Proposition d'un modele de silicification superficielle des gres neogenes de Montjuic, Barcelone (Espagne); parageneses minerales, environments geochimiques et circulation des fluides

The Montjuic hill is part of the Neogene horst and graben system of the Catalan Coastal Ranges at the northwestern edge of the Valencia Trough. It is located to the SE of Barcelona City and consists of a 200 m thick strongly silicified detrital succession (mainly conglomerate and sandstone units alternating with lutitic units) of Miocene age. The geological constraints of this area (young age, shallow depositional environment and no evidence of burial processes) ensure that authigenic minerals formed during silicification have not been modified by further diagenetic processes and allow to constrain the age and nature of the silicification. Silicification has strongly increased the hardness of the original sediment. Textural effects of mechanical compaction are rare, testifying that burial processes had no effect on diagenesis and pointing towards an early and/or shallow cementation. Two main diagenetic facies with characteristic associations of authigenic minerals can be identified, namely: (1) non silicified facies are present in ochre-coloured, fine-grained sandstones with high clay and carbonate content. In these facies, cementation is scarce and generally forms minor feldspar overgrowths around detrital K-feldspar as well as layers or nodules of calcite spar cement mainly filling interparticle porosity; (2) silicified facies are red, purple-coloured and characterized by the presence of opal, microquartz and quartz overgrowths as well as other minor authigenic minerals such as Ti and Fe oxides and alunite. Particularly, alunite and opal appear often at the boundary of the silicified/non silicified facies, coming with the development of bleached facies and are replaced by silica. In this paper, a detailed sampling of the silicification fronts has been made, in order to establish the main silicification pathways. In the sampled zone the non silicified sandstones are mainly made up of quartz, K-feldspar, muscovite, phyllite fragments and bioclasts and cemented by thin K-feldspar overgrowths and decimetric concretions of intergranular calcite spar with spherical and tabular shapes. Sandstones contain some pyrite pseudomorphs and 20 to 30% of clay minerals, essentially illite-mica. Samples collected perpendicular to the silicification fronts reveal significant textural, compositional and petrographical transformations, namely: (1) The color of the sample varies strongly from ochre in the non silicified facies to white and red in the bleached weakly silicified front and finally to red, purple and grey in the massively silicified facies; (2) The siliciclastic framework of Montjuic sandstones remains stable during the silicification, only detrital feldspars are partially altered into illite, and biotites are completely altered. The detrital carbonate components disappear quickly towards silicified facies; (3) Within the silicification front, either bleached or not, authigenic minerals show quite important variability. Calcite disappears progressively. The first silicification stage is built by incipient quartz overgrowths, then microquartz develops towards the massively silicified facies. Alunite and opal are usually present in samples collected in this silicification boundary; (4) In the massively silicified facies quartz overgrowths and microquartz take up almost all the intergranular volume of sandstones. Clay content is strongly reduced to 5-10% (mainly illite), so the primary clay-carbonate matrix has been replaced and/or transformed to microquartz. Iron oxides appear around feldspar and phyllite fragments. Because of the geological constraints Montjuic sandstones silicification was a surface/sub-surface phenomenon. Therefore, silicification occurred at relatively low temperature and pressure conditions. Partly, silica may have an internal origin (supplied by clay and feldspar hydrolysis). Supposing that diagenetic transformations inside sandstones are made at steady state conditions it is necessary to consider a strong external supply of silica. The presence of alunite points to acidic fluids with pH between 1,5 and 4. In these conditions, quartz solubility is unaffected, but the aluminium becomes mobile, thus aluminosilicate minerals (like feldspars) are hydrolyzed and clay minerals are transformed into opal CT. A feasible process which may have contributed to the acidification is the oxidation of the pyrite and organic matter present in the original sediments, testified by numerous pyrite ghosts in the non silicified and silicified sandstones. Silicification occurred in an oxidizing environment where sulfides were oxidized and iron oxides precipitated, explaining the colour of these materials. At the basin scale, different models can be considered: (a) a topographic driven flow that moved groundwater from the horst towards the basin; (b) a thermoconvective driven flow that moved phreatic and formation waters along the main faults of the graben or (c) a compaction driven flow that also moved formation waters. Only shallow systems driven by topographic flows can explain the oxidizing nature of the silicification solutions of Montjuic. Conclusions. The Montjuic sandstone silicification is remarkable in several aspects. (1) The lack of compaction and the oxidizing nature of the silicification indicate that this diagenesis was induced by subsurface groundwater, in shallow environments. (2) Silicification is pervasive in medium and coarse-grained sandstones and conglomerates. On the contrary, silicification is restricted to fracture zones in fine-grained sandstones.