The Proterozoic Vazante hypogene zinc silicate district, Minas Gerais, Brazil: a review of the ore system applied to mineral exploration

The Proterozoic Vazante zinc silicate district in Minas Gerais, Brazil, hosts world-class hypogene willemite deposits in dolomitic rocks interbedded with siliciclastic rocks deposited in subtidal to supratidal environments. Willemite ore bodies are structurally controlled along regional NE-trending structures which are interpreted as being active during the Neoproterozoic Brasiliano orogeny. The hydrothermal alteration is characterized by an early stage of Fe-dolomite, which replaced the host dolomitic rocks, followed by precipitation of minor sphalerite and then hematite and willemite. Elements commonly enriched in the zinc ore include As, Ba, Be, Bi, Cd, Co, Fe, Ge, In, Mo, Ni, Sb, Se, U, V and W. Mineralogical, fluid inclusion and isotopic data indicates that mixing of S-poor metalliferous saline fluids with meteoric water favored the formation of willemite ore. Carbonaceous phyllites from the underlying thick siliciclastic sequence show evidence of early enrichment in zinc (and ore-related metals) and remobilization, respectively, prior to and during the Brasiliano orogenic event. This unit is interpreted as a possible source of ore-related elements. It is proposed that during the Brasiliano orogeny, hot (T > 170 degrees C) saline fluids (>15 wt % eq. NaCl) leached metals from siliciclastic source rocks and precipitated willemite ore in the overlying dolomitic sequence along structures that favored mixing with oxidizing meteoric water

An?lise estrutural e principais controles de mineraliza??o do dep?sito de Ambr?sia Sul-Paracatu/MG.

Programa de P?s-Gradua??o em Evolu??o Crustal e Recursos Naturais. Departamento de Geologia, Escola de Minas, Universidade Federal de Ouro Preto.O dep?sito de Zn-(Pb) sulfetado de Ambr?sia Sul ocorre encaixado no setor meridional da faixa Vazante, regi?o lim?trofe entre a faixa Bras?lia e a borda sudoeste do cr?ton S?o Francisco. Tal regi?o ? caracterizada por t?pico ambiente c?rstico em zona de clima tropical, que comumente ? representada por intenso desenvolvimento de cobertura espessa e err?tica de solo o que dificulta o mapeamento geol?gico da regi?o bem como as correla??es litoestratigr?ficas e estruturais. Neste contexto, a sondagem explorat?ria, aliada ? recupera??o de testemunhos, permitiu o mapeamento litoestrutural em profundidade da zona de carstifica??o e a identifica??o de 6 litof?cies, agrupadas em 4 associa??es de f?cies. As caracter?sticas sedimentares das associa??es de f?cies sugere um ambiente marinho com varia??es relativas do n?vel mar, associado com o desenvolvimento de uma plataforma carbon?tica em contexto de bacia de antepa?s. A correla??o estratigr?fica regional permitiu reconhecer na pilha estratigr?fica da ?rea as forma??es Serra do Garrote, Serra do Po?o Verde e Morro do Calc?rio, definindo uma discord?ncia estratigr?fica e/ou a n?o deposi??o de rochas do membro Pamplona m?dio e inferior, presente no setor meridional da faixa Vazante. Por sua vez, a sondagem com orienta??o de testemunhos permitiu uma investiga??o estrutural completa, onde foram descritas estruturas como falhas, fraturas, veios, acamamento, dobras, folia??es e linea??es. A an?lise estrutural local sugere uma fase de extens?o com fluxo de massa para oeste, onde teriam sido geradas as falhas normais e veios de alto ?ngulo, seguida de uma fase de contra??o, com reativa??o de planos de acamamento e nuclea??o de falhas reversas e veios de baixo ?ngulo, al?m de dobras com verg?ncia para leste. As mineraliza??es hidrotermais est?o encaixadas em uma zona de brecha hidrotermal, preferencialmente em veios de alto ?ngulo e em catacl?sticos gerados durante a extens?o local. Os dolomitos constituem a principal rocha encaixante, todavia, pr?ximo ao contato de base com os filitos carbonosos, observou-se o enriquecimento local de sulfetos o que sugere o controle da mineraliza??o ligado uma barreira litoqu?mica. Veios de baixo ?ngulo, raramente mineralizados em Zn, cortam veios de alto ?ngulo, sugerindo que o evento mineralizante tenha perdurado inclusive durante a fase de invers?o tect?nica. Este contexto indica que o dep?sito de Ambr?sia Sul tenha se formado na regi?o do bulge da bacia de antepa?s, afetado primeiramente por uma fase extensional, durante a sua nuclea??o, e posteriormente compressional, com o avan?o dos cavalgamentos da faixa Bras?lia para leste.The Ambr?sia Sul sulfide Zn (Pb) deposit is hosted in the southern sector of the Vazante belt, a border region between the Brasilia belt and the southwest edge of the S?o Francisco craton. This area is characterized by a typical karstic environment in a tropical climate zone generated a thick and erratic soil cover that hinders the geological mapping of the region as well as the lithostratigraphic and structural correlations. In this context, an assessment survey, combined with the collection of core samples, allowed lithostructural mapping at the karstification zone depth and the identification of six lithofacies, which were grouped into four facies associations. The sedimentary characteristics of the facies associations suggest a marine environment with relative fluctuations in sea level that was associated with the development of a carbonate platform in a foreland basin context. The regional stratigraphic correlation allowed the Serra do Garrote, Serra do Po?o Verde and Morro do Calc?rio formations to be recognized in the stratigraphic stack, defining a stratigraphic unconformity and/or non-deposition of rocks of the middle and lower Pamplona member in the southern section of the Vazante belt. On the other hand, the assessment survey with structural core orientation allowed a complete structural investigation, where structures such as faults, fractures, veins, bedding, folds, foliations and lineaments were describe. The local structural analysis suggests a extension phase with mass flow to the west, where the normal faults and high angle veins would have been generated, followed by a contraction phase, with reactivation of bedding planes and nucleation of reverse faults and veins of low angle, plus folds with vergence to the east. The hydrothermal Zn-(Pb) mineralizations are hosted in a hydrothermal breccia zone, rather in high angle veins and in cataclasites generated during the local extension. The dolomites constitute the main host rock, but near the base contact with the carbonaceous phyllites, local enrichments of sulfides were observed, which suggests a mineralization control linked to a lithochemical barrier. Low-angle veins, rarely mineralized in Zn, cut high-angle veins, suggesting that the mineralizing event persisted even during the tectonic inversion phase. This context indicates that the Ambr?sia Sul deposit formed in the bulge region of the foreland basin, first affected by an extensional phase, during its nucleation, and later compressional, with the advancement of the ridges of the Bras?lia to the east