The occurrence and economic potential of nodular monazite in south-central Wales

Review of all Mineral Reconnaissance Programme panned concentrate data confirmed that very high (> 5000 ppm) levels of cerium, caused by the presence of nodular monazite, are characteristic of samples collected from catchments containing sedimentary rocks of Upper Cambrian to Silurian age deposited in the Welsh Basin. The largest panned concentrate anomalies (>l% Ce) are associated with rocks of Upper Ordovician age and the most extensive area containing these very high values is in south-central Wales, near Newcastle Emlyn. Follow-up panned concentrate sampling in this area showed that cerium anomalies caused by nodular monazite extend from tributary drainage into the main river systems and, in the Afon Teifi, persist for over 20 km into the estuary near Cardigan. Levels of monazite in stream sediment locally exceed minimum grades exploited in placer deposits, reaching 1.65% in the ~2 mm (sand, silt and clay) fraction of the samples collected. Consequently it is recommended that further work is undertaken to assess the concentrations of monazite and other heavy minerals in river and estuarine sediments, dune sands and beach deposits associated with the Afon Teifi and other rivers draining Upper Ordovician and Lower Silurian sedimentary rocks deposited in the Welsh Basin. Few other heavy resistate minerals were recorded in the sediments collected from the Afon Teifi, but the placer deposit potential of some other mom&e-bearing estuarine sediments in Wales is likely to be enhanced by the presence of additional economic minerals, notably gold. Mineralogical studies of nodular monazites found in the rocks of the Newcastle Emlyn area showed that they have very similar properties to those described from Central Wales, Belgium, France and Spain. They occur in mudrocks subjected to low-grade metamorphism, are less than 2 mm in size, ovoid to discoid in shape, dark grey in colour, and have a prominent inclusion fabric indistinguishable from the host-rock. The nodules are characterised by a low thorium and high europium content compared with monazites of igneous origin, and are compositionally zoned with LREE-rich rims. However, the monazites from the Newcastle Emlyn area have some distinctive properties: they are notably smaller and more ragged, and commonly contain more inclusions than those from Central Wales. They contain lower thorium and higher europium levels than many other nodular mom&es, and display complex chemical zonation with up to seven concentric zones, lo- 100 pm wide, distinguishable within a single nodule. In contrast to Central Wales, where nodules are concentrated in Llandovery-age hemipelagic mudstone horizons showing considerable enrichment in REE, nodules appeared to be dispersed in the mudstone-dominated succession of the Newcastle Emlyn area. Bulk analysis of these nodulebearing rocks showed that they do not contain unusually high overall levels of REEs and no evidence of stratabound REE enrichment was found in the survey area. The concentrations of nodules recorded both here and in the hemipelagic mudstones of Central Wales suggest that an economic deposit of nodular monazite in bedrock is unlikely to exist in Wales. It is believed that the nodules formed by post-depositional, pre-metamorphic diagenetic growth under physico-chemical conditions that are poorly understood, but which involved at least local saturation of REE with respect to REE phosphate coprecipitation in the pore-fluids. Under the anoxic conditions likely to have prevailed during early diagenesis, REE may have been released from iron-manganese hydrous oxides and other phases, and fmed by phosphate released during the decomposition of dispersed organic matter.