Generation of Ce anomalies in SW Sardinian Mn ores

Sedimentary (pedogenic) and hydrothermal Mn ores are hosted by a volcanic succession in southwestern Sardinia. The sedimentary Mn ores are characterized by chondrite-normalized Ce anomalies (Ce/Ce*) of 1.08-3.40, whereas the hydrothermal Mn ores have Ce anomalies of 0.35-3.15. The presence of both negative and positive Ce anomalies in the hydrothermal Mn ores is related to the remobilization of rare earth elements (REEs) within the hydrothermal system, which occurred via the dissolution and redistribution of REE-bearing accessory minerals by mineralizing fluids. The REEs were removed from wall-rocks and redeposited within the Mn ores as apatite, monazite, and cerianite, which have textures characteristic of growth into a Mn-oxide matrix during mineralization. In addition, the redistribution of cerium led to a progressive increase in the magnitude of the Ce anomaly from E to W, suggesting that mineralizing fluids moved along an E-W-striking fault close to the Mn ore. In the sedimentary Mn ores, the spatial variability of Ce/Ce* values probably relates to changes in the deposition environment, from organic-rich to organic-poor. Positive Ce anomalies with Ce/Ce* values of 1.08-1.75 are associated with samples from thick sedimentary layers within a paleosoil or samples from veins related to paleosoils, and potentially formed in a environment where organic speciation of the REEs acted to limit cerianite precipitation. Conversely, spheroidal concretions of Mn-oxides within pumice and ash layers have larger positive Ce anomalies (Ce/Ce* > 2.5) and likely formed in an organic-free environment matter, with REE speciation in circulating fluids dominated by inorganic species. Finally, given the large range in Ce/Ce* ratios of Mn ores, we stress the need to carefully consider sampling methodologies, to ensure the accurate interpretation and determination of ore deposit models, the identification of mechanisms involved in Ce distribution, and the relationships between Ce and Mn phases.