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We present a high resolution spectroscopic analysis of 62 red giants in theMilky Way globular cluster NGC5286. We have determined abundances ofrepresentative light proton-capture, alpha, Fe-peak and neutron-capture elementgroups, and combined them with photometry of multiple sequences observed alongthe colour-magnitude diagram. Our principal results are: (i) a broad, bimodaldistribution in s-process element abundance ratios, with two main groups, thes-poor and s-rich groups; (ii) substantial star-to-star Fe variations, with thes-rich stars having higher Fe, e.g. <[Fe/H]>_s-rich - <[Fe/H]>_s-poor ~0.2~dex; and (iii) the presence of O-Na-Al (anti-)correlations in both stellargroups. We have defined a new photometric index, c_{BVI}=(B-V)-(V-I), tomaximise the separation in the colour-magnitude diagram between the two stellargroups with different Fe and s-element content, and this index is notsignificantly affected by variations in light elements (such as the O-Naanticorrelation). The variations in the overall metallicity present in NGC5286add this object to the class of "anomalous" GCs. Furthermore, the chemicalabundance pattern of NGC5286 resembles that observed in some of the anomalousGCs, e.g. M22, NGC1851, M2, and the more extreme Omega Centauri, that also showinternal variations in s-elements, and in light elements within stars withdifferent Fe and s-elements content. In view of the common variations ins-elements, we propose the term s-Fe-anomalous GCs to describe this sub-classof objects. The similarities in chemical abundance ratios between these objectsstrongly suggest similar formation and evolution histories, possibly associatedwith an origin in tidally disrupted dwarf satellites.

Iron and s-elements abundance variations in NGC5286: comparison with anomalous globular clusters and Milky Way satellites