Boletín de la Sociedad Geológica Mexicana / 2019 / 321 Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area, Catalonia, NE Iberian Peninsula ABSTRACT Yael Díaz-Acha, Marc Campeny, Joan Carles Melgarejo, Josep Bosch, Saleh Lehbib, Lisard Torró, Joaquín A. Proenza, Montgarri Castillo-Oliver, Antoni Camprubí, Cristina Villanova-de-Benavent, Aleu Andreazini, Míriam Pastor, Núria Pujol-Solà, Sandra Amores, Jingyao Xu, Júlia Farré Yael Díaz-Acha ydiaz@bcn.cat ABSTRACT RESUMEN Marc Campeny Museu de Ciències Naturals de Barcelona. Parc de la Ciutadella Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area s/n, 08003 Barcelona, Spain. Mining for iron resources in the Gavà area La minería de los recursos de hierro del área de Gavà of Catalonia occurred intermittently during (Cataluña) fue activa de forma intermitente durante Joan Carles Melgarejo Joaquín A. Proenza the Iberian and Roman epochs, the Middle las épocas Ibérica y Romana, la Edad Media, y hasta Aleu Andreazini Ages, and continuing until the industrial era, la era industrial, lo cual se ha probado mediante datos Míriam Pastor as evidenced by historical and archaeological Núria Pujol-Solà arqueológicos y documentos históricos. Es posible que, Sandra Amores documents. Iron mining in this area could have inclusive, la minería de hierro en el área haya existi- Jingyao Xu occurred even earlier, during the Neolithic pe- do durante el periodo Neolítico. Las mineralizaciones Júlia Farré Departament de Mineralogia, Petrologia i Geologia Aplicada, riod. Iron ores were formed in two stages: (1) de hierro se formaron en dos estadios: (1) alteración Facultat de Ciències de la Terra, Universitat de Barcelona. Martí a regional hydrothermal alteration associated hidrotermal regional asociada a cabalgamientos Her- i Franquès s/n, 08028 Barcelona, Spain. with Hercynian thrusts that produced the an- cínicos que produjo ankeritizaciones de calizas en las keritization of limestones within the Paleozoic Josep Bosch series del Paleozoico, y (2) el reemplazamiento kárstico Museu de Gavà. Plaça de Dolors Clua, 13, 08850 Gavà, Bar- series, and (2) the karstic replacement of these celona, Spain. de los carbonatos ricos en hierro durante el Plioceno y el iron-rich carbonates during the Pliocene and Cuaternario por medio de fluidos supergénicos que pro- Saleh Lehbib Quaternary by means of supergenic fluids that Departament de Mineralogia, Petrologia i Geologia Aplicada, produced ochres with goethite and hematite. dujeron ocres con goethita y hematites. El estilo de mine- Facultat de Ciències de la Terra, Universitat de Barcelona. Martí The style of mineralization largely depends ralización depende en gran medida de las características i Franquès s/n, 08028 Barcelona, Spain. on the characteristics of the replaced proto- del protolito reemplazado, lo cual lleva a la definición de Ministery of Water and the Environment, Sahrawi Arab Dem- ocratic Republic. lith, and three styles of mineralization can be tres estilos de mineralización: (1) El reemplazamiento defined: (1)The supergenic replacement of supergénico de calizas masivas ankeritizadas del Pri- Lisard Torró Departament de Mineralogia, Petrologia i Geologia Aplicada, ankeritized massive Pridolian limestones only doliense produjo únicamente reemplazamientos locales Facultat de Ciències de la Terra, Universitat de Barcelona. Martí produced local replacements that were restrict- restringidos a discontinuidades estructurales o estrati- i Franquès s/n, 08028 Barcelona, Spain. Departament de Geologia, Facultat de Ciències, Universitat ed to structural or stratigraphic discontinuities, gráficas por lo tanto, las mineralizaciones son de dimen- Autònoma de Barcelona. Edifici Cs, 08193 Bellaterra (Cerdan- therefore, the mineralization has reduced di- siones reducidas en forma de vetillas o masas verticales yola del Vallès), Spain. Departamento de Ingeniería de Minas, Pontificia Universidad mensions and occurs as irregular veinlets or irregulares; (2) El reemplazamiento de alternancias de Católica de Perú. Avenida Universitaria 1801, San Miguel, Lima pipes; (2) The replacement of interbedded an- calizas ankeritizadas con pizarras que contienen pirita 32, Peru. keritized limestones and pyrite-bearing shales diseminada (Lockoviense) produjo bolsadas de minera- Montgarri Castillo-Oliver (Lockovian) produced massive ores in pod- lizaciones masivas ricas en sílice derivadas de las piza- Departament de Mineralogia, Petrologia i Geologia Aplicada, shaped bodies rich in silica impurities derived rras alteradas; (3) El reemplazamiento de carbonatos Facultat de Ciències de la Terra, Universitat de Barcelona. Martí i Franquès s/n, 08028 Barcelona, Spain. from the altered shales; and (3) The replace- cabalgados por pizarras ricas en sulfuros y fosfatos fa- ARC Centre of Excellence for Core to Crust Fluid Systems ment of carbonates overthrust by pyrite- and voreció la formación de mineralizaciones estratoligadas and GEMOC, Department of Earth and Planetary Sciences, phosphate-rich shales favored the formation masivas, que son las de mayor tamaño y grado en el Mac-quarie University.NSW 2019, Australia. of massive stratabound deposits, which are the área de estudio, pudiendo estar localmente enriquecidas Antoni Camprubí largest and highest grade deposits in the study en minerales del supergrupo de la alunita y en fosfatos Instituto de Geología, Universidad Nacional Autónoma de area, and may be locally enriched in minerals Méx-ico. Ciudad Universitaria, 04510 Coyoacán, CDMX, de Ca y Fe. Afloramientos de todos estos estilos de mi- of the alunite supergroup and Ca- and Fe-rich Mexico. neralización fueron explotadas tanto en la antigüedad phosphates. Outcrops of all of these styles Cristina Villanova-de-Benavent como en la Edad Media, aprovechando la alta calidad of mineralization were mined by the Iberian School of Environment and Technology, University of metalúrgica de las menas, pese a que los dos primeros Brighton. Mithras House, Lewes Rd., Brighton BN2 4AT, cultures, during the roman period and in the estilos de mineralización presentan cuerpos de reducido United Kingdom. Middle Ages, taking advantage of the relative- ly high metallurgical quality of the ores.There- tamaño. Por ello, en dichas épocas la explotación fue fore, the exploitation during these epochs was artesanal mediante trincheras o pocillos. En cambio, en BOL. SOC. GEOL. MEX. 2019 artisanal by means of trenches or small pits. la época industrial sólo se beneficiaron los recursos ma- VOL. 71 NO. 2 In contrast, during the industrial era only the sivos del tercer estilo de mineralización a cielo abierto y P. 321 ‒ 342 massive stratabound deposits were exploited in a través de galerías. http://dx.doi.org/10.18268/BSGM2019v71n2a6 open pits and underground galleries. Palabras clave: minería de hier- Manuscript received: January 15, 2018. Keywords: iron mining, karst, ro, karst, supergénico, fosfatos, Corrected manuscript received: April 25, 2018. supergenic, phosphates, al- alunita, cultura Íbera. Manuscript accepted: May 8, 2018. unite, Iberian culture. 322 / Boletín de la Sociedad Geológica Mexicana / 2019 INTRODUCTION 1. Introduction to archeological evidence, were mined in the Ibe- rian towns of Les Guàrdies near El Vendrell, 60 The Iberian culture settled along the Mediterra- km SW of Barcelona (Asensio et al., 1996), and nean coast between the southeastern part of the Fontscalents near Castellar del Vallès, 40 km N Iberian Peninsula and southeastern France be- of Barcelona (Coll et al., 1993). Many others were tween the sixth century BC and the third century mined during the Middle Ages, like the Els Cros- AC. Many small cities and towns of this period sos mines located in a magnetite skarn deposit are found along the coastal area of Catalonia (Be- near Les Borges del Camp, 120 km SW of Barce- larte et al., 2001; Sanmartí, 2014). This period rep- lona (Jové, 1981), and during the nineteenth cen- resents the Iron Agein this area, and the existence tury, as the Fontscalents mines that were exploited of iron mines was a key factor for the development again (Planas and Portolés, 1995). Among karstic Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area of many settlements nearby. deposits, those of Gavà have the best outcropping Iron deposits of many different styles, such as conditions, favored by intensive mining and arche- skarns, hard grounds, and karstic deposits, are ological excavations. Therefore, these mines allow found along the Catalan Coastal Ranges (CCR) studying the iron deposits as a representative of (Figure 1). Some of the iron deposits, according the karstic iron deposits in Catalonia. Figure 1 Geological map of the Catalan Coastal Ranges (CCR), with the distribution of the known iron mines and outcrops with archaeological interest. INTRODUCTION / GEOLOGICAL Boletín de la Sociedad Geológica Mexicana / 2019 / 323 The Gavà area is famous worldwide for the ex- netic model, particularly of those iron ores found BACKGROUND istence of ancient Neolithic mines (Alonso et al., in the archeological sites, in order to understand 1978; Villalba et al., 1986, 1989, 1990; Blasco et how the Iberian and Iberian-Roman population al., 1991; Camprubí et al., 1994, 2003; Costa et al., might have used these natural resources. 1994; Bosch et al., 1996; Villalba, 2002; Díaz-Acha et al., 2019) and the important archeological ob- jects found therein, such as the Gavà Venus (Bosch 2. Geological background and Estrada, 1994) and variscite ornaments (Bor- rell and Estrada, 2009). However, many other The study area is locatedin the central part of types of mining operations for iron exist in the the CCR. The CCR are classically divided in two same area, in many cases in the immediate vi- NE–SW-trending ranges (Prelittoral and Littoral) cinity of the Neolithic works and separated from Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area separated by the Prelittoral depression (Anadón et them by only tens of meters. The archeological al., 1979). These geological units are separated by objects, such as pottery, found inside these mines, regional faults which acted as NE–SW strike-slip and some absolute carbon dating, established that faults during the Alpine compressive event (Paleo- no less than 16 of these mines were active during gene) and as normal faults during the Neogene, the Iberian and Iberian-Roman periods (Villalba thus generating alternate faults and associated and Alonso, 1984; Burjachs et al., 1991; Molinàs, grabens as well as the present coastline (Anadón 2007; Álvarez and Estrada, 2009; Bordas et al., et al., 1985a). The study area lies in the Littoral 2009, 2010). There are evidences of other small range, where a Paleozoic metamorphic basement open-shaft operations of these iron ores, during is unconformably overlain by Mesozoic sedimen- the Middle Ages (Campmany et al., 2004) and the tary rocks. All of them are covered by Quaternary Modern Age (Bordas et al., 2009), but the largest sediments towards the SE (Costa et al., 1994). mining operation on these ores was developed The iron deposits in the CCR are generally during the industrial era since the mid-nineteenth found in the Paleozoic basement. The stratigra- century (Maureta and Thos, 1881) and reworked phy of the Paleozoic basement has been studied during the early twentieth century). The last stag- in detail in the areas near Barcelona (Julivert and es of mining produced open pit extractions and Duran, 1990a; García-López et al., 1990, 1996). large galleries, as in the case of the Elvira mines in The Paleozoic series starts with Late Ordovician the Rocabruna range, NW Gavà (Maluquer, 1905; orthoquartzites overlain by 100 m of interbedded Bareche and Viñals, 1994). The Rocabruna mines black shales with cherts and alkaline volcanics of were probably worked since antiquity. However, Llandoverian age, 50 m of black shales with inter- large-scale modern mining destroyed any evidence bedded thin quartzites (Wenlockian), and 50 m of of earlier exploitation. There are two essential black shales (Ludlowian). This unit is overlain by points those agree with the fact that these mines La Creu Formation which is 30-40 m thick and it were exploited in Antiquity: the outcrops of iron is made of Pridolian nodular limestones (Julivert et ores are larger than any other known Iberian or al., 1985). These limestones are conformably cov- Iberian-Roman settlement in Catalonia and, as ered by the Olorde unit, which starts with 2-3 m of already mentioned, mining existed in this specific Lower Lockovian black shales overlain by an inter- area (the Les Ferreres range and the Can Tintorer bedding of limestones and reddish shales (Middle complex) since the Neolithic. Lockovian) that grades to green to bluish marls The aim of this contribution is to describe the geo- with carbonate nodules (15-20 m thick, Upper logical context of the Gavà iron ores and their ge- Lockovian) and limestones that alternate with 324 / Boletín de la Sociedad Geológica Mexicana / 2019 GEOLOGICAL BACKGROUND Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area Figure 2 Geological map of the Gavà-Rocabruna area with indication of the iron mines and iron outcrops of this sector. shales (10-15 m thick, Pragian). The series at the place in the epizone, and axial-plane foliation is top consists of green marls. The contact with the poorly developed. There is a very low-grade re- Carboniferous materials (Figure 2) has not been gional metamorphism. Contact metamorphism established in the area because all the contacts are is absent in the study area, owing to the lack of tectonic. However, they are described at the south- Hercynian intrusives in this sector of the CCR. ern part of the study area as black cherts (“lidites”) However, the Hercynian thrusts were associated of Tournaisian age and greenish graywackes of with low-temperature hydrothermal circulation, the Visean (Anadón et al., 1985b). which is more evident in Silurian and Devonian The Hercynian deformation largely affected the dolomitized-ankeritized limestones. The resulting Paleozoic series of the area. It is described in the rocks are yellowish to brownish and consist of fer- CCR domain by Julivert and Duran (1990b). A rous saddle dolomite or ankerite, with crystals up first stage of deformation produced NW–SE tight to 2 mm in diameter. In addition, these rocks may folds with the axial planes dipping to the NE. contain disseminated pyrite with cubic crystals These folds have thrusts and retrothrusts on their about 500 µm in size, but in some cases they con- flanks, and the black shales are the most common stitute nodules up to 3 cm in diameter. The sulfide detachment level. A second phase of deformation content in these rocks rarely attains ~1% modal. produced folds with similar axial direction but The Paleozoic rocks are unconformably overlain opposite vergence. These folds can also contain by Triassic sedimentary rocks that dip gently to- detachments in the flanks. The deformation took wards the SW (IGME, 1976a, 1976b). The Tri- Boletín de la Sociedad Geológica Mexicana / 2019 / 325 GEOLOGICAL BACKGROUND of the Paleozoic basement. Pyrite-bearing black assic series starts with a thick (more than 100 m) shales were oxidized, and the resulting acidic and Early Triassic detrital Buntsandstein facies that oxidizing fluids leached off the organic matter in starts with conglomerates followed by sandstones the black shales. As a result, these rocks turned and clays (Virgili, 1958). This is followed by about pale grayish near the surface, and typical sulfates 50 m of limestones and dolomites of the Lower and phosphates of the alunite supergroup were Muschelkalk facies (Calvet and Ramón, 1987), 50 formed. These minerals were dated at 1.2 and 1.3 m of red clays and gypsum of the Middle Mus- Ma (Calabrian/Villafranchian; Camprubí et al., chelkalk facies (Morad et al., 1995), and 50 m of 2003). Weathering also affected Paleozoic Fe-rich limestones, dolostones, and marls of the Upper carbonates, thus producing karstic iron replace- Muschelkalk facies (Calvet et al., 1987). Towards ments (Costa et al., 1993) that were favored by the SW the materials are covered by thick series Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area jointing in the dolostones. of Jurassic and Cretaceous carbonates of the Gar- The Quaternary sediments are more common in raf basin (Salas, 1987; Albrich et al., 2006). Large the lowest part of the range, near the plain asso- lull episodes in sedimentation favored the devel- ciated with the Llobregat River (Figure 1). Un- opment of hard ground mineralizations along the consolidated gravels are common in the flanks of stratigraphic profiles of the Mesozoic sediments. the mountains, and sequences of calcretes, sedi- The Paleozoic rocks are unconformably overlain mentary clay deposits, and breccias can be found by the Paleogene series, which in the base start near the border between Paleozoic rocks and the as continental rocks that can be related with the graben (i.e., Solé-Benet et al., 1988; Casassas and development of karstification and Fe-Mn miner- Riba, 1992). The Llobregat delta plain consists of alization in the underlying materials. These were river terrace and flood plain deposits, and Pleisto- followed by evaporitic and marine series that cor- respond to the transition with the marine series of cene-Holocene paleochannels (Daura et al., 2013). the Ebro basin. The iron mines in the Gavà area strike NW-–SE The Alpine deformation, as in other regions of along a 2 km stretch section, between the Gavà the CCR, produced a dense network of Oligocene town and the Bruguers chapel, 20 km west of transpressive faults that strike mainly NE–SW and Barcelona in Catalonia (Figure 2). Many of these NW–SE, thus resulting in complex block move- mines were destroyed during the dense urbanis- ments in the area (Guimerà, 1984). Several sets tic operations of the 1960s- decade. However, in of subvertical joints, related with such faults, are recent decades, the municipality of Gavà and the widely distributed and affect all the materials in Catalan government enforced the study of all old the area, in particular the dolostones. These joints mines in the area and, in many cases, the preser- represent good pathways for fluid circulation. vation of the Neolithic and Iberian mines. In this Most transpressive faults were reactivated during particular case, the Can Badosa area, which is an the Neogene extension as normal faults, thus pro- urbanistic polygon around an old farm in the Les ducing a compartmentalization of pre-existing Ferreres range, at the NW corner of the town of blocks into horsts and grabens. The syntectonic Gavà, comprises both Neolithic variscite mines sediments associated with this stage are mainly and Iberian to modern iron mines. The archae- conglomerates due to graben infilling, along with ological excavations in recent years allowed the sands and clays. Some Fe-Mn karstic mineraliza- study of many of these mines, but many of the tions can also be associated with this episode. recently discovered Iberian mines are now covered During the early Pleistocene, weathering pro- by buildings and no further studies are possible duced strong changes in the outcropping materials anymore. 326 / Boletín de la Sociedad Geológica Mexicana / 2019 METHODS / STYLES OF IRON 3. Methods ticular when the beds are subvertical), or structur- al (joints, thrusts, faults). In particular, joints were DEPOSITS Samples were studied using thin polished sections effective in favoring supergenic processes due to by transmitted and reflected light optical mineralo- their penetrativity in dolomites. Further, dolomite gy, coupled with X-ray powder diffraction (XRD). rocks have additional porosity that was associat- The XRD measurements were performed using a ed with the dolomitization processes, which in- Bragg-Brentano θ/2 Siemens D-500 diffractom- creased the likeliness for these rocks to react with eter (radius = 200.5 mm) with Cu Kα radiation, supergenic fluids. The supergenic mineralization selected by means of a secondary graphite mono- is scarcely developed in depth and was confined chromator, and a NaI(Tl) scintillation counter. to the first 3 to 5 m from the paleosurface, none- The divergence slit was 1° and the receiving slit theless. The morphology of the contacts between the mineralization and the host carbonates clearly Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area 0.05°. The starting and the final 2θ angles were 4 and 70°, respectively. The step size was 0.05° 2θ corresponds to replacement processes and are ir- and the measuring time was 3 seconds per step. regularly shaped. The analyses were performed at the Scientific and The iron oxide ore bodies occur as lenses, pipes, Techcnological Centres of the University of Bar- and pods. In most cases, lenses are developed celona (CCiTUB). along joints. The mineralized areas have a rough- ly lensoidal shape that are only a few centimeters wide on either side of the joints, but rarely up to 3 4. Styles of iron deposits in the Gavà m wide. The length of these lenses is less than 15 m, generally 2 to 3 m long. The pipes were usu- area ally formed along the intersection of two systems of joints or along the intersection of a joint and Iron deposits in the study area were developed by other types of discontinuity, such as faults or verti- replacement of three rock ensembles, which con- cal stratification surfaces. Pipes are subvertical and trol the morphology and volume of the deposit their diameter varies between a few cm and 3 m. as well as its resulting mineralogy: (a) deposits Pods are very irregular and are generally located developed on weathered profiles in carbonates on the surface or in its vicinities. Their forma- of the La Creu Formation (Pridolian), (b) depos- tion was probably favored by the intersection of its developed on weathered profiles in carbonates three or more discontinuities in the rock. In fact, interbedded with shales of the Ludlow-Pridolian non-outcropping bodies appear to occur in the transition or within the Olorda Formation (Locko- path of pipes or lenses from the surface. The size vian-Pragian), and (c) deposits in weathered pro- of pods is highly variable, but always less than 5 m files on carbonates overthrusted by Silurian shales. in diameter. 4.1. DEPOSITS IN WEATHERED PROFILES ON THE LA The mineral filling started with a short period CREU FORMATION (PRIDOLIAN) of calcite precipitation that could represent spe- leothems. The next stage may contain breccias, The Pridolian carbonate rocks were strongly dolo- which are cemented by mixtures of cryptocrys- mitized or even ankeritized (Figure 3A), and easily talline goethite and hematite with minor amounts affected by supergenic processes in their surface of wad. Therefore, the color of the ores may vary exposures. Also, some limestones were rich in among different hues of yellow, brown, red, or pyrite nodules that underwent oxidation (Figure black, hence their labeling as “ochres” following 3B) and produced supergenic acidic fluids. The the definition of Elias et al. (2006). The breccias replacement progresses along any discontinuities, grade laterally and vertically to massive aggregates either sedimentary (stratification surfaces, in par- of these minerals. These minerals are usually ar- Boletín de la Sociedad Geológica Mexicana / 2019 / 327 STYLES OF IRON DEPOSITS Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area Figure 3 General aspects of the geological context of karst in Pridolian carbonates. (A) Dolostones (pale gray) replaced by ankerite (dark gray) in a quarry, 1 km NW of Gavà. (B) Dolomitized limestones (light gray) with pyrite nodules pseudomorphosed by goethite (dark brown). (C) Detail of the irregular replacement of the ankeritized limestones by iron oxides (black and yellow) near Can Badosa. ranged in boxwork textures when replacing iron- dations of a building in the immediate vicinity of rich carbonates and the accompanying pyrite, the Neolithic mines. This type ofiron ores were and botryoidal textures are also common in these locally mined (Figure 3C). circumstances. However, in most cases, ore bod- ies are unconsolidated earthy masses of hydrat- 4.2. DEPOSITS IN WEATHERED PROFILES ON CARBONATES INTERBEDDED WITH SHALES ed amorphous iron oxyhydroxides (“limonite”), mixed with cryptocrystalline goethite, hematite, As in the previous case, some examples of weath- and jarosite. Sedimentary textures and pisoliths are ered profiles on carbonates interbedded with absent in these deposits. Late calcite veinlets and shales deposits were found at the foundation of a speleothems may occur in some deposits onto the building in the Can Badosa area (Figure 4). These remaining iron oxide ores left after their exploita- deposits occur in the Lockovian series but have tion, thus pointing to a very recent formation. many similarities with others found in the tran- Some representative ores of this type were found sition unit between the Ludlovian shales and the in the Can Badosa, area when excavating the foun- 328 / Boletín de la Sociedad Geológica Mexicana / 2019 STYLES OF IRON DEPOSITS Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area Figure 4 General aspects of the iron ores developed by alteration of carbonates interbedded with shales in the Can Badosa area. (A) Meter-thick altered dolostone beds (pale gray) interbedded with shales (dark). Note the mine at the right-hand side, partly shut. (B) Pervasive replacement of the shale and carbonate rock ensemble by iron oxides (yellowish and dark brown). Note some mine workings on the left-hand side. Boletín de la Sociedad Geológica Mexicana / 2019 / 329 STYLES OF IRON DEPOSITS Pridolian carbonates. In all cases the host rocks a higher mineralogical and structural complexity are thin limestone lenses interbedded within py- than in the above mineralization styles. The main rite-rich shales, that may be interbedded with reason for this is that large volumes of carbonate phosphate beds as well, made up with cryptocrys- rocks were fully replaced in the proximity of thrust talline fluorapatite. Therefore, the altered rock is surfaces. The resulting oxyhydroxide deposits are geochemically more complex than in the previous massive and irregularly shaped, and their size is style and the resulting ores after weathering con- on the order of hectometers in all directions. The tain abundant silicates and phosphates. The de- distribution of massive ores is basically restricted posit is mainly controlled by bedding surfaces as to replaced carbonates, but minor enrichments are the most suitable channels for water infilling but found in altered shales. also by joints, thus resulting in pod-shaped ore The oxyhydroxide ores in this style of deposits are bodies (Figure 4). rich in different types of phosphates (Costa et al., Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area The Lockovian carbonate beds are also replaced 1993), in particular Fe-Ca phosphates as white by dolomite and ankerite, and weathering produc- botryoidal hydroxylapatite [Ca5(PO4)3(OH)], es similar mixtures of oxyhydroxides and cryp- greenish microcrystalline calcioferrite [Ca2Fe3+2(- tocrystalline goethite, hematite, and jarosite as in PO4)3 (OH)·7H2O], yellowish cryptocrystalline the Pridolian limestones. However, the shales were tinticite [Fe3+5.34(PO4)3.62(VO4)0.38(OH)4· 6.7H2O] enriched in pyrite and the resulting acidic fluids (Bareche and Viñals, 1994; Camprubí et al., 1994; would have reacted with illite from the associated Melgarejo et al., 1988; Rius et al., 2000; Diez and shales, which resulted in iron ores with relative- Rosell, 2013), and reddish cryptocrystalline du- ly high silica contents in the form of opal. With frénite Ca0.5Fe2+Fe3+5(PO4)4(OH)6· 2H2O (Tritlla, higher degrees of replacement of the protoliths, 1987). These phosphates formed in a late stage the resulting ores may become massive, up to 10 and occur either as nodules or veinlets filling small m in diameter pods with high iron oxides and opal cracks across the goethite bodies, mainly along 2 contents. In other cases, the ore bodies are roughly m below the contact with the Silurian shales (Fig- tabular in shape and a few centimeters thick, as ure 5B). Moreover, jarosite is a common miner- they mimic the shape of the replaced carbonates. al in the contact between the thrust shales and In the cases where this process occurred in rocks the ore bodies, and is found forming yellowish with carbonate/shale ratios above 1, the mineral- or greenish cryptocrystalline massive aggregates ization became almost massive and consists of a (Figure 5C) that can be associated with calciofer- mixture of iron oxides, quartz, and illite or kaolin- rite, tinticite and dufrénite. Alunite is also found ite. This style of mineralization occurs in the form as white, earthy cryptocrystalline masses along the of cryptocrystalline or even earthy aggregates, and thrust contact (Figure 5D; Camprubí et al., 2003). consists mainly of goethite and “limonite”-type The oxide ores are the same as those described mineral mixtures. in the Can Badosa mines and show the same tex- tures. The occurrence of carbonate minerals is less 4.3. DEPOSITS IN WEATHERED PROFILES ON common in this style of mineralization than in the CARBONATES OVERTHRUST BY SILURIAN SHALES previous two. A significant example of this style is represent- 4.4. DISTRIBUTION OF IRON MINING OPERATIONS ed by the deposits in the Rocabruna area (2 km IN THE GAVÀ AREA NW of Gavà; Figure 1) in which large volumes of iron oxyhydroxides were formed. The Pridolian The geological mapping of the area combined limestones were overthrusted by Silurian shales with bibliographic research and archeological and supergenic alteration processes affected both studies performed during the urban developments lithologies (Figure 5A). This scenario resulted in in the last 30 years allow the identification of more 330 / Boletín de la Sociedad Geológica Mexicana / 2019 STYLES OF IRON DEPOSITS Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area Figure 5 Field aspect of the massive iron ores developed on Pridolian carbonates overthrusted by pyrite-rich Silurian black shales at the Rocabruna mines, Gavà. (A) Top of the Rocabruna quarry, with the overthrust altered shales, enriched in iron ores. (B) Tinticite veins (yellow) cutting massive goethite bodies produced by replacement of ankeritized Pridolian limestones. (C) Subhorizontal contact between the overthrust shales (strongly replaced by goethite, upper half of the image) and the Pridolian carbonates replaced by goethite (dark brown, lower half of the image). (D) Alunite intergrowths (white coloured) crystallized over the thrust surface that roughly follows the top of the mining gallery. The lower part of the image shows entirely massive iron oxides. Boletín de la Sociedad Geológica Mexicana / 2019 / 331 STYLES OF IRON DEPOSITS than 50 old iron mines in the Gavà municipali- the brownish color of iron-rich rocks and is typi- ty. The mines occur within a 2 km long and 0.5 cally used for areas with iron mines in Catalonia km wide belt between Gavà and Bruguers, NW since the Middle Ages. of Gavà (Figure 2). There are two main mining The geological study of most of the iron mines areas concentrated at the Les Ferreres range (the was performed as the archeological excavations “Ironworks” range) and the Rocabruna area ( the progressed. This aspect was important in order to “Brownish Rock” area). This toponym is related to discriminate between the Neolithic variscite mines Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area Figure 6 Top: Geological section along the foundation of a building on Pompeu Fabra Avenue in Gavà showing the location of the iron mines. Bottom: Geological section of an iron mine in the same area. 332 / Boletín de la Sociedad Geológica Mexicana / 2019 DESCRIPTION OF THE MINING STYLES OF IRON DEPOSITS / (see Díaz-Acha et al., 2019) and more recent iron I and II centuries AD based on a coin found inside mines, especially where both types of mines occur the mine. (Arenas et al., 1989; Álvarez et al., 1991). nearby each other. Both artisanal and industrial This mine consists of a large trench (16.5 m long mineral operations can be recognized in Gavà. It is and 5.3 m wide). A small prospecting pit (mine 63, not clear, however, whether industrial mines were interpreted as older than mine 65) was found in its developed in areas that were previously mined in proximity. Similar open pits were found thereafter an artisanal manner. All the exploitations were de- during archeological excavations (i.e., mine 82; Es- veloped on altered carbonate-bearing rocks (Fig- trada and Bosch, 1997), and some of the outcrops ures 4 to 6). of the southern part of Can Badosa represent this style of mine operations (Figure 3C). A large number of small artisanal operations (mines 97–101, 103–105) were found when con- Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area 5. Description of the mining operations structing the foundations of a building near Pom- 5.1. ARTISANAL MINING IN THE CAN BADOSA AREA peu Fabra Avenue on the outskirts of Gavà. Many small karstic cavities were found in the study area, One of the first Iberian mining operations, discov- but they do not show credible evidence for histori- ered in Gavà, was mine 65 (Figure 2 and 7), which cal mining operations. Most of the artisanal mines was minted during the rule of the Roman emper- were developed as vertical pits and, therefore, are or Trajan. This mine has been dated between the filled with natural debris. Although many of the Figure 7 Archeological excavation on the Iberian or Iberian-Roman trench mine 65 in Gavà. Boletín de la Sociedad Geológica Mexicana / 2019 / 333 DESCRIPTION OF THE MINING mines have not been completely excavated, they 5.2. ARTISANAL TO INDUSTRIAL MINING IN THE have been sealed and protected in order to facil- ROCABRUNA MINES itate future archeological research. These mines consist of small pits (1 to 3 m deep) that were dug Campmany et al. (2004) describe historical docu- directly on outcropping ore bodies (i.e., mine 105 ments about the mining activity and the ore pro- in Figure 6). These pits can give access to short cessing by the Catalan forge system in the area galleries, less than 3 m long. The galleries and pits during the Middle Ages. The forges were located section tends to be rounded and it is less than 1.5 both in Gavà and Bruguers; hence, the forges were m in diameter, However it galleries are generally most likely fed by material extracted from the Ro- irregular because the miners carefully extracted as cabruna mines. The oldest activity registered in the much as possible the iron ores. Therefore, the ex- area corresponds to the year 1068. At that time the traction of ores and the general work inside these Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area area was near the frontier between the county of mines was complicated, and the geometric char- Barcelona and Al‒Andalus, and was considered a acteristics of some of the mines suggest that they territory in conflict. Therefore, the products of the were probably exploited by children. After the iron mines and the associated forges could have extraction of iron ore, some mines experienced had a strategic importance in war. The mining water infiltration that produced the development and forge activities continued at least during the of calcite speleothems (Figure 6). twelfth and thirteenth centuries, and ceased at the Figure 8 Open pit of the nineteenth century, and upper levels of the 1940s mining works, at the Rocabruna mines. Notice the control on the location of ores exerted by Hercynian thrusts. 334 / Boletín de la Sociedad Geológica Mexicana / 2019 DESCRIPTION OF THE MINING end of the Middle Ages (Campmany et al., 2004). formed in other iron outcrops of the area during The Rocabruna mines were reopened in 1770 and the period 1876–1878. The mines were exploited ceased again shortly thereafter (Campmany et al., once more following the same system during the 2004). There is no data about the exploitation sys- period 1888–1908 and at that time the mineral tem during this period, but the ore bodies were was exported to be processed in France (Maureta probably exploited by means of trenches. and Thos, 1881). However, the dimensions of the During the period 1860–1861 the mines were mines exploited between the nineteenth century reopened to furnish iron mineral to a forge at and the beginning of the twentieth century allows Sants (Bauzá, 1861), today a district of Barcelo- estimating a production of less than 50000 tons na. During this period the mines were exploited by in total. open pits of reduced dimensions (less than 50 m The last and more important period of activity in diameter, Figure 5A), but the mining company of these mines was during the autarchy period of Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area went bankrupt and mining ceased (Bauzá, 1861). the Francoist dictatorship, during the 1940s and New operations using small open pits were per- 1950s, at the end of this period mining ceased Figure 9 Open pit of the nineteenth century, and upper levels of the 1940s mining works, at the Rocabruna mines. Notice the control on the location of ores exerted by Hercynian thrusts. DESCRIPTION OF THE MINING / Boletín de la Sociedad Geológica Mexicana / 2019 / 335 definitively in the area (Campmany et al., 2004). The morphology of the iron ores at Can Bado- DISCUSSION During this period some horizontal tunnels were sa corresponds to vertical karstic shafts and small built starting from the bottom of the open pit chamber rooms, following the terminology of (Figure 8). One of the tunnels gave service to a Loucks (1999). These deposits were formed by large stope to exploit the iron ores. A gallery of replacement of the carbonate rocks within the va- reconnaissance started from the stope (gallery 1 in dose zone near the Quaternary paleosurface. This Figure 8) and found small irregular pods of ore re- domain is not able to produce large cavities in the placing dolostones at the contact with Hercynian karstic realms (Loucks, 1999). The volume of in- thrusts, but the replacement of carbonate rocks dividual iron ore bodies in the study area is limited was only partial and therefore the distribution of in most cases due to the small thickness of carbon- the ore was very irregular, and the gallery was sub- ate beds or by the low amount of available iron in these rocks. Therefore, these deposits do not meet Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area sequently closed. Other exploratory galleries were planned without geological criteria. For instance, the current economic criteria for industrial min- the gallery starting at the adit 2 (Figure 8) also ing operations, but their mining would have been found ore, but the exploitation did not continue feasible by artisanal exploitations, as shown by entered the black shales. those mines active during the Iberian and Roman The mineral obtained from these galleries was times, and during the Middle Ages. sent through shafts to a lower plant mainly used All the iron oxyhydroxide deposits in the study for mineral evacuation. This lower plant consisted area are karstic in origin (common around Barce- of an access tunnel with some lateral galleries for lona), similar to those described in many locations ore reconnaissance (Figure 9). The continuity at along the CCR (Mata-Perelló and Montori- depth of the ore bodies was demonstrated in these ol-Pous, 1975; Mata-Perelló, 1990). These type of galleries but they never were exploited at this level. iron mines were exploited in industrial times with- The mineral was extracted through the adit of this out significant profit. Other iron deposits, also mine and sent to the processing plant. developed on weathered limestones, are found in the vicinity of El Vendrell (60 km SW of Gavà). These deposits also have reduced dimensions and 6. Discussion were also exploited during the pre-Roman Iberian epoch (Morer and Rigo, 1999). 6.1. FORMATION OF IRON ORES IN THE GAVÀ AREA The formation model for iron deposits along the CCR is variable. A sedex-type origin was proposed The iron ores in the Can Badosa–Les Ferreres for the Malgrat iron mines near the Catalan coast range area are essentially hosted by Pridolian mas- (Carmona and Viladevall, 1983, 1985) and an ep- sive limestones, which are the main guides for find- ithermal origin was suggested for the Celrà iron ing old iron mines in the Gavà region (Costa et al., mines in NE Catalonia (Ferrer-Modolell, 1991). 1993). Also, some artisanal works were established However, the irregular morphology produced by on minor deposits that are hosted by dolostones replacement, the botryoidal and earthy textures, interbedded with shales of the Silurian-Devonian and the poorly crystalline character of most iron transition. The outcrops of the carbonate rocks ores of the CCR suggest that these deposits were are not continuous and the Paleozoic tectonic produced by edaphic processes, in part coupled structure in the area is complex. The limestones with karstic development. The controls of the ores are segmented because of thrusting and Alpine are lithologic (availability of iron-rich carbonates), faults, factors that limit the development of large paleogeographic (proximity of the Quaternary deposits. paleosurface, less than 5 m deep), and structural 336 / Boletín de la Sociedad Geológica Mexicana / 2019 (presence of discontinuities as joints and faults). genic acidic fluids, whereas the likeliest source of DISCUSSION The Can Badosa karstic deposits could be classi- iron in the Can Badosa deposits would be the dis- fied as immature, as the karst was developed by solution of Fe-rich dolostones alone. Consequent- fluids, circulating vertically by gravity, within the ly, the downward incorporation of acidic fluids vadose zone (as described by Loucks, 1999). The helped to produce larger deposits at Rocabruna Can Badosa and Rocabruna deposits exhibit sig- than in Can Badosa by increasing the replaced nificant mineralogical differences between them, volume and supplying additional Fe. Moreover, such as the occurrence of minerals of the alunite the dissolution of apatite in the overthrusting Si- supergroup and iron phosphates in Rocabruna, lurian shales by acidic fluids is responsible for the which are absent in Can Badosa. Also, the deposits formation of descending H3PO4-enriched fluids, are much larger in the Rocabruna than in the Can which would have reacted with Ca-Fe carbonates Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area Badosa group of deposits. and oxyhydroxides to form iron-rich phosphates. The occurrence of large amounts of alunite and The existence of deep weathering profiles suggests jarosite in the Rocabruna deposits is a key factor to that these deposits were formed under a humid recognize the effects of acidic fluids involved in the climate. Dating of the alunite associated with su- genesis of the deposit and to explain its size and pergenic ores in the Gavà area yielded Calabri- grade. A hydrothermal origin is not an exclusive an-Villafranchian ages (late Pliocene; Camprubí et source for acidic and oxidizing fluids, and such al., 2003). These ages are in accordance with paly- solutions can be produced by weathering below nological studies in sediments from other areas of paleosurfaces of sulfide-rich rocks (Melgarejo et Catalonia, which suggest that the climate during al., 1997; Borruel-Abadía et al., 2016), a role that the early to middle Pleistocene was warm and is played effectively by the overthrusting Silurian humid (Postigo Mijarra et al., 2007). shales in the study area. Weathering processes may Iron ore deposits in paleokarsts have been inten- develop economic deposits of alunite-group min- sively mined worldwide between prehistoric times erals, such as those developed on similar Silurian and the nineteenth century because they were black shales in central Spain (Caballero et al., 1974; highly suitable for metallurgical extraction of iron Cañada, 1974). Such processes may also produce (Bárdossy et al., 2015). In some cases, as in Bel- economic iron oxyhydroxide deposits on the sur- gium, the richest deposits occur in deposits simi- face, as is the case of the Riaza deposits in central lar to Rocabruna; that is, limestones overthrust by Spain (Leal-Meca and Lillo-Ramos, 2009). There- shales (Denayer, 2016). In fact, pre-existing con- fore, the acidic oxidizing fluids that result from the centrations of iron ores are considered necessary oxidation of pyrite are able to transport leached to produce large iron deposits by supergenic pro- iron towards deeper parts of the weathering pro- cesses, as proposed to explain the iron enrichments file where, in the case of the studied deposits, Pri- in the large mines of Marquesado, Alquifes and dolian carbonates were found. The neutralization Las Piletas near Granada, southern Spain (Tor- of the acidic fluids when reacting with the car- res-Ruiz, 1983). bonates would have produced (1) the precipitation of sulfates of the alunite group at the contact be- 6.2. CONDITIONS FOR EARLY IRON MINING IN THE tween the shales and the carbonate rocks, and (2) GAVÀ AREA the replacement of iron-rich carbonates and pre- cipitation of iron oxyhydroxides at higher pH and The morphology and mineralogy of the deposits below the contact. Therefore, the sources of iron are crucial factors that control the feasibility of at the Rocabruna deposit could be both the iron exploitations through history. The existence of coming from the dissolution of Fe-rich dolostones relatively soft oxyhydroxide ores, similar in texture and the iron transported by the descending super- and mineralogy to those classically known as “bog Boletín de la Sociedad Geológica Mexicana / 2019 / 337 DISCUSSION / CONCLUSIONS ore”, could have favored artisanal mining between 7. Conclusions the Iberian and Roman historical periods. Such is the case of many other locations in Europe during The iron ores of the Gavà domain were formed pre-Roman times (i.e., Thelemann et al., 2017). by karstic processes developed during the late The porous nature of the ores could have favored Pliocene and the Quaternary on Fe-rich dolostone reducing reactions (Tylecote, 1992) as such as ores units of Silurian-Devonian age. The dimensions were suitable for iron reduction in bloomery (i.e., of these deposits are quite reduced owing to the Sauder and Williams, 2002). The studied iron mines were near populated cen- scarce thickness of the Paleozoic carbonates and ters since the Iberian period. Despite the reduced the compartmentalization of the outcrops by their dimensions of the mined ore deposits, these could structural arrangement. have been large enough to feed the local demand The ore grade decreases with increasing silicate Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area within a subsistence economic scheme. Therefore, content in the protolith (Lockovian or Ludlovian) such ores would have supported the sort of mining due to interbedding between carbonate rocks and operations that could be developed during the Ibe- shales. Also, the ore size and grade ores increase, rian or Iberian-Roman periods, and such a possi- and the mineralization become massive, when ble age for the earliest iron mining in the region is carbonates were replaced by acidic fluids due to also supported by archeological evidence (Arenas the oxidation of pyrite in overthrusting pyrite-rich et al., 1989; Álvarez et al., 1991). Also, the forges shales. that would have been fed by mining activities in Karstic ores consist mainly of cryptocrystalline these ores were active during the Middle Ages, as goethite and hematite, often as unconsolidated documented by Campmany et al. (2004). aggregates. Such textures and mineralogy favored Some of the oldest mines in Europe were opened artisanal mining of these ores during antiquity. to exploit iron ochres for pigments, such as a mine Archeological evidence indicates that all the styles in Greece that was dated at 2,000 BP (Levato, of iron ores in the Gavà area have been exploit- 2016). The studied iron mines share the same ed at least since the Iberian historical period. Iron geological context and mineralized areas with the mining has been intermittently active in this area Neolithic variscite mines of Gavà, which makes a through history, essentially during the Roman strong case for both types of mines to be exploit- epoch, the Middle Ages, and the industrial period. ed simultaneously, although there is no convincing For the latter two cases, historical documental re- evidence for mining of iron ores in the Gavà area cords are available. The earliest artisanal iron ore earlier than the Iberian period. exploitations were designed using trenches, pits, The Can Badosa iron ores are silica-poor in the or shafts along all the available outcrops and all Pridolian carbonates, but silica-rich in the Locko- the styles of mineralization. Iron mining during vian carbonates. The silica-rich ores would have the Neolithic cannot be ruled out. hindered the Iberian period metallurgy by gener- Between the nineteenth and twentieth centuries ating large volumes of slag. The Rocabruna ores, with high contents in phosphates, could have pro- the development of industrial mining was restrict- duced lower quality metal objects that would have ed to the largest deposits by means of open pits. In been harder than P-poor objects but slightly more these cases, it is likely that any evidence for earlier brittle (Sarabia, 1994). mining activities has been destroyed. 338 / Boletín de la Sociedad Geológica Mexicana / 2019 ACKNOWLEDGEMENTS / Acknowledgements 1979, Evolución tectonoestratigráfica de los REFERENCES Catalánides: Acta Geologica Hispanica, 14, This paper is a contribution to the projects 2014 242–270. SGR 1661 and 2014/100820 of the Government Anadón, P., Cabrera, L., Guimerà, J., Santanach, of Catalonia (Generalitat de Catalunya), which P., 1985a, Paleogene strike-slip deformation provided the necessary funds. The Municipality of and sedimentation along the Southeastern Gavà and the Gavà Museum helped in the study margin of the Ebro basin, in Biddle, of the archeological sites and Elisenda Miquel of K.T., Christie-Blick, N., (eds.), Strike- the Gavà Environmental Department helped to slip deformation, basin formation and allow access to many areas of the municipality for sedimentation: Oklahoma, USA, Society of geological mapping. Some of the samples used in Economic Paleontologists and Mineralogists Special Publication, 37, 303–318. Geological context and origin of the mineralization of the historic and prehistoric iron mines in the Gavà area this study were provided by the Mineralogy Col- lection of the University of Barcelona and by the Anadón, P., Julivert, M., Sáez, A., 1985b, Natural Sciences Museum of Barcelona. The au- Aportación al conocimiento del Carbonífero thors acknowledge the revisions by the referees Dr. de las Cadenas Costeras Catalanas (abstract), María del Sol Hernández Bernal and Dr. Giovan- in Compte rendu X Congrès International de ni Sosa Cevallos. Stratigraphie et de Géologie du Carbonifère, 1983: Madrid, Spain, Instituto Geológico y Minero de España, 1, 99–106. 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