Sierra Menera


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The Sierra Menera Mg-Fe carbonate deposits are located in the Iberian Range of northeasten Spain.

These Mg-Fe carbonate deposits and the overlying goethite gossan have historically been mined for iron, with about 400 Mt of iron ore of various grades and qualities having been produced from the Vizcaya-Santander, Coto Vivaldi, Coto Wagner, Cala, Marquesado and Sierra Menera mines. Iron was first extracted in the Sierra Menera district from artisan ironworks of Iberian and Roman ages. Between 1906 and 1986, the Sierra Menera district produced nearly 45 Mt of ore with a grade of 45 to 53 percent Fe. Although there are estimated reserves of approximately 33 Mt of ore, mining ceased in 1986 because the product was not of high enough quality and grade and because of the high cost of transport.

The Iberian Range, is located between the Pyrenean and Betic alpine belts, and consists of small Mesozoic massifs cored by Palaeozoic basement separated by basins filled by Tertiary and Quaternary detrital rocks. The Hercynian basement is composed of Precambrian to Permian rocks folded in northwest-southeast to north-south directions and overthrust during the Hercynian orogeny.

The Mg-Fe carbonate deposits of the of Sierra Menera district lie within a sequence of Ordovician to Silurian basement rocks. The basal portion of the stratigraphic sequence consists of quartzites and graywackes of Early Ordovician age. This is overlain by a Late Ordovician carbonate dominated succession, known as the El Pobo limestones, which is interpreted to have been deposited in an epicontinental sea after Caledonian tectonism. The Sierra Menera deposits are hosted within this sequence of shallow-marine carbonates. The El Pobo limestones have been divided into three members: i). a 70 to 140 m thick package of quartzites and sandstones with silty intercalations; ii). an irregularly distributed sequence of marls and shales, which is intercalated with marly dolomites and sandstones locally containing reefal deposits; and iii). a 25 to 100 m thick succession of carbonates (Cabezo limestones), including Mg-Fe carbonates and dolostones, iron oxides and clays.

The strata-bound orebodies at Sierra Menera are preferentially located on the flanks of the Sierra Menera anticline. They are mainly composed of Mg-Fe carbonates, belonging to the siderite-magnesite series, and dolomite, with a gangue of quartz, clay minerals, and calcite. Mg-Fe carbonates at Sierra Menera occur as lenticular to irregular orebodies, which are interfingered laterally with the host dolostone. Replacement of host dolostone by massive Mg-Fe carbonates is observed at all scales, and relicts of host dolostone are commonly found inside the Mg-Fe carbonate bodies. Although the thickness of the orebodies is typically from less than 1 up to 10 m, in some locations the entire 120 m thick host dolostone sequence has been replaced by Mg-Fe carbonates. The upper part of the Mg-Fe carbonate bodies was affected by oxidation processes linked to late Pliocene karstification, which gave rise to goethite gossans. The contacts between Mg-Fe carbonates and their oxidation products are sharp (faults and joints define many contacts), and dissolution features, including mogotes, karren, caves, shafts, and relicts of Mg-Fe carbonates in gossan bodies are evident.

Three generations of carbonate have been recognized from textural criteria: host dolomite, Mg-Fe carbonates, and late dolomite. Mg-Fe carbonate mineralization typically consists of medium to coarse and very coarse anhedral and cloudy spar crystals arranged in a xenotopic fabric. Pinolitic crystals are also common. Relicts of coarse dolomite can be found within the Mg-Fe carbonates. MgO and FeO contents of the carbonates range between 13.74 and 44.02 and 5.52 and 41.73 wt percent, respectively, corresponding to the breunnerite and pistomesite content.

The characteristics of the Mg-Fe carbonates are consistent with a metasomatic hydrothermal origin by replacement of a dolomitic protolith as the result of the influx of hot Fe-Mg rich fluids under acid and reducing conditions. These deposits have similarities to Alpine-type magnesite deposits (Austria, Slovakia, western Carpathian Mountains, Pyrenees) and Mississippi Valley-type Fe deposits (Basque-Cantabrian basin, northern Spain).

The Iberian Range also hosts several Ba-Zn-Pb-Cu, low-temperature vein and stratabound deposits as well as several different types of Fe deposits. These include stratiform and vein type deposits of hematite that occur at a specific stratigraphic level of Early Cambrian age and at a pre-Triassic unconformity. In addition, several massive ironstone horizons occur in Ordovician siltstones with oolitic textures, containing iron oxides, silicates, and carbonates.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2003.    
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:
Fernandez-Nieto C, Torres-Ruiz J, Perez I S, Gonzalez I F and Gonzalez Lopez J M  2003 - Genesis of Mg-Fe Carbonates from the Sierra Menera Magnesite-Siderite Deposits, Northeast Spain: Evidence from Fluid Inclusions, Trace Elements, Rare Earth Elements, and Stable Isotope Data: in    Econ. Geol.   v98 pp 1413-1426

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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