The braunite (3Mn2O3·MnSiO3)-rich mineralization in the metasedimentary succession from southern Apennines (Italy): Genesis constraints

Abstract

A massive Mn-bearing mineralization, extensively exploited until the 70 s, locally occurs in the metasedimentary succession of the southern Apennines (at Mormanno site, Calabria-Lucania border, Italy). The mineralization mainly consists of three types distinguished by the relative percentages of the principal mineral phases: braunite, quartz, Mn oxi-hydroxides and Mn-rich epidote (piemontite). Petrographic observations revealed that the ore underwent medium- to high-grade metamorphism that was responsible of the growth of braunite at the expense of “primary” amorphous Mn oxi-hydroxides and quartz. During metamorphism, piemontite also formed where altered phyllosilicates are present in addition to braunite and quartz, suggesting that the aluminosilicates were source of Al necessary to the epidote crystallization. The most important geochemical features of the ore are high Mn contents (up to 71.6 wt% of MnO), low concentrations of transitional elements (Co + Ni+Cu + Zn < 0.1 wt%), high Ba (up to 4455 ppm) and variable total REEs (14.7 < ΣREEs < 329.7 ppm) contents. These are the typical characteristics of the ocean-floor hydrothermal Mn deposits, similarly to what observed in braunite ores occurring in the Alps and northern Apennines. The high LREE/HREE fractionations and the negative Ce and Eu anomalies characterizing the studied ore, suggest the mineralizing fluid was formed by a mixing solution consisting of a Mn-rich hydrothermal fluid and seawater, which attained the ideal condition for the ore precipitation in a distal oceanic area, far from the submarine hydrothermal vent.