Genesis of gray monazites; evidence from the Paleozoic of Belgium

Systematic mineralogical investigations, carried out on panned stream concentrates from the northwestern part of the Cambro-Ordovician Stavelot massif (Belgium), led to the discovery of nodular gray monazites. These were later identified in situ in black slates associated with conglomeratic levels containing phosphatic material. Microscopic investigation shows that gray monazites are authigenic and predate cleavage. To test the hypothesis of a sedimentary origin for rare earth elements, samples of shales, phosphatic onglomerates, and monazite concentrates were submitted to sequential inductively coupled plasma analysis. Results show that the rare earth element pattern in gray monazites is a reflection of the abundance of these elements in the host rock. This suggests a model invoking desorption of the rare earth elements from their clayey support and differential precipitation of rare earth phosphates inthe sediment initiated by P mobility during diagenesis. The genetic model for gray monazite mineralization assumes three essential nd complementary phenomena: (1) sedimentation f clays in a reducing marine environment, (2) a phosphate anomaly in the same lithological sequence, and (3) evo-lution to low-grade epizonal metamorphic onditions.