Stratigraphy of amethyst geode-bearing lavas and fault-block structures of the Entre Rios mining district, Paraná volcanic province, southern Brazil

Léo A Hartmann,Lucas M Antunes,Leonardo M Rosenstengel

doi:10.1590/0001-37652014201120004

Léo A Hartmann, Lucas M Antunes + Show 1 more

Open Access

https://doi.org/10.1590/0001-37652014201120004

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Abstract

The Entre Rios mining district produces a large volume of amethyst geodes in underground mines and is part of the world class deposits in the Paraná volcanic province of South America. Two producing basalt flows are numbered 4 and 5 in the lava stratigraphy. A total of seven basalt flows and one rhyodacite flow are present in the district. At the base of the stratigraphy, beginning at the Chapecó river bed, two basalt flows are Esmeralda, low-Ti type. The third flow in the sequence is a rhyodacite, Chapecó type, Guarapuava subtype. Above the rhyodacite flow, four basalt flows are Pitanga, high-Ti type including the two mineralized flows; only the topmost basalt in the stratigraphy is a Paranapanema, intermediate-Ti type. Each individual flow is uniquely identified from its geochemical and gamma-spectrometric properties. The study of several sections in the district allowed for the identification of a fault-block structure. Blocks are elongated NW and the block on the west side of the fault was downthrown. This important structural characterization of the mining district will have significant consequences in the search for new amethyst geode deposits and in the understanding of the evolution of the Paraná volcanic province.

Highlights

The large-scale amethyst geode production (600 ton/month) from mines in the Paraná volcanic province, mostly Ametista do Sul (Brazil) and Los Catalanes (Uruguay) makes the understanding of the structure of mining districts a major topic in volcanic geology
The district is located in a transitional region between low-Ti basalt flows in the south and high-Ti flows in the north of the Paraná volcanic province (Bellieni et al 1984, Peate et al 1992)
The integrated use of field relationships, geochemistry and gamma-spectrometry led to the identification of the lava flows present, their stratigraphy and the overall fault-block structure of the mining district (Table I)

Summary

INTRODUCTION

The Paraná volcanic province is a major geological unit (Fig. 1) in southeastern South America (917,000 km2; Frank et al 2009) and is composed of many (possibly 120) lava flows in each stratigraphic section. The Entre Rios mining district (Fig. 2) is approximately 1,000 m above the contact with the paleo-erg. The district is located in a transitional region between low-Ti basalt flows in the south and high-Ti flows in the north of the Paraná volcanic province (Bellieni et al 1984, Peate et al 1992). In spite of extensive weathering and soil cover of the studied region, silicified sand dikes in basalt and massive sandstones inside geodes are observed in the Entre Rios mining district (Fig. 3). The Entre Rios district in the state of Santa Catarina (Fig. 1) offers a rare opportunity for the study of the varied processes related to volcanism, hydrothermal alteration, sand injection, geode mineralization, lava stratigraphy and block-faulting in a context of a major continental flood basalt province.

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