Hydrothermal origin of manganese in the high-pressure metasedimentary sequence ofPraborna (Aosta Valley, Western Alps)

The manganese ore deposit of Praborna crops out in the Zermatt-Saas unit of the Western Alps, in the St. Marcel valley. It represents a Jurassic ophiolitic sedimentary cover subducted at high-pressure conditions during the Alpine orogeny. Major- and trace-element analyses of representative samples of the ore and the host metasediments were collected to geochemically characterise the deposit. Selected phases (piemontite, braunite, garnet, clinopyroxene, white mica and manganiandrosite) were investigated with ion and electron microprobes to link the mineral chemistry to the bulk-rock chemistry. Compared to shales, Praborna is enriched in Mn (up to 38.7 wt% Mn2O3) and in many trace elements (Sc, Co, Ni, Cu, Ge, As, Sr, Ag, Sb, Te, Ba, Tl, Pb and Bi). The bulk-rock REE pattern suggests 20% hydrogenous and 80% hydrothermal inputs in the proto-ore. Compared to the shale, the hanging-wall Mn-poor schists share with the Mn ore body the enrichment in Sc, Mn, Co, Sr and Te, suggesting a common enrichment process involving these elements. The REE pattern suggests a sedimentary origin for these schists, which were probably composed of clay mixed with components of volcanic origin. In order to confirm the hydrothermal origin of the Praborna Mn ore deposit, we built up a database of more than 5000 data of modern hydrogenous and hydrothermal oceanic Mn deposits worldwide, adding data of oceanic Mn-rich sediments and of the Ligurian Mn ore deposits, which are thought to be the unmetamorphosed geological equivalent of Praborna. The classic ternary Mn-Fe-(Cu + Co + Ni) diagram, the agglomerative hierarchical clustering and the principal-component analysis, which takes into account a larger set of elements, strongly support the hypothesis of an oceanic hydrothermal origin for manganese in the Praborna and in the Ligurian ore deposits.