Petrographic and geochemical characterization of the magmatic sequence of Seival Mine, Hilario Formation (Neoproterozoic Camaquã Basin), Rio Grande do Sul, Brazil

A Mina do Seival é constituída por rochas vulcânicas e diques de composição andesítica e traqui-andesítica, dispostas em duas sequências. A sequência I inclui rochas piroclásticas e efusivas, e a sequência II é representada pelos diques de composição andesítica. Ambas são incluídas na Associação Shoshonítica de Lavras do Sul. Este magmatismo é relacionado ao estágio pós-colisional do ciclo Brasiliano/Pan-Africano, situando-se estratigraficamente no Alogrupo Bom Jardim, pertencendo à Formação Hilário na Bacia do Camaquã (Neoproterozoico). A área possui intensa alteração hidrotermal e mineralizações de Cu. A mineralização e o magmatismo da Sequência II são controlados por estruturas tectônicas orientadas segundo N/NE e NO, que são relacionadas à distensão regional no período pós-colisional da Orogênese Brasiliano/Pan-Africana. Processos hidrotermais em diferentes temperaturas atuaram sobre estas rochas originando produtos de alteração pervasiva, principalmente clorita, corrensita e esmectita, com veios preenchidos por quartzo, carbonato, barita e minerais de cobre. Em ambas as sequências encaixantes da Mina do Seival é possível identificar a afinidade shoshonítica das rochas. Os elevados teores de Cu, Zn e Ni nos diques em relação às rochas piroclásticas e às efusivas, mesmo nas mais hidrotermalizadas, sugerem que as principais ocorrências de mineralização de Cu têm origem magmática. Os dados químicos de rocha total indicam que o enriquecimento dos elementos componentes da mineralização, Ag, Au, Cu e Zn, está relacionado aos diques da Sequência II. Os teores de Au e Cu são mais elevados nas amostras com menores concentrações de carbonatos, sugerindo que a carbonatação não tem relação com a deposição dos minérios.The Seival Mine consists of andesitic volcanic and trachy-andesitic volcanic and hipabissal rocks grouped into two volcanic sequences. The Sequence I is composed of pyroclastic rocks and effusive rocks, and sequence II by andesite dikes. The area shows intense hydrothermal alteration and Cu mineralization. This magmatism is related to the post-collisional stage of the cycle Brasiliano/Pan-African, and it is stratigraphically positioned in the Bom Jardim Allogroup, Hilário Formation, which is part of the Neoproterozoic Camaquã Basin. Sequence II magmatism and mineralization are controlled by tectonic structures with N/NE and NW orientation, which are related to regional extension during the post-collisional stage of Brasiliano/Pan-African Orogeny. Processes involving different temperatures caused pervasive alteration products, with generation of chlorite, corrensite and smectite, with veins filled by quartz, carbonate, barite and copper minerals. In both sequences of Seival Mine is possible to identify the shoshonitic affinity of host sequences, as pointed out by geochemical data. The higher contents of Cu, Zn, and Ni in andesite dikes in relation to pyroclastic and effusive rocks, even in the hydrothermally altered types, suggest that the main Cu mineralizations have magmatic origin. The Au and Cu contents are higher in samples with lower carbonate concentrations, which suggests than carbonate alteration have no relation with ore deposition

Mafic magmatism associated with Encruzilhada do Sul Granite, RS: implications for the geration of the post-collisional granitc magmatism of the Dom Feliciano Belt

. A integração entre o mapeamento geológico com estudos petrográficos e geoquímicos permitiu caracterizar a evolução petrológica do Granito Encruzilhada do Sul (GES). Este maciço ocorre na parte norte do Batólito Pelotas, porção leste do Cinturão Dom Feliciano. No interior do GES ocorrem pequenos corpos de dioritos e zonas ricas em enclaves máficos, representando uma interação de mistura física entre os magmas ácido e básico. Fácies graníticas com alto teor de minerais máficos e de composição intermediária a ácida são interpretados como rochas híbridas e relacionadas a mistura química entre os magmas. Com base nos dados geoquímicos é possível definir que o magmatismo máfico mostra afinidade tholeiítica, enquanto o magmatismo félsico é caracterizado por granitos do tipo-A. O GES apresenta uma fácies de bordo caracterizada por monzogranitos porfiríticos de matriz fina a média com alto teor de enclaves máficos. A porção central do maciço é composta por sieno a monzogranitos heterogranulares grossos ricos em quartzo e com baixo teor de minerais máficos. Uma fácies de sienogranitos equigranulares ocorre na porção centro-norte e representa a cúpula parcialmente preservada do maciço. A zonação textural e composicional observada foram geradas a partir do resfriamento de uma única câmara magmática. O posicionamento do granito foi controlado por uma fase cinemática extensional da Zona de Cisalhamento Dorsal de Canguçu que permitiu a ascensão dos magmas até sua estabilização em níveis superiores da crosta. Os dados obtidos sugerem que a geração do GES ocorreu no período pós-colisional da orogênese Dom Feliciano, associada à fusão parcial de uma crosta granulítica, promovida pelo aumento do gradiente geotérmico devido ao magmatismo máfico. As condições de alta temperatura do evento magmático permitiram, em estágio precoce da cristalização, a mistura química entre os magmas félsico e máfico e a geração de tipos híbridos. Durante o processo da cristalização, entretanto, a viscosidade e a densidade dos magmas são modificadas, favorecendo o desenvolvimento de estruturas geradas pela mistura física. The integration of geological mapping with petrographic and geochemical studies allowed the characterization of petrological evolution of the Encruzilhada do Sul Granite (ESG). This pluton occurs in the northern part of the Pelotas Batholith, eastern portion of the Dom Feliciano Belt. In the interior of the ESG small bodies of diorites and zones rich in mafic enclaves occur, representing an interaction of physical mixture between the acid and basic magmas. Granite facies with high content of mafic minerals and intermediate to acid composition are interpreted as hybrid rocks and related to the chemical mixture between the magmas. Based on the geochemical data, it is possible to define that mafic magmatism shows tholeiitic affinity, while felsic magmatism is characterized by A-type granites. The ESG presents an outer facies characterized by porphyritic monzo- to granodiorites of fine to medium matrix with high content of mafic enclaves. The central portion of the pluton is composed of thick heterogranular sieno to monzogranites rich in quartz and with low content of mafic minerals. A facies of equigranular sienogranites occurs in the center-north portion and represents the partially preserved cupola of the massif. Based on geochemical data, it is possible to define that the mafic magmatism shows tholeiitic affinity, whereas the felsic magmatism is characterized by A-type granites. The emplacement of the granite was controlled by an extensional kinematic phase related to the regional Dorsal do Canguçu Shear Zone, which allowed the rise of magmas up to higher levels of the crust. The data suggest that the generation of the ESG took place in the post-collisional period of Dom Feliciano orogeny and was associated with partial melting of a granulite crust promoted by increasing of the geothermal gradient due to the mafic magmatism. The high temperature conditions of the magmatic event allowed the chemical mixing between felsic and mafic magmas and the generation of hybrid types during an early stage of crystallization. The evolution of crystallization modified the viscosity and density of these magmas and imply on the development of mingling structures

Petrology of Gameleira potassic lamprophyres, São Francisco Craton

Gameleira lamprophyres are dykes and mafic microgranular enclaves associated with the shoshonitic Gameleira monzonite. This association belongs to the Paleoproterozoic alkaline magmatism from Serrinha nucleus, northeast Brazil. The liquidus paragenesis is diopside, pargasite, apatite and mica. Reverse zoning was identified in the groundmass alkali feldspar and was related to the undercooling of lamprophyric magma during the emplacement, with high growth rate of pargasite/edenite inducing disequilibrium between feldspars and liquid. Chemical data indicate that the lamprophyres are basic rocks (SiO2 < 48 wt%), with alkaline character (Na2O + K2O &gt; 3 wt%) and potassic signature (K2O/Na2O &#8776; 2). High contents of MgO and Cr are consistent with a signature of a primary liquid, and such concentrations, as well as Al, K, P, Ba, Ni- and light rare earth elements, are consistent with an olivine-free metasomatic mantle source enriched in amphibole, clinopyroxene and apatite. By contrast, the ultrapotassic lamprophyres from Morro do Afonso, contemporaneous alkaline ultrapotassic magmatism in Serrinha nucleus, were probably produced by melting of a clinopyroxene-phlogopite-apatite enriched-source. The identification of different mineral paragenesis in the source of potassic and ultrapotassic lamprophyres from Serrinha nucleus can contribute to the understanding of the mantle heterogeneities and tectonic evolution of this region

Biogeochemistry of REE elements and tetrad effect in the soil-plant system: a study on volcanic rock covers in southernmost Brazil

This paper deals with the distribution of REE in rock, soil and plant in an area of monzonitic rocks from southernmost Brazil. The REE patterns in Schinus lensticifolius show a negative-Ce anomaly and a prominent tetrad effect, characterized as W-type that are not present in rock and soil samples. The REE patterns in the soils and rocks sampled are very similar and there is no fractionation of REE during the processes of soil formation. The W-type patterns are interpreted as indicating that REE were absorved by S. lentiscifolius as simple ions rather than as complex ions, or, alternatively, that the transport of REE in the plant metabolic processes was as free ions. The recognition of tetrads, either, M- or W-type patterns, is an additional tool for understanding the biogeochemistry of REE and can contribute to the study of monitoring processes of contaminated environment or to mineral prospecting

Geochemistry and petrogenesis of post-collisional ultrapotassic syenites and granites from southernmost Brazil: the Piquiri Syenite Massif

The Piquiri Syenite Massif, southernmost Brazil, is part of the post-collisional magmatism related to the Neoproterozoic Brasiliano-Pan-African Orogenic Cycle. The massif is about 12 km in diameter and is composed of syenites, granites, monzonitic rocks and lamprophyres. Diopside-phlogopite, diopside-biotite-augite-calcic-amphibole, are the main ferro-magnesian paragenesis in the syenitic rocks. Syenitic and granitic rocks are co-magmatic and related to an ultrapotassic, silica-saturated magmatism. Their trace element patterns indicate a probable mantle source modified by previous, subduction-related metasomatism. The ultrapotassic granites of this massif were produced by fractional crystallization of syenitic magmas, and may be considered as a particular group of hypersolvus and subsolvus A-type granites. Based upon textural, structural and geochemical data most of the syenitic rocks, particularly the fine-grained types, are considered as crystallized liquids, in spite of the abundance of cumulatic layers, schlieren, and compositional banding. Most of the studied samples are metaluminous, with K2O/Na2O ratios higher than 2. The ultrapotassic syenitic and lamprophyric rocks in the Piquiri massif are interpreted to have been produced from enriched mantle sources, OIB-type, like most of the post-collisional shoshonitic, sodic alkaline and high-K tholeiitic magmatism in southernmost Brazil. The source of the ultrapotassic and lamprophyric magmas is probably the same veined mantle, with abundant phlogopite + apatite + amphibole that reflects a previous subduction-related metasomatism

Mineral assemblages and temperature associated with Cu enrichment in the Seival area (Neoproterozoic Camaquã Basin of Southern Brazil)

International audienceThe Neoproterozoic sequence of volcanic rocks in the Camaquã Basin included in the Lavras do Sul Shoshonitic Association hosts disseminated Cu deposits. The volcanic sequence in the Seival Mine area includes andesitic lava flows, lapilli tuff, volcanic agglomerate and andesitic dikes with pervasive alteration. The hydrothermal alteration is interpreted to be a product of late–magmatic fluids or the mixing of magmatic with meteoric fluids or basinal brines. The late–magmatic hydrothermal system started after volatile oversaturation and degassing in a magmatic system, which was partially segregated into vesicles. The similar REE contents in the andesine–labradorite and albite indicate that the fluid temperatures ranged from 600 to 250 °C. The crystallization of titanite and Fe–clinochlore to Mg–chamosite followed the albitization process. The temperature range associated with the chloritization process was estimated using a chlorite geothermometer and varied from 251 ± 56 to 183 ± 39 °C. The precipitation of Mg–saponite and smectite–rich chlorite/smectite mixed–layers in some andesitic lava flows and the precipitation of quartz, calcite, barite and hematite in fractures suggest fluid circulation with temperatures lower than the chloritization process. The alteration minerals are represented by chlorite and albite associated with pyrite–chalcopyrite, while chlorite/smectite mixed–layers and barite or hematite are associated with bornite–chalcocite–covellite. Thermodynamic calculations confirm the potential coprecipitation of pyrite–chalcopyrite with chlorite and albite. The occurrence of bornite–chalcocite–covellite and barite seems to be favored by low–temperature chlorite/smectite with neutral to mildly acidic water influx. Consequently, albitization and the process of chloritization at high temperatures (>ca. 251 ± 56 °C) can be used as an exploration guide for primary pyrite–chalcopyrite enrichment, and chlorite/smectite at low temperatures (ca. 250 to 50 °C) can be related to bornite–chalcocite–covellite or the process of Cu enrichment