Genesis of gray monazites; evidence from the Paleozoic of Belgium

Abstract

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 conglomerates, 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 in the sediment initiated by P mobility during diagenesis. The genetic model for gray monazite mineralization assumes three essential and complementary phenomena: (1) sedimentation of clays in a reducing marine environment, (2) a phosphate anomaly in the same lithological sequence, and (3) evolution to low-grade epizonal metamorphic conditions. These nonrestrictive genetic conditions clearly suggest that gray monazites have been generally overlooked and must be much more widespread than believed. The economic interest of gray monazite lies in potential stream or beach placers. Their low Th content and comparatively high Eu content are interesting features in the scope of ore beneficiation. (Figs. 1 and 2). Discovery of Gray Monazite in the Stavelot Massif NODULAR gray monazite has been identified in numerous panned stream sediment concentrates collected in an area located south of Spa where a 15-km ridge (Vecqu•e) composed of Middle and Upper Cambrian sedimentary rocks extends northeastward from the Ambl•ve valleyOther Belgian occurrences of gray monazites were recently reported by Nonnon (1984) in the Cambrian of the Rocroi massif and by Limbourg (1986) in Lower Devonian rocks of the NeufchSteau synclinorium (Fig. 1). Elsewhere in Europe, gray monazite has been recorded in the Ordovician and Dinantian of Brittany (Donnot et al., 1973), in the Ordovician of northwest Spain (Vaquero, 1979), and in lower Paleozoic rocks of Wales and southwest England (Read et al., 1987). Other occurrences of nodular gray monazites were reported by Rosenblum and Mosier (1983) notably from Taiwan, USSR, Zaire, Gabon, Alaska, and Montana.

In situ U–Pb dating and element mapping of three generations of monazite: Unravelling cryptic tectonothermal events in low-grade terranes

“…Neodymium shows the greatest variation of the MREE, from $19 wt% Nd 2 O 3 in the cores to $10% in the rims, while La 2 O 3 ranges from $8 wt% in the cores to $18 wt% in the rims. The monazite grains have elevated Eu 2 O 3 values ($0.5 wt% in the cores; $0.15 wt% in the rims), which are typical of monazites from slate belts (Rosenblum and Mosier, 1983;Burnotte et al, 1989;Cabella et al, 2001) and other low-temperature settings.…”

“…Monazite has long been considered to be thermodynamically unstable during diagenesis and sub-amphibolite facies metamorphism (Overstreet, 1967), however, a number of recent studies have identified monazite growth during diagenesis (Burnotte et al, 1989;Milodowski and Zalasiewicz, 1991;Lev et al, 1998), prehnitepumpelleyite facies metamorphism (Cabella et al, 2001;Rasmussen et al, 2001Rasmussen et al, , 2005 and greenschist facies metamorphism (Franz et al, 1996;Rasmussen et al, 2002;Wing et al, 2003).…”

MONAZITE-(Sm), A NEW MEMBER OF THE MONAZITE GROUP FROM THE ANNIE CLAIM #3 GRANITIC PEGMATITE, SOUTHEASTERN MANITOBA

“…Otherwise, Sm invariably is subordinate to minor with respect to the major REE, as is Gd. This is abundantly documented in general reviews of monazite (e.g., Neumann et al 1966, Fleischer & Altschuler 1969, as well as in statistically extensive studies of monazite from a variety of specific petrological environments: granitic suites, granitic pegmatites, tin deposits, Alpine fissures, sediments and metamorphic rocks (e.g., Lee & Bastron 1967, Donnot et al 1973, Petruk & Owens 1975, Burnotte et al 1989, Wark & Miller 1993, Watt 1995, Förster & Rhede 1995, Franz et al 1996, van Emden et al 1997, Förster 1998. What may be the reason for the unique enrichment of the Annie Claim #3 monazite-(Sm) in Sm and Gd?…”

REE-bearing minerals in Drava river sediments, Slovenia, and their potential origin

“…Detrital monazite texture, mode of occurrence and its geochemistry have been found to point toward its genetic environment, therefore, enabling separation of igneous monazite from its hydrothermal counterpart (buRnotte et al, 1989;scHandl et al, 1994;WanG et al, 1994). scHandl & GoRton (2004 demonstrated that hydrothermal monazite can be distinguished from igneous monazite by combining petrographic analyses with advanced electron microscopy techniques, performed on monazite mineral grains, and concluded that hydrothermal monazite (or greenschist-amphibolite metamorphic facies monazite) generally contains lower concentrations of ThO 2 (< 1 wt%), compared to igneous monazite, in which concentrations of ThO 2 were found to be significantly higher (3 ->5 wt%).…”

“…Knowledge of drainage area and therefrom eroded rocks, of the river and its tributaries combined with in-detail chemical analyses of minerals, both from parent rock and sediments, is crucial for determining the provenance of the grains downstream. The region of Hohe Tauern, in detail discussed by scHiMd et al (2013) andscHaRF et al (2013), is considered to be the source of gold found in fluvial sediments of river Drava (beRManec et al, 2014), therefore broader area of Hohe Tauern could also be considered as a source of minerals in heavy mineral sands.…”