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Geothermal mineralized scales in the pipe system of the geothermal Piancastagnaio power plant (Mt. Amiata geothermal area): a key to understand the stibnite, cinnabarite and gold mineralization of Tuscany (central Italy)

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Abstract

The CO2-rich geothermal fluids produced in the Piancastagnaio geothermal field (Mt. Amiata geothermal area, Southern Tuscany, Italy) show temperatures up to 360°C and pressures of about 200 bar at depths of around 3,500 m (Giolito, Ph.D. thesis, Università degli Studi di Firenze, Italy, pp 1–147, 2005). CaCO3- and/or SiO2-dominated scales are deposited in the pipes leading to the pressure and atmospheric separators of the geothermal wells. High content of metastibnite and/or stibnite in both calcite and silica scales and Sb contents of up to 50 mg/L in the fluids indicate their mineralising potential. The red or black colours of the scales depend on the predominance of red metastibnite or black stibnite, respectively. In our condensation experiments, as well as during deposition of the scales, metastibnite is the first Sb2S3 mineral to form. In a second stage, metastibnite is transformed to stibnite. During depressurization the Hg content of geothermal fluids partitions preferentially into the gas phase, whereas Sb and As remain in the liquid phase. This separation explains the often observed areal separation of Hg and Sb mineralization. The multistage deposition of Sb in the mining district of Tuscany is due to a periodic restoration of the permeability of the ore-bearing faults by microseismic events and subsequent host rock brecciation. The still ongoing microseismic events are induced by the accumulation of high-pressure CO2-rich fluids along faults followed by mechanical failure of the faults.

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Acknowledgements

Many thanks to the ENEL-GREEN ENERGY company for the permission to sample fluids and scales. Thanks also to the staff of ENEL-GREEN ENERGY at the Larderello and Mt. Amiata facilities for many help during sampling. We thank Dr. Culivicchi (Larderello) for the discussions and advice. The manuscript has benefited from stimulating review by two anonymous reviewers and a careful editorial handling by B. Lehmann.

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  1. Gmain 1, 84424, Isen, Germany

    Giulio Morteani

  2. CNR-Istituto di Geoscienze e Georisorse, Via G. La Pira, 4, 50121, Florence, Italy

    Giovanni Ruggieri

  3. Helmholtz-Zentrum Potsdam, Deutsches Geoforschungszentrum, Telegrafenberg, 14374, Potsdam, Germany

    Peter Möller

  4. Institut für Rechtsmedizin, Ludwig-Maximilians Universität München, Nussbaumstrasse 26, 80336, Munich, Germany

    Christine Preinfalk

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  1. Giulio Morteani
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Correspondence to Giulio Morteani.

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Morteani, G., Ruggieri, G., Möller, P. et al. Geothermal mineralized scales in the pipe system of the geothermal Piancastagnaio power plant (Mt. Amiata geothermal area): a key to understand the stibnite, cinnabarite and gold mineralization of Tuscany (central Italy). Miner Deposita 46, 197–210 (2011). https://doi.org/10.1007/s00126-010-0316-5

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