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Iglesias District - Buggerru, Planu Sartu, Masua, Iglesias, Nebida, Monteponi, San Giovanni, Margana |
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Sardinia, Italy |
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Zn Pb
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Super Porphyry Cu and Au
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The old mines of the Iglesias District in southwestern Sardinia exploited both sulphide and non-sulphide zinc-lead ores from the 13 th century until the late 1970's. Total output is unclear, although in the 1970's production was at a rate of around 84 000 tonnes of 20 to 35% Zn per year. Historic mines include: Buggerru, Planu Sartu, Masua, Iglesias, Nebida, Monteponi, San Giovanni, Campo Piasno, Marganai, Oridda and others.
The host succession in this part of Sardinia commenced with 400 to 500 m of lower Cambrian siliciclastics with interbedded carbonates towards the top, grading into the overlying 300 to 600 m of shallow water platformal carbonates. These were followed in turn by two units of middle to upper Cambrian to lower Ordovician age, the first of which was 50 to 80 m of nodular limestone and then 400 m of slate. All of these were deformed during an Ordovician orogenic cycle, accompanied by erosion and then unconformably overlain by upper Ordovician to Silurian sediments. During the Devonian to Carboniferous Variscan Orogeny the sequence was further deformed and tilted. The intrusion of late Carboniferous to Permian granitoids produced a strong pervasive hydrohermal dolomitisation of the Palaeozoic carbonates. Late Variscan uplift and subsequent pulses of extension further fractured the rocks and allowed for Mesozoic hydrothermal circulation. This promoted karstic development in the Cambrian carbonates and deposition of calcite in the openings and collapse breccias. The terrane was subjected to pulses of Tertiary volcanism, sedimentation (shallow marine and continetal), erosion, lateritisation, deep oxidation and tectonism, accompanied by karst development, brecciation and circulation of fluids.
Sulphide mineralisation had been deposited in the lower Cambrian sediments, mainly the carbonates which are distributed around the flanks of a domal structure cored by the lower Cambrian siliciclastics. The majority of this mineralisation was stratabound, comprising i). stratabound early diagenetic massive sulphides (pyrite > sphalerite > galena) in the tidal dolomites of the upper member of the lower Cambrian units, and ii). void-filling breccia cement and replacement deposits (sphalerite > galena > pyrite). Both of these were strongly deformed and sheared. At the end of the Variscan Orogeny, granitoids developed iii). calcic skarn associated Zn-Pb(-Cu) mineralisation within the carbonates. Between the end of the Variscan and the beginning of the Tertiary Alpine Orogeny widespread pervasive hydrothermal alteration of the lower Palaeozoic rocks produced iv). small Ag rich galena and barite occurrences.
Non-sulphide mineralisation includes the carbonate hosted calamine ores composed of smithsonite, hydrozincite and hemimorphite, with cerussite and anglesite (often associated with galena nodules) as well as Fe and Mn oxyhydroxides with red staining (goethite, lepidocrocite and hematite). The ore grade was very variable, ranging from a few percent in the upper sections (or barren in the leached surface profile) to 30% in the base of the non-sulphide ore where all of the primary sulphides had been replaced. Mineralisation is considered to result from in situ oxidation of primary sulphides by acid meteoric fluids (from dissolution of the abundant pyrite in the rock), and replacement of the sulphides and surrounding carbonates. These secondary deposits comprise 1). "calamine roche" from replacement of primary sulphide and country rock, to 2). "calamine terre" detrital, ferruginous "earthy" smithsonite and hemimorphite rich clays filling a maze of interconnecting karst cavities and openings metres to tens of metres long in the upper mine. These occurred to depths of between 200 and 500 m, with partial oxidation for a further 100 m.
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription 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.
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Selected References:
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Boni M, Gilg H A, Aversa G, Balassone G 2003 - The Calamine of southwest Sardinia: Geology, mineralogy and stable isotope geochemistry of supergene Zn mineralisation: in Econ. Geol. v98 pp 731-748
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Boni M, Large D 2003 - Nonsulfide zinc mineralization in Europe: an overview: in Econ. Geol. v98 pp 715-729
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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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