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The Reocin carbonate hosted zinc deposit lies is located 30 km SW of Santander in northern Spain.

  The deposit was discovered in 1856 and was first mined by the Compagnie Royale Asturianne des Mines and, since 1981, by its affiliate, Asturiana de Zinc, S.A. The mine was closed in 2003.

  The Reocin deposit is one of many Zn-Pb carbonate-hosted orebodies and prospects in the Santillana syncline of the late Triassic-Jurassic to Cretaceous Basque-Cantabria Basin. This basin is located between the Basque Palaeozoic and Asturian Paleozoic massifs to the east and west respectively. The centre of the basin contains in >14 km of Mesozoic marine and continental sedimentary rocks, whereas the thickness rarely exceeds the 3 km in the Santillana area (Vadala, 1981).

  The dominant structural features within the basin are three main fault trends (NW-SE, east-west and NE-SW in orientation), subdividing the basin during the Lower Cretaceous into well-defined highs and troughs (García-Mondéjar, 1990). These fault systems have been interpreted as reactivated Hercynian structures, that were subjected to sinistral strike-slip displacement in the Late Triassic and Cretaceous (García-Mondéjar, 1990) during the opening of the Bay of Biscay.

  The sequence within the Basque-Cantabria Basinis characterised by shallow to fresh water sediments, saline deposits, with gypsum and clay, marls, limestone, dolomite and sandstone. The Triassic sequence was deposited during the first rifting stage, and is composed of red clastic sedimentary rocks, platform carbonates and evaporite-rich shales. Jurassic rocks typically comprise platformal limestone and marl during a sag phase. Intense fault activity during a renewed rifting event that began in the Late Jurassic led to the deposition of a thick sequence of sedimentary rocks, mainly during the ensuing Early Cretaceous. The Aptian and Albian successions may reach up to 7000 m in the western and central parts of the Basque-Cantabrian basin (García-Mondéjar et al., 2005). This period was characterised by the deposition of shallow-platform limestones with abundant rudist-rich reefs of the Urgonian facies (Rat, 1959). In the central and eastern regions of the Basque-Cantabrian basin, the Urgonian facies are mainly terrigenous, whereas reefal carbonates predominate in the west (García-Mondéjar et al., 1996). Between the Albian and Santonian, the Cretaceous sedimentation was accompanied by submarine alkaline-type volcanism (Castañares et al., 2001). Sedimentary rocks directly overlying the Urgonian facies are commonly siliciclastic (Utrillas and Valmaseda Formations) and were deposited in continental, deltaic and talus environments. Upper Cretaceous and Paleogene rocks range from continental clastics to platform limestones to flysch-type marls.

  The ~460 m thick Aptian-Albian sequence in the Reocín area consists of 9 alternating units, numbered for 1 at the base, to the uppermost 9. These units are either i). predominantly calcareous rocks or ii). terrigenous marl, marly limestones or sandstone.

  The Reocín deposit is immediately hosted by the 150 m thick Cretaceous (Aptian) carbonate bed, unit 5, that has been completely dolomitised. Two stages of dolomitisation are recognised in the Reocín area. The first is a pervasive alteration of the limestone country rocks that was controlled by faulting and locally affected the upper part of the Aptian and the complete Albian sequence. The second event occurred after erosion and was controlled by karstic cavities. This later dolomitisation event was accompanied by ore deposition and, locally, filling of dolomite sands and clastic sediments in karstic cavities.

  The main deposit covers an area of around 3500 x 800 m with a number of distinctive zones.  Two of these zones are composed of a total of 4 lenses, each of 2 to 6 m in thickness, while the third is 20 m and the fourth 45 to 60 m thick. The ore is both stratabound and controlled by structure, with richer ore following one particular fault line direction.

  The bulk of the economic zinc-lead mineralisation at Reocín is found in the western part of the ore deposit, hosted within a mineralised palaeokarst network roughly paralleling bedding in the lower few metres of the dolomitised unit 5. The eastern part of the mineralised zone is strata bound, and is made up of strongly discordant zones of disseminated mineralisation, and small, dispersed mineralised lenses that fill voids and cement breccias of the host dolostone. Morphologically, the Reocín orebody has been laterally subdivided into three areas, depending on the dominant geometry of mineralisation:
• The most extensive orebody Capa Sur is predominantly stratabound and occurs in the southwestern and central part of the deposit, with a strike length of >2000 m and thickness of between 1 and 5 m. It is roughly bounded by a set of 45 to 60° trending faults. The mineralisation is closely related to a very thin but continuous carbon-rich clay horizon or black dolostone at the top of the marly limestone of unit 4. The centre of this section of the deposit can be vertically divided into three bands, from the base upwards: i). Capa Sur as described above, ii). Capa Norte, located ~12 m above, and iii). Tercera Capai, located ~10 m higher. These latter two lenses form distinct layers, but are more or less interconnected, with average thicknesses of 4 to 6 m and 1 to 6 m respectively. These lenses pass laterally into weakly mineralised or barren dolostones, and are connected to the east by the N-10 fault, which is part of a 100° trending fault system (Velasco et al., 2003).
• The central mineralised zone is both stratabound and discordant, and is broadly tabular to irregular and subhorizontal, resulting from infilling of open karstic cavities. The mineralisation occurs as peneconcordant lenses and small irregular pods of ore that are laterally discordant with the host dolostones. The orebody is located 15 m above the boundary with the Unit 4 marly dolostone, extending for near 1000 m roughly parallel to the dominant east-west fault system. The orebody pinches out laterally, with abrupt changes in thickness of up to 20 m (Velasco et al., 2003).
• The eastern part of the Reocín deposit, Barrendera, consists of numerous small lenses and disseminations of sulphides that formed by replacement and infilling of pre-existing vugs and pore spaces in breccias. In this zone, the mineralised sequence has an average thickness of 50 m, but ranges up to 150 m locally. In Unit 5, the thickness of mineralisation may reach as much as 240 m before it fades to the NE (Velasco et al., 2003).

  Two main mineral styles are recognised in all three mineralised areas, irrespective of the ore morphology: i). disseminated sphalerite and/or galena replacing the host dolostone and ii). open-space filling of veins, breccias and a wide variety of vugs resulting from dissolution and fracturing of host carbonates. All are generally composed of fine-grained sphalerite layers, sometimes appearing as colloform bands that contain grains of dolomite and quartz. Variations in grain size and composition of the sphalerite affect the colour and detail of the colloform banding. Within each style of mineralisation, crackle breccias, ore-matrix breccias, trash zone accumulations, mineralised internal sediments, and the 'snow-on-roof' textures of Leach and Sangster (1993) are recognised. Most of the ores have undergone a late stage of brecciation and have been cemented by internal sediments or hydrothermal Fe-rich dolomite (Velasco et al., 2003).

  Four main ore textures occur in the different mineralisation styles of Reocín deposit: i). colloform, layered sphalerite ('schalenblende') exhibiting colour and grain size variations; ii). fine- to coarse-grained, disseminated to massive sphalerite aggregates that replaced the dolostone; iii). botryoidal to pendulous sphalerite growths; and iv). rare, coarse-grained sphalerite infilling late veins.

  Fine dolomite and sphalerite intergrowths, grading into colloform sphalerite layers with a similar chemical zonation imply that the colloform and disseminated sphalerite were broadly coeval, and probably resulted from similar physical and chemical processes. The pendulous sphalerite occurs as a late depositional stage that partially replaced previously deposited banded sphalerite or formed pendulous growths from the roof of some interlayered vugs and open cavities (Velasco et al., 2003).

  In addition to sphalerite the principal minerals are marcasite, galena, melnikovite and sparse pyrite, with a gangue of dolomite and calcite, and secondary smithsonite, melanterite and epsomite in oxidised zones.

  The ore was being mined by open cut and by underground at rates of 0.77 and 0.87 Mt per annum respectively in 1988.

  The total resource at Reocin amounted to 87 Mt @ 11% Zn, 1% Pb, of which proven reserves in 1989 were 29.8 Mt @ 11% Zn, 1.7% Pb (Mine visit, 1992).

  According to Velasco et al. (2003), production to the end of 2002 was ~60 Mt @ 8.7% Zn, 1.0% Pb, with remaining reserves of 2.5 Mt @ 8.5% Zn, 0.9% Pb.

The most recent source geological information used to prepare this decription was dated: 2018.     Record last updated: 31/3/2019
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Anonymous  1993 - Various plans and diagrams: in    Reocin Mine Handout - Unpub.    9p
Boni M, Large D  2003 - Nonsulfide zinc mineralization in Europe: an overview: in    Econ. Geol.   v98 pp 715-729
Perez A A, Arias C P  1989 - Reocin mine: in    Mining Magazine - August 1989    pp 102-103, 105-106, 109-110, 113, 115-116
Perona, J., Canals, A. and Cardellach, E.,  2018 - Zn-Pb Mineralization Associated with Salt Diapirs in the Basque-Cantabrian Basin, Northern Spain: Geology, Geochemistry, and Genetic Model: in    Econ. Geol.   v.113, pp. 1133-1159.
Symons, D.T.A., Lewchuk, M.T., Kawasaki, K., Velasco, F. and Leach, D.L.,  2009 - The Reocin zinc-ead deposit, Spain: paleomagnetic dating of a late Tertiary ore body: in    Mineralium Deposita   v.44, pp. 867-880.
Vazquez Guzman F  1989 - Spain (Extracts): in Dunning F W, Garrard P, Haslam H W, Ixer R A (Eds.),  Mineral Deposits of Europe IMM, London   v 4/5: Southwest and Eastern Europe, with Iceland pp 105-127, 194-196
Velasco, F., Herrero, J.M., Yusta, I., Alonso, J.A., Seebold, I. and Leach, D.,  2003 - Geology and Geochemistry of the Reocin Zinc-Lead Deposit, Basque-Cantabrian Basin, Northern Spain: in    Econ. Geol.   v.98, pp. 1371-1396.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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