Evolution of unconformity-related Mn - Fe - As vein mineralization, Sailauf (Germany): Insight from major and trace elements in oxide and carbonate minerals

The Sailauf Mn Fe As vein mineralization located in the Spessart district (central Germany) is characterized by complex hydrothermal carbonate and oxide assemblages. The mineralization is hosted by a Permian rhyolite body and is structurally related to the Variscan unconformity that separates Permian sedimentary rocks from the underlying Variscan crystalline basement. The hydrothermal vein system has been studied by optical microscopy electron microprobe and LA ICPMS analysis of major and trace elements (including the REE). Four distinct mineralization stages that are characterized by diagnostic carbonate oxide assemblages are identified which are (I) pre ore stage (2) ore stage 1 (3) ore stage 2 and (4) the replacement stage. Hydrothermal carbonates show complex compositional trends in Ca Mn (Fe+Mg) space and comprise calcite Mn calcite Fe Mg calcite Ca rhodochrosite and Ca kutnahorite. Oxide assemblages are dominated by braunite and hematite with minor amounts of manganite and hausmannite. The mineralization is enriched in a distinct suite of trace elements including As W Pb Zn and Cu. Analysis of the paragenetic evolution in conjunction with the major and trace element data allows to reconstruct the fluid evolution of the hydrothermal system. The first order change in mineralogy between the two main ore stages (Mn oxides and calcite evolving into hematite and Mn rich calcite) records a pronounced shift in fluid pH and silica activity of the system. This interpretation is also supported by variations in the behavior of Ce in different carbonate generations. The late stage replacement carbonates relate to destabilization of the primary ore assemblages. The distinct geological setting immediately below the Permian unconformity in conjunction with the mineralogical and chemical data suggests that dynamic fluid mixing processes involving basement derived brines and more shallow groundwaters were important in the formation of the Sailauf Mn Fe As mineralization. The significant enrichment in the As W Pb Zn Cu element suite resembles that of other Mn Fe deposits and points to felsic lithologies as the main metal source of the mineralization. (C) 2012 Elsevier B.V. All rights reserved.