag et al. (2012). These parameters were plotted along complete cross-sections, as well as on individual drill holes, to visualize patterns or trends, both in depth and laterally throughout of the system studied.
3.5 SWIR hyperspectral imaging system
For representative samples collected in the drill cores, hyperspectral images were obtained by a SisuCHEMA SWIR XLTM hyperspectral camera (SPECIM, Finland) at the Laboratory of Chemistry Department at UNICAMP. Taking advantage of its higher spatial and spectral resolution to measure diagnostic spectral absorption traits in the SWIR wavelength region, SisuCHEMA device was employed to identify mineral variability, as well
as subtle compositional variations into each drill hole samples. This push-broom imaging technology recorded 256 spectral bands at increments of 6.26 nm (spectral resolution) that resulted in images consisting of 156 x 156 μm pixels. The instrument is equipped with a high-magnification lens with a field of view of 50 mm. Images were collected by scanning each rock sample line by line covering a spectral range of 940-2500 nm with 10 nm resolution. ChemaDAQ data acquisition software was used for saving spectral data (imagery) in real-time. Calibration (i.e., raw image data to reflectance) was carried out using an internal standard reference configured to automatic fashion.
On resulting images, a spatial and spectral subset was performed in order to mask out non-interest areas (i.e., sample platter background and uneven sample edges) and to suppress the noise (stripping) respectively, thereby eliminating the first and last 13 channels of the spectra.
Besides acquiring images, this imaging technology collects an entire spectrum for each image pixel, thus allowing detailed mapping based on the spectroscopic characteristics of minerals (Goetz et al., 1985, Boardman et al., 1995).
3.6 Hyperspectral image processing
3.6.1 “Spectral Hourglass” approach
This processing methodology for hyperspectral imagery analysis was applied for extraction of spectral endmembers (or key spectra) from sisuCHEMA dataset. This approach, incorporated within ENVI® software as the “Spectral Hourglass Wizard” tool, combines different convex geometry-based procedures interactively using the derived endmembers (Boardman and Kruse, 2011). Practical applications and a detailed description of this technique are found in the works of Boardman (1993); Kruse et al., (1996); Kruse et al., (2002); Kruse and Perry (2009) and Boardman and Kruse (2011), among others. According to Boardman and Kruse (2011), this method permits the reduction of spectral and spatial size (dimensionality) of imagery in order to find, to define and to select scanty and specific endmembers. Thereupon, these few key spectra are used to map their abundances and spatial distribution. The “step by step” involved in the hyperspectral imagery processing is outlined below and simplified in Figure 1.
(1) MNF Transformation: the minimum noise fraction (MNF) transformation (Green et al. 1988), is a type of principal component analysis that is enable to reduce spectral
dimensions and to eliminate noise in the data, partitioning the data space into two groups: one consisting of coherent images (MNF bands), which are used for next steps, and a second involving noise-dominated images that was sidelined for subsequent processing.
(2) Pixel Purity Index (PPI): making use ofthe MNF bands selected, a “Pixel Purity Index” (PPI) method were utilized in order to determine the endmembers, which represent the most spectrally extreme pixels (purest) from a dataset (Boardman et al., 1995). Thus, selecting only the purest pixels, it is possible to reduce the unique spectra to a minimum for forthcoming separation of specific endmembers.
(3) Interactive N-dimensional visualization: the pixels aforementioned was used in an interactive “N-dimensional” visualization technique that permits to extract the endmembers, where “N” is the number of MNF bands (Boardman, 1993; Boardman and Kruse, 2011). Within ENVI® software, spectra are visualized as points distributed in an N-space scatterplot or “data cloud”, thus allowing to estimate the number of spectral endmembers and their pure spectral signatures. Then, the extremities on the cloud (corresponding to a pure endmember) pointed out and separated for ensuing identification.
(4) Identification of Endmembers: the recognition of the key spectra was carried out as explained in Section 4.4 (i.e. a combination of visual inspection and comparison to spectral libraries). These endmembers or a subset of them were used for further mineral classification/mapping.
(5) Mineral Mapping: The Spectral Angle Mapper (SAM) technique was used as a final step in producing mineral maps for the hyperspectral SisuCHEMA imagery. SAM algorithm is an automated method for comparing input spectra to reference spectra based on spectral similarity (J. W. Boardman, unpublished data; Kruse et al., 1993b). The input and reference spectra are projected as vectors into n-dimensional space (where n is equal to the number of input spectral bands). For each pixel, the algorithm determines the angle between the vector defined by the pixel values and each endmember vector (Kruse et al., 1993b). In addition, rule images are computed by displaying the angular distance (in radians) between each spectrum in the image endmember spectrum. Darker pixels in these rule images represent smaller spectral angles, meaning spectra with more similarity to the endmember spectra. Then, the resulting image is inverted, whereby the best matches are shown as bright pixels. It provides a means of identifying and spatially classification minerals, which final product is an image that shows the best SAM match at each pixel (a classes map).
Fig. 1. Depiction of the “Spectral Hourglass” approach showing each step involved in the imaging hyperspectral data analysis (Adapted from Boardman and Kruse, 2011).
3.7 Petrography
Petrographic analysis of twenty polished thin sections was performed using a LEICA DM750P microscope, which operates with both transmitted and reflected light modes. A particular focus was to evaluate alteration assemblages and textural relationship, as well as for recognition of minerals that permitted to validate (or not) those identified using reflectance spectroscopy. The criteria for the sample selection were based on recognition of different rock and alteration types, ore mineral assemblages, textural pattern and distinct mineralization styles observed.
3.8 Scanning electron microscopy
Ore and accessory minerals were analyzed with Scanning Electron Microscope (SEM) using a Zeiss Leo 430i including EDS (Energy-Dispersive X-Ray Spectrometer), at the laboratory of Scanning Electron Microscopy of the Institute of Geosciences, UNICAMP, Brazil. The acceleration voltage was 20 Kv at 19 mm working distance, and with a flow de electrical current of 500 pA and 6000 pA for images and microanalysis respectively. The EDS device attached to the SEM allowed obtaining semi-quantitative compositional data from certain minerals. These analyses seek to identify non-recognized mineral phases through the optical microscope and at documenting microtextures, distribution and relationships between gangue and ore minerals.
3.9 Whole-rock geochemistry
Chemical analyses on 1-m intervals of drill core samples were provided by Yamana Gold company. Samples were crushed and analyzed using total (4 acid) and partial digestion (aqua regia) methods, in the ALS Chemex laboratories, Peru. Partial digestion is preferred for the investigation of mobile elements, as it is less aggressive than the total digestion. Trace elements were assayed by inductively coupled plasma mass spectrometry (ICP-MS) and major elements were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Gold was analyzed via lead collection fire assay on 50g samples.
4. LOCATION OF STUDY AREA AND ACCESS
The study area is located 20 km north of the Ibitiara town, in the center-west portion of the Bahia State (Brazil), approximately 500 km west of Salvador capital city (Fig. 2). The access to Ibitiara from the Salvador city is done through the highway 324 to Feira de Santana town, where route 116 is taken until the junction with road 242. Then, continue west on road 242, passing through the Chapada Diamantina National Park and Seabra city, for finally to take route 152 to head toward Ibitiara town. Once there, the entrance to LavraVelha area is made via dirt roads.
Fig. 2. Location map of the study area showing main access (routes) and nearby cities to LavraVelha.
5. REGIONAL GEOLOGY AND TECTONIC EVOLUTION
The LavraVelha deposit is regionally located in the western edge of the physiographic domain of the Chapada Diamantina, nearly in the geographical centre of the São Francisco Craton, Bahia State, Eastern Brazil (Fig. 3). The São Francisco Craton is bounded by the Neoproterozoic Brasília, Araçuaí, Rio Preto, Riacho do Pontal and Sergipano fold-thrust belts (Almeida et al., 1981); It is broadly covered by Precambrian and Phanerozoic successions comprising two morphotectonic units known as the Paramirim Aulacogen and the São Francisco Basin (Fig. 3) (Cruz and Alkmim, 2006). The basement of São Francisco Craton is formed by a nucleus of Archean age and two segments corresponding to a Paleoproterozoic orogen (Alkmim, and Martins-Neto, 2012 and references therein). The archean nucleus is represented by the Itabuna-Salvador-Curaçá Belt, and Serrinha, Jequié, and Gavião blocks (Fig. 3).
The Gavião Block, represented by its homonymous complex, comprises Archean TTG orthogneisses and migmatitic-gneisses terranes, as well as greenstones belts, granitoids and supracrustal sequences of Paleoproterozoic age (Cordani et al., 1992; Bastos Leal et al., 1998; Cruz et al., 2009). Dating carried out on zircons from these TTG terranes, yielded ages between 3.4-3.1 Ga, coupled with TDM Sm-Nd model ages of 3.6 Ga, which allowed characterizing these rocks as the oldest in South America (Cunha et al., 1996, 2000; Santos Pinho et al., 1998). The Gavião Block is bordered on the north by the Paramirim Block, defined by Arcanjo et al. (2000) as a suite of tonalite-trondhjemite-granodiorite rocks of Archean age belonging to Paramirim Complex, which experimented anatexis at ca. 2.7 Ga (Cordani et al., 1992). The Paramirim Block depicts the cluster where the LavraVelhadeposit is confined.
Over the basement rocks described above, were settled two diachronic ensialic sedimentary basins; (i) the Staterian Eastern Espinhaço Basin of rift-sag type; (ii) and the syneclise-type Chapada Diamantina Basin of Calymnian age (Guimarães et al., 2005). These two intracratonic basins are grouped into a major morphotectonic unit so-called the Paramirim Aulacogen (after Pedrosa Soares et al., 2001), initially defined by Costa and Inda (1982) as the “Espinhaço aulacogen”. It corresponds to an intracratonic rift basin generated from a succession of rift/syneclise stages developed between 1.75 and 0.68 Ga, with a long subsidence history recorded by the Espinhaço and São Francisco supergroups of Paleo/Mesoproterozoic and Neoproterozoic ages respectively, and by the
TonianMacaúbas-Santo Onofre Group (Guimarães et al., 2008; Danderfer Filho et al., 2009; Pedrosa-Soares &Alkmim, 2011).
The rifting event that leaded to the Paramirim Aulacogen establishment is also so-called Statherian traphogenesis and can be divided in different episodes that describe its evolution (Fig. 4). The first early-rift phase is defined by the implantation of the Espinhaço and Chapada Diamantina rift system that affected the basement comprising by Paramirim and Gavião Complexes. Thus, two main domains were structured, the Espinhaço domain to the west and the Chapada Diamantina Western to the east, both separated by the Paramirin Block (Fig. 4) (Loureiro et al., 2009). The following sin-rift episode is represented by an A-type magmatism corresponding to the Lagoa Real Complex, Matinos Granite and the volcanism of the Novo Horizonte Formation that intruded the basement along the axis of the Paramirim aulacogen (Fig. 4) (Pedrosa-Soares &Alkmim, 2011, and references therein). Subsequently, a post-rift phase of Calyminan age is marked by the deposition of continental/marine sediments of the Paraguaçu Group and Chapada Diamantina Group and by the intrusion of mafic dikes and sills dated at ca. 1.5 Ga using the U-Pb zircon method (Fig. 4) (Babinski et al., 1994; Guimarães et al., 2005).
In Ediacaran times, the Paramirim Aulacogen experienced a pronounced inversion that generated a system of NNW-trending faults and folds (Fig. 4) in response to a shortening related to collisions of São Francisco/Congo plate in the context of assembly of the West Gondwana (Cruz &Alkmim, 2006; Cruz et al., 2012). Muscovite/sericite from mylonitic rocks, generated during the inversion stage, were dated by Guimarães et al. (2005) and Campos (2013) using the Ar-Ar method and yielded ages of 497 ± 2.3 Ma and 516 ± 2 Ma, respectively (Fig. 4). According to Cruz (2004), this Neoproterozoic partial inversion of the Paramirim Aulacogen is expressed on basement from 12° 45’ S parallel towards the south, thus bringing implications on the São Francisco craton extension in these latitudes. By doing so, the author suggested a new limit located within the interference zone between the Paramirim aulacogen and the Araçuaí Belt (Fig. 3).
Within the Paramirim Aulacogen, Alckmin et al. (1993) and Cruz &Alckmin (2006) have recognized a zone of maximum positive inversion, denominated as Paramirim Corridor that cuts across the craton in the NNW direction (Fig. 3). This large structural feature is characterized by reverse-to-reverse dextral shear zones and dissimilar folding (Cruz et al., 2015, and references therein). Regarding the origin of the Paramirim Corridor, Alkmim et al.
(1993) proposed that it was caused by indentation of basement wedges and its imbrication over supracrustal rocks, as a consequence of the cratonward migration of the Brasiliano front.
Fig. 3. Simplified geologic map of the São Francisco Craton showing the Paramirim Aulacogen, the Paramirim Corridor, Archean segments and the surrounding Neoproterozoic belts, as well as the location of the study area. The boundary of the São Francisco Craton around latitude 12° 45’ is that proposed by Cruz (2004) (Based on Cruz and Alkmim, 2006).
Fig. 4. Geological model representing the rifting and basin inversion events developed during the Mesoproterozoic and Neoproterozoic, respectively. Ages: 1, Cunha et al. (2000); 2, Leal et al. (1996); 3, Guimarães et al. (2005); 4, Schobbenhaus et al. (1994); 5, Babinski et al. (1994); 6-7, Guimarães et al. (2005); 8, Campos (2013). (Figure based on Guimarães et al., 2005 and Loureiro et al., 2009).
6. GEOLOGICAL SETTING OF THE IBITIARA REGION
In this section, it will be described those lithostratigraphic units involved in the deposit area and surroundings, which are summarized in Figure 5 and showed on the map of Figure 6.
Fig.5. Lithostratigraphic sequence of the Ibitiara region (BA). Picture taken and modified from the Geological Survey of Brazil report ProjetoIbitiara-Rio de Contas (Guimarães et al., 2005).
6.1 Basement
6.1.1 Paramirim Complex
The basement in the region of the LavraVelha deposit is represented by migmatites and granodioritic orthogneisses of Archean age (Arcanjo et al., 2000) corresponding to the Paramirim Complex (Fig. 6) (Sá et al., 1976). Anatectic granites also occur cross-cutting these migmatitic banding. Granodioritic orthogneisses outcrop occasionally as medium-grained metric-scale bodies, exhibiting southwestern steeply dip (Guimarães et al., 2005). According to Texeira (2000), these rocks are peraluminous and have K calc-alkaline affinity, probably due to the partial melting of a crust of TTG composition, with the assimilation of sedimentary material.
6.1.2 Ibitiara Granitoid
The Ibitiara Granitoid occurs as an elongated body in the NNW-SSE direction. It outcrops in the central part of a large anticline structure (Ibitiara Anticline). These granitoids are surrounded by metavolcano-sedimentary rocks belonging to the Rio dos RemédiosGroup, whose erosion has allowed their current exposure (Fig. 6 and 7a). Tonalites, diorites and granites with peraluminous and calc-alkaline sodic signature, comprise the igneous suite that defines the different units of theIbitiara Granitoid (Guimarães et al., 2005). In addition, a porphyritic granodiorite facies with phenocrysts of sericitized feldspar and quartz also is present, often containing xenoliths of biotite gneiss. Hydrothermal processes such as potassic, epidote and sericite alteration affected these rocks in different degrees of intensity, although a strong alteration is the usual. U-Pb dating performed by Guimarães et al. (2005) in zircons, yielded crystallization ages of 2.09 ± 0.6 Ga. Teixeira (2005), from geochemical results, suggested an origin associated with partial melting of the metasomatized mantle in magmatic arc environment.
6.2 Espinhaço Supergroup
6.2.1 Serra da Gameleira Depositional Sequence (Pre-Rift)
According to Guimarães et al. (2005), this depositional sequence is lying on an erosive and angular discordance over the Archean gnesisses and Paleoproterozoic granitoids. It is formed by continental siliciclastic lithofacies that were deposited in a desertic environment via aeolian process, consisting of quartz metarenites, metagreywackes and metarkoses and metaconglomerates.
6.2.2 Rio dos Remédios Group (Syn-Rift)
The Rio dos RemédiosGroup represents the syn-rift stage during which took place an alkaline volcanism episode corresponding to the Novo Horizonte Formation followed by the settling of an alluvial fan system that deposited the lacustrine and conglomeradic sequences of the Ouricuri do Ouro/Lagoa de Dentro formations.
6.2.2.1 Novo Horizonte Formation
The Novo Horizonte Formation comprises rhyolites, dacites, quartz porphyry and phenoandesites, together with volcaniclastic rocks (Guimarães et al., 2005). According to Guimarães et al. (2005), this volcanic/subvolcanic sequences exhibits several patterns of
hydrothermal alteration, such as potassic, propylitic, sericite and hematite alteration, as result of the action of hydrothermal fluids stemming from different sources. Evidence of deformation that took place during basin inversion stage are recorded by the presence of sericite schists, mylonites and ultramylonites generated in shear zones.
Volcanism ages were obtained through the U-Pb method in zircon of metarhyolites and porphyritic dacites, yielding ages of 1.75 ± 0.4 Ga (Schobbenhaus et al., 1994) and 1.74 ± 0.4 Ga (Babinski et al., 1994). According to these data, the volcanic event would have occurred in contemporaneity with the Espinhaço rift development. The units comprising the Novo Horizonte Formation are associated to an A-type peraluminous alkaline to sub-alkaline magmatism (e.g. Matinos Granite), with crustal contribution developed in rifting setting (Teixeira, 2005).
Gold, barite and rutile quartz occurrences are found within this lithostratigraphic formation. Gold occurs mainly hosted in quartz veins, and subordinately as placer-type deposits. Mineralized veins appear concordant to main regional structures, represented by shear zones, fault and fractures with predominant NNW-SSE orientation (Guimarães et al., 2005).
6.2.2.2 Matinos Granite
The Matinos Granite is represented by granodiorites and monzogranite, which can present porphyritic or foliated texture (Guimarães et al., 2005). According to Teixeira (2005), they are classified as A-type potassic, metaluminous to peraluminous granitoids and were ascribed to be equivalents to the Novo Horizonte Formation.Arcanjo et al. (2000) showed that the granitoids are affected by narrow ductile shear zones, wherein were transformed to quartz-sericite-chlorite schist.
Guimarães et al. (2005) found occurrences of copper in the Matinos Granite, that take place in the form of malachite, chrysocolla, azurite, chalcopyrite, bornite and chalcocite hosted by quartz veins bearing magnetite, limonite and carbonate associated E-W-trending fault zones.
6.2.2.3 Ouricuri do Ouro/Lagoa de Dentro formations
The Ouricurí do Ouro and Lagoa de Dentro formations constitute one of the tectono-sequences defined by Guimarães et al. (2005), which culminated the first filling cycle of the Chapada Diamantina basin. Ouricurí do Ouro Formation is formed by
metaconglomerates, lithic metarenites, metarkoses and metagreywackes that lies on the volcanic rocks of the Novo Horizonte formation by means of an erosive discordance. Laterally, the Ouricurí do Ouro Formation is interdigitated with the rhythmic and lacustrine sediments of the Lagoa de Dentro formation. Recently, de Souza (2017) interpreted the Ouricuri do Ouro Formation as an alluvial fan system composed of distinct facies associations involving proximal deposits of non-cohesive debry-flows, proximal and intermediate sheet-floods, and distal sandy flood plains.
6.2.3 Paraguaçú Group (Pós-rift)
The sedimentary rocks of the Paraguaçu Group corresponding to Mangabeiras and Araçuaí formations mark the expiry of the alluvial systems controlled by mechanic subsidence and the onset of a passive sedimentation stage in arid conditions. (Guimarães et al., 2005). The Paraguaçu Group is constituted by coarse and impure metarenites deposited in desert coastal environment (Mangabeiras Formation), and rhythmic intercalations of metarenites-metapelites in shallow littoral environment (Araçuaí Formation). Both units are metamorphosed in low greenschist facies and show hydrothermal alteration (sericitization, silicification) mainly associated with shear zones.
6.3 Mafic intrusive rocks
According to Guimarães et al. (2005), mafic and tholeiitic dikes and sills intrude rocks of Espinhaço Supergroup. The dikes and sills are composed of medium- to coarse-grained isotropic dark greyish and greenish gabbros, that exhibit an intergranular texture with crystals of saussuritized plagioclase up to 1 cm length. Generally, these rocks present a preferred orientation NNW parallel to major structural trends. Chemically these rocks were classified by Teixeira (2005) as belonging to tholeiitic magmas series, with involvement of crustal contamination. Guimarães et al. (2005) obtained the age of 1499 ± 3.2Ma. by U-Pb method in zircons, while dating carried out by Babinski et al. (1999) for same rocks yielded age around 1510 ± 2.7 Ma
6.4 Structures
All the units described above, are affected in varying degrees by NNW-SSE-trending reverse-to-reverse dextral shear zones (with subsidiary NNE-SSO branches) and by dissimilar folding developed, mainly, in greenschist facies (Cruz &Alkmim, 2006). Shearing
with sinistral dynamic also is described in the area. These deformation belts exhibit a branched pattern and, according to Guimarães et al. (2005), are responsible by the Ibitiara Granitoid uplifting, as well as by the formation of the Ibitiara Anticline (Fig. 6). In the Ibitiara region stand out the Ibitiara (Cruz, 2004) and Ibiajara (Guimarães et al., 2005) Shear Zones, both of contractional nature (Fig. 6).
Fig. 6. Simplified geologic map of the Ibitiara region showing main lithological units and major structures. The study area is also indicated (Modified from Guimarães et al., 2005).
7. MINERAL OCCURRENCES REPORTED IN THE IBITIARA REGION
Mineral resources in the Ibitiara region were assessed and inventoried by the Geological Survey of Brazil (Guimarães et al., 2005). The register includes gold, barite and rutile quartz as main occurrences, with subordinate copper, tin and manganese. Gold occurs mainly hosted in quartz veins, and to a lesser extent as placer-type deposits. Mineralized veins are concordant to main regional structures, represented by shear zones, fault and fractures with predominant NNW-SSE-trending. Barite veins housed by N-S-trending shear zones were mapped at the deposit area by Yamana Gold team (Fig. 7b). As considered per same authors, mineralization of gold is tectonically controlled and linked to low-T metamorphic-hydrothermal fluids that circulated, evolved and precipitated as veins during the regional inversion event (~0.6 Ga).
In the region, gold associated with iron oxides were carved at BaixaFunda mine during mid-1990s. According to Mello (1991), the mineralization at BaixaFunda is hosted by carbonaceous schist occurring as four types: (1) massive bodies of banded magnetite-quartz; (2) massive banded pyrite-magnetite bodies; (3) quartz-siderite sulfide bodies; and (4) disseminated sulfides in surrounding hydrothermal alteration zones. These halos of alteration envelop the structurally-controlled orebodies and consist of mineralogy resulting from chloritization, sericitization, carbonatization, turmalinization and pyritization processes. Mello (1991) proposed an epigenetic model for Au mineralization involving hydrothermal fluids, wherein the origin of the gold would be related to mafic rocks (metabasites) lying on the base of the deposit.
Gold occurrences hosted by the Ibitiara Granitoid are confined to the so-called mine Beta do Tatu, whose activities ceased in the 1950s (Guimarães et al., 2005). The mineralization took place as discontinued-fractured Au-bearing quartz veins embedded in sheared and weathered domains, showing impregnations of Fe-oxides on these veins (Guimarães et., 2005).
8. THE LAVRA VELHA DEPOSIT
The LavraVelha deposit (LVD) is an early-stage Au-Cu prospect owned by Yamana Gold company, located in the central part of Bahia State, around 540 kilometers west of Salvador city, Brazil (see Figure 2 at section 4). LavraVelha is placed at the northern part of the Ibitiara Granitoid, close to the hinge of the Ibitiara Anticline, which represents a structural-erosional window that exposes the Paleoproterozoic basement surrounded by younger volcanic and sedimentary rocks of the Novo Horizonte and Ouricuri do OuroFormations, belonging to Rio dos Remedios Group (Fig. 7a, b). According to Guimarães et al. (2005), this basement is represented by Paleoproterozoic granitic intrusions (Ibitiara Granitoid) and late mafic dikes/sills (Fig. 7b). The LVD consists of high gold and copper grades (Au>Cu) hosted by hydrothermal vein and breccia system, associated with altered facies of Ibitiara Granitoid. Metric-wide barite veins also outcrop at the area of the project (Fig. 7b). In recent years, some work has been conducted on the LavraVelha deposit by Campos (2013), Carlin (2016) and Carlin et al. (2018).
Campos (2013) grouped the hydrothermal breccias (either Au-Cu-bearing or barren breccias) that occur in LVD into four types: calc-silicate, sulphide, hematite and sericite breccias. According to the author, the breccias exhibit mineral zoning from the base to the top of the deposit and a strong structural control. Elements such as silver, arsenic, cobalt, bismuth, uranium, barium, manganese, cerium and lanthanum are highly correlated and associated with the gold and copper mineralization (Campos, 2013). Zircon grains from a mineralized breccia unit dated by Campos (2013) via U-Pb LA-ICP-MS yielded ages of 2.16 ± 0.5 Ga. These ages are coincident with the crystallization age of the Ibitiara Granitoid (e.g. U-Pb zircon, 2.09 ± 0.6 Ga; Teixeira, 2005), suggesting contemporaneity between the genesis of the mineralization, the Ibitiara Granitoid and the hydrothermal breccias formation (Campos, 2013). Isotope analysis of C, O and S were carried out by Campos (2013) for LavraVelha. From these isotopic results, Campos (2013)intepretedas magmatic the source of the mineralizing fluids (δ13C ~-2‰ in carbonates), although oxygen isotopic composition (δ18O from +9.95 to +15.3‰) would indicate mixing with lower temperature fluids (Campos, 2013). The sulfur isotopic signature of chalcopyrite and pyrite expressed a derivation from cooling magmas for sulfur from the mineralized breccias, with the involvement of more oxidized fluids or host rocks (δ34S from +2.22 to + 3.89 ‰).
Carlin et al. (2018) characterized the host rocks at LavraVelha classifying them mainly as meta-tonalite and meta-quartz diorite. These hosts are hydrothermally altered under
deformation at low-strain rate, showing an intense sericitization and iron oxide formation, as well as saussuritization and locally, albitization. The authors, also concluded that these host rocks correspond to deformed and hydrothermalized portions of the Ibitiara Granitoid.
Despite the fact that an IOCG-style mineralization has been proposed as a potential model for the Au (Cu) deposit of LavraVelha, there is still controversy about its genesis. Campos (2013) suggested it may have formed from the partial melting of a metasomatized subcontinental lithospheric mantle (SCLM) in an environment of magmatic arc, around 2.1 Ga, involving dominantly magmatic-hydrothermal fluids associated with copper, iron, arsenic and bismuth sulphides. Conversely, Carlin et al. (2018) proposed an origin for Au mineralization related to the Espinhaço basin later deformation stage, in the framework of the inversion of the Paramirim Aulacogen that took place during the late of the Brasilian Orogeny. Guimarães et al. (2005) obtained Ar-Ar age of ca. 0.5 Ga at mineralized veins from neighbouring areas to deposit. This age. also is consistent with the inversion age of the Paramirim Aulacogen (Cruz &Alkmim, 2006), thus positioning the mineralization in a late stage of the evolutionary history of the region.
Initially, the mantle-type arrangement of the mineralization at LavraVelha was thought as a structural/lithological boundary since its close affinity to the volcanic-plutonic contact that separate the Ibiatira Granitoid from the overlaying rocks of the Novo Horizonte Formation. Since Carlin (2016) postulated that the host rocks represent altered portions of the same Ibitiara Granitoid (rather than a volcanic sequence), new insights about the mineralization control were postulated. At present, the Yamana Gold´s exploration team support a fluid mixing model involving a hot reduced fluid and a cooler oxidized fluid, as responsible for gold precipitation, while the varying stratigraphically mineralized levels would be a consequence of fluctuations at paleo-groundwater table (L. de Oliveira, 2018, pers. comm., September 11th).
Fig. 7. a) Anticlinal Ibitiara, with the Ibitiara Granitoid outcropping in the center of the structure and the Rio dos Remedios Group (RDRG) bordering on its flanks. The LavraVelha deposit is located at northern portion of the anticlinal (hinge region), close to the contact between two units. Yellow square indicates roughly the area of local map in figure (b) (Modified from Campos, 2013). b) Local map displaying the main units, structures and the location of the holes that conform the section analyzed in this work (Adapted from Yamana Gold and from Passos et al., 2019 submitted for publication)
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