ALBERT

Description: Bleaching of red beds by the migration of reducing pore fluids is a widespread phenomenon, with most previous work on the topic focused on detailed studies in the southwestern United States, and to a lesser extent Germany. Herein, we report on widespread bleaching of Cretaceous red beds in the Neuquén Basin and the relationship of this bleaching to hydrocarbon migration. In the Cerro Granito area of the Huincul High, Neuquén Basin, the basal deposits of the Neuquén Group (Candeleros and Huincul Formations) are bleached. This alteration was apparently controlled by fluid and host-rock composition as well as by the intensity and span of the fluid-rock interaction. Red, fine-grained sandstones and mudstones of the Candeleros Formation were deposited in fluvial and swamp environments and contain authigenic hematite, Na-rich corrensite, micro- and mesoquartz, calcite, and analcite. Coarse-grained sandstones and mudstones of the Huincul Formation, which overlie the Candeleros Formation, were deposited in a braided fluvial system, under more humid and acidic conditions than the Candeleros Formation. Sandstones are similar in composition and texture to the Candeleros sandstones, except they contain less basic volcanic detritus and lack corrensite. Bleached facies in both formations contain bitumen and clusters of calcite + bitumen concretions, and partially dissolved detrital clasts and most cements, including early Fe-oxides, which resulted in an increase in secondary porosity and bleaching of the red beds. Alteration of the Candeleros Formation is confined to medium- to coarse-grained sandstone in which corrensite is partially dissolved and its interlayer charge is satisfied by Ca instead of Na, as in corrensite from the red sandstones. Extensive dissolution occurred in the Huincul Formation, which favored the precipitation of new authigenic minerals such as smectite, mixed-layer chlorite-smectite, and pyrite. Bleaching and associated alteration most likely resulted from interaction of hydrocarbons with the red beds. Hydrocarbon paleomigration in the Cerro Granito area occurred in higher-permeability portions of the Candeleros and Huincul Formations. The seal provided by the Lisandro Formation was breached during uplift driven by Tertiary tectonism, resulting in the exhumation of the Huincul and Candeleros Formations. This study highlights the importance of a variety of alterations, including complex clay mineral paragenesis, contemporaneous with bleaching of the red beds. Such alterations can potentially be used to provide evidence for the passage of chemically reducing fluids associated with hydrocarbons in other regions, including cases where the original red color has been completely removed.

Description: Detailed textural and mineralogical investigations carried out in mineralized veins of the New Caledonian peridotites reveal three major periods of infilling and alteration closely linked to post-obduction tectonic activity. The first two periods of infillings are related to the alteration of hydrothermal serpentines, mostly found in the thick serpentine network of the peridotites, into fine-grained serpentine-like residues and newly formed talc-like minerals of weak but variable swelling capacity. The alteration of serpentine into talc-like minerals is limited during the first period of infilling and almost completed during the second one. In some fault zones of the New Caledonian peridotites, talc-like minerals are replaced by sepiolite. Such alterations are reported in both the Ni-free and Ni-rich zones of the infillings ( i.e. in the white ‘deweylite’ and greenish ‘garnierite’ of the fault zones, respectively). They led to the individualization of the hydrous Mg/Ni silicate ore, which is nowadays found in fractures of the saprock and saprolite units of thick lateritic profiles. The third and last period of infilling is assigned to the accumulation of silica and crystallization of quartz. This succession of clay minerals (serpentine-like and talc-like minerals, sepiolite) and quartz in the infillings is interpreted as the result of sequential exportation of Mg and redistribution of Ni along the reactivated faults, generating periods of increasing Si activity in solutions. In this paragenetic model, the meteoric water infiltrating repeatedly the permeable network created by post-obduction tectonic activity would have interacted with a low-temperature hydrothermal field following the exhumation and cooling of the ophiolite nappe. In the less permeable parts of the fault network, restricted leaching conditions would have generated greater alkalinity in solutions and therefore favored the crystallization of sepiolite instead of finely divided talc-like minerals.

Description: The Kiggavik project, located 70 km west of Baker Lake (Nunavut), is a major uranium exploration project in the Canadian arctic, with three significant basement-hosted uranium deposits (Kiggavik, End, and Andrew) which spread along a NE–SW trend a few kilometers from the south-eastern border of the Thelon Basin. These deposits are closely associated with alteration zones in which clay minerals are abundant. At the scale of the whole structural trend, the alteration paragenesis is composed of illite ± sudoite ± hematite ± aluminum phosphate sulfate minerals (APS). Alteration petrography and mineral paragenesis are similar to those identified in basement-hosted uranium deposits related to Paleoproterozoic unconformities in the Athabasca Basin (Canada) or Alligator River (Australia). The alteration haloes are characterized by two similar types of phyllosilicate assemblages (dioctahedral micas or illite and chlorites) corresponding to a regional retrograde metamorphic stage that was overprinted by hydrothermal alteration during the mineralization event. These two assemblages can be distinguished on the basis of crystallographic and chemical properties and mapping of structural parameters, such as the variation of crystallinity along the c -axis or the polytypes of phyllosilicates, which can be used as a vector to mineralization. The crystal chemistry of the hydrothermal phyllosilicates replacing the previous metamorphic minerals indicates a release of iron. This last point is fundamental to the occurrence of hematite in alteration zones and points out the potential effects of the iron redox state in the control of uranium precipitation during the hydrothermal event.

Description: A method to characterize and map both kaolinite and dickite polytypes in sandstone thin sections using infrared microspectroscopy (IRMS) was developed. Minerals identification using IRMS can be performed using the hydroxyl-stretching band of most clay minerals (3500–4000 cm –1 ) in spite of infrared (IR) interferences caused by the embedding resin and glass substratum. Emphasis was placed on determining the optimum analytical conditions for IR data acquisition. The best data-acquisition parameters for Fourier-transform infrared (FTIR) measurements ( i.e. spectra quality as a function of beam size and the number of scans) were obtained from a series of single spectra. Then, spatial resolution was explored as a function of the IR beam size (from 50 μm x 50 μm to 15 μm x 15 μm) and the step-scan interval ( i.e. the distance between two successive analysis points). The IRMS measurements were performed on thin sections of materials characterized previously using scanning electron microscopy (SEM) and chemical analysis. Using IRMS, locations on the thin sections containing nearly pure dickite or kaolinite polytypes were identified and mapped. Most spectra collected using IRMS represented kaolin mineral aggregates rather than individual crystals, however, and mixing of kaolin polytypes was common at the spatial resolution of the IR beam size used. The spatial resolution of the IRMS was comparable to optical petrography and made possible the identification of areas on the thin section for further ‘ in situ ’ investigation using other methods ( e.g. microprobe, Laser Ablation Inductively Coupled Plasma Mass Spectrometry – LA-ICP-MS, etc .). Also, the use of blocky crystal morphology to identify dickite was questioned, as kaolinite with blocky habit was identified. Mineral mapping using IRMS seems particularly suited for investigating petrographic relationships between kaolinite and dickite in sandstone diagenesis, but could also be used for clay minerals in other rock types or soils.

Description: Surficial indicators of recent to present geothermal activity are present around Bouillante (Guadeloupe, French West Indies). Until lately, most mineralogical parageneses identified in this area were typical of low temperature alteration (〈 100°C) with the occurrence of dioctahedral aluminous smectites accompanied by zeolites (heulandite-clinoptilolite) ± calcite ± silica ± kaolinite, as an evidence of the cap rock of the geothermal system. Recently, numerous boulders of highly silicified breccias containing high temperature minerals (〉 200°C) have been identified in the Bouillante bay (Anse Marsolle). Their petrographic study revealed several hydrothermal parageneses typical of a multistage alteration process. Stage 1) An early potassic alteration facies typical of a high-temperature geothermal system characterized by K-feldspar/adularia + quartz + pyrite. K-feldspars have been shown to be present both in the clasts (replacement) and cement of these breccias. They are associated with quartz whose textural properties have revealed fracturing associated with boiling. Stage 2) An acid-sulphate advanced argillic alteration facies with the occurrence of jarosite, gypsum ± silica (≥ 150°C) as usually observed in the upper part of epithermal systems worldwide as marker of deeper boiling. Stage 3) An argillic alteration facies (illite/smectite mixed layers ± smectite ± calcite). With temperatures typically ranging from 100 to 200°C, this alteration facies is associated with near neutral fluids of mainly meteoric origin as known in the present geothermal reservoir (pH = 5.4). This alteration is the later one as evidenced by petrographic observations. These results highlight mineral assemblages and mineral textures characteristic of high temperature hydrothermal alteration in epithermal settings. The occurrence of these breccias involved the existence of eruptive events (magmatic/hydrothermal explosion) which ejected this material. Isotopic (oxygen, argon) and geochemical (trace elements) studies are now necessary to clarify the timing of these breccias and the nature and the connection of the original fluids with current geothermal fluids

Description: 〈span〉〈div〉ABSTRACT〈/div〉This paper describes a detailed petrographic and isotopic study of hypabyssal sheets of quartz-syenite that represent the ultimate differentiation product of the oceanite alkaline magmatic reservoir of the Piton des Neiges stratovolcano (Réunion Island). Clay minerals of the corrensite to chlorite series crystallized during the late-magmatic activity, with quartz, carbonates and accessory minerals from juvenile fluids filling the primary porosity of the quartz-syenite. It is proposed that a double chemical transfer occurred at the end of the crystallization process: degassing of the exsolved CO〈sub〉2〈/sub〉-rich and SiO〈sub〉2〈/sub〉-rich fluid from the magmatic chamber through the porous quartz-syenite and diffusion of Al, Fe and Mg from the intruded basalts affected by the juvenile fluids towards the primary porosity of the quartz-syenite, feeding the crystallization of late-magmatic clays in the residual primary pores after quartz and carbonate deposition. This process may be generalized to alkaline plutonism, as well as to the primitive crusts of terrestrial planets, and may be the first source of clays in early planets.〈/span〉

Description: The Portezuelo Formation is part of a red bed sequence in the Neuquén Basin assigned to the Neuquén Group (lower Cenomanian–middle Campanian). In outcrops of the Portezuelo Formation in the Barda González region, iron oxide and hydroxide cements are only preserved in the less permeable claystone layers. Paleomigration of hydrocarbons and formation waters along the most permeable layers bleached the rocks due to partial to total dissolution of cements and grains and formation of several authigenic minerals: (1) montmorillonite and pyrite in the medium- to fine-grained sandstones and mudstones, and (2) multistage calcite and pyrite in the coarse-grained sandstones and conglomerates. The $${\delta }^{34}{\mathrm{S}}_{\mathrm{CDT}}$$ (Canyon Diablo troilite) of –24.4 to –60 values in pyrite point to microbial reduction of a sulfate precursor. The oxidation of hydrocarbons and fluctuations in pH and carbon dioxide pressure caused by organic acids resulted in multiple stages of dissolution and precipitation of calcite. Calcite crystals host hydrocarbon-bearing fluid inclusions (± pyrite) with variable fluorescence color, indicating that hydrocarbon composition changed with time. The intermediate $${\delta }^{13}{\mathrm{C}}_{\mathrm{PDB}}$$ (Peedee belemnite) (–8.11 to –8.52) and $${\delta }^{18}{\mathrm{O}}_{\mathrm{PDB}}$$ (–9.76 to –9.83) isotope composition of calcites may have resulted from mixtures of $$^{13}\mathrm{C}$$ -rich $${\mathrm{CO}}_{2}$$ after the dissolution of local calcrete with $$^{13}\mathrm{C}$$ -poor $${\mathrm{CO}}_{2}$$ from oxidizing hydrocarbons. This contribution provides for the first time key information to predict the distribution of authigenic minerals in reservoir rocks in the basin and improves understanding of the diagenetic history of the Portezuelo Formation.