Mineralium Deposita (v.36, #3-4)

Preface by David I. Groves; Richard J. Goldfarb; Luiz Carlos da Silva (205-206).

The Brazilian Shield extends over much of South America to the east of the Andean Chain, and is partly covered by Phanerozoic sedimentary rocks. The three major tectonic units (older than 900 Ma) of the shield are the Amazon, São Francisco, and Rio de la Plata Cratons, whereas rocks of the Neoproterozoic (900–550 Ma) Brasiliano Cycle belts surround the cratons in the eastern half of Brazil. Paleoarchean rocks occur as small cratonic nuclei in northeastern Brazil, but the cratons contain voluminous 3.0–2.6-Ga granitic and greenstone belts and a large volume of Paleoproterozoic rocks. Late Mesoproterozoic belts (~1.2 Ga) occur in the western and northwestern Amazon Craton and possibly in northeastern Brazil. There are voluminous Paleoproterozoic and Mesoproterozoic intrusive and volcanic rocks in the Amazon Craton. Belts of reworked crust are common, but the Amazon Craton grew to the north and west mainly during accretionary orogens. The Neoproterozoic Brasiliano Cycle belts contain 5% of juvenile crust in central and southern Brazil, but are dominantly derived from reworking of older crust. It is possible that the cratons were continuous in a super-continent during the Mesoproterozoic prior to the Brasiliano Cycle. Gold deposits are concentrated in the Archean and Paleoproterozoic terranes, including the Carajás and Quadrilátero Ferrífero gold provinces of the Amazon and São Francisco Cratons, respectively, but some also occur in younger terranes. Recent alluvial gold deposits and supergene deposits in the weathering zone are widespread in Brazil.

Major Brazilian gold deposits – 1982 to 1999 by Charles H. Thorman; Ed DeWitt; Marcos A. Maron; Eduardo A. Ladeira (218-227).
Brazil has been a major but intermittent producer of gold since its discovery in 1500. Brazil led the world in gold production during the 18th and early 19th centuries. From the late 19th century to the late 20th century, total mining company and garimpeiro production was small and relatively constant at about 5 to 8 t/year. The discovery of alluvial deposits in the Amazon by garimpeiros in the 1970s and the opening of eight mines by mining companies from 1983 to 1990 fueled a major boom in Brazil's gold production, exceeding 100 t/year in 1988 and 1989. However, garimpeiro alluvial production decreased rapidly in the 1990s, to about 10 t/year by 1999. Company production increased about tenfold from about 4 t/year in 1982 to 40 t in 1992. Production from 1992 to the present remained relatively stable, even though several mines were closed or were in the process of closing and no new major mines were put into production during that period.Based on their production history from 1982–1999, 17 gold mines are ranked as major (>20 t) and minor (3–8 t) mines. From 1982–1999, deposits hosted in Archean rocks produced 66% of the gold in Brazil, whereas deposits in Paleoproterozoic and Neoproterozoic rocks accounted for 19% and 15%, respectively. Deposits in metamorphosed sedimentary rocks, especially carbonate-rich rocks and carbonate iron-formation, yielded the great bulk of the gold. Deposits in igneous rocks were of much less importance. The Archean and Paleoproterozoic terranes of Brazil largely lack base-metal-rich volcanogenic massive sulfide deposits, porphyry deposits, and polymetallic veins and sedimentary exhalative deposits. An exception to this is in the Carajás Mineral Province.

Brazil's premier gold province. Part I: The tectonic, magmatic, and structural setting of the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero by Lydia Lobato; Luiz Ribeiro-Rodrigues; Márcia Zucchetti; Carlos Noce; Orivaldo Baltazar; Luiz da Silva; Claiton Pinto (228-248).
Rocks of the Rio das Velhas Supergroup comprise one of the most significant Archean greenstone-belt successions in Brazil, in both their appreciable mineral productivity and extensive mineral potential. A large part of this greenstone belt is contained within the Quadrilátero Ferrífero (Iron Quadrangle) region, Minas Gerais state, southeastern Brazil, which occupies the southernmost portion of the São Francisco craton. The Nova Lima Group rocks, at the base of the Rio das Velhas greenstone belt, host important orogenic gold deposits. The group contains lithological associations from bottom to top as follows: (1) mafic-ultramafic volcanic, (2) volcanic-chemical, (3) clastic-chemical, (4) volcaniclastic, and (5) resedimented rocks. Rocks of the resedimented, volcanic-chemical, and mafic-ultramafic volcanic associations mainly host the most important gold deposits. An early compressional deformation occurs in the rocks of the Rio das Velhas greenstone belt and basement gneisses, with tangential thrusting from the north to the south or southwest. Structures generated during a second, compressional deformation, encompass NW-striking thrust faults and SW-vergent, tight to isoclinal folds, inferring a general southwest transport direction. In the central portion of the Quadrilátero Ferrífero, the Paciência lineament, which strikes northwest and dips to the northeast in the south, or strikes northeast and dips to the southeast in the north, is a thrust-related, oblique ramp fault that hosts important gold deposits. The convergence of these two trends in the Nova Lima region is accommodated by roughly E–W-striking transcurrent faults, which are the most favored sites for large gold concentrations. Intracratonic extension in Late Archean to early Paleoproterozoic times and NW-vergent, Trans-Amazonian compressional deformation post-date gold deposition. Late extension during the Paleoproterozoic led to basin formation and the prominent dome-and-keel architecture of the Quadrilátero Ferrífero region. Deformation related to the west-vergent thrust-and-fold belts of the Brasiliano orogeny is recognized at the eastern margin of the Quadrilátero Ferrífero region. Defining structures as Archean, Trans-Amazonian, and Brasiliano in age is still difficult, although it is accepted that the gold-related Archean structures are best preserved in the central and western parts of the Quadrilátero Ferrífero. The principal source for the Rio das Velhas sedimentary rocks was probably the trondhjemite–tonalite–granodiorite crust that formed in multiple episodes after ~3,500 Ma, and was widely metamorphosed and intruded at 2,880–2,850 Ma. Felsic volcanism at 2,772±6 Ma formed the Rio das Velhas greenstone belt. The volcanic succession was a source for some overlying sedimentary rock units, as indicated by the presence of detrital zircons dated at 2,777–2,771 Ma. Strongly foliated granitic plutons range between 2,712+5/–4 and 2,698±18 Ma. The age of gold mineralization is inferred between 2,698 and 2,670 Ma. A metamorphic overprint during the Trans-Amazonian orogeny is estimated at ~2,050 Ma. There is evidence of local isotopic disturbances because of post-Trans-Amazonian tectonic events, at ~1.8–1.7 and 0.6 Ga.

Orogenic, gold deposits are hosted by rocks of the Archean Rio das Velhas greenstone belt in the Quadrilátero Ferrífero region, Minas Gerais state, Brazil, one of the major gold provinces in the world. The gold deposits occur at the base of the mafic–ultramafic succession, with the most important orebodies controlled by E–W-striking, strike-slip faults. The main mineralization styles are (1) structurally controlled, sulfide replacement zones in banded iron formation (BIF); (2) disseminated sulfide minerals and gold in hydrothermally altered rocks along shear zones; and (3) auriferous quartz–carbonate–sulfide veins and veinlets in mafic, ultramafic, and felsic volcanic rocks, and also in clastic sedimentary rocks. The most common host rocks for ore are metamorphosed oxide- and carbonate-facies banded iron (± iron-rich metachert) formations (e.g., the Cuiabá, São Bento and Raposos deposits) and the lapa seca unit, which is a local term for intensely carbonatized rock (e.g., the giant Morro Velho mine with >450 t of contained gold). Metabasalts host most of the remaining gold deposits. Mineralogical characteristics and fluid inclusion studies suggest variations in the H2O/CO2 ratio of a low-salinity, near-neutral, reducing, sulfur-bearing, ore fluid. The presence of abundant CH4-rich inclusions is related to reduction of the original H2O–CO2 fluid via interaction with carbonaceous matter in the wallrocks. Oxygen fugacity was close to that of graphite saturation, with variations likely to have been influenced by reaction with the carbonaceous matter. Carbon-rich phyllites and schists, which commonly bound ore-bearing horizons, seem to have played both a physical and chemical role in localizing hydrothermal mineral deposition. Microtextural studies indicate that gold deposition was mainly related to desulfidation reactions, and was paragenetically coeval with precipitation of arsenic-rich iron sulfide minerals. Carbon isotope data are compatible with dissolution of CO2 from pre-existing mantle-derived carbonation zones, and indicate fluids of metamorphic origin. A major episode of hydrothermal fluid introduction into different rock types caused epigenetic gold formation and wallrock alteration at about 300 to 400 °C during the late stages of regional deformation and metamorphism of the greenstone belt. The age of gold mineralization is constrained to be younger than 2,698 and perhaps closer to 2,670 Ma.

Gold deposits of the Tapajós and Alta Floresta Domains, Tapajós–Parima orogenic belt, Amazon Craton, Brazil by João Orestes Schneider Santos; David Ian Groves; Léo Afraneo Hartmann; Márcia Abrahão Moura; Neal Jesse McNaughton (278-299).
The Tapajós region is one domain of a major Paleoproterozoic orogenic belt, named Tapajós–Parima and is discussed in the context of the evolution of the Amazon Craton. The orogenic belt is composed of a back-arc sequence, four volcano-plutonic arcs, intra-arc sedimentation and is limited to the east by the cratonic rocks of the Central Amazon Province. The evolution and timing of the main events is established by zircon, baddeleyite, and titanite SHRIMP U–Pb geochronology of 29 rock samples, while lead and argon isotopes are used to study the age and source of the gold mineralization. Based on the mesoscopic nature of the orebodies, and, in some cases, on microthermometric and stable isotope data, the Tapajós gold deposits are classified as (1) orogenic and (2) intrusion-related, and may be grouped into four deposit-type categories: (1) orogenic, turbidite-hosted: disseminated and quartz–pyrite veinlet deposits, hosted by metaturbidites (lower greenschist-facies, Jacareacanga Group) and emplaced in ductile structures; (2) orogenic, magmatic arc-hosted: disseminated and pyrite–quartz–carbonate vein deposits, hosted by metamorphic rocks (Cuiú-Cuiú Complex) and formed under a ductile–brittle regime, with the Ouro Roxo deposit as a type example; (3) intrusion-related, epizonal quartz-vein deposits: vertical to subvertical quartz-pyrite veins and pyrite disseminations filling extensional brittle faults; and (4) intrusion-related, epizonal, disseminated/stockwork deposits, the type-example being the Serrinha deposit. Gold mineralization of type 3 is similar to that of Korean-type, while type 4 mineralization shows some similarities to porphyry-type deposits. Galena Pb–Pb and muscovite Ar–Ar data indicate an age of ~1,860 Ma for the intrusion-related gold mineralization. Preliminary Pb isotope data on K-feldspar indicate that the fluid source was more likely to have been from within the Jacareacanga, Cuiú-Cuiú and Tropas units than the Creporizão, Maloquinha, and Iriri units. This study shows the existence of two main types (orogenic and intrusion-related) of gold deposits, which are related to specific tectono-magmatic events that occurred during a limited period of time in the orogenic belt evolution. This information may be useful as a guide for gold exploration along the orogenic belt.

The Carajás mineral province is located in the south-eastern part of the Amazon craton and is divided into the southern and northern tectonic blocks, which are referred to as the Rio Maria granitoid–greenstone terrane and the Itacaiúnas shear belt, respectively. The geological evolution of this province occurred almost entirely in the Archean, with no tectonism from ~2.4 to 1.9 Ga and then an episode of very extensive intraplate granitic magmatism. Distinct types of gold deposits, most associated with Archean metavolcano-sedimentary sequences, including greenstone belts as old as 2.9 Ga, comprise (1) Fe-oxide-poor Cu–Au, (2) Fe-oxide–Cu–Au–U–REE, (3) shear-zone related lode-gold, and (4) sedimentary rock-hosted Au–PGE. The shear-zone related deposits are more typical of the Rio Maria granitoid–greenstone terrane, whereas the other types are more abundant in the Itacaiunas shear belt. The first type, represented by the Serra Verde deposit, shows most characteristics of volcanogenic massive sulfide (VMS) deposits, except for the dominance of chalcopyrite over pyrite and/or pyrrhotite. Some deposits of the second type (Igarapé Salobo and Igarapé Bahia) are also syngenetic, but their ore mineralogy and paucity of pyrite and/or pyrrhotite are inconsistent with, or at least unusual for, classic VMS deposits. Other Fe-oxide–Cu–Au–U–REE deposits, such as the Cristalino and Sossego, occur as epigenetic stockwork mineralization related to Archean granitoid stocks. Most shear-zone related lode-gold deposits are classified as orogenic gold deposits (e.g. Sapucaia, Babaçu and Tucumã) that cut greenstone belt rocks. The Cumaru deposit is also hosted by a shear zone, but may have a genetic association with a granitoid intrusion. The Águas Claras deposit, although also hosted in a shear zone and perhaps intrusion-related, differs from the orogenic gold deposits by its extensional tectonic setting, lack of CO2 in the ore fluids, and voluminous chalcopyrite. The sedimentary rock-hosted Serra Pelada Au–PGE deposit is structurally controlled and occurs in an Archean clastic-chemical sedimentary rock sequence.

Gold mineralization in the Serra de Jacobina region, Bahia Brazil: tectonic framework and metallogenesis by João Batista Guimarães Teixeira; Jorge André Braz de Souza; Maria da Glória da Silva; Carlson Matos Maia Leite; Johildo Salomão Figueiredo Barbosa; Carlos Eduardo Silva Coelho; Maisa Bastos Abram; Valter Mônaco Conceição Filho; Sundaram S.S. Iyer (332-344).
Gold deposits in the Serra de Jacobina region in Bahia, northeastern Brazil, occur in a belt of siliciclastic metasedimentary rocks intercalated with mafic and ultramafic rocks, and underlain by a tonalite–trondhjemite–granodiorite gneiss-dominated (TTG) basement. The siliciclastic sequence probably represents the remnants of a sedimentary basin, which formed in an Archean passive margin-type setting. The basin was subsequently subjected to a complex history of deformation, metamorphism, and hydrothermal activity, as a result of oblique collisional events in Late Archean and Paleoproterozoic. The majority of the auriferous occurrences in the area are hosted by quartz-pebble conglomerates, and have been noted to resemble placer-type deposits. However, structurally-controlled hydrothermal orebodies, and the formation of gold occurrences also in quartzites and mafic and ultramafic rocks, support an epigenetic model for the mineralizing event. Gold mineralization is interpreted to be an integral part of the late (~1.9 Ga) tectonothermal evolution of the Serra de Jacobina region. It was roughly coeval with the emplacement of large volumes of post-collisional type, peraluminous granitic magmas, during a regional strike-slip regime.

The Rio Itapicuru greenstone belt, Bahia, Brazil: geologic evolution and review of gold mineralization by Maria da Glória da Silva; Carlos Eduardo Silva Coelho; João Batista Guimarães Teixeira; Fernando César Alves da Silva; Roberto Albuquerque Silva; Jorge André Braz de Souza (345-357).
Shear-hosted gold deposits in the Rio Itapicuru greenstone belt occur within supracrustal rocks metamorphosed under greenschist to amphibolite facies conditions. Integration of host-rock geochemistry, fluid-inclusion studies, geochronology, and structural geology supports a model of metallogenic evolution intimately associated with a Paleoproterozoic history of tectonic convergence, involving early arc/back-arc magmatism, and subsequent collision and granitoid emplacement. In this context, gold mineralization occurred mainly during late collisional tectonism at ~2,050 Ma, and resulted from a crustal-scale hydrothermal system characterized by carbonic and low salinity aqueous-carbonic fluids of distinct sources. The fluids migrated and interacted with host rocks of different compositions. In the southern part of the belt, structurally controlled fluid circulation within iron-rich mafic rocks gave rise to the Weber gold belt, which contains the largest gold deposits of northeastern Brazil. In contrast, fluid–rock interaction with volcaniclastic–carbonaceous sedimentary rocks, both in the southern and northern parts of the belt, resulted in the development of relatively smaller deposits, suggesting that local-scale structural and lithological attributes were critical controls in the size of the deposits.

Gold deposits and occurrences of the Crixás Goldfield, central Brazil by Hardy Jost; Paulo de Tarso Ferro de Oliveira Fortes (358-376).
The Crixás Goldfield is a part of the Tocantins province and consists of a ~2.9 Ga Archean granite–greenstone terrane. The gold field contains one large deposit (Mina III, 65 tons Au) and several smaller occurrences, which are structurally controlled by regional shear zones that are either thrust or strike-slip faults. The deposits can be classified as auriferous massive sulfide (pyrrhotite + arsenopyrite) hosted ores within sulfidized banded iron formations, auriferous quartz veins, and disseminated gold ores in carbonaceous schists. On a more local scale, the ore bodies coincide with dilation zones surrounded by hydrothermal alteration haloes of various widths within carbonaceous schists, iron formations, and metabasalts. Hydrothermal alteration related to deposits within thrust faults consists of sulfidization and carbonatization, whereas, within strike-slip faults, propylitic, potassic, and sericitic alteration zones occur in concentric haloes. In both cases, quartz veins and veinlets may be abundant. Absolute ages of mineralization are uncertain, but structural data suggest multiple events probably occurred during Archean, Paleoproterozoic and Neoproterozoic tectonic events.