The Monte Amiata area (Tuscany) was the third mining district in the world for the production of mercury (total 104473 t). Among the several mines of the district, the Abbadia San Salvatore mine takes a prominent position for having provided about 50% of the total production. The Abbadia San Salvatore mercury deposit is one of the largest in the world (after Almaden in Spain and Idrija in Slovenia) and it is the only Tuscan metalliferous deposit that can be included in the category of the "World-Class Ore Deposits". Despite the remarkable economic importance and the huge amount of mining data produced, Abbadia San Salvatore and the other smaller ore deposits of the Amiata District (Solforate, Morone-Selvena, Siele, Cornacchino, Cortevecchia, Monte Labbro, Cerreto Piano) still wait for a detailed scientific characterization. The most deficient aspect is the description of geometry, texture, and type of hydrothermal alteration of the ore bodies. On the occasion of the preparation of this volume, a thorough study of the scientific-mining literature and part of the mining documentation has been performed. The result is the definition of more accurate descriptive models of the Monte Amiata mercury deposits and the formulation of a general interpretive model that fits coherently in the framework of mercury deposits worldwide and, more generally, in the conceptual model of epithermal systems. Most of the Hg ore (mainly cinnabar with traces of meta-cinnabar and native mercury) that made the fortune of this district was made of masses of cinnabar-rich clay, the socalled "biocca" in the miner's jargon. The "biocca" constituted small replacement bodies (mantos, Hg up to 8%) in marly limestone lenses or, as an alternative, the matrix of breccia bodies both with a sub-vertical (hydrothermal breccia pipes, Hg content up to 3%), and a sub-horizontal (conformable hydrothermal breccia bodies, Hg content <= 1%) attitude. The few available analyses of the clay minerals are consistent with a kaolinite ± montmorillonite assemblage, typical of argillic and advanced argillic hydrothermal alteration. The detailed study of mining maps/sections of the three main deposits (Abbadia San Salvatore, Solforate-Siele and Morone-Selvena) allowed the definition of the following general interpretive model. The Hg deposits show a sub-horizontal, basal mineralised zone (conformable hydrothermal breccia bodies, stock-works, disseminations; low-medium Hg grade), from which sub-vertical breccia chimneys (hydrothermal breccia pipes, medium-high Hg grade) rise for 100-150 m up to, or near to the paleo-surface. In favourable case - presence of reactive lenses of marly limestone - the hydrothermal breccia pipes can be surrounded by manto-type replacement bodies (high Hg grade). The extraordinarily high mercury concentrations of this district transformed through time from a great economic-social opportunity to an environmental problem. Today, the awareness of the environmental remediation costs should not hide the great scientific and cultural potential of this mining region. The Monte Amiata World-Class Ore Deposits still deserve to be studied to help the understanding of hydrothermal processes in volcanic settings, and to provide new knowledge to Mining Parks that are currently trying to turn a "problem" into an "opportunity" for territorial development.
Miniere e minerali del distretto mercurifero del Monte Amiata
Dini A
2017
Abstract
The Monte Amiata area (Tuscany) was the third mining district in the world for the production of mercury (total 104473 t). Among the several mines of the district, the Abbadia San Salvatore mine takes a prominent position for having provided about 50% of the total production. The Abbadia San Salvatore mercury deposit is one of the largest in the world (after Almaden in Spain and Idrija in Slovenia) and it is the only Tuscan metalliferous deposit that can be included in the category of the "World-Class Ore Deposits". Despite the remarkable economic importance and the huge amount of mining data produced, Abbadia San Salvatore and the other smaller ore deposits of the Amiata District (Solforate, Morone-Selvena, Siele, Cornacchino, Cortevecchia, Monte Labbro, Cerreto Piano) still wait for a detailed scientific characterization. The most deficient aspect is the description of geometry, texture, and type of hydrothermal alteration of the ore bodies. On the occasion of the preparation of this volume, a thorough study of the scientific-mining literature and part of the mining documentation has been performed. The result is the definition of more accurate descriptive models of the Monte Amiata mercury deposits and the formulation of a general interpretive model that fits coherently in the framework of mercury deposits worldwide and, more generally, in the conceptual model of epithermal systems. Most of the Hg ore (mainly cinnabar with traces of meta-cinnabar and native mercury) that made the fortune of this district was made of masses of cinnabar-rich clay, the socalled "biocca" in the miner's jargon. The "biocca" constituted small replacement bodies (mantos, Hg up to 8%) in marly limestone lenses or, as an alternative, the matrix of breccia bodies both with a sub-vertical (hydrothermal breccia pipes, Hg content up to 3%), and a sub-horizontal (conformable hydrothermal breccia bodies, Hg content <= 1%) attitude. The few available analyses of the clay minerals are consistent with a kaolinite ± montmorillonite assemblage, typical of argillic and advanced argillic hydrothermal alteration. The detailed study of mining maps/sections of the three main deposits (Abbadia San Salvatore, Solforate-Siele and Morone-Selvena) allowed the definition of the following general interpretive model. The Hg deposits show a sub-horizontal, basal mineralised zone (conformable hydrothermal breccia bodies, stock-works, disseminations; low-medium Hg grade), from which sub-vertical breccia chimneys (hydrothermal breccia pipes, medium-high Hg grade) rise for 100-150 m up to, or near to the paleo-surface. In favourable case - presence of reactive lenses of marly limestone - the hydrothermal breccia pipes can be surrounded by manto-type replacement bodies (high Hg grade). The extraordinarily high mercury concentrations of this district transformed through time from a great economic-social opportunity to an environmental problem. Today, the awareness of the environmental remediation costs should not hide the great scientific and cultural potential of this mining region. The Monte Amiata World-Class Ore Deposits still deserve to be studied to help the understanding of hydrothermal processes in volcanic settings, and to provide new knowledge to Mining Parks that are currently trying to turn a "problem" into an "opportunity" for territorial development.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
