Sulfur-rich monazite with high common Pb in ore-bearing schists from the Schellgaden mining district (Tauern Window, Eastern Alps)

Abstract

The compositional variation of accessory monazite in ore bearing micaschists from the Schellgaden mining district, Tauern Window, Eastern Alps, was studied by means of the electron microprobe. In ore-rich domains monazite yields unusually high sulfur contents (up to 2.5 wt.% SO3), which enter the monazite structure together with Ca and Sr as “anhydrite-celestine” component replacing P and REEs. The exchange reaction is S6++ (Ca, Sr)2+ = REE3++ P5+. Sulfur-rich monazite is intergrown with anglesite, pyromorphite or galena and shows oscillatory zoning indicating growth from S-bearing fluids. This type of S-enriched monazite yields very high common lead contents (up to 0.5 wt.% PbO) and unrealistic high apparent Th-U-total Pb single dates (> 1 Ga). However, S-enriched monazite grains provide a flat trendline in the Th* vs. Pb isochron diagram similar to the trendline defined through low-S, and low-Pb monazite crystals (0.1–1 wt.% SO3, < 0.05 wt.% PbO), which were observed in ore-poor parts of micaschists. Results from this study imply an Alpine rather than a pre-Alpine formation age for monazite and a strong S-rich fluid activity during the Alpine orogeny. Apart from this geological aspect, the current study also shows that the detection of sulfur in monazite may serve as a warning for a possible presence of common Pb.