Context : NGC 6558 is a bulge globular cluster with a blue horizontal branch ( BHB ) , combined with a metallicity of [ Fe/H ] \approx -1.0 .
It is similar to HP 1 and NGC 6522 , which could be among the oldest objects in the Galaxy .
Element abundances in these clusters could reveal the nature of the first supernovae .

Aims : We aim to carry out detailed spectroscopic analysis for four red giants of NGC 6558 , in order to derive the abundances of the light elements C , N , O , Na , Al , the \alpha -elements Mg , Si , Ca , Ti , and the heavy elements Y , Ba , and Eu .

Methods : High-resolution spectra of four stars with FLAMES-UVES @ VLT UT2-Kueyen were analysed .
Spectroscopic parameter-derivation was based on excitation and ionization equilibrium of Fe I and Fe II .

Results : This analysis results in a metallicity of [ Fe/H ] = -1.17 \pm 0.10 for NGC 6558 .
We find the expected \alpha -element enhancements in O and Mg with [ O/Fe ] =+0.40 , [ Mg/Fe ] =+0.33 , and low enhancements in Si and Ca .
Ti has a moderate enhancement of [ Ti/Fe ] =+0.22 .
The r-element Eu appears very enhanced with a mean value of [ Eu/Fe ] =+0.63 .
Ba appears to have a solar abundance ratio relative to Fe .

Conclusions : NGC 6558 shows an abundance pattern that could be typical of the oldest inner bulge globular clusters , together with the pattern in the similar clusters NGC 6522 and HP 1 .
They show low abundances of the odd-Z elements Na and Al , and of the explosive nucleosynthesis \alpha -elements Si , Ca , and Ti .
The hydrostatic burning \alpha -elements O and Mg are normally enhanced as expected in old stars enriched with yields from core-collapse supernovae , and the iron-peak elements Mn , Cu , Zn show low abundances , which is expected for Mn and Cu , but not for Zn .
Finally , the cluster trio NGC 6558 , NGC 6522 , and HP 1 show similar abundance patterns .