Geology – Tapajos

The Tapajós Gold Province is located in the western portion of Pará State, central northern Brazil and covers a total of some 100,000 km2. The Tapajós is in the southern-central portion of the Amazon Craton, generally termed the Brazilian Shield, as opposed to the northern portion of the Craton referred to as the Guyanian Shield and extends into the littoral countries of the northern South American continent.

The Brazilian Shield is nucleated on the Archaean granite-greenstone terrain of the Carajás-Imataca Province in eastern Pará State, and progressively becomes younger and shallower towards the west, grading into granite dominated then into granite-volcaniclastic terrain of Paleoproterozoic age rocks of the eastern Amazonas State. In the Jardim do Ouro region lithologies are dominated by granitoids of Paleoproterozoic age.

The Tapajós Province represents a tectonically controlled geological evolution attributed to the Orosirian Proterozoic period, comprising four plutonic events, over a 140 Ma period.

In the Tapajós Province two main units form the basement, the Paleoproterozoic Cuiú-Cuiú metamorphic suite (2.0 -2.4 Ga) and the Jacareacanga metamorphic suite (>2.1 Ga). The Jacareacanga is considered to be the older suite; however the relationship is not yet well defined.

The Jacareacanga suite is comprised of a sedimentary-volcanic sequence, deformed and metamorphosed to a regional greenschist facies, with units of sericitic and chloritic schists and rare banded iron formations.

The Cuiú-Cuiú suite, which is the basement for the Palito area, is comprised of orthogneisses of dioritic to grandioritic composition, locally mylonitized, deformed tonalitic granitoids and enclaves or rafts of amphibolites.

Both the Cuiú-Cuiú and Jacareacanga suites are intruded by monzogranites of the Paráuari suite (2000 -1900 Ma), tonalites, diorites and granodiorites of the Tropas suite (1907 Ma -1898 Ma) and granites and granodiorites of the Creporizão suite (1893 -1853 Ma). These three intrusive suites are considered to have calc-alkaline affiliations and may be considered remnants of a magmatic back arc system interpreted for the region.

Regional structural analysis of the Tapajós Province has identified various compressive deformation regimes including ductile, brittle-ductile and brittle. The deformation is interpreted to have occurred as two separate events, the first compressive event, with peak deformation around 1.96 Ma, resulting in the development of ductile and brittle-ductile deformation regimes. The second event occurring at 1.88 Ma resulted in brittle deformation. These events resulted in major north-south, north west-south east and east-west lineament sets.

The geometry of the lineament and structures are compatible with a combination of Riedel fracturing and strike slip fault systems, where the principle vector of compression is oriented in an east-west and ENE-WSW direction

Gold mineralization is not restricted to a particular suite, with deposits located in all suites including; Cuiú-Cuiú Suite (Cuiú-Cuiú), Paráuari Suite (Tocantinzinho, São Jorge and Palito), Tropas Suite (Ouro Roxo), Salustiano and Bom Jardim Formation (V-series deposits, Bom Jardim), Maloquinha Suite (Mamoal). Gold mineralization associated with quartz and hydrothermal alteration assemblages is reported in all the fracture orientations of the Reidel system, and are dominated by fractures oblique to the principle strike-slip shear orientation.