Abstract

Iron played a decisive part when uranyl phosphates (‘yellow U ores’) formed during supergene alteration of the aplitic and pegmatitic rocks of the Hagendorf Pegmatite Province. Three different supergene U mineral successions, referred to in the current study as pathways I to III, were identified. Pathway I began with the release of PO42− by the dissolution of rockbridgeite and ended after short-distance transport under alkaline reducing or slightly acidic oxidizing conditions in the precipitation of bassetite. Pathway II is an advanced form of pathway I that did not stop with the precipitation of bassetite, but progressed by acidic Cu-bearing meteoric waters under oxidizing conditions up to the torbernite precipitation stage. Minor amounts of Mn or Ca may have led to a deviation from the normal pathway into the stability fields of lehnerite or autunite, respectively, both of which may occur either as solid solution series or in a layered intergrowth with torbernite. Limonite-cored torbernite has been described for the first time and only exists in pathway III. Unlike its counterpart pathways I and II, which appeared at the end of a complex polystage element recycling process of secondary Fe phosphates under fluctuating redox and pH conditions, limonite-cored torbernite resulted from a monostage transformation of primary ‘black Fe-U ore minerals’ under strongly oxidizing conditions and short-distance element transport. These restricted physicochemical conditions caused the immediate stabilization of the Fe-U-P system and by doing so the U-Pb ratios of the black ore progenitor were well preserved in the limonitic core. Torbernite was analysed for U, Th and Pb isotopes by laser ablation inductively coupled plasma mass spectrometry techniques. For one domain the data yielded a formation age of 4.55±0.02 Ma, which corresponds to Miocene-Pliocene weathering and geomorphological processes in the study area. A second domain gave a discordia with an upper intercept age of 549±12 Ma, interpreted to represent a thermal event at the Precambrian-Cambrian boundary. Ferrocolumbite is found exclusively in the mineralization of pathway III. Due to the proximity of ferrocolumbite to torbernite, limonite-cored torbernite probably inherited the 549 Ma age from ferrocolumbite during supergene alteration.

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