Abstract Sinter overlying the first quartz andesite flow at the base of the Serra Geral Group is a surface manifestation of intense hydrothermal processes operating in the Paraná Basin during the Early Cretaceous. The coarse quartz sinter from western Rio Grande do Sul state was studied in satellite images, field surveying, optical petrography, scanning electron microscopy, and electron probe microanalyses, including backscattered electron images and chemical analyses of rocks. Quartz forms large crystals (10 cm) because it was either deposited in a dilute aqueous solution or recrystallized from fine-grained sinter. Well-crystallized chamosite — an iron aluminosilicate (Fe-chlorite) — occurs in quartz crystals, partly associated with fractures. The composition of chamosite is akin to that in ore deposit associations. Noble metals in two sinter samples are present in concentrations of 0.1 ppm Ag and 15 ppb Au. The contents of Ba, Bi, Cu, Mo, S, and W are low but are significant. The present description of sinter quartz signals the presence of a major paleo-hotspring field in the Fronteira Oeste Rift, Rio Grande do Sul, Brazil, that mertis further study to fully characterize the extent and metallogenetic endowment (Au-Ag-Cu) of the epithermal province.

Abstract Fascinating circular hydrothermal bowls 0.5 m in diameter occur in paleodunes of the 1.2 million km2 Botucatu paleoerg in the Fronteira Oeste of Brazil, Rio Grande do Sul, and were exposed upon erosion of presumed 2,000 m of basalt and rhyodacite cover of the Paraná Volcanic Province (1 million km2) since the Cretaceous (135 Ma). The tops of the paleodunes display low-temperature (150-50°C) hydrothermal structures comparable to those in basalts and rhyodacites. The source beds of sand injectites display evidence of sequential fault-valve action. The bowls represent a new structure in continental paleodunes. They present circular rims sealed by quartz and a flat horizontal bottom with a central sealed vent. The inclined internal rims of the bowls exhibit a radial set of linear fractures filled with quartz, forming prominent, sharp, positive structures. Their origin is ascribed to the work of water vapor originated in the heated freshwater aquifer. The concave ceiling was lifted a few millimeters along circular fractures long enough for quartz to be deposited. Erosional exhumation of the buried Botucatu hyperdry sand sea has revealed the effects, including the new structures described here, of hydrothermalism associated with a major igneous province upon a giant paleoerg turned-aquifer.

Abstract The isotopic and geochemical evolution of tourmaline constrain the processes of paleo-oceanic lithosphere in ophiolites. The Brasiliano Orogen is a major structure of South America and requires characterization for the understanding of Gondwana supercontinent evolution. We made a pioneering investigation of tourmaline from a tourmalinite in the Ibaré ophiolite by integrating field work with chemical analyses of tourmaline by electron microprobe (EPMA) and δ11B determinations via laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Remarkably massive tourmalinite (>90 vol.% tourmaline, some chlorite) enclosed in serpentinite has homogeneous dravite in chemical and isotopic composition (δ11B = +3.5 to +5.2‰). These results indicate a geotectonic environment in the altered oceanic crust for the origin of the tourmalinite. This first δ11B characterization of tourmaline from tourmalinite sets limits to the evolution of the Neoproterozoic to Cambrian Brasiliano Orogen and Gondwana evolution.

Abstract Mineralogical evolution of ophiolites is significant to understand paleo-oceanic crust and mantle requiring multi-proxy techniques to identify steps in the processes. We studied the Bossoroca ophiolite from the southern Brasiliano Orogen, a prime example of Tonian accretion to an oceanic island arc. Integration of field geology, aeromagnetometry, aerogamaspectrometry, electron microprobe analyses, and compositional maps of minerals led to the decoding of oceanic and continental processes. The ophiolite is highly magnetic and low-K and is positioned at the base of the superstructure. We studied amphibolite, tourmalinite, and chromite-talc-magnesite granofels from the ophiolite, Capivaras diorite from the Cambaí Complex infrastructure and one metavolcanoclastic rock from the Vacacaí Group superstructure. Honblende is zoned in all rock types. Low-Ti hornblende is compatible with medium-pressure metamorphism at 7 kbar. This M1 to M2 amphibolite facies resulted in the widespread association of olivine + talc in metaserpentinite. Dravite is similar to tourmaline from the Ibaré ophiolite. Andesine and oligoclase are dominant and albite minor. Cr-spinel in granofels recrystallized in greenschist facies; host rock originated by carbonatization of serpentinite formed in the oceanic crust along with chloritite and tourmalinite. Serpentinite rare earth elements (REE) suggest origin in depleted mantle peridotite. The ophiolite evolved in the Adamastor Ocean until incorporation into the island arc.

Resumo: No presente estudo, dados detalhados de microanálise (elementos maiores e traços) em minerais de sills da Formação Serra Geral representam a primeira tentativa de avaliar o coeficiente de partição de Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Cs, Ba, REE, Hf, Ta, Pb, Th e U entre plagioclásio, augita e pigeonita coexistentes em 12 sills basálticos toleíticos ao longo da borda leste da Bacia do Paraná, Brasil. Estas 12 amostras são dos estados do Rio Grande do Sul, Paraná, São Paulo e Goiás. Coeficientes de partição em plagioclásio foram comparados em zonas de núcleo, intermédio e borda, usando LA-ICP-MS, e os resultados foram comparados com as variações das áreas de distribuição dos elementos Ca, Al, Na e Si. O coeficiente de partição do Sr e Eu diminui com o aumento da concentração de Ca no plagioclásio e aumenta para a borda, relacionado com o aumento da concentração de Na e Si e diminuição da temperatura. O KD do Ni e V nos clinopiroxênios analisados possui correlações positivas para Mg2+, Ca2+ e Al3+ devido a altas concentrações destes elementos no núcleo, diminuindo para a borda, e correlações negativas de KD Sc, Cr, Co, Pb e Lu, devido a substituição preferencial pelo Fe2+ em coordenações octaédricas. O uso de LA-ICP-MS e EPMA possibilitam a caracterização química de elementos maiores e traços e a determinação do coeficiente de partição em minerais coexistentes e o líquido.

Abstract: In the present study, detailed micro-analytical data (major and trace elements) in minerals from sills of Serra Geral Formation represents the first attempt to evaluate the partition coefficient of Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Cs, Ba, REE, Hf, Ta, Pb, Th and U among coexisting plagioclase, augite and pigeonite from 12 tholeiitic basaltic sills along the eastern border of the Paraná Basin, Brazil. These 12 samples come from Rio Grande do Sul, Paraná, São Paulo and Goiás states. Plagioclase/melt partition coefficients KD were determined in core, intermediate and rim zones using LA-ICP-MS and the results were compared with the variations of the elements Ca, Al, Na and Si in those areas. Partition coefficients of Sr and Eu decrease with the increase of Ca concentration in plagioclase. A reduction in temperature and consequently a rise in Na and Si levels increase the partition coefficients of Sr and Eu at the rim of mineral The KD of Ni and V in clinopyroxene has positive correlations with Mg2+, Ca2+ and Al3+ due to higher concentrations of those elements at the core and lower concentration towards the rim and negative correlations of KD of Sc, Cr, Co, Pb and Lu, due to preferential substitutions by Fe2+ in sixfold coordination. The use of LA-ICP-MS and EPMA made possible the chemical quantification of major and trace elements and the determination of partition coefficients in coexisting plagioclase and clinopyroxene in basaltic melts.