LITHOS 0 ELSEVIER Lithos 37 (1996) 309-333 Breccia-hosted manganese-rich minerals of Alpi Apuane, 1taly:A marine, redox-generated deposit M. Francesclnelli a* * , M. Puxeddu b, G. Carcangiu ‘, M. Gattiglio d, F. Pannuti a a zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Dipa.rtimento di Science delta Terra, uia Trentino 51, Uniuersith di Cagliari, I-09127 Cagliari, Italy h Istituto Internazionale per le Ricer&e Geotermiche C.N.R., Piazza Solferino, 2 56100, Pisa, Italy ’ Centro Studi Geominerari e Mineralurgici C.N.R., Piazza d’Armi, 09127 Cagliari, Italy ’ Diparrimento di Scienze della Terra, Via Valperga Caluso 37. Uniuersitd di Torino, 10125 Turin, Italy Received 8 February 1995; accepted 12 September 1995 zyxwvutsrqponmlkjihgfedcbaZYXWVUT Abstract 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 f0, > lOAs for T= 350°C and fO,, 2 lOm6 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 w1 th 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 Al,O, + SiO, 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 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 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 sepamted 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 * Corresponding author 0024-4937/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDlOO24-4937(95)00026-7