Genetic concepts on the formation of the Austrian magnesite and siderite mineralizations in the Eastern Alps of Austria

A consanguineous origin of the sparry siderite and magnesite mineralizations of the Eastern Alps has been repeatedly discussed in the past, often without the back-up of sound scientific arguments. Here, it is shown that the characteristics of these mineralizations including structures, fluid parameters, timing etc. are amazingly similar suggesting in fact the genetic linkage of these deposits. The siderite as well as the magnesite mineralizations of the Eastern Alps exhibit metasomatic-epigenetic structures of lens-shaped orebodies with dolomitic alteration rims as dominant features. The basic chemical characteristics of the ore forming fluids in these mineralizations are those of highly fractionated hypersaline fluids exhibiting all the features of residual bittern brines. Fluid invasion and mineralization structures strictly depend on the original lithology of the host rocks, mainly carbonatic sedimentary rocks. In Late Triassic times, these buried evaporitic brines were mobilized either by magmatic/metamorphic processes in the underlying crystalline units or by the superimposed sedimentary upload of the Triassic platform carbonates thus leading to magnesite formation. Initially these residual, bittern brines were completely free of Fe and rich in Mg suitable for the formation of magnesite. Deeper and more extensive circulation of these fluids and their interaction with the host-rock, resulted in the uptake of Fe and the formation of siderite.