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Location: 8 km S from Jacobina, Brazil
Fazenda Itapicuru, s/nJacobinaBrazil44700-000
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The Jacobina gold district is defined by a 40 km-long belt that extends from Campo Limpo in the south to Santa Cruz do Coqueiro in the north. Gold mineralization at Jacobina occurs in two principal styles: (i) conglomerate-hosted gold, generally interpreted as paleoplacer deposits and representing the most important mineralization type in the Jacobina district; and (ii) post-depositional modification by structural and hydrothermal events, expressed as gold-bearing stockworks, shear zones, and associated extensional quartz veins. Gold mineralization at Jacobina is hosted almost entirely within quartz-pebble conglomerates of the Serra do Córrego Formation, the lowermost sequence of the Proterozoic-age Jacobina Group. This formation is typically about 500 m thick but locally reaches thicknesses of up to 1 km. The gold-bearing conglomerate units, known as reefs, range from less than 1.5 m to 25 m in thickness and can be traced along strike for hundreds of metres and, in some cases, for more than 1 km. Some contacts between the reefs and crosscutting mafic and ultramafic intrusive rocks are enriched in gold.The Jacobina gold district coincides with most of the Jacobina Range, where quartzite, conglomerate, and schist units of the Paleoproterozoic Jacobina Group form a series of north–south-trending mountain ranges that rise up to 1,200 m above sea level (masl). The longitudinal north–south valleys, as well as the east–west-oriented valleys, often correspond to recessive ultramafic sills and dykes.The Mairi Complex consists of Archean-aged tonalitic, trondhjemitic, and granodioritic gneiss-dominated basement rocks and related remnants of supracrustal rocks of the Gavião Block. It underlies the flatter terrain east of the Jacobina Range. East of the Mairi Complex, fine-grained biotite gneisses of the Archean Saúde Complex also underlie a flat landscape. The transition between the hilly topography of the Jacobina Range and the flatter eastern domains corresponds to exposures of the Archean Mundo Novo Greenstone Belt.Different styles of deformation are recognized within the Jacobina Group and the surrounding Archean rocks along and across the northern portion of the 50-kilometre-long, north-trending Contendas–Mirante–Jacobina lineament. Thrust faults, oblique sinistral-reverse faults, and regional tight to open folds developed in response to a strong westward-verging mass-transport event caused by Paleoproterozoic continent–continent collision. To the west, the Jacobina Group is thrust over the Archean Mairi Complex, the Campo Formoso Mafic–Ultramafic Complex, and late- to post-tectonic granitic intrusions (Miguel Calmon–Itapicurú, Mirangaba–Carnaíba, and Campo Formoso intrusions) along a thrust fault known as the Jacobina Fault. Eastward, this structural setting changes to a series of steeply east-dipping blocks bounded by subparallel, east-dipping reverse faults.Gold mineralization at Jacobina occurs in two principal styles:• Conglomerate-hosted gold, generally interpreted as paleoplacer deposits and representing the most important mineralization type in the Jacobina district;• Post-depositional structural and hydrothermal modification, expressed as gold-bearing stockworks, shear zones, and associated extensional quartz veins. This style is relatively minor and does not contribute significantly to the established mineral resources at Jacobina.This type of deposit is analogous to the Witwatersrand and Tarkwa deposits of southern and western Africa.Conglomerate-Hosted Placer Gold MineralizationGold mineralization at Jacobina is hosted almost entirely within quartz-pebble conglomerates of the Serra do Córrego Formation, the lowermost unit of the Paleoproterozoic Jacobina Group. This formation is typically about 500 m thick but locally reaches thicknesses of up to 1 km. The gold-bearing conglomerate units, known as reefs, range from less than 1.5 m to 25 m in thickness and can be traced along strike for hundreds of metres and, in some cases, for several kilometres. Some contacts between the reefs and crosscutting mafic and ultramafic intrusive rocks are enriched in gold.Although the reefs are relatively homogeneous along their strike and dip extensions, individual gold-bearing conglomerates differ in stratigraphic position and patterns of gold distribution. These differences likely reflect variations in sediment source regions, erosion and transport mechanisms, and depositional environments. Not all conglomerates within the Serra do Córrego Formation are gold-bearing.Conglomerate-hosted deposits contain very fine-grained native gold, typically 20–50 µm in size, hosted within the matrix of the conglomerate. Gold may also be associated with rounded pyritic aggregates interpreted to be of sedimentary origin. No other economically significant elements are present. Detailed geochemical studies of the reefs show only minor enrichment in iron, titanium, and uranium in some reefs, commonly associated with rounded grains of uraninite, ilmenite, and rutile.Mineralization is typically hosted by well-sorted, clast-supported conglomerates and may comprise microfractured, gold-bearing, recrystallized, silicified, and pyritic conglomerate units of the Serra do Córrego Formation. These rocks commonly exhibit a greenish fuchsite-rich matrix and hematite coatings along shear planes, joints, and fracture surfaces. Gold mineralization does not show a consistent correlation with pyrite or fuchsite content; however, well-mineralized reefs are typically enriched in hematite and may contain red-colored, oxidized pebbles.Post-Depositional Gold-Bearing Stockworks, Shear Zones, and Extensional Quartz VeinsThis group includes gold-bearing extensional quartz veins and veinlets related to semiconcordant shear zones hosted by quartzites, andalusite–graphite–quartz schists, and locally conglomerates of the Rio do Ouro Formation (e.g., the Goela da Ema, Biquinha, Cercadinho, and Guardanapo gold workings). This style of mineralization represents a very minor volumetric component at Jacobina and does not contribute significantly to the mineral resource. The main hydrothermal alteration types associated with this style are silicification, sericitization, chloritization, pyritization (locally with chalcopyrite), and local tourmalinization.Ultramafic and mafic rocks also host mineralization in the form of narrow shear zones, up to 4 m thick, developed within north–south-oriented ultramafic sills and dykes, commonly near their footwall and hanging-wall contacts with quartzite and conglomerate units of the Serra do Córrego, Rio do Ouro, and Serra da Paciência formations. These mineralized shear zones are characterized by the development of gold-bearing quartz veins and/or stockworks. The principal hydrothermal alteration types include silicification, fuchsitization, pyritization, and sericitization, with local tourmalinization.Numerous examples of this mineralization style are documented at mine sites and surrounding areas, including Canavieiras, Itapicurú, Serra do Córrego, Morro do Vento, and João Belo, as well as at Serra da Paciência (Mina Velha, Várzea Comprida, Cinquenta e Um, Cabeça de Nego, and Milagres gold workings) in the northern part of the district. As with other post-depositional styles, this mineralization does not contribute significantly to the overall mineral resource at Jacobina.The overprinting hydrothermal alteration event at the Jacobina deposit is characterized by the development of pyrite, pyrrhotite, quartz, chrome-sericite (fuchsite), chrome-rutile, and chrome-tourmaline. The chromium-rich nature of this alteration assemblage is attributed to leaching of mafic–ultramafic intrusive rocks by circulating hydrothermal fluids.
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