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Franceschelli M., Puxeddu M., Carcangiu G., Gattiglio M., Pannuti F. Breccia-hosted manganese-rich minerals of Alpi Apuane, Italy: a marine, redox-generated deposit. In: Lithos, vol. 37 pp. 309 - 333. Elsevier, 1996.
 
 
Abstract
(English)
Mn-rich beds occur within the “Brecce di Seravezza” Formation (BSFm) intercalated between Norian metadolostone-Megalodontic marble and Hettangian marble in the Alpi Apuane, (Italy). The BSFm is a rockfall talus deposit that accumulates at the base of the footwall scarp of normal faults bordering extensional marine coastal basins. The type sequence of metabreccia ends with a bed of chloritoid schist interpreted as the metamorphic equivalent of lateritic soils. The four main types of Mn mineral assemblages are: (1) braunite, piemontite, and rare hausmannite; (2) piemontite, braunite, hollandite and minor rhodochrosite and kutnahorite; (3) braunite, hollandite and rare piemontite. The other minerals are: quartz, calcite, muscovite, phlogopite, baryte and minor hematite, rutile and apatite. During the Alpine orogeny pressures of 4-6 kb and temperatures of 350°-380°C were attained; Mn assemblages with hematite and phlogopite indicate fO2 > 10−8 for T = 350°C and fO2 ≥ 10−6 when braunite appears. On the Mn-rich rocks Ti, Fe, Nb, Sn, Ta, Hf, Th, Ga, Rb, Pr, K, Na, Nd were identified as detrital in origin and are positively correlated with Al. U is positively correlated with Co, Pb, Bi, Mo. The rare earth element patterns normalized to seawater show a negative slope towards the heavy rare earth elements, with positive Ce anomalies in some samples, positive Ce and Eu anomalies in other samples and negative Ce and positive Eu anomalies in a third group. The Eu concentrations are nearly constant and the Eu anomalies reflect differences in the concentration of other rare earth elements; for Ce, a positive correlation with Al2O3 + SiO2 indicate that a detrital signature prevails on that produced by duration of seawater exposure. But a significant good correlation of La, Tm, Lu with Mg could indicate, at least for these elements a seawater signature. Only in some samples, Ba, Eu, Sr show anomalously high concentrations, likely of hydrothermal origin. However, generally in all the chemical variation diagrams the BSFm samples fall outside the field of hydrothermal and hydrogenous deposits as defined in the literature for other Mn deposits. In addition, metabasites and metavolcaniclastic beds are absent from the BSFm sequence and BSFm coastal basins were for the most part separated from the open ocean and related streams, that must be excluded as possible Mn-sources. Owing to the arid climate of Late Triassic the runoff was negligible and the groundwater, oxidizing and alkaline, were unable to mobilize Mn. The BSFm basins are then the most suitable depositional environment for Mn. The water column was stagnant and stratified. Only where bottom of the basin was deeper than the oxic-anoxic boundary, reducing marine bottom waters percolated within the fractured basement and leached Mn from disseminated minerals of metamorphic rocks and, then, rising along fault and fractures, supplied Mn to the bottom waters. Afterwards, when tectonic uplift and sealevel drop brought the bottom into the oxic field, Mn precipitated and formed the Mn-rich matrix of the BSFm deposits.
URL: http://https://www.journals.elsevier.com/lithos
Subject petrography and mineralogy
Alpi Apuane


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