minerals Article Fracturing and Near-Surface Diagenesis of a Silicified Miocene Deltaic Sequence: The Montjuïc Hill (Barcelona) Irene Cantarero 1, * , David Parcerisa 2 , Maria Alexandra Plata 1 , David Gómez-Gras 3 , Enrique Gomez-Rivas 1 , Juan Diego Martín-Martín 1 and Anna Travé 1 1 Departament de Mineralogia, Petrologia i Geologia Aplicada, Facultat de Ciències de la Terra, Universitat de Barcelona (UB), C/Martí i Franqués s/n, 08028 Barcelona, Spain; alexa854@gmail.com (M.A.P.); e.gomez-rivas@ub.edu (E.G.-R.); juandiegomartin@ub.edu (J.D.M.-M.); atrave@ub.edu (A.T.) 2 Departament d’Enginyeria Minera, Industrial i TIC, Escola Politècnica Superior d’Enginyeria de Manresa, Universitat Politècnica de Catalunya, Av. Bases de Manresa 63-71, 08242 Manresa, Spain; dparcerisa@epsem.upc.edu 3 Departament de Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra (Cerdanyola del Vallès), Spain; David.Gomez@uab.cat * Correspondence: i_cantarero@ub.edu; Tel.: +34-934031165  Received: 29 November 2019; Accepted: 31 January 2020; Published: 4 February 2020  Abstract: Near-surface diagenesis has been studied in the Langhian siliciclastic rocks of the Montjuïc Hill (Barcelona Plain) by means of petrographical (optical and cathodoluminescence) and geochemical (electron microprobe, δ18 O, δ13 C, δ34 S and 87 Sr/86 Sr) analyses. In the hill, these rocks are affected by strong silicification, but the same unit remains non-silicified at depth. The results reveal that fracturing took place after lithification and during uplift. Fracture cementation is clearly controlled by the previous diagenesis of the host rock. In non-silicified areas, cementation is dominated by calcite, which precipitated from meteoric waters. In silicified areas, fractures show multiepisodic cementation produced firstly by barite and secondly by silica, following the sequence opal, lussatite, chalcedony, and quartz. Barite precipitated only in fractures from the mixing of upflowing seawater and percolating meteoric fluids. The presence of silica stalactites, illuviation, and geopetal structures, and δ18 O values indicate that silica precipitation occurred in the vadose regime from low-temperature percolating meteoric fluids, probably during a glacial period. Moreover, the presence of alunite suggests that silica cement formed under acidic conditions. Karst features (vugs and caverns), formed by arenisation, reveal that silica was derived from the dissolution of surrounding silicified host rocks. Keywords: silicification; meteoric diagenesis; fractures; deltaic sequence; karst; glacial period 1. Introduction Depending on the depositional environment, interstitial waters in deltaic depositional systems can be of meteoric or marine origin, or a mixture of both [1] but can be modified due to relative sea-level fluctuations [1–5]. However, other external factors not related to the depositional system, such as fracturing, can also control the type and distribution of fluids. Fractures can act as conduits for fluids, which can be external fluids with different chemical and/or thermal properties or as barriers, causing compartmentalization of the system. Thus, the quality and heterogeneity of deltaic reservoirs depend on the depositional facies, diagenesis, and the presence of fractures [6–11]. Accordingly, the systematic study of cements, in both host rocks and fractures, can provide valuable information about the origin of fluids, the degree of fluid-rock interaction, the paleofluid flow paths, and the role of fractures for fluid transport [12–18]. Minerals 2020, 10, 135; doi:10.3390/min10020135 www.mdpi.com/journal/minerals Minerals 2020, 10, 135 2 of 22 Common early diagenetic processes in sandstones are burrowing and boring, cementation, often as concretions (typically of carbonate minerals, iron oxides, and phosphates), and soil formation [5,19]. Quartz and silica minerals are some of the most abundant cements in sandstones. Despite that, silicification processes are usually related to burial diagenesis and hydrothermalism [18,20–22], silicification can also develop in the form of chert or silcrete in near-surface conditions at relatively low temperatures [23–26]. Recent studies have demonstrated that different silicification events, starting with low temperature and followed by high-temperature silicification, had strongly modified the petrophysical properties of pre-salt carbonate reservoirs of the Campos and Santos Basin (offshore Brazil) and in the Kwanza Basin (offshore Angola) [27,28]. In the Catalan Coastal Ranges (NE Spain), previous studies show that the main diagenetic processes linked to Cenozoic and Mesozoic deformation events are calcite cementation and dolomitization ([15,29–37]). Only two locations have been found to show extensive silicification, both related to the Neogene extension: the Camp dels Ninots (Caldes de Malavella) and the Montjuïc Hill (in the city of Barcelona). In the Camp dels Ninots, silicification is represented by opaline chert nodules developed in maar-diatreme lake sediments, that is, in Pliocene sediments deposited in a lake that formed in a volcanic crater caused by a phreatomagmatic eruption. This silicification was produced by hydrothermal fluids associated with Neogene NW–SE normal faults and Pliocene volcanism [38]. On the contrary, the Montjuïc hill is a horst delimited by Neogene NE–SW normal faults and the silicification of its deltaic sequence has been attributed to low-temperature meteoric fluids [39–42]. In this contribution, we focus on the complex fracture diagenesis developed in previously silicified sandstones of a Miocene deltaic sequence in the Montjuïc Hill. Specifically, the aims of this study are fivefold: (i) to petrologically and geochemically characterize the successive generations of fracture-filling cements; (ii) to determine the composition and origin of fluids that circulated along the faults; (iii) to unravel the paleofluid pathways; (iv) to infer possible paleoclimatic implications from silica cements in fractures; and, (v) to determine the source of silica in fractures. 2. Geological Setting The Catalan Coastal Ranges (CCR) in the NE of Spain, constitutes the northwestern edge of the Valencia Trough, which opened during the Neogene extensional event (Oligocene–Middle Miocene, [43]) (Figure 1A). During this extension, the CCR acquired a well-developed horst and graben structure limited by listric faults striking NE–SW and NNE–SSW with a detachment level at 12–16 km [44,45] (Figure 1B). This structure was also segmented by later faults trending NW–SE to NNW–SSE. The Neogene extensional event has been divided into a syn-rift stage (Aquitanian–Late Burdigalian), an early post-rift stage (Langhian–Serravallian), and a late post-rift stage (late Serravallian–Pliocene) [46]. Two small compressional episodes are recognized during the Neogene extension: one between the late Langhian–Serravalian (early post-rift) and the other during the Messinian (late post-rift) [45]. One of the grabens that formed at this time is the Barcelona Plain, a ~40 km long and ~2–10 km wide graben situated in the central sector of the CCR, and mainly filled with Miocene continental-transitional siliciclastic deposits and Quaternary fluvio-deltaic deposits. Its northern boundary is limited by the Collserola-Montnegre Horst, which is mainly formed by Paleozoic rocks consisting of Cambro-Ordovician shales and phyllites, Silurian black shales and phyllites, Devonian carbonates, Carboniferous Culm facies, and late Hercynian leucogranites, tonalites, and granodiorites [47]. The Montjuïc Hill is a tilted horst formed within the Miocene sediments filling the Barcelona Plain. It is bounded to the SE by the Morrot fault, a NE–SW normal fault that dips to the SE with a minimum vertical throw of 215 m [41] (Figure 2). It has been postulated that this block is also bounded to the NW by another NE–SW normal fault that dips to the NW [48,49]. Minerals 2020, 10, 135 3 of 22 Minerals 2020, 10, x FOR PEER REVIEW 3 of 22 Figure 1. Geological Geological setting. setting. (A) (A)Location Locationand and schematic schematic map map of of the the Catalan Catalan Coastal Coastal Ranges Ranges and magnification magnification of of the the Littoral Littoral Chain Chain around around the study area. The The Montjuï Montjuïc c Hill Hill is is marked marked with with aa black black square. (B) (B) Cross-section Cross-section of of the the CCR indicated in (A) [45]. The Montjuï The Montjuïc block is formed by alternating sandstones c block sandstones andand conglomerates conglomerates units units and and minor minor lutitic beds interpreted as a prograding deltaic sequence of Serravallian age [40,41]. However, However, recent biostratigraphy studies biostratigraphy studiesplace placethe theplanktonic planktonic zonations zonations N9N9andand N10N10 of [50] of [50] in theinLanghian the Langhian [51]. [51]. Four Four lithostratigraphic lithostratigraphic units units from to from base base toptohave top been have defined been defined [40] (Figure [40] (Figure 2): (1)2): The(1)basal The basal Morrot Morrot unit, unit, formed formed by 80 by 80conglomerates m of m of conglomerates and sandstones, and sandstones, interpreted interpreted as deltaasplain deltadeposits; plain deposits; (2) The (2) The Castell Castell unit, unit, represented represented by 100 bym 100 m of siltstones of siltstones and mudstones and mudstones and well-cemented and well-cemented conglomerates conglomerates and and sandstones, sandstones, interpreted interpreted as delta-front as delta-front deposits; deposits; (3) (3) TheThe Miramar Miramar unit, unit, defined defined byby1515mmof of marls, marls, interpreted as interpreted as prodelta prodelta deposits, deposits, and and finally, finally, (4) The Mirador unit, formed formed by by 20 20 m of massive conglomerates and sandstones, interpreted as proximal delta-front delta-front deposits. deposits. The uplift of the hillhill took took place between the Pliocene and the Quaternary, according to the presence of faulted Pliocene between the Pliocene and the Quaternary, according to the presence of faulted Pliocene rocks rocks [52]. [52]. Host Rock Petrology and Diagenesis 2.1. Host The Rock Petrology petrology and and Diagenesis diagenesis of sandstones and conglomerates of Montjuïc have been studied in detailThe before [39–42]. petrology A brief and summary diagenesis is given inand of sandstones this conglomerates subsection. of Montjuïc have been studied Siliciclastic rocks range from lithic to arkosic arenites/rudites in detail before [39–42]. A brief summary is given in this subsection. to greywackes. Detrital grains are well-rounded and are composed of quartz (35%), potassium feldspar Siliciclastic rocks range from lithic to arkosic arenites/rudites to greywackes.(9%), rock fragments Detrital (20%, grains are mainly quartzites and schists) and minor muscovite, zircon, tourmaline, and biotite. well-rounded and are composed of quartz (35%), potassium feldspar (9%), rock fragments (20%, The shale matrix, composed mainly of illite-mica, quartzites is frequently and schists) transformed and minor muscovite,into opal and zircon, microquartz tourmaline, and with variable biotite. amounts The shale of matrix, composed of illite-mica, is frequently transformed into opal and microquartz with variable amounts of remaining clay minerals. Porosity is low (1–3%), primary as intergranular or secondary due to the Minerals 2020, 10, 135 4 of 22 Minerals 2020, 10, x FOR PEER REVIEW 4 of 22 remaining clay minerals. Porosity is low (1–3%), primary as intergranular or secondary due to the dissolution of altered dissolution altered feldspar and shaleshale matrix. matrix. Detrital Detrital fragments fragments indicate indicate that that the the source source area area most most probably was the Collserola Massif (Figure 1), where Paleozoic rocks rocks crop crop out. out. Lithification of Lithification of the the Montjuï Montjuïc sandstones was c sandstones was mostly mostly duedue toto the the authigenesis authigenesisof of silica silica minerals. minerals. Silicification of these sandstones only occurred Silicification of these sandstones only occurred on high on high ground, as demonstrated by as demonstrated by numerous numerous boreholes around boreholes around thethe mountain mountain where where these units units appear appear non-silicified. non-silicified. On On high high ground, ground, specifically specifically in the in the Morrot Morrot area, the the presence presence of of some some lenticular lenticular remnants remnants of of non-silicified non-silicified rocks rocks allows allows the the observation that observation that silicification silicification fronts fronts preferentially preferentially used used coarse-grained coarse-grained sediments sediments and and fractures. fractures. Consequently, two Consequently, twomainmaindiagenetic diageneticfacies facieswithwith characteristic characteristic associations associations of authigenic of authigenic minerals minerals are are identified: identified: (i)non-silicified (i) the the non-silicified and (ii)and (ii) silicified silicified facies.facies. Non-silicified Non-silicified facies facies are are constituted constituted by ochre- by ochre-colored colored fine-grained fine-grained sandstones. sandstones. In theseInfacies, these cementation facies, cementation is scarce is and scarce and generally generally forms forms minor minor feldspar feldspar overgrowths overgrowths around around detrital detrital k-feldspar k-feldspar as well as well layersasorlayers or nodules nodules of calciteofspar calcite spar cement cement mainly filling interparticle porosity. Additionally, they present moldic mainly filling interparticle porosity. Additionally, they present moldic porosity filled with calcite porosity filled with calcite cement. cement. Silicified Silicified facies facies are characterized are characterized by red by red to topurple-colored purple-coloredcoarse-grained coarse-grained sandstones sandstones containing opal, containing opal, microquartz microquartz and and quartz quartz overgrowths overgrowths as as well well as as other other minor minor authigenic authigenic minerals minerals such as such as Ti and Fe oxides and alunite. Sparitic Sparitic calcite calcite cement cement filling filling residual residual porosity porosity of of silicified silicified sandstones has also sandstones also been been described. described. Alunite Alunite andand opal opal often often appear appear at at the the boundary boundary between between silicified silicified and non-silicified and non-silicified facies. According totothe According theaforementioned aforementioned authors [39–42], authors the geological [39–42], setting and the geological the lack setting andofthe compaction lack of suggest thatsuggest compaction cementation occurred at shallow that cementation occurredconditions at shallowby oxidizingby conditions groundwaters. oxidizing groundwaters. Figure 2. (A) Figure (A) General General stratigraphic stratigraphic section section ofof Montjuï Montjuïcc Hill [40]. (B) Arrangement [40]. (B) Arrangement of of the the lithostratigraphic lithostratigraphic units in the Montjuïc Hill [41] and Schmidt lower-hemisphere stereoplot in the Montjuïc Hill [41] and Schmidt lower-hemisphere stereoplot projectionsprojections of the of the different different generations of fractures found in the study area. The The three three generations generations of of fractures fractures are are indicated with indicated with different colors: set set 11 (black), set 2 (blue), and set 3 (red). 3. Methodology Five outcrops (Far, Sot del Migdia, La Foixarda, Ferrocarrils, and Grec) were chosen because of the good exposure of Miocene rocks affected by fracturing to study the fluid flow events related to Minerals 2020, 10, 135 5 of 22 3. Methodology Five outcrops (Far, Sot del Migdia, La Foixarda, Ferrocarrils, and Grec) were chosen because of the good exposure of Miocene rocks affected by fracturing to study the fluid flow events related to fracturing occurring in previously silicified rocks of the Montjuïc Hill. Moreover, access to four boreholes (L2-29, Castell, Mirador, and TCB16) allowed us to have a more representative lateral and vertical distribution of fracture-filling cements. Fractures were measured in the field and crosscutting relationships were established. Structural data were plotted and analyzed in lower hemisphere Schmidt stereoplots (Figure 2B). The different fracture-filling minerals developed in the different fracture systems were sampled in outcrops and boreholes for petrographic observations and geochemical analyses. Thirty samples were collected from the outcrops and forty-eight samples were obtained from the four different boreholes. Thin sections were studied using optical and cathodoluminescence microscopes. A Technosyn Cold Cathodoluminescence Model 8200 MkII (Technosyn Limited, Cambridge, UK) operating at 16–19 kV and 350 µA gun current were used to establish the different calcite cement generations. Some thin sections were also examined under ESEM Quanta 200 FEI, XTE 325/D8395 scanning electronic microscope (FEI Europe B.V., Eindhoven, The Netherlands). X-ray diffraction of bulk rock and oriented aggregates have been performed with a Bragg-Brentano PAnalytical X’Pert PRO MPD alpha 1(PANalytical B.V., Almelo, The Netherlands) operating at 1.5406 Å, 45 kV, and 40 mA in order to establish the mineralogy of fine-grained fracture fillings. After the petrographic study, geochemical analysis of the different fracture-filling cements and host rock intergranular and moldic cements (elemental geochemistry, stable isotopes, and 87 Sr/86 Sr ratios) were carried out to characterize the different fluids (i.e., origin, pathways, fluid-rock interaction, temperature). Carbon-coated thin sections were used for elemental analyses of carbonate and barite cements with a CAMECA model SX-50 microprobe (CAMECA SAS, Gennevilliers, France). It was operated using 15 nA of current intensity, 20 kV of acceleration voltage, and a beam diameter of 10 µm. The detection limits for carbonates are 99 ppm for Na, 554 ppm for Ca, 491 ppm for Mg, 223 ppm for Fe, 158 ppm for Mn, and 141 ppm for Sr. Detection limits (d.l.) for barites are 105 pp for Ca, 298 ppm for Co, 793 ppm for Sr, 1516 ppm for Ba, and 805 ppm for Pb. The precision of major elements is about 0.64% (at 2σ level). Microsamples for oxygen, carbon, sulfur, and strontium isotope analysis were powdered with a 4 mm-diameter micro drill. Collected samples for carbon and oxygen stable isotopes in carbonates were reacted with 100% phosphoric acid at 70 ◦ C for two minutes in an automated Kiel Carbonate Device attached to a Thermal Ionization Mass Spectrometer Thermo Electron MAT-252 (Thermo Fisher Scientific, Bremen, Germany). The International Standard NBS-18 and the internal standard RC-1, traceable to the International Standard NBS-19, were used for calibration [53,54]. The results are expressed in δ% VPDB standard (Vienna Pee Dee Belemnite). Standard deviation is ±0.02% for δ13 C and ±0.05% for δ18 O. Oxygen isotopes were analyzed in ten samples of chalcedony and quartz cement and speleothems. The CO2 to be analyzed was obtained by laser fluorination, using a CO2 laser of 25 W and ClF3 as a reactant, following the method of [55]. The obtained CO2 was analyzed in a Dual Inlet SIRA-II mass spectrometer (VG-Isotech, Cheshire, UK). The results are expressed in δ% VSMOW standard (Vienna Standard Mean Ocean Water). These analyses were performed at the stable isotopic service of the Universidad de Salamanca. Sulphur and oxygen isotopes were analyzed in ten barite samples to establish the origin of the precipitating fluid. The CO and SO2 gases produced from the barites were analyzed on a continuous-flow elemental analyzer Thermo Delta Plus XP mass spectrometer (Thermo, Bremen, Germany), with a TC/EA pyrolyser for oxygen and a Finnigan MAT CHNS 1108 analyzer (Finnigan, Bremen, Germany) for sulfur. Results were calibrated with NBS-127, SO-5, and SO-6 international standards [56] and the internal standard YCEM (+12.78% CDT). The analytical error is ±0.4% CDT (Canyon Diablo Troilite) for δ34 S and ±0.5% VSMOW for δ18 O. Minerals 2020, 10, 135 6 of 22 The 87 Sr/86 Sr ratios of three barite samples were obtained. The Sr contained in the powder was Minerals obtained by2020, 10, x FOR means PEER REVIEW of chromatography using a SrResinTM (Triskem International). After evaporation, 6 of 22 samples were loaded onto a Re filament with 1 µL of 1M phosphoric acid and 2 µL of Ta2 O5 . evaporation, samples were loaded onto a Re filament with 1 μL of 1M phosphoric acid and 2 μL of Isotopic ratio measurements were carried out in a TIMS-Phoenix mass spectrometer (Isotopx, Cheshire, Ta2O5. Isotopic ratio measurements were carried out in a TIMS-Phoenix mass spectrometer (Isotopx, UK) Cheshire, with a dynamicUK) with multicollector during 10 blocks a dynamic multicollector duringof1016 sweeps blocks each, of 16 sweeps an 88with witheach, Sr beam intensity an 88Sr beam of 3V. Isotopic 87 88 Sr/86 Sr intensityratios of 3V. were corrected Isotopic from ratios were Rb interferences corrected and normalized from 87Rb interferences using a value and normalized using aofvalue = 0.1194 of 88Sr/to86Sr correct thetopossible = 0.1194 mass correct the fractionation possible during loading mass fractionation and analysis during loading of the and analysis of sample. the During sample sample. analysis, During samplethe isotopic analysis, thestandard NBS-987NBS-987 isotopic standard was measured was measuredseven seven timestimes obtaining a media obtaining a mediaand of 0.710247 of 0.710247 and deviation a standard a standard 2σ deviation 2σ of 0.000008. of 0.000008. Thesehave These values values have been beenfor used used thefor the correction correction of the analyzed of values the analyzed in thevalues in the samples. samples. Analytical Analytical error in theerror in 86 87 Sr/ the Sr/86Sr Sr87ratio isratio is 0.01%. 0.01%. The The standard standard deviation deviationThese is 0.000003. is 0.000003. These analyses wereanalyses were at performed performed the CAI at the CAI (Centro de (Centro Apoyo de a laApoyo a la Investigación) Investigación) of Geochronology and Isotopic Geochemistry from the Universidad Complutense de of Geochronology and Isotopic Geochemistry from the Universidad Complutense de Madrid. Madrid. 4. Results 4. Results 4.1. Fracturing 4.1. Fracturing The Montjuïc Hill is affected by intense fracturing produced during the Neogene post-rift period The Montjuïc Hill is affected by intense fracturing produced during the Neogene post-rift period (Langhian to present). (Langhian Fractures to present). are well-developed Fractures within are well-developed the silicified within areas areas the silicified whereas they disappear whereas they into the disappear into the non-silicified areas. Three fracture sets have been distinguished accordingorientations, non-silicified areas. Three fracture sets have been distinguished according to their to their kinematics, and crosscutting orientations, relationships kinematics, and crosscutting (Figures 2B and relationships 3). 2B and 3). (Figures Figure 3. Outcrop Figure 3. Outcrop photographs photographsofofthe thedifferent different fracture sets.(A) fracture sets. (A)Interpreted Interpreted outcrop outcrop viewview of the of the subsidiary subsidiary faultfault zonezone of the of the Morrotfault Morrot faultin inthe the far far outcrop. outcrop. Red Redareas areashighlight main highlight fault main planes fault and and planes blackblack arrows arrows indicate indicate thethe orientationand orientation anddisplacement displacement direction directionofofslickenlines. Rock slickenlines. hammer Rock for for hammer scale. (B) Outcrop view perpendicular to the image shown in A. Sigmoidal shapes and scale. (B) Outcrop view perpendicular to the image shown in A. Sigmoidal shapes and alteration halos alteration halos produced by iron oxides are observed. (C) Outcrop view of the second fracture set characterized by produced by iron oxides are observed. (C) Outcrop view of the second fracture set characterized by straight and discrete planes coated by orange and purple iron oxides. Ferrocarrils outcrop. (D) straight and discrete planes coated by orange and purple iron oxides. Ferrocarrils outcrop. (D) Horsetail ending of a fault from the third fracture set partially filled with silica. Pen for scale. Ferrocarrils outcrop. Minerals 2020, 10, 135 7 of 22 The first set of fractures is represented by NE–SW to NNE–SSW trending joints and normal and strike-slip faults with highly variable dips (Figure 2B). This array has been mainly observed in the southeastern part of the hill. In the far outcrop, a 20 m thick fault zone parallel and subsidiary to the Morrot fault is observed (Figure 3A,B). Individual planes are discrete and undulating, forming an arrangement of sigmoidal shapes that indicate dip-slip movement of the fault zone (Figure 3A,B). Fault planes show both dip-slip and strike-slip slickenlines, the latter being probably the result of the re-activation of the previous normal faults during the Messinian. Fault planes are cemented by silica and are surrounded by red to purple alteration halos towards the host rocks caused by the precipitation of iron oxides (Figure 3B). The second set of fractures is defined by E–W to WNW–ESE trending normal faults and joints dipping between 60 and 85◦ to both N and S (Figure 2B). They are characterized by straight to slightly undulating discrete planes (Figure 3C). Finally, the third set is constituted by N–S to NNW–SSE trending normal faults and joints with high dips (80–90◦ ) to the E and W (Figure 2B). Commonly, these faults show horsetail endings (Figure 3D). Some fault planes of the second and third sets appear enlarged by dissolution and are cemented by silica minerals or filled with breccias cemented by silica, whereas other planes are coated by orange to purple iron oxides (Figure 3C,D). 4.2. Fracture Fillings The fractures of the Montjuïc Hill contain different fillings depending on whether they are located in the non-silicified area, silicification front or in the silicified area. Fractures in the non-silicified area are cemented by euhedral equant calcite crystals, ranging from 90 µm to 1 mm in size, that display a drusy texture (Figure 4A,B). Two calcite cement generations have been recognized. The first one, Cc1, shows a zoned bright orange to brown cathodoluminescence, similar to host rock intergranular cement, whereas the second generation, Cc2, has a uniform bright orange luminescence similar to that of cement filling molds (Figure 4B,C). The change between the two calcite cement generations in fractures is gradual. Fractures within the silicification front are filled with clays with isolated grains of quartz and feldspar from the host rock (Figure 4D). This clayey filling has white or yellow to brownish color and it consists of sericite, kaolinite, goethite, and minor smectite and alunite. In the silicified area, fractures display multi-episodic cementation produced by: (1) barite and (2) several silica varieties. Barite cementation has been only identified in core L2-29 (Figure 2B) and in the Sot del Migdia outcrop (Figure 2B). Specifically, it is found along the uppermost 40 m below the present-day surface. It usually forms crusts up to 1 cm thick but in the L2-29 core, it completely fills a 30 cm-wide fracture. Barite cementation is characterized by the first stage of up to 300 µm-long prismatic crystals that grow decussated from the fracture wall and the second stage of up to 1 mm-long prismatic crystals that grow perpendicular to the fracture wall (Figure 5A). In some samples, in addition to barite, millimeter-sized cubic pyrite crystals and 20 to 50 µm long prismatic jarosite crystals are present. Pyrite crystallized either before the first barite generation or coevally during the last growth stage of this barite. Barite crystals of the second generation commonly have their margins replaced by chalcedony (Figure 5B). The presence of prismatic crystals with low relief next to quartz grains implies the complete replacement of barite by quartz. With regard to silica cementation, the sequence opal → lussatite → chalcedony →quartz is recognized (Figure 5C), representing an increase in crystallinity and purity towards the inner part of fractures. Purity is defined by the presence of cations other than silica (mostly in low crystallinity silica varieties) but also by the presence of clay-sized particles of other minerals coming from the sandstone matrix. Three successive phases of opal precipitation have been identified according to the amount of lithic fragments, the content in titanium oxides, and the different degree of recrystallization to microquartz (Figure 5C,D). In some of the thickest veins, opal shows millimeter-scale cracks and the silicic clasts develop both quartz rims and corrosion pits (Figure 5D,E). Lussatite has a fibrous texture, arranged in pseudospherulites, and usually forms single 20 to 35 µm-thick bands (Figure 5C,F). Minerals 2020, 10, x FOR PEER REVIEW 8 of 22 Minerals 2020, 10, 135 8 of 22 scale cracks and the silicic clasts develop both quartz rims and corrosion pits (Figure 5D,E). Lussatite has a fibrous texture, arranged in pseudospherulites, and usually forms single 20 to 35 μm-thick bands (Figure Chalcedony 5C,F).multiple shows Chalcedony shows multiple precipitation episodesprecipitation defined by 30episodes defined bands to 600 µm-thick by 30 to 600 μm- constituted thick bands by fibers withconstituted by fibers spherulites radial disposition, with radial or disposition, spherulites semispherulites (Figureor 5F). semispherulites It presents two (Figure 5F). varieties, It presents two chessboard varieties,and chalcedony chessboard chalcedony chalcedonite. andquartz Finally, chalcedonite. Finally, precipitates quartz in the precipitates remaining in the porosity as remaining porosity euhedral crystals as euhedral (Figure crystals 5F). Quartz (Figure crystals 5F). Quartz normally rangecrystals between normally 20–70 µm range between in size but in20–70 some μm in size moldic but in porosity ofsome moldic the Castell porosity cores (Figure of2B), the they Castellcancores reach(Figure 2 mm. This 2B), silica they can reach 2 mm. precipitation This sequence silica locallyprecipitation alternates with sequence locally alternates precipitation of goethitewith (Figureprecipitation 5G,H). of goethite (Figure 5G,H). Breccias Breccias andand internal internalsediment, sediment,cemented cementedbyby thethe mineralogical mineralogical sequence sequence described described above, above, are are also also present present fillingfilling the fractures the fractures in the in the silicified silicified area. Breccias area. Breccias are made areofmade angular of angular to subrounded to subrounded clasts up clasts to 3 cmupintosize. 3 cmSediments in size. Sediments show a brownish show a brownish color and color were and were deposited deposited as fine laminae as fine laminae at the at the base of base of fractures fractures and on top andofon top ofclasts breccia breccia clasts6A,B). (Figure (Figure 6A,B). structures Geopetal Geopetal are structures usuallyare usually present present (Figure 6C). (Figure 6C). Thesecomprise These sediments sedimentsopal, comprise opal,iron smectite, smectite, oxides, iron oxides, barite, barite, alunite andalunite and K-feldspar, K-feldspar, and and variable variable contentscontents of host rockof host rock grains grains (0–60%). (0–60%). In Foixarda,Grec In La Foixarda, Grecandand SotSot deldel Migdia Migdia outcrops, outcrops, silicasilica crusts,crusts, and stalactites and stalactites are observed are observed coating coating the faultthe fault planes planes (Figures (Figures 2B and2B6D,G). and 6D,G). Moreover, Moreover, in theinGrec the Grec outcrop outcrop flowstone flowstone speleothems speleothems are are alsoalso present present (Figure (Figure 6D). 6D). AtAt least least three three generationsofofcrusts generations crustsarearepresent presentand andconsist consist of of 1 mm thick thick wavy wavy layers layers of of opal opal andand chalcedony chalcedony with with different different amounts amounts of of iron iron oxides oxides inclusions inclusions(Figure (Figure 6E). 6E). Most Most stalactites stalactites areare small small (up (up to 7 mm long) and present several morphologies. Some Some of of them them areare cylindrical, cylindrical, up uptoto22mm mminindiameter, diameter,and and formed formed bybyseveral severalconcentric concentric layers (Figure layers 6F).6F). (Figure They do not They do show a drip-water not show a drip-water channel channel through through their theirlength. length. Other Other stalactites stalactitesdisplay displaydiscontinuous discontinuousand and thin thin wavy “curtains” “curtains”(Figure (Figure 6G), 6G), whilstwhilst some planes some fracture fractureareplanes are mammillary coated with coated withmicrostalactites. mammillary microstalactites. The color The color of stalactites of stalactites varies between grey, varies between yellow grey, yellow and orange, and due probably orange, probably to the influencedueofto the small influence amounts of of iron smallcompounds. amounts of iron compounds. Figure 4. Figure Fracture fillings 4. Fracture fillings from from the the non-silicified non-silicified area area and and silicification silicification front. front. (A) (A) Fracture Fracture from from the the non-silicified area cemented by a drusy mosaic of equant calcite crystals (XPL). (B,C) non-silicified area cemented by a drusy mosaic of equant calcite crystals (XPL). (B,C) Cross-polarized Cross-polarized and cathodoluminescence and cathodoluminescence images imagesof ofaafracture fracturefrom fromthethenon-silicified non-silicifiedarea areafilled filledwith withtwo generations two generationsof calcite of cement. calcite TheThe cement. fracture wall wall fracture is marked with awith is marked whiteathick whitedashed-line. Note the Note thick dashed-line. gradual thetransition gradual from the bright orange-brown zoned luminescence (Cc1) to the uniform bright transition from the bright orange-brown zoned luminescence (Cc1) to the uniform bright orange orange luminescence (Cc2). The white luminescence thin (Cc2). dashed-line The white thinshows the limit dashed-line between shows the two the limit calcite between thegenerations. (D) Fracture two calcite generations. (D) Fracture from the silicification front, affecting reddish fine-grained sandstones due to theiron from the silicification front, affecting reddish fine-grained sandstones due to the presence of oxide presence cement, filled with brownish clays and grains from the host rock (white arrows) of iron oxide cement, filled with brownish clays and grains from the host rock (white arrows) (PPL). (PPL). The limit between The limit the host rock between and the the host rockclayey and thefilling hasfilling clayey been marked has beenwith a redwith marked dashed-line. a red dashed-line. Minerals 2020, 10, 135 9 of 22 Minerals 2020, 10, x FOR PEER REVIEW 9 of 22 Figure Figure 5. 5. Cementsininthe Cements thefractures fracturesin inthe thesilicified silicified area of the the Castell CastellUnit. Unit.(A) (A)Fracture Fracture cemented cemented byby two generations of barite (Ba1 and Ba2) and a later generation of chalcedony (Ch) (XPL). two generations of barite (Ba1 and Ba2) and a later generation of chalcedony (Ch) (XPL). Fracture walls Fracture arewalls markedare marked with a with white a white dashed-line. dashed-line. (B) (B) Detail Detail of of thethe replacementof replacement of barite barite (Ba2) (Ba2)by bychalcedony chalcedony (Ch) (XPL). (C) Silica varieties cementing a fracture according to their purity and crystallinity:two (Ch) (XPL). (C) Silica varieties cementing a fracture according to their purity and crystallinity: two opalopal generations generations (Op1and (Op1 andOp2), Op2),lussatite lussatite(Lu) (Lu) and and chalcedony chalcedony (Ch) (Ch)(XPL). (XPL).(D) (D)Fracture Fracturefilled byby filled twotwo generations generations ofof opalaffected opal affectedbybymillimeter-scale millimeter-scale cracks cracks (white (white arrows). arrows).(E)(E)Quartzite Quartziteclast clastcontained contained in ainfracture a fracture cementedbybyopal cemented opalthat thathas hasdeveloped developed aa quartz quartz rim rim (Qtz). (Qtz).(F) (F)Fracture Fracturecemented cemented byby thethe silica sequence, lussatite, three generations of chalcedony and quartz (XPL). (G) Fracture filled by silica sequence, lussatite, three generations of chalcedony and quartz (XPL). (G) Fracture filled by goethite (Gt) (and minor barite), alternation of goethite and chalcedony and finally pure chalcedony. goethite (Gt) (and minor barite), alternation of goethite and chalcedony and finally pure chalcedony. The sequence is later cut by a fracture (yellow dashed- line) cemented by iron oxides (Gt2) (XPL). (H) The sequence is later cut by a fracture (yellow dashed- line) cemented by iron oxides (Gt2) (XPL). Fracture cemented by several episodes of chalcedony and later goethite botryoids (PPL). (H) Fracture cemented by several episodes of chalcedony and later goethite botryoids (PPL). Minerals 2020, 10, 135 10 of 22 Minerals 2020, 10, x FOR PEER REVIEW 10 of 22 Figure Figure6.6.(A) (A)Fracture Fracturefilled filledwith withsediment sediment(Sed) (Sed)(deposited (depositedatatthe thefracture fracturebottom) bottom)andanda amicrobreccia microbreccia cemented cemented by opal and lussatite (PPL). (B) Fracture filled with sediment and a microbrecciacemented by opal and lussatite (PPL). (B) Fracture filled with sediment and a microbreccia cemented by bychalcedony. chalcedony.Some Someofofthe theclasts clastscontain containearlier earliersediment sedimentfragments. fragments.V:V:voidvoid(PPL). (PPL).(C) (C)Geopetal Geopetal structure structurefilled filledwith withsediment sedimentatatthe thebase baseand andchalcedony chalcedonycement cement(Ch) (Ch)atatthe thetop topdeveloped developedafter afteropal opal cementation (Op) (PPL). (D) Cascade speleothems on a fault plane of the Grec cementation (Op) (PPL). (D) Cascade speleothems on a fault plane of the Grec outcrop. (E) Hand outcrop. (E) Hand sample sampleofofseveral severalcrust crustgenerations generationsthat thatpre-date pre-datestalactite stalactiteformation. formation.(F) (F)Hand Handsample sampleofofcylindrical cylindrical microstalactites. (G) Hand sample of small silica curtains. microstalactites. (G) Hand sample of small silica curtains. 4.3.Geochemistry 4.3. Geochemistry 4.3.1.Elemental 4.3.1. Elementaland andIsotopic IsotopicComposition CompositionofofBarite Barite EPMA analyses EPMA analyses revealed revealeda alarge variation large in barite variation composition, in barite with Ba composition, contents with varying varying Ba contents between 58.2% and 60.24%, Sr contents between 800 ppm and 1.36%, Ca contents between 100 and between 58.2% and 60.24%, Sr contents between 800 ppm and 1.36%, Ca contents between 100 and 700 ppm and Pb ppm 700 contents andfrom below the Pb contents detection from limit below the to 700 ppm. detection limitStrontium andStrontium to 700 ppm. barium show a good negative and barium show a correlation (R2 = 0.832, n = 16), which reflects the isomorphous substitution of these elements in the good negative correlation (R2 = 0.832, n = 16), which reflects the isomorphous substitution of these elements in the barite-celestite solid solution (Figure 7A). Barites from the second generation that were replaced by silica show lower Sr contents than those that were not replaced. S29-1 19.4 13 0.712446 L2-29 core at 37.85 m (+chalcedony) S29-2 21.1 13.3 L2-29 core at 37.85 m (+chalcedony) S29-14 B 22.7 15.4 L2-29 core at 39 m (+chalcedony) S29-20 A 24.7 14.8 L2-29 core at 47 m. Next to fracture wall (+ch) S29-20 B 13.1 13.2 L2-29 core at 47 m. Far from fracture wall (+ch) MTJ-322020, Minerals A 10, 135 42 16.8 0.712939 Sot del Migdia. Next to fracture wall 11 of 22 MTJ-32 B 28.2 12.6 Sot del Migdia. Central part of the fracture MTJ-32 C 34.2 13.8 0.715218 Sot del Migdia. Far from fracture wall FMP-21a 30.7 barite-celestite solid solution 12.9 (Figure 7A). Barites from Sot del Migdia the second generation that were replaced by FMP-21b 41.9 15.2 Sot del Migdia silica show lower Sr contents than those that were not replaced. (A)Barite Figure7.7.(A) Figure Baritecompositions compositionsplotted plottedinina a%Sr %Srvsvs%Ba %Babinary binarydiagram. diagram.(B)(B)δδ3434 δ18 S Svsvsδ18 OObinary binary diagram. Grey boxes represent the isotopic composition of seawater for the Late Miocene diagram. Grey boxes represent the isotopic composition of seawater for the Late Miocene and from and from Pliocene to present according to [57]. Pliocene to present according to [57]. Barite cement has δ34 S values ranging between +19.4% to +24.7% CDT in the core L2-29, whereas 4.3.2. Elemental and Isotopic Composition of Calcite Cements more δ34 S-enriched values, from +28.2% to +42% CDT, have been measured in the Sot del Migdia outcrop Calcite (Table cement 1). These Cc1,docharacterized values by zonedwith not show covariation δ18 O data, which ranges cathodoluminescence, has Mg areas from +12.5% contents in both below the +16.8% SMOW to detection limit to(Figure 2584 ppm, Mn and 7B). Barite 87 has FeSr/ 86 from Sr below the detection ratios varying between limit up to and 0.712446 70790.715218 and 2028 ppm, (Table 1). respectively, and low Sr content (from below the detection limit to 665 ppm) (Table 2). The elemental composition of this calcite 1. Sulphur, Tablecement oxygen, and in fractures andstrontium isotopes intergranular of bariteprior porosity cement. to silicification is very similar,Sample althoughδ34the latter S (%CDT) reaches 18 higher δ O (%SMOW) Fe contents 87 86 Sr/ Sr (up to 24,189 ppm). Calcite Location and Description cement Cc1 is characterized by δ18O values between −10.5‰ and −6.7‰VPDB and δ13C values between −5.8‰ and S29-12 A 23.8 13.4 L2-29 core at 35.20 m (±jarosite) −1.7‰VPDB S29-12 B (Figure 22.68). Calcite Cc2, 12.5 which cements fractures and L2-29molds, has higher core at 35.20 m (+ironMn contents, oxides) from 1300S29-1 to 14,272 ppm, 19.4 than cement Cc1.13 It has low 0.712446 Mg contentsL2-29 core atbelow (average 37.85 mthe (+chalcedony) detection limit), Fe fromS29-2 21.1 below the detection 13.3 limit to 3100 ppm, and very low Sr L2-29 core at 37.85 m (+chalcedony) contents. Na is below the detection S29-14 B 22.7 15.4 L2-29 core at 39 m (+chalcedony) limit in bothA cement 24.7 S29-20 generations. Cement14.8 Cc2 has δ O values 18 L2-29between −18.2‰ core at 47 m. Next toand −12.1‰VPDB fracture wall (+ch) and δ13S29-20 B C values between13.1 −5.5‰ and −3.5‰VPDB 13.2 (Figure 8).L2-29 core at 47 m. Far from fracture wall (+ch) MTJ-32 A 42 16.8 0.712939 Sot del Migdia. Next to fracture wall MTJ-32 B 28.2 12.6 Sot del Migdia. Central part of the fracture MTJ-32 C 34.2 13.8 0.715218 Sot del Migdia. Far from fracture wall FMP-21a 30.7 12.9 Sot del Migdia FMP-21b 41.9 15.2 Sot del Migdia 4.3.2. Elemental and Isotopic Composition of Calcite Cements Calcite cement Cc1, characterized by zoned cathodoluminescence, has Mg contents from below the detection limit to 2584 ppm, Mn and Fe from below the detection limit up to 7079 and 2028 ppm, respectively, and low Sr content (from below the detection limit to 665 ppm) (Table 2). The elemental composition of this calcite cement in fractures and intergranular porosity prior to silicification is very similar, although the latter reaches higher Fe contents (up to 24,189 ppm). Calcite cement Cc1 is characterized by δ18 O values between −10.5% and −6.7%VPDB and δ13 C values between −5.8% and −1.7%VPDB (Figure 8). Calcite Cc2, which cements fractures and molds, has higher Mn contents, from 1300 to 14,272 ppm, than cement Cc1. It has low Mg contents (average below the detection limit), Fe from below the detection limit to 3100 ppm, and very low Sr contents. Na is below the detection limit in both cement generations. Cement Cc2 has δ18 O values between −18.2% and −12.1%VPDB and δ13 C values between −5.5% and −3.5%VPDB (Figure 8). Minerals 2020, 10, 135 12 of 22 Minerals 2020, 10, x FOR PEER REVIEW 12 of 22 Table 2. Elemental composition of calcite cements (