Tapajos Mineral Province Research Articles

Hydrothermal alteration, mineralization and fluid inclusions in the Pista and Fofão prospects: implications for the genetic model of the Coringa polymetallic deposit, SE Tapajós Mineral Province, Amazonian Craton, Brazil

Pista and Fofao are prospects located near the main orebodies of the Coringa polymetallic deposit, in southeastern Tapajos Mineral Province of the Amazonian Craton. In both prospects, the mineralization is hosted in an Orosirian alkali feldspar granite, which is associated with coeval volcanic rocks at Pista. The paleo-hydrothermal system comprises an early post-magmatic stage, characterized by albite and K-feldspar alterations. Silicified and brecciated zones, sericitization of wall-rocks, both with adularia, as well as carbonatization (siderite and Mn-siderite rich stage) are the proximal ore alteration zones. Chlorite – hematite – rutile – epidote veinlets represent weak and distal zones of hydrothermal alteration. The mineralization is polymetallic (Au and subordinate Ag, Zn, Pb, and Cu) and occurs as dissemination and filling fissure veinlets. Gold occurs predominantly as Au-Ag-tellurides included in pyrite, and as free gold in association with pyrite, sphalerite and hematite within quartz veinlets. Fluid inclusions trapped in quartz veinlets show H2O, H2O-CO2-NaCl and CO2-only inclusions. Microthermometric data indicate an H2O-CO2-NaCl fluid, likely of magmatic derivation, with low-salinity (5.4 ± 1.3 wt. % NaCl equiv.) and moderate density (0.36-0.94 g/cm3) trapped between 264 and 370oC, and a low-salinity aqueous fluid (≤11.2 wt % NaCl equiv.) trapped at 280 to 360oC, possibly evolved from effervescence followed by isothermal mixing with an external heated H2O-NaCl fluid, probably meteoric. After this, the fluids were continuously diluted and cooled. This stage is represented by the liquid-rich H2O-NaCl fluid inclusions of low salinity (up to 5.3 wt. % NaCl equiv.) and relatively low trappingtemperature (177 to 113°C). The petrographic and fluid inclusions data suggest that the Au, Ag and base metals precipitation resulted from fluid mixing and dilution, which led to the breakdown of Au(HS)2−. We interpret the Pista and Fofao prospects as part of a magmatic-hydrothermal mineralized system, with some characteristics compatible with those of low- to intermediate-sulfidation epithermal systems, despite the absence of some typical textures from these systems.

New Crustal Framework in the Amazon Craton Based on Geophysical Data: Evidences of Deep East-West Trending Suture Zones

The Tapajos mineral province (TMP), in the Brazilian Amazon Craton, comprises NW-SE Paleoproterozoic insular magmatic arcs accreted to the Carajas Archean Province (CAP). We present new geological and geophysical data pointing toward a different evolutionary model for the TMP. Results obtained from magnetic data indicate that NNW-SSE trending structures occur at shallow crustal levels. Furthermore, an E-W structural framework shows up at 15.4 km depth, in disagreement with the accreted island arc orientation. These E-W structures are associated with north-dipping blocks, reflecting ductile compressive tectonics, similar to the tectonic setting found in the CAP. We interpret these E-W structures of the TMP as the continuity westwards of similar structures from the CAP, under the Paleoproterozoic volcanic rocks of the Uatuma Supergroup. Based on this evidence, we propose that Paleoproterozoic arcs have been formed in an Archean active continental margin, instead of in island arcs. This novel tectonic setting for the TMP has significant implications for the tectonic evolution and the metallogenic potential of the southern portion of the Amazon craton, particularly for Paleoproterozoic magmatic-hydrothermal (epithermal and porphyry) precious and base metal systems.

Granito Aragão — Suíte intrusiva Nhandú — um granito oxidado, tipo A2, de 1967 a 1964 Ma na Província Aurífera Alta Floresta — Cráton Amazônico

O Granito Aragão, localizado na Província Aurífera Alta Floresta (PAAF), consiste de um stock granítico de ~ 80 km2, de granulação média a grossa, textura porfirítica e composição modal monzogranítica a sienogranítica. Os dados geoquímicos indicam afinidade com magmas graníticos tipo-A2, similares àqueles gerados em margens continentais ativas evoluídas. As idades de cristalização U-Pb SHRIMP em zircão de 1964 ± 11 Ma e 1967 ± 2 Ma permitem incluí-lo na Suíte Intrusiva Nhandú (SINH) e correlacioná-lo com a Suíte Intrusiva Creporizão (SIC) na Província Mineral do Tapajós (PMT), no Pará. Associado ao Granito Aragão ocorre um megaenclave fanerítico médio a grosso, de textura porfirítica e composição modal granodiorítica a monzogranítica. O enclave apresenta padrão geoquímico semelhante a magmas graníticos shoshoníticos, e sua idade de cristalização U-Pb SHRIMP em zircão (1995 ± 5 Ma) indica relação com granitoides do Complexo Cuiú-Cuiú. O Granito Aragão ocorre alojado em granitoides foliados com idade de cristalização U-Pb SHRIMP de 2009 ± 4 Ma. Estes resultados sugerem que as rochas hospedeiras do Granito Aragão fazem parte do Complexo Cuiú-Cuiú. As correlações estratigráficas estabelecidas no presente trabalho indicam que a porção leste da PAAF representa a extensão da Província Tapajós-Parima, ao sul do Gráben do Cachimbo, norte do Estado de Mato Grosso.

“We are all Garimpeiros:” Settlement and Movement in Communities of the Tapajós Small‐Scale Gold Mining Reserve

ResumenEste artigo aborda o papel da mineração de ouro em pequena escala na urbanização da Amazônia brasileira, enfocando os processos locais em dois assentamentos de mineração na Província Aurífera Tapajós. Nossa análise revela três tendências na mudança de mobilidade para permanência. Em primeiro lugar, evidencia‐se que, ao longo dos anos, os assentamentos de mineração irregulares cresceram em aldeias formalizadas. Mobilidade e permanências temporais, características tradicionais de mineração de ouro em pequena escala, gradualmente, dá lugar a estadias mais longas em assentamentos de mineração. Fatores cruciais, identidificados nesse processo, são a presença de representação do governo e de infra‐estrutura material. Em segundo lugar, essa transição de assentamentos de forma irregular e provisória para uma condição mais estável e formalizado não é um processo unilateral. Este processo está tomando forma através da sinergia das iniciativas locais e informais e um governo local e federal, entendido nesta pesquisa no sentido passivo‐ reativo. Em terceiro lugar, a permanência é em grande medida construída com a manutenção de uma “comunidade garimpeira” através de ligações com centros urbanos regionais.

Análise prospectiva para ouro nas regiões Ouro Roxo-Cantagalo e Chico Torres, Província Mineral do Tapajós

Spatial modeling (fuzzy logic) was used to predict key areas for gold exploration in the Tapajos Mineral Province (Brazilian Amazon), specifically in the regions here named Ouro Roxo-Cantagalo and Chico Torres. The data used were regional airborne geophysical and SAR-R99B data. The airborne geophysical data processing allowed the generation of evidence layers for modeling approaches. The best evidences in current models were anomalous potassium, F parameter, radioelement ratios, radioelement normalization, and amplitude of the analytical signal of the magnetic anomalous field. The SAR images were used in the photointerpretation stage. In the case of regions of the Ouro Roxo-Cantagalo and Chico Torres shear zones, geophysical images and interpretations (when necessary) were used to application of fuzzy logic, and the gold locations were used to evaluate the quality of the modeling results. The main contributions of this research include the observation of potential gamma-ray spectrometry signatures, the refining interpretation of the digital data, and improving the information of the current geological maps. The prospective Ouro Roxo-Cantagalo and Chico Torres modeling results using the conceptual fuzzy logic method exemplify the generation of regional exploration models for the cartography and selection of gold potential areas. This methodology can be extended to others regions of the Tapajos Mineral Province using airborne geophysical data current available by CPRM and the SAR images of the Centro Tecnico Operacional do Sistema de Protecao da Amazonia (CENSIPAM).

Adularia associated with epithermal gold veins in the Tapajós Mineral Province, Pará State, Northern Brazil

The potassium feldspar variety adularia was found in the Davi and Joel primary gold prospects located in the Tapajos Mineral Province, southwestern Para State, Brazil. In both areas adularia occupies fractures and vugs intimately associated with mineralized quartz veins hosted by hydrothermally altered Proterozoic subvolcanic mafic rocks. Adularia constitutes small (1-4 mm) red to pink crystals of pseudo-orthorhombic form. The Felsobanya habit was identified in specimens from the Joel prospect. Microprobe analyses show that adularia has a composition near to pure KalSi3O8. X-ray diffraction data indicate that the feldspars correspond to predominantly triclinic types with structural state near that of maximum microcline. This high degree of ordering is relatively rare in adularia and may be attributed to the extensive action of late fluids over originally metastable, disordered crystals whose primitive monoclinic form is still preserved. Fluid inclusion studies in quartz crystals adjacent to adularia show that the feldspar formed under temperatures of 240° -320°C at Davi and 220° - 340°C at Joel. The trapped fluids are poor in or devoid of CO2. Fluid salinity was low, averaging 1.15 at Davi and 0.43 at Joel, attaining exceptionally values up to 11 wt.% NaCl equivalent. Considering that the presence of adularia is indicative of boiling, it is possible to estimate a maximum depth of 1.5 km for the formation of adularia and the associated quartz veins. The presence of adularia in the Davi and Joel prospects is important in the definition of the typology of primary gold of the Tapajos Province as this feldspar is a typical mineral of epithermal deposits of the adularia-sericite category. Other features also point to the epithermal nature of the mentioned occurrences. These include the non-metamorphic character of the host rocks and their hydrothermal alteration pattern; the open-space filling textures of the veins and their mineral assemblage; and the brittle nature of the deformation imprinted in both veins and wallrocks. The composition of fluids in inclusions also lies within the limits admitted for adularia-sericite epithermal deposits. The temperatures of formation of the adularia and quartz veins, in turn, belong to a high temperature rank in epithermal deposits, suggesting that the Davi and Joel prospects correpond to exposures of the deeper, higher temperature sectors of their original hydrothermal systems. Two other aspects should be emphasized concerning the definition of an epithermal typology for gold occurrences in the Tapajos region. The Proterozoic age is an unusually old one for this type of deposit, as the majority of their representatives in the world are Cenozoic; and the definition is interesting for gold exploration in connection with the potential for bulk-tonnage deposits.