Summary:
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 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 a group of Archean-aged tonalitic, trondhjemitic, and granodioritic gneiss-dominated basement and related remnants supracrustal rocks of the Gavião Block; it underlies the flatter terrain east of the Jacobina range. East of the Mairi Complex, the fine-grained biotite gneisses of the Archean Saúde Complex also underlie a flat landscape. The transition between the hilly and the flatter topography of the eastern domains corresponds to the exposures of the Archean Mundo Novo Greenstone Belt.
Different styles of deformation are recognized within the Jacobina Group and 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 and open folds were developed in response to the strong westward-verging mass transport event caused by the Paleoproterozoic continent/continent collision. To the west, the Jacobina Group is thrust over the Archean Mairi Complex, the Campo Formoso Mafic–Ultramafic Complex, and the late-to-post-tectonic granitic intrusions (Miguel Calmon-Itapicurú, Mirangaba-Carnaíba and Campo Formoso intrusions), along a thrust fault named the Jacobina Fault. This structural setting changes eastwards to a series of steeply east-dipping blocks, bounded by east-dipping subparallel reverse faults.
The mineralization at Jacobina consists of two mineralization styles:
1) conglomerate-hosted gold, generally interpreted to represent paleoplacer deposits (the most important mineralization type in the Jacobina district), and
2) post-depositional modification by structural and hydrothermal events evident as gold-bearing stockwork, shear zones, and associated extensional quartz veins (relatively minor and do not contribute to the established mineral resources at Jacobina).
This type of deposit is similar to the Witwatersrand and Tarkwa deposits in South and West Africa.
Conglomerate-Hosted Placer Gold Mineralization
The 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 500 metres thick but locally achieves thicknesses of up to one kilometre. The gold-bearing conglomerate units, known as reefs, range from less than 1.5 metres to 25 metres in width and can be followed along strike for hundreds of metres, and in some cases for kilometres. Some contacts between the reefs and crosscutting mafic and ultramafic intrusive rocks are enriched in gold. Although they are quite homogeneous along their strike and dip extensions, the gold-bearing conglomerates differ from one another in stratigraphic position and pattern of gold distribution. The differences are likely due to variations in the sedimentary source regions, in the erosion and transportation mechanisms, and in the nature of the depositional environments. Not all conglomerates of the Serra do Córrego Formation are gold-bearing.
Conglomerate-hosted deposits contain very fine grains of native gold, typically 20 to 50 µm in size, hosted in the matrix of the conglomerate. Gold may also be associated with rounded pyritic aggregates believed to be of sedimentary origin. There are no other significant elements present, with detailed studies of the reef chemistry showing only very minor enrichment in iron, titanium and uranium in some reefs associated with rounded grans of uraninite, ilmenite and rutile. Mineralization is typically hosted by well sorted, clast-supported conglomerate and may comprise micro-fractured, gold-bearing, recrystallized, silicified, and pyritic conglomerate units of the Serra do Córrego Formation, with a greenish fuchsite matrix and common hematite coatings along shear planes, joints, and fracture surfaces. Gold mineralization does not display a correlation with the pyrite or fuchsite content of the rock, although well-mineralized reefs are typically enriched in hematite and may contain red colored, oxidized pebbles.
Post-Depositional Gold-Bearing Stockwork, Shear Zones and Extensional Quartz Veins
This group encompasses gold-bearing extensional quartz veins and veinlets related to semiconcordant shear zones hosted by quartzites, andalusite-graphite-quartz schists, and local conglomerates of the Rio do Ouro Formation (e.g., Goela da Ema, Biquinha, Cercadinho and Guardanapo gold workings). This style of gold mineralization is a very minor volumetric component at Jacobina and does not contribute significantly to the mineral resource. The main hydrothermal alterations associated with this style of mineralization are silicification, sericitization, chloritization, and pyritization (locally with chalcopyrite), and local tourmalinization.
The ultramafic and mafic rocks also host mineralization as narrow shear zones up to 4 m- thick in north-south oriented ultramafic sills and dikes, close to their footwall and hangingwall contacts with the hosting quartzite and conglomerate units of the Serra do Córrego, Rio do Ouro, and Serra da Paciência Formations. The mineralized shear zones are characterized by the development of gold-bearing quartz veins and/or stockwork. The main hydrothermal alteration types are silicification, fuchsitization, pyritization, and sericitization, with local tourmalinization. A number of examples of this group are known at the mine sites and surrounding areas (Canavieiras, Itapicurú, Serra do Córrego, Morro do Vento, and João Belo), and at Serra da Paciência (Mina Velha, Várzea Comprida, Ciquenta e Um, Cabeça de Nego and Milagres gold workings), in the north. This style of mineralization does not contribute significantly to the mineral resource at Jacobina.
The overprinting hydrothermal alteration event at the Jacobina deposit consists 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 the maficultramafic intrusive rock by circulating hydrothermal fluids.