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Structure- and tectonic-related gas migration into Ordovician sandstone reservoirs and its impact on diagenesis history were reconstructed in two gas fields in the Sbaa Basin, in SW Algeria. This was accomplished by petrographical observations, fluid inclusion microthermometry and stable isotope geochemistry on quartz, dickite and carbonate cements and veins. Two successive phases of quartz cementation ( CQ1 and CQ2) occurred in the reservoirs. Two phase aqueous inclusions show an increase in temperatures and salinities from the first CQ1 diagenetic phase toward CQ2 in both fields. Microthermometric data on gas inclusions in quartz veins reveal the presence of an average of 92 ± 5 mole% of CH4 considering a CH4- CO2 system, which is similar to the present-day gas composition in the reservoirs. The presence of primary methane inclusions in early quartz overgrowths and in quartz and calcite veins suggests that hydrocarbon migration into the reservoir occurred synchronically with early quartz cementation in the sandstones located near the contact with the Silurian gas source rock at 100-140°C during the Late Carboniferous period and the late Hercynian episode fracturing at temperatures between 117 and 185°C, which increased in the NW-direction of the basin. During the fracture filling, three main types of fluids were identified with different salinities and formation temperatures. A supplementary phase of higher fluid temperature (up to 226°C) recorded in late quartz, and calcite veins is related to a Jurassic thermal event. The occurrence of dickite cements close to the Silurian base near the main fault areas in both fields is mainly correlated with the sandstones where the early gas was charged. It implies that dickite precipitation is related to acidic influx. Late carbonate cements and veins (calcite - siderite - ankerite and strontianite) occurred at the same depths resulting from the same groundwater precipitation. The absence of methane inclusions in calcite cements result from methane flushing by saline waters. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic Thelon Basin, located on the border between Nunavut and the Northwest Territories of Canada, is a contemporaneous analog of the uranium-rich Paleoproterozoic Athabasca Basin in Canada. Early diagenesis resulted in precipitation of extensive hematite on the surfaces of detrital quartz grains throughout the Thelon Formation and minor hydroxy-phosphate in veins locally. Continued diagenesis then resulted in syntaxial quartz cementation of detrital quartz at 130°C from fluids having ca. 17 wt.% equivalent NaCl, similar to the Athabasca Basin. Cementation of this type is most pronounced in fine-grained sequences in the Thelon Basin. A period of extensive desilicification during continued burial was followed by formation, at ca. 200°C, of peak-diagenetic illite having Ar–Ar ages of ca. 1400–1690 Ma in the Thelon Formation. This illite was associated with fluids with δ[sup 18] O and δD values of ca. 6‰ and –50‰, respectively, similar to those during peak diagenesis of the Athabasca Basin. Although the timing, salinity, and isotopic composition of the peak-diagenetic fluids in the Thelon and Athabasca Basins are similar, the peak-diagenetic mineral assemblage in the Athabasca Formation is dickite and illite, with minor dravite and goyasite rather than simply illite. Consequently, the fluids at peak diagenesis, which in the Athabasca Basin are synchronous with formation of world-class unconformity-type uranium deposits, had different compositions in each basin. Post-peak diagenesis in the Thelon Basin was quite distinct from that in the Athabasca Basin in that illite was replaced in the central portion of the basin by K-feldspar and then sudoite, which crystallized from saline brines at ca. 1000 Ma and 100°C. Evidence for later infiltration of these brines is absent in the Athabasca Basin, although uranium mobilization at ca. 900 Ma from fluids having the same characteristics as those at peak diagenesis was pronounced in the Athabasca Basin. Recent incursion of meteoric waters along reactivated structures into the Athabasca Basin has variably affected hydrous and uraniferous minerals, but evidence for this is lacking in the Thelon Basin. The Thelon Basin reflects less intensive fluid–rock interaction in its early history than that recorded in the basal units of the Athabasca Basin.Le bassin de Thelon (Paléoprotérozoïque), situé sur la frontière entre le Nunavut et les Territoires du Nord-Ouest du Canada, est l'analogue contemporain du bassin riche en uranium d'Athabasca (Paléoprotérozoïque), au Canada. Une diagenèse précoce a fait précipiter une grande quantité d'hématite sur les surfaces des grains de quartz détritique à travers toute la Formation de Thelon et, localement, une faible quantité d'hydroxy-phosphate dans les veines. La poursuite de la diagenèse a ensuite résulté en la cémentation quartzique par surcroissance du quartz détritique à 130 ºC à partir de fluides qui avaient environ 17 % (poids) d'équivalent NaCl, semblable au bassin d'Athabasca. La cémentation de ce type est le plus prononcé dans les séquences à grains fins du bassin de Thelon. Une période de désilicification extensive, au cours d'un enfouissement continu, a été suivie de la formation d'illite au sommet de la diagenèse, à environ 200 ºC; les âges Ar–Ar de l'illite sont d'environ 1400–1690 Ma dans la Formation de Thelon. Cette illite était associée à des fluides ayant des valeurs de δ[sup 18] O et δD d'environ 6‰ et – 50 ‰ respectivement, semblables a celles présentes durant le sommet de la diagenèse du bassin d'Athabasca. Bien que la séquence, la salinité et la composition isotopique des fluides au sommet de la diagenèse dans les bassins de Thelon et d'Athabasca soient semblables, l'assemblage minéralogique du sommet de la diagenèse dans la Formation de l'Athabasca consiste en dickite et en illite avec de la dravite et de la goyasite plutôt que simplement de l'illite. Par conséquent, les fluides au sommet de la diagenèse, lesquels dans le bassin d'Athabasca sont synchrones à la formation de dépôts d'uranium de classe mondiale et de type discordance, ont des compositions différentes dans chaque bassin. Dans le bassin de Thelon, la diagenèse après le sommet a été assez différente de celle dans le bassin d'Athabasca en ce que l'illite dans la partie centrale du bassin a été remplacée par un feldspath-K puis par de la sudoïte, qui a cristallisé à partir de saumures vers 1000 Ma et à 100 ºC. Il n'existe aucune preuve d'une infiltration postérieure de ces saumures dans le bassin de l'Athabasca, bien que la mobilisation de l'uranium, vers 900 Ma et à partir de fluides ayant les mêmes caractéristiques, ait été forte dans le bassin de l'Athabasca. Une récente incursion d'eaux météoriques dans le bassin de l'Athabasca, le long de structures réactivées, a affecté les minéraux hydratés et uranifères à divers degrés, mais il n'y a pas d'évidence de cela dans le bassin de Thelon. Le bassin de Thelon reflète une interaction fluide–roche moins intense au début de son histoire que celle enregistrée dans les unités à la base du bassin de l'Athabasca.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

The orogenic pyrite–rich Au (As, Bi, Ag) mineralization (up to 31 g/t) of the Buracão area is one of several orogenic deposits in the Brasilia Fold Belt (BFB; Central Brazil), exposed to tropical weathering that shows extensive oxidized profiles. The distribution and thickness of these gossan profiles lead to variable gold enrichments and uncertainties to gold prospecting in this region. In order to investigate physicochemical processes related to the evolution of the gossan cover and the supergene gold enrichment, we present mineralogical, textural and geochemical data from each zone of twenty gossan profiles in the Buracão area. These results show that the mineralogy and textures along the gossan profile reflect oxidation, hydrolysis, dissolution and precipitation processes controlled by different Eh–pH conditions, which are controlled by supergene conditions and hypogene ore composition. The geometry, distribution, and thickness of gossan profiles, as well as the style of gold occurrence, reflects textures of the hypogene orebodies, while the weathering has an important role in the gold chemical refining (i.e., increasing ca. 25% in gold fineness) and the distribution of gold grades along the profile. These points allowed us to identify that the leached spongy hematite–rich zone in the upper indigenous profile contains the higher fineness gold grades (i.e., the "bonanza level"), and Bi–minerals such as waylandite and bismutite that may be important pathfinders for high gold grades in these gossan profiles. Our results demonstrate that ore prospection in gossans over Au–As–Bi–Ag deposits should be done focusing on leached zones, different to that traditionally conducted in gossans related to base metal sulfide deposits that have focused on precipitation zones. The similarities between the hypogene mineralization of the Buracão area and many deposits in the BFB and worldwide encourage the application of this approach elsewhere, which improve gold prospecting in weathering profiles over orogenic deposits. • Gossan evolution over orogenic pyrite–rich Au (As, Bi, Ag) mineralization • The role of environment, climate and primary ore composition on the gossan evolution • Mineralogy and textures related to physicochemical processes controlled by contrasting Eh–pH conditions • The nature of supergene gold enrichment and its distribution along the gossan profile • The implication of mineralogy and textures as prospecting guides in weathered profiles [ABSTRACT FROM AUTHOR]

The Trindade Island is located in the South Atlantic Ocean, 1170 km from the Brazilian coast and represents the eastern end of the submarine E–W Vitória–Trindade Chain. This is interpreted as the Trindade plume track beneath the South American plate during the Cenozoic. Almeida (1961) recognized five volcanogenic successions at Trindade in decreasing age: the Trindade Complex (TC) and the Desejado (DF), Morro Vermelho (MV), Valado (VF) and Paredão (PF) formations, composed of nephelinitic to phonolitic effusive-pyroclastic deposits, dykes and necks. Here, we present new 40 Ar/ 39 Ar ages and re-evaluation of available 40 K/ 40 Ar* data that, coupled with previous petrological information, allowed us to reconstruct the volcanic history of the island (i.e., 3.9–0.25 Ma) distinguishing near synchronous volcanic episodes and solving several stratigraphic uncertainties reported in the literature. The geochronological dataset show that the nephelinitic–phonolitic volcanism at Trindade was discontinuous throught time, being marked by periods of high volcanic activity and periods of quiescence, which suggests variable melt production and eruption rates. The peak of the volcanic activity occurred between 3.9 and 2.5 Ma (i.e., Lower Pliocene to Lower Pleistocene) that is represented by the TC, which correspond to the largest volume of volcanic deposits preserved in the island. The volcanic activity slows down progressively from 2.5 Ma to cease at ca. 1.6 Ma, period that is represented by the DF. This volcanism was followed by a quiescent period that lasted until the ephemeral nephelinitic volcanism of the MV (no age), VA (no age) and PF (ca. 0.25 Ma). Thus, the volcanic activity in Trindade was ceased completely at ca. 0.25 Ma, event registered in the uppermost volcanic deposits of the PF and that represents the last volcanic activity in the Brazilian territory. [ABSTRACT FROM AUTHOR]