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Tracking mineral evolution and element mobility during albitisation and subsequent kaolinisation of phyllite rocks; a case study from the Verrucano of Monti Pisani, Tuscany, Italy

Massimo D'Orazio, Paolo Fulignati, Anna Gioncada and Francesco Cavalcante
Tracking mineral evolution and element mobility during albitisation and subsequent kaolinisation of phyllite rocks; a case study from the Verrucano of Monti Pisani, Tuscany, Italy
Mineralogical Magazine (June 2022) 86 (3): 459-477

Abstract

This work describes the first occurrence of albitite rocks in the Middle Triassic Verruca Formation, Monti Pisani, Northern Apennines, northern Tuscany, Italy. The albitite formed by Na-metasomatism of phyllites ('potassic white mica' + quartz + 'chlorite' + hematite + albite) in an amagmatic environment. The albitisation process took place after the Miocene main phases of Apenninic deformation and was followed by the formation of veins of Fe-carbonate + quartz. Hydrothermal alteration progressed with the ingression, possibly favoured by the increase of permeability due to albitisation, of a slightly acidic, oxidising, aqueous fluid that led to the pervasive kaolinisation of the albitite and to the complete transformation of the Fe-carbonate of the veins into Fe-hydroxides. This stage was followed by supergene alteration that led to the formation of a pervasive network of halloysite veinlets and colloform (P-Al-Si)-bearing Fe-hydroxides. Finally, the hydrothermally altered rock underwent a localised brittle fracturing without new minerals being formed. The prominent compositional changes occurring during this multi-stage hydrothermal process were the inversion of the Na (sub 2) O/K (sub 2) O ratio of the whole rock (from 0.07 in the pristine phyllite to up to 200 for the kaolinised albitite), the loss of Fe and Mg, and the enrichment of Sb. The MREE were partially lost, whereas LREE and HREE behaved conservatively. Though pervasive hydrothermal alteration occurrences are common in central-southern Tuscany, mostly related to the post-collisional extensional regime, lithospheric thinning and emplacement of magmatic bodies in the crust, the rare Monti Pisani kaolinised albitite described in this investigation expands the effects of post-collisional hydrothermal activity in Tuscany northwards, far from potential magmatic sources.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 86
Serial Issue: 3
Title: Tracking mineral evolution and element mobility during albitisation and subsequent kaolinisation of phyllite rocks; a case study from the Verrucano of Monti Pisani, Tuscany, Italy
Affiliation: Universita di Pisa, Dipartimento di Scienze della Terra, Pisa, Italy
Pages: 459-477
Published: 202206
Text Language: English
Publisher: Cambridge University Press on behalf of the Mineralogical Society of the UK and Ireland, London, United Kingdom
References: 51
Accession Number: 2022-053830
Categories: General mineralogy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, sketch maps
N43°00'00" - N44°19'60", E09°40'00" - E11°00'00"
Secondary Affiliation: CNR Istituto di Metodologie per l'Analisi Ambientale, ITA, Italy
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2022, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2022
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