Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by MARESCOTTI, P. Articles by FREZZOTTI, M. L. Search for Related Content GeoRef GeoRef Citation

Alteration of braunite ores from Eastern Liguria (Italy) during syntectonic veining processes

¹ Dipartimento per lo Studio del Territorio e delle sue Risorse. Università di Genova, 16132 Genova, Italy. e-mail: marescot{at}dipleris.unige.it
² Dipartimento di Scienze della Terra, Università di Siena, 53100 Siena, Italy. E-mail: frezzotti{at}dst.unisi.it

The manganese ores of the Northern Apennine ophiolites (Val Gravegila, Eastern Liguria, Italy) occur within chert sequences, mainly as stratiform layers (0.1-1 m thick) or massive lenses (5-20 m thick). The ores originated in the Ligurian-Piedmont oceanic basin (Middle Callovian) during turbiditic resedimentation of hydrothermally Mn-enriched submarine muds. During Upper Cretaceous-Lower Cenozoic orogenic events, the primary sedimentary oxide assemblages were completely recrystallized to braunite + quartz assemblages under prehnite-pumpellyte facies conditions (T = 275 ± 25 °C; P = 2.5 ± 0.5 kbar). This tectonic event induced mobilisation of Mn and Si, and thickening of the mineralised layers. A complex network of sigmoidal quartz veins formed at this stage. The interaction between the Mn mineralisation and the circulating fluids generated centimetric to decimetric reaction rims in the wall rock, where the braunite + quartz assemblage is replaced by Mn silicates (mainly bementite, johannsenite, parsettensite and rhodonite) and carbonates (mainly Mn-bearing calcite and rhodochrosite) with a zoned distribution. Mineral zoning points to an early interaction between the Mn mineralisation and H₂O-CO₂ fluids with high water activity. Two distinct types of fluids are present in syntectonic quartz veins, namely a low-salinity water-rich fluid, and a Mn-Ca-Na-Mg-Fe-bearing aqueous solution that in all probability represent a relict of an early fluid phase circulating during breakdown of braunite. The isochore distribution related to low-salinity fluids indicates that veining processes took place during the main tectono-metamorphic events, at peak P-T conditions.

Key-words: Liguria, manganese ore, braunite, fluid inclusions, metamorphic fluids.

This article has been cited by other articles:

				J. M. Hughes, J. M. Hughes, J. Rakovan, R. Bracco, and M. E. Gunter The atomic arrangement of the ganophyllite-group modulated layer silicates as determined from the orthorhombic dimorph of tamaite, with the elusive 16.8 A ganophyllite-group superstructure revealed American Mineralogist, August 1, 2003; 88(8-9): 1324 - 1330. [Abstract] [Full Text] [PDF]

				M. Haruna, H. Satoh, Y. Banno, M. Kono, and M. Bunno MINERALOGICAL AND OXYGEN ISOTOPIC CONSTRAINTS ON THE ORIGIN OF THE CONTACT-METAMORPHOSED BEDDED MANGANESE DEPOSIT AT NAGASAWA, JAPAN Can Mineral, August 1, 2002; 40(4): 1069 - 1089. [Abstract] [Full Text] [PDF]